This cadaveric biomechanical study tested whether a frictionless contact model can accurately predict forces across the patellar mechanism from 0–120° of flexion. Geometric parameters were measured in 42 specimens; ligamentum patellae tension was measured in 10 specimens during simulated leg raising and static lifting. The central question: does the PFJR act as a pulley bisector, or normal to the contact surface — and does the answer matter for surgery?
Before this paper, tibial tubercle elevation (Maquet procedure) was analyzed assuming the patella acts as a frictionless pulley — meaning surgeons predicted PFJR reduction based solely on how much they moved the tubercle anteriorly (Δβ). That model predicts the patellar ligament and quadriceps tendon always carry equal tension, which this study shows is wrong at virtually every flexion angle.
When planning or evaluating a Maquet-type tubercle elevation, the reduction in PFJR depends on both the change in patellar mechanism angle (Δβ) AND the resulting shift in contact surface orientation (Δα). A 10° increase in Δβ cuts PFJR by 16% if contact geometry stays fixed. But only by 10% if contact geometry shifts adversely by 10°.
In practice: you cannot predict the mechanical benefit of tubercle elevation from the elevation distance alone. The contact geometry change (which depends on where the patella contacts the femur at a given flexion angle) modulates the outcome. This is why variable clinical results after Maquet procedures may reflect differences in Δα across patients, not just differences in how far the tubercle was moved.
The frictionless contact assumption holds for intact cartilage. But this model explicitly breaks down in advanced patellofemoral arthritis, where friction is non-negligible and a different analytical framework is required.
This cadaveric biomechanical study tested whether a frictionless contact model can accurately predict forces across the patellar mechanism from 0–120° of flexion. Geometric parameters were measured in 42 specimens; ligamentum patellae tension was measured in 10 specimens during simulated leg raising and static lifting. The central question: does the PFJR act as a pulley bisector, or normal to the contact surface — and does the answer matter for surgery?
Before this paper, tibial tubercle elevation (Maquet procedure) was analyzed assuming the patella acts as a frictionless pulley — meaning surgeons predicted PFJR reduction based solely on how much they moved the tubercle anteriorly (Δβ). That model predicts the patellar ligament and quadriceps tendon always carry equal tension, which this study shows is wrong at virtually every flexion angle.
When planning or evaluating a Maquet-type tubercle elevation, the reduction in PFJR depends on both the change in patellar mechanism angle (Δβ) AND the resulting shift in contact surface orientation (Δα). A 10° increase in Δβ cuts PFJR by 16% if contact geometry stays fixed. But only by 10% if contact geometry shifts adversely by 10°.
In practice: you cannot predict the mechanical benefit of tubercle elevation from the elevation distance alone. The contact geometry change (which depends on where the patella contacts the femur at a given flexion angle) modulates the outcome. This is why variable clinical results after Maquet procedures may reflect differences in Δα across patients, not just differences in how far the tubercle was moved.
The frictionless contact assumption holds for intact cartilage. But this model explicitly breaks down in advanced patellofemoral arthritis, where friction is non-negligible and a different analytical framework is required.