The patella component of a total knee arthroplasty is the most frequent component to show signs of radiographic loosening. The incidence of loosening of the patella implant after total knee arthroplasty has been reported to be approximately 1%. 4,9 The overall incidence of radiographic loosening in the current series was 4.2% (180 of 4287) and the longitudinal process leading to loosening was characterized.
From this investigation, the authors propose a combined biologic and biomechanical mechanism leading to loosening of the all-polyethylene patella component. The majority of the patients in the current series had femoral components implanted with posterior referencing instrumentation. Therefore, the components were placed into relative internal rotation compared with the epicondylar axis. This increased the need for intraoperative lateral retinacular release to balance the soft tissues of the patellofemoral articulation, which increased the risk of subsequent avascular necrosis of the patella and eventual component loosening. The femoral component rotation also would biomechanically predispose the patella component to lateral subluxation and increased lateral patella biomechanical forces that may contribute to loosening of the patella component.
The radiographic features that were documented in the current series follow the biologic pathway of osteonecrosis such as those recognized in the talus, scaphoid, and the femoral head. These features include increased density of the bone, subchondral and trabecular bone resorption, progressive radiolucency, and fracture and fragmentation of the bone. Once loose, there was lateral subluxation of the residual patella bone in 146 of 180 (81%) of the knees caused by the biomechanical forces resulting from relative internal rotation of the femoral component. The process of loosening of the patellar component is not entirely biologic and not fully attributable to abnormal biomechanical forces but has features of both mechanisms contributing to the failure process. The current study may serve as a worst case scenario for future cohorts of patients to compare patellar loosening when the femoral component rotation is determined with the epicondylar axis and no lateral retinacular release is performed.
The design of patella components including the role of three-peg versus one-peg has been suggested to play a role in implant failure. In the current series, patellar component breakage always was associated with other radiographic features of loosening and avascular necrosis. Francke and Lachiewiez, 12 reported peg shearing of three-peg patellar components at the interface of the peg with the component. Using single-peg patellar components, Huang et al 19 reported four cases of failure of the peg junction with the patella component as a cause of failure. Berry and Rand 1 reported an additional seven cases of fracture of the fixation peg in a large series of isolated patellar component revisions. Fatigue fractures at the peg-plate junction were reported in 12 knees by Rosenberg et al 30 in ingrowth-type implants. It remains unclear which is a more durable implant design.
Thickness of the patella has not been shown to have an effect on patella fracture or loosening or both. 29 Biomechanical studies by Hsu et al 17 have shown that reapproximation of the original patella thickness may lead to more normal contact areas and help restore postoperative range of motion and decreased the incidence of patella subluxation. Patella thickness and patella tilt were not measured in the current series because they have been documented previously. 29
Instability and subluxation of the patella have been reported to be related to errors in surgical technique, quadriceps imbalance, and trauma. 5,14,24 Proposed treatment of instability includes correction of proximal and distal malalignment with or without component revision, which has been associated with a serious complication rate including patella tendon rupture. 14 Bindelglass et al 2 reported that subluxation was not associated with decreased pain scores. Briard and Hungerford 5 suggested that treatment of patellar instability should be focused on correcting the underlying cause of a patellofemoral instability and not solely patella component revision. The authors found that lateral instability or subluxation was the final radiographic feature to appear in the process of loosening of the patellar component and not a primary source of patellar complications.
Patella tilt has not been correlated with poor outcome or increased incidence of radiolucency or loosening of the patella component. The incidence of patella tilting after knee replacement surgery varies and has been reported to be as high as 31%. 2,25 Chan and Gill 8 reported that the greater the preoperative patellar tilt, the more likely the patella would tilt laterally postoperatively. Tilt has not been found to be associated with the incidence of lateral release or decrease in pain scores 2 although it may predispose to increase loading at the lateral periphery of the component and wear of metal-backed components. 2 In an earlier series of total knee replacements from the authors’ institution, lateral retinacular release was shown to decrease lateral tilting of the patella. 27 From the current study, one may conclude that proper balancing of the patellofemoral articulation through proper component rotation with efforts to alleviate the need for lateral release may be the best way to prevent tilt, subluxation, and decrease the overall likelihood of patellar loosening.
Fracture of the residual patella beneath the patella component may be a catastrophic complication after total knee arthroplasty if the extensor mechanism is disrupted. The incidence of patella fracture after total knee replacement surgery ranges from 3% to 21%. 31,33 These results compare with the results of the current series in which a fracture was seen in 133 patellas for a rate of 3.1%. The revision of such fractures and reconstruction of the extensor mechanism has been found to have a high complication rate. 1 Fracture type and function of the extensor mechanism has been shown to affect outcome of operative and nonoperative treatment of patients with patella fractures. 13 In general, it is recommended that patients with nondisplaced fractures of the patella can be treated conservatively with immobilization. 15,16 Even patients with displaced fractures without extensor lag can be treated nonoperatively. Keating et al reported on fractures with and without patellar loosening and found that operative treatment with excision had a high complication rate with four of nine patients having deep infection and two patients treated with internal fixation had nonunion. (Keating, EM, Berend, ME, Crites, BM, et al: Patella Fracture Post Total Knee Replacements. Presented at the Knee Society Meeting, Boston, September 2000.)
Much attention has been given to the role of lateral release and its effects on patella complications and the disruption in the patellar blood supply postoperatively. 3,6,21,27,28,32 The relative role of the biomechanical imbalance that leads to the need for lateral release predisposing to patellar loosening in comparison with the vascular disruption that results after lateral retinacular release is unclear. Lateral release may alter the patellar tracking and joint contact characteristics 18 but may lead to other more detrimental effects on the viability of the patellar bone. Lateral release has been associated with decreased patella viability and osteonecrosis. 23,32 Sacrificing or saving the lateral geniculate artery during lateral release does not seem to influence patellofemoral complications after total knee arthroplasty. 28 In an early series, Ritter and Campbell 28 found no increased evidence of osteonecrosis, bone-cement radiolucency, or patella fracture after lateral release. Subsequent studies 28,29 with longer-term followup reported that lateral retinacular release was associated with increased incidence of initial patella fractures within 1 year. Both of these series had a high incidence of lateral retinacular release ranging from 15% to 35%. In the current series, lateral retinacular release was associated with a significantly increased rate of loosening of the patellar component. This correlates with the radiographic features of an avascular process leading to patellar component loosening.
One limitation of the current study as it describes the timing of the failure process is the timing of the postoperative radiographs. The authors standard followup protocol is for radiographs to be obtained at 2 months, 6 months, 1 year, 3 years, 5 years, 7 years, and 10 years. The majority of the radiographic features were found between the radiographs obtained at 6 months and 3 years. Perhaps with more frequent intervals of followup the exact timing could be identified more precisely. The progressive nature of the failure process including progression through the phases of bonecement radiolucency, increased density, collapse, fracture and fragmentation, and finally lateral subluxation was reproducible and predictable with the interval of followup used in the current study.
Pain scores and Knee Society scores 11,20 were not affected adversely by the presence of a loose patella averaging 45 points and 85 points, respectively. Revision or excision of the patellar component was done in only 15 patients (0.3%) in the series when the component completely dislodged from the patellar bone or when there was a complete disruption of the extensor mechanism with significant extensor lag.
Patellar component loosening occurred in 4.2% of AGC total knee replacements in the current series. A loose patellar component did not adversely affect the functional outcome or increase knee pain and infrequently requires revision. Loosening was strongly associated with lateral retinacular release and was present in 73% of the cases. The mechanism of failure was found to have characteristics of an avascular process with bone-cement radiolucency, increased density, collapse, fracture, and eventual lateral subluxation. The radiographic features and timing of this process were described. It is concluded that patellar loosening can be reduced by establishing proper patellofemoral balancing through appropriate femoral component rotation and alleviating the need for lateral release and maintaining patellar viability.
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Merrill A. Ritter, MD; and John B. Meding, MD, Guest Editors© 2001 Lippincott Williams & Wilkins, Inc.