TABLE VII -
PROMs of Patients with Anterior Knee Pain Compared with Those with No Pain at 10 Years by Follow-up Time Point
||Anterior Knee Pain (N = 73)
||No Pain (N = 89)
||41.9 (10.0) (19 to 61)
||44.6 (11.0) (14 to 58)
||51.4 (9.8) (29 to 65)
||53.2 (11.1) (24 to 66)
||35.5 (8.5) (16 to 48)
||37.2 (9.1) (11 to 48)
||40.9 (11.1) (19 to 57)
||43.1 (11.4) (19 to 61)
||51.1 (9.7) (27 to 71)
||52.7 (10.7) (26 to 67)
||35.7 (10.2) (5 to 48)
||39.6 (9.2) (14 to 48)
||35.5 (11.6) (15 to 57)
||43.4 (10.6) (21 to 57)
||48.5 (9.4) (26 to 67)
||51.5 (9.7) (28 to 65)
||29.6 (10.9) (7 to 48)
||40.1 (7.1) (17 to 48)
The values given as the mean with the standard deviation in the first parentheses and the 95% CI in the second.
Mann-Whitney U test.
The values are given as the number of patients with the percentage in parentheses.
A quarter of patients alive with an intact single-radius cruciate-retaining TKA who had not undergone routine patellar resurfacing reported anterior knee pain at 10 years. Patients with anterior knee pain at 10 years reported worse PROMs (OKS) beginning at 1 year. Radiographic measures including femoral component flexion, anterior femoral offset ratio (absolute and >15% of the femoral diameter), whether the femoral component was flush with the distal part of the native femur, and the medial proximal tibial angle, all with strong interobserver agreement, were significantly associated with anterior knee pain. Multivariate analysis indicated that, in this TKA design, femoral component flexion, tibial component coronal alignment (medial proximal tibial angle), and patella baja independently predicted long-term anterior knee pain. When the analysis was corrected for those variables, the anterior femoral offset ratio and a flush femoral component were no longer significant predictors, possibly reflecting the relationship between femoral flexion and the anterior femoral offset ratio. ROC curve analysis confirmed that femoral component extension of ≥0.5° correctly identified patients with anterior knee pain 87% of the time.
Postoperative anterior knee pain is the most common complication following TKA, and its association with PROMs confirms its importance. Post-TKA anterior knee pain has been reported in 80% to 85% of patients during chair rising and in 90% on stair climbing18. There have been few reports on anterior knee pain in 10-year cohorts1, but the rates reported in association with multi-radius designs (26% after cruciate-retaining TKA3 and 30% after posterior-stabilized TKA with resurfacing2) are comparable with our results. A number of variables have been considered as potential causes of anterior knee pain, including patellar resurfacing, “overstuffing,” denervation, fat-pad excision or retention, component rotation, joint-line alteration, sagittal alignment, and medial/lateral translation1. The roles of these variables have not been consistently reported, and the multitude of different TKA designs and resurfacing combinations makes comparisons difficult1. When present, anterior knee pain is difficult to manage, with 60% of cases persisting after secondary patellar resurfacing19.
Routine patellar resurfacing was not performed for our patient cohort. Meta-analysis of numerous randomized controlled trials demonstrated no difference in anterior knee pain between resurfaced and non-resurfaced patellae20, although reoperation rates were higher after TKAs that did not include patellar resurfacing, a fact confounded by the bias inherent in secondary resurfacing being possible20. Primary resurfacing rates vary internationally, with rates of 4% in Norway and 82% in the United States6. Across multiple national joint registries, the rate of primary resurfacing in TKAs was 35% in 20106; thus, the results of TKAs without resurfacing are applicable to the majority of TKA cases worldwide.
The influence of patellofemoral overstuffing and anterior femoral offset on anterior knee pain has been investigated previously17,21,22. Pierson et al.21 examined changes in anterior femoral offset in 838 patients (86% with a cruciate-retaining TKA, all with patellar resurfacing), concluding that overstuffing (arbitrarily defined as any anterior femoral offset increase or anterior patellar displacement of >15%) had no effect on range of motion or Knee Society Scores in comparative groups with different sample sizes (ranging from 19 to 41 in the “stuffed” group versus 723 to 769 in the “unstuffed” group). Sagittal femoral alignment was not considered. Matz et al.22 evaluated 970 patients who underwent posterior-stabilized TKA with resurfacing and divided them into 3 groups: increased, decreased, and unchanged anterior femoral offset. They found no difference in Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores among the groups, concluding that there were no consequences of patellofemoral overstuffing. Beldman et al.17 investigated overstuffing (any increase in anterior femoral offset or posterior condylar offset) in 193 patients treated with posterior-stabilized TKA with resurfacing and found anterior overstuffing in 43%, posterior overstuffing in 87%, and total overstuffing in 80%. They reported no effects of overstuffing on anterior knee pain or WOMAC scores at 1 year. In all 3 studies, the authors used arbitrary definitions of overstuffing, considered only absolute values, and identified associations with overstuffing rather than anterior knee pain. Defining any increase in offset as overstuffing may mask effects of truly significant overstuffing by dilution.
Despite the patellofemoral-friendly features of the TKA design used in our study, anterior knee pain was reported in 25% of our patients at 10 years. Although modern femoral component trochleae are designed to reproduce anatomical patellar tracking, cadaveric studies suggest that physiological kinematics are not restored8. Artificially maintained patellar offset throughout motion increases patellofemoral pressures and may cause anterior knee pain8. Limiting the anterior femoral offset ratio by femoral component flexion may reduce this effect. Tibial component rotation was found to affect peak retropatellar pressures in cadavers23. However, a recent study of 46 TKAs performed with computer navigation showed sagittal alignment to have a greater effect on patellofemoral kinematics (patellar tilt and medialization) than did rotational alignment7. Although we did not measure component rotation, an important study weakness, this study supports the importance of femoral sagittal alignment on patellofemoral biomechanics. We are unable to comment on the effect of patellar resurfacing as we did not include a comparison group with that procedure; however, a beneficial effect of resurfacing has not been proven20.
The cohort in this study consisted of the first single-radius TKAs performed at our institution, so it includes our learning curve. Initially, the 7° anterior femoral flange was often implanted more parallel to the anterior aspect of the femur than we would now advocate, resulting in component extension and an increased anterior femoral offset ratio. Femoral component flexion is now achieved by utilizing a posterior femoral entry point. The results of this study appear to support this strategy. The importance of sagittal component alignment in predicting long-term anterior knee pain, and thus PROMs, in patients with this TKA is a novel finding and is relevant in an age of precision implantation and robotic technology. Although these data identify sagittal component positioning as important in the long-term success of single-radius TKA, it cannot be ascertained whether this variable alone causes anterior knee pain. Further research is required to investigate additional variables such as joint-line restoration, coronal alignment, and component rotation, which were not assessed here.
Limitations of this study include no comparison with preoperative radiographs and no measurement of implant rotation or joint-line restoration. Hip-knee-ankle radiographs were not used for measurement of coronal alignment, making interpreting medial proximal tibial angle results difficult. Lateral radiographs were adequate to define anterior and posterior femoral cortex alignment and thus the distal femoral axis (Fig. 2), but full femoral bowing was not measured. Fat-pad resection was not documented, although its effect on anterior knee pain has not been proven in the longer term24. The patella was rarely resurfaced, so conclusions cannot be drawn regarding TKA with resurfacing. Postoperative skyline radiographs were unavailable, and patellar offset and tilt were not assessed. There was no formal recording of intraoperative patellar tracking. Anterior knee pain rates were measured at 10 years only. Previous studies have shown variation in anterior knee pain over time1. However, as implant survival is routinely reported at 10 years this was considered an acceptable time point. Nine percent of patients were lost to follow-up.
Despite a patellofemoral-friendly design, anterior knee pain was reported by 25% of patients alive with an intact prosthesis at 10 years after receiving a single-radius cruciate-retaining TKA without routine patellar resurfacing. When anterior knee pain was present it was associated with inferior PROMs, including an OKS that was worse starting at 1 year. Multivariate analysis showed femoral component flexion, tibial component coronal alignment (medial proximal tibial angle), and patella baja to independently predict long-term anterior knee pain in patients treated with this TKA design. ROC curve analysis demonstrated that femoral component extension predicted anterior knee pain with 87% sensitivity.
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