Purpose of this study is to create an experimental model of electrophysologic evaluation of the supraspinatus muscle on rats, after traumatic rupture of its tendon.

The population of this study consisted of 10 male Sprague Dawley rats weighting 300–400g. Under general anaesthesia we proceeded with traumatic rupture of the supraspinatus tendon and exposure of the muscle. The scapula was immobilized, and the supraspinatus tendon was attached to a force transducer using a 3–0 silk thread. A dissection was performed in order to identify the suprascapular nerve, which was then stimulated with a silver electrode. Stimulations were produced by a stimulator (Digitimer Stimulator DS9A) and were controlled by a programmer (Digitimer D4030). Fiber length was adjusted until a single stimulus pulse elicited maximum force during a twitch under isometric conditions. Rectangular pulses of 0.5 ms duration were applied to elicit twitch contractions. During the recordings, muscles were rinsed with Krebs solution of approximately 37 8C (pH 7.2–7.4) and aerated with a mixture of 95% O2 and 5% CO2. The output from the transducer was amplified and recorded on a digital interface (CED).

The following parameters were measured at room temperature (20–21 8C): single twitch tension; time to peak; half relaxation time; tetanic tensions at 10, 20, 40, 80 and 100 Hz; and fatigue index, which was evaluated using a protocol of low frequency (40 Hz) tetanic contraction, during 250 ms in a cycle of 1 s, for a total time of 180 s. The fatigue index value was then calculated by the formula [fatigue index=(initial tetanic tension − end tetanic tension) ∗ 100/(initial tetanic tension)]. In the end, the transducer was calibrated with standard weights and tensions were converted to grams.

The mean single twitch was 8.2, the time to peak 0.034 msec and the half relaxation time 0.028 msec. The strength of titanic muscle contractures was 5.7 msec at 10Hz and 17.7 at 100Hz. Finally, the fatigue index was calculated at 48.4.

We believe that electrophysiologic evaluation of the supraspinatus muscle in rats will help us understanding the pathology of muscle atrophy after rotator cuff tears and possibly the functional restoration after cuff repair

Background

Tears of the rotator cuff are a common pathology and poorly understood. Achieving a good functional outcome for patients may be difficult, and the degree of fat infiltration into the muscle is known to be a major determining factor to surgical repair and post operative function. It is the hypothesis of this study that the degree of retraction of the common central tendon as seen on MRI corresponds to the amount of fat infiltration classified according to the Goutallier grading System.

Introduction. Following tear of its tendon, the muscle undergoes retraction, atrophy and fatty infiltration. These changes are inevitable and considered irreversible and limit the potential of successful repair of musculotendinous units. It was the purpose of this study to test the hypothesis that administration of anabolic steroids can prevent these muscular changes following experimental supraspinatus tendon release in the rabbit. Methods. The supraspinatus tendon was experimentally released in 20 New Zealand rabbits. Musculotendinous retraction was monitored over a period of 6 weeks. The seven animals in group I had no additional intervention, six animals in group II had local and seven animals in group III had systemic administration of nandrolone deconate during six weeks of retraction. At the time of sacrifice, in-vivo muscle performance as well as radiologic and histologic muscle changes were investigated. Results. Supraspinatus retraction was significantly higher in group I (1.8 ± 0.2cm) than in group II (1.5 ± 0.3cm, p = 0.044) or III (1.2 ± 0.3cm, p = 0.001). The reduction in radiological cross sectional area, as a measure for atrophy, was significant in groups I (p = 0.013) and II (p = 0.030) and insignificant in group III (p = 0.149). Histologically, there was no fatty infiltration in the treated groups II (p = 1.000) and III (p = 0.812), but in the untreated group I (p = 0.0312). The work of the respective muscle during one standardized contraction with supramaximal stimulation decreased markedly in groups I (p = 0.056) and II (p = 0.0528), and also but less in group III (p = 0.23). Conclusion. This is the first documentation of prevention of important muscle alterations after chronic retraction of the musculotendinous unit caused by rotator cuff tear. Nandrolone deconate administration in the post tendon release phase prevented fatty infiltration of the supraspinatus muscle and reduced functional muscle impairment caused by myo-tendinous retraction