Histopathological changes in patellar tendon enthesis of rabbit induced by electrical stimulation intensity

Enthesis injury is a common problem in athletes and workers, which is considered closely related to overuse. However, the early pathophysiologic changes of osteotendinous junction are not well understood, and moreover, few studies investigated the relationship between intensity and pathological changes. The purpose of this study was to evaluate microstructural changes induced by different loading intensities and to find out the threshold intensity.

Forty-eight New Zealand White rabbits were randomly divided into six groups. One control group, the others were electrically stimulated to contract repetitively for 2 h per day, three days a week. 30% of peak tetanic force (7.06 N) was adopted to stimulate the rabbits in the 100% cyclic loading group. Other groups were stimulated with 20%, 40%, 60% and 80% of this force. After four weeks, prepared samples were stained with hematoxylin and eosin.

After 4 weeks of cyclic loading, the shape and the distribution of tendon cells in patellar enthesis changed, the arrangement of collagen fibers became disordered and the tidemark had become irregular or even disappeared. Different stimulus intensity caused a significant change of cell density in different groups (F = 10.19, P < 0.001). The cell densities of tendon were 34.3 ± 7.9 cells/100 μm² (L60), 38.2 ± 5.9 cells/100 μm² (L80), 43.8 ± 10.3 cells/100 μm² (L100) respectively, which had significant difference with CON group 22.5 ± 3.5 cells/100 μm². The thickness of fibrocartilage zone had no significant difference among the groups.

The changes of histomorphology with the increasing exercise intensity elucidated that the degree of enthesis microdamage was directly related to the intensity of exercises. The findings demonstrated that 18% (used in L60 group) of peak tetanic force was the threshold intensity which could induce pathological changes in enthesis in four weeks.