Proprioception of the cruciate ligaments: receptor mapping in an animal model

Ten anterior and posterior cruciate ligaments (ACL and PCL) harvested from adult sheep were investigated under light microscopy for data on the frequency and localisation of neural structures. Serial sections of 25 microns thickness were stained with a modified gold chloride technique. Receptors were classified according to their histological structure. Topographic distribution and frequency within the ligament texture were determined with the help of computerized image analysis. Three distinct neural structures could be identified: Ruffini endings, Ruffini corpuscles of the Golgi tendon organ-like type and Pacinian corpuscles. Golgi tendon organs were not found. In total, 274 and 238 neural structures were present in the 10 ACL and 10 PCL, respectively. Pacinian receptors were the most common structures, with a mean frequency of 13.6 +/- 5.3 (ACL) and 12.4 +/- 5.1 (PCL), followed by Ruffini endings with 8.9 +/- 3.2 (ACL) and 7.8 +/- 2.9 (PCL), whereas Ruffini corpuscles had the lowest frequency with a mean value of 4.9 +/- 2.1 (ACL) and 3.4 +/- 1.1 (PCL). The majority of the neural structures were located in the subsynovial sheath or closely associated with endotenon structures. The tibial and femoral insertion areas had a significantly increased receptor density compared with the midpart of the ACL and PCL (P < 0.001), where only 19.3% and 23.7% of the receptors were located. These results emphasise the complex sensory structure of the cruciate ligaments and provide a valid morphological basis for further neurophysiological investigations.