Neer Award 1999 - Overuse activity injures the supraspinatus tendon in an animal model: A histologic and biomechanical study

Overuse activity has been implicated as an etiologic factor in injury to th e rotator cuff and to the supraspinatus tendon in particular Due in part to the lack of an appropriate animal model, experimental studies have not add ressed this issue. With the use of a rat model, we measured the effects of an overuse running regimen on 36 Sprague-Dawley rats after 4 (n = 12), 8 (n = 12), or 16 (n = 12) weeks of exercise and compared them with a control g roup of ran (n = 10) who were allowed normal cage activity The histologic c haracteristics, the gross morphologic characteristics, and the mechanical p roperties of the tendon tissue were evaluated. The supraspinatus tendons in the exercised animals demonstrated significant changes as a result of over use at all time points compared with the normal group. There was an increas e in cellularity and a loss of the normal collagen fiber organization consi stent with what has been seen in human tendinopathy. The tendons From the e xercise groups were larger than normal in cross-sectional analysis at 4 wee ks (129% of control, P <.07) and continued to increase in size with time to 16 weeks (164% of control, P =.01). The mechanical properties of the tendo ns deteriorated in response to overuse exercise with a decreased modulus of elasticity ranging from 52% to 61% of control (P =.07 at 4 weeks, P <.05 a t 8 and 16 weeks) and a decreased maximum stress at failure ranging from 51 % to 63% of control (P <.007). These findings support overuse activity as a n etiologic factor in the development of supraspinatus tendinopathy and beg in to describe the changes in the tendons as a result of such activity. Thi s model can now be used to study the effect of various treatment modalities on these injuries.