Summary

The patellar contact pressures after dynamic MPFL reconstructions were similar to those of the native knee, whereas a static reconstruction results in higher pressures and thus increases the risk of patellofemoral osteoarthritis in the long-term. Therefore, dynamic MPFL reconstruction could be a safer option than static reconstruction from a biomechanical point of view.

Abstract

Background

Medial patellofemoral ligament reconstruction (MPFLr) is the "gold standard" in treating chronic lateral patellar instability (CLPI). Different MPFLr techniques have been described: static in which both graft attachments are fixed rigidly to the bone and dynamic in which one of the attachments is fixed to soft tissue.

Hypothesis

Static MPFLr (anatomical technique) using gracilis tendon is the most commonly performed. However, a dynamic MPFLr functions more like the native MPFL. Therefore, this type of reconstruction will result in patellar contact pressures closer to those generated in a native knee.

Objective

To evaluate the effect of different MPFL fixation techniques on patellar pressures compared with the native knee.

Methods

A finite element model of the patellofemoral joint consisting of around 33,800 nodes and 25,550 elements was created based on the Computed Tomography data from 24 knees with CLPI. Patellar contact pressures and maximum MPFL-graft stress at five positions of flexion (0º, 30º, 60º, 90º and 120º) were analyzed in three types of MPFLr: (1) static, (2) dynamic using the adductor magnus tendon (AMT) as femoral fixation, and (3) dynamic using the quadriceps tendon as attachment (medial quadriceps tendon-femoral ligament –MQTFL- reconstruction).

Results

In the static/anatomical technique using gracilis, the patellar contact pressures at 0 and 30º are greater than in the native knee (1.61 MPa vs 0,18 at 0º and 0.11 MPa vs 0.016 MPa at 30º). As in a native knee, the contact pressures at 60, 90 and 120º are very low. The maximum MPFL-graft stress at 0º and 30º is higher than in a native knee (58.78 MPa vs 8.85 MPa at 0º and 5.12 MPa vs 0.78 MPa at 30º). As in a native knee, at 60, 90 and 120º the MPFL-graft is loose, meaning it has no tension. In the dynamic MPFLr with gracilis using the AMT as a pulley, the patellar contact pressures are similar to those of a native knee in all the range of motion. However, the maximum stress of the MPFL-graft is less at 0º than that of a native ligament (8.85 MPa vs 6.35 MPa). However, at 30° of flexion, the maximum MPFL-graft stress is significantly higher than in a native ligament (0.78 MPa vs 2.10 MPa). After 30º of flexion the MPFL-graft loosens as occurs in a native knee. In the dynamic MQTFL reconstruction using semitendinosus the maximum patellar contact pressure is slightly higher than in a normal knee. The maximum stress of the MPFL-graft is much higher at 0º and 30º than that of a native MPFL (8.85 MPa vs 66.7 MPa at 0º and 0.78 MPa vs 9.36 MPa at 30º). After 30º of flexion, the MQPFL-graft loosens as occurs in the native knee.

Conclusion

The patellar contact pressures after dynamic MPFL reconstructions were similar to those of the native knee, whereas a static reconstruction results in higher pressures and thus increases the risk of patellofemoral osteoarthritis in the long-term. Therefore, dynamic MPFLr could be a safer option than static reconstruction from a biomechanical point of view.