Rabbit supraspinatus tendon detachment: effects of size and time after tenotomy on morphometric changes in the muscle.

We studied the effects of size and time after rabbit supraspinatus tenotomy (group A--small tenotomy, group B--large tenotomy) on muscle morphometric changes in 48 rabbits. Animals were killed 6 (subgroups A1 and B1), 12 (subgroups A2 and B2) and 24 weeks (subgroups A3 and B3) after tendon detachment. Statistically significantly greater increases in interstitium volume were noted in subgroups A1-A3 and B1-B3 than in controls. Reductions in type I and II fiber diameters were mainly due to the length of observation. However, statistically significant differences in comparison with controls appeared earlier after large tenotomy. The size of the tenotomy primarily affected muscle fiber composition. Our results suggest that these changes were caused by fiber transformation from type I to type II and vice versa. These findings indicate that the interdigitations between the supraspinatus and infraspinatus and between the supraspinatus and subscapularis tendons are important in dynamics and the degree of morphometric changes in the rabbit supraspinatus after tenotomy.