Isometry measurements in the knee with the anterior cruciate ligament intact, sectioned, and reconstructed.

When assessing isometry during anterior cruciate ligament surgery, it is assumed that points determined to be isometric remain so after reconstruction. We sought to evaluate if isometric measurements vary with the status of the anterior cruciate ligament. A computerized electronic isometer was used to measure the magnitude and pattern of change in separation distance between a constant point in the tibial insertion of the anterior cruciate ligament and five positions within the femoral insertion with the anterior cruciate ligament intact, sectioned, and reconstructed. For the center position, the magnitude and pattern of the change in separation distance was physiologically isometric in all conditions (maximal length change, 3.0 mm). For the posterior position, the isometry pattern remained physiologic in each condition, and the magnitude of the separation distance was nearly isometric in all conditions (maximal length change, 3.7 mm). The superior and inferior positions had similar isometric measurements in the intact and sectioned conditions but significantly different measurements after anterior cruciate ligament reconstruction. Intraoperative assessment of isometry at positions in the center or posterior portion of the anterior cruciate ligament's femoral insertion provides useful information that is not altered by reconstruction. For superior and inferior positions, however, points found to be isometric in the anterior cruciate ligament-deficient knee did not remain isometric after reconstruction.