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STRYKER CORP. v. INTERMEDICS ORTHOPEDICS

July 11, 1995

STRYKER CORPORATION and OSTEONICS CORPORATION, Plaintiffs, against INTERMEDICS ORTHOPEDICS, INC. and MARLI MEDICAL SUPPLIES, INC., Defendants.

ARTHUR D. SPATT, United States District Judge


The opinion of the court was delivered by: ARTHUR D. SPATT

Table of Contents

  INTRODUCTION

 I. BACKGROUND

 1. The 023 Patent and Claims

 2. The APR II and the Parties' Contentions

 II. AS TO PATENT INFRINGEMENT

 1. Literal Infringement

 
A. The Legal Standard
 
B. The Infringed Claims
 
(1) Limitation (iii):
 
"Enabling Flexing" of the Shaft
 
(a) Claims Construction
 
(b) Infringement Determination
 
(2) Limitation (ii): "Distal Tip
 
Integral With The Distal End of the
 
Stem"
 
(a) Claims Construction
 
(b) Infringement Determination
 
(3) Limitation (v): A Fixation
 
Resistant Surface-Finish
 
(a) Claims Construction
 
(b) Infringement Determination
 
C. Other Evidence of Infringement:
 
Development of the APR II Distal Sleeve
 
After the February 1988 AAOS Meeting
 
D. Conclusions and Findings as to Literal
 
Infringement

 2. Infringement Under the Doctrine of Equivalents

 
A. The Legal Standard
 
B. The Defendant's Equivalency
 
Contentions
 
C. Equivalence of the APR II
 
and the 023 Patent
 
D. Prosecution History Estoppel
 
E. Conclusion and Findings as to
 
Equivalents

 III. AS TO UNENFORCEABILITY:

 INEQUITABLE CONDUCT AND INVALIDITY

 1. Inequitable Conduct

 
A. Applicable Legal Standard
 
B. Determination of Inequitable Conduct
 
(1) Materiality of the Whiteside Device
 
(2) Intent to Deceive

 2. Invalidity

 
A. Invalidity Under 35 U.S.C. §§ 102 and 103
 
(1) Legal Standard
 
(a) The Hoffman-Daimler Patent
 
(b) The Lee and Ling Patents
 
B. Invalidity Under 35 U.S.C. § 112
 
(1) The Legal Standard
 
(2) Definiteness Analysis

 3. Conclusion and Findings as to Unenforceability

 IV. WILLFUL INFRINGEMENT

 V. DAMAGES

 1. Lost Profits

 
A. The The Legal Standard
 
B. Evidence Regarding Osteonics's Lost Profits
 
(1) Panduit Factor One: Demand for
 
the Patented Pruduct
 
(2) Panduit Factor Two: The Absence
 
of Acceptable Non-Infringing
 
Substitutes
 
(3) Panduit Factor Three: Osteonics's
 
Manufacturing and Marketing
 
Capacity
 
(4) Panduit Factor Four: The Amount of
 
Profit Osteonics Would Have Made
 
C. Conclusions and Findings on Lost Profits

 2. Reasonable Royalty

 3. Prejudgment Interest

 4. Enhancement of Damages and Attorneys' Fees

 
A. Enhancement of Damage Award
 
B. Attorneys' Fees

 5. Injunctive Relief

 VI. CONCLUSION

 SPATT, District Judge:

 INTRODUCTION

 In years past, a person with an arthritic hip was generally relegated to a wheelchair. The advent of modern medical prosthesis technology now permits such a person to walk and resume a normal life through the use of an artificial socket, ball and neck of the hip joint, known as a hip implant prosthesis or femoral prosthesis. Today, the manufacture and supply of prosthetic devices is a growing and profitable industry. The marketing of such devices is estimated to have generated $ 2.2 billion dollars in revenue to manufacturers in 1990, and has averaged approximately a 15 percent annual dollar growth since 1988. Approximately one quarter of the orthopedic market involves hip implants.

 The present case concerns the alleged infringement of a femoral prosthesis patent. The patent in suit is United States Patent No. 4,888,023 ("the 023 patent"), which is entitled "Femoral Prosthesis with Uncoupled Distal Tip." The 023 patent was filed with the United States Patent Office ("Patent Office") on January 19, 1988 and assigned by its inventor to the plaintiff Osteonics Corporation ("Osteonics" or "plaintiff"). The Patent Office issued the patent on December 19, 1989.

 Among the key features of the 023 patent is a metal distal (lower end) tip adapted for engagement with the prosthesis's stem by means of complementary tapers. The term "distal" means furthest away from the point of attachment. The term "proximal" means nearest the point of attachment. As related to a hip implant or femoral prosthesis and as the terms are used in this opinion, distal means at the lower end of the prosthetic device, and proximal means at the upper end of the device. A diagram of the 023 patent is annexed as Appendix A.

 Osteonics is a subsidiary of the Stryker Corporation and is in the business of "researching, developing, designing, manufacturing, marketing and selling hip and knee implants." (Tr. at 368). *fn1" Since 1988, Osteonics has manufactured and supplied a femoral prosthesis known as the Omniflex, which is the commercial embodiment of the 023 patent. A diagram of the Omniflex is annexed as Appendix B.

 The defendant Intermedics Orthopedics, Inc. (the "defendant" or "Intermedics" or "IOI") has, since January 1990, manufactured and supplied to the medical and orthopedic industry a femoral prosthesis known as the APR II. A diagram of the APR II is annexed as Appendix C. Each APR II has a metal distal sleeve which engages with the distal portion of the stem by means of a complementary taper. The successor to the APR II is the APR II-T, which was designed in 1991 and introduced in 1992. The three changes embodied in the APR II-T are (1) a tapered neck to fit the ceramic ball, (2) wrapped porous coating all around the proximal portion of the device, and (3) a multi-sized hollowed stem. The APR II-T retained the tapered stem and the distal sleeve which "serves the same purpose as the sleeve made available with the [APR II]." (Tr. at 762).

 Both Osteonics and Intermedics supply a full complement of Omniflex and APR II stem sizes and correspondingly sized distal tips or sleeves to the hospital operating room, so that the surgeon is able to assemble and use the stem and the tip or sleeve when medically appropriate. The complete APR II hip system sells for between $ 3800 and $ 4000.

 This infringement suit was brought by Stryker and Osteonics against Intermedics and Marli Medical Supplies, Inc. ("Marli"), an Intermedics distributor. Osteonics charges Intermedics with literal and willful infringement of claims 8, 10, and 12 of the 023 patent by reason of its manufacture and sale of the APR II and its successor, the APR II-T. (Unless otherwise indicated, references to both the APR II and APR II-T will be to the "APR II"). Osteonics seeks damages, treble damages, injunctive relief and attorneys' fees pursuant to 35 U.S.C. §§ 284, 285 (1988). The defendants deny the allegations of literal and willful infringement. Intermedics also asserts that the 023 patent is invalid in light of prior art and because its claims are indefinite, and further asserts that the patent is unenforceable due to inequitable conduct by Osteonics.

 This case was tried by the Court over a substantial period of time on all the issues, including liability and damages. This Opinion includes the Court's findings of fact and conclusions of law on all the issues of liability and damages. Because the parties raise a large number of separate issues, the Court's findings and conclusions will be integrated and set forth in the discussion of each particular issue.

 Although there are two named plaintiffs and two named defendants, the Court will refer to the two plaintiffs as "the plaintiff" or "Osteonics," and to the two defendants as "the defendant" or "Intermedics" or "IOI."

 I. BACKGROUND

 1. The 023 Patent and Claims

 The socket of the implant is screwed into the acetabulum of the pelvis. The ball is then fitted into the socket. The ball and socket are joined to the femur through the neck on the stem of the prosthesis. The stem itself is inserted longitudinally into the intermedullary canal of the resected femur. In order to prepare the femur, the surgeon must ream out the canal for a length equal to the stem. The distal portion of the canal is reamed until only the harder cortical bone is left. The proximal portion of the canal is reamed by a broach that allows for some of the softer cancellous bone to remain. When the stem is inserted into the canal, the wedge-shaped affixation surface allows the stem to be affixed to the proximal portion of the femur by a tight press fit, with the prosthesis seated on the femoral calcar. The porous surface on the affixation surface is conducive to ingrowth of the softer cancellous bone onto the proximal end of the prosthesis. The stem may also be cemented into the intermedullary canal.

 When a person moves, various forces act on the femur. In a normal hip joint, the load (force) on the femur is transmitted through the ball and socket to the proximal portion of the femur, which carries the brunt of the load. In the case of a stem-type femoral implant, the load on the femur from the joint is transmitted down the metal stem of the implant and is borne by the femur along the entire length of the stem, and not just at the proximal portion of the femur.

  Significantly, forces acting on the ball of the prosthesis will tend to cause the prosthesis to swing at the proximal end of the stem, thus causing a bending stress on the stem and creating forces tending to move the distal end of the stem axially (up and down) and transversely (side to side), relative to the femur. This side to side movement of the stem is called "toggling" in the hip implant industry. In addition, and particularly in cases where the stem is cemented to the canal, the stresses acting on the stem tend to concentrate at the distal end of the stem, thus reducing the load stress at the proximal end of the stem. This is called "stress shielding" in the hip implant industry. Stress shielding is undesirable because reducing stress in the proximal end of the stem, where the stress is best accommodated relative to the bone, causes the proximal portion of the femur to atrophy, thereby loosening the affixation of the proximal end of the stem to the femur and requiring replacement of the entire prosthesis. Both toggling and the loosening of the prosthesis resulting from stress shielding cause severe thigh pain to the implant patient.

  On January 19, 1988, Robert G. Averill and Robert C. Cohen filed the 023 patent application. The patent specifies that it is an improvement of the stem-type femoral prosthesis, because it contains a distal tip which interlocks with the distal end of the stem by means of complementary tapers. The distal tip is made from a fixation resistant metal alloy, which prevents the tip from affixing to the intermedullary canal. According to Osteonics the 023 patent is a "break-through invention in the field of artificial hips," because it solves the problems of toggling and stress shielding. Toggling is prevented by placing the distal tip within the harder cortical bone of the femur, such that it fits tightly within the reamed canal. In this way, transverse movements of the distal end of the stem, which is locked onto the tip, are precluded. Proximal stress shielding is prevented by maintaining the tip unaffixed to the wall of the canal, thus allowing axial and rotational movement of the stem relative to the femur. Allowing this movement prevents the concentration of stress at the distal end of the stem.

  The patent description summarizes the invention as follows:

  
The above objects and advantages . . . are attained by the present invention [through] the improvement comprising: a distal tip at the distal end of the stem, the distal tip being spaced axially downwardly from the affixation surface a distance sufficient to enable seating of the distal tip within harder portions of the bone in the wall of the femur when the affixation surface is properly seated in the prepared femur and having an external peripheral surface for engaging the harder portions of bone to confine the distal tip against transverse movements within the passage upon completion of the implantation, a fixation-resistant surface finish on the external peripheral surface for maintaining the distal tip unaffixed to the femur and moveable axially relative to the wall of the femur to permit axial displacement of the distal tip and the distal end in response to forces applied to the prosthesis during use of the prosthesis.

  Plaintiff's Exh. 2, at Col. 1, line 65 to Col. 2, line 50. *fn2"

  The 023 patent lists 14 claims of exclusivity. The relevant claims which are at issue in this suit will be described and discussed below.

  Finally, the patent specifies that having complementary tapers between the stem and the distal tip allows for removal and replacement of the distal tip with one of another sized diameter. According to the patent specification, the interchangeability of distal tips allows a wide variety of distal tip diameter sizes to be fitted onto one stem. This interchangeability is called "modularity" in the industry. One of the several benefits of modularity is that the number of stems necessary to accommodate the different sized tips is reduced.

  The Omniflex is the preferred embodiment of the 023 patent. It was introduced in 1988, and apparently was a financial success. During the period from 1988 through April 30, 1993, Osteonics sold almost 29,000 of the Omniflex hip implant devices amounting to a sales volume in excess of one hundred million dollars.

  The parties agree to the following uncontested facts concerning the Omniflex:

  1. The Omniflex hip prosthesis is manufactured and sold by the plaintiff Osteonics and is a preferred commercial embodiment of the prosthetic hip implant device described in the 023 patent and embraced by at least representative claims 8, 10 and 12 of the 023 patent.

  2. As of May 31, 1992, Osteonics had sold 24,640 Omniflex Hip System hip prostheses embodying the 023 patent claims.

  2. The APR II and the Parties' Contentions

  During the time period that the Omniflex was being introduced, the defendant Intermedics was having problems with its own hip implant, called the APR I. In 1988, the defendant marketed the APR I without a distal tip component. This device apparently caused a 24 percent incident of hip pain and, with its sales fading fast, Intermedics would soon find itself out of the hip implant business. Intermedics therefore embarked on the development of the APR II. As ultimately developed, the APR II included a distal sleeve component.

  The parties agree to the following uncontested facts concerning the APR II:

  1. Each of the APR II and APR II-T hip prostheses includes a femoral stem-type component for implantation in the resected proximal end of a femur. Each of the APR II and APR II-T hip prostheses includes a stem component to be received within the prepared femur (collectively referred to hereinafter as the "APR II and APR II-T stem[s]").

  2. Each APR II and APR II-T stem has a proximal end.

  3. Each APR II and APR II-T stem has an affixation surface adjacent to the proximal end for enabling the stem to be affixed in place when seated properly within the prepared femur.

  4. Each APR II and APR II-T stem has a distal end spaced axially downwardly from the proximal end for reception within a passage created in the wall of the prepared femur.

  5. At all relevant times, Intermedics has manufactured � and  modular distal sleeve components (collectively referred to as "APR II modular distal sleeve[s]", or "distal sleeve[s]") for optional use during surgery with each size APR II and APR II-T titanium stem.

  6. Each APR II modular distal sleeve component is made from a titanium metal alloy.

  7. Each APR II modular distal sleeve is formed with a Morse-type taper on the internal bore thereof which is complementary in design to a Morse-type taper formed on the correspondingly sized APR II or APR II-T titanium stem. The complementary Morse-type tapers formed on the APR II and APR II-T stem/modular distal sleeve components are specially adapted to lockingly engage the modular distal sleeve onto the stem.

  8. Each APR II modular distal sleeve component is adapted to be secured to the APR II or APR II-T stem such that, if the sleeve is used, all movements of the stem during use in the body result in corresponding movements of the modular distal sleeve.

  9. Each APR II modular distal sleeve has an external peripheral surface so that, if the sleeve is used, it engages the harder cortical bone portions of the femoral canal and confines the distal sleeve against transverse movements within the canal upon completion of the implantation.

  10. Each APR II modular distal sleeve is cylindrical.

  The plaintiff contends that the APR II infringes the 023 patent. According to the plaintiff, in early February 1988 Intermedics officials attended a conference of the American Association of Orthopedic Surgeons ("AAOS") in Atlanta, Georgia. At this conference, they obtained a brochure describing the Omniflex. Two weeks later, the plaintiff contends the defendant began designing an APR II distal sleeve modeled on the Omniflex distal tip. Prior to the conference, the plaintiff claims the APR II did not include a distal sleeve component and Intermedics did not have any plan for developing such a sleeve. The February 1988 AAOS meeting is discussed in more detail later in this Opinion.

  Osteonics proceeds with its claim of patent infringement on two theories. First, it contends that there was a literal infringement in that the APR II infringes claims 8, 10 and 12 of the 023 patent. Second, the plaintiff also contends that the APR II infringes claims 8, 10 and 12 of the 023 patent under the doctrine of equivalents.

  On the other hand, the defendant contends that it devised and created the design of APR II by the ideas of its own personnel and without any reference to the Omniflex. Further, according to the defendant, the APR II functions in a manner "contrary to the design philosophy embodied in the Omniflex implant." According to the defendant, the Omniflex was designed to promote flexibility of the stem, while the APR II was designed to eliminate movement in the femur and promote "solid, stable proximal (upper) fit." Moreover, the defendant contends that its distal sleeve has been used in less than half of the cases in which the APR II was implanted, and that the percentage of cases in which the distal sleeve was actually implanted is as low as 5 to 20 percent with most surgeons.

  In addition, as a defense, the defendant asserts that prior to the plaintiff's patent, a Dr. Leo Whiteside designed a hip implant with a plastic distal sleeve, which was intended to prevent the prosthesis from moving within the femoral canal. The defendant asserts that because of the non-disclosure of the "prior art" Whiteside distal sleeve to the Patent Office, the plaintiff engaged in inequitable conduct and its patent is unenforceable. With regard to this defense of inequitable conduct, the plaintiff contends that the Whiteside device is not "prior art" to the 023 patent in suit.

  The defendant further asserts as defenses that the 023 patent is invalid, because (1) certain of its claims are vague and indefinite, and (2) various prior art render the patent obvious. The plaintiff denies that the patent is invalid on these grounds.

  II. AS TO PATENT INFRINGEMENT

  1. Literal Infringement

  A. The Legal Standard

  Section 271(a) of the Patent Statute, 35 U.S.C. § 271(a) (1988 & Supp. IV 1992), provides in relevant part that "whoever without authority makes, uses or sells any patented invention, within the United States during the term of the patent therefor, infringes the patent." In addition, section 271(b) provides that "[w]hoever actively induces infringement of a patent shall be liable as an infringer." 35 U.S.C. § 271(b). See also Joy Technologies, Inc. v. Flakt, Inc., 6 F.3d 770, 774 (Fed.Cir. 1993); Moleculon Research Corp. v. CBS, Inc., 793 F.2d 1261, 1272 (Fed. Cir. 1986), cert. denied, 479 U.S. 1030, 93 L. Ed. 2d 829, 107 S. Ct. 875 (1987).

  In a patent infringement case the burden of proof is on the plaintiff to prove the defendant's infringement by a preponderance of the evidence. Conroy v. Reebok Int'l, Ltd., 14 F.3d 1570, 1573 (Fed. Cir. 1994); Rite-Hite Corp. v. Kelley Co., Inc., 819 F.2d 1120, 1123 (Fed. Cir. 1987).

  The determination of infringement is a two-step process. The first step involves determining the meaning and scope of the patent claims asserted to be infringed. The second step entails comparing the properly construed claims to the product accused of infringement. Markman v. Westview Instruments, Inc., 52 F.3d 967, 976 (Fed. Cir. 1995) (in banc) (citing Read Corp. v. Portec, Inc., 970 F.2d 816, 821 (Fed. Cir. 1992)). The first step, referred to as claim construction or interpretation, is a question of law. Markman, 52 F.3d at 976; Electro Medical Systems, S.A., v. Cooper Life Sciences, Inc., 34 F.3d 1048, 1053 (Fed.Cir. 1994). The second step, application of the construed claim to the accused product, is a question of fact. Id.; Read, 970 F.2d at 822.

  The United States Court of Appeals for the Federal Circuit has analogized the interpretation of patent claims to statutory interpretation. Markman, 52 F.3d at 987. To determine the meaning of the patent claims, the Court looks to three sources: the claims, the patent specification, and the patent's prosecution history. Id. at 979 (citing cases); Electro Medical, 34 F.3d at 1054. Moreover, expert testimony, including evidence of how those skilled in the art would interpret the claims, may also be used to assist the court in coming to a correct conclusion as to the true meaning of the language claims. Markman, 52 F.3d at 979-80 (internal quotations and numerous citations omitted).

  According to the Federal Circuit, the numerically paragraphed claims of the patent "provide the concise formal definition of the invention," and "particularly point out and distinctly claim" the subject matter which the applicant regards as his or her invention. It is to "these wordings that one must look to determine whether there has been infringement." Corning Glass Works v. Sumitomo Elec. U.S.A., Inc., 868 F.2d 1251, 1258 (Fed. Cir. 1989) (quoting Autogiro Co. of America v. United States, 181 Ct. Cl. 55, 384 F.2d 391, 395-96 (Ct. Cl. 1967)).

  Further, the wordings of a claim "describe and point out the invention by a series of limiting words or phrases," which are called limitations. Corning Glass, 868 F.2d at 1258. In construing the claim, "all the limitations of a claim must be considered meaningful." Unique Concepts, Inc. v. Brown, 939 F.2d 1558, 1562 (Fed.Cir. 1991); accord Texas Instruments Inc. v. United States Int'l Trade Comm'n, 988 F.2d 1165, 1171 (Fed.Cir. 1993) (claim cannot be construed so as to render the disputed claim language mere surplusage).

  Claims must also be read in view of the specification of which they are a part. "The specification contains a written description of the invention that must enable one of ordinary skill in the art to make and use the invention." Markman, 52 F.3d at 979. However, claims are not to be interpreted by adding limitations appearing only in the specification. Thus, "although the specification may well indicate that certain embodiments are preferred, particular embodiments appearing in the specification will not be read into the claims when the claim language is broader than such embodiments." Electro Medical, 34 F.3d at 1054.

  If in evidence, the prosecution history of the patent should also be considered because it is the "undisputed public record" of the patent proceedings, and confirms the construction of the patent by the patentee. Markman, 52 F.3d at 980. The prosecution history limits the interpretation of claim terms so as to exclude any interpretation that was disclaimed during prosecution. Southwall Technologies, Inc. v. Cardinal IG Co., 54 F.3d 1570, 34 U.S.P.Q.2D (BNA) 1673, 1995 WL 274383 at *4 (Fed.Cir. May 10, 1995) (citing cases).

  In addition, extrinsic evidence may be used to help the court understand the claims language. Such evidence consists of all evidence external to the patent and prosecution history, including expert and inventor testimony, dictionaries, and learned treatises. This evidence is helpful to explain scientific principles, the meaning of technical terms, and terms of art that appear in the patent and prosecution history. The extrinsic evidence may be used only for the court's understanding of the patent, and not for the purpose of varying or contradicting the terms of the claim. Markman, 52 F.3d at 981 (citing U.S. Indus. Chem. Inc. v. Carbide & Carbon Chems. Corp., 315 U.S. 668, 678, 62 S. Ct. 839, 844, 86 L. Ed. 1105 (1942)). It should be noted that, by using such extrinsic evidence to assist in its construction of the patent claims, the court is not crediting certain evidence over other evidence or making factual evidentiary findings. Markman, 52 F.3d at 981.

  Once the claims have been properly construed, they must be applied to the accused product to determine infringement. Conroy, 34 F.3d at 1573. As stated in the seminal patent case of Graver Tank & Mfg. Co. v. Linde Air Prods. Co. :

  
In determining whether an accused device or composition infringes a valid patent, resort must be had in the first instance to the words of the claim. If accused matter falls clearly within the claim, infringement is made out and that is the end of it.

  Graver Tank, 339 U.S. 605, 607, 70 S. Ct. 854, 855, 94 L. Ed. 1097 (1950).

  In conducting this second step of the infringement inquiry, the physical similarities between the accused device and the patentee's commercial embodiment of a patented invention are not to be compared. Rather, the proper comparison is between the patent claims and the accused device. International Visual Corp. v. Crown Metal Mfg. Co., 991 F.2d 768, 771-72 (Fed.Cir. 1993); Datascope Corp. v. SMEC, Inc., 879 F.2d 820, 824 (Fed.Cir. 1989) ("the district court lost sight of this court's repeated caution that it is claims, not commercial embodiments, that are infringed.") cert. denied, 493 U.S. 1024, 107 L. Ed. 2d 747, 110 S. Ct. 729 (1990). As explained by the Federal Circuit, in determining infringement, "the words of the claim must first be interpreted, and, as properly interpreted, they must be 'read on' the accused structure to determine whether each of the limitations recited in the claim is present in the accused structure." Corning Glass, 868 F.2d at 1258 (citations omitted).

  B. The Infringed Claims

  The plaintiff contends that the APR II literally infringes on claims 8, 10 and 12 of the 023 patent. Claim 8 incorporates claims 1, 4 and 5, and is delineated as follows:

  
1. (i) In a stem-type femoral prosthesis for implantation in a resected proximal end of a femur, the prosthesis including a stem to be received within the prepared femur, the stem having a proximal end, an affixation surface adjacent the proximal end for enabling the stem to be affixed in place when seated properly within the prepared femur, and a distal end spaced axially downwardly from the proximal end for reception within a passage created in the wall of the prepared femur, the improvement comprising:
  
(ii) a distal tip integral with the distal end of the stem, the distal tip being spaced axially downwardly from the affixation surface a distance sufficient to enable seating of the distal tip within harder portions of the bone in the wall of the femur when the affixation surface is properly seated in the prepared femur,
  
(iii) the stem including a shaft portion located between the affixation surface and the distal tip, the shaft portion having a diameter smaller than the corresponding diameter of the distal tip for enabling flexing in the shaft portion,
  
(iv) and the distal tip having an external peripheral surface for engaging said harder portions of bone to confine the distal tip against transverse movements within the passage upon completion of the implantation,
  
(v) and a fixation-resistant surface-finish on the external peripheral surface for maintaining the distal tip unaffixed to the femur and moveable axially relative to the wall of the femur to permit axial displacement of the distal tip and the distal end in response to forces applied to the prosthesis during use of the prosthesis.
  
4. The improvement of claim 1 including securing means for securing the distal tip to the stem for selective removal and replacement of the distal tip.
  
5. The improvement of claim 4 wherein the securing means includes a securing mechanism responsive to relative axial movement between the stem and the distal tip for securing and releasing the distal tip and the stem.
  
8. The improvement of claim 5 wherein the securing mechanism includes an internal bore in the distal tip and complementary tapered surfaces on the stem and on the internal bore of the distal tip.

  Claim 10 includes all of the elements of claims 8 and 9, and is delineated as follows:

  
Claim 10:
  
Same elements as claim 8.
  
9. The improvement of claim 8 wherein the external peripheral surface of the distal tip includes means in the form of an axial length long enough to provide an area of contact between the distal tip and the wall of the femur, when the stem is seated properly in the passage for maintaining a relatively low level of stress on the wall of the femur at the location of the distal tip in response to forces applied to the prosthesis which result in transverse forces at the distal tip.
  
10. The improvement of claim 9 wherein the external peripheral surface is cylindrical such that the distal tip is rotatable about the central axis thereof relative to the wall of the femur.

  Claim 12 includes the elements of claims 1 and 11, and is delineated as follows:

  
Claim 12:
  
All the elements of claim 1.
  
11. The improvement of claim 1 wherein the fixation-resistant finish is a smooth finish on the external peripheral surface of the distal tip.
  
12. The improvement of claim 11 wherein the distal tip is constructed of a metal alloy and the fixation-resistant finish comprises a polished surface.

  The defendant contends that the APR II does not infringe claims 8, 10 and 12. According to the defendant, these claims contain the following limitations of claim 1: (1) limitation (iii): "the stem including a shaft portion . . . having a diameter smaller than the corresponding diameter of the distal tip for enabling flexing in the shaft portion."; (2) limitation (ii): "a distal tip integral with the distal end of the stem"; and (3) limitation (v): "a fixation-resistant surface-finish on the external peripheral surface [of the distal tip]." Because each of these limitations is contained in independent claim 1, on which claims 8, 10 and 12 are dependant, Intermedics contends that the APR II does not infringe these claims.

  The Court will construe the claim limitations, and review each of the contentions with respect to the claim alleged to be infringed.

  (1) Limitation (iii):

  "Enabling Flexing" of the Shaft

  (a) Claims Construction

  (i) Patent Language and Specification

  Limitation (iii) of claim 1 states that the 023 patent improvement consists of the stem shaft portion located between the affixation surface and the distal tip "having a diameter smaller than the corresponding diameter of the distal tip for enabling flexing in the shaft portion." Patent at 7:3-7.

  Although the term "enabling flexing" is not defined in the patent specification, understanding the problems the patent sought to address and a reading of the patent specification enables the Court to interpret the meaning of the term.

  In describing the problems which led to the invention, the patent states in its "Background of the Invention" that certain components of the forces exerted upon the prosthesis during use are transferred to distal portions of the stem, rather than being accommodated by the proximal portion of the prosthesis. This results in concentration of stresses at non-proximal portions of the femur and a corresponding reduction in stresses at the proximal portion of the femur, a condition described above as stress shielding. Moreover, forces on the ball of the prosthesis will tend to swing the prosthesis about the proximal end of the stem, "thereby exerting bending stresses on the stem and establishing forces tending to move the distal end of the stem" in a transverse direction relative to the femur, a condition described above as toggling, and axially relative to the femur. Patent at 1:31-36.

  The patent specification explains that the invention's distal tip is essentially locked onto the distal end of the stem by a system of complementary tapers, so that movement by the distal end of the stem will result in a corresponding movement by the distal tip. By using a distal tip that is the same diameter as the reamed passage inside the femur, the distal tip engages or presses against the harder portion of cortical wall inside the femur. By so pressing against the wall, the tip prevents transverse movements of the stem within the reamed passage, i.e., prevents toggling, while at the same time allowing the transverse force at the distal end to be transmitted to the femur. Significantly, by creating a cylindrically shaped distal tip with a fixation-resistant surface that prevents the tip from affixing to the femur, the distal tip prevents stress shielding by (1) allowing the bending movements of the stem to displace the distal tip, and thus the distal end of the stem, axially upwards, (2) allowing the bending forces to displace the distal tip axially downward in certain cases, and (3) allowing the tip to rotate about its central axis in response to torsional forces. Patent at 4:4-44.

  Moreover, in order to reduce the stress in the area of contact of the distal tip's surface and the cortical bone of the femur, the external surface of the distal tip must be of sufficient length to maintain a relatively low stress in that area. However, the length of the surface of the distal tip must also be short enough "to preserve flexibility in the shaft of the stem." As described in the specification:

  
Flexibility in the shaft enables the stem to respond in the manner indicated [bending, and moving axially upward] so that the combination of the prosthesis and the femur can manage the forces to be applied to the prosthesis. It is desirable that the shaft have sufficient flexibility to preclude excessive stiffening of the femur by the prosthesis along the implant site. In the preferred configuration, the diameter of the shaft of the stem above the distal tip is made smaller than the diameter of the external peripheral surface of the distal tip itself so as to render the shaft flexible, while still providing a contact area between the distal tip and the wall of the femur great enough to maintain relatively low contact stresses.

  Patent at 5:44-57 (emphasis supplied).

  (ii). The Patent's Prosecution History.

  The patent's prosecution history (Plaintiff's Exh. 568; Defendant's Exh. AKR) corroborates this interpretation of the term "enabling flexing" found in the third limitation of claim 1. The language of the third limitation was found in claim 14 of initial disclosure of the patent filed with the Patent Office on January 19, 1988. The initial patent disclosure described the bending of the shaft and corresponding axial movement of the distal tip. (Plaintiff's Exh. 568 at 8-10, 24).

  After reviewing the disclosure, the Patent Examiner rejected claims 1 and 14, along with other claims, in a letter dated July 14, 1988. The grounds for the rejection of the relevant claims were that claim 1 was anticipated by the Lord patent, and claims 1 and 14 were unpatentable over the Witzel and Freeman patents. (Plaintiff's Exh. 568 at 33, 35).

  The Patent Examiner did not discuss the Lord patent. In discussing the Witzel patent, the Patent Examiner concluded that the Witzel device performed the same function as the 023 patent, because the stem of the Witzel patent moved axially within a distal tip:

  
Wherein Witzel's tip (25 resin) moves axially within the tip in response to axial and bending loads applicant's device, as claimed, has a tip (metal) fixed to the stem and the stem moves axially in the medullary canal in response to axial and bending loads. It is noted that both devices perform the same function: they both transfer and distribute stresses at the tip to the calcar bone . . . by permitting the stem of the prosthesis to move axially with respect to the bone thereby transferring and distributing bending stresses from the offset loading on the hip socket ball to the calcar bone through the tip.

  (Plaintiff's Exh. 568 at 35).

  With regard to the Freeman patent, the Examiner concluded that patent also performs the same function as the 023 patent:

  
Freeman discloses a typical stem-type femoral prosthesis made of metal with a substantially circular stem. End tip fits onto the tapered end of the stem with a matching taper surface. . . . It is also clear . . . that [the] stem is narrower (at least in portions) than the tip. . . . It is presumed that the taper surfaces of the tip and stem lock the two together. . . . It is also noted that Freeman also, as Witzel, uses the tip for stress distribution of the forces at the distal end of the stem due to the loading of the ball.

  (Plaintiff's Exh. 568 at 37).

  In response to the Patent Examiner's rejection, the 023 applicant responded by, among other things, amending claim 1 in the initial patent disclosure to include the language of limitation (iii) of the present claim 1. The applicant also distinguished the 023 patent from the Lord, Witzel and Freeman patents, based on stem shaft flexibility.

  In distinguishing the 023 patent from the Lord patent, the applicant indicated that the Lord device failed to enable flexibility in the stem shaft, because the diameter of the Lord stem was greater than or equal to the diameter of the distal tip:

  
[W]hatever the function of the Lord distal tip, the distal tip disclosed in Lord has a diameter which is the same as, or less than, the diameter of the remainder of the femoral stem, and no part of the stem has a diameter less than any diameter of the distal tip. Accordingly, the reference does not contemplate flexing of the stem and axial movement of the distal tip in the medullary canal in response to loads placed on the implant during service. The subject matter of the present claims includes structures which enables the femoral stem to flex in response to loads placed on the implant during service, and provides an uncoupled distal tip which is displaced axially within the medullary canal, relative to the bone of the femur, as the femoral stem is flexed, to accommodate the flexing without deleterious effects. Thus, all of the present claims set forth that the femoral stem includes a shaft portion with a diameter smaller than the corresponding diameter of the distal tip for enabling the aforesaid flexing of the shaft portion and axial displacement of the distal tip.

  (Plaintiff's Exh. 568 at 49-50) (emphasis supplied).

  Similarly, in distinguishing the Witzel patent the applicant indicated that the distal sleeve in the Witzel device was fixed to the canal, and thereby did not provide any axial displacement of the sleeve. This results in relatively higher stresses at the distal end of the Witzel device:

  
In Witzel, a small sleeve of synthetic resin material is first secured in the medullary canal, and then the shaft of the femoral stem is inserted into the sleeve. The shaft can move relative to the sleeve, but there is no displacement of the sleeve relative to the bone of the femur during service. In the construction of the present claims, the distal tip is affixed to the distal end of the stem . . . and flexing of the shaft . . . during service is accommodated by displacement of the distal tip relative to the surrounding bone of the femur. The difference in structure yields several advantages. Among those advantages is the distribution of stresses during service. . . . Thus, in the Witzel construction, the articulating surfaces between the stem and the sleeve are of limited area, resulting in higher stresses[.]

  (Plaintiff's Exh. 568 at 50-51).

  With regard to the Freeman patent, the applicant explained that the Freeman device was not similar to the 023 patent, because it used a distal tip affixed to the bone that acted as a "stop" to centralize the stem shaft, and which was not intended to move during use of the stem. (Plaintiff's Exh. 568 at 52).

  These claims were ultimately accepted by the Patent Office and the 023 patent was issued.

  Thus a review of the patent's prosecution history corroborates that the term "enabling flexing" is construed as allowing the stem shaft of the prosthesis to flex and move axially upwards, by the use of a distal tip secured to the distal end of the stem which (i) is of a larger diameter than the stem shaft, and (ii) is not affixed to the bone.

  (iii). Evidence from Extrinsic Sources.

  The scientific principles of shaft flexing were also explained by DR. ALBERT H. BURSTEIN, who is a biomechanical engineer and expert in the field of hip implant devices. Dr. Burstein has "fabricated over 3,000 total joint implants" of which 25 percent were hip implants. Dr. Burstein is a person considered skilled in the art of the subject matter of the 023 patent during the relevant time period involved in this law suit.

  During direct examination concerning the 023 patent teachings as compared to the APR II, he explained that the flexing movement of the shaft -- which is only on the order of a thousandth of an inch, i.e., less than a hair's breadth -- follows well established bio-mechanical principles:

  
A Now the way you prevent bending loads from being transmitted distally is to make the stem flexible. Then if it deforms a little bit it doesn't transmit much bending load. The stiffer the stem, the more bending load it can transmit when the bone deforms.
  
So the patent teaches that the way you prevent the transmission of bending loads is to make a portion of the stem of diameter smaller than the sleeve so that it is flexible, so that you have enabled flexing of that portion of the stem that's above the sleeve.
  
Now when you load it, this portion flexes more freely than if it would have if it were a constant big diameter and transmit proportionately less bending load down here on the bone. It transmits more bending load up on the proximal area, it transmits more, it transmits more proximally. That's a well-known principle in biomechanics and taught by the invention and it shows you how to practice that invention by reducing this diameter.
  
A The patent teaches in order for this to work it must be free to move up and down in the bone in accordance with how much the prosthesis normally wants to move. That is, under load this prosthesis will move on the order of a thousandth of an inch or two, and it must be free to do so, and in order to do so, you cannot allow the distal end sleeve or tip to become fixed to the bone. If it becomes fixed to the bone it transects the bone. So by having an affixation resisting surface, that will not allow the bone to grow into or onto the surface, it becomes free to move up and down.
  
Q Dr. Burstein, would you look at Column 5, lines 34-44, and tell the Court in your own words what that means or what that teaching is?
  
A What the patent is . . . teaching . . . is it is telling the practitioner of the art [of] how long the sleeve can be made and still function according to this patent. It is saying the sleeve or tip has to be made long enough, that is, it must have some minimum length, so that when it is in the body and you put the load on the prosthesis [the tip] pushes against the side of the bone. . . . The tip should not create a stress condition on the femur that the femur cannot accept. In plain language it shouldn't push to hard.
  
The way you control it is to make the tip long enough so it has enough conduct [sic contact] area on the surface so it shouldn't push too hard.
  
[The patent] also says there's another limit. The longer you make it, the less stem there is left over to produce the flexibility you want. So don't make it so long as to not leave enough stem left over so that you have flexibility, and it tells you essentially the longer you make it, the more reduced the flexibility will be in the leftover portion of the system.

  (Tr. at 131-135) (emphasis supplied). Dr. Burstein's elucidation of flexibility is consistent with the Court's interpretation of the patent limitation.

  (b) Infringement Determination

  The defendant makes a sophisticated argument that the claim limitation language at issue requires a stem shaft of lesser diameter than the distal tip for the purpose of enabling flexing. According to the defendant, any flexing of the stem shaft that is the natural, inherent result of a smaller diameter shaft relative to the tip is not excluded by the claim. Thus, according to Intermedics, "it would be unfair to extend the patent to cover products that do not have a reduced shaft diameter that exists for the purpose of enabling flexing. " Defendant's Post-Trial Brief at 3. Indeed, Intermedics goes so far as to contend that the language of the claim limitation probably requires that the stem must be significantly narrower over a substantial portion of the shaft length than the diameter of the distal tip.

  Based on its interpretation of the claim limitation language, the defendant contends that the APR II does not infringe the first claim of the 023 patent, because the stem geometry of the APR II relative to the distal sleeve does not purposefully enable flexing of the stem. According to the defendant, flexibility was not a design criteria of the APR II. Rather, the inventor incorporated the optional distal sleeve to achieve rigid cortical fit and fill against the cortical bone in the diaphyseal (shaft section) area of the femur for patients with a certain bone geometry. Moreover, Intermedics contends that the purpose of having exposed stem shaft above the distal sleeve in the APR II is to prevent impingement by a full-length sleeve covering the entire shaft on the anterior cortical portion of the femur. Any flexing of the APR II stem shaft due to its having a smaller diameter than the distal sleeve is, according to the defendant, the result of natural, inherent physical principles and not intentional in the design of the APR II stem and distal sleeve.

  More importantly, the Court is not persuaded by Intermedics's contention that the APR II stem shaft flexing is the result of inherent forces as opposed to purposeful design. Notwithstanding the testimony of DR. LAWRENCE DORR, the chief designer of the APR II prosthesis and a major witness on behalf of Intermedics, and other witnesses on behalf of Intermedics, the record, including admissions and the documentary evidence, shows -- and the Court so finds -- that the defendant's contentions concerning shaft flexing are belied by the defendant's own admissions that the APR II distal sleeve was intended to enable flexibility in the stem shaft above the sleeve, as well as to add bulk or fill to the canal.

  In a document prepared by Intermedics entitled "The APR II System -- Developed by Larry Dorr, M.D. -- Manufactured by Intermedics Orthopedics, Inc.", Dr. Dorr states:

  
This is Dr. Larry Dorr. I would like to discuss with you the design rationale of the APR II Hip System.
  
What we can try to achieve, is the best relationship we can get between the stem and the bone with the least stiff stem possible in the canal to give the bone it's best chance to be able to respond optimally when the stem and the bone have to move together. That is to say, we would like to decrease the differential deflection that occurs between stem and the bone when load is applied to the hip through the stem. By using a sleeve, we allow some increased flexibility in the stem which will improve this relationship.

  (Plaintiff's Exh. 210 at 13) (emphasis supplied).

  In a videotape presentation with regard to the APR II, Dr. Dorr states:

  
DORR: Now the broach with the sleeve attached there and there's a screw mechanism that screws on the sleeve here so it cannot disengage in the canal and with the sleeve you can see the difference in the regard to the amount of substrate metal that would be involved if we had to go to an implant that was all that size. So that we have improved the flexibility of the implant also by using the sleeve and only having to use this much bulk for the remainder of the implant.

  (Plaintiff's Exh. 117 at 13) (emphasis supplied).

  Testimony from Intermedics company officials reveals that the reason for designing the APR II was the commercial failure of the APR I, in part because it caused thigh pain resulting from toggling. The development of the APR II was, thus, also geared towards preventing toggling.

  NICHOLAS CINDRICH, the President of Intermedics, testified that: "I don t read patents," and that in 1989 "APR I sales were fading fast," and Intermedics required a new hip system:

  
Q Prior to leaving IOI, did you ever read the 023 patent?
  
A Huh-uh, I don't read patents.
  
Q At the time that IOI began its commercial launch of the APR II, was the APR I still on the market?
  
A Yes, whose sales were fading fast.
  
Q Was it your understanding that the APR II product with the distal sleeve component was important to IOI from the standpoint of rejuvenating the sales that the PAR (sic APR) product was losing?
  
A For all practical purposes, IOI was finding itself out of the hip business with the APR I. We had to get back into the hip business and I knew that it required a new hip system, and we just happened to call it the APR II.
  
Q And why was [the APR II] better?

  (Tr. at 285-286) (emphasis supplied).

  WARLONEX JOSE BELEN, the Intermedics Director of Marketing, testified in a deposition as follows:

  
Q Prior to the use of a distal sleeve component for the APR II, what was the alternative for preventing the toggling problem in connection with a hip prosthesis stem?
  
A We didn't have one.

  (Plaintiff's Exh. 759, Deposition Transcript at 79). Testifying at the trial, Belen agreed that toggling and thigh pain were problems with the APR I, and that the APR II was designed to prevent toggling at the distal end of the stem:

  
Q Now, during this entire design process, coming to the APR II Hip System, as part of your review and case studies that you talked about, you also became aware that the APR I system had an extremely high occurrence of thigh pain; is that correct?
  
A That's correct.
  
Q And one of the objectives in designing the APR II was to stop the thigh pain; is that correct?
  
A Yes.
  
Q And did you have an understanding at the time you were involved in the design of APR II that toggling motion at the distal end of the stem may have contributed to thigh pain?
  
A That's correct. That was my hypothesis.
  
Q And, the APR II prosthesis, wasn't that designed so as to prevent toggling of the distal end of the stem?
  
A Yes.
  
Q And isn't it true that a design feature of the option of a distal sling (sic sleeve) were designed to put on the distal stem the prevented (sic) the transverse motion or toggling to occur?
  
A Yes.
  
Q And prior to this use of a distal sleeve for the APR II, isn't it true that IOI had no alternative product for preventing the toggling problem in connection with the distal end of a hip prosthesis?
  
A True.
  
Q And isn't it true prior to the use of the distal sleeve on the APR II device IOI did not have a practical alternative for solving this toggling problem?
  
A I will say true.

  (Tr. at 1018-1020). Belen also testified that an additional product feature of the APR II was to "reduce the stiffness at the distal end of the stem." (Tr. at 997).

  Other documents prepared by Intermedics corroborate that the distal sleeve is designed to permit some flexibility. For example, in an Intermedics document called "An Introduction to the APR II Hip System," BRIAN OLIND, the Product Manager for the APR II Hip System, discussed the APR II with RONNIE BURCHETT, the group product manager for IOI Hip Systems:

  
BRIAN OLIND: Well, how does the APR II system address stem/bone ratio?
  
RON BURCHETT: Stem/bone ratio is addressed by allowing the distal end of the prothesis to be adjusted in diameter with the sleeves. By adding a 2mm or 4mm sleeve you can adjust the distal diameter. . . . But this stem is not as stiff as a 19mm stem because it still has the 15mm core diameter.

  (Plaintiff's Exh. 219 at 3) (emphasis supplied).

  Burchett also discussed the flexibility of the APR II in his deposition:

  
Q Based on that example is it fair to say that the use of the APR II distal sleeve allows IOI to achieve better distal fill, yet, at the same time, use a smaller diameter and, thus, more flexible stem than it otherwise would be able to use to achieve that same fill without a sleeve.
  
A The design parameters for the II were based on APR I. When we designed the II we had the APR I clinical results to compare to.
  
The stem itself has a different geometry than the APR I; in that, the distal cross-section is circular, not a rectangle. Because of that, this stem is more flexible than the APR I.
  
Q Mr. Burchett, please look at Plaintiff's Exhibit 6 for a moment, which is the sample APR II stem and sleeve.
  
Do you have any reason to doubt that the shaft portion of the APR II stem located above the distal sleeve is capable of flexing in the body in response to bending stresses applied to the prosthesis?
  
A No. There should be a certain amount of flexibility. Yes.

  (Plaintiff's Exh. 760, Deposition Testimony at 91-92, 195) (emphasis supplied).

  FREDERICK S. GEORGETTE, the Intermedics Manager of Material, testified in a deposition as follows:

  
Q During the time you worked at IOI, was it your understanding that the APR II distal sleeve, when implanted in the body would be capable of a small amount of axial motion relative to the femur bone in response to bending stresses applied to the prosthesis?
  
A Yes.

  (Plaintiff's Exh. 772 at 13). Georgette also testified to the following:

  
Q During the time you worked for IOI was it also your understanding that the APR II distal sleeve when implanted in the body [would] also be capable of a small amount of rotational motion relative to the femur bone in response to rotational or . . . torsional motions applied to the prosthesis?
  
A Yes.

  (Tr. at 1244-1245) (Plaintiff's Exh. 772 at 14).

  KEVIN DELANEY, formerly the defendant's Vice President of Marketing and Sales, hedged when asked about the distal sleeve and increased flexibility. He didn't recall and he didn't remember anything on that subject. (Tr. at 2171). However, he did say:

  
Q Has Intermedics in the past addressed the issue of flexibility in any of the stems?
  
A I can recall it being a topic of discussion for the revision system.
  
Q Did Intermedics undertake any design changes to achieving increased --
  
THE COURT: To achieve.
  
MR. JONES: Yes.
  
Q -- to achieve increased flexibility in its revision stem?
  
A I believe so.
  
Q What design changes were those?
  
A The feature of a slotted distal stem in the revision prosthesis was done to achieve that.

  (Tr. at 2178).

  Finally, the APR II Design Rationale states: "What is the purpose of the modular sleeves? Decreases stiffness, saves bone, increases surgical options." (Plaintiff's Exh. 14 at 1).

  Similarly, an APR II-T document distributed at its national sales meeting in November 1992, included the following statement:

  
Another clinic problem often associated with non-cemented hip prostheses is thigh pain. It has been suggested that distal stiffness mismatch is one of the causes of end of stem pain. It is widely accepted that more flexible implants are needed to reduce stress shielding and post-operative thigh pain.

  (Tr. at 1398) (Defendant's Exh. AAO at 079100).

  All of the above evidence indicating that the APR II was designed to intend the shaft to be flexible by keeping the shaft diameter smaller than the distal sleeve diameter was corroborated by Dr. Burstein, who conducted bending tests on the APR II shaft. He found that by using the 4mm distal sleeve, the stem shaft above the distal sleeve is "300 percent more flexible" than a solid stem having the same diameter as the sleeve, (Tr. at 194, 229-30), or "a threefold increase in the flexibility of the stem when you compare it with and without the sleeve with the proper diameter." (Tr. at 359). Using the mm sleeve will increase flexibility in the stem shaft by 100 percent. (Tr. at 194).

  
Q. You mentioned stress shielding. Does the APR II practice or achieve the advantages of the 023 patent with respect to stress shielding?
  
A. Yes, it does, and it does it in the same way.
  
In addition, [the APR II] prevents loads from being transmitted distally.
  
Now the way you prevent bending loads from being transmitted distally is to make the stem flexible. Then if it deforms a little bit it doesn't transmit much bending load. The stiffer the stem, the more bending load it can transmit when the bone deforms.
  
So the patent teaches that the way you prevent the transmission of bending loads is to make a portion of the stem of diameter smaller than the sleeve so that it is flexible, so that you enable flexing of that portion of the stem that's above the sleeve.
  
Q. Does the APR II system, including the sleeve, practice the teachings with respect to the length of the sleeve contained in the 023 patent at 5, lines 34 through 44?
  
A. Yes.
  
The APR II has a sleeve that is long enough so that it does not hurt the bone, it is sufficiently long, and it is sufficiently short to leave over a portion of the stem that provides greatly increased flexibility of the stem.
  
The APR II device has a sleeve, which according to the patent is long enough to result in low stresses and short enough to result in a section of stem of reduced diameter which increases flexibility.

  (Tr. 131-32 and 135-36).

  Thus, a review of the evidence indicates that (1) the APR II stem is designed with a taper so that it can be fitted with a complementary tapered distal sleeve that is not affixed to the bone, (2) the diameter of the stem shaft above the distal tip is smaller than the diameter of the distal sleeve, and (3) the intention of designing the distal sleeve to have a greater diameter than the stem shaft was not only to fill the canal, but also to allow or enable the stem to be less rigid and more flexible. In this way, both toggling and stress shielding would be prevented.

  (2) Limitation (ii): "Distal Tip Integral With The Distal End of the Stem"

  (a) Claims Construction

  (i) Patent Language and Specification

  Limitation (ii) of claim 1 states that the 023 patent improvement consists of "a distal tip integral with the distal end of the stem." Patent at 6:65. In order to construe the limitation, the terms "distal tip," "integral" and "distal end of the stem" have to be interpreted.

  Referring to the drawings included in the specification, the patent describes the stem, reference number "22", as that portion of the femoral prosthesis that is inserted into the resected femur. At the proximal end of the stem are the prosthetic neck and the wedge-shaped affixation area, both of which are "unitary with the stem." Patent at 3:23-26. The stem is inserted into the passage reamed by the surgeon (called the "cavity"), and is advanced downwards until the stem is fully within the passage. Patent at 3:35-38. By definition, the "distal" end of the stem is that portion of the stem furthest from the point of attachment at the hip joint.

  Located at the distal end of the stem is a tip, which the patent calls the "distal tip," referred to as number "40" in the specification references. The specification describes the location of the distal tip in the following manner:


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