Component Design Affecting Patellofemoral Complications After Total Knee Arthroplasty

Abstract

Three hundred one primary cemented total knee arthroplasties were performed in 289 patients. Two different prostheses were used; 148 knees received the Miller-Galante I prosthesis and 153 knees received the Press Fit Condylar prosthesis. Minimum followup was 2 years. The groups were similar in all parameters both preoperatively and postoperatively, with the exception of the patellofemoral complication rate. Knees that were implanted with the Miller-Galante I prosthesis experienced a complication rate of 10.1%, while those with the Press Fit Condylar prosthesis experienced a complication rate of 0.7%. The distinct difference in the patellofemoral complication rate may be due to differences in design of the femoral component. Features that may have contributed to increased patellofemoral morbidity included a short, narrow anterior flange; a shallow patellar groove; and an abrupt anterior to distal transition with a smaller radius of curvature. Because subtle design differences can have a profound effect on clinical outcome, long term evaluation of new designs should be completed before wide-spread use. The clinician also should be aware of the desirable design features when choosing a component.

It has been estimated that patellofemoral problems account for 50% of all complications following total knee arthroplasty.¹ The complications can be divided into several categories, including patellar fractures, loosening of the patellar prosthesis, patellar component wear, soft tissue impingement, and patellar instability.⁵ Of these, patellar instability and fracture are the most significant short-term complications.^1,6,9,11 The cause of the complication often is multifactorial. Preoperative malalignment, particularly in patients with valgus deformities, increases the incidence of patellofemoral complications.⁵ Technical errors, resulting in malposition of the components or inadequate patellar resection, also have been cited.^4,5,10 Yet, few studies have addressed the influence of prosthetic design on postoperative patellofemoral morbidity. Upon reviewing experience with 2 different prostheses, a marked difference was noted in the incidence of patellofemoral complications. This study addresses the design differences in the patellofemoral articulation which may lead to the increased complications.

MATERIALS AND METHODS

Three hundred and twenty eight total knee replacements in 289 patients were performed within the study period. Twenty seven knees in 27 patients were excluded for inadequate followup, death, or infection. This left 134 patients (148 knees) in Group I who received the Miller-Galante I, and 128 patients (153) knees in Group II who received the Press Fit Condylar to be analyzed for patellofemoral function. Preoperative population comparisons are shown in Table 1. No significant differences existed between the groups.

The study population included all patients who underwent primary total knee arthroplasty between January 1987 and June 1989 at the Hospital of the University of Pennsylvania (Philadelphia) and Delaware County Memorial Hospital. Patients were divided into 2 groups based on their choice between 2 hospitals, where 2 different total knee systems were used. Concurrently, 1 group received Miller-Galante I prostheses (Zimmer, Warsaw, IN) while the other group received Press Fit Condylar prostheses (Johnson & Johnson, New Brunswick, NJ). Surgical technique was similar for each group, with all procedures being performed under the supervision of the senior surgeon (P.A.L.). A median parapatellar approach was performed with modified designer guides to make the femoral and tibial cuts. Resection of the patella was accomplished to obtain a level surface parallel to the quadriceps tendon insertion. All components were fixed with polymethylmethacrylate. Dome, all polyethylene, patellar buttons were used in all knees. Patellar tracking was assessed by the no thumbs technique before closure. If lateral subluxation was noted, a lateral retinacular release was performed to obtain satisfactory tracking. The superior lateral geniculate artery was not preserved. Postoperatively, a knee immobilizer was used for 48 hours, followed by the progressive increase of active and passive range of motion exercises. A continuous passive motion machine augmented physical therapy. If 70 ° of flexion was not reached by 7 days postoperatively, closed manipulation was performed under general anesthesia. Patients were discharged when at least 80 ° flexion was obtained and independent ambulation with crutches or a walker was possible. Protected weightbearing was continued for 6 weeks.

Clinical and radiographic evaluations were performed postoperatively at 6 weeks, 6 months, 1 year, and then yearly. All patients were evaluated both pre- and postoperatively by measuring range of motion, as well as determining the knee and function score using the Knee Society clinical rating system.⁷ Preoperatively, knee alignment was directly measured from standing anteroposterior radiographs, while postoperatively it was included in an overall radiograph score. This score was derived by measuring the alignment of the components with deductions for malposition, radiolucencies, patellar tilt, or change of joint line height. A total score then was derived, with 100 being the maximum score possible. The geometry of the patellofemoral joint was evaluated by measuring similarly sized femoral components. The anterior width, proximal length, and arc of curvature of the patellofemoral articulation were compared. Also, the depth and configuration of the patellar groove was determined at 1-cm intervals moving proximally from the joint line. A minimum of 2 years followup was required for inclusion in the study.

RESULTS

The results of the clinical and radiographic followup evaluations are shown in Table 2. Again, by these parameters, the groups are quite similar. However, a significant difference did exist between the 2 groups concerning the postoperative patellofemoral complication rate. In Group I, the complication rate was 10.1%, including 3 patellar fractures, 5 patellar subluxations, and 7 dislocations. These patients required 5 subsequent operative procedures, including 1 component revision. This differs from the single patellar fracture in Group II, which translated to an overall patellofemoral complication rate of 0.7%. The preoperative parameters of the 15 patients in Group I with patellofemoral complications are representative of the entire Group I sample, as shown in Table 3.

Analysis of the patellofemoral geometry showed several differences in design. The proximal extension of the anterior femoral flange was 20% longer on the Press Fit Condylar prosthesis than on the Miller-Galante I prosthesis and was wider by 20% to 37% at various levels above the joint line (Fig 1). The patellar groove is 20% deeper in the Press Fit Condylar design (Fig 2) than in the Miller-Galante I prosthesis. Also, the distal slope descends earlier from the anterior cortex on the Press Fit Condylar than it does on the Miller-Galante I prosthesis and has a smaller radius of curvature (Figs 3, 4). Finally, the Miller-Galante I patellar groove has an elevated lateral flange (Fig 2). In general, the Press Fit Condylar design extends more proximally and has a wider patellar groove in the femoral component, allowing for earlier capture of the patella during flexion. A longer and deeper anterior to distal transition, and a deeper patellar groove secure the patellar tracking as flexion increases. The elevated lateral wall of the patellar groove in the Miller-Galante I did not appear to prevent patellar subluxation.

DISCUSSION

While numerous authors have described the etiology of patellofemoral complications following total knee arthroplasty,^4,5,10 few have addressed the contribution of component design. Van Kampen and Huiskes¹⁵ have conducted detailed cadaveric studies demonstrating the 3-dimensional movements of the patella during tracking. Despite the complexity of the tracking, they concluded that the motion constraints of the patella are determined by the anatomic characteristics of the distal femur and by the balance of forces along the soft tissues. Similarly, the patellofemoral design appears to be a critical determinant of patellar tracking after total knee arthroplasty.

Others have demonstrated some desirable anatomic features of the prosthetic patellofemoral articulation. For example, experience with constrained devices has shown that the lack of internal tibial rotation with flexion leads to increased patellofemoral morbidity.¹² Early capture of the patella in the patellar groove also has been suggested to reduce patellar subluxation and dislocation.^3,8 Freeman et al reported their experience with several designs and concluded that the patellofemoral complication rate was lowest when a long anterior flange, of at least 60 mm, was used to capture the patella in the patellar groove early in flexion.³ The depth of the groove also is important for optimal tracking. Freeman et al suggested a groove at least 5 mm deep.³ Johnson and Eastwood attributed patellofemoral complications to a shallow groove.⁸

In this study, the patellofemoral complications that occurred after total knee arthroplasty was performed using the Miller-Galante I and the Press Fit Condylar prostheses were compared. The 2 patient populations were comparable both preoperatively and postoperatively because the procedures were performed concurrently, by the same surgeon, and by using similar techniques. Therefore, many important technical variables that affected the patellofemoral joint were controlled, including depth and angle of patellar resection and component rotation. Although each type of prosthesis had its own alignment guides, the same judgment regarding component rotation was necessary concerning each patient. Postoperatively, both groups experienced similar radiographic and clinical results, except for the incidence of patellofemoral complications. Therefore, the differences in patellofemoral complications may be due to differences in prosthesis design. Others also have reported a high incidence of patellofemoral complications with the Miller-Galante I prosthesis, again implicating component design as a major factor.^2,8,13,14 Although there is now greater awareness of the techniques required to assure correct component rotation, the concurrent surgical experience indicates that the Miller-Galante I was more likely to result in patellofemoral morbidity. It is necessary to place the femoral component in slight external rotation to compensate for the changes imposed on normal anatomy by prosthetic design; but, the Miller-Galante I design was less forgiving, with any patellofemoral imbalance likely to result in a complication.

Compared with the Press Fit Condylar femoral component, the anterior flange of the Miller-Galante I component is shorter and narrower, and does not trap the patella until later during flexion. The groove in the Miller-Galante I prosthesis also is more shallow allowing greater freedom of the patellofemoral articulation. Finally, the Miller-Galante I prosthesis has a more abrupt posterior declination of the patellar groove with a larger radius of curvature (hence a larger curvature). This most likely maintains a tight retinaculum as flexion is increased. The raised lateral wall of the Miller-Galante I prosthesis did not prevent subluxation.

Patellofemoral design is an important factor in preventing short-term complications of this articulation. It appears that more proximal capture of the patella in a deeper groove with more gradual proximal-to-distal transition reduces morbidity. While this particular model of the Miller-Galante I prosthesis has been revised, the basic concepts of patellofemoral design remain. The clinician should be aware of desirable design features when evaluating a prosthesis for potential use. Subtle design differences can have significant long term effects. These subtleties can be demonstrated only by long term evaluations, which should be completed before a new prosthesis is used widely.

Fig 1: . View of the anterior flange of the Pess Fit Condylar prosthesis (left) and the Miller-Galante I prosthesis (right) shows the Press Fit Condylar flange to extend 20% more proximal. Also, the Press Fit Condylar flange is wider by 20% to 37% at various levels above the joint line.

Fig 2: . The patellar groove of the Press Fit Condylar, seen in a tangential view (left), is deeper and wider than the groove of the Miller-Galante I (right). Note the raised lateral wall on the Miller-Galante I. Points A, B, C, and D correspond to the indicated distance from the joint line as represented in Figure 3.

Fig 3: . The anterior to distal transition of the Press Fit Condylar prosthesis is more gradual, with a smaller radius of curvature. Again, the longer anterior flange of the Press Fit Condylar also can be seen.

Fig 4: . A comparison of the sagittal contours shows the more gradual distal transition of the Press Fit Condylar. This allows for smoother tracking of the patella with increasing flexion.