The results of this study showed that a 2 mm increase or decrease in patellar thickness resulted in an approximately 20% increase or decrease in the patellofemoral pressure. The advantage of this study is very accurate measurements of patellofemoral pressure with various patella component thicknesses. There are various methods for measuring patellofemoral joint pressure [8–16]. In the method of measuring pressure by placing a sensor between the patellofemoral joint, the pressure sensor may shift, and the patellofemoral joint pressure may be increased due to the thickness of the pressure sensor [8, 9, 11, 12]. On the other hand, with a pressure sensor built into the patella, patellofemoral joint pressure can be measured accurately [13–15]. However, because the osteotomy and joint capsule sutures are redone when changing the patellar thickness, the conditions are not constant. In our method, the sensor is built into the femoral component, and the patellofemoral joint pressure can be measured without inserting a thick sensor in the patellofemoral joint. Furthermore, since the thickness of the patellar component is adjustable, patellofemoral joint pressures at various patellar thicknesses can be made under identical conditions without re-suturing of the joint capsule. Thus, this study has the advantage of obtaining very accurate measurements.
Although previous reports have reported that a 2 mm increase in patellar thickness resulted in a 2.7-fold increase in patellofemoral pressure [17], the increase in patellofemoral pressure in the present experiment was 14.8%. We believe that this difference can be attributed to differences in measurement methods. The amount of patellar osteotomy is associated with postoperative knee joint function, and it has been reported that for every 1 mm increase in patellar thickness, joint range of motion is reduced by 3 degrees [18]. It has also been reported that patellofemoral pressure is associated with patient satisfaction and clinical scores, for example, an increase in patellofemoral pressure is associated with a decrease in clinical scores such as the Knee Surgery Score, Forgotten Joint Score, and Patella Score [14].
In general, the osteotomy volume of the patella should be the same as the thickness of the patellar component. The results of the present study showed that resection of an additional 2 mm of patella reduced patellofemoral joint pressure by 20%. However, increasing the osteotomy volume to decrease patellofemoral joint pressure is not recommended and should be done cautiously.
Thinning the patella by more than 2 mm compared to the original patella thickness has been reported to increase the risk of postoperative patellar clunk/crepitus. [19]. Also, if the native patella thickness is thin, there is a risk of patellar fracture when the patella thickness after osteotomy is less than 12 mm [20]. In such cases, the choice of non-patellar replacement or the use of an in-lay type or a thinner patella component should be considered. Patellofemoral load pressure does not only increase with patellar thickness, but also with an anterior position of the femoral implant. Thus, it is necessary to resect the femur near flush to the anterior cortex to avoid patellofemoral overstuffing and resulting unsatisfactory flexion.
Our study has several limitations. First, the sample size was small so a type II error might not be fully excluded in our study. However, the number of cadavers used in this study, five, seems to be a reasonable number compared to similar studies in the past. [5, 8, 9, 12]. Second, we were not able to simulate physiological load of the quadriceps or hamstring muscles in this study. It is inferred that the actual patellofemoral pressure is much higher than in our experiment due to the action of the muscles and the weight bearing. Highly accurate and ethically acceptable measurement of the knee under weight-bearing conditions in vivo would be ideal. Finally, we have only experimented with one type of knee joint prosthesis in the present study. The knee joint prosthesis used in this study has a single-radius shape for the femoral implant and a disc shape for the patella. Patellofemoral joint pressure may be different with an implant with a multi-radius shape of the femoral implant or an anatomical shape of the patella.