CHANGES OF SUBCHONDRAL MINERALIZATION PAT TERNS AFTER RECONSTRUCTION OF THE ACL IN SHEEP

The cruciate ligaments are generally regarded as regulators of the joi nt kinematics, but also increasingly as a ''sensory organ'' regulating the periarticular musculature, which influences the position of the c ontact surfaces and the direction and magnitude of the resultant force s. Indirectly, therefore, they influence the long-term stress distribu tion, which is itself reflected in the distribution of the subchondral mineralisation. An experimental investigation was therefore undertake n to see whether, in terms of this distribution, the distribution of t he stress alters postoperatively, since this could be regarded as an e arly cause of degenerative change. We examined the distribution of the subchondral mineralisation of six tibiae from six adult Merino sheep 1 year after anterior cruciate ligament reconstruction with patellar t endon. The contralateral tibia we used as controls. Maximum subchondra l mineralisation of both the medial and the lateral joint surfaces in all the control joints was found to be central. In the treated knees, however, the medial joint surface maxima were found to have been displ aced towards the dorsomedial edge of the surface in every case. If the density pattern on the control side is accepted as the expression of a normal stress distribution, the distribution patterns on the treated side must indicate a pathologic distribution of stress. Reconstructio n of the anterior cruciate ligament is therefore - at least in the ani mal model - not able to prevent the development of a pathologic stress distribution. There are two possible explanations for the alteration in joint stress following reconstruction of ACL: (1) a change in the k inematics (caused by the poorer mechanical properties of the transplan t or the different fibre structure), and therefore in the extent of th e contact surfaces; (2) less effective regulation of the musculature d ue to loss of proprioception, with consequent alteration of the positi on of the resultant forces.