Background: Progression of fatty infiltration of the suscapularis muscle subsequent to total shoulder arthroplasty is frequent and may be an underestimated problem. The approach with osteotomy of the lesser tuberosity led to consistent bone to bone healing with neither retraction nor overtensioning of the musculotendinous unit. However, in a previous study, fatty infiltration of the subscapular muscle had progressed at least by one stage in 45% of the patients’ shoulders. We hypothesized that anterior approach to the shoulder joint with release of the subscapularis muscle would lead to a direct or indirect subclinical damage of the subscapular nerve and would be an explanation for the progression of fatty infiltration of the muscle.

Methods: Nine Shoulders in eight consecutive patients had received a total shoulder arthroplasty using an anterior approach with osteotomy of the lesser tuberosity. The mean age at time of operation was 67 years. Patients were followed clinically including the Constant score and a detailed neurological examination, as well as radiographically with pre–and postoperative MRI’s, CT’s and standard radiographs at 6–and 12 months thereafter. Neurophysiological assessment was performed using a new pre–intra–and postoperative electromyographic technique for the subscapular muscle. Fibrillation and sharp waves as spontaneuous activities of the motor unit potentials (MUP) indicated direct signs of denervation. Incomplete interference patterns (IP’s) indicated an incomplete innervation pattern as an indirect sign of denervation.

Results: After a one year follow up, 89% of the patients were very satisfied or satisfied with the result. The relative Constant Score improved from an average of 50 percent preoperatively to an average of 96 percent postoperatively (p=0,008). That corresponds to a postoperative subjective Shoulder value of 89 percent. From preoperative to 6–and 12-months postoperative, the average degree of fatty infiltration of the subscapular muscle progressed in an almost significant extent (0.6, 1.1, and 1.6 respectively; p=0.056).

Intra–and postoperatively, there were neither fibrillations and sharp waves of the MUP’s as direct signs, nor incomplete (IP’s) of the motor unit of the subscapular nerve as indirect signs of denervation.

Conclusion: Total Shoulder Arthroplasties perfomed by an anterior approach using an osteotomy of the lesser tuberosity yields good results with a high satisfaction rate. Comparable to our previous study, there are signs of progression of fatty infiltration of the subscapularis muscle within the first year subsequent to total shoulder arthroplasty. However, there were no signs of direct or indirect subclinical damage of the subscapular nerve during total shoulder arthroplasty.

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.