Quadriceps Muscle and Medial Retinaculum Combinate Effects on Patellar Instability during Knee Flexion

Background: Patellar instability can cause pain and disability. A finite element model of a healthy human knee was performed to analyze the role of quadriceps and medial retinaculum imbalance in patellar instability. Methods: The model was created by matching magnetic resonance and computed tomography images of a normal adult patient’s knee. Muscle force intensities were calculated by static optimization, considering the lower limb muscles, knee movement and the ground reaction during walking. Patellar instability was experimentally generated by progressively uncoupling muscular forces, (90 N versus 110 N), while at the same time, the load derived from the quadriceps was gradually reduced by 20%. Results: This loss in force symmetry of 10 N on the retinaculum may produce a displacement of approximately 7 mm, with an increase in patellar contact forces of approximately 44%. When the quadriceps force is reduced by 10% and the unbalanced medial retinaculum acts together, the displacements are in the order of 14 mm, and the patellar contact forces increase by 84%. Conclusion: A reduced quadriceps force alone is not able to cause significant patellar instability, while an imbalance of forces at the level of the retinaculum could lead to patellar instability, especially when the two effects are combined. A better understanding of joint relationships and muscle synergies can help to improve clinical approaches to patella instability.