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Mechanical Stability of Well-Functioning Tibial Baseplates From Postmortem-Retrieved Total Knee Arthroplasties
The Journal of Arthroplasty Vol. 25 No. 3 2010
Mechanical Stability of Well-Functioning Tibial Baseplates From Postmortem-Retrieved Total Knee Arthroplasties Anand S. Rao, MS, Johnathan A. Engh, MD, Gerard A. Engh, MD, and Nancy L. Parks, MS
Abstract: In joint replacement, cyclic motion at the bone-prosthesis interface is considered a precursor to component loosening. This study characterized the mechanical stability of 13 total knee arthroplasties harvested postmortem after an average time in situ of 10.3 years. With loads applied to the medial and then the lateral tibial plateau, motion between the tibial component and underlying bone was measured with extensometers. The amount of motion between the tibial component and underlying bone under medial and lateral loads of 500 N and then twice body weight was typically less than 20 μm. Tray depression under load application and the liftoff on the contralateral side indicated that the tibial stems limited implant rotation and that implant fixation did not deteriorate with time in situ. Keywords: total knee arthroplasty, retrievals, mechanical testing, tibia, micromotion. © 2010 Elsevier Inc. All rights reserved.
Although the efficacy of polymethyl methacrylate for tibial fixation in total knee arthroplasty (TKA) has been established by the excellent results reported in long-term clinical studies [1-5], tibial component loosening remains one of the most commonly cited reasons for revision surgery [6,7]. Many studies have characterized the stability of total knee tibial components and the changes that occur at the fixation interface. Most notably, roentgen stereophotogrammetric analysis (RSA) has shown that even cemented tibial trays migrate after implantation [8,9], moving permanently into a new position. More than 150 μm of recoverable motion between an implant and bone affects tissue differentiation and encourages the formation of fibrous tissue and fibrocartilage at the fixation interface [10-12]. Such changes have been characterized by hard tissue histology [13-16] and are represented by isolated radiolucencies commonly seen on postoperative follow-up radiographs
From the Investigation performed at the Anderson Orthopaedic Research Institute, Alexandria, Virginia. Submitted March 14, 2008; accepted January 1, 2009. Benefits or funds were received in partial or total support of the research material described in this article. These benefits or support were received from the following sources: Inova Health System. Reprint requests: Nancy L. Parks, MS, Anderson Orthopaedic Research Institute, PO Box 7088, Alexandria, VA 22307. Or via Express Mail, Anderson Orthopaedic Research Institute, 2501 Parkers Lane, Suite 200, Alexandria, VA 22306. © 2010 Elsevier Inc. All rights reserved. 0883-5403/08/2503-0025$36.00/0 doi:10.1016/j.arth.2009.01.006
[16,17]. A component with recoverable motion less than 150 μm would be considered stable in a clinical setting. The RSA studies indicate that implants with large amounts of early migration are prone to fail by aseptic loosening [8,18,19]. However, the mechanism of aseptic loosening of total knee implants has not been established. Although mechanical factors alone may be severe enough to result in early loosening, late-onset loosening may correlate to a biologic host tissue response to debris that migrates along the fixation interface [16,20-22]. The increasing frequency of tibial radiolucencies with increasing length of follow-up suggests that ongoing biological changes may be taking place [23,24]. These changes may be a response to debris. Because the ingress and egress of fluid and particulate are largely determined by the mechanical stability of the component, we might conclude that poorly bonded components that experience greater cyclic motion with load encourage debris migration, the formation of increasing amounts of fibrous tissue, and ultimately aseptic loosening. To our knowledge, there has been no study characterizing the stability of the tibial interface with cemented and cementless postmortem retrievals from patients who underwent TKA. This study was designed to characterize the mechanical stability of well-functioning, cadaveric tibial components among both cemented and cementless implants to learn more about long-term tibial tray fixation. Our goal was to record relative motion between implant and bone with tibial loading. In addition, we inspected the radiographic appearance of the fixation interface compared with implant stability, and we
481
482 The Journal of Arthroplasty Vol. 25 No. 3 April 2010 Table 1. Mechanical Testing Motion Values Protocol 1: 500 N Medial Load Case
Implant
Tray Material
1 2 3 4 5* 6 7 8 9* 10 * 11 12 13
AMK CoCr AMK Ti PFC Ti AMK Ti SYN CoCr AMK Ti AMK Ti AMK Ti SYN CoCr SYN CoCr AMK Ti AMK Ti AMK Ti Average Standard deviation
Stem Length (mm)
In Situ (y)
Average Medial Depression (μm)
Average Lateral Liftoff (μm)
50 30 50 50 38 50 30 50 38 38 50 50 30 42.6 8.77
5.1 6.9 8 8.3 9 9.2 9.5 10.8 12.8 13.2 13.2 13.6 13.9 10.3 2.87
−5.24 (−4.51 to −6.16) −5.06 (−4.7 to −5.51) −3.83 (−3.39 to −4.55) −7.8 (−7.4 to −8.4) −57.67 (−53.8 to −63.2) −13.6 (−11.2 to −15.3) −2.12 (−1.93 to −2.42) −5.19 (−0.64 to −7.53) −14.06 (−13.29 to −15.03) −3.57 (−3.46 to −3.75) −9.04 (−8.06 to −10.07) −15.37 (−14.99 to −16) −5.32 (−4.99 to −5.88) −11.37 14.56
1.88 (1.61 to 1.87) 0.46 (0.36 to 0.65) −2.44 (−2.26 to −2.63) 0.2 (0.1 to 0.4) 16.97 (14.8 to 20.1) 0.13 (0 to 0.3) 6.91 (4.3 to 12) 0.41 (0.40 to 0.42) 0.76 (0.27 to 1.21) 5.19 (4.84 to 5.5) 10.58 (7.69 to 16.04) −2.23 (−1.57 to −2.69) 10.83 (8.86 to 14.77) 3.82 5.9
Values in parentheses are ranges. PFC indicates Press Fit Condylar; SYN, Synatomic. *Denotes that the implant was cementless.
examined the relationship between mechanical stability of cemented tibial components and time in situ.
Materials and Methods Study Group The study group of 13 proximal tibial specimens came from 10 TKA patients within 48 hours of death. Table 1 shows the implants, type of metal from which they were made, stem length, and time in situ. The retrieved prostheses included 9 Anatomic Modular Knee (AMK) implants (DePuy Inc, Warsaw, IN), 3 Synatomic implants (DePuy), and 1 Press Fit Condylar implant (DePuy). These were all primary modular components that had stems between 30 and 50 mm in length. The 3 cementless implants (Synatomic, cases 5, 9, 10) were stored in 10% neutral buffered formalin (Surgipath, Richmond, Ill). The other 10 implants, which had cemented baseplates, were not placed in any preservative before mechanical testing but were stored in a freezer at −20°C. Freezing does not compromise the structural integrity of trabecular bone [25]. We excluded all cemented specimens in our collection that were stored in formalin, alcohol, or any other preservative because these agents could degrade the mechanical integrity of the cement. We also excluded a revision case and a case that had only been implanted for 2 weeks. The donor population consisted of 4 females and 6 males with primary TKAs. Three patients had bilateral TKAs, whereas 7 had a unilateral TKA. The mean patient weight was 80.3 kg (range, 55.8-112 kg). The mean patient age at the time of death was 71 years (range, 6177 years). Specimens were retrieved after a mean of 10.3 years in situ (range, 5.1-13.9 years). Anteroposterior and lateral serial clinical and postmortem radiographs of each specimen were examined
for radiolucencies at the bone-cement interface and osteolysis in the adjacent bone. All implants were functioning well at the time of the patient's death. Patients had been followed every 2 to 3 years postoperatively with good to excellent Knee Society Scores at their last clinical follow-up. Mechanical Loading The polyethylene inserts were removed from the metal tibial baseplates. The bony distal end of each specimen was mounted in a self-hardening acrylic resin (Bondo/ Mar-Hyde Corporation, Atlanta, GA), leaving the metaphyseal segment of the tibia and the tibial prosthesis exposed. The tibial trays were marked for axial loading on both the medial and the lateral plateaus using loading sites defined by Finlay et al [26]. The medial loading site was designated at a distance from the medial border of the baseplate that was 20% of the mediolateral baseplate dimension and at one half the anteroposterior dimension of the tray. In a similar fashion, the lateral loading site was designated at a distance from the lateral border that was 20% of the mediolateral baseplate dimension and at one half the anteroposterior diameter of the tray. The specimen was secured on an 858 Mini Bionix servo hydraulic testing machine (MTS, Eden Prairie, MN) with a U clamp. Extensometers measured the motion under load at the bone-cement interface in an inferior-superior direction. Two separate extensometers with a gauge length of 10 mm were attached: one on the lateral side of the specimen and one on the medial side of the specimen. Each extensometer was set up such that one arm contacted the metal tray while the other arm contacted the underlying bone (Fig. 1). Both extensometers spanned the entire tibial interface. To stabilize the upper extensometer blade on the tray, a
Stability of Post-Mortem TKA Tibial Components Rao et al
Protocol 2: 500 N Lateral Load
483
Protocol 3: 2× Body Weight Medial Load
Average Lateral Depression (μm)
Average Medial Liftoff (μm)
Average Medial Depression (μm)
Average Lateral Liftoff (μm)
−8.95 (−8.19 to −9.94) −1.66 (−1.65 to −1.69) −75.82 (−70.27 to −84.11) −5.03 (−4.7 to −5.6) −13.3 (−13.2 to −13.4) −9.6 (−8.7 to −11.1) −61.77 (−50.2 to −81.1) −8.24 (−7.62 to −9.15) −34.65 (−31.02 to −40.96) −23.83 (−21.4 to −27) −4.42 (−0.653 to −6.45) −9.91 (−9.69 to −10.34) −283.11 (−268.7 to −306.7) −41.56 76.15
0.08 (0 to 0.121) 20.17 (17.3 to 23.6) 3.83 (3.39 to 4.55) 0.7 (0.69 to 0.70) 20.83 (20.7 to 21) 0.67 (0.6 to 0.8) 0.27 (0.12 to 0.4) −1.21 (−1.20 to −1.21) 1.32 (1.09 to 1.53) 3.97 (3.22 to 4.95) −1.92 (−1.61 to −2.12) −0.87 (−0.6 to −1.04) 1.28 (1.17 to 1.41) 3.78 7.61
−11.23 (−10.56 to −12.09) −15.8 (−15.3 to −16.1) −16.88 (−15.34 to −19.11) −6.63 (−6.4 to −7) −136.67 (−129 to −148) −36.9 (−35.9 to −38.2) −2.68 (−2.62 to −2.74) −5.28 (−4.16 to −7.39) −40.89 (−36.94 to −45.92) −8.5 (−8.14 to −8.86) −31.19 (−30.74 to −32.08) −34.54 (−24.82 to −40.57) −21.17 (−16.4 to −23.93) −28.33 35
6 (5.51 to 6.71) 1.38 (1.33 to 1.45) −6.23 (−5.91 to −6.46) 11.88 (9.4 to 15.2) 42.77 (40.5 to 46.3) 1.2 (0.9 to 1.6) 26.27 (19.6 to 36.8) 3.31 (2.82 to 4.2) 4.25 (2.28 to 8.06) 21.63 (21.1 to 21.9) 24.93 (24.24 to 26.16) −3.62 (−2.69 to −4.16) 18.57 (9.53 to 24.97) 11.72 14.23
notch was made using a small rotary saw (Leco Corporation, St. Joseph, MI) on the tray. The lower arm of the extensometer, which contacted the underlying bone, was stabilized by first drilling a 7/64th-in pilot hole below the notched tibial tray and applying a layer of orthodontic resin less than 1 mm thick (Orthodontic Resin, Milford, DE). A notch was made in the resin before hardening to stabilize the lower extensometer blade. Axial loads were applied to each specimen using 3 separate force-controlled loading protocols: protocol 1 applied a vertical compressive load to the medial plateau of the tibial tray at a quasi-static rate of 20 N/s up to a maximum load of 500 N. Protocol 2 applied the same loading sequence as protocol 1 to the lateral plateau of the tibial tray. Protocol 3 applied a load of 2 times the patient's body weight to the medial plateau of each specimen at a rate of 40 N/s. The extensometers recorded any motion of the tray relative to the bone on the medial
and lateral sides for each loading sequence. All 3 protocols were run 3 times, and averages were computed for each test. The data were stored using the TestStar IIs version 2.1 system (MTS, Eden Prairie, MN). Recorded data included time (seconds), axial load (newtons), medial motion (micrometers), and lateral motion (micrometers). To insure the reproducibility of the test measurements, displacement curves were plotted (Fig. 2) and inspected for any irregularities. Validation of the test setup was performed by measuring the compression of a cylindrical block of polymethyl methacrylate with the extensometers. We then calculated the modulus of elasticity of the cement block to be 3.37 GPa and found that comparable to the published modulus of 3.28 GPa [27].
Fig. 1. Mechanical test setup.
Fig. 2. Example displacement curve.
Results We were able to record downward recoverable motion (“depression”) and upward recoverable motion (“liftoff”) in all specimens (Table 1). None of the trays migrated permanently to a new position. Under a 500 N medial
484 The Journal of Arthroplasty Vol. 25 No. 3 April 2010 load (protocol 1), the medial depression averaged 11.37 μm (range, 0.64-63.2 μm), whereas the mean lateral liftoff was 3.82 μm (range, –2.4 to 20.1 μm). Only one specimen (specimen 5) had more than 17 μm of depression on the medial side, and none had more than 17 μm of lateral liftoff. Under a 500 N lateral load (protocol 2), the mean lateral depression was 41.56 μm (range, 0.65-306.7 μm), whereas the medial liftoff was 3.78 μm (range, –2.12 to 23.6 μm). Under the medial loading protocol of 2 times body weight (protocol 3), 12 of 13 specimens had depression less than 50 μm. The mean medial depression was 28.33 μm (range, 2.62-148 μm). When the medial load was increased to twice body weight, the mean lateral liftoff was 11.72 (range, –6.23 to 46.3). As with protocol 1 medial loading, in this higher level of protocol 3 medial loading, specimen 5 moved the greatest amount and was the only specimen with a medial depression greater than 50 μm. From Table 1, the results above, and the radiographs, specimen 5 clearly had the most medial depression. Examination of clinical and postmortem radiographs revealed specimen 5 had significant osteolysis. It was a cementless component in situ for 9 years with a medial autograft and a 34 cm2 area of tibial screw hole osteolysis visible on postmortem radiographs. Case 13, in situ nearly 14 years, exhibited a 2.5 cm2 lucent area under the lateral plateau, which may be osteolysis. This was of interest because of the 283 μm of lateral depression for that specimen in protocol 2. Small radiolucencies and even contained areas of osteolysis were visible on 7 other specimens but did not reflect increased tray motion. The remainder of the radiographs showed well-bonded trays. Statistically there was no difference in motion between Co-Cr or Ti alloy trays (P = .206, Mann-Whitney U test), cemented and cementless trays (P = .06, Mann-Whitney U test), or between males and females (P = .189, MannWhitney U test). There was no correlation between in situ times and average motion values (P = .147, Spearman correlation) or between weight and the average motion (P = .273, Spearman correlation).
Discussion This study documents the amount of upward and downward motion at the tibial tray interface and the overall stability of postmortem TKA specimens. Even in cases that had a radiolucent line or a small (less than 5 cm2) area of osteolysis, the amount of tray motion relative to the tibia was only tens of micrometers. It seems that as long as most of the tray is supported, small areas of compromised fixation do not affect the tray stability. We conclude that tibial components are well fixed in the long term, in the presence of adequate bone quality. Most of our specimens (10 of 13) were cemented TKAs, and fixation was not a factor that influenced motion in this study. The motion we measured between the implant and bone includes cement-implant debonding as well as motion between cement and bone. The cement seems to
have provided a stable medium between the bone and the implant. Other studies of cemented tibial components suggest this as well [28,29]. However, most published studies on tibial tray motion have implanted tibial components into cadaveric bone or foam bones and cyclically loaded them, then measured the motion with loads up to 1500 N using a Linear Variable Differential Transformer. Branson et al [28] reported values of 200 μm of motion for cementless knees without bone ingrowth compared to 0 to 100 μm of motion with initially cemented knees under cyclic loads of 10 to 2000 N. Volz et al [30] tested cementless components implanted in cadaveric bones and subjected to 300 000 loading cycles and found less than 100 μm of motion in AMK tibial components. In vivo studies of component migration and inducible displacement have been performed using RSA [8,9,18,19]. According to RSA studies of Ryd et al [31], tibial component migration is greatest in the first year and then levels off. Inducible displacement of 20 cemented and cementless porous coated anatomico (PCA) knees was found to be from 0 to 0.6 mm. That motion is greater than what we measured with extensometers at 500 N loads. The accuracy of the RSA technique however is 0.2 mm [31] and also reflects movement of the proximal tibial bone, the size of the tantalum bead, and motion in 3 dimensions. As we have learned from RSA and our own clinical experience, good clinical stability does not necessarily mean perfectly rigid fixation. To determine how much motion would be predicted from our test setup in the case of no interface motion, we calculated the amount of depression that would occur at the interface using stress equations. Our mathematical result was lower than our test results by 100-fold. We can conclude that although these implants were clinically stable, perfectly rigid fixation between implant and bone was not present in our retrieved specimens.
References 1. Font-Rodriguez DE, Scuderi GR, Insall JN. Survivorship of cemented total knee arthroplasty. Clin Orthop Relat Res 1997;79. 2. Meding JB, Ritter MA, Faris PM. Total knee arthroplasty with 4.4 mm of tibial polyethylene: 10-year followup. Clin Orthop Relat Res 2001;112. 3. Ranawat CS, Boachie-Adjei O. Survivorship analysis and results of total condylar knee arthroplasty. Eight- to 11-year follow-up period. Clin Orthop Relat Res 1988;6. 4. Ranawat CS, Flynn Jr WF, Saddler S, et al. Long-term results of the total condylar knee arthroplasty. A 15-year survivorship study. Clin Orthop Relat Res 1993;94. 5. Vince KG, Insall JN. Long-term results of cemented total knee arthroplasty. Orthop Clin North Am 1988;19:575. 6. Fehring TK, Odum S, Griffin WL, et al. Early failures in total knee arthroplasty. Clin Orthop Relat Res 2001;315. 7. Sharkey PF, Hozack WJ, Rothman RH, et al. Insall Award paper. Why are total knee arthroplasties failing today? Clin Orthop Relat Res 2002;7.
Stability of Post-Mortem TKA Tibial Components Rao et al 8. Ryd L. Roentgen stereophotogrammetric analysis of prosthetic fixation in the hip and knee joint. Clin Orthop Relat Res 1992;56. 9. Toksvig-Larsen S, Ryd L, Lindstrand A. Early inducible displacement of tibial components in total knee prostheses inserted with and without cement: a randomized study with roentgen stereophotogrammetric analysis. J Bone Joint Surg Am 1998;80:83. 10. Tompkins GS, Rosenberg AG. Porous ingrowth and implant fixation. Semin Arthroplasty 1993;4:223. 11. Lee RW, Volz RG, Sheridan DC. The role of fixation and bone qualit6y on the mechanical stability of tibial knee components. Clin Orthop Relat Res 1991;273:177. 12. Bryan RS, Rand JA. Revision total knee arthroplasty. Clin Orthop Relat Res 1982;170:116. 13. Peters Jr PC, Engh GA, Dwyer KA, et al. Osteolysis after total knee arthroplasty without cement. J Bone Joint Surg Am 1992;74:864. 14. Cook SD, Barrack RL, Thomas KA, et al. Quantitative histologic analysis of tissue growth into porous total knee components. J Arthroplasty 1989;4(Suppl):S33. 15. Collier JP, Bauer TW, Bloebaum RD, et al. Results of implant retrieval from postmortem specimens in patients with well-functioning, long-term total hip replacement. Clin Orthop Relat Res 1992;274:97. 16. Engh GA, Bobyn JD, Peterson TL. Radiographic and histologic study of porous coated tibial component fixation in cementless total knee arthroplasty. Othopedics 1988;11: 725. 17. Tibrewal SB, Grant DA, Goodfellow JW. The radiolucent line beneath the tibial components of the Oxford meniscal knee. J Bone Joint Surg Br 1984;66:523. 18. Ryd L, Linder L. On the correlation between micromotion and histology of the bone-cement interface. Report of three cases of knee arthroplasty followed by roentgen stereophotogrammetric analysis. J Arthroplasty 1989;4: 303. 19. Ryd L, Albrektsson BE, Carlsson L, et al. Roentgen stereophotogrammetric analysis as a predictor of mecha-
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Mechanical Stability of Well-Functioning Tibial Baseplates From Postmortem-Retrieved Total Knee Arthroplasties Anand S. Rao, MS, Johnathan A. Engh, MD, Gerard A. Engh, MD, and Nancy L. Parks, MS
Abstract: In joint replacement, cyclic motion at the bone-prosthesis interface is considered a precursor to component loosening. This study characterized the mechanical stability of 13 total knee arthroplasties harvested postmortem after an average time in situ of 10.3 years. With loads applied to the medial and then the lateral tibial plateau, motion between the tibial component and underlying bone was measured with extensometers. The amount of motion between the tibial component and underlying bone under medial and lateral loads of 500 N and then twice body weight was typically less than 20 μm. Tray depression under load application and the liftoff on the contralateral side indicated that the tibial stems limited implant rotation and that implant fixation did not deteriorate with time in situ. Keywords: total knee arthroplasty, retrievals, mechanical testing, tibia, micromotion. © 2010 Elsevier Inc. All rights reserved.
Although the efficacy of polymethyl methacrylate for tibial fixation in total knee arthroplasty (TKA) has been established by the excellent results reported in long-term clinical studies [1-5], tibial component loosening remains one of the most commonly cited reasons for revision surgery [6,7]. Many studies have characterized the stability of total knee tibial components and the changes that occur at the fixation interface. Most notably, roentgen stereophotogrammetric analysis (RSA) has shown that even cemented tibial trays migrate after implantation [8,9], moving permanently into a new position. More than 150 μm of recoverable motion between an implant and bone affects tissue differentiation and encourages the formation of fibrous tissue and fibrocartilage at the fixation interface [10-12]. Such changes have been characterized by hard tissue histology [13-16] and are represented by isolated radiolucencies commonly seen on postoperative follow-up radiographs
From the Investigation performed at the Anderson Orthopaedic Research Institute, Alexandria, Virginia. Submitted March 14, 2008; accepted January 1, 2009. Benefits or funds were received in partial or total support of the research material described in this article. These benefits or support were received from the following sources: Inova Health System. Reprint requests: Nancy L. Parks, MS, Anderson Orthopaedic Research Institute, PO Box 7088, Alexandria, VA 22307. Or via Express Mail, Anderson Orthopaedic Research Institute, 2501 Parkers Lane, Suite 200, Alexandria, VA 22306. © 2010 Elsevier Inc. All rights reserved. 0883-5403/08/2503-0025$36.00/0 doi:10.1016/j.arth.2009.01.006
[16,17]. A component with recoverable motion less than 150 μm would be considered stable in a clinical setting. The RSA studies indicate that implants with large amounts of early migration are prone to fail by aseptic loosening [8,18,19]. However, the mechanism of aseptic loosening of total knee implants has not been established. Although mechanical factors alone may be severe enough to result in early loosening, late-onset loosening may correlate to a biologic host tissue response to debris that migrates along the fixation interface [16,20-22]. The increasing frequency of tibial radiolucencies with increasing length of follow-up suggests that ongoing biological changes may be taking place [23,24]. These changes may be a response to debris. Because the ingress and egress of fluid and particulate are largely determined by the mechanical stability of the component, we might conclude that poorly bonded components that experience greater cyclic motion with load encourage debris migration, the formation of increasing amounts of fibrous tissue, and ultimately aseptic loosening. To our knowledge, there has been no study characterizing the stability of the tibial interface with cemented and cementless postmortem retrievals from patients who underwent TKA. This study was designed to characterize the mechanical stability of well-functioning, cadaveric tibial components among both cemented and cementless implants to learn more about long-term tibial tray fixation. Our goal was to record relative motion between implant and bone with tibial loading. In addition, we inspected the radiographic appearance of the fixation interface compared with implant stability, and we
481
482 The Journal of Arthroplasty Vol. 25 No. 3 April 2010 Table 1. Mechanical Testing Motion Values Protocol 1: 500 N Medial Load Case
Implant
Tray Material
1 2 3 4 5* 6 7 8 9* 10 * 11 12 13
AMK CoCr AMK Ti PFC Ti AMK Ti SYN CoCr AMK Ti AMK Ti AMK Ti SYN CoCr SYN CoCr AMK Ti AMK Ti AMK Ti Average Standard deviation
Stem Length (mm)
In Situ (y)
Average Medial Depression (μm)
Average Lateral Liftoff (μm)
50 30 50 50 38 50 30 50 38 38 50 50 30 42.6 8.77
5.1 6.9 8 8.3 9 9.2 9.5 10.8 12.8 13.2 13.2 13.6 13.9 10.3 2.87
−5.24 (−4.51 to −6.16) −5.06 (−4.7 to −5.51) −3.83 (−3.39 to −4.55) −7.8 (−7.4 to −8.4) −57.67 (−53.8 to −63.2) −13.6 (−11.2 to −15.3) −2.12 (−1.93 to −2.42) −5.19 (−0.64 to −7.53) −14.06 (−13.29 to −15.03) −3.57 (−3.46 to −3.75) −9.04 (−8.06 to −10.07) −15.37 (−14.99 to −16) −5.32 (−4.99 to −5.88) −11.37 14.56
1.88 (1.61 to 1.87) 0.46 (0.36 to 0.65) −2.44 (−2.26 to −2.63) 0.2 (0.1 to 0.4) 16.97 (14.8 to 20.1) 0.13 (0 to 0.3) 6.91 (4.3 to 12) 0.41 (0.40 to 0.42) 0.76 (0.27 to 1.21) 5.19 (4.84 to 5.5) 10.58 (7.69 to 16.04) −2.23 (−1.57 to −2.69) 10.83 (8.86 to 14.77) 3.82 5.9
Values in parentheses are ranges. PFC indicates Press Fit Condylar; SYN, Synatomic. *Denotes that the implant was cementless.
examined the relationship between mechanical stability of cemented tibial components and time in situ.
Materials and Methods Study Group The study group of 13 proximal tibial specimens came from 10 TKA patients within 48 hours of death. Table 1 shows the implants, type of metal from which they were made, stem length, and time in situ. The retrieved prostheses included 9 Anatomic Modular Knee (AMK) implants (DePuy Inc, Warsaw, IN), 3 Synatomic implants (DePuy), and 1 Press Fit Condylar implant (DePuy). These were all primary modular components that had stems between 30 and 50 mm in length. The 3 cementless implants (Synatomic, cases 5, 9, 10) were stored in 10% neutral buffered formalin (Surgipath, Richmond, Ill). The other 10 implants, which had cemented baseplates, were not placed in any preservative before mechanical testing but were stored in a freezer at −20°C. Freezing does not compromise the structural integrity of trabecular bone [25]. We excluded all cemented specimens in our collection that were stored in formalin, alcohol, or any other preservative because these agents could degrade the mechanical integrity of the cement. We also excluded a revision case and a case that had only been implanted for 2 weeks. The donor population consisted of 4 females and 6 males with primary TKAs. Three patients had bilateral TKAs, whereas 7 had a unilateral TKA. The mean patient weight was 80.3 kg (range, 55.8-112 kg). The mean patient age at the time of death was 71 years (range, 6177 years). Specimens were retrieved after a mean of 10.3 years in situ (range, 5.1-13.9 years). Anteroposterior and lateral serial clinical and postmortem radiographs of each specimen were examined
for radiolucencies at the bone-cement interface and osteolysis in the adjacent bone. All implants were functioning well at the time of the patient's death. Patients had been followed every 2 to 3 years postoperatively with good to excellent Knee Society Scores at their last clinical follow-up. Mechanical Loading The polyethylene inserts were removed from the metal tibial baseplates. The bony distal end of each specimen was mounted in a self-hardening acrylic resin (Bondo/ Mar-Hyde Corporation, Atlanta, GA), leaving the metaphyseal segment of the tibia and the tibial prosthesis exposed. The tibial trays were marked for axial loading on both the medial and the lateral plateaus using loading sites defined by Finlay et al [26]. The medial loading site was designated at a distance from the medial border of the baseplate that was 20% of the mediolateral baseplate dimension and at one half the anteroposterior dimension of the tray. In a similar fashion, the lateral loading site was designated at a distance from the lateral border that was 20% of the mediolateral baseplate dimension and at one half the anteroposterior diameter of the tray. The specimen was secured on an 858 Mini Bionix servo hydraulic testing machine (MTS, Eden Prairie, MN) with a U clamp. Extensometers measured the motion under load at the bone-cement interface in an inferior-superior direction. Two separate extensometers with a gauge length of 10 mm were attached: one on the lateral side of the specimen and one on the medial side of the specimen. Each extensometer was set up such that one arm contacted the metal tray while the other arm contacted the underlying bone (Fig. 1). Both extensometers spanned the entire tibial interface. To stabilize the upper extensometer blade on the tray, a
Stability of Post-Mortem TKA Tibial Components Rao et al
Protocol 2: 500 N Lateral Load
483
Protocol 3: 2× Body Weight Medial Load
Average Lateral Depression (μm)
Average Medial Liftoff (μm)
Average Medial Depression (μm)
Average Lateral Liftoff (μm)
−8.95 (−8.19 to −9.94) −1.66 (−1.65 to −1.69) −75.82 (−70.27 to −84.11) −5.03 (−4.7 to −5.6) −13.3 (−13.2 to −13.4) −9.6 (−8.7 to −11.1) −61.77 (−50.2 to −81.1) −8.24 (−7.62 to −9.15) −34.65 (−31.02 to −40.96) −23.83 (−21.4 to −27) −4.42 (−0.653 to −6.45) −9.91 (−9.69 to −10.34) −283.11 (−268.7 to −306.7) −41.56 76.15
0.08 (0 to 0.121) 20.17 (17.3 to 23.6) 3.83 (3.39 to 4.55) 0.7 (0.69 to 0.70) 20.83 (20.7 to 21) 0.67 (0.6 to 0.8) 0.27 (0.12 to 0.4) −1.21 (−1.20 to −1.21) 1.32 (1.09 to 1.53) 3.97 (3.22 to 4.95) −1.92 (−1.61 to −2.12) −0.87 (−0.6 to −1.04) 1.28 (1.17 to 1.41) 3.78 7.61
−11.23 (−10.56 to −12.09) −15.8 (−15.3 to −16.1) −16.88 (−15.34 to −19.11) −6.63 (−6.4 to −7) −136.67 (−129 to −148) −36.9 (−35.9 to −38.2) −2.68 (−2.62 to −2.74) −5.28 (−4.16 to −7.39) −40.89 (−36.94 to −45.92) −8.5 (−8.14 to −8.86) −31.19 (−30.74 to −32.08) −34.54 (−24.82 to −40.57) −21.17 (−16.4 to −23.93) −28.33 35
6 (5.51 to 6.71) 1.38 (1.33 to 1.45) −6.23 (−5.91 to −6.46) 11.88 (9.4 to 15.2) 42.77 (40.5 to 46.3) 1.2 (0.9 to 1.6) 26.27 (19.6 to 36.8) 3.31 (2.82 to 4.2) 4.25 (2.28 to 8.06) 21.63 (21.1 to 21.9) 24.93 (24.24 to 26.16) −3.62 (−2.69 to −4.16) 18.57 (9.53 to 24.97) 11.72 14.23
notch was made using a small rotary saw (Leco Corporation, St. Joseph, MI) on the tray. The lower arm of the extensometer, which contacted the underlying bone, was stabilized by first drilling a 7/64th-in pilot hole below the notched tibial tray and applying a layer of orthodontic resin less than 1 mm thick (Orthodontic Resin, Milford, DE). A notch was made in the resin before hardening to stabilize the lower extensometer blade. Axial loads were applied to each specimen using 3 separate force-controlled loading protocols: protocol 1 applied a vertical compressive load to the medial plateau of the tibial tray at a quasi-static rate of 20 N/s up to a maximum load of 500 N. Protocol 2 applied the same loading sequence as protocol 1 to the lateral plateau of the tibial tray. Protocol 3 applied a load of 2 times the patient's body weight to the medial plateau of each specimen at a rate of 40 N/s. The extensometers recorded any motion of the tray relative to the bone on the medial
and lateral sides for each loading sequence. All 3 protocols were run 3 times, and averages were computed for each test. The data were stored using the TestStar IIs version 2.1 system (MTS, Eden Prairie, MN). Recorded data included time (seconds), axial load (newtons), medial motion (micrometers), and lateral motion (micrometers). To insure the reproducibility of the test measurements, displacement curves were plotted (Fig. 2) and inspected for any irregularities. Validation of the test setup was performed by measuring the compression of a cylindrical block of polymethyl methacrylate with the extensometers. We then calculated the modulus of elasticity of the cement block to be 3.37 GPa and found that comparable to the published modulus of 3.28 GPa [27].
Fig. 1. Mechanical test setup.
Fig. 2. Example displacement curve.
Results We were able to record downward recoverable motion (“depression”) and upward recoverable motion (“liftoff”) in all specimens (Table 1). None of the trays migrated permanently to a new position. Under a 500 N medial
484 The Journal of Arthroplasty Vol. 25 No. 3 April 2010 load (protocol 1), the medial depression averaged 11.37 μm (range, 0.64-63.2 μm), whereas the mean lateral liftoff was 3.82 μm (range, –2.4 to 20.1 μm). Only one specimen (specimen 5) had more than 17 μm of depression on the medial side, and none had more than 17 μm of lateral liftoff. Under a 500 N lateral load (protocol 2), the mean lateral depression was 41.56 μm (range, 0.65-306.7 μm), whereas the medial liftoff was 3.78 μm (range, –2.12 to 23.6 μm). Under the medial loading protocol of 2 times body weight (protocol 3), 12 of 13 specimens had depression less than 50 μm. The mean medial depression was 28.33 μm (range, 2.62-148 μm). When the medial load was increased to twice body weight, the mean lateral liftoff was 11.72 (range, –6.23 to 46.3). As with protocol 1 medial loading, in this higher level of protocol 3 medial loading, specimen 5 moved the greatest amount and was the only specimen with a medial depression greater than 50 μm. From Table 1, the results above, and the radiographs, specimen 5 clearly had the most medial depression. Examination of clinical and postmortem radiographs revealed specimen 5 had significant osteolysis. It was a cementless component in situ for 9 years with a medial autograft and a 34 cm2 area of tibial screw hole osteolysis visible on postmortem radiographs. Case 13, in situ nearly 14 years, exhibited a 2.5 cm2 lucent area under the lateral plateau, which may be osteolysis. This was of interest because of the 283 μm of lateral depression for that specimen in protocol 2. Small radiolucencies and even contained areas of osteolysis were visible on 7 other specimens but did not reflect increased tray motion. The remainder of the radiographs showed well-bonded trays. Statistically there was no difference in motion between Co-Cr or Ti alloy trays (P = .206, Mann-Whitney U test), cemented and cementless trays (P = .06, Mann-Whitney U test), or between males and females (P = .189, MannWhitney U test). There was no correlation between in situ times and average motion values (P = .147, Spearman correlation) or between weight and the average motion (P = .273, Spearman correlation).
Discussion This study documents the amount of upward and downward motion at the tibial tray interface and the overall stability of postmortem TKA specimens. Even in cases that had a radiolucent line or a small (less than 5 cm2) area of osteolysis, the amount of tray motion relative to the tibia was only tens of micrometers. It seems that as long as most of the tray is supported, small areas of compromised fixation do not affect the tray stability. We conclude that tibial components are well fixed in the long term, in the presence of adequate bone quality. Most of our specimens (10 of 13) were cemented TKAs, and fixation was not a factor that influenced motion in this study. The motion we measured between the implant and bone includes cement-implant debonding as well as motion between cement and bone. The cement seems to
have provided a stable medium between the bone and the implant. Other studies of cemented tibial components suggest this as well [28,29]. However, most published studies on tibial tray motion have implanted tibial components into cadaveric bone or foam bones and cyclically loaded them, then measured the motion with loads up to 1500 N using a Linear Variable Differential Transformer. Branson et al [28] reported values of 200 μm of motion for cementless knees without bone ingrowth compared to 0 to 100 μm of motion with initially cemented knees under cyclic loads of 10 to 2000 N. Volz et al [30] tested cementless components implanted in cadaveric bones and subjected to 300 000 loading cycles and found less than 100 μm of motion in AMK tibial components. In vivo studies of component migration and inducible displacement have been performed using RSA [8,9,18,19]. According to RSA studies of Ryd et al [31], tibial component migration is greatest in the first year and then levels off. Inducible displacement of 20 cemented and cementless porous coated anatomico (PCA) knees was found to be from 0 to 0.6 mm. That motion is greater than what we measured with extensometers at 500 N loads. The accuracy of the RSA technique however is 0.2 mm [31] and also reflects movement of the proximal tibial bone, the size of the tantalum bead, and motion in 3 dimensions. As we have learned from RSA and our own clinical experience, good clinical stability does not necessarily mean perfectly rigid fixation. To determine how much motion would be predicted from our test setup in the case of no interface motion, we calculated the amount of depression that would occur at the interface using stress equations. Our mathematical result was lower than our test results by 100-fold. We can conclude that although these implants were clinically stable, perfectly rigid fixation between implant and bone was not present in our retrieved specimens.
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