Length Change Behavior of Virtual Medial Patellofemoral Ligament Fibers During In Vivo Knee Flexion.

Background: In vivo length change behavior of native medial patellofemoral ligament (MPFL) fibers throughout the range of knee motion has not been reported in vivo. Purpose: To measure the length changes of various fibers of the MPFL and to determine their length change patterns during in vivo passive knee flexion. Study Design: Descriptive laboratory study. Methods: The right knees of 11 living subjects were scanned with a high-resolution computed tomography scanner at 0, 30, 60, 90, and 120 of knee flexion, and 3-dimensional (3D) models were constructed using customized software. Five patellar points were determined: 20% (point 20), 30% (point 30), 40% (point 40), 50% (point 50), and 60% (point 60) from the superior pole of the patella. The Schöttle femoral point (point F) was marked on a translucent 3D model of a true lateral view. Five virtual fibers connecting these points on the 3D knee model were created, and the lengths of various fibers were digitally measured. Results: The average length changes were 9.1 6 2.5 mm in F20, 9.1 6 2.5 mm in F30, 8.1 6 2.6 mm in F40, 6.9 6 2.4 mm in F50, and 6.9 6 1.7 mm in F60. There were significant differences in length changes of these 5 fibers (P\.001). The lengths of 2 superior fibers (F20 and F30) increased as the knee flexed from 0 to 30 and decreased as the knee flexed over 30. The lengths of a middle fiber (F40) and an inferior fiber (F50) increased from 0 to 30, reached a plateau from 30 to 60, and then decreased from 60 to 120. F60 showed an increase from 0 to 30, and then a plateau pattern from 30 to 90, followed by a decrease during further flexion. Conclusion: Superior fibers exhibited their maximum lengths at low flexion angles, and inferior fibers exhibited their maximum lengths at midflexion angles. The MPFL is a complex of functionally various fibers with some taut and others slack over the whole range of knee motion. Clinical Relevance: The results for lengths and length change patterns of various MPFL fibers are expected to serve as a theoretical background for anatomic double-bundle MPFL reconstruction. [ABSTRACT FROM AUTHOR]