Further observations on the isometricity of the anterior cruciate ligament: an anatomical study using a 6-mm diameter replacement

Six anatomical specimens were used to evaluate the ligament length pattern and mean maximal ligament length change (MMLLC) of the anterior cruciate ligament (ACL) using a ligament replacement with a 6-mm diameter. Three femoral and three tibial attachment sites were selected. Nine different combinations were studied in intact knees and also in knees whose posterior capsule and arcuate ligament complex were sequentially divided, whose menisci were resected, and whose capsule and arcuate complex were repaired. The smallest ligament length change (5.6 mm) was obtained in the intact knee with the anatomic femoral and tibial attachments. Moving the femoral origin either anteriorly or to the over-the-top position significantly increased the MMLLC. Moving the tibial insertion posteriorly had a negligible effect, but changing to an anterior attachment site increased (but not significantly) the MMLLC. Even for anatomically positioned ligaments, the instability caused by sectioning the posterior capsule and arcuate ligament complex increased the MMLLC by more than 80%. Resection of both menisci did not result in a further increase in the MMLLC, but repairing the posterior capsule and arcuate complex restored the length changes to those of the intact knee. These data confirm the importance of precise anatomic placement of the femoral and tibial tunnels when performing intraarticular ACL grafts for anterolateral rotatory instability. The adverse effect of certain other associated instabilities on the ligament length change, even with correct tunnel placement, has been demonstrated and may be one explanation for the variability in the clinical results of these procedures.