Scapula kinematics and associated impingement risk in manual wheelchair users during propulsion and a weight relief lift

Abstract

Background

Shoulder impingement syndrome is a common upper extremity pathology in manual wheelchair users. Central to impingement is the orientation of the scapula and humerus as they determine the available subacromial space. The purpose of this study was to examine the scapulothoracic and glenohumeral internal/external rotation kinematics during the time of peak shoulder loading of propulsion and weight relief lift conditions to assess possible risk of impingement.

Methods

Scapula, humerus and trunk kinematics were measured for twelve manual wheelchair users over three conditions: level propulsion, ramp propulsion, and a weight relief lift. Scapulothoracic and glenohumeral kinematic variables were characterized for the full cycle of each condition as well as at the period of peak loading.

Findings

Common to all activities was an externally rotated glenohumeral joint and an anteriorly tilted and internally rotated scapula. At peak loading, glenohumeral internal/external rotation showed a significant difference between conditions, and post hoc analysis revealed that the weight relief lift displayed significantly less external rotation at peak loading when compared to level and ramp propulsion.

Interpretation

All activities placed the scapula in a potentially dangerous orientation for development of shoulder impingement. The weight relief lift, with a decrease in glenohumeral external rotation and large superior forces at the shoulder, potentially places the shoulder of the manual wheelchair user at the greatest risk for impingement soft tissue injury. Preventative strength training and activity modification may provide measures to slow progression of impingement development and associated pain in the manual wheelchair user.

Introduction

In the United States (US), approximately 12,000 people suffer from a spinal cord injury each year (National Spinal Cord Injury Statistical Center, 2010). Of the roughly 260,000 people living in the US with a spinal cord injury, many use manual wheelchairs. Manual wheelchair users (MWU) rely on their upper extremities (UE) for mobility, activities of daily living, and vocation performance. With the primary load bearing role shifting from the lower extremities to the smaller musculature of the UE (Requejo and Mulrouy, Spring, 2008), manual wheelchair users are at a high risk for UE pain and pathology limiting not only their ambulation but all areas of function (Bayley et al., 1987, Gellman et al., 1988a, Gellman et al., 1988b, Nichols et al., 1979, Pennes et al., 1983, Pentland and Twomey, 1991). Shoulder impingement syndrome has been reported as the most common UE pathology in MWU (Bayley et al., 1987, Lee and Mcmahon, 2002) and is described by Neer (Neer, 1972) as a compromise of the space between the humeral head and the acromial arch affecting the soft tissue contained within.

Central to the occurrence of impingement syndrome is the orientation of the scapula and humerus as they determine the available subacromial space. In the non-weight bearing shoulder, certain scapular and glenohumeral orientations have been identified to reduce the subacromial space; and thus, increase the risk of shoulder impingement. Scapular orientations of concern have been reported as an increase in anterior tilting (decrease in posterior tilt), an increase in internal rotation (decrease in external rotation), and a decrease of upward rotation (increase in downward rotation) (Ludewig and Cook, 2000, Lukasiewicz et al., 1999, Solem-Bertoft et al., 1993, Warner et al., 1992). Additionally, increased humeral internal rotation has been reported to contribute to greater deformation of the soft tissue within the subacromial space (Flatow et al., 1994).

Previous investigations into scapular orientation in MWU have been focused on transfers and weight relief lifts (Finley et al., 2005, Nawoczenski et al., 2003, Riek et al., 2008). Common to these studies was the characterization of scapula and humerus orientations during transfers and weight relief lifts that were associated with a decrease in the subacromial space. The rate at which these activities are performed in MWU places the shoulder at a high risk of developing an injury. Finley et al. (2005) observed that MWU with and without impingement perform transfers with different scapular kinematics, and some of the differences could be attributed to compensatory changes in those with impingement to allow for a maintenance of subacromial space during the tested activity. Riek et al. (2008) explored a larger variety of activities in MWU and concluded that the standing frame posture resulted in the most favorable scapular and humeral orientations when compared to a sitting rest posture, transfers, weight relief lifts, and standing depression lifts. While many activities have been characterized in terms of MWU shoulder health, propulsion tasks have not been investigated.

Another crucial variable in impingement risk beyond kinematics alone is the loading associated with an activity. The load of an activity performed by MWU can affect the musculature resulting in altered kinematics as well as contributing to a harmful migration of the humeral head into the subacromial space. The high loading and repetitive nature of manual wheelchair activities of daily living and mobility have been reported to result in an increase of scapular protraction on the thoracic wall from the fatigue of the scapular muscles. In protraction, the scapula is translated away from the midline and rotated to conform to the thoracic wall (Lee and McMahon, 2002). This altered scapular location and orientation results in an increase in scapula internal rotation and possible increase in scapula anterior tilting: leading to a decrease of the available subacromial space. While previous studies on scapular kinematics in MWU have focused on traditionally high load activities (transfers and weight relief lifts), loads occurring during the kinematic data collection were not reported. This piece is essential to not only being able to comment on global impingement risk, but specifically the risk in MWU who are utilizing their shoulders in a predominantly load-bearing state. Therefore, the purpose of this investigation was to characterize scapular kinematics during periods of high loading in a spectrum of wheelchair activities: level propulsion, ramp propulsion, and a weight relief lift. We hypothesized that the loading and kinematics associated with weight relief lifts would highlight its potential risk for impingement in comparison to level and ramp propulsion.