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Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene
THEKNE-02630; No of Pages 6 The Knee xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
The Knee
Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene☆ Owen J. Diamond ⁎, Lisa Howard, Bassam Masri University of British Columbia, Department of Orthopaedics, Adult Hip and Knee Reconstruction Service, Gordon & Leslie Diamond Health Care Centre, 3rd Floor, 2775 Laurel Street, Vancouver, BC V5Z 1M9, Canada
a r t i c l e
i n f o
Article history: Received 6 March 2018 Received in revised form 2 May 2018 Accepted 4 May 2018 Available online xxxx Keywords: Prolong Highly cross linked polyethylene Posterior stabilized total knee arthroplasty Tibial post fracture NexGen LPS-Flex total knee arthroplasty
a b s t r a c t Background: Fracture of a polyethylene tibial post after a posterior stabilized total knee arthroplasty (PS-TKA) is an uncommon but severe complication. We report five cases of nontraumatic fracture of a tibial polyethylene post that occurred with Prolong highly crosslinked polyethylene with the NexGen LPS-Flex total knee prosthesis. Methods: A Joint Reconstruction database for a high volume arthroplasty unit was used to identify all cases of revision of Prolong polyethylene used in PS-TKA. Five cases were identified as being revised because of a broken tibial post. Results: All five cases presented with a combination of sudden and increasing pain, instability and giving way, in previously well-functioning TKAs. There was no history of trauma or precipitating incident. Mean time from primary TKA to presentation and diagnosis of post fracture was 67.7 months (range 24–108). All five cases were successfully treated by revision, in the form of a liner exchange to a standard ultra-high molecular weight polyethylene (UHMWPE) bearing of the same thickness. The five cases occurred from a consecutive series of 955 total PS-TKAs with Prolong. This gives a conservative estimate of the frequency of this complication of 0.52%. This would give a risk of a tibial post fracture in approximately one in every 200 TKAs with this specific implant and bearing combination. Conclusions: To our knowledge, this is the first report of a non-traumatic fracture to the tibial post with this bearing type. We would advocate against the routine use of Prolong highly crosslinked polyethylene in PS-TKA. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Aseptic loosening remains a major long-term cause for revision following total knee arthroplasty (TKA). Highly cross-linked polyethylene (XLPE) was developed because of its superior resistance to wear and therefore potentially lower wear particle volume and osteolysis. XLPE introduction to clinical use has been accepted as a definite improvement for outcomes for total hip replacement (THR) [1–3]. In total knee arthroplasty despite improved wear characteristics and extremely encouraging in vitro results, XLPE has not been as ubiquitously accepted [4]. The unique biomechanical environment seen in TKA, characterized mainly by larger contact stresses and shear forces, differs from the highly conforming articulation and primarily abrasive and adhesive wear found in THR. For this reason, XLPE, with its decreased fatigue resistance and fracture toughness compared to ultra-high
☆ Institutional Review Board Statement: All information included in this article has met institution review board approval: “Observational Study of Functional Outcome and Quality of Life Following Major Joint Replacement Arthroplasty” (UBC CREB NUMBER: - H02-70250). ⁎ Corresponding author. E-mail addresses: [email protected], (O.J. Diamond), [email protected]. (B. Masri).
https://doi.org/10.1016/j.knee.2018.05.005 0968-0160/© 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Diamond OJ, et al, Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene, Knee (2018), https://doi.org/10.1016/j.knee.2018.05.005
2
O.J. Diamond et al. / The Knee xxx (2018) xxx–xxx
molecular weight polyethylene (UHMWPE), may not be the best material to withstand these unique forces in TKA, especially those with a polyethylene post. In posterior stabilized total knee arthroplasty (PS-TKA) there are concerns about the risk of fracture of the tibial post due to the potential weakness of the post in XLPE. The posterior-stabilizing tibial post in PS-TKA was designed as a mechanism to reduce shear forces on the cement–implant interface and to maintain flexion stability in the replaced knee, when the posterior cruciate ligament is resected. The femoral CAM articulates with the back of the tibial post so as to facilitate stable femoral rollback and to prevent posterior subluxation. This prominent area of polyethylene in PS-TKA can be a focal point for stress within the knee and there are several case reports of tibial post fracture from a variety of manufacturers, even with conventional polyethylene [5–9]. XLPE is a modified form of UHMWPE that has a higher cross-link density achieved by irradiation beyond that is necessary for sterilization and by thermal treatment to remove any potentially remaining oxygen free radicals. Prolong (Zimmer, Warsaw Ind) polyethylene is Electron beam-crosslinked at a radiation dose of 65 kGy, that is then remelted and ultimately gas plasma sterilized. It was introduced as the XLPE bearing option for use with the NexGen High Flex LPS-TKA (Zimmer, Warsaw, IN). With the introduction of new technology and materials into Orthopaedic practice, it is important for any complications or reasons for revision to be reported [10]. To date there has not been a case reported of a fracture of the tibial post with Prolong XLPE. This is paper a report of five cases of tibial post fracture from a single surgeon series of 955 consecutive PS-TKAs using Prolong XLPE. 2. Methods Between 1st April 2007 and 17th January 2016, 955 posterior stabilized total knee arthroplasty cases were performed using Prolong highly cross-linked polyethylene under the care of a single high volume arthroplasty surgeon. The departmental arthroplasty database identified 10 cases (one percent) of these 955 TKAs, which had revision knee surgery by October 2017, with a mean follow-up of 88 months (range 17–122). Reasons for revision were polyethylene fracture — five knees (in four patients) (all were tibial post fractures), instability — two knees, infection — one knee, periprosthetic fracture — one knee and aseptic loosening — one knee. Patient notes, x-rays and outcomes were all reviewed for all four patients (five knees) with tibial post fracture. X-ray measurements of alignment (both coronal and sagittal) were made on focused knee x-rays because full-length films were not available. The database also recorded that the same surgeon has also performed 2566 cases between December 2004 and October 2017, using the same techniques, the same posterior stabilized total knee arthroplasty system but with conventional UHMWPE polyethylene, with no known cases of revision for tibial post fracture or polyethylene wear. 3. Cases Five cases of tibial post fracture from 955 total knees with Prolong polyethylene give a conservative estimate of the frequency of this complication of 0.52%. Patient clinical and radiographic information on all five cases of tibial post fracture is presented in Table 1. In this system, the recommended tibial slope is seven degrees and this is what the surgeon aimed for. In addition, the planned coronal alignment was six degrees of valgus in all patients. The surgeon aimed for neutral flexion/extension of the femoral component in the sagittal plane. There was one female patient who had bilateral episodes at 71 and 93 months after operation, and the other three cases were in male patients, who presented with post fractures at 24, 42, and 108 months after operation. In all cases presentation was with the same complaints of a sudden onset of pain and giving way in a previously well-functioning TKA. Patients were aware, that their knee was functioning differently than before. No patient had a history of severe trauma or precipitating event prior to presentation. Mean time to presentation and diagnosis of was 67.7 months (range 24–108). All patients had the NexGen LPS-High Flex total knee implants (Zimmer, Warsaw, IN). All patients had liners of a thickness of 10 mm, which is the most common liner thickness in the surgeon's experience. In all five cases the revision procedure involved exchange of the liner to a non-highly cross-linked UHMWPE of the same thickness. All five TKAs had a range of movement clinically, which had 120° of flexion or more in their last clinic visit prior to the broken post and at most recent follow-up since revision. Two out of the five knees have now had UHMWPE in situ for longer since their revision surgery than the initial Prolong XLPE was in place before it broke. No other complications have occurred with any of the five TKAs in these four patients. Table 1 Patient information on the five cases of tibial post fractures.
Case 1 Case 2 Case 3 Case 4 Case 5
Sex
Age at TKA (years)
Side
Coronal alignment (post-TKA)
Flexion angle of femoral component (post-TKA)
Posterior tibial slope post-TKA
ROM post-TKA
Time (months) between primary TKA and Prolong fracture
Time (months) since revision of fractured Prolong to standard poly
F F M M M
55 55 55 47 56
R L L R R
6° valgus 8° valgus 5° valgus 5° valgus 7° valgus
3° flexion 5° flexion 4° flexion 4° flexion 3° flexion
6° 7° 6° 5° 3°
0–130° 0–120° 5–120° 0–130° 0–120°
71 93 24 42 108
38 17 33 81 7
Please cite this article as: Diamond OJ, et al, Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene, Knee (2018), https://doi.org/10.1016/j.knee.2018.05.005
O.J. Diamond et al. / The Knee xxx (2018) xxx–xxx
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4. Discussion All five cases presented with increasing pain plus instability and giving way, in previously well-functioning total knees. There was no history of trauma or precipitating incident. Mean time to presentation and diagnosis of post fracture was 67.7 months (range 24–108). Case 1 and Case 2 are the right and left knees on the same female patient who had bilateral PS-TKA performed on the same day (Figures 1 and 2). The right side failed first, at a time of 71 months post-TKA when she felt something gave in in her knee without any significant trauma. Presentation was with a feeling of pain and instability, particularly when walking down stairs. Intra-operatively the diagnosis was confirmed as a fractured tibial post (Figures 3 and 4). A liner of the same thickness was required to maintain soft tissue and flexion–extension balance and there was no indication to increase the thickness of the liner. The instability had been coming from the CAM/post articulation and not a flexion/extension gap mismatch or impingement of the post in extension due to sagittal malalignment of the femoral component or an excessive posterior tibial slope. The patient then developed the same type of symptoms on the left side at a time of 93 months from the primary TKA. Having been through the same experience with the opposite knee the patient was able to tell us that she thought the same thing had happened again. Clinical examination was in keeping with the patient's suspicion and the diagnosis was confirmed at the time of left knee revision when the liner was changed to a UHMWPE of the same size as her XLPE original insert which fractured. Diagnosis can be made based on a history of a previously well-functioning PS-TKA (known to have Prolong-XLPE) with a sudden increase in the symptoms of pain and giving way. A history of preceding trauma was not necessarily present. Clinical examination is key to the diagnosis and an intact tibial post can be confirmed by performing the posterior drawer test at approximately 70–90° of flexion and hanging the leg over the examination table. If the tibial post is intact then there will be an audible clunk and palpable firm endpoint as the tibia is translated posteriorly. This represents the back of the tibial post coming in contact with the CAM of the femoral component. For all five knees successful treatment involved revision surgery by exchange of the polyethylene liner to a conventional polyethylene liner. All such liners were modern liners that were not sterilized in the presence of oxygen in order to minimize the risk of oxidation. In no cases was the thickness of the liner changed. In two of the five cases the revised conventional liner has now been in situ (time since the liner exchange revisions) under the same loading TKA component orientation, alignment and loading conditions for longer than the XLPE liner, which fractured. Case 3 has survived 33 months successfully with conventional poly whereas Prolong poly fractured at 24 months. Case 4 has survived 81 months successfully with conventional poly whereas Prolong poly fractured at 42 months. This suggests that the conventional UHMWPE may be less susceptible to fatigue fracture under the same loading conditions. Alignment has been previously hypothesized to be a risk factor for polyethylene fracture. None of the five cases reported here were judged to have been malaligned. Intra-operatively at the time of liner revision, femoral rotation and tibial rotation were
Figure 1. Pre-operative AP knees of the female patient who's right knee was Case 1 and left knee was Case 2.
Please cite this article as: Diamond OJ, et al, Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene, Knee (2018), https://doi.org/10.1016/j.knee.2018.05.005
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O.J. Diamond et al. / The Knee xxx (2018) xxx–xxx
Figure 2. Post-operative AP knees of the female patient who's right knee was Case 1 and left knee was Case 2.
assessed and thought to be within normal limits. Coronal alignment for all five knees was a mean of 6.2° (range five to eight). Posterior slope of the tibia was a mean of 5.4° (range three to seven) (see Table 1). All patients had a clinical range of movement with flexion of more than 120°. This may have been a contributory factor as to why these patients suffered this complication. All patients had the high flexion (HF) NexGen PS-TKA. In this implant system, as in many others, HF implants were introduced to minimize the risk of excessive stresses on the polyethylene in deep flexion, to allow a safe increase in range of motion. Nonetheless, these design changes to the implant may potentially have deleterious effects on polyethylene wear. A study from London, Ontario has shown that HF inserts are more susceptible to post damage, possibly as a result of higher contact stresses from greater flexion [11]. Although none of the patients in this study were found to have hyperextension of their knees, this could be a potential risk factor for post fracture. In a cadaveric study of kinematics and tibial post contact pressures of the NexGen PS-TKA, anterior post impingement was found at low flexion angles of simulated heel strike and hyperextension [12]. Being a dual radius femoral
Figure 3. Intraoperative finding of broken tibial post still sitting within the femoral notch.
Please cite this article as: Diamond OJ, et al, Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene, Knee (2018), https://doi.org/10.1016/j.knee.2018.05.005
O.J. Diamond et al. / The Knee xxx (2018) xxx–xxx
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Figure 4. Broken tibial post from Prolong-XLPE.
component the NexGen LPS has a smaller-radius of femoral component in flexion compared to extension and a tibial insert with a flat surface. Therefore it results in a curve-on-flat design in flexion which is less constrained and places more stress on the post [13]. Hyperextension of the knee is thought to cause increased wear by impingement of the anterior post on the femoral component. Increased contact areas of this area could also be increased by sagittal misaligning the femoral component in too much flexion. Measurement of the flexion angle of the femoral component for these patients is a mean of 3.8° (range three to five) (see Table 1), which is within normal limits [14]. Five cases of tibial post fracture from 955 total knees with Prolong polyethylene give a conservative estimate of the frequency of this complication of 0.52%. This would give a risk of a tibial post fracture in approximately one in every 200 TKAs with this specific implant and bearing combination. To date from the 2566 TKAs performed by the senior author with the same knee system with non-XLPE UHMWPE there are no known cases of tibial post fracture. To date there have been no case reports of fracture of the tibial post with Prolong XLPE. Given our conservative estimate of this complication of 0.52%, we have concerns that the occurrence of this complication may be both under-diagnosed and under-reported. There are several case reports of tibial post fracture with a variety of PS-TKA. A review article in 2010 found 27 total reported cases of tibial post fractures in 18 published articles between 2000 and 2010 [15], with a presenting complaint of instability, pain and patellar clunking. Mean time between index surgery and revision was 40 months (range, 24–83 months). Three cases were identified with UHMWPE in the NexGen LPS-TKA. Since then there have been other case reports of fractured UHMWPE tibial post with the NexGen PS-TKA but up until now none describing the complication with Prolong. One article by Kumar et al. in 2015 described a fracture of the tibial post in NexGen LPS-TKA but in their text did not specify what type of polyethylene had fractured [16]. Clinical studies comparing the two options of polyethylene for the NexGen LPS-TKA have not identified a clinical benefit of XLPE over UHMWPE. Neither have they reported any cases of tibial post fracture within the XLPE arms of their series. In one randomized controlled trail (RCT) of Prolong (n = 94) versus UHMWPE (n = 99) in NexGen PS-TKA there was no difference in revision rate, functional outcome and no incidence of fractured poly at a mean of 4.5 years (range two to eight years) [17]. A second study of 308 patients who had bilateral PS-TKA using the NexGen LPS with Prolong on one side and UHMWPE on the other, showed at a minimum follow-up of five years, no difference on any outcome measures and no tibial post fracture in either group [18]. Results from national joint registries suggest that XLPE may have a survival benefit in Cruciate retaining (CR) TKA over conventional polyethylene for the same manufacturer system. However in PS-TKA, XLPE has not shown the same improvement in survival compared to UHMWPE for the same manufacturer. For instance in the Australian Joint Registry 2017 report the cumulative percentage revision for NexGen-LPS with XLPE and UHMWPE was 4.6 (95% confidence interval (CI) 4.0–5.2) and 5.1 (95% CI 4.7–5.6) respectively [19]. The Kaiser Permanente Total Joint Replacement Registry reported a five-year revision risk for NexGen PS-TKA with XLPE and UHMWPE as 3.5% and 2.5% respectively (Hazard Ratio 1.14, 95% CI 8.86–1.51, p = 0.35). They commented that no revisions resulting from tibial insert dislodgement, fracture, fatigue failure, or dislocation were observed [20]. Please cite this article as: Diamond OJ, et al, Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene, Knee (2018), https://doi.org/10.1016/j.knee.2018.05.005
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5. Conclusion Given the increased risk of tibial post fracture with Prolong XLPE presented here, higher cost for the Prolong XLPE and no definite clinical advantage displayed over conventional polyethylene in PS-TKA, we would advocate against the routine use of Prolong-XLPE in PS-TKA. It is understood that fabrication techniques vary widely among highly cross-linked polyethylene products and therefore these results are representative of the risk only with this particular polyethylene in this posterior stabilized total knee system.
References [1] Devane PA, Horne JG, Ashmore A, Mutimer J, Kim W, Stanley J. Highly cross-linked polyethylene reduces wear and revision rates in total hip arthroplasty: a 10-year double-blinded randomized controlled trial. J Bone Joint Surg Am 2017;99:1703–14. [2] Gaudiani MA, Ranawat AS, Ranawat CS. Wear analysis of highly cross-linked polyethylene in young and active patients at average fourteen years: a concise follow-up of a previous report. J Arthroplasty 2018;33(2):586–9. [3] Hanna SA, Somerville L, McCalden RW, Naudie DD, MacDonald SJ. Highly cross-linked polyethylene decreases the rate of revision of total hip arthroplasty compared with conventional polyethylene at 13 years' follow-up. Bone Joint J 2016;98-B:28–32. [4] Sakellariou VI, Sculco P, Poultsides L, Wright T, Sculco TP. Highly cross-linked polyethylene may not have an advantage in total knee arthroplasty. HSS J 2013;9: 264–9. [5] Bal BS, Greenberg D. Failure of a metal-reinforced tibial post in total knee arthroplasty. J Arthroplasty 2007;22:464–7. [6] Boesen MP, Jensen TT, Husted H. Secondary knee instability caused by fracture of the stabilizing insert in a dual-articular total knee. J Arthroplasty 2004;19: 941–3. [7] Chiu YS, Chen WM, Huang CK, Chiang CC, Chen TH. Fracture of the polyethylene tibial post in a NexGen posterior-stabilized knee prosthesis. J Arthroplasty 2004; 19:1045–9. [8] Lee CS, Chen WM, Kou HC, Lo WH, Chen CL. Early nontraumatic fracture of the polyethylene tibial post in a NexGen LPS-Flex posterior stabilized knee prosthesis. J Arthroplasty 2009;24(1292):e5–9. [9] Clarke HD, Math KR, Scuderi GR. Polyethylene post failure in posterior stabilized total knee arthroplasty. J Arthroplasty 2004;19:652–7. [10] Zywiel MG, Johnson AJ, Mont MA. Graduated introduction of orthopaedic implants: encouraging innovation and minimizing harm. J Bone Joint Surg Am 2012;94: e158. [11] Paterson NR, Teeter MG, MacDonald SJ, McCalden RW, Naudie DD. The 2012 Mark Coventry award: a retrieval analysis of high flexion versus posterior-stabilized tibial inserts. Clin Orthop Relat Res 2013;471:56–63. [12] Li G, Papannagari R, Most E, Park SE, Johnson T, Tanamal L, et al. Anterior tibial post impingement in a posterior stabilized total knee arthroplasty. J Orthop Res 2005;23:536–41. [13] Tamaki M, Tomita T, Yamazaki T, Yoshikawa H, Sugamoto K. Factors in high-flex posterior stabilized fixed-bearing total knee arthroplasty affecting in vivo kinematics and anterior tibial post impingement during gait. J Arthroplasty 2013;28:1722–7. [14] Hood B, Blum L, Holcombe SA, Wang SC, Urquhart AG, Goulet JA, et al. Variation in optimal sagittal alignment of the femoral component in total knee arthroplasty. Orthopedics 2017;40:102–6. [15] Lachiewicz PF. How to treat a tibial post fracture in total knee arthroplasty? A systematic review. Clin Orthop Relat Res 2011;469:1709–15. [16] Kumar N, Yadav C, Raj R, Yadav S. Fracture of the polyethylene tibial post in a posterior stabilized knee prosthesis: a case report and review of literature. J Orthop 2015;12(3):160. [17] Lachiewicz PF, Soileau ES. Is there a benefit to highly crosslinked polyethylene in posterior-stabilized total knee arthroplasty? A randomized trial. Clin Orthop Relat Res 2016;474:88–95. [18] Kim YH, Park JW. Comparison of highly cross-linked and conventional polyethylene in posterior cruciate-substituting total knee arthroplasty in the same patients. J Bone Joint Surg Am 2014;96:1807–13. [19] (aoanjrr) AOA. Australian National Joint Registry Annual Report. https://aoanjrr.sahmri.com/en/annual-reports-2017; 2017. [20] Paxton EW, Inacio MC, Kurtz S, Love R, Cafri G, Namba RS. Is there a difference in total knee arthroplasty risk of revision in highly crosslinked versus conventional polyethylene? Clin Orthop Relat Res 2015;473:999–1008.
Please cite this article as: Diamond OJ, et al, Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene, Knee (2018), https://doi.org/10.1016/j.knee.2018.05.005
Contents lists available at ScienceDirect
The Knee
Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene☆ Owen J. Diamond ⁎, Lisa Howard, Bassam Masri University of British Columbia, Department of Orthopaedics, Adult Hip and Knee Reconstruction Service, Gordon & Leslie Diamond Health Care Centre, 3rd Floor, 2775 Laurel Street, Vancouver, BC V5Z 1M9, Canada
a r t i c l e
i n f o
Article history: Received 6 March 2018 Received in revised form 2 May 2018 Accepted 4 May 2018 Available online xxxx Keywords: Prolong Highly cross linked polyethylene Posterior stabilized total knee arthroplasty Tibial post fracture NexGen LPS-Flex total knee arthroplasty
a b s t r a c t Background: Fracture of a polyethylene tibial post after a posterior stabilized total knee arthroplasty (PS-TKA) is an uncommon but severe complication. We report five cases of nontraumatic fracture of a tibial polyethylene post that occurred with Prolong highly crosslinked polyethylene with the NexGen LPS-Flex total knee prosthesis. Methods: A Joint Reconstruction database for a high volume arthroplasty unit was used to identify all cases of revision of Prolong polyethylene used in PS-TKA. Five cases were identified as being revised because of a broken tibial post. Results: All five cases presented with a combination of sudden and increasing pain, instability and giving way, in previously well-functioning TKAs. There was no history of trauma or precipitating incident. Mean time from primary TKA to presentation and diagnosis of post fracture was 67.7 months (range 24–108). All five cases were successfully treated by revision, in the form of a liner exchange to a standard ultra-high molecular weight polyethylene (UHMWPE) bearing of the same thickness. The five cases occurred from a consecutive series of 955 total PS-TKAs with Prolong. This gives a conservative estimate of the frequency of this complication of 0.52%. This would give a risk of a tibial post fracture in approximately one in every 200 TKAs with this specific implant and bearing combination. Conclusions: To our knowledge, this is the first report of a non-traumatic fracture to the tibial post with this bearing type. We would advocate against the routine use of Prolong highly crosslinked polyethylene in PS-TKA. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Aseptic loosening remains a major long-term cause for revision following total knee arthroplasty (TKA). Highly cross-linked polyethylene (XLPE) was developed because of its superior resistance to wear and therefore potentially lower wear particle volume and osteolysis. XLPE introduction to clinical use has been accepted as a definite improvement for outcomes for total hip replacement (THR) [1–3]. In total knee arthroplasty despite improved wear characteristics and extremely encouraging in vitro results, XLPE has not been as ubiquitously accepted [4]. The unique biomechanical environment seen in TKA, characterized mainly by larger contact stresses and shear forces, differs from the highly conforming articulation and primarily abrasive and adhesive wear found in THR. For this reason, XLPE, with its decreased fatigue resistance and fracture toughness compared to ultra-high
☆ Institutional Review Board Statement: All information included in this article has met institution review board approval: “Observational Study of Functional Outcome and Quality of Life Following Major Joint Replacement Arthroplasty” (UBC CREB NUMBER: - H02-70250). ⁎ Corresponding author. E-mail addresses: [email protected], (O.J. Diamond), [email protected]. (B. Masri).
https://doi.org/10.1016/j.knee.2018.05.005 0968-0160/© 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Diamond OJ, et al, Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene, Knee (2018), https://doi.org/10.1016/j.knee.2018.05.005
2
O.J. Diamond et al. / The Knee xxx (2018) xxx–xxx
molecular weight polyethylene (UHMWPE), may not be the best material to withstand these unique forces in TKA, especially those with a polyethylene post. In posterior stabilized total knee arthroplasty (PS-TKA) there are concerns about the risk of fracture of the tibial post due to the potential weakness of the post in XLPE. The posterior-stabilizing tibial post in PS-TKA was designed as a mechanism to reduce shear forces on the cement–implant interface and to maintain flexion stability in the replaced knee, when the posterior cruciate ligament is resected. The femoral CAM articulates with the back of the tibial post so as to facilitate stable femoral rollback and to prevent posterior subluxation. This prominent area of polyethylene in PS-TKA can be a focal point for stress within the knee and there are several case reports of tibial post fracture from a variety of manufacturers, even with conventional polyethylene [5–9]. XLPE is a modified form of UHMWPE that has a higher cross-link density achieved by irradiation beyond that is necessary for sterilization and by thermal treatment to remove any potentially remaining oxygen free radicals. Prolong (Zimmer, Warsaw Ind) polyethylene is Electron beam-crosslinked at a radiation dose of 65 kGy, that is then remelted and ultimately gas plasma sterilized. It was introduced as the XLPE bearing option for use with the NexGen High Flex LPS-TKA (Zimmer, Warsaw, IN). With the introduction of new technology and materials into Orthopaedic practice, it is important for any complications or reasons for revision to be reported [10]. To date there has not been a case reported of a fracture of the tibial post with Prolong XLPE. This is paper a report of five cases of tibial post fracture from a single surgeon series of 955 consecutive PS-TKAs using Prolong XLPE. 2. Methods Between 1st April 2007 and 17th January 2016, 955 posterior stabilized total knee arthroplasty cases were performed using Prolong highly cross-linked polyethylene under the care of a single high volume arthroplasty surgeon. The departmental arthroplasty database identified 10 cases (one percent) of these 955 TKAs, which had revision knee surgery by October 2017, with a mean follow-up of 88 months (range 17–122). Reasons for revision were polyethylene fracture — five knees (in four patients) (all were tibial post fractures), instability — two knees, infection — one knee, periprosthetic fracture — one knee and aseptic loosening — one knee. Patient notes, x-rays and outcomes were all reviewed for all four patients (five knees) with tibial post fracture. X-ray measurements of alignment (both coronal and sagittal) were made on focused knee x-rays because full-length films were not available. The database also recorded that the same surgeon has also performed 2566 cases between December 2004 and October 2017, using the same techniques, the same posterior stabilized total knee arthroplasty system but with conventional UHMWPE polyethylene, with no known cases of revision for tibial post fracture or polyethylene wear. 3. Cases Five cases of tibial post fracture from 955 total knees with Prolong polyethylene give a conservative estimate of the frequency of this complication of 0.52%. Patient clinical and radiographic information on all five cases of tibial post fracture is presented in Table 1. In this system, the recommended tibial slope is seven degrees and this is what the surgeon aimed for. In addition, the planned coronal alignment was six degrees of valgus in all patients. The surgeon aimed for neutral flexion/extension of the femoral component in the sagittal plane. There was one female patient who had bilateral episodes at 71 and 93 months after operation, and the other three cases were in male patients, who presented with post fractures at 24, 42, and 108 months after operation. In all cases presentation was with the same complaints of a sudden onset of pain and giving way in a previously well-functioning TKA. Patients were aware, that their knee was functioning differently than before. No patient had a history of severe trauma or precipitating event prior to presentation. Mean time to presentation and diagnosis of was 67.7 months (range 24–108). All patients had the NexGen LPS-High Flex total knee implants (Zimmer, Warsaw, IN). All patients had liners of a thickness of 10 mm, which is the most common liner thickness in the surgeon's experience. In all five cases the revision procedure involved exchange of the liner to a non-highly cross-linked UHMWPE of the same thickness. All five TKAs had a range of movement clinically, which had 120° of flexion or more in their last clinic visit prior to the broken post and at most recent follow-up since revision. Two out of the five knees have now had UHMWPE in situ for longer since their revision surgery than the initial Prolong XLPE was in place before it broke. No other complications have occurred with any of the five TKAs in these four patients. Table 1 Patient information on the five cases of tibial post fractures.
Case 1 Case 2 Case 3 Case 4 Case 5
Sex
Age at TKA (years)
Side
Coronal alignment (post-TKA)
Flexion angle of femoral component (post-TKA)
Posterior tibial slope post-TKA
ROM post-TKA
Time (months) between primary TKA and Prolong fracture
Time (months) since revision of fractured Prolong to standard poly
F F M M M
55 55 55 47 56
R L L R R
6° valgus 8° valgus 5° valgus 5° valgus 7° valgus
3° flexion 5° flexion 4° flexion 4° flexion 3° flexion
6° 7° 6° 5° 3°
0–130° 0–120° 5–120° 0–130° 0–120°
71 93 24 42 108
38 17 33 81 7
Please cite this article as: Diamond OJ, et al, Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene, Knee (2018), https://doi.org/10.1016/j.knee.2018.05.005
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4. Discussion All five cases presented with increasing pain plus instability and giving way, in previously well-functioning total knees. There was no history of trauma or precipitating incident. Mean time to presentation and diagnosis of post fracture was 67.7 months (range 24–108). Case 1 and Case 2 are the right and left knees on the same female patient who had bilateral PS-TKA performed on the same day (Figures 1 and 2). The right side failed first, at a time of 71 months post-TKA when she felt something gave in in her knee without any significant trauma. Presentation was with a feeling of pain and instability, particularly when walking down stairs. Intra-operatively the diagnosis was confirmed as a fractured tibial post (Figures 3 and 4). A liner of the same thickness was required to maintain soft tissue and flexion–extension balance and there was no indication to increase the thickness of the liner. The instability had been coming from the CAM/post articulation and not a flexion/extension gap mismatch or impingement of the post in extension due to sagittal malalignment of the femoral component or an excessive posterior tibial slope. The patient then developed the same type of symptoms on the left side at a time of 93 months from the primary TKA. Having been through the same experience with the opposite knee the patient was able to tell us that she thought the same thing had happened again. Clinical examination was in keeping with the patient's suspicion and the diagnosis was confirmed at the time of left knee revision when the liner was changed to a UHMWPE of the same size as her XLPE original insert which fractured. Diagnosis can be made based on a history of a previously well-functioning PS-TKA (known to have Prolong-XLPE) with a sudden increase in the symptoms of pain and giving way. A history of preceding trauma was not necessarily present. Clinical examination is key to the diagnosis and an intact tibial post can be confirmed by performing the posterior drawer test at approximately 70–90° of flexion and hanging the leg over the examination table. If the tibial post is intact then there will be an audible clunk and palpable firm endpoint as the tibia is translated posteriorly. This represents the back of the tibial post coming in contact with the CAM of the femoral component. For all five knees successful treatment involved revision surgery by exchange of the polyethylene liner to a conventional polyethylene liner. All such liners were modern liners that were not sterilized in the presence of oxygen in order to minimize the risk of oxidation. In no cases was the thickness of the liner changed. In two of the five cases the revised conventional liner has now been in situ (time since the liner exchange revisions) under the same loading TKA component orientation, alignment and loading conditions for longer than the XLPE liner, which fractured. Case 3 has survived 33 months successfully with conventional poly whereas Prolong poly fractured at 24 months. Case 4 has survived 81 months successfully with conventional poly whereas Prolong poly fractured at 42 months. This suggests that the conventional UHMWPE may be less susceptible to fatigue fracture under the same loading conditions. Alignment has been previously hypothesized to be a risk factor for polyethylene fracture. None of the five cases reported here were judged to have been malaligned. Intra-operatively at the time of liner revision, femoral rotation and tibial rotation were
Figure 1. Pre-operative AP knees of the female patient who's right knee was Case 1 and left knee was Case 2.
Please cite this article as: Diamond OJ, et al, Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene, Knee (2018), https://doi.org/10.1016/j.knee.2018.05.005
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Figure 2. Post-operative AP knees of the female patient who's right knee was Case 1 and left knee was Case 2.
assessed and thought to be within normal limits. Coronal alignment for all five knees was a mean of 6.2° (range five to eight). Posterior slope of the tibia was a mean of 5.4° (range three to seven) (see Table 1). All patients had a clinical range of movement with flexion of more than 120°. This may have been a contributory factor as to why these patients suffered this complication. All patients had the high flexion (HF) NexGen PS-TKA. In this implant system, as in many others, HF implants were introduced to minimize the risk of excessive stresses on the polyethylene in deep flexion, to allow a safe increase in range of motion. Nonetheless, these design changes to the implant may potentially have deleterious effects on polyethylene wear. A study from London, Ontario has shown that HF inserts are more susceptible to post damage, possibly as a result of higher contact stresses from greater flexion [11]. Although none of the patients in this study were found to have hyperextension of their knees, this could be a potential risk factor for post fracture. In a cadaveric study of kinematics and tibial post contact pressures of the NexGen PS-TKA, anterior post impingement was found at low flexion angles of simulated heel strike and hyperextension [12]. Being a dual radius femoral
Figure 3. Intraoperative finding of broken tibial post still sitting within the femoral notch.
Please cite this article as: Diamond OJ, et al, Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene, Knee (2018), https://doi.org/10.1016/j.knee.2018.05.005
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Figure 4. Broken tibial post from Prolong-XLPE.
component the NexGen LPS has a smaller-radius of femoral component in flexion compared to extension and a tibial insert with a flat surface. Therefore it results in a curve-on-flat design in flexion which is less constrained and places more stress on the post [13]. Hyperextension of the knee is thought to cause increased wear by impingement of the anterior post on the femoral component. Increased contact areas of this area could also be increased by sagittal misaligning the femoral component in too much flexion. Measurement of the flexion angle of the femoral component for these patients is a mean of 3.8° (range three to five) (see Table 1), which is within normal limits [14]. Five cases of tibial post fracture from 955 total knees with Prolong polyethylene give a conservative estimate of the frequency of this complication of 0.52%. This would give a risk of a tibial post fracture in approximately one in every 200 TKAs with this specific implant and bearing combination. To date from the 2566 TKAs performed by the senior author with the same knee system with non-XLPE UHMWPE there are no known cases of tibial post fracture. To date there have been no case reports of fracture of the tibial post with Prolong XLPE. Given our conservative estimate of this complication of 0.52%, we have concerns that the occurrence of this complication may be both under-diagnosed and under-reported. There are several case reports of tibial post fracture with a variety of PS-TKA. A review article in 2010 found 27 total reported cases of tibial post fractures in 18 published articles between 2000 and 2010 [15], with a presenting complaint of instability, pain and patellar clunking. Mean time between index surgery and revision was 40 months (range, 24–83 months). Three cases were identified with UHMWPE in the NexGen LPS-TKA. Since then there have been other case reports of fractured UHMWPE tibial post with the NexGen PS-TKA but up until now none describing the complication with Prolong. One article by Kumar et al. in 2015 described a fracture of the tibial post in NexGen LPS-TKA but in their text did not specify what type of polyethylene had fractured [16]. Clinical studies comparing the two options of polyethylene for the NexGen LPS-TKA have not identified a clinical benefit of XLPE over UHMWPE. Neither have they reported any cases of tibial post fracture within the XLPE arms of their series. In one randomized controlled trail (RCT) of Prolong (n = 94) versus UHMWPE (n = 99) in NexGen PS-TKA there was no difference in revision rate, functional outcome and no incidence of fractured poly at a mean of 4.5 years (range two to eight years) [17]. A second study of 308 patients who had bilateral PS-TKA using the NexGen LPS with Prolong on one side and UHMWPE on the other, showed at a minimum follow-up of five years, no difference on any outcome measures and no tibial post fracture in either group [18]. Results from national joint registries suggest that XLPE may have a survival benefit in Cruciate retaining (CR) TKA over conventional polyethylene for the same manufacturer system. However in PS-TKA, XLPE has not shown the same improvement in survival compared to UHMWPE for the same manufacturer. For instance in the Australian Joint Registry 2017 report the cumulative percentage revision for NexGen-LPS with XLPE and UHMWPE was 4.6 (95% confidence interval (CI) 4.0–5.2) and 5.1 (95% CI 4.7–5.6) respectively [19]. The Kaiser Permanente Total Joint Replacement Registry reported a five-year revision risk for NexGen PS-TKA with XLPE and UHMWPE as 3.5% and 2.5% respectively (Hazard Ratio 1.14, 95% CI 8.86–1.51, p = 0.35). They commented that no revisions resulting from tibial insert dislodgement, fracture, fatigue failure, or dislocation were observed [20]. Please cite this article as: Diamond OJ, et al, Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene, Knee (2018), https://doi.org/10.1016/j.knee.2018.05.005
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5. Conclusion Given the increased risk of tibial post fracture with Prolong XLPE presented here, higher cost for the Prolong XLPE and no definite clinical advantage displayed over conventional polyethylene in PS-TKA, we would advocate against the routine use of Prolong-XLPE in PS-TKA. It is understood that fabrication techniques vary widely among highly cross-linked polyethylene products and therefore these results are representative of the risk only with this particular polyethylene in this posterior stabilized total knee system.
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Please cite this article as: Diamond OJ, et al, Five cases of tibial post fracture in posterior stabilized total knee arthroplasty using Prolong highly cross-linked polyethylene, Knee (2018), https://doi.org/10.1016/j.knee.2018.05.005