Suprascapular nerve block disrupts the normal pattern of scapular kinematics
Introduction
In the United States, it is estimated that the incidence of rotator cuff tears in patients over 40 years of age may be as high as 40%, making it one of the most prevalent complications associated with the adult shoulder (Matsen et al., 2004, Soslowsky et al., 1997). Although the etiology of rotator cuff tears is multifactorial, it has been suggested that if the synchronous pattern of motion between the scapula and humerus is disrupted, the rotator cuff tendons might become impinged under the coracoacromial arch (Fu et al., 1991). This may in part be due to the fact that alterations in scapular orientation can affect the amount of clearance in the subacromial space, as demonstrated in vivo with Magnetic Resonance Imaging (MRI) (Solem-Bertoft et al., 1993) and in cadavera models (Karduna et al., 2005, Wong et al., 2003).
In the clinic, alterations of scapular movement patterns are associated with several conditions that can accompany rotator cuff tears, such as muscle weakness (Nicholson, 1989), fatigue (Cohen and Williams, 1998), and paralysis (Matsen and Arntz, 1990). Several research studies have quantitatively demonstrated that patients with cuff tears have increased scapular motion when compared to healthy controls (Yamaguchi et al., 2000, Paletta et al., 1997, Deutsch et al., 1996, Mell et al., 2005). Since it is not practical to observe patients before they develop cuff tears, we do not know whether these abnormal patterns are causal or compensatory in nature. In this situation, controlled models are often useful in understanding an underlying pathology. To date, there has been extensive work in the development of animal (Soslowsky et al., 1996, Dejardin et al., 2001, Tillander et al., 2001) and cadavera (Parsons et al., 2002, Halder et al., 2002, Thompson et al., 1996) models of rotator cuff tears. However, an in vivo human model would provide additional information.
The majority of full thickness tears start with the supraspinatus tendon and then progress posteriorly to the infraspinatus tendon (Matsen et al., 2004, Sher, 1999). Therefore, interventions that result in dysfunction of these muscles are candidate models. One possibility is to selectively fatigue these muscles. Although we have had success with this approach for the infraspinatus in our laboratory (Tsai et al., 2003), the supraspinatus is more of a challenge. Another approach would be the use of a pharmacological nerve block. The suprascapular nerve branches from the superior trunk of the brachial plexus, and after passing inferiorly and laterally, goes deep to the trapezius muscle. The nerve then passes through the suprascapular notch and innervates both the supraspinatus and infraspinatus muscles (Pratt, 1991). Suprascapular nerve blocks are commonly performed clinically for pain relief of the shoulder due to conditions such as adhesive capsulitis and nerve entrapment (Tan et al., 2002, Shanahan et al., 2003, Karatas and Meray, 2002). However, several investigators have taken advantage of its innervation to perform nerve block studies for biomechanical evaluations of strength (Kuhlman et al., 1992, Howell et al., 1986, Colachis and Strohm, 1971) and kinematics (Howell and Kraft, 1991).
We propose the use of a suprascapular nerve block as an appropriate model of dysfunction of the supraspinatus and infraspinatus muscles. Specifically, the aim of this project is to examine the effect of a suprascapular nerve block on scapular kinematics. We hypothesize that this block will result in a compensatory increase in scapular rotations and decrease in glenohumeral motion.
Section snippets
Subjects
Fifteen subjects participated in this study (age range 20–33 years). There were seven females and eight males, with a mean age of 26 (SD 4) years, a mean height of 174 (SD 9) cm and a mean mass of 70 (SD 10) kg. Subjects were excluded from the study if they had any of the following: (1) less than 135° of active humeral elevation in the scapular plane; (2) prior shoulder surgery; (3) shoulder injury in the past six months; (4) presence of shoulder pain preventing the correct execution of tests;
Results
Out of the original 15 subjects tested, 10 were included for the purposes of data analysis. Four subjects did not meet the 50% reduction in external rotation torque and one subject was so affected by the block that she could not elevate her arm without assistance.
The ICC values were 0.85 or better for all kinematic variables, except for clavicular plane values below 80° of humeral elevation. The SEM was 2° or less for all dependent variables, except for external rotation (Table 1). For external
Discussion
The results of the current study support our original hypothesis that a suprascapular nerve block results in a compensatory increase in scapular rotations and decrease in glenohumeral motion. Interestingly, when analyzing mean data, these changes were only observed for upward rotation and external rotation. However, when the absolute magnitude of the changes was analyzed, all six kinematic variables demonstrated significant changes. This was due to the fact that for some subjects there was an
Conclusions
The results of this study, especially those for upward rotation, are in general agreement with what has been found for patients with rotator cuff tears. Although the supraspinatus and infraspinatus do not directly control the movement of the scapula, they appear to result in a compensatory change the scapulothoracic rhythm. While more work needs to be done, it appears that abnormal scapular motion patterns observed in patients with cuff tears may be compensatory in nature.
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