Strain within the native and reconstructed MPFL during knee flexion.

There is little published data on the strain within the medial patellofemoral ligament (MPFL) and medial retinaculum through knee motion. This study was undertaken to evaluate the three-dimensional strain across the MPFL in the native state, using a proprietary visible-light stereophotogrammetry (VLS) system, and to compare the findings to the strain in a MPFL injury model and in two different reconstructed states. This is a controlled laboratory study. Eight cadaveric knees were marked along the MPFL and medial retinaculum, placed in an activity simulator, and taken through a range a motion. A proprietary VLS system was used to calculate the strain across the medial retinaculum and MPFL at 10 different degrees of knee flexion. This process was repeated in an MPFL injury model, as well as after standardized reconstruction of the MPFL using hamstring autograft performed in both 20 and 45 degrees of flexion. Averaged over all the measurement sites, the maximum principal strain (ε1) within the native MPFL increased rapidly from full extension to 120 degrees of flexion. The highest value of ε1 (87%) was observed at 120 degrees of knee flexion in the MPFL region. The largest change in strain occurred between 25 and 30 degrees (10% increase). The strain patterns in the knees reconstructed at 45 degrees of flexion more closely resembled the strain in the native state than did the strain in the knees reconstructed at 20 degrees. Strain within the native MPFL increases as the knee flexion angle increases, with the largest change occurring between 25 and 30 degrees. Reconstruction of the MPFL at 45 degrees is preferable to reconstruction at 20 degrees as the strain across the medial retinaculum more closely resembles the strain in the native state. Knowledge of the strain across the MPFL should allow for more accurate reconstruction of the MPFL, potentially reducing the risk of patellar maltracking or cartilage overload. The proprietary VLS system used in this study has many potential uses for experimental analysis of strain in the human body. Thieme Medical Publishers 333 Seventh Avenue, New York, NY 10001, USA.