Background: Graft reconstructions of the lateral collateral ligament, popliteus tendon, and popliteofibular ligament are frequently performed in conjunction with a reconstruction of the posterior cruciate ligament to restore knee stability. The purpose of this study was to determine the femoral fixation sites resulting in the optimum isometry of popliteus tendon, popliteofibular ligament, and lateral collateral ligament grafts in a knee with a reconstruction of the posterior cruciate ligament.

Methods: Relative length changes (isometry measurements) were recorded between sutures fixed at femoral grid points and appropriate fibular or tibial graft tunnel sites; sites resulting in the least change in suture length as the knee was moved from 0° to 90° of flexion were identified as optimum isometric points. Bone blocks of Achilles tendon grafts were fixed with the midpoint of the tissue's leading edge adjacent to the optimum isometric point (optimum placement). Isometry measurements were repeated with a lateral collateral ligament graft placed in a fibular tunnel and with popliteus tendon and popliteofibular ligament grafts alternately placed in appropriate tibial and fibular tunnels. The graft isometry measurements were then repeated with the bone block centered over the femoral footprint of the lateral collateral ligament or popliteus tendon.

Results: For all reconstructions, there was no difference between the relative length changes of the suture placed at the optimum isometric point and the relative length changes of the graft with an optimally placed bone block. The mean location of the optimally placed bone-block center of the lateral collateral ligament graft was within 1.85 mm of the mean center of the footprint of the lateral collateral ligament; the mean graft isometry measurements with the optimally placed bone block were not significantly different from those with the bone block centered over the lateral collateral ligament footprint. The mean optimally placed bone-block center of the popliteus tendon and popliteofibular ligament reconstructions was 11 mm anterior and 2.7 mm proximal to the center of the popliteus tendon footprint. The mean relative length changes of the popliteus tendon and popliteofibular ligament grafts with the bone block optimally placed were <0.9 mm and <1.2 mm, respectively; the means with the popliteus tendon and popliteofibular ligament bone blocks centered over the popliteus tendon footprint were 3.7 mm and 5.0 mm, respectively.

Conclusions: A popliteus tendon or popliteofibular ligament reconstruction with the bone block centered over the femoral footprint of the popliteus tendon was highly non-isometric. If the graft were fixed at 30° of flexion, it would elongate approximately 4 mm when the knee was extended to 0° and possibly stretch out.

Clinical Relevance: We found suture isometry to be a good indication of graft isometry. In situ measurements of relative suture-length changes at the time of surgery may be helpful in determining a femoral fixation site that will result in graft isometry.