An increase in anterior patellar displacement was associated with a lower probability of the need for a lateral retinacular release (odds ratio = 0.50/cm; p = 0.0022) (Table V). Furthermore, an increase in measured anteroposterior femoral size was associated with a higher probability of the need for a lateral retinacular release (odds ratio = 1.9/cm; p = 0.0010) (Table V). These relationships, even when combined, explained only 10.1% of the observed variance in the need for a lateral release.
Increases in anterior femoral offset also increased the odds of lateral release (odds ratio = 2.2/cm; p = 0.0006) (Table V). Measurement B (anteroposterior femoral size) overlaps and encompasses measurement C (anterior femoral offset). The two measures were analyzed in separate regressions to ensure that the effect of length C was uniquely represented. The results indicated that anteroposterior femoral size (odds ratio = 1.9/cm; p = 0.0010) and anterior femoral offset (odds ratio = 2.2/cm; p = 0.0006) equivalently increased the likelihood of lateral release.
Table VI demonstrates additional factors associated with the probability of lateral retinacular release. The forward/stepwise selection procedure of the logistic regression revealed that gender, large as opposed to medium patellar size, and absolute femoral component size were more important than either anterior patellar displacement or measured anteroposterior femoral size (Table VI).
There were no differences in outcome between the AGC or LPS knee systems with regard to the Knee Society Knee Score, the Knee Society Function Score, the Knee Society Pain Score, or the need for lateral release (p = 0.9949, p = 0.7898, p = 0.0851, and p = 0.0612, respectively). There was a +4.4° difference in the postoperative range of motion for the LPS prosthesis compared with the AGC prosthesis (p < 0.0001), and this effect was accounted for in the analysis. Interactions between stuffing measures that had a significant effect on range of motion and the use of either prosthesis were not significantly related to range of motion (anterior patellar displacement, p = 0.5755; anteroposterior femoral size, p = 0.0924).
Increasing the anteroposterior size of the patella or the distal part of the femur, or both, during total knee arthroplasty is referred to as stuffing the patellofemoral joint1,2. Some authors have observed that stuffing of the patellofemoral joint adversely affects the outcome of knee arthroplasty1-7,9-11. Shoji et al. reported that 17% of 141 knees with a ≥10% increase in anteroposterior patellar thickness after total knee arthroplasty achieved >120° of flexion postoperatively, compared with 50% of ninety knees with a <10% increase in patellar size4. Ryu et al. observed that postoperative patellar thickness was significantly greater in twenty-one knees with flexion of ≤90° after total knee arthroplasty than in twenty-nine knees with flexion of ≥120°10. Daluga et al. reported an increased need for manipulation following total knee arthroplasty in ten of ten patients with a postoperative increase of 12% in combined patellofemoral anteroposterior size3. Koh et al. compared fifty-six patients with residual patellar bone thickness of ≤12 mm with sixty-six patients with residual patellar bone thickness of >12 mm12. Postoperative differences in the Knee Society Score, Knee Society Function Score, and range of motion were not observed, but the authors noted that a higher rate of patellar complications may occur when the patellofemoral articulation is “excessively” increased. It is important to note that those studies involved relatively small numbers of patients and knees3,4,10,12.
Our study evaluated the effect of stuffing the patellofemoral compartment on the outcome of total knee arthroplasty in a large sample of 556 patients and 830 knees. For every millimeter of increase in anterior patellar displacement, we observed a corresponding 0.18° decrease in range of motion (p = 0.0079) and a 0.52-point increase in the Knee Society Function Score (p < 0.0001). It is possible that anterior displacement shifted the extensor mechanism, limiting its full excursion and adversely affecting range of motion, and improved the efficiency of the quadriceps mechanism by increasing the moment arm of the quadriceps. Clinically, however, our findings indicate that even a 1-cm change in anterior patellar displacement, which rarely (if ever) occurs, could result in only a 1.8° change in the range of motion and a 5.2-point change in the function score. For every millimeter of increase in anteroposterior femoral size, the range of motion increased by 0.14° (p = 0.0182) and the Knee Society Function Score decreased by 0.23 points (p = 0.0094). Combined anterior patellar displacement and anteroposterior femoral size had no effect on outcomes. Both anterior femoral offset and posterior femoral offset had small negative effects on function scores. Posterior offset alone was related to range of motion, with a small positive correlation being observed. None of the measures were associated with significant changes in the Knee Society Knee Score or the Knee Society Pain Score.
The patellar components used in the present study ranged in thickness from 7.5 to 10 mm, and our attempts to resolve even a 5° loss in range of motion due to anterior patellar displacement was only marginally successful (p = 0.0512, F-test, H0: β ≥ 5). Similarly, an effect size of ≥5 points in the Knee Society Function Score for a 1-cm change in anterior patellar displacement or anteroposterior femoral size was equally unattainable in this population (p < 0.0001, F-test, H0: β ≥ 5). Thus, the significant effects that we observed do not appear to be clinically relevant in terms of patient outcomes.
The probability of the need for lateral retinacular release decreased as the anterior patellar displacement increased and as anteroposterior femoral size increased. The latter finding is consistent with the concept that increasing anteroposterior femoral size tightens the lateral retinaculum and increases the likelihood of lateral patellar subluxation and tilt. However, increasing anterior patellar displacement theoretically produces a similar negative effect on patellar tracking, and yet we found that an increase in anterior patellar displacement was associated with a lower probability of the need for a lateral release. Moreover, the probability of needing a lateral release was more significantly related to gender, large as opposed to medium patellar size, and absolute femoral component size than it was to either anterior patellar displacement or anteroposterior femoral size. This observation suggests that the probability of requiring a lateral release most likely involves a more complex set of factors than just stuffing of the patellofemoral joint.
The present study was limited by its retrospective design, the potential for measurement errors on the radiographs, the inability to correct for patellar erosion and the effect of patellar tilt and/or subluxation on measured anterior patellar displacement, and the absence of an outcome survey specific to patellofemoral problems and function. Multiple surgeons performed the operations but, with the exception of the two different implant designs, all surgeons employed standardized procedures and approaches to patient care based on shared physician practice protocols and evidence-based standards of care. Although the present study reflects a retrospective review of historical cases and not a prospective collection of data from new cases, the outcome data were collected by the operating surgeons and are therefore subject to observer bias.
Our study evaluated the outcomes associated with a posterior cruciate-retaining prosthesis (AGC Knee) and a posterior cruciate-substituting prosthesis (LPS Knee). With the numbers available, the analysis did not indicate differences in outcomes with the exception of range of motion. Thus, we believe that our findings are relevant to both cruciate-retaining and cruciate-substituting total knee arthroplasty designs.
Our data do not support the concept that stuffing of the patellofemoral joint is associated with adverse outcomes. As discussed earlier, it is not likely that any of the small effect sizes that we observed would result in meaningful changes in flexion, function, or pain postoperatively. Although we still advocate the practice of attempting to reproduce anterior patellar displacement, anteroposterior femoral size, and the combined patellofemoral anteroposterior size during a total knee arthroplasty, our data show that precision of reproduction of these sizes did not affect the outcome of the procedure.
The findings of the present study have important clinical implications. In the evaluation of a patient who has pain or stiffness following a total knee arthroplasty, caution should be exercised in attributing these problems to stuffing of the patellofemoral joint even if radiographs show evidence of an increase in anterior patellar displacement, anteroposterior femoral size, or the combination of anterior patellar displacement and anteroposterior femoral size. Because our data do not demonstrate a relationship between stuffing and pain, we do not recommend a revision to correct stuffing of the patellofemoral joint in the absence of another identifiable cause of the pain, such as prosthetic loosening, osteolysis, infection, or instability. ▪
Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from Biomet. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Biomet and Zimmer: royalties, consulting). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
Investigation performed at the Center for Hip and Knee Surgery, St. Francis Hospital, Mooresville, Indiana, and the Center for Joint Replacement, St. Vincent Hospital, Indianapolis, Indiana
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