INTRAOPERATIVE ELECTROMYOGRAPHIC ANALYSIS OF SUBSCAPULAR MUSCLE DURING TOTAL SHOULDER ARTHROSPLASTY: A PROSPECTIVE CONSECUTIVE STUDYABSTRACT

Abstract

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.