Scapular positioning assessment: Is side-to-side comparison clinically acceptable?

Abstract

Clinicians routinely assess scapular position and motion of the symptomatic shoulder taking as reference for the contralateral asymptomatic side. A different positioning between sides (scapular asymmetry) is often assumed as pathological, however, the symmetry of scapular kinematics in healthy individuals is yet to be demonstrated. This study tested the hypothesis of scapular symmetry during arm elevation. The 3-dimensional scapular positioning of the dominant and non-dominant shoulders of fourteen healthy young adults was simultaneously measured by a 6 degrees of freedom electromagnetic tracking device at three positions of arm elevation: rest, hands on hips, and 90° of shoulder abduction with internal rotation. The scapula on the dominant shoulder showed greater retraction (P < 0.001; η²_p = 0.68) and upward rotation (P < 0.001; η²_p = 0.70) at all positions of arm elevation. From rest to 90° of shoulder abduction, the mean (±SD) amount of scapular angular displacement was, respectively for dominant and non-dominant shoulders, 7.2° (±7.8°) and 7.2° (±4.4°) for retraction, 17.4° (±5.1°) and 17.8° (±6.4°) for upward rotation, and 3.8° (±3.6°) and 0.9° (±3.6°) for posterior tilting. These findings suggest that scapular positioning on the thorax are not the same despite the observation of an identical kinematic pattern during arm elevation. This should be taken into consideration when comparing scapular position and motion of symptomatic and contralateral shoulders.

Introduction

Patients with subacromial impingement syndrome and glenohumeral instability often exhibit abnormal scapular kinematics of the ipsilateral shoulder during arm elevation, an impairment known as scapular dyskinesis or scapulothoracic dysfunction (Lukasiewicz et al., 1999; Ludewig and Cook, 2000; Matias and Pascoal, 2006; McClure et al., 2006; Ogston and Ludewig, 2007). Potential biomechanical mechanisms for such deviations include the tightness of the pectoralis minor muscle and of the glenohumeral joint peripheral connective tissue, augmented thoracic kyphosis, and impaired motor control of the scapulothoracic muscles (Kebaetse et al., 1999; Ludewig and Cook, 2000; Finley and Lee, 2003; Borstad, 2006; Matias and Pascoal, 2006; Yang et al., 2009). It is widely accepted that executing specially designed exercises and manual techniques to correct these mechanisms is essential for improving function and reducing pain (Wang et al., 1999; Voight and Thomson, 2000; Sahrmann, 2002; Burkhart et al., 2003; Ludewig and Borstad, 2003; Borstad and Ludewig, 2006; Cools et al., 2007); therefore, identifying abnormal patterns of scapular kinematics is a key element of shoulder examination. However, the assessment of a bi-directional gliding mechanism combined with 3 rotations that is inter-dependent of the position and motion of the thorax, clavicle and arm, and covered by muscles and skin, can be challenging. Motion tracking devices allow 3-dimensional (3D) analysis of the shoulder motion with accuracy but these are very expensive and time-consuming to use in most clinical settings. A clinical instrument that can easily measure 3D scapular kinematics is yet to be developed, and the combination of several tape measurements between the dorsal spine and the scapula to estimate scapular position and orientation has shown disappointing correlations with tracking devices (Borstad, 2006; Morais, 2009). In addition, the relatively large between-subject variability of scapular position and motion seen in both healthy and symptomatic individuals poses difficulties to create valid and reliable cut-off values between what may be considered normal and what may be assumed as abnormal (de Groot, 1997; Ludewig and Cook, 2000; Borstad and Ludewig, 2002; Nijs et al., 2005; Lewis and Valentine, 2007).

A convenient solution to overcome part of these issues may involve comparing the ipsilateral shoulder kinematics with the contralateral side. Side-to-side differences in several aspects of the shoulder mechanics such as the range of motion (ROM), strength and proprioception are, to some extent, negligible (Ellenbecker and Davies, 2000; Aydin et al., 2001; Crockett et al., 2002; Valentine and Lewis, 2006; Roy et al., 2009). Moreover, the contralateral asymptomatic shoulder may provide a reliable basis for comparisons over time (e.g. to assess the result of the intervention) because the within-subject variability of scapular kinematics between measurements is low (1°–2°) (Borstad and Ludewig, 2002). Indeed, this procedure is commonly used in clinical settings, where the pattern of scapular position and motion during arm elevation is assessed using the contralateral side as a reference of the normal behavior. For clinical decision making, it is assumed that side-to-side scapular kinematics in healthy individuals is relatively identical or symmetrical, therefore, a different positioning between sides (scapular asymmetry) is deemed as dyskinetic (Kibler, 1998; Kibler et al., 2002; Donatelli and Wooden, 2010). However, robust evidence supporting symmetric 3D scapular position and motion during arm elevation in healthy individuals is lacking, confounding the interpretation (Lukasiewicz et al., 1999; Oyama et al., 2008; Uhl et al., 2009; Yoshizaki et al., 2009; Yano et al., 2010; Matsuki et al., 2011).

The purpose of this preliminary study was to describe and compare 3D scapular kinematics of dominant and non-dominant shoulders in healthy individuals. Preliminary data about scapular orientation at rest and during arm elevation between sides in this population was deemed necessary to clarify side-to-side comparisons performed in clinical settings to assess scapular positioning and dyskinesis. Our hypothesis was that in healthy subjects the scapula on the dominant side has the same 3D positioning as the contralateral scapula during arm elevation, supporting the clinical belief that the non-affected side could be assumed as reference of the normal scapular kinematics.