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Retrieval Analysis of Posterior Stabilized Polyethylene Tibial Inserts and Its Clinical Relevance
The Journal of Arthroplasty 29 (2014) 365–368
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The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
Retrieval Analysis of Posterior Stabilized Polyethylene Tibial Inserts and Its Clinical Relevance Hee-Nee Pang, MBBS, MRCS (Edin), MMED (Orth), FRCS (Orth)(Edin), Paul Jamieson, Matthew G. Teeter, PhD, Richard W. McCalden, MD, FRCSC, Douglas D.R. Naudie, MD, FRCSC, Steven J. MacDonald, MD, FRCSC Division of Orthopaedic Surgery, London Health Sciences Centre, Ontario, Canada
a r t i c l e
i n f o
Article history: Received 10 April 2013 Accepted 21 May 2013 Keywords: knee posterior stabilized damage arthroplasty
a b s t r a c t This was a retrieval analysis of 83 PS inserts to assess the effect of limb alignment, implant position and joint line position on the pattern of wear in posterior stabilized (PS) tibial inserts. The total damage score was significantly higher in knees with postoperative varus alignment more than 3° (P = 0.03). The total damage score to the post was significantly more in knees with joint line elevation more than 5 mm (9.7 ± 3.9, compared to 6.5 ± 3.7 in knees with less joint line elevation) (P = 0.05). Limb malalignment and joint line elevation resulted in more damage in PS inserts. An external rotation subluxation damage pattern was found in joint line elevation. © 2014 Elsevier Inc. All rights reserved.
Total knee arthroplasty (TKA) has excellent survivorship of more than 90% at ten years [1,2]. One of the main reasons for revision surgery is polyethylene wear with resultant osteolysis and aseptic loosening [2–4]. Wear in a posterior stabilized (PS) polyethylene tibial insert depends on component position, limb alignment and ligament balancing [5]. Postoperative varus knee alignment has been shown to result in poor clinical outcomes and accelerated wear [6–8], while more recent work has demonstrated that a postoperative mechanical axis of 0° ± 3° did not improve the fifteen-year implant survival rate in modern total knee arthroplasties [9]. Retrieval analysis can provide valuable information on the in-vivo wear characteristics of implants and allow clinicians to correlate damage pattern with radiographic findings in an objective and reproducible manner. Previous reports in the literature have focused on the effect of limb alignment and implant position on polyethylene wear in cruciateretaining designs [5,10–12]. Wasielewski et al [5] had demonstrated external rotation subluxation wear patterns in cruciate-retaining TKA. To our knowledge, there has not been a report on the effect of joint line elevation on polyethylene damage in PS TKA. The objective of this study was to report on the pattern of damage, and investigate the effect of postoperative limb alignment, implant position and joint line elevation on polyethylene damage in tibial inserts that had been retrieved from PS TKA. It was hypothesized that (1) polyethylene damage would increase with postoperative limb and component
The Conflict of Interest statement associated with this article can be found at http:// dx.doi.org/10.1016/j.arth.2013.05.029. Reprint requests: Hee-Nee Pang, MBBS, FRCS (Orth)(Edin), Division of Orthopaedic Surgery, London Health Sciences Centre, University Hospital, 339 Windemere Road, London, Ontario, Canada N6A 5A5. 0883-5403/2902-0023$36.00/0 – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.arth.2013.05.029
malalignment, as well as joint line elevation, and (2) external rotation subluxation damage patterns would be present in cases with joint line elevation. Materials and Methods Study Design There were eighty three Genesis II posterior stabilized tibial inserts available from our Institutional Review Board-approved implant retrieval laboratory between 1999 and 2011. These inserts were selected because of their prevalence within the collected retrieved implants. The components were all cemented. Inserts that had been implanted for less than 3 months were excluded from analysis. Most patients would require 3 months to regain their normal gait. The mean age of patients was 72 years old (range, 51–96 years old). The retrieved specimens had a mean duration of implantation of 3.5 years (range, 0.3 to 10.3 years, and the mean patient BMI was 32.2 kg/m 2 (range, 20.8–58.8 kg/m 2). Damage Assessment Three examiners were blinded to the demographic data of the patients and performed damage analysis of the polyethylene tibial inserts. The articulating and non-articulating surfaces of the insert were divided into sixteen zones [13] and scored based on the protocol recommended by Hood and Wright et al [14]. The damage modes assessed included burnishing, abrasion, scratches, pitting, cold flow, embedded debris and delamination.
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Radiographic Measurements Two examiners analysed the pre-revision radiographs for the anatomic tibiofemoral angle (TFA) and femoral and tibial component positions on weight-bearing anteroposterior (AP), lateral and skyline films of the knee. The joint line measurement was the distance of the tibio-femoral articulating surface from the tip of the fibula [15]. The amount of joint line elevation was determined by comparison with the contralateral side. Statistical Analysis Statistical analysis was performed with SPSS statistical software (version 11.0; SPSS, Chicago, Illinois). Univariate analysis was performed with Chi-square or the Fisher’s exact test for comparison of proportions between two categorical data. The Mann–Whitney U test was used to compare the non-parametric data between two independent samples. A P value b 0.05 was considered significant. Results The majority of implants were revised as a result of infection (fiftynine of eighty-three patients, 71%). Five (6%) were revised for aseptic loosening, five (6%) for instability, four (4%) for peri-prosthetic fractures and the rest (eleven of eighty-three patients, 13%) for other reasons such as persistent pain or stiffness. Most of the PS inserts were retrieved during the first revision (fifty-three of eighty-three, 64%), fourteen (17%) during the second revision, twelve (15%) during the third revision, and the rest (four of eighty-three, 5%) during the fourth or fifth revision. All inserts demonstrated burnishing and pitting. The next most common modes of damage were scratches (seventy-nine of eightythree, 96%), abrasions (seventy-four of eighty-three, 89%), cold flow (sixty-five of eighty-three, 78%), debris (thirteen of eighty-three, 16%) and delamination (two of eighty-three, 2%). The embedded debris consist of bone fragments. The total damage score was highest in the medial compartment (mean 25, SD ± 10, range 6 to 50), followed by lateral compartment (mean 24, SD ± 9, range 3 to 50), post (mean 8, SD ± 5, range 1 to 25), and backside (mean 5, SD ± 4, range 0 to 16). Backside damage was present in seventy-two inserts (87%). The most common mode of backside damage was burnishing (sixty-seven of eighty-three, 82%), followed by scratches (thirty-nine of eighty-three, 46%), and pitting (twenty-four of eighty-three, 28%). Most of the damage was located in the anterior half of the backside (mean 3, SD ± 2, range 0 to 10), compared to posterior backside (mean 2, SD ± 2, range 0 to 9). There was damage to the post in all of the inserts retrieved (100%). The most common modes of damage were burnishing, pitting and scratches. There was damage on the posterior aspect of the post in all tibial inserts. The next most common area of damage was the medial aspect of the post (sixty-four of eighty-three, 77%), followed by lateral post (sixty-one of eighty-three, 75%), and anterior post (forty-one of eighty-three, 49%). Factors Affecting Polyethylene Damage Scores in PS Tibial Inserts (Table 1) Alignment of Lower Limb The mean TFA of the lower limb was 4.5 o (SD ± 3.4°, range − 6.0° varus to 7.7°). Damage score was significantly higher in knees with postoperative varus deformity (P = 0.03). Most of the damage was located in the medial compartment of the tibial insert (Table 2). Femoral Component Position The mean coronal alignment of the femoral component was 0.1° valgus (SD ± 2.3°, range 5.5° varus to 4.9° valgus). In the sagittal
Table 1 Factors Affecting Damage Scores in PS Tibial Inserts.
Variable Postoperative tibiofemoral angle Neutral (3°–7°) Varus (b3°) Femoral component position in relation to mechanical axis of lower limb Neutral N 3° varus N 3° valgus Flexion femoral component Neutral Flexed Extended Tibial component position in relation to mechanical axis of lower limb Neutral N 3° varus Tibial slope Posterior slope Anterior slope Patellar subluxation No Yes Joint line elevation b 5 mm ≥ 5 mm
No. of Kneesa
Total Damage Score
P Value
69 (83) 66 ± 24 (23 to 122) 14 (17) 72 ± 19 (41 to 92) 0.03
72 (87) 66 ± 24 (23 to 122) 8 (10) 69 ± 18 (52 to 92) 3 (4) 30 ± 16 (19 to 122) 0.12 49 (59) 61 ± 24 (19 to 109) 28 (34) 68 ± 26 (34 to 122) 6 (7) 75 ± 20 (55 to 95) 0.42
77 (93) 65 ± 24 (19 to 122) 6 (7) 53 ± 22 (34 to 85) 0.32 68 (82) 64 ± 25 (19 to 122) 15 (18) 69 ± 25 (23 to 98) 0.42 63 (76) 63 ± 23 (23 to 122) 20 (24) 69 ± 28 (19 to 109) 0.44 55 (66) 61 ± 21 (34 to 122) 28 (34) 59 ± 24 (19 to 109) 0.82
PS = posterior-stabilized. All others listed as mean ± SD (range). a The data are given as the number of patients with the percentage in parenthesis.
profile, the mean was 0.2° extension (SD ± 1.8°, range 8.6° flexion to 4.0° extension). There was no correlation between the coronal alignment, sagittal profile of the femoral components and the damage scores in the numbers studied. Tibial Component Position None of the tibial component was implanted in more than 3° valgus. The mean coronal alignment of the tibial component was 0.2° varus (SD ± 1.8°, range 4.3° varus to 3.0° valgus). The mean posterior slope was 3.9° (SD ± 3.6°, range 4.4° anterior slope to 8.6° posterior slope). Seven patients (39%) had posterior tibial slope (mean 4.0°, SD ± 2.1°, range 2.0° to 7.0°. There was no correlation between the coronal alignment, posterior slope of the components and the damage scores in the numbers studied. Patellar Subluxation/Dislocation There was no significant difference in the total damage score for patients with patellar subluxation. However, scratches (mean 9, SD ± 7, P = 0.03) and debris (mean 2, SD ± 1, P = 0.01) were significantly more common in cases of patellar subluxation. Joint Line Elevation The mean joint line elevation was 2 mm (SD ± 6 mm, range 0 to 13 mm). Joint line elevation was strongly correlated with damage to the post (r 2 = 0.7, P = 0.02). In patients with joint line elevation more than 5 mm, there was significantly more damage especially in the posterior (mean 2, SD ± 1, P = 0.01) and medial (mean 3, SD ± 2, P = 0.03) aspects of the post (Table 3). Discussion Accelerated wear in polyethylene inserts can be the result of surgeon, patient and implant factors [10,16,17]. The aim of this study was to investigate the relationship between limb alignment, implant position, joint line elevation and polyethylene damage in PS inserts. Damage was found in all the posts, and backside wear was
H.-H. Pang et al. / The Journal of Arthroplasty 29 (2014) 365–368 Table 2 Comparison of Scores by Region Across All Modes of Damage for Postoperative Limb Alignment. Postoperative Tibiofemoral Angle Region
Neutral (3°–7°)
Varus (b3°)
Valgus (N7°)
Lateral 26 ± 10 (7 to 49) 28 ± 8 (17 to 37) 12 ± 9 (3 to 21) Articular Medial 25 ± 10 (10 to 50) 29 ± 10 (11 to 44) 11 ± 2 (10 to 12) Articular Post 8 ± 5 (2 to 22) 9 ± 3 (4 to 7) 4 ± 2 (2 to 7) Backside 7 ± 4 (0 to 16) 6 ± 4 (0 to 11) 3 ± 2 (0 to 4)
P Value 0.05 0.03 0.25 0.27
Data listed as mean ± SD (range).
demonstrated in most inserts. Damage scores were higher in TKA with suboptimal postoperative limb alignment and joint line elevation. Sharkey et al reported poly wear as the most common mode of TKA failure 10 years ago, and that significant wear could be detected in revision TKR as early as 2 years postoperatively [4]. In our study, most of the knees were revised for infection. This is similar to the findings by Mortazavi et al [18]. Backside wear is a common finding in retrieval studies and Rao et al [19] reported backside wear in all their retrieved implants. There was backside wear in 87% of our retrieved implants, which were locked into highly polished titanium baseplates found in Genesis II. The primary modes of damage were burnishing, pitting and scratching, and this was in keeping with literature [18–20]. PS designs are subject to greater stress at the modular tibial interface because of shear forces transmitted directly to the post instead of the posterior cruciate ligament [18]. Attempts have been made to reduce the incidence of backside wear to little avail. Rotating platform TKA has no surface damage advantage to fixed bearing ones and has reportedly greater backside wear as a result of third body debris scratching [21]. The concept of highly polished baseplate was applied to fixed bearing prostheses in the hope of reducing the surface roughness. Backside wear persists despite these efforts. Improved locking designs may reduce relative micromotion, and seal off third body debris from the modular interface. Wear was evident on the posts of all retrieved implants and the most prevalent location of wear was the posterior surface. These findings were similar to those of Puloski et al [22]. The post acts as a contact guide for promoting femoral rollback and limiting tibial subluxation. It is therefore not surprising that most damage was found on the posterior surface. Hyperextension may lead to more damage on the anterior post. One of the postulated reasons for the decreased survival of PS TKA in comparison to the cruciate-retaining TKR is that of post-cam wear [2]. These wear particles may have been smaller and more bioactive, and resulted in increased osteolysis [2], and can lead, although very infrequently, to post fracture and instability, requiring revision surgery [6]. Collier et al reported greater polyethylene loss in TKA with more postoperative varus alignment [7]. A well aligned TKA purportedly has better longevity while those placed in varus have increased rates of failure and revision [8,23]. However, recent studies failed to demonstrate any correlation between postoperative limb alignment and
Table 3 Comparison of Scores by Region Across All Modes of Damage for Joint Line Elevation. Region
Joint Line Elevation b 5 mm
Joint Line Elevation ≥ 5 mm
P Value
Lateral Articular Medial Articular Post Backside
24 ± 9 (3 to 44)
22 ± 10 (7 to 37)
0.82
24 ± 9 (10 to 47)
22 ± 10 (10 to 40)
0.62
6 ± 3 (1 to 18) 6 ± 4 (0 to 16)
9 ± 4 (3 to 14) 7 ± 5 (0 to 16)
0.04 0.32
Data listed as mean ± SD (range).
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survivorship [9,24]. In our study, the damage scores were significantly higher in TKA with postoperative varus alignment and the wear was predominantly in the medial compartment. Therefore, we believe it would still be a good practice to restore the limb alignment to reduce the wear debris generated from the tibio-femoral articulation. Wasielewski et al [5] reported severe rotational wear patterns in CR inserts when the tibial joint line was elevated, leading to severe localized delamination damage at the tibia spine and the component edge. It was postulated that joint line elevation adversely affected patellofemoral tracking, which caused the patella to sublux in flexion and exert an external rotation force on the proximal tibia. In turn, this led to abnormal tibio-femoral kinematics and early wear and failure. Similarly, Callaghan et al [25] described tibial post impingement in joint line elevation and the associated large extension gap. This resulted in knee hyperextension and femoral impingement on the anterior post. In our study, it was observed that joint line elevation was associated with higher damage scores. The damage was predominantly located over the posterior and medial aspect of the post, which was pathognomonic for an external rotation subluxation wear pattern. There were several limitations to the study. It was a relatively small sample study where we examined only one design. Therefore, the findings of this study may not apply to other designs. We were only able to apply univariate analysis due to the small numbers, and future multivariate statistical studies may be required to isolate and prioritize factors leading to accelerated polyethylene wear. The damage analysis was a subjective assessment of the surface degradation and may not correlate directly with in-vivo wear. However, interobserver bias was minimized by taking the average of the damage scores from three observers. As in all retrieval studies, the implants may not be representative of well-functioning prosthesis. Revision diagnosis of instability and aseptic loosening might lead to abnormal knee kinematics and accelerated damage. However, studies have reported unintended tibial post damage occurring in-vivo regardless the revision diagnosis [22,26]. In conclusion, limb malalignment and joint line elevation resulted in more wear in PS inserts. Backside wear and post wear were demonstrated in 87% and 100% of retrieved inserts respectively. In cases with joint line elevation, an external rotation subluxation wear pattern was found with more damage over the posteromedial aspect of the post. References 1. Font-Rodriguez DE, Scuderi GR, Insall JN. Survivorship of cemented total knee arthroplasty. Clin Orthop Relat Res 1997;345:79. 2. Abdel MP, Morrey ME, Jensen MR, et al. Increased long-term survival of posterior cruciate-retaining versus posterior cruciate-stabilizing total knee replacements. J Bone Joint Surg Am 2011;93:2072. 3. Knutson K, Lewold S, Robertsson O, et al. The Swedish knee arthroplasty register. A nation-wide study of 30,003 knees 1976–1992. Acta Orthop Scand 1994;65:375. 4. Sharkey PF, Hozack WJ, Rothman RH, et al. Insall Award paper. Why are total knee arthroplasties failing today? Clin Orthop Relat Res 2002;404:7. 5. Wasielewski RC, Galante JO, Leighty RM, et al. Wear patterns on retrieved polyethylene tibial inserts and their relationship to technical considerations during total knee arthroplasty. Clin Orthop Relat Res 1994;299:31. 6. Hendel D, Garti A, Weisbort M. Fracture of the central polyethylene tibial spine in posterior stabilized total knee arthroplasty. J Arthroplasty 2003;5:672. 7. Collier MB, Engh Jr CA, McAuley JP, et al. Factors associated with the loss of thickness of polyethylene tibial bearings after knee arthroplasty. J Bone Joint Surg Am 2007;89:1306. 8. Ritter MA, Davis KE, Meding JB, et al. The effect of alignment and BMI on failure of total knee replacement. J Bone Joint Surg Am 2011;93:1588. 9. Parratte S, Pagnano MW, Trousdale RT, et al. Effect of postoperative mechanical axis alignment on the fifteen-year survival of modern, cemented total knee replacements. J Bone Joint Surg Am 2010;92:2143. 10. Srivastava A, Lee GY, Steklov N, et al. Effect of tibial component varus on wear in total knee arthroplasty. Knee 2012;19:560. 11. Ritter MA, Faris PM, Keating EM, et al. Postoperative alignment of total knee replacement. Its effect on survival. Clin Orthop Relat Res 1994;299:153. 12. Feng EL, Stulberg SD, Wixson RL. Progressive subluxation and polyethylene wear in total knee replacements with flat articular surfaces. Clin Orthop Relat Res 1994;299:60.
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13. Paterson NR, Teeter MG, Macdonald SJ, et al. The 2012 Mark Coventry Award: a retrieval analysis of high flexion versus posterior-stabilized tibial inserts. Clin Orthop Relat Res 2013;471:56. 14. Hood R, Wright T, Burstein AH. Retrieval analysis of total knee prostheses: a method and its application to 48 condylar prostheses. J Biomed Mater Res 1983;17:829. 15. Ee G, Pang HN, Chong HC, et al. Computer navigation is a useful intra-operative tool for joint line measurement in total knee arthroplasty. Knee 2013;20:256. 16. Dolan MM, Kelly NH, Nguyen JT, et al. Implant design influences tibial post wear damage in posterior-stabilized knees. Clin Orthop Relat Res 2011;469:160. 17. Blunn GW, Walker PS, Joshi A, et al. The dominance of cyclic sliding in producing wear in total knee replacements. Clin Orthop Relat Res 1991;273:253. 18. Mortazavi SM, Molligan J, Austin MS, et al. Failure following revision total knee arthroplasty: infection is the major cause. Int Orthop 2011;35:1157. 19. Rao AR, Engh GA, Collier MB, et al. Tibial interface wear in retrieved total knee components and correlations with modular insert motion. J Bone Joint Surg Am 2002;84:1849.
20. Williams IR, Mayor MB, Collier JP. The impact of sterilization method on wear in knee arthroplasty. Clin Orthop Relat Res 1998;356:170. 21. Stoner K, Jerabek SA, Tow S, et al. Rotating-platform has no surface damage advantage over fixed-bearing TKA. Clin Orthop Relat Res 2013;471:76. 22. Puloski SK, McCalden RW, MacDonald SJ, et al. Tibial post wear in posterior stabilized total knee arthroplasty: an unrecognized source of polyethylene debris. J Bone Joint Surg Am 2001;83:390. 23. Fang DM, Ritter MA, Davis KE. Coronal alignment in total knee arthroplasty: just how important is it? J Arthroplasty 2009;24:39. 24. Bonner TJ, Eardley WG, Patterson P, et al. The effect of post-operative mechanical axis alignment on the survival of primary total knee replacements after a follow-up of 15 years. J Bone Joint Surg Br 2011;93:1217. 25. Callaghan JJ, O'Rourke MR, Goetz DD, et al. Tibial post impingement in posteriorstabilized total knee arthroplasty. Clin Orthop Relat Res 2002;404:83. 26. Furman BD, Lipman J, Kligman M, et al. Tibial post wear in posterior-stabilized knee replacements is design dependent. Clin Orthop Relat Res 2008;11:2650.
Contents lists available at ScienceDirect
The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
Retrieval Analysis of Posterior Stabilized Polyethylene Tibial Inserts and Its Clinical Relevance Hee-Nee Pang, MBBS, MRCS (Edin), MMED (Orth), FRCS (Orth)(Edin), Paul Jamieson, Matthew G. Teeter, PhD, Richard W. McCalden, MD, FRCSC, Douglas D.R. Naudie, MD, FRCSC, Steven J. MacDonald, MD, FRCSC Division of Orthopaedic Surgery, London Health Sciences Centre, Ontario, Canada
a r t i c l e
i n f o
Article history: Received 10 April 2013 Accepted 21 May 2013 Keywords: knee posterior stabilized damage arthroplasty
a b s t r a c t This was a retrieval analysis of 83 PS inserts to assess the effect of limb alignment, implant position and joint line position on the pattern of wear in posterior stabilized (PS) tibial inserts. The total damage score was significantly higher in knees with postoperative varus alignment more than 3° (P = 0.03). The total damage score to the post was significantly more in knees with joint line elevation more than 5 mm (9.7 ± 3.9, compared to 6.5 ± 3.7 in knees with less joint line elevation) (P = 0.05). Limb malalignment and joint line elevation resulted in more damage in PS inserts. An external rotation subluxation damage pattern was found in joint line elevation. © 2014 Elsevier Inc. All rights reserved.
Total knee arthroplasty (TKA) has excellent survivorship of more than 90% at ten years [1,2]. One of the main reasons for revision surgery is polyethylene wear with resultant osteolysis and aseptic loosening [2–4]. Wear in a posterior stabilized (PS) polyethylene tibial insert depends on component position, limb alignment and ligament balancing [5]. Postoperative varus knee alignment has been shown to result in poor clinical outcomes and accelerated wear [6–8], while more recent work has demonstrated that a postoperative mechanical axis of 0° ± 3° did not improve the fifteen-year implant survival rate in modern total knee arthroplasties [9]. Retrieval analysis can provide valuable information on the in-vivo wear characteristics of implants and allow clinicians to correlate damage pattern with radiographic findings in an objective and reproducible manner. Previous reports in the literature have focused on the effect of limb alignment and implant position on polyethylene wear in cruciateretaining designs [5,10–12]. Wasielewski et al [5] had demonstrated external rotation subluxation wear patterns in cruciate-retaining TKA. To our knowledge, there has not been a report on the effect of joint line elevation on polyethylene damage in PS TKA. The objective of this study was to report on the pattern of damage, and investigate the effect of postoperative limb alignment, implant position and joint line elevation on polyethylene damage in tibial inserts that had been retrieved from PS TKA. It was hypothesized that (1) polyethylene damage would increase with postoperative limb and component
The Conflict of Interest statement associated with this article can be found at http:// dx.doi.org/10.1016/j.arth.2013.05.029. Reprint requests: Hee-Nee Pang, MBBS, FRCS (Orth)(Edin), Division of Orthopaedic Surgery, London Health Sciences Centre, University Hospital, 339 Windemere Road, London, Ontario, Canada N6A 5A5. 0883-5403/2902-0023$36.00/0 – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.arth.2013.05.029
malalignment, as well as joint line elevation, and (2) external rotation subluxation damage patterns would be present in cases with joint line elevation. Materials and Methods Study Design There were eighty three Genesis II posterior stabilized tibial inserts available from our Institutional Review Board-approved implant retrieval laboratory between 1999 and 2011. These inserts were selected because of their prevalence within the collected retrieved implants. The components were all cemented. Inserts that had been implanted for less than 3 months were excluded from analysis. Most patients would require 3 months to regain their normal gait. The mean age of patients was 72 years old (range, 51–96 years old). The retrieved specimens had a mean duration of implantation of 3.5 years (range, 0.3 to 10.3 years, and the mean patient BMI was 32.2 kg/m 2 (range, 20.8–58.8 kg/m 2). Damage Assessment Three examiners were blinded to the demographic data of the patients and performed damage analysis of the polyethylene tibial inserts. The articulating and non-articulating surfaces of the insert were divided into sixteen zones [13] and scored based on the protocol recommended by Hood and Wright et al [14]. The damage modes assessed included burnishing, abrasion, scratches, pitting, cold flow, embedded debris and delamination.
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H.-H. Pang et al. / The Journal of Arthroplasty 29 (2014) 365–368
Radiographic Measurements Two examiners analysed the pre-revision radiographs for the anatomic tibiofemoral angle (TFA) and femoral and tibial component positions on weight-bearing anteroposterior (AP), lateral and skyline films of the knee. The joint line measurement was the distance of the tibio-femoral articulating surface from the tip of the fibula [15]. The amount of joint line elevation was determined by comparison with the contralateral side. Statistical Analysis Statistical analysis was performed with SPSS statistical software (version 11.0; SPSS, Chicago, Illinois). Univariate analysis was performed with Chi-square or the Fisher’s exact test for comparison of proportions between two categorical data. The Mann–Whitney U test was used to compare the non-parametric data between two independent samples. A P value b 0.05 was considered significant. Results The majority of implants were revised as a result of infection (fiftynine of eighty-three patients, 71%). Five (6%) were revised for aseptic loosening, five (6%) for instability, four (4%) for peri-prosthetic fractures and the rest (eleven of eighty-three patients, 13%) for other reasons such as persistent pain or stiffness. Most of the PS inserts were retrieved during the first revision (fifty-three of eighty-three, 64%), fourteen (17%) during the second revision, twelve (15%) during the third revision, and the rest (four of eighty-three, 5%) during the fourth or fifth revision. All inserts demonstrated burnishing and pitting. The next most common modes of damage were scratches (seventy-nine of eightythree, 96%), abrasions (seventy-four of eighty-three, 89%), cold flow (sixty-five of eighty-three, 78%), debris (thirteen of eighty-three, 16%) and delamination (two of eighty-three, 2%). The embedded debris consist of bone fragments. The total damage score was highest in the medial compartment (mean 25, SD ± 10, range 6 to 50), followed by lateral compartment (mean 24, SD ± 9, range 3 to 50), post (mean 8, SD ± 5, range 1 to 25), and backside (mean 5, SD ± 4, range 0 to 16). Backside damage was present in seventy-two inserts (87%). The most common mode of backside damage was burnishing (sixty-seven of eighty-three, 82%), followed by scratches (thirty-nine of eighty-three, 46%), and pitting (twenty-four of eighty-three, 28%). Most of the damage was located in the anterior half of the backside (mean 3, SD ± 2, range 0 to 10), compared to posterior backside (mean 2, SD ± 2, range 0 to 9). There was damage to the post in all of the inserts retrieved (100%). The most common modes of damage were burnishing, pitting and scratches. There was damage on the posterior aspect of the post in all tibial inserts. The next most common area of damage was the medial aspect of the post (sixty-four of eighty-three, 77%), followed by lateral post (sixty-one of eighty-three, 75%), and anterior post (forty-one of eighty-three, 49%). Factors Affecting Polyethylene Damage Scores in PS Tibial Inserts (Table 1) Alignment of Lower Limb The mean TFA of the lower limb was 4.5 o (SD ± 3.4°, range − 6.0° varus to 7.7°). Damage score was significantly higher in knees with postoperative varus deformity (P = 0.03). Most of the damage was located in the medial compartment of the tibial insert (Table 2). Femoral Component Position The mean coronal alignment of the femoral component was 0.1° valgus (SD ± 2.3°, range 5.5° varus to 4.9° valgus). In the sagittal
Table 1 Factors Affecting Damage Scores in PS Tibial Inserts.
Variable Postoperative tibiofemoral angle Neutral (3°–7°) Varus (b3°) Femoral component position in relation to mechanical axis of lower limb Neutral N 3° varus N 3° valgus Flexion femoral component Neutral Flexed Extended Tibial component position in relation to mechanical axis of lower limb Neutral N 3° varus Tibial slope Posterior slope Anterior slope Patellar subluxation No Yes Joint line elevation b 5 mm ≥ 5 mm
No. of Kneesa
Total Damage Score
P Value
69 (83) 66 ± 24 (23 to 122) 14 (17) 72 ± 19 (41 to 92) 0.03
72 (87) 66 ± 24 (23 to 122) 8 (10) 69 ± 18 (52 to 92) 3 (4) 30 ± 16 (19 to 122) 0.12 49 (59) 61 ± 24 (19 to 109) 28 (34) 68 ± 26 (34 to 122) 6 (7) 75 ± 20 (55 to 95) 0.42
77 (93) 65 ± 24 (19 to 122) 6 (7) 53 ± 22 (34 to 85) 0.32 68 (82) 64 ± 25 (19 to 122) 15 (18) 69 ± 25 (23 to 98) 0.42 63 (76) 63 ± 23 (23 to 122) 20 (24) 69 ± 28 (19 to 109) 0.44 55 (66) 61 ± 21 (34 to 122) 28 (34) 59 ± 24 (19 to 109) 0.82
PS = posterior-stabilized. All others listed as mean ± SD (range). a The data are given as the number of patients with the percentage in parenthesis.
profile, the mean was 0.2° extension (SD ± 1.8°, range 8.6° flexion to 4.0° extension). There was no correlation between the coronal alignment, sagittal profile of the femoral components and the damage scores in the numbers studied. Tibial Component Position None of the tibial component was implanted in more than 3° valgus. The mean coronal alignment of the tibial component was 0.2° varus (SD ± 1.8°, range 4.3° varus to 3.0° valgus). The mean posterior slope was 3.9° (SD ± 3.6°, range 4.4° anterior slope to 8.6° posterior slope). Seven patients (39%) had posterior tibial slope (mean 4.0°, SD ± 2.1°, range 2.0° to 7.0°. There was no correlation between the coronal alignment, posterior slope of the components and the damage scores in the numbers studied. Patellar Subluxation/Dislocation There was no significant difference in the total damage score for patients with patellar subluxation. However, scratches (mean 9, SD ± 7, P = 0.03) and debris (mean 2, SD ± 1, P = 0.01) were significantly more common in cases of patellar subluxation. Joint Line Elevation The mean joint line elevation was 2 mm (SD ± 6 mm, range 0 to 13 mm). Joint line elevation was strongly correlated with damage to the post (r 2 = 0.7, P = 0.02). In patients with joint line elevation more than 5 mm, there was significantly more damage especially in the posterior (mean 2, SD ± 1, P = 0.01) and medial (mean 3, SD ± 2, P = 0.03) aspects of the post (Table 3). Discussion Accelerated wear in polyethylene inserts can be the result of surgeon, patient and implant factors [10,16,17]. The aim of this study was to investigate the relationship between limb alignment, implant position, joint line elevation and polyethylene damage in PS inserts. Damage was found in all the posts, and backside wear was
H.-H. Pang et al. / The Journal of Arthroplasty 29 (2014) 365–368 Table 2 Comparison of Scores by Region Across All Modes of Damage for Postoperative Limb Alignment. Postoperative Tibiofemoral Angle Region
Neutral (3°–7°)
Varus (b3°)
Valgus (N7°)
Lateral 26 ± 10 (7 to 49) 28 ± 8 (17 to 37) 12 ± 9 (3 to 21) Articular Medial 25 ± 10 (10 to 50) 29 ± 10 (11 to 44) 11 ± 2 (10 to 12) Articular Post 8 ± 5 (2 to 22) 9 ± 3 (4 to 7) 4 ± 2 (2 to 7) Backside 7 ± 4 (0 to 16) 6 ± 4 (0 to 11) 3 ± 2 (0 to 4)
P Value 0.05 0.03 0.25 0.27
Data listed as mean ± SD (range).
demonstrated in most inserts. Damage scores were higher in TKA with suboptimal postoperative limb alignment and joint line elevation. Sharkey et al reported poly wear as the most common mode of TKA failure 10 years ago, and that significant wear could be detected in revision TKR as early as 2 years postoperatively [4]. In our study, most of the knees were revised for infection. This is similar to the findings by Mortazavi et al [18]. Backside wear is a common finding in retrieval studies and Rao et al [19] reported backside wear in all their retrieved implants. There was backside wear in 87% of our retrieved implants, which were locked into highly polished titanium baseplates found in Genesis II. The primary modes of damage were burnishing, pitting and scratching, and this was in keeping with literature [18–20]. PS designs are subject to greater stress at the modular tibial interface because of shear forces transmitted directly to the post instead of the posterior cruciate ligament [18]. Attempts have been made to reduce the incidence of backside wear to little avail. Rotating platform TKA has no surface damage advantage to fixed bearing ones and has reportedly greater backside wear as a result of third body debris scratching [21]. The concept of highly polished baseplate was applied to fixed bearing prostheses in the hope of reducing the surface roughness. Backside wear persists despite these efforts. Improved locking designs may reduce relative micromotion, and seal off third body debris from the modular interface. Wear was evident on the posts of all retrieved implants and the most prevalent location of wear was the posterior surface. These findings were similar to those of Puloski et al [22]. The post acts as a contact guide for promoting femoral rollback and limiting tibial subluxation. It is therefore not surprising that most damage was found on the posterior surface. Hyperextension may lead to more damage on the anterior post. One of the postulated reasons for the decreased survival of PS TKA in comparison to the cruciate-retaining TKR is that of post-cam wear [2]. These wear particles may have been smaller and more bioactive, and resulted in increased osteolysis [2], and can lead, although very infrequently, to post fracture and instability, requiring revision surgery [6]. Collier et al reported greater polyethylene loss in TKA with more postoperative varus alignment [7]. A well aligned TKA purportedly has better longevity while those placed in varus have increased rates of failure and revision [8,23]. However, recent studies failed to demonstrate any correlation between postoperative limb alignment and
Table 3 Comparison of Scores by Region Across All Modes of Damage for Joint Line Elevation. Region
Joint Line Elevation b 5 mm
Joint Line Elevation ≥ 5 mm
P Value
Lateral Articular Medial Articular Post Backside
24 ± 9 (3 to 44)
22 ± 10 (7 to 37)
0.82
24 ± 9 (10 to 47)
22 ± 10 (10 to 40)
0.62
6 ± 3 (1 to 18) 6 ± 4 (0 to 16)
9 ± 4 (3 to 14) 7 ± 5 (0 to 16)
0.04 0.32
Data listed as mean ± SD (range).
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survivorship [9,24]. In our study, the damage scores were significantly higher in TKA with postoperative varus alignment and the wear was predominantly in the medial compartment. Therefore, we believe it would still be a good practice to restore the limb alignment to reduce the wear debris generated from the tibio-femoral articulation. Wasielewski et al [5] reported severe rotational wear patterns in CR inserts when the tibial joint line was elevated, leading to severe localized delamination damage at the tibia spine and the component edge. It was postulated that joint line elevation adversely affected patellofemoral tracking, which caused the patella to sublux in flexion and exert an external rotation force on the proximal tibia. In turn, this led to abnormal tibio-femoral kinematics and early wear and failure. Similarly, Callaghan et al [25] described tibial post impingement in joint line elevation and the associated large extension gap. This resulted in knee hyperextension and femoral impingement on the anterior post. In our study, it was observed that joint line elevation was associated with higher damage scores. The damage was predominantly located over the posterior and medial aspect of the post, which was pathognomonic for an external rotation subluxation wear pattern. There were several limitations to the study. It was a relatively small sample study where we examined only one design. Therefore, the findings of this study may not apply to other designs. We were only able to apply univariate analysis due to the small numbers, and future multivariate statistical studies may be required to isolate and prioritize factors leading to accelerated polyethylene wear. The damage analysis was a subjective assessment of the surface degradation and may not correlate directly with in-vivo wear. However, interobserver bias was minimized by taking the average of the damage scores from three observers. As in all retrieval studies, the implants may not be representative of well-functioning prosthesis. Revision diagnosis of instability and aseptic loosening might lead to abnormal knee kinematics and accelerated damage. However, studies have reported unintended tibial post damage occurring in-vivo regardless the revision diagnosis [22,26]. In conclusion, limb malalignment and joint line elevation resulted in more wear in PS inserts. Backside wear and post wear were demonstrated in 87% and 100% of retrieved inserts respectively. In cases with joint line elevation, an external rotation subluxation wear pattern was found with more damage over the posteromedial aspect of the post. References 1. Font-Rodriguez DE, Scuderi GR, Insall JN. Survivorship of cemented total knee arthroplasty. Clin Orthop Relat Res 1997;345:79. 2. Abdel MP, Morrey ME, Jensen MR, et al. Increased long-term survival of posterior cruciate-retaining versus posterior cruciate-stabilizing total knee replacements. J Bone Joint Surg Am 2011;93:2072. 3. Knutson K, Lewold S, Robertsson O, et al. The Swedish knee arthroplasty register. A nation-wide study of 30,003 knees 1976–1992. Acta Orthop Scand 1994;65:375. 4. Sharkey PF, Hozack WJ, Rothman RH, et al. Insall Award paper. Why are total knee arthroplasties failing today? Clin Orthop Relat Res 2002;404:7. 5. Wasielewski RC, Galante JO, Leighty RM, et al. Wear patterns on retrieved polyethylene tibial inserts and their relationship to technical considerations during total knee arthroplasty. Clin Orthop Relat Res 1994;299:31. 6. Hendel D, Garti A, Weisbort M. Fracture of the central polyethylene tibial spine in posterior stabilized total knee arthroplasty. J Arthroplasty 2003;5:672. 7. Collier MB, Engh Jr CA, McAuley JP, et al. Factors associated with the loss of thickness of polyethylene tibial bearings after knee arthroplasty. J Bone Joint Surg Am 2007;89:1306. 8. Ritter MA, Davis KE, Meding JB, et al. The effect of alignment and BMI on failure of total knee replacement. J Bone Joint Surg Am 2011;93:1588. 9. Parratte S, Pagnano MW, Trousdale RT, et al. Effect of postoperative mechanical axis alignment on the fifteen-year survival of modern, cemented total knee replacements. J Bone Joint Surg Am 2010;92:2143. 10. Srivastava A, Lee GY, Steklov N, et al. Effect of tibial component varus on wear in total knee arthroplasty. Knee 2012;19:560. 11. Ritter MA, Faris PM, Keating EM, et al. Postoperative alignment of total knee replacement. Its effect on survival. Clin Orthop Relat Res 1994;299:153. 12. Feng EL, Stulberg SD, Wixson RL. Progressive subluxation and polyethylene wear in total knee replacements with flat articular surfaces. Clin Orthop Relat Res 1994;299:60.
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