The sensitivity of joint afferents to knee translation

The cruciate ligaments contain mechanoreceptors which putatively contribute to knee function and dysfunction. However, the interpretation of studies showing neural responses to traction loads applied to the cat cranial cruciate ligament (CCL analogous to the anterior cruciate ligament in humans) depends upon demonstrating that non-CCL periarticular receptors are not stimulated. We assessed the capability to rigidly fix the knee against traction loads applied to the feline CCL. The tibia and femur were fixed either with clamps or Steinmann pins. Motion of the bones was monitored with liquid metal strain gages (LMSG) and the activity of the posterior articular nerve (PAN) was recorded while traction loads of up to 20-30 N were applied to the CCL. Joint afferents recorded from the PAN were insensitive to the CCL loads in the rigidly fixed preparation. Motion of the proximal tibia and distal femur was less than 100 micrometers for both methods of fixation, with neither method demonstrating more rigid fixation. In contrast, we observed vigorous discharges with focused light pressure on the capsule and under conditions allowing 200-500 micrometers of tibial displacement on the femur. This suggests that clinically undetectable instability may give rise to aberrant mechanoreceptor activity contributing to dysfunction.