Mechanoreceptor Re-innervation Following Allograft versus Autograft Anterior Cruciate Ligament Reconstruction

OBJECTIVES: Functional instability following anterior cruciate ligament (ACL) rupture is due to both loss of mechanical restraint, and loss of proprioception that restricts the ability of coordinated muscle activity to compensate and stabilize the knee joint. This lack of coordinated muscle control is thought to be due to diminished or absent sensory feedback from the ACL mechanoreceptors to the neuromuscular system. Clinical, motor and proprioceptive function is known to improve following ACL reconstruction but does not return to normal. While histological studies of human ACL allografts have been unable to demonstrate mechanoreceptor re-innervation, animal data suggests re-innervation occurs when an autograft is used. The aim of this study was to compare the presence or absence of mechanoreceptors in allograft versus autograft in following ACL reconstruction in humans. METHODS: Eleven patients with previous ACL reconstruction presenting for either revision ACL surgery or knee arthroscopy for other reasons were enrolled in a prospective, comparative study. Six patients had a previous autograft ACL and five patients had an allograft. In the case of revision patients, the previous ACL graft tissue was excised within 6 weeks of ligament rupture, and in arthroscopy patients 2mm biopsies of the ACL graft were taken from the tibial and femoral insertions. ACL tissue was also taken from 2 additional patients within 6 weeks following acute primary ACL rupture as a control. The specimens were fixed with 10% buffered formalin solution and frozen section was performed. Specimens were stained with hematoxylin-eosin (H-E) and monoclonal antibodies against S-100 and neurofilament protein (NFP). Immunohistochemical examination was performed using Freeman and Wyke's criteria to morphologically classify mechanoreceptors. RESULTS: Ruffini corpuscles and free nerve endings were shown to be present in the specimens of the control group. Histological examination for mechanoreceptors for six patients with previous autografts and five patients with previous allografts were compared. The average time between original graft and biopsy was 10.7 years (range 0.5 ### 27). There were significantly more mechanoreceptors visible in control (native) ACL tissue than graft tissue. CONCLUSION: We present comparative data demonstrating reduced concentration of mechanoreceptors in ACL grafts compared to native ACL tissue. This may explain the continued proprioceptive deficit known to exist even following ACL reconstruction.