Contact between an anterior cruciate ligament graft and the intercondylar roof has been termed roof impingement. Grafts with impingement sustain permanent damage, and if the injury is extensive enough, then the graft may fail, causing recurrent instability. This study evaluated two mechanical factors that could be responsible for the graft injury associated with roof impingement: an increase in graft tension or elevated pressures between the graft and the roof, or both. An anterior cruciate ligament reconstruction was performed using an Achilles tendon graft in five fresh-frozen cadaveric knees. Using a six-degree-of-freedom load application system, the anterior displacement of the knee with the native anterior cruciate ligament was restored in the reconstructed knee at a flexion angle of 30 degrees and with an anterior force of 200 N applied. Pressure between the graft and intercondylar roof, graft tension, and flexion angle were measured during passive knee extension for three tibial tunnel placements (anterior, center, and posterior). Intercondylar roof impingement increased the contact pressure between the graft and the roof but had no significant effect on graft tension. Therefore, during passive knee extension, the contact pressure between the anterior cruciate ligament graft and the intercondylar roof is a more likely cause of graft damage than increased graft tension.

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Because of the complications of impingement of anterior cruciate ligament grafts on the intercondylar roof and because current surgical procedures locate the tibial tunnel such that impingement is avoided during passive but not active extension, the objectives of this study were to determine if (a) active extension precipitates and aggravates roof impingement, and (b) a roofplasty mitigates the effects of impingement. The tibial translation, flexion angle defining the onset of roof impingement, graft-roof contact pressure, and graft tension were measured for six cadaveric specimens. In each specimen, two tibial tunnel positions were studied: one customized for the slope of the intercondylar roof, and the other translated 6 mm anteriorly from the customized position. For a quadriceps load of 1,500 N, the flexion angle defining the onset of impingement, the peak contact pressure, and the graft tension increased significantly for both tunnel positions. The increases occurred because of the anterior tibial translation caused by the active load. Although a roofplasty decreased the onset of the angle of impingement, the graft tension remained unaffected. Thus, to mitigate the effect of impingement during active rehabilitative knee extension exercises, the position of the tibial tunnel must be customized to the angle of the intercondylar roof and a roofplasty must be performed. The extent of bone removed must be customized as well and can be determined by removing bone from the intercondylar roof in excess of that required to freely pass a rod, the same diameter of the graft, through the tibial tunnel into the intercondylar notch with the knee in full passive extension.

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