A previously developed experimental model for producing nonunions in r
ats was used to study the biochemical changes of connective tissue par
ameters in impaired fracture repair. The model is based on rotational
instability between the fracture fragments. A mid-diaphyseal femoral o
steotomy was performed on 30 male rats and fixed with a loose-fitting
intramedullary nail. The rats were killed 1, 2, 3, 7, 9, and 12 weeks
postoperatively, and the development of nonunions was verified with ra
diographs. The calluses were dissected free and set for biochemical an
alysis, The contents of nitrogen, hydroxyproline, calcium, and phospho
rous, as well as the RNA/DNA ratio, were determined. It appeared that
in the impaired fracture repair there is an extended matrix production
phase continuing until 7 weeks postoperatively, Simultaneously, the n
umber of callus cells increased, indicating an extended expression of
the mitotic signals for callus cells. The net synthesis of collagen ma
trix seemed to be sufficient, but the mineral binding capacity of the
newly synthetised collagen was impaired. Later, the cessation of chond
rogenic and osteogenic activity could be observed with the formation o
f nonmineralized fibrous tissue between the fracture fragments.