The most important finding of the present study was that the cartilage thickness in the patella was reduced to less than half its preoperative level within 5 years after non-patellar resurfacing TKA despite the use of a zirconia ceramic implant with high biocompatibility. Furthermore, the functional knee score significantly worsened with a reduction in cartilage thickness in the patella. These results suggest that non-patellar resurfacing TKA, despite the use of a zirconia ceramic implant, might cause PF joint problems.
Although there have been many reports that have compared the clinical outcomes and complication rates between patellar resurfacing and non-patellar resurfacing in TKA [6, 9, 19, 24, 25], to our knowledge, there are no reports on the longitudinal observation of changes in patellar cartilage thickness after non-patellar resurfacing TKA. Assessment of patellar cartilage thickness using MRI has high technical validity and is appropriate . Previously, Kumahashi et al. reported that dGEMRIC allowed the quantitative evaluation of the patellar cartilage after TKA . In that study, the postoperative dGEMRIC values of the outer medial half of the superficial zone of the patellar cartilage in the non-resurfacing group were significantly decreased compared to the preoperative values, and the postoperative cartilage thickness of the outer zone of the patella was significantly thinner than the preoperative thickness. However, the study by Kumahashi et al. showed changes in the thickness of the patellar cartilage only 1 year postoperatively and did not evaluate longitudinal changes. In contrast, our data included longitudinal measurements of the thickness of the patellar cartilage over a 5-year period, and clearly suggest that non-patellar resurfacing TKA may result in a decrease in the thickness of the patellar cartilage despite using a zirconia ceramic implant. The strength of our study was the longitudinal observation of changes in patellar cartilage thickness over a 5-year period after non-patellar resurfacing TKA.
In our study, we used a zirconia ceramic implant (LFA®) with high biocompatibility. Nevertheless, cartilage thickness in the patella was reduced to less than half of the preoperative value within 5 years after non-patellar resurfacing TKA. Decreasing patellar cartilage thickness mainly depends on the increasing pressure on the PF joint. The cause of increasing pressure on the PF joint is dependent on body mass index , implant design, implant position, radiological factors including patella height , and patella tracking course. Regarding implant design, LFA®, which was designed to accommodate the non-resurfaced patella, was used in our study. The patella groove design has an anatomical femoral groove angle of approximately 130° . Therefore, it does not seem to be associated with an increase in pressure on the PF joint and implant design. Next, the implant position was appropriate in all cases, and radiological factors, such as patellar height, did not change before and after TKA. Finally, in the patella tracking course, the patella is inclined to shift laterally due to the change in knee alignment from varus to normal, so that the pressure of the PF joint might be changed. This was consistent with the fact that the cartilage thickness of the lateral facet decreased compared to that of the central and medial facets postoperatively. A previous study reported that the cartilage thickness of the lateral zone was significantly thinner postoperatively, and the patella was inclined laterally (lateral shift ratio was 10.8) . The PF joint pressure may be localised and concentrated on the lateral facet of the patella. This finding suggested that the pressure on the PF joint, especially the lateral zone, may have been increased in our study.
A previous systematic review showed that most meta-analyses unanimously reported equivalent results after patellar resurfacing compared to non-resurfacing in terms of functional scores and complication rates; however, an increased risk of reoperation after patellar non-resurfacing was reported . Persistent AKP remains an important clinical issue after non-resurfacing patella TKA . Its exact aetiology remains subtle, and the effects of prosthesis design, surgical technique, the degree of patellar chondromalacia, preoperative AKP, and patellar tracking alteration on the prevalence of postoperative AKP remain undefined . A previous study reported an average AKP incidence of 10% in non-resurfaced cases compared to 3.3% in resurfaced cases . After 5 years of follow-up, 5% of patients in that study had persistent AKP in the resurfaced side compared to 23% of patients who complained of such pain in the non-resurfaced side. The overall incidence of AKP was higher in patients who underwent TKA and had a non-resurfaced patella than in those who had a resurfaced patella. In our study, the incidence of AKP 5 years postoperatively was 18.7%, which was similar to previously reported data. However, it can be clinically difficult to determine the cause of AKP in a patient without patellar resurfacing, as many other pathologies can have similar presentations (subclinical infection, patellar maltracking, mid-flexion instability, and component malrotation). We should also interpret these results with caution because the use of different scoring systems has resulted in variations in objective AKP assessment and has contributed to the observed heterogeneity. Moreover, the rate of reoperation for patellar non-resurfacing was the main concern for the investigation. In most previous studies, a lower risk for reoperation was reported after patellar resurfacing compared to that after non-resurfacing [13, 19, 25]. In contrast, Parvizi et al.  reported no significant difference in the re-intervention rate between the resurfaced and non-resurfaced patella. The cumulative percentage revision rate for patellar resurfacing after non-resurfacing patella TKA was reported to be 10–15% after primary TKA within a 5–10-year follow-up period [2, 19]. The revision rate in our study (6.7%) was lower than that in previous studies. Furthermore, patients’ activity levels must be considered. The mean age of our patients (72.2 years) was higher than that of patients in other studies (the mean age ranged from 60 to 70 years) [2, 19]. Therefore, the activity level of patients in our study may have been lower; hence, close attention had to be paid to our patients to avoid overloading the implants.
We must consider the effect of decreasing cartilage thickness in the patella on clinical outcomes. This study demonstrated a correlation between decreasing cartilage thickness in the patella and functional knee scores. Decreasing cartilage thickness in the patella can cause PF joint problems. Decreasing cartilage thickness is caused by vigorous physical activities, such as jogging, squatting, and climbing stairs, that subjects the knee to repeated stress. It can also be caused by abnormal tracking of the patella in the trochlear groove. Hence, we must consider that the scoring system is not specific for PF disorders. The total JOA score for osteoarthritic knees was a total of 100 points: pain and walking, 30 points; pain and stairs up and down, 25 points; range of motion, 35 points; and swelling, 10 points . The PF joint problem accounts for 25% of the total points. There is a possibility that another factor is related to worsening functional knee score; hence, it may be difficult to determine whether decreasing cartilage thickness in the patella leads to degraded clinical outcomes. To our knowledge, the Kujala score is a specific scoring system for PF disorders to evaluate subjective symptoms and functional limitations in PF disorders . However, it includes high-load activities, such as squatting, running, and jumping. Therefore, it is not suitable to evaluate the PF joint outcome after TKA in elderly patients. Although it was necessary to assess the correlation between decreasing cartilage thickness in the patella and the functional knee score precisely, it might be inadequate to use only the current scoring system.
This study has several limitations. First, the number of patients was small due to the limited criteria for indication in this study. Therefore, further studies with larger sample sizes are required to verify the findings of this study. Second, this study lacked a control, which could have been a non-resurfaced patella CoCr implant in TKA. We did not evaluate the difference between the effect of ceramic implants and that of metal implants on cartilage thickness in non-resurfaced patella in TKA. However, in case of metal implants, it is difficult to assess the cartilage thickness accurately using MRI due to metal artefacts and make comparisons between the ceramic and metal implants. Third, we evaluated only one slice of the patella using MRI. We selected an axial MR image slice, which was the centre of the patella, to evaluate patellar thickness. Therefore, errors in image slice selection may occur and influence the measurement of patellar cartilage thickness. Fourth, we evaluated the implant positioning using conventional X-rays and 2D-CT. The most accurate method to evaluate implant positioning is 3D-CT ; however, we did not perform a full limb CT in this series. We could not rule out the effect of rotational or sagittal malalignment of the femoral component precisely on clinical outcomes. Finally, we did not compare and evaluate the natural course of degeneration of the patellar cartilage in intact knees of elderly patients with annual MRI.