Summary
The femoral footprint of the MPFL is ribbon shaped, suggesting that the function of its most proximal fibers may vary from its most distal fibers based on their locations on the femur.
Abstract
The medial patellofemoral ligament (MPFL) has a narrow and elongated femoral footprint on the medial femur. Many surgeons utilize a bone tunnel and inference screw for femoral fixation of the MPFL graft during reconstruction. Because a screw within the tunnel can eccentrically place the graft, the aim of this study is to 1) describe the ribbon shaped footprint of the MPFL on the femur with regard to anatomic landmarks, and 2) describe the difference between native MPFL anatomy and optimal femoral tunnel placement to recreate the femoral footprint during MPFL reconstruction.
20 paired fresh frozen cadaveric knees were dissected. The MPFL was exposed and followed to its footprint on the medial femur. Images of the medial femur were analyzed using Image J software. The MPFL footprint was described in terms of size, position of its most proximal and distal points, and its angle relative to the axis of the femoral shaft. The midpoint of the footprint (M1) was described in relation to the adductor tubercle and surrounding bony landmarks. A reference line between the adductor tubercle (AT) and the medial epicondyle (ME) (AT-ME line) was created, and M1 was categorized based on its position relative to this line, in Zone 1 (anterior half, and proximal to AT line), Zone 2 (posterior half, and proximal to AT line), Zone 3 (anterior half, distal to AT-ME line), or Zone 4 (posterior half, distal to AT-ME line). A digital overlay of a simulated 8mm bone tunnel with screw was placed to match the native femoral footprint, and the midpoint of this tunnel (M2) was described using the same measurements. The positions of M1 and M2 were compared using paired t tests.
17 knees from 10 cadavers (7M, 3F, mean age 73.1) were included in this study. All knees had visible MPFL fibers, and the femoral origin was clearly identified with a length of 11.7+/-1.8mm (Range 9.6,15.7) and a width of <2mm. The long axis of the footprint was at an angle 14.6+/-16.6 anterior to the axis of the femoral shaft, with the distal margin of the footprint being 10.9mm distal and 2.6mm more posterior than the proximal margin. The midpoint of the native MPFL (M1) was found to be 6.2+/-3.9mm anterior and 7.6+/-4.5mm distal to the AT. After simulation of a 8mm tunnel, the midpoint of this tunnel (M2) was 2.5mm posterior and 0.8mm proximal to M1 [p<0.001, p=0.011]. When using surgical landmarks, 29.4%, 0%, 41.1% and 29.4% of M1 were found in Zones 1, 2, 3 and 4, compared to 0%, 5.9%, 23.5% and 70.6% of M2, respectively.
The femoral footprint of the MPFL is ribbon shaped, suggesting that the function of its most proximal fibers may vary from its most distal fibers based on their locations on the femur. An eccentrically placed graft and screw within a tunnel can recreate this footprint, and this study found the center of this corresponding tunnel to be 2.5mm posterior to the native midpoint. When classified based on surgical landmarks of the AT and ME, the center of the corresponding bone tunnel was primarily found in Zone 4 (in the posterior half, and distal to, the AT-ME line). Further biomechanical studies and radiographic correlations are needed to clarify this relationship between tunnel position and native MPFL anatomy, as well as to evaluate the reproducibility of using these references to guide MPFL reconstruction.