24/09/2023
Patellofemoral contact areas & contact stresses 🦵🦵
🦵 Patellofemoral biomechanics intimately participate in the function of knee extension and flexion. While the proximal medial patellar restrains (medial quadriceps tendon to femur ligament (MQTFL) and medial patellofemoral ligament (MPFL) provide the main restraint to lateral translation in early flexion, starting at 15–20° of knee flexion, the trochlea increasingly contributes with flexion.
🦵 At greater than 30° of knee flexion, the stability of the patella depends largely on the trochlea [https://pubmed.ncbi.nlm.nih.gov/26733595/, https://pubmed.ncbi.nlm.nih.gov/22928430/, https://pubmed.ncbi.nlm.nih.gov/16254736/].
🦵 As flexion increases, the contact area on the patella moves both proximally and laterally. The largest contact area is at 45°, where it forms an ellipse across the central portion of the medial and lateral facets [https://link.springer.com/chapter/10.1007/978-3-662-61097-8_41].
🦵 Progressing to 90°, the contact area shifts to the proximal aspects of the medial and lateral patellar facets. At 130–135° of knee flexion, the patellar facets contact the articular surfaces of the femoral condyles (FCs).[https://link.springer.com/chapter/10.1007/978-3-662-61097-8_41].
🦵 The odd facet (most medial part of the patella) only makes contact with the femur in extreme flexion (i.e., squatting). [https://link.springer.com/chapter/10.1007/978-3-662-61097-8_41].
🦵 Stress to the patellofemoral joint is the force per area of contact. In a closed kinetic chain activity, the joint reaction forces increase as the knee moves from extension to 90 degrees [https://pubmed.ncbi.nlm.nih.gov/6725318/, https://pubmed.ncbi.nlm.nih.gov/2213343/].
🦵 This increase in patellofemoral force would markedly increase the contact pressure if not for the accompanying increase in contact area with flexion to 90 degrees. The increase in contact area protects the patellofemoral joint by limiting the increase in contact pressure with increasing patellofemoral force. The resultant patellofemoral force does, however, increase disproportionately to the contact area, causing the contact pressure to increase modestly with flexion[https://pubmed.ncbi.nlm.nih.gov/6725318/,https://pubmed.ncbi.nlm.nih.gov/2394060/].The greatest compressive forces occur in 60- to 90-degree positions. [https://pubmed.ncbi.nlm.nih.gov/9809277/, https://pubmed.ncbi.nlm.nih.gov/8346760/, https://pubmed.ncbi.nlm.nih.gov/17224441/]
📷 Picture: https://pubs.rsna.org/doi/abs/10.1148/rg.220177?journalCode=radiographics
Patellofemoral contact areas. Normal contact areas between the patella and femoral trochlea vary at different angles of knee flexion. This results in variable contact force at the patellofemoral articulation in different degrees of flexion. In general, the contact pressure increases with increasing flexion angle degree.
