Absence of sensory function in the reconstructed anterior cruciate ligament.

Cruciate ligaments provide sensory information that cause excitatory as well as inhibitory effects to the activity of the muscles around the knee. The aim of the study was to determine whether these muscular reflexes are reestablished after anterior cruciate ligament (ACL) re-construction. Wire electrodes were inserted during arthroscopy into the normal posterior cruciate ligament (PCL) and the reconstructed ACL in 11 patients who had a successful ACL re-construction 8 months to 12 years earlier. After the anesthesia had subsided, the PCL was stimulated electrically through the electrodes and the sensory threshold was determined. Stimulus amplitudes were increased to 1.5-2.0 times the sensory threshold, and inhibitory reflexes could be elicited from PCL in the quadriceps during active extension and in the hamstrings muscles during active flexion in all patients. Subsequently the ACL re-constructions were stimulated. The sensory threshold was 3.4 times higher in the ACL than in the PCL. Stimulus amplitudes were increased to 1.5-2.0 times the sensory threshold, and a typical inhibitory reflex could be elicited in 9 patients. The latency was the same as for the reflex from the PCL. The stimulus amplitudes necessary to elicit reflexes from the ACL re-constructions were 2, 9 times higher than amplitudes that elicited reflexes from the PCL. Sensation and afferent reflex activity required a much stronger stimulus in the ACL graft compared to normal PCL. We suggest that the reason for this is that the ACL grafts were not reinnervated, and that the reflexes were elicited by spread of stimulus current to the PCL.

Reflexes in the shoulder muscles elicited from the human coracoacromial ligament.

Morphological studies have demonstrated mechanoreceptors in the capsuloligamentous structures of the shoulder joint, however knowledge of the role these joint receptors play in the control of shoulder stability is limited. We therefore investigated the effect of electrically induced afferent activity from mechanoreceptors in the coracoacromial ligament (CAL) on the activity of voluntary activated shoulder muscles in healthy humans. In study I, wire electrodes, for electrical stimulation, were inserted into the CAL in eight normal shoulders. In study II, a needle electrode was inserted into the CAL in seven normal shoulders. Electric activity was recorded from eight shoulder muscles by surface and intramuscular electrodes. During isometric contractions, electrical stimulation was applied to the CAL at two different stimulus intensities, a weak stimulus (stim-1) and a stronger stimulus (stim-2). In both experiments, electrical stimulation of the CAL elicited a general inhibition in the voluntary activated shoulder muscles. In study I the average latencies (mean+/-SE) of the muscular inhibition were 66+/-4 ms (stim-1) and 62+/-4 ms (stim-2) during isometric flexion and 73+/-3 ms (stim-1) and 73+/-5 ms (stim-2) during isometric extension. In study II the average latency (mean+/-SE) of the response was 66+/-4 ms (stim-1) during isometric flexion. Our results demonstrated a response, probably of reflex origin, from mechanoreceptors in the CAL to the shoulder muscles. The existence of this synaptic connection between mechanoreceptors in CAL and the shoulder muscles suggest a role of these receptors in muscle coordination and in the functional joint stability.

Cruciate ligament reflexes.

The idea of muscular reflexes elicited from sensory nerves of the cruciate ligaments is more than 100 years old, but the existence of such reflexes has not been proven until the recent two decades. First in animal experiments, a muscular excitation could be elicited in the hamstrings when the anterior cruciate ligament (ACL) was pulled, and tension in the ligament caused activity of the gamma motor neurones of the muscles around the knee. Impulses from the sensory nerves in ACL were activated during motion of the knee, in particular overstretching and combined extension and rotation. In humans, proprioception in the knee is decreased after ACL rupture. By mechanical or electrical stimulation of the ACL, an excitation in the hamstrings muscles can be elicited. During muscular activity, stimulation of the ACL or PCL results in a clear inhibition of the ongoing activity, both during static isometric and isokinetic muscle work, and also during dynamic activity (gait). This inhibitory reflex subjectively resembled giving way. The latency of the reflex was short in animals (about 3 ms) and long in humans (60-120 ms), probably caused by differences in the experimental setup and between species. The long latency in humans makes it unlikely that it is a directly protective reflex. Instead it may be involved in the updating of motor programs. Further research may characterize the reflex in details and map its pathways. The existence of this reflex indicate that the cruciate ligaments have an afferent function, which influences knee dynamics.