Summary. Data of 663 patients with three different pathologies were examined. We found that using patients with significant symptoms and functional difficulty in the opposite shoulder will not bias the results of observational studies if outcomes are based on routine disability measures such as ASES or Constant-Murley scores. Introduction. Recently, using patients with bilateral limb problems as independent cases has raised concerns in orthopaedic research due to violating the assumption of independence. If observations are too similar in characteristics, they become highly correlated which leads to lowering the variance and biasing the results. Type of pathology (impingement, cuff tear, osteoarthritis) and aging are expected to affect the incidence of bilateral shoulder complaints and should be considered when examining potential bias in this area. In addition, the impact of dominant side pathology has not been investigated primarily in patients with shoulder problems. The objectives of this study were: 1) to examine the incidence of bilateral shoulder complaints and pathology on the dominant side in patients with impingement syndrome, rotator cuff tear and osteoarthritis of the glenohumeral joint, 2) to explore the role of sex and age in developing bilateral shoulder complaints, and 3) to examine the impact of bilaterality and hand dominance on pre and one year post-operative disability. Patients and Methods. This study involved review of data of patients with a diagnosis of impingement syndrome, rotator cuff tears and osteoarthritis (OA) of the gleno-humeral joint who had undergone surgery and had returned for their one year follow-up. Two outcome measures were used; the American Shoulder & Elbow Surgeons (ASES) and the Constant Murley score (CMS). Results. Data of 663 patients (317 females; 269 impingement syndrome, 290 rotator cuff tear, 104 osteoarthritis) were included in the analysis. There was a difference in the incidence of bilateral symptoms in patients with different pathologies: osteoarthritis 46%, impingement 26%, and rotator cuff tears 23% (p<0.0001). The incidence of dominant side involvement was 70%, 68% and 50% in patients with rotator
