Original articlePathological muscle activation patterns in patients with massive rotator cuff tears, with and without subacromial anaesthetics
Introduction
Muscle activation patterns (coordination) are bound to change after mechanical deficits like massive rotator cuff tears. Subacromial injection with lidocaine reduces pain and has been shown to coincide with an increase in active forward flexion and muscle strength in patients with specific subacromial disorders like impingement (Ben Yishay et al., 1994). In a comparable intervention it was found that patients with massive rotator cuff tears were well capable of arm abduction despite the absence of supraspinatus force, but were actively hampered to do so due to pain (van de Sande et al., 2006; de Groot et al., 2006). Their findings also showed that supraspinatus muscle force was not per se required to produce the necessary glenohumeral abduction torque.
Both series used active and isometric loading by a constant force in a direction rotating perpendicular around the longitudinal axis of the humerus. This so-called principal action method made it possible to define the direction of maximum muscle activation, in combination with the additional compensating muscle activity needed to produce force in exactly that direction (Flanders and Soechting, 1990; Arwert et al., 1997; de Groot et al., 2004; Meskers et al., 2004). The principal action method quantifies shoulder muscle contributions during an isometric force task and facilitates the analysis of the activation patterns of shoulder muscles.
This study was set up to analyse shoulder muscle coordination using the principal action method in patients with massive cuff tears. We analysed activation patterns prior to and after subacromial anaesthetics. In addition to de Groot et al. (2006) we addressed more muscles in order to explain the observed enhancement of external arm force, viz.; the deltoideus (three parts), the latissimus dorsi, the pectoralis major pars clavicularis and the teres major.
Section snippets
Subjects
Six male and four female patients (Table 1) with an average age of 61 years (SD=8) with MRI-proven massive rotator cuff tears were included in the study. All patients were informed and signed informed consent.
Procedure
The principal muscle activation patterns of six muscles were recorded as described by de Groot et al. (2004), and Meskers et al. (2004). Patients were seated with their injured arm in a splint with the humerus positioned in 30° of forward rotation relative to the frontal plane, about 45°
Results
Subacromial lidocaine injection led to an average significant reduction on the VAS scale , from 7.7 (SD 1.2) to 0.9 (SD 1.6), indicating a strong reduction in pain, although some patients still experienced pain after treatment (Fig. 2a). The exerted arm force during the task could significantly be increased by factor 1.6 after pain reduction, from 10.4 N (SD 5.7) to 15.7 N (SD 7.4) (Fig. 2b). Patient no. 7 did not respond to the lidocaine injection on any of the three outcome
Discussion
As reported earlier (van de Sande et al., 2006; de Groot et al., 2006) and in agreement with impingement (Ben Yishay et al., 1994), external forces increased significantly after subacromial lidocaine injection in patients with massive rotator cuff tears, despite the (partially) absent supraspinatus forces.
The lidocaine intervention did lead to large changes in principal action, but not consistent for all subjects and therefore not significant for the whole patient group. No statistical
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