Excellent Fixation Achieved With Cementless Posteriorly Stabilized Total Knee Arthroplasty

Transcription

1 The Journal of Arthroplasty Vol. 28 No Excellent Fixation Achieved With Cementless Posteriorly Stabilized Total Knee Arthroplasty Steven F. Harwin, MD, FACS,* Mark A. Kester, PhD,y Arthur L. Malkani, MD,z and Michael T. Manley, PhD Abstract: Cementless posteriorly stabilized (PS) total knee arthroplasty has not been widely accepted primarily because of prior unpredictable results and concern about micromotion at the tibial fixation interface caused by the cam/post interaction. A prospective consecutive series of 114 cementless, tricompartmental periapatite-coated single-radius PS implants in 110 patients with a mean age 62 years was performed to determine if initial stability and biologic fixation could be achieved. At a mean follow-up of 36 months, all implants demonstrated radiographic evidence of stable biologic fixation with no evidence of loosening, osteolysis, stress shielding, or progressive radiolucent lines. Based on these early results, cementless, periapatite-coated single-radius PS total knee arthroplasty offers marked promise. Keywords: cementless, total knee arthroplasty, posteriorly stabilized Elsevier Inc. All rights reserved. From the *Total Hip and Knee Arthroplasty Service, Beth Israel Medical Center, New York, New York; ystryker Orthopaedics, Mahwah, New Jersey; zdepartment of Orthopaedic Surgery, University of Louisville School of Medicine, Louisville, Kentucky; and Homer Stryker Center for Orthopaedic Education, Mahwah, New Jersey. Submitted April 15, 2012; accepted June 8, The Conflict of Interest statement associated with this article can be found at doi: Disclosures: Drs Harwin and Malkani are product designers and consultants for Stryker Orthopaedics and receive royalties and consulting fees. Dr Kester is an employee of Stryker Orthopaedics, and Dr Manley is a consultant/employee of Stryker Orthopaedics. No funds were received in support of this study. Reprint requests: Steven F. Harwin, MD, FACS, 910 Park Ave, New York, NY Elsevier Inc. All rights reserved / $36.00/0 Practice patterns regarding fixation strategies of total knee arthroplasties (TKAs) and total hip arthroplasties (THAs) differ considerably. In a recent survey of the American Association of Hip and Knee Surgeons [1], experienced surgeons performing a relatively high volume of THAs and TKAs were queried about implant fixation preferences. In the hip, all surgeons used cementless femoral and cementless acetabular components in at least 75% of their cases. By comparison, less than 10% of surgeons questioned used cementless fixation for all components of the knee. Other than some poor outcomes reported in the early days of patch pad fixation technology [2], cementless fixation in the hip has proven to be durable and long lasting for well-designed implants with porous [3], roughened [4], and hydroxylapatite (periapatite [PA]) [5] fixation interfaces. The cementless hip procedure allows simplification in the operating room and less waiting time for the surgeon when compared with the cemented approach. If revision is necessary, the chore of removing cement from bone is avoided in the knee; early cementless results suffered from inadequate implant design that led to complications and revisions [6-8]. Later reports of cruciateretaining (CR) knees, often outside the United States, with hydroxylapatite and porous fixation interfaces showed durability that matches the standard achieved in the hip with early stable fixation and clinically successful outcomes in the intermediate (5-8 years) period [9-11]. The advantages of the cementless approach in the knee match those of the hip with reduced surgical complexity, the potential for lifelong survivorship, and the conservation of bone if revision is necessary [12]. Although cementless implants in both hip and knees are typically more expensive than cemented components, the total cost per patient has been shown to be similar to cemented procedures when taking into consideration the decreased operating time; lack of a need for cement, mixers, and bowls; and other fixed costs associated with cemented implants [13]. When cemented posteriorly stabilized (PS) knees were first introduced, there was concern that tibial loosening would occur because of the out-of-plane forces imposed at the tibial fixation interface by the cam and post interaction [14]. With cement fixation of PS knees, these concerns appear unsubstantiated [15-18]. In the cementless knee literature, reports of clinical results with PS knees are scant either because of implant 7

2 8 The Journal of Arthroplasty Vol. 28 No. 1 January 2013 unavailability from the manufacturers or because of the concerns for tibial stability immediately after surgery, in the period before biologic fixation occurs to secure the implant to bone. With a modern tibial component stabilized by a broad fixation keel, we hypothesized that the tibial stability achieved at surgery will allow bone ingrowth into a porous fixation surface that is enhanced by a highly crystalline hydroxylapatite coating. The clinical experience reported in the literature with hydroxylapatite-coated implants in the hip [5] and with CR knees [9] demonstrated that interface stability is achieved at 2 or more years of follow-up and is a necessary precursor to long-term survivorship. Experience with beaded, porous-coated cementless knee implants has shown that once the implant achieves satisfactory early fixation, there is very little likelihood of later aseptic loosening (D. Hungerford, communication, 2012). The purpose of this prospective study was to report the clinical and radiographic results of a consecutive series of porous-coated, hydroxlyapatite-enhanced cementless knees to determine whether interface stability necessary for longterm survivorship can be demonstrated at 2 years or more after implantation. Methods and Materials With approval from our institutional review board, between March 2008 and February 2010, 110 patients were enrolled for the study. A single surgeon (S.F.H.) implanted 114 consecutive cementless PS Triathlon PS- PA total knee prostheses (Stryker Orthopaedics, Mahwah, New Jersey). There were 106 unilateral and 4 bilateral procedures. Inclusion criteria included patients requiring tricompartmental TKA, a willingness to comply with our postoperative follow-up and evaluations, and desiring a cementless knee rather than a cemented one. Exclusion criteria included patients who had prior infection and refusal to agree to a cementless knee. Importantly, patients with radiographic evidence of osteopenia or inflammatory disease and those with a greater-than-normal body mass index, limited range of motion, and/or large angular deformities were not excluded. During the same period, 413 cemented knees were performed. The patient cohort was 65 men and 45 women, with a mean age of 62 years (range, years) and a mean body mass index of 33.5 kg/m 2 (range, kg/m 2 ). Follow-up ranged from 24 to 50 months, with a mean of 36 months. There were 101 knees with osteoarthritis, 10 with rheumatoid arthritis, and 3 with osteonecrosis of the medial femoral condyle. Based on the preoperative femorotibial angle on standing anteroposterior (AP) radiographs, there were 85 varus knees (mean, 7.8 [range, 3-35 ]), 22 valgus knees (mean, 14.7 [range, ]), and 7 neutral knees (Table 1). Table 1. Demographic Information of the Patient Population Parameter Gender, n (female) 65 Gender, n (male) 45 Age (y), mean (range) 61.8 (38-85) Diagnosis, n 114 Osteoarthritis 101 Rheumatoid arthritis 10 Osteonecrosis of medial femoral condyle 3 Preoperative varus/valgus/neutral knees, n 85/22/7 One hundred eight patients were evaluated in person for follow-up. Range of motion was recorded, and radiographs were reviewed. Two patients (2 knees) who lived distant from the office were interviewed by telephone. Their range of motion and radiographic findings were examined and determined by reviewing digital images sent to the office. No patients were lost to follow-up. The prosthesis used is a cobalt-chromium implant with anthropometric sizing designed to improve the fit and avoid overhang or uncovered bone [19] (Fig. 1). The implant is porous coated with multiple layers of 0.4-mm diameter cobalt-chromium beads, giving a porosity of 35%, a mean pore size of 425 μm, and a coating thickness of 1.5 mm. The beads were coated with a 20-μm layer of PA, a highly crystalline solution deposited form of hydroxylapatite [10]. The femoral component incorporates an open PS box with smooth lugs that aid with placement and initial stability. Primary tibial stability is provided with a broad nonbeaded, PA-coated fixation keel that is intended to resist the out-of-plane forces generated when the cam and post interact during flexion. Provision is made for 4 adjunct fixation screws. The baseplate is beaded and PA coated. The patella component has 3 beaded lugs on a beaded metal backing, both PA coated. The articular geometry of the femorotibial joint has a unique single center of rotation Fig. 1. Triathlon PS-PA beaded cementless total knee implant with PA coating. Beads cover the entire metal substrate except the sidewalls of the box, the pegs, and the tibial keel.

3 Excellent Fixation Achieved With Cementless PS TKA Harwin et al 9 intended to provide ligament isometry throughout the arc of motion and prevent midflexion instability. This single-radius design minimizes dynamic stress transfer to the tibial fixation interface by providing minimal resistance to internal and external motion in hyperextension and throughout flexion, and by locating the bearing sulcus directly over the tibial keel to reduce sagittal rocking during ambulation [20]. Clinical data including preoperative Knee Society pain and function scores, range of motion, and body mass index were recorded by research assistants. Radiographs were reviewed by the operating surgeon. Postoperatively, patients were evaluated at 6 weeks, 3 months, and yearly thereafter. Preoperative standing AP radiographs in extension, PA standing radiographs in 30 of flexion, lateral views in 45 of flexion, and Merchant patella views were taken. At the initial 6-week examination, a similar standing knee radiographic series was performed. At subsequent visits, recumbent AP and lateral and Merchant views were taken. All procedures were performed under epidural anesthesia. A minimally invasive midvastus approach was used in all cases via a midline skin incision. Intramedullary guides were used for the femur and extramedullary guides for the tibia. The patella was everted only for osteotomy and insertion of the implant. The distal femur was cut in 5 valgus with 3 of flexion. The proximal tibia was cut at 90 to the longitudinal axis with 3 flexion in the sagittal plane. Flexion and extension gaps were equalized. The holes created for the patella pegs, the uncoated femoral component lugs, and the slot for the tibial keel were prepared to provide a 0.75-mm interference fit. For osteopenic bone, a drill that provided a 1-mm press-fit for the patella was used. A medially offset dome patella component was used in all cases. A trial reduction was performed before the final implantation. Cysts were curetted and packed with autologous bone. Sclerotic bone not resected was abraded with a saw blade or drilled with a sharp pointed drill to a depth of 5 mm. All components were implanted without any slurry or bone paste applied to the bone surfaces. The patella component was implanted first, followed by the tibial component with 4 screws for adjunct fixation. The femoral component was then inserted, followed by the polyethylene insert. Reduction was then accomplished, and a final check of range of motion, stability, and patella tracking was performed. Closure was performed in a routine fashion. No drains were used. Continuous passive motion was begun on the first postoperative day and used in the hospital only. All patients left the hospital with flexion of 90 or greater. For patients of normal risk, deep vein thrombosis prophylaxis was enteric-coated aspirin (325 mg twice daily for 6 weeks) starting on the night of surgery. Higher-risk patients were treated with either enoxaperin, started after the epidural catheter was removed, or warfarin, started on the night of surgery. Below-the-knee sequential compressive devices were used on all patients. Starting on the first postoperative day, an accelerated physical therapy program with full weight-bearing and full range-of-motion exercises as tolerated was performed. Most patients initially used a walker or crutches and then transitioned to a cane as tolerated. Patients were told that they had no postoperative restrictions and were encouraged to bear full weight as tolerated and to return to normal activities of daily living as soon as comfortable. The postoperative physical therapy program included range-of-motion and isometric exercises for the first 6 weeks and then progressive resistive exercises thereafter for another 6 weeks. Results At most recent follow-up, the mean postoperative Knee Society pain score was 94 (range, ) after a preoperative mean score of 51 (range, 20-71). The mean postoperative Knee Society function score was 84 (range, ) after a mean preoperative score of 53 (range, 20-77). The mean range of motion improved to 1 of extension (range, 0-8 ) to 127 of flexion (range, ) from a preoperative mean of 5 extension (range, 0-30 ) to 101 of flexion (range, ). The postoperative femorotibial angle improved to a mean of 5.3 (range, 3-9 ) (Table 2). A lateral retinacular release was necessary in 2 patients (1.75%). The mean length of stay was 3.1 days (range, 2-5 days). Complications included 1 case of superficial wound necrosis that was successfully treated by surgical debridement. One patient developed a postoperative tense hemarthrosis that was surgically evacuated without sequelae. There was 1 deep infection with methicillin-sensitive Staphylococcus aureus presenting at 6 months, of 3-day duration, treated by arthrotomy, debridement, poly exchange, removal of screws from the tibial baseplate, and intravenous antibiotics for 6 weeks. This patient at 3.5 years postoperatively is infection-free, with a pain score of 93, function score of 90, and a range of motion of 0 to 125. Two patients Table 2. Preoperative and Postoperative Range of Motion and Knee Society Scores of the Patient Population Preoperative Postoperative Range of motion (deg), mean (range) 5 (0-30) to 101 (85-125) 1 (0-8) to 127 (90-150) Knee Society Score, 51 (20-71) 94 (82-100) pain, mean (range) Knee Society Score, function, mean (range) 53 (20-77) 84 (72-100)

4 10 The Journal of Arthroplasty Vol. 28 No. 1 January 2013 with inadequate flexion at 6 weeks underwent manipulation. There were no revisions or reoperations for any mechanical or technical failure. Survivorship excluding the poly exchange and screw removal for infection was 100%. Radiographic analysis at last follow-up revealed no evidence of loosening of any component. There has been no evidence of stress shielding, osteolysis, or progressive radiolucent lines. There were 2 cases with a narrow (b1 mm) nonprogressive dentate line, one on the medial side of the tibia in a varus knee and one on the lateral side in a valgus knee (Fig. 2A, B). The presence or absence of radiolucent or dentate lines cannot be stated with certainty because the radiographs were not taken using fluoroscopic control. However, all radiographs were taken by 2 experienced technicians following a standard protocol for positioning. There was 1 tibial baseplate that migrated into extension, seen at 6 weeks postoperative. The patient was managed by protected weight bearing with a walker, and at 12 weeks, no further displacement was noted. At 2.5 years, the component has been stable without any further migration and appears biologically fixed. The patient is asymptomatic with a Knee Society pain score of 95, function score of 85, and a range of motion of 0 to 115. One patient showed a radiodense reaction about 1 of the screws that penetrated the cortex by about 5 mm. This was noted at 3 months and has remained unchanged at 3 years (Fig. 3). The baseplate appears biologically fixed with no other reactive changes. No other patients with similar screw placement showed any such reaction. Eight patients showed some minor reactive bone growth medially or laterally about the tibial or patella component, confirmed as reactive bone growth and not subsidence by comparing follow-up radiographs to the immediate postoperative radiographs Fig. 3. Reactive bone surrounding a screw placed through the cortex (oval). (Fig. 4). At 4 years, these biologically fixed interfaces typically show no interval changes, with maintenance of normal periprosthetic bone morphology (Fig. 5A-C). Discussion The reasons for the relatively low volume of cementless TKAs being performed today are multifactorial and based on issues with early press-fit technology. Historically, cementless hip and knee arthroplasty evolved with similar goals of elimination of a cement interface, improved fixation, and prolonged longevity. Although the early cementless approaches to TKA were innovative, there was a significant failure rate and cementless fixation fell out of favor. The PCA TKA (Howmedica Corporation, Rutherford, New Jersey) was the first fully cementless knee using beaded technology. When failure occurred, it was mostly due to tibial loosening and patella complications. Moran et al [21] reported a 20% Fig. 2. (A) Preoperative radiograph of a valgus knee showing sclerosis of the lateral tibial plateau (oval). (B) Postoperative radiograph of the same knee showing a thin dentate line at 1 year. There has been no change at 3 years (arrow).

5 Excellent Fixation Achieved With Cementless PS TKA Harwin et al 11 Fig. 4. Minor bone upgrowth medially and laterally around the tibial tray (arrows). failure rate at 5 years. Berger et al [22] reported on the Miller-Galante I (Zimmer, Inc, Wausau, Indiana) implant using fiber-mesh technology and screws and had an 8% tibial loosening rate at 11 years. The Ortholoc (Wright Medical Technology, Inc., Arlington, TN) modular TKA with patch porous coating and smooth metal ridges had a significant failure rate due to osteolysis. Femoral component fractures and stress shielding found in implants using thin metal, sharp angles, and porous-coated lugs were also reported [23-25]. Several reports of patella failures included peg shearing [26] polymetal disassociation [27], and polyethylene wear [28]. These negative reports led to a rapid decline in the use of cementless knee implants, but design improvements over the years allowed excellent results and satisfactory long-term outcomes mainly outside the United States. Improvements included stronger implants, tibial keels, tibial screws, or pegs; improved articular geometry, bearing surfaces, and kinematics; and adjunct surface coatings. Several successful cementless designs emerged including the Natural Knee [29], with survivorship of 95% to 99% at 14 years; the LCS rotating platform [30], with a 98% survivorship at 16 years; the Osteonics 3000 [31], with a 100% survivorship at 10 years and 97% at 13 years; Osteonics HA [9], with a 98% survivorship at 15 years; the AGC [32], with a 97% survivorship at 20 years; and another report of the AGC [33], with a 97% survivorship of the tibia and 100% survivorship of the femur at 20 years. These results of 95% survivorship at 14 to 20 years approach or equal that of the gold standard of cemented TKA, which is approximately 95% at 15 years [34-36]. There are a number of factors, both design and technique related, that have contributed to improved cementless outcomes. At surgery, better knowledge of alignment and component positioning techniques, gap balancing, and more reliable and reproducible instrumentation favorably influence outcomes. Surfaces designed for biologic fixation, enhanced with HA coatings, have shown excellent results with fixation of the femoral, tibial, and patella components of CR knees. Recent attempts to use highly porous implants as the fixation interface have shown varied results. For example, highly porous tantalum implants have shown less than satisfactory results in some reports. In a study of PCL-retaining TKA (NexGen TKA; Zimmer, Inc) using a porous tantalum monoblock tibial component, an unacceptable early failure rate was found [37]. The failure mode was characteristic in all cases with subsidence of the tibial component into varus in 7.5% of cases at a mean of only 18 months (range, 3-41 months), resulting in 6 early revisions. Another study using the same implant found similar early tibial fixation failures with the same tibial failure mode [38]. Another report revealed poor performance of the same type of highly porous patella component with a high early failure rate [39]. These authors began a series of cementless PCL-retaining TKA (NexGen TKA; Zimmer) using highly porous tantalum components for the patella and tibia. They were able to obtain satisfactory fixation of the femoral and tibial components, but the authors had to abandon the use of the cementless patella because of an unacceptably high early failure Fig. 5. (A-C) Anteroposterior, lateral, and patella views of bone-prosthetic interfaces at 4 years.

6 12 The Journal of Arthroplasty Vol. 28 No. 1 January 2013 rate. The study continued using only cemented patella components. In a 5-year outcome study of a bicompartmental cementless PS knee design using the same highly porous substrate for the tibia (NexGen TKA; Zimmer), the authors reported satisfactory results for the tibia. In their study, they elected not to use the system's cementless patella, opting for cemented fixation for all patients [13]. Cementless patellofemoral articulation has been problematic both in the early and in the modern designs. In our study, the patellofemoral articulation was based on that of the Duracon TKA (Stryker Orthopaedics), a proven successful design with excellent clinical and radiographic outcomes [40]. At 4-year follow-up, there are no patella failures in our patient cohort. For the tibial component, 100% of our components appear biologically fixed and stable by radiographic assessment, and the concerns for out-of-plane force transmission leading to tibial loosening appear unsupported. In addition, this cam-post mechanism provides minimal resistance to rotation throughout the range of motion. Earlier work has been shown that the beaded PAcoated porous surface contributes to satisfactory radiographic and functional outcomes [11]. In our study, this fixation interface has shown no evidence of loosening, progressive radiolucencies, polyethylene wear, osteolysis, or stress shielding. Our study was designed to look at the early results of cementless fixation, the crucial period for this technology. Although our relatively short follow-up period cannot predict long-term survivorship with certainty, results with hip components and CR knees show that early stability with cementless devices tends to suggest successful long-term outcomes. We can report that the out-of-plane forces imposed on the tibial fixation interface does not prevent biologic fixation from occurring, at least with this particular fixation technology and this particular single-radius design implant. We will continue to follow-up this patient cohort to determine whether these favorable early results will lead to successful long-term outcomes. Conclusion This implant has demonstrated satisfactory clinical and radiographic outcomes at 4-year follow-up, and continued use is justified. Further monitoring of this cohort is underway. To our knowledge, this is the first prospective study of a completely cementless tricompartmental PS total knee. Concerns that tibial fixation would be prevented or disrupted by the forces applied to the PS post-cam mechanism appear unfounded. References 1. Berry DJ, Bozic KJ. Current practice patterns in primary hip and knee arthroplasty among members of the American Association of Hip and Knee Surgeons. J Arthroplasty 2010;25(6 Suppl):2. 2. Thanner J, Kärrholm J, Malchau H, et al. Poor outcome of the PCA and Harris-Galante hip prostheses. Randomized study of 171 arthroplasties with 9-year follow-up. Acta Orthop Scand 1999;70: Belmont Jr PJ, Powers CC, Beykirch SE, et al. Results of the anatomic medullary locking total hip arthroplasty at a minimum of twenty years. A concise follow-up of previous reports. J Bone Joint Surg Am 2008;90: Greene JW, Malkani AL, Kolisek FR, et al. Ceramic-onceramic total hip arthroplasty. J Arthroplasty 2009;24(6 Suppl): Capello WN, D'Antonio JA, Jaffe WL, et al. Hydroxyapatite-coated femoral components: 15-year minimum followup. Clin Orthop Relat Res 2006;453: Tsao A, Mintz L, McRae CR, et al. Failure of the porouscoated anatomic prosthesis in total knee arthroplasty due to severe polyethylene wear. J Bone Joint Surg Am 1993; 75: Chockalingam S, Scott G. The outcome of cemented vs. cementless fixation of a femoral component in total knee replacement [TKR] with the identification of radiological signs for the prediction of failure. Knee 2000;7: Stulberg BN, Watson JT, Stulberg SD, et al. A new model to assess tibial fixation in knee arthroplasty I. Histologic and roentgenographic results. Clin Orthop Relat Res 1991; 263: Epinette JA, Manley MT. Hydroxyapatite-coated total knee replacement: clinical experience at 10 to 15 years. J Bone Joint Surg Br 2007;89: Hansson U, Ryd L, Toksvig-Larsen S. A randomized RSA study of peri-apatite HA coating of a total knee prosthesis. Knee 2008;15: Chana R, Shenava Y, Nicholl AP, et al. Five- to 8-year results of the uncemented Duracon total knee arthroplasty system. J Arthroplasty 2008;23: Whiteside LA. Cementless total knee replacement. Nine to 11-year results and 10-year survivorship analysis. Clin Orthop Relat Res 1994;309: Kamath AF, Lee GC, Sheth NP, et al. Prospective results of uncemented tantalum monoblock tibia in total knee arthroplasty: minimum 5-year follow-up in patients younger than 55 years. J Arthroplasty 2011;26: Mikulak SA, Mahoney OM, de la Rosa MA, et al. Loosening and osteolysis with the press-fit condylar posteriorcruciate substituting total knee replacement. J Bone Joint Surg Am 2001;83: Oh KJ, Goodman SB, Yang JH. Prospective, randomized study between Insall-Burstein II and NexGen legacy with a minimum 9-year follow-up. J Arthroplasty 2011;26: Scuderi GR, Clarke HD. Cemented posterior stabilized total knee arthroplasty. J Arthroplasty 2004;19(4 Suppl 1): Abdeen AR, Collen SR, Vince KG. Fifteen-year to 19-year follow-up of the Insall-Burstein-1 total knee arthroplasty. J Arthroplasty 2010;25: Mahoney OM, Kinsey TL. 5- to 9-year survivorship of single-radius, posterior-stabilized TKA. Clin Orthop Relat Res 2008;466: Hitt K, Shurman II JR, Greene K, et al. Mont M Anthropometric measurements of the human knee: correlation

7 Excellent Fixation Achieved With Cementless PS TKA Harwin et al 13 to the sizing of current knee arthroplasty systems. J Bone Joint Surg Am 2003;85: Bhimji S, Kester M, Schmalzried T. Rotational constraint of posterior-stabilized total knee prostheses. J Knee Sur 2008;21: Moran CG, Pinder IM, Lees TA, et al. Survivorship analysis of the uncemented porous-coated anatomic knee replacement. J Bone Joint Surg Am 1991 Jul;73: Berger RA, Lyon JH, Jacobs JJ, et al. Problems with cementless total knee arthroplasty at 11 years followup. Clin Orthop Relat Res 2001;392: Whiteside LA. Effect of porous-coating configuration on tibial osteolysis after total knee arthroplasty. Clin Orthop Relat Res 1995; Whiteside LA, Fosco DR, Brooks Jr JG. Fracture of the femoral component in cementless total knee arthroplasty. Clin Orthop Relat Res 1993;286: Campbell MD, Duffy GP, Trousdale RT. Femoral component failure in hybrid total knee arthroplasty. Clin Orthop Relat Res 1998;356: Rosenberg AG, Andriacchi TP, Barden R, et al. Patellar component failure in cementless total knee arthroplasty. Clin Orthop Relat Res 1988;236: Stulberg SD, Stulberg BN, Hamati Y, et al. Failure mechanisms of metal-backed patellar components. Clin Orthop Relat Res 1988;236: Kraay MJ, Darr OJ, Salata MJ, et al. Outcome of metalbacked cementless patellar components: the effect of implant design. Clin Orthop Relat Res 2001;392: Hofmann AA, Evanich JD, Ferguson RP, et al. Ten- to 14-year clinical followup of the cementless Natural Knee system. Clin Orthop Relat Res 2001;388: Buechel Sr FF, Buechel Jr FF, Pappas MJ, et al. Twentyyear evaluation of meniscal bearing and rotating platform knee replacements. Clin Orthop Relat Res 2001;388: Watanabe H, Akizuki S, Takizawa T. Survival analysis of a cementless, cruciate-retaining total knee arthroplasty. Clinical and radiographic assessment 10 to 13 years after surgery. J Bone Joint Surg Br 2004;86: Ritter MA, Meneghini RM. Twenty-year survivorship of cementless anatomic graduated component total knee arthroplasty. J Arthroplasty 2010;25: Eriksen J, Christensen J, Solgaard S, et al. The cementless AGC 2000 knee prosthesis: 20-year results in a consecutive series. Acta Orthop Belg 2009;75: 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;286: Font-Rodriguez DE, Scuderi GR, Insall JN. Survivorship of cemented total knee arthroplasty. Clin Orthop Relat Res 1997;345: Ritter MA, Herbst SA, Keating EM, et al. Long-term survival analysis of a posterior cruciate-retaining total condylar total knee arthroplasty. Clin Orthop Relat Res 1994;309: Meneghini RM, de Baubien BC. Early failure of cementless porous tantalum monoblock tibial components is likely related to patient height, weight and gender. J Arthroplasty 2010;25:e Deshmane PP, Keen JS, Cushner FD. Early onset failure of porous tantalum monoblock tibial component. Presented at Annual Meeting; February, 2012; American Academy of Orthopaedic Surgeons; San Francisco, CA. 39. Unger AS, Duggan JP. Midterm results of a porous tantalum monoblock tibia component clinical and radiographic results of 108 knees. J Arthroplasty 2011;26: Mont MA, Yoon TR, Krackow KA, et al. Eliminating patellofemoral complications in total knee arthroplasty: clinical and radiographic results of 121 consecutive cases using the Duracon system. J Arthroplasty 1999;14:446.