EXTENDED EXPRESSION OF CARTILAGE COMPONENTS IN EXPERIMENTAL PSEUDARTHROSIS

The healing of femoral fractures in an experimental rat pseudoarthrosi s model was followed by studying the expression of cartilage specific genes coding for type II and X collagens and aggrecan, soft tissue and bone specific type I collagen, and decorin. Severe impairment of heal ing was observed with cartilage gene expression continuing until the s eventh week and then declining rapidly. The abnormal healing pattern r esults in an inactive scar-like callus after the ninth week of healing even though house-keeping (e.g., GAPDH) genes are continuously expres sed in the tissue. These results could be explained on the basis of co ntinuous chondrogenic stimulus extending much beyond the normal range. If union is not achieved because of mechanical instability, signal of endochondral ossification persists until it becomes exhausted and cal lus at the fracture gap becomes an inactive fibrous scar. The disturbe d matrix gene expression was confirmed by histology.