Towards improving clinical evaluation of the shoulder: Defining upper limb biomechanics of breast cancer survivors during functional evaluation tasks

Breast cancer is the most commonly diagnosed cancer among women in Canada, and survivors are often affected by post-treatment upper limb sequelae. Some limitations, such as range of motion restrictions, strength reductions, and presence of pain, have been well documented in survivors but the definition of biomechanical changes following treatment are not as robust. Further, status as breast cancer survivor has been indicated as a risk factor for secondary rotator cuff disorders, but the causes of such secondary morbidity are unidentified. Characterization of biomechanical shoulder alterations could provide insight to the higher prevalence of rotator cuff disorders in breast cancer survivors. Historically, scapular motion tracking has been difficult for biomechanists, and currently used strategies were developed and tested on young, unimpaired participants. The utility of current methods had not yet been tested in a pathological population. Therefore, the first study of this dissertation quantified error of the acromion marker cluster (AMC) in the study sample. Data were collected from 25 non-cancer controls and 25 post-mastectomy breast cancer survivors. Tracking the scapula with the AMC was most successful when using the double calibration method. This method resulted in errors of approximately 5-10˚ throughout full arm range of motion, with highest errors in scapular protraction. This error magnitude is within the previously reported range from younger populations, suggesting that the AMC is an acceptable strategy for tracking scapula in this sample. The second study of this dissertation defined the upper limb kinematics of breast cancer survivors during return-to-work focused functional tasks. The motion of the torso, humeri and scapulae were tracked during six different functional tasks: overhead reach, repetitive reach, fingertip dexterity, hand and forearm dexterity, overhead lift, and overhead work. Mean, maximum, and minimum values for each degree of freedom were extracted from each movement cycle and compared across groups. Post-hoc analyses determined that presence of impingement pain in breast cancer survivors, as determined by pain on at least one of three impingement provocation tests, was associated with clinical, performance, and kinematic differences. Breast cancer survivors with pain had higher disability scores, lower range of motion, and lower performance scores. During the overhead reach and overhead lift, scapular upward rotation was reduced at the top of the movements in the pain group. Additionally, at the extremes of the repetitive reach and overhead lift, breast cancer survivors with pain had reduced humeral abduction and humeral internal rotation. These compensations are associated with impingement pain diagnosis suggesting a potential link between biomechanical risk factors, pain, and future development of rotator cuff disorders. A measure of muscle activation would clarify the influence of altered muscle force strategy on identified movement compensations, and this was the objective of the third study. Motion data from the six functional tasks were used as input for a biomechanical model. A modified version of the Shoulder Loading Analysis Module (SLAM) was used to estimate individual muscle forces. The model was modified to accept measured scapular orientations, then pectoralis capacity adjustment for breast cancer survivors was tested to determine the best strategy for modelling this group. Model outputs were compared to measured electromyography (EMG) from select muscles to assess model efficacy, and then maximum muscle forces for each task were compared between the three groups. Model outputs with these participants, task parameters, and modifications differed from experimental EMG, but within accepted error ranges. Maximum forces during task performance differed for the breast cancer survivors with pain: upper trapezius, supraspinatus and pectoralis major muscles were consistently higher for this group, suggesting that rehabilitation should focus on preventing potentially harmful scapular and humeral kinematics by reducing activity in several key muscles, notably the upper trapezius and supraspinatus. To determine the applicability of these data for current biomechanical and clinical practice and arm assessments, the relationship of scapular motion during arm elevation and functional tasks was evaluated. While alterations were identified during functional tasks, it is not clear if these same alterations are present in arm elevations, even though this is the prevailing scapular motion assessment method. First, scapular upward rotation at five levels of arm elevation was compared between the three groups, and then the correlation of upward rotation and scapulohumeral rhythm (SHR) in both types of motion at corresponding arm elevation levels was assessed. Decrements in upward rotation were identified in the pain group, but at lower arm elevations. Upward rotation was moderately to strongly correlated between the two types of movements, but SHR did not demonstrate the same strength and significant relationships. Overall, sagittal arm elevation was most strongly correlated with functional task performance, but the differing results from the group comparisons and inconsistency of the SHR relationship suggest that a simple functional task could be a more robust clinical assessment method. These studies combine to enhance both fundamental and clinical definitions of post-treatment shoulder dysfunction in breast cancer survivors. Shoulder kinematics, as measured by the AMC, are altered in breast cancer survivors with impingement-related pain, and subsequent muscle force predictions highlighted important compensatory muscle strategies that could be targeted in rehabilitation to treat dysfunction and prevent rotator cuff disorders in this population. Finally, while evaluation during arm elevation is the predominant method both in laboratory and clinical evaluations, a loaded reach is recommended for improved assessment of scapular motion for return-to-work focused rehabilitation