A PHOTOELASTIC MODEL OF THE PATELLOFEMORAL JOINT TO STUDY PATELLAR HEIGHT

Abstract

Background: Patellar height has been related to anterior knee pain and Osgood–Schlatter disease. To study the influence of patellar height on knee biomechanics, a bidimensional photoelastic prototype of the patellofemoral joint was developed. Methods: Nine tests were performed at different knee flexion angles and patellar heights. A free body diagram was constructed for each test. The following parameters were calculated: lengths (patella, patella thickness and ligamentum patellae); patellar height index; angles (knee flexion, patellar flexion, quadriceps and ligamentum patellae force and reaction force at the ankle); moment arms (extensor, quadriceps and ligamentum patellae in relation to both the tibiofemoral and patellofemoral contact points), effective moment arm and forces around the extensor mechanism relative to the applied force. Results: Femoropatellar and femorotibial reaction forces were greater with increased knee flexion and a fixed patellar height. With fixed knee flexion and different patellar heights, these forces were greater if the patella was elevated. A decrease in the angle between the tibial axis and ligamentum patellae was observed when patellar height increased. Patellar flexion angle increased when patellar height increased. This was accompanied by an increase in the angle of action of the quadriceps force. Extensor moment arm decreased with increased patellar height when knee flexion and the tibiofemoral contact point were fixed. Conclusion: A new application of photoelasticity is presented. The preliminary findings obtained confirm the influence of patellar height on patellar biomechanics, and specifically on forces around the extensor mechanism of the knee.