Tibial Post Failures in a Condylar Posterior Cruciate Substituting Total Knee Arthroplasty

The Journal of Arthroplasty Vol. 23 No. 5 2008

Tibial Post Failures in a Condylar Posterior Cruciate Substituting Total Knee Arthroplasty B. Sonny Bal, MD,* David Greenberg, MD,* Stephen Li, PhD,y David R. Mauerhan, MD,z Loren Schultz, DVM,§ and Kenneth Cherry, MD‖

Abstract: In posterior-stabilized total knee arthroplasties, a femoral cam and polyethylene tibial post are commonly used to restore posterior stability after sacrifice of the posterior cruciate ligament. This article reports a high incidence of early tibial post failures in one design of prosthesis and examines the variables that may have contributed to such. Five hundred sixty-four consecutive posteriorstabilized total knees were implanted in 512 patients, using a total knee prosthesis with a polyethylene tibial post and femoral cam. Clinical and radiographic outcomes were measured at a mean follow-up of 40 months after surgery (range, 24-83 months). At follow-up, 70 knees in 62 patients (12%) had undergone revision surgery because of symptoms related to catastrophic failure of the tibial post. Key words: tibial post failure, clinical outcomes, total knee arthroplasty, posterior cruciate substituting components. © 2008 Elsevier Inc. All rights reserved.

Posterior cruciate substituting components are commonly used in primary total knee arthroplasty (TKA) [1-3]. After sacrifice of the posterior cruciate ligament (PCL), posterior stability can be restored by a polyethylene tibial post that articulates against a cam in the femoral component [4]. During knee flexion, the cam and post mechanism induces femoral rollback, allows the femoral-tibial contact area to move posteriorly, and prevents posterior tibial subluxation [5,6]. Satisfactory outcomes with posterior-stabilized TKA (PS-TKA) using the cam

and post mechanism have been described in several reports [1,2,7]. Despite its advantages, the cam and post mechanism in some implant designs has been associated with knee instability and patellar clunking [8-10]. Plastic deformation and gross damage to the tibial post can occur from anterior or posterior impingement against the femoral component [6,11,12]. Catastrophic in vivo failures of the tibial post have also been reported, manifesting as knee pain and instability [5,13-15]. In this report, we describe a high incidence of tibial post failures that were associated with one design of PSTKA and examine the risk factors that may have contributed to the failures.

From the *Department of Orthopaedic Surgery, University of Missouri-Columbia; yMedical Device Testing and Innovations, LLC, Sarasota, Florida; zDepartment of Orthopaedic Surgery, Carolinas Medical Center, Charlotte, North Carolina; §College of Veterinary Medicine, University of Missouri-Columbia; and OUniversity Orthopaedics Center, State College, Pennsylvania. Submitted November 28, 2006; accepted August 1, 2007. No benefits or funds were received in support of the study. Reprint requests: B. Sonny Bal, MD, MBA, Department of Orthopaedic Surgery, University of Missouri, MC213, DC053.00, One Hospital Drive, Columbia, Missouri 65212. © 2008 Elsevier Inc. All rights reserved. 0883-5403/08/2305-0002$34.00/0 doi:10.1016/j.arth.2007.08.002

Materials and Methods Between April 1997 and December 2000, 564 consecutive primary TKAs were performed by one surgeon in 512 patients. Knee implants from the Foundation-100 series Total Knee System (Encore Orthopaedics, Austin, Tex) were used in every patient. The femoral component was made of cobalt chrome, with a closed box to accommodate the tibial

650

Tibial Post Failures in a Condylar Posterior Cruciate Substituting TKA
Bal et al

post. The bearing insert was made of ultrahigh molecular weight polyethylene, with anterior tabs that locked into the metal tibial plate. All polyethylene components were machined from standard industry ram-extruded bar stock. Sterilization was done with γ irradiation either in nitrogen or in air, followed by vacuum packaging. Shelf aging of polyethylene was less than 5 years from the date of manufacture, although the exact shelf life and sterilization environment of individual components was unknown. The tibial post was designed with a central hole to allow supplemental screw fixation of the insert to the tibial plate (Fig. 1). According to the manufacturer, the tibial post was designed to engage the femoral cam at approximately 42° of knee flexion and through the end point of flexion. Furthermore, the post could accommodate up to 8° of internal or external rotation between the femur and tibia from the neutral axis before impinging against the femoral box. The height of the tibial post was the same for all polyethylene insert thicknesses. All operations were performed by one surgeon who was experienced in TKA, using standard instrumentation, and surgical exposure through a medial parapatellar arthrotomy with routine sacrifice of the PCL. The distal femur was cut at 5° of valgus relative to the intramedullary axis of the femur. The proximal tibia was cut perpendicular to the tibial shaft in the coronal plane, with a 6° posterior slope in the sagittal plane. The patella was routinely resurfaced, and a lateral release was done only if necessary to improve patella tracking. All components were cemented to bone using standard techniques. After approval from the institutional review board, data were collected from chart review and

Fig. 1. An example of a retrieved tibial insert with failed post. The tibial post has a central screw hole for fixation to the tibial tray.

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Table 1. Comparison of Total Knees With and Without Tibial Post Failures Number of TKAs = 564 (512 patients)

Variables

Post Failure, n = 70 TKAs (62 Patients)

Patient age (y) 64 (46-81) Weight (lb) 204 (145-305) Sex (proportion of 71 males) (%) 31.1 (21.4-48.8) BMI (kg/m2) Preoperative knee 5 (0-10) extension y Preoperative knee 108 (90-140) flexion y Postoperative knee 0 (0-5) extension y Postoperative knee 120 (90-130) flexion y Mean polyethylene 12 (8-19) insert thickness (mm) AP knee alignment y 1.5 valgus Posterior slope of 3.5 (0-9) tibial cut y Flexion of femoral 8.1 (1-15) component y Tibial cut–fibula 6.5 (1-14) distance (mm)

No Post Failure, n = 494 TKAs (450 Patients) 68 (35-83) 181 (112-250) 42 30.6 (20.5-41.6) 6 (0-10)

P .035 * .002 * .036 * .690 .230

103 (90-125)

.311

0 (0-5)

.329

123 (100-135)

.188

12 (9-17)

.226

2.7 valgus 3.5 (0-7)

.148 .652

8.7 (3-14)

.540

6.3 (1-15)

.848

* P b .05 (statistically significant). y Specified in degrees, with range in parentheses.

knee radiographs (Table 1). Patient data included the age, sex, body mass index (BMI), and range of knee motion before and after surgery. Knee radiographs in the anteroposterior (AP), lateral, and sunrise patella projections were obtained preand postoperatively, at 6 months after surgery, and annually thereafter. Because femoral component flexion, posterior slope of the tibia, varus alignment, and joint line alterations can be associated with anterior tibial post impingement and damage [16,17], these variables were measured in the present study. Joint line alterations were assessed by measuring the distance from the tibial cut to the proximal fibula on the postoperative AP radiograph. On the lateral radiograph, femoral component flexion was the angle between the anterior flange of the femoral component and the anterior cortex of the femur, and tibial slope was the angle between the tibial tray and the long axis of the tibia, approximated from the anterior tibial cortex on the lateral knee radiograph. Varus or valgus alignment was measured on the AP radiograph. Radiographic failure was defined as a complete radiolucent line greater than 2 mm wide at the bone-cement interface, greater than 3° change in alignment, or greater than 3 mm migration of any component.

652 The Journal of Arthroplasty Vol. 23 No. 5 August 2008 Study variables were compared between the knees that had a failed post, and those without, to detect significant differences between these 2 groups. Data were analyzed by an independent statistician using the Student's t test or 1-way analysis of variance to compare variables of interest. Statistical significance was set at P b .05.

Results Sixty-two patients (70 knees) of the cohort of 564 knees (12%) had undergone repeat surgery because of symptoms after sudden failure of the tibial post at an average of 40 months after the index procedure (range, 24-83 months). Eight of the 62 patients (13%) with bilateral TKA had tibial post fractures in both knees as separate events. Each patient with symptoms related to a failed post elected to have revision surgery, which consisted of replacing the damaged tibial insert with a congruent deep-dish design made by the same manufacturer. With the exception of these 70 knees, no other knee in this series underwent repeat surgery. Table 1 summarizes the data compared between the knees with a failed tibial post and those without. Pertinent differences are described further below. Clinical Data In the group with post failures, the proportion of males was 71% (20 women and 50 men); this was significantly greater than the proportion of males in the group without post failures (42%; 189 men and 261 women, P = .036). Among patients without post failures, the mean patient age was significantly older (68 years; range, 35-83 years; P = .035), and mean patient weight was significantly less (181 lb; range, 112-250 lb; P = .002), although mean patient BMI was not different from the group with post failures (30.6 kg/m2; range, 20.5-41.6 kg/m2; P = .690). Knee range of motion was nearly identical in both study groups (Table 1). The mean polyethylene insert thickness in both study groups was 12 mm, with a range of 8 to 19 mm in knees with post failures and 9 to 17 mm in knees without post failures (P = .226).

with the knees without post failures (Table 1). No knee in either group had radiographic evidence of component loosening or migration. The symptoms described by patients after post failure included knee instability, patella clunking, pain, or a combination of these (Fig. 2). Patients with patella clunking could reproduce a sharp, snapping sensation during examination as the knee was brought into extension from a flexed position. Symptoms typically began with the knee in mid to deep flexion during routine daily activities. In some patients, the first sign of post failure was a sensation of the knee giving out while rising from a seated position. Seven patients associated a distinct traumatic event, such as a fall, with the onset of symptoms of post failure. Physical examination of knees with post failures demonstrated posterior tibial subluxation when the knee joint was flexed more than 90°. With less knee flexion, posterior instability could not be consistently demonstrated. Plain radiographs were not useful in diagnosing post rupture. Early in this series, knee arthroscopy was sometimes necessary to confirm the diagnosis until the history and clinical findings (positive posterior drawer beyond 90° of flexion) were consistent and familiar enough to make the diagnosis without arthroscopy. All patients with a failed post elected to have revision with a deep-dish congruent design made by the same manufacturer. Satisfactory fixation, alignment, and rotation of metal components were carefully verified during each revision procedure, and the metal components did not require revision in any knee with a failed tibial post. After insert exchange, patients regained an average knee extension of 1° (range, 0°-5°), and an average knee flexion of 118° (range, 100°-130°) at a mean followup of 28 months (range, 13-61 months) after the revision operation.

Radiographic Data There were no differences in radiographic variables between study groups. In knees with tibial post failures, the mean femoral component flexion, mean posterior tibial slope, mean AP alignment, and mean distance between the tibial cut and fibula were not significantly different when compared

Fig. 2. Symptoms of tibial post failure.

Tibial Post Failures in a Condylar Posterior Cruciate Substituting TKA
Bal et al

The articulating surfaces of all retrieved inserts had surface discoloration and polyethylene delamination to varying degrees. The tibial post sheared off close to the base, leaving a small portion of the post on the insert. The post sometimes broke into multiple pieces. In some inserts, an anterior groove on the remaining post was present; this resulted from contact against the femoral component during knee extension and suggested that anterior post impingement was a separate phenomenon from post failure. Despite failure, enough post remained on the retrieved inserts to confer posterior stability to the knee, especially in less than 90° of flexion (Fig. 1). Flexion beyond 90° disengaged the failed post from the femoral cam, allowing posterior subluxation to occur.

Discussion The Total Condylar knee prosthesis had a highly conforming polyethylene tibial insert to substitute for the PCL [18]. Survivorship and pain relief were excellent with this design, but limited knee flexion was a concern [4]. The Insall-Burstein prosthesis improved femoral rollback and knee flexion by incorporating a prominence on the tibial insert that could engage a cam on the femoral component during knee flexion [19]. This cam and post design was used in many subsequent designs of PS-TKA, with excellent outcomes in terms of pain relief, survivorship, wear, and knee range of motion [2,3,7,20]. Complications related to the cam and post have included patellar clunking [8,10], gross wear of the tibial post [1,6], post damage from anterior impingement [11,21], and catastrophic failures of the post [14,22]. A broken tibial post can manifest as patellar clunking, knee instability, pain, and even prosthesis dislocation [13,14,22,23]. The diagnosis of a broken post can easily be overlooked because symptoms are usually intermittent and nonspecific. Posterior tibial subluxation in association with a failed post may not be evident unless the knee is placed in greater than 90° of flexion [13]. Previous reports have described the necessity of surgical exploration [14], arthroscopic surgery [13,24], plain and computed tomographic arthrography [22,25], and plain radiographs of the knee [5] to correctly diagnose a broken tibial post. Thirteen of the 70 knees in the present series had patella clunking and the patients could demonstrate this finding during physical examination. In a separate report, post failures were associated with patella clunking in 5 total knees of the same design

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as the present series [13]. Patella clunking is not specific for post failures, however, because it can occur in PS-TKA with an intact tibial post as well [8,23]. Although not verified fluoroscopically, we hypothesize that in some knees, after post failure, the femoral cam moved anterior to the broken post as the tibia subluxated posteriorly in knee flexion. With knee extension, the damaged post moved anterior to the femoral cam with a clunking sensation. Our findings suggest that the acute onset of patella clunking in an otherwise wellfunctioning PS-TKA should be investigated further. Because tibial post deformation and damage are common in retrieved PS-TKA implants, the cam and post design may not be suitable for high-demand patients or those who want greater knee flexion after TKA [26-28]. Retrieved tibial posts from many total knee designs show anterior impingement and damage from contact against the femoral component during knee extension [6,11,15,21]. Anterior impingement can occur in both the tibial posts of PSTKAs and the polyethylene tibial eminences of PCLretaining TKAs [6,11,12,21,29]. In knee flexion, the tibial post can contact the metal femoral cam posteriorly instead, resulting in damage and wear that can be severe enough to result in failure of the arthroplasty [12]. Because tibial post impingement can occur in well-functioning total knees, the design and geometry of the cam and post are critical to the longterm performance of the implant [12,13]. The high incidence of post failures reported here probably reflects the combined effect of several adverse variables. One, the sharp corners of the post increased the risk of impingement in the femoral box during knee rotation. Two, the structural integrity of the post was compromised by the central screw hole. Three, the post was further weakened by oxidative degradation in the polyethylene. The tibial posts in the present study failed either in knee flexion, when the femoral cam exerted a tensile lift-off force on the post, or during knee rotation as the patient got up from a seated position. These mechanisms have been previously implicated in post failures associated with fatigue strength reduction of polyethylene from γ irradiation and oxidation [30]. Although not demonstrated by our data, it is possible that post failures occurred in a subset of patients with excessive rotation between the femur and tibia. This mechanism is associated with tibial post impingement in the femoral box, polyethylene wear, implant loosening, and osteolysis [31]. Up to 15° of external tibial rotation can occur after TKA [32]. Normal and reverse axial rotation in excess of

654 The Journal of Arthroplasty Vol. 23 No. 5 August 2008 20° after routine primary TKA have also been demonstrated [33]. We speculate that in combination with suboptimal implant design and post weakening, excessive knee rotation may have led to early post failures in a selected subset of patients in the present series. Although the patients with tibial post failures were more likely to be male, heavier, and younger compared with patients without post failures, the numerical differences in these variables were not meaningful. A limitation of the present study is that precise data relating to the sterilization method of polyethylene and shelf aging of polyethylene components was unavailable. Although γ irradiation decreases the fatigue strength of polyethylene, 30 tibial post failures have occurred in non–crosslinked γ-inert sterilized polyethylene as well [5]. Furthermore, PS-TKA survivorship of 90% to 95% after 10 years has been reported with polyethylene components sterilized by conventional γ irradiation in air [34-36]. Postirradiation shelf aging of polyethylene is associated with oxidative degradation and clinical failures in total knees [2,37-40]. Independent of shelf aging, oxidative degradation of polyethylene can continue even after TKA implantation from dissolved oxygen in the synovial fluid, albeit at a slower rate [41-43]. Thus, the extent to which shelf aging and the sterilization of polyethylene contributed to post failures in the present study is uncertain, although it is possible that the tibial post may have been weakened by oxidation. Surgical variables such as excessive femoral component flexion, anterior positioning of the tibial tray, excessive posterior tibial slope, and joint line alterations of 8 mm or greater can predispose to anterior tibial post impingement and failure [6,11,16,17]. In the present series, these variables were similar between knees with post failures and those without. Furthermore, post failures usually occurred during knee flexion rather than extension, and many retrieved inserts had anterior grooves on the remaining post that were distinct from the level of post fracture, suggesting that anterior impingement was unrelated to post failure. The high rate of early post failures reported here is concerning and requires careful surveillance of patients who have received this design of implant. Because of the multifactorial etiology of post failures, it is possible that the number of failures will increase with longer follow-up, despite appropriate surgical implantation of the components. If metal component alignment and rotation is satisfactory, isolated revision of the failed insert with a deep-dish design that has a raised anterior rim can salvage the arthroplasty without removing the metal implants [4].

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