Stability Results After Anterior Cruciate Ligament Reconstruction

The Definition of Stability After Anterior Cruciate Ligament Reconstruction

The truest definition of stability after anterior cruciate ligament reconstruction (ACLR) would be the normalization of the pivot shift in the anesthetized patient, which is obviously never measured in a follow-up study. The pivot shift in the conscious patient is not reliable. The next best measure is the instrumented Lachman test. This is usually a KT-1000 (Medmetric Corporation, San Diego, California) measurement, but other arthrometers are also used. These measurements are routinely performed and included in publications of ACLR results. The weakness of the instrumented Lachman test is that a vertical graft can normalize the Lachman test while still permitting some degree of rotatory instability. However, most surgeons publishing in the anterior cruciate ligament literature are experts in their craft and are unlikely to place vertical grafts. Therefore the instrumented Lachman is both the best available test and also one that is generally a satisfactory index of knee stability, or laxity, after ACLR.

Instrumented Lachman tests are also useful in that they can help quantify the degree of instability of the knee, in increasing millimeters of side-to-side knee difference after maximum manual testing. The studies considered in this chapter all use this standard. Older studies, which used 20- or 30-lb force, have been excluded.

The Significance of Stability After Anterior Cruciate Ligament Reconstruction

The primary purpose of ACLR is to restore knee stability in order to preserve function and help prevent degeneration. Restoration of stability clearly mitigates degeneration with both hamstring (HS) and bone–tendon–bone (BTB) grafts, although the degree to which it does so is a matter of continuing investigation. Overtightening has the potential to produce stiffness and an adverse result. However, there is universal agreement that restoration of normal stability without overtightening is desirable. The normalization of laxity such that side-to-side difference is zero is desirable, but generally less than half of the knees in studies with excellent clinical results achieve this standard. For this reason, standardized ratings generally consider 0–2 mm side-to-side difference to be normal, 3–5 mm to be nearly normal, and greater than 5 mm to be abnormal. It is clear that a small disparity in laxity between sides is consistent with an excellent clinical result. This is also consistent with the finding that most less than 50% partial tears have side-to-side differences of approximately 2 mm. Most such partial tears are consistent with normal function, and they have not been shown to predispose to premature degeneration.