INTRODUCTION: Chronic experimental rotator cuff tears are associated with muscle retraction, atrophy, fatty infiltration, a pronounced change in the pennation angle of the muscle and consequent shortening of muscle fibres. It was the purpose of this investigation to study whether slow, continuous elongation of the musculotendinous unit can revert the pennation angle and elongate the shortened muscle fibres.

MATERIAL AND METHODS: The infraspinatus tendons of twelve sheep were released. After retraction of the tenotomised musculotendinous unit, the infra-spinatus was elongated one mm per day using a new elongation-apparatus. After restoring the approximate original length, the tendon was repaired back. Muscular architecture (retraction and pennation angle), fatty infiltration (in Hounsfield units=HU) and muscular cross sectional area (in % of the mean control side) were analyzed at start, at the time before elongation, at the time of repair and six and eighteen weeks thereafter.

RESULTS: In four sheep the elongation failed technically. In the other eight sheep, elongation could be achieved as planned. After retraction of 29mm ± 6 mm after 16 weeks (14% of original length, p=0.008), the mean traction time was 24 days ± 6 days with a mean traction distance of 19 mm ± 4 mm. At sacrifice the mean pennation angle increased in the failed sheep from 30° ± 6° up to 55° ± 14° (p=0.035). In those sheep in which traction was applied, the mean pennation angle was not different to the control side (29.8° ± 7.5° vs. 30° ± 6°, p=0.575). Compared to preoperative, there was a significant increase in fatty infiltration (36 HU, p=0.0001) and decrease of the muscular cross sectional area of 43 % (range 21% to 67%, p=0.0001) at 4 months. In the sheep in which traction could be achieved, fatty infiltration remain unchanged (36 HU ± 6 HU vs 38 HU ± 4 HU, p=0.438) and atrophy decreased by 22% (range 10% to 33%) after 6 weeks of continuous traction (p=0.008).

CONCLUSION: Continuous experimental elongation of a retracted musculotendinous unit is technically feasible and might lead to recovery of the muscle architecture, partial reversibility of atrophy and, arrest of progression of fatty infiltration.

Purpose of the study: Muscles contract after a full thickness tear their tendon. The muscle then undergoes atrophy and fatty degeneration. These changes produce effects well described by histology, computed tomography and magnetic resonance imaging (MRI). To date however, the correlation of this process with the future conractile force of the muscle and the prognosis after cuff repain has been poorly understood.

Material and methods: Thirteen patients with a full thickness tear of the supraspinatus muscle were treated surgically by cuff suture. The shoulders were examined clinically and MRI. During the operation, the supra-scapular nerve was stimulated with a supramaximal voltage to obtain maximal muscle contraction which was measured. Biopsy specimens of the supraspinatus were taken before and after surgical repair in order to determine whether muscle activity during stimulation have any histological impact. The intraoperative measurements were compared with the MRI findings and the physical examination performed preoperative and at six weeks, six months and twelve months after surgery.

Results: Maximal force of the supraspinatus muscle was 200N, which is greater than the force of a direct suture repair. The maximal force was clearly correlated with muscle atrophy and fatty degeneration: by surface area, force was 12N/cm² for Goutallier grade 3 and 42 N/m² for grade 0. Five of the thirteen repairs ended with a secondary tear, the muscle in four of the five patients was among the six strongest muscles. The fifth case was the weakest muscle of all. The histological study revealed a larger quantity of lipofuchin in the muscle with atrophy and a change in the fiber structure. Analysis of the results did not demonstrate any lesions caused by the tension during the operation. MRI demonstrated one case of repair without secondary tear, the fatty infiltration had not improved and the atrophy only partially. In muscles with secondary tears, atrophy and fatty degeneration progressed significantly.

Discussion and conclusion: There is a risk of rupture of the supraspinatus tendon in the event of muscle atrophy but also for good quality muscle. The capacity of a muscle to develop force strongly depends on the state of atrophy and fatty degeneration. Atrophy can regress after reconstruction without secondary tear, but fatty degeneration is irreversible.