Central Motor Deficits of the Deltoid Muscle in Patients with Chronic Rotator Cuff Tears

PURPOSE OF THE STUDY Previous surface EMG studies have shown that chronic rotator cuff tears (RCT) may be associated with a altered activation of adjacent shoulder muscles. The effect of RCT on central neuromuscular control mechanisms of the shoulder girdle muscles such as the deltoideus muscle (MID), a key muscle of shoulder function, has as yet not been studied in detail. This study investigated the cortico-spinal excitability of the MID to assess the effects of RCT on the central neuromuscular function of upper limb muscles. MATERIAL AND METHODS The motor evoked potentials (MEP) in response to transcranial magnetic stimulation of MID and first dorsal interosseus muscle (FDI) on both sides were obtained of six right-handed men with chronic, symptomatic, full-thickness RCT on the dominant sides. Stimulus response curves at four different levels were measured at two tasks (MID at rest and during activity). RESULTS Different interactions were found between stimulus intensity, task and side for MEP of the MID (F = 3.9, P = 0.03), indicating that MID excitability on the affected side were lower when compared with the non-affected side. No correlation was found between the correspondent MEP amplitudes of MID and FDI at rest (r = 0.1, P = 0.44) and MID activation (r = 0.3, P = 0.05) on the affected side whereas a correlation existed on the non-affected side at rest (r = 0.5, P = 0.007) and during activation (r = 0.8, P < 0.001). CONCLUSIONS These decreased cortico-motoneuronal excitability of the MID on the affected side seems to related to adaptive changes in motor cortex as a consequence of chronic RCT The data suggest an involvement of central mechanisms and seem to precede severe changes of osteoarthritis of the shoulder.