Summary

Currently there is no method of characterising the patellofemoral loading occurring dynamically in the native knee or during knee replacement. We describe a novel apparatus to dynamically measure patella loading in the native and replaced knee, offering the possibility of reducing AKP by more accurate balancing and replication of the patellofemoral forces.

Abstract

Introduction. Twenty percent of patients report dissatisfaction following TKR, 45% of this group characterise anterior knee pain (AKP) as a source of their discomfort. Therefore, there is interest in studying the ‘Third Space’ or patellofemoral joint and the pressures and function of the surrounding extensor hood. Currently there is no method of characterising the patellofemoral loading occurring dynamically in the native knee or during knee replacement. We describe a novel apparatus to dynamically measure patella loading in the native and replaced knee, and the effect of varying the depth and angle of patella resection.
Method. A sensory apparatus was attached to the patella undersurface recording pressures through a range of full flexion in the patellofemoral joint in four native cadaveric knees (unpreserved, pelvis to toe preparations). Sensors were positioned at superior, inferior, medial and lateral positions on the patella surface.Sixteen range of motion studies from full extension to full extension were completed. A TKR was then performed under optimal conditions with robot assistance (MAKO, Stryker inc.) to control accuracy and reproducibility between the four cadavers. In this way surgeon variability was reduced. The patellofemoral sensor was reintroduced and the measurements repeated. The effect of different depths and angles of patella resection were noted. Reliability and reproducibility was shown in an in vitro test rig and verified in the four cadaveric studies. Sensor data was compared for all 4 quadrants using ANOVA with alpha error 0.05.
Results. A clear, reproducible pattern of patellofemoral loading occurs in the native cadaveric knee. Following TKR this was significantly changed in both pattern and magnitude (p<0.01). Changing the depth and angle of patella resection altered patellofemoral loading (p< 0.05). In some cases, by the surgeon selecting appropriate depths and angles of patellofemoral resection to address aspects of the abnormal patterns observed after TKR, it was possible to achieve the same patterns and magnitude of patellofemoral forces observed in the native knee therefore replicating natural patellofemoral loading.
Conclusions. A characteristic pattern of patellofemoral loading is shown in the native knee which is significantly altered following TKR, suggesting abnormal loading of the patella and extensor hood apparatus may be responsible in AKP seen after TKR. It has been possible to characterize for instance, the lateral loading that occurs in lateral maltracking and subsequently address and reduce the overload by altering the depth and angle of subsequent patella resections. Altering patella resection depth and angles subsequently allows loading in TKR to approach that of the native knee, offering the possibility of reducing AKP by more accurate balancing and replication of the patellofemoral forces.