Patella Apex Influences Patellar Ligament Forces and Ratio.

The relationship between three-dimensional shape and patellofemoral mechanics is complicated. The Wiberg patella classification is a method of distinguishing shape differences in the axial plane of the patella that can be used to connect shape differences to observed mechanics. This study uses a statistical shape model to relate the Wiberg patella classification to patella height and investigates its role in force distribution within the patellofemoral joint. The Wiberg Type I patella is shortest and has a more symmetrical medial and lateral facet while the Type III patella is longest with a larger lateral facet compared to medial. We generated patellofemoral morphologies from the statistical shape model and integrated them into a musculoskeletal model with a twelve degrees-of-freedom knee. We simulated an overground walking trial with these morphologies and recorded patellofemoral mechanics and ligament forces. An increase in patellar ligament force corresponded with an increase in patella height. Wiberg Type III patellas had a sharper patella apex which related to lower ratios of quadriceps tendon forces to patellar ligament forces. The change in pivot point of the patella affects the ratio of forces as well as the patellofemoral reaction force. This study provides a better understating of how patella morphology affects fundamental patella mechanics which may help identify at-risk populations for pathology development.