Graft isometricity in unitunnel anterior cruciate ligament reconstruction: analysis of influential factors using a radiographic model

A radiographic model was developed to investigate the influence of three surgical variables on the change in attachment point distance (CAPD) of a hypothetical graft using the unitunnel technique of anterior cruciate ligament (ACL) reconstruction. Using three different femoral target points, we tested the hypothesis that varying the angle of knee flexion between 70° and 110° and varying the tibial starting point over a 4‐cm range do not result in a significant variation in CAPD. We also tested the hypothesis that the CAPD from 0° to 135° is greater than the CAPD from 0° to 90°. There was a statistically significant correlation ( r =0.8465, P <0.0001) between radiographically estimated and isometer‐measured values of CAPD. The tibial starting point and the femoral target point were found to affect the CAPD significantly ( P <0.005). A more proximal tibial starting point was associated with a lower CAPD. Both the center of the anatomic femoral attachment of the ACL, and a point 1 mm medial to the junction of the roof and lateral wall of the femoral intercondylar notch and 6 mm anterior to its posterior margin, were associated with lower CAPD values than a target point 5 mm superior and posterior to the center of the femoral ACL attachment. The angle of knee flexion did not significantly affect the CAPD. The CAPD [0°–135°] was significantly greater than the CAPD [0°–90°] for all combinations of variables ( P <0.0005). An acceptable CAPD can be achieved with the unitunnel technique but it is a technically demanding procedure with a narrow margin for error. A modification of the technique to allow independent selection of tibial and femoral attachments may be preferable. This may be achieved by drilling the tibial tunnel first and then creating the femoral tunnel secondarily using a drill with a reduced‐diameter shaft.