Both groups had improvements (p < 0.001 for all) in the patellar score of Feller et al., The Knee Society score, and WOMACTM score at last followup. Postoperative scores were similar (p > 0.05 for all comparisons) in the two groups at last followup. No knees had radiolucent lines wider than 2 mm or probable loosening on any prosthetic components. Of 17 patients in the PC group who underwent bilateral TKAs with postoperative patellar crepitus on one side, no major differences were observed between knees in terms of clinical scores, functional abilities, or radiographic factors.
The major predictors of the development of patellar crepitus were patellar cartilage of Outerbridge Grade IV (odds ratio [OR], 11.9; 95% CI, 2.2-65.3) and joint line elevation (OR, 5.1; 95% CI, 1.9-8.6) (Table 4).
Patellar crepitus is a manifestation of a spectrum of peripatellar fibrosynovial formations and usually is encountered after PS-TKA, although this also may occur with the cruciate-retaining design. The incidence of painful or painless patellar crepitus has been reported to range from 0% to 21% [9, 16, 22, 24, 29, 31, 35]. Numerous factors have been implicated as an etiology of this phenomenon after PS-TKA [1, 4, 9, 11, 15, 16, 20, 22, 24, 28-31, 35], but it is unclear whether and to what degree crepitus influences pain and function. In addition, these studies did not specifically explore patellar crepitus in patients having TKAs without patellar resurfacing. We therefore determined (1) the frequency of crepitus; (2) which factors predicted the occurrence of crepitus; and (3) whether crepitus influenced pain and function in a control-matched clinical evaluation.
Several limitations of our study should be considered. First, because we did not directly compare other prosthetic designs with different patellofemoral geometries and kinematic patterns, our findings cannot be applied to all PS-TKA designs. In addition, although this implant had functioned well in our patient cohort, further study is necessary to assess how it would function in a group with patellar resurfacing. Second, this study is negatively affected by its nonrandomized and retrospective design or some missing data but positively affected by the control-matched study groups with TKAs without patellar resurfacing. Third, the followup duration (2.0-4.5 years) was short, but time from index surgery to patellar crepitus onset has been mostly before 12 months [4, 22, 26, 29, 35]. Fourth, the size of the analyzed cohort may be insufficient to provide statistical power for all factors. However, to our knowledge, no large-scale studies have addressed patellar crepitus after PS-TKA without patellar resurfacing. Finally, Asian populations adopt the lotus position and squatting in daily activities compared with Western populations. These differences in lifestyle may affect the development or implication of patellar crepitus.
The femoral component design is reportedly an important factor in the development of patellar crepitus [1, 4, 9, 16, 22, 24, 25, 28-30]. Ranawat et al.  noted a major reduction in the rate of painful patellar crepitus development for the PFC Modular Knee (DePuy, Warsaw, IN, USA) as compared with the IB/II (Zimmer, Warsaw, IN, USA) design. Pollock et al.  reported the rate of grinding patellar crepitus was 13.5% for the AMK Congruency (DePuy) knee, 3.8% for the anatomic modular knee, and 0% for the fixed-bearing PFC Sigma® knee (DePuy). Improvements in the designs of femoral components with deepening of the trochlear groove and elevation of the lateral flange have been applied to some PS-TKA implants to reduce peripatellar problems [9, 26, 30]. Proximally positioned femoral box designs with an intercondylar box height ratio greater than 0.7 (ratio of the length of the intercondylar box to the AP width of the femoral component) and a sharp transition into the intercondylar notch are at greater risk of this phenomenon. When this ratio is larger, the superior pole of the patella and the anterior edge of the intercondylar box come into contact during earlier flexion. The Vanguard PS complete knee (Biomet) used in this study was designed to optimize patellar tracking and to limit the impingement of soft tissue between the patella and the intercondylar notch with the intercondylar box ratio less than 0.7. In this study, the incidence of patellar crepitus (8.5%) was similar to that of equivalent fixed-bearing PS-TKAs and no patient had patellar clunk syndrome develop. Although we are unsure of the exact cause for no manifestation of patellar clunk during the study period, we speculate elaborate excision of the suprapatellar synovial lining and use of this design result in decreased entrapment of fibrosynovial hyperplasia in the intercondylar box, limiting patellar excursion.
We found advanced cartilage tear was related to the development of patellar crepitus. Although the exact cause for this correlation is doubtful, we assume advanced patellofemoral arthritis results in increased intraarticular scar formation and increased soft tissue tension, which may increase the risk of development of fibrosynovial proliferation. The study also shows 9.3% of patients with postoperative patellar crepitus had peripatellar pain but that all patients in the PC group eventually achieved relief of their symptoms without a surgical procedure and no patients required revision surgery for secondary patellar resurfacing. Our findings do not support routine use of patellar resurfacing to reduce patellar crepitus in patients with advanced patellofemoral osteoarthritis. Previous studies [3, 7, 8] have reported patellar cartilage status does not influence clinical results, regardless of whether the patella is resurfaced. Although our study did not include knees without patelloplasty, we believe these favorable findings may be related to the patelloplasty to enhance patellofemoral tracking. The patella includes an elastic layer that enables plastic deformation allowing adaptation of the patellar contact surface to the femoral trochlea, which enhances biologic remodeling and decreases peripatellar pain [2, 18, 23]. Thus, a properly remodeled articular patellar surface may result in better patellofemoral tracking, even in knees with high-grade chondromalacia, if patelloplasty and soft tissue balancing are properly performed. Further study is warranted to determine whether patellar resurfacing can reduce the risk of patellar crepitus in patients with a patella of advanced grade.
We also found joint line elevation, which can lead to patella baja, was related to the development of patellar crepitus. Joint line elevation may increase contact stress between the quadriceps tendon and the superior portion of the intercondylar box and cause fibrosynovial tissue formation and entrapment in the box [15, 35]. We recommend excising the synovial lining overlying the top of the femoral component and peripatellar soft tissue by electrocautery to remove nidus to scar tissue formation.
The natural history of patellar crepitus after PS-TKA usually is associated with gradual amelioration of pain. However, problems often are persistent in patients with moderate-to-severe peripatellar pain or patellar clunk syndrome [12, 25, 29, 33, 34] and, thus, open or arthroscopic débridement of the fibrosynovial hyperplasia should be considered in these patients. Previous studies [12, 33, 34] have reported rates of 82% to 100% for successful resolution of patellar crepitus or clunk without recurrence. In our study, no patient required a surgical procedure and all knees were well functioning and symptom-free without evidence of recurrence.
Our study suggests the development of patellar crepitus is associated with advanced patellofemoral osteoarthritis and joint line elevation. Furthermore, all patients achieved complete symptom relief without an arthroscopic procedure or arthrotomy. Patellar crepitus is self-limited and a benign problem. We continue not to resurface arthritic patellae and counsel patients with patellar crepitus that their symptoms will improve without intervention.
We thank Hye-Sun Ahn MS and Ji-Won Park RS for help in collecting and organizing the data analyzed in this study.
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