Primary stabilizers such as the anterior cruciate ligament (ACL) bear load in concert with surrounding secondary stabilizers to resist pivoting maneuvers. When the ACL is compromised, secondary stabilizers such as the anterolateral ligament (ALL) must bear increased load to compensate for the injured primary stabilizer. The stabilizing role of primary and secondary stabilizers is commonly assessed through serial sectioning studies where changes in primary and coupled motions are measured in response to an applied load or by measuring the changes in joint reaction forces before and after sectioning a tissue in response to a known displacement. Unfortunately, these previous approaches do not quantify where within the envelope of joint motion these secondary restraints bear load (i.e., their engagement patterns). Such knowledge would provide a framework for understanding the effect of surgical reconstruction of ligaments on knee function.

Methods

Nine fresh-frozen human cadaveric knees were acquired for testing. Specimens were mounted to a robot with a universal force-moment sensor. A simulated pivot test at 30° flexion was applied in the ACL-intact and -sectioned conditions consisting of combined valgus (8 Nm) and internal rotation (4 Nm) torques. The kinematics were subsequently repeated before and after serially sectioning the ALL. To quantify the engagement patterns of the ALL, it was first categorized as either load-bearing or non-load-bearing based on whether it carried at least a third of the load seen in the ACL in the same knee. Then, load in the ALL was used to identify the anterior position of the tibia where the ALL began to carry load in the ACL-sectionedknee. This anterior position was termed the ‘engagement point’.

Results

In the ACL-intact condition, the ALL was load-bearing in none of the nine knees that were tested. In the ACL-sectioned condition, the ALL was load-bearing in seven of nine knees. The engagement point of the ALL in the ACL-sectioned knee was 4.3 ± 2.4 mm beyond the maximum coupled anterior translation that occurred with the ACL intact (p < 0.005) with the difference ranging from 0.1 to 7.1 mm.

Discussion

We established a new method to characterize the function of secondary stabilizers of the knee by assessing where within the passive envelope of motion they begin to carry load. The ALL consistently engaged (i.e., bore load) beyond the range of coupled anterior translation of the ACL-intact knee in response to multiplanar valgus and internal rotation torques that cause anterior tibial subluxation in the ACL-sectioned knee at 30° flexion . That is, the ALL has slack at this flexion angle, thus the tibia begins to sublux anteriorly before load carried by the ALL begins to build in the ACL-sectioned knee. Knowing where within the envelope of motion ligaments bear load may aid establishing patient-specific guidelines for surgical reconstruction of intra and extra-articular stabilizers including rotation angle, graft pretension and flexion angle to import the desired engagement patterns of the graft.