Causes for patella maltracking and its result on anterior knee pain were extensively researched previously but there are limited studies that provide evidence on patient satisfaction and its relationship with patella tilt. Various reasons were given to explain the lateral patella tilt. These include internal rotation of femoral component, external rotation of tibia component, valgus malalignment and soft tissue imbalance, etc. Combined femorotibial internal rotation found to be correlated with lateral patella tilt as 1–4 degree correlated with greater patella tilt, 3–8 degree can be correlated with patella displacement (greater than 0) and 7–18 degree can correlate with patella dislocation or late prosthesis failure [30]. Our aim was to find out the effect of patella tilt on patient satisfaction by using 3 patient-reported outcome scores at 2 years. The most important finding from the present study is that there was significantly poor KSS functional score at year 1 and KSS objective score at 2 years associated with patella tilt more than 10 degrees. There was no significant difference in the other two functional outcomes including SF 36 (Both physical and functional component), WOMAC scores associated with patella tilt more than 10 degrees after surgery till 2 years follow up.
These findings were contradictory to a previous study done by Narkbunnam et al. In a retrospective review of 138 primary TKA's with patellar resurfacing, they found the odds ratio of a poor outcome score with suboptimal patellofemoral mechanics as 3.4 (95% CI 1.6–7.2) for KSS, 6.4 (95% CI 2.9–14.2) for KOOS, and 5.9 (95% CI 2.6–13.5) for WOMAC [31]. Suboptimal patellar tilt was defined as a patellar tilt more than 5 degrees and lateral patellar displacement more than 5 mm [32, 33]. They supported the results from CT based study done by Bells et al in which they identified internal rotation malalignment of tibial (p = 0.0003) and femoral (p = 0.014) component individually as well as combined component rotation (p = 0.0003) and component rotation mismatch (p = 0.0001) to be a factor in pain following TKA [34]. The above results supported the finding of a CT based on another study by Matsuda et al [35]. However, in a retrospective study done by Young et al for the evaluation of unexplained knee pain following TKA, there was no difference in the incidence of tibial or femoral component malalignment in painful versus well-functioning TKAs [36]. Later this finding was supported by Becker et al, who reported internal and external malrotation of the femoral component does not correlate automatically with poor knee function [37]. However, in the same study patient with more internally rotated femoral component score worse in the physical function category of WOMAC at postoperative 6 and 24 months. Recently, Corona et al in a systemic review showed that malrotation of the femoral component does not correlate with poor functional outcome automatically while another systemic review by Shiavone Panni et al showed that internal rotation of tibia more than 10 degrees may be a significant factor for pain and inferior functional outcome [38, 39].
Patella tilt and patella displacement were also studied in relation to patella thickness and patella facets angle and its effect on postoperative functional outcome and postoperative osteonecrosis. Pre-operative patella facet angle < = 126 degrees were found to be correlated to increased post-operative patella tilt as compared to > 126 degrees [18]. The former group was found to have more frequent development of progressive osteosclerosis of the patellar ridge at 5 years follow-up associated with pain and functional impairment. The above study was done in cruciate retaining (CR) TKA. Kim et al did a similar study in PS TKA and found patella shape evaluated by patellar facet angle can partially affect the pre-operative patellofemoral alignment. The study result indicated insignificant clinical relevance of the patella shape in PS TKA. Also, radiologic and clinical outcomes evaluated after PS TKA showed no difference according to the patella shape [40].
We have used the skyline view for the assessment of patellofemoral congruence via the merchant technique. This technique is well adopted in hospital protocol in both pre and postoperative evaluation. In a retrospective study of 90 patients following primary TKS by White et al, have used end on axial view for the calculation of patella tilt, lateral patella displacement, and patella overstuffing. They evaluated the relationship between radiographic risk factors for anterior knee with an anatomic patella button and found that an increased combined patella tilt was risk factors for developing AKP and painless noise at two years follow up. However, they failed to find patellar tilt, displacement, or overstuffing as risk factors for adverse clinical outcomes [41]. Ranawat et al failed to find a correlation between patella tilt and post-operative functional outcome after patella resurfacing supporting results from the previous studies with different designs of the patella button [13, 17, 42]. However, they found higher patella resection angle only statistically significant independent risk factors both anterior knee pain and painless noise.
Clearly, there is controversial evidence regarding the effect of patella tracking on functional outcomes. To our knowledge, this is the largest study that compared radiological parameters of patella tracking with functional outcomes using 3 scores. The strength of the study includes a large sample size with heterogeneous distribution according to age. Secondly, data were drawn from prospectively maintained institutional registry databases, reducing the risk of observer bias. Thirdly, the surgeon, data observer of the registry, and statistician were blind.
There were several limitations and biases in the study. Firstly, the study framework was a retrospective analysis of consecutively operated patients from a single institute. We based our result strictly on data from the registry at only one and 2 years follow up and we did not correlate it with a clinical sign including pain and range of motion. Our purpose of the study was only identification of the relationship of functional outcome with post-operative patella tilt. Secondly, a CT scan is a better modality to identify component malposition and to analyze reasons for patella maltracking. However, we have used best end on skyline view to identify patella tilt and to identify its correlation with functional outcome, avoiding more radiation exposure to the patients. Third, since only one type of implant was used in this study, selection bias should be considered when interpreting the results. Lastly, we have not identified the relationship of other factors including age, gender, and BMI with post-operative functional outcome and patella tilt. Our study clearly suggests an improvement in functional outcomes in the post-operative period along with post-operative patella tilt.