A Prospective Analysis of Patients With Anterior Versus Posterior Shoulder Instability: A Matched Cohort Examination and Surgical Outcome Analysis of 200 Patients
Authors: Bernhardson Andrew S. , Murphy Colin P. , Aman Zachary S. , LaPrade Robert F. , Provencher Matthew T. 
Published: January 2019

Background: Anterior and posterior shoulder instabilities are entirely different entities. The presenting complaints and symptoms vastly differ between patients with these 2 conditions, and a clear understanding of these differences can help guide effective treatment. Purpose: To compare a matched cohort of patients with anterior and posterior instability to clearly outline the differences in the initial presenting history and overall outcomes after arthroscopic stabilization. Study Design: Cohort study; Level of evidence, 2. Methods: Consecutive patients with either anterior or posterior glenohumeral instability were prospectively enrolled; patients were excluded if they had more than 10% anterior or posterior glenoid bone loss, multidirectional instability, neurologic injury, or prior surgery. Patients were assigned to anterior or posterior shoulder instability groups based on the history and clinical examination documenting the primary direction of instability, with imaging findings to confirm a labral tear associated with the specific direction of instability. Preoperative demographic data, injury history, and overall clinical outcome scores (American Shoulder and Elbow Surgeons [ASES], Single Assessment Numeric Evaluation [SANE], and Western Ontario Shoulder Index [WOSI]) were assessed and compared statistically between the 2 cohorts. Patients were indicated for surgery if they elected to proceed with surgical management or did not respond to a course of nonoperative management. Results: The study included 103 patients who underwent anterior stabilization (mean age, 23.5 years; range, 18-36 years) and 97 patients who underwent posterior stabilization (mean age, 24.5 years; range, 18-36 years). The mean follow-up was 39.7 months (range, 24-65 months), and there were no age or sex differences between the groups. No patients were lost to follow-up. The primary mechanism of injury in the anterior cohort was a formal dislocation event (82.5% [85/103], of which 46% [39/85] required reduction by a medical provider), followed by shoulder subluxation (12%, 12/103), and “other” (6%, 6/103; no forceful injury). No primary identifiable mechanism of injury was found in the posterior cohort for 78% (75/97) of patients; lifting and pressing (11%, 11/97) and contact injuries (10% [all football blocking], 10/97) were the common mechanisms that initiated symptoms. Only 10 patients (10.3%) in the posterior cohort sustained a dislocation. The most common complaints for patients with anterior instability were joint instability (80%) and pain with activities (32%). In the posterior cohort, the most common complaint was pain (90.7%); only 13.4% in this cohort reported instability as the primary complaint. Clinical outcomes after arthroscopic stabilization were significantly improved in both groups, but the anterior cohort had significantly better outcomes in all scores measured: ASES (preoperative: anterior 58.0, posterior 60.0; postoperative: anterior 94.2 vs posterior 87.7, P < .005), SANE (preoperative: anterior 50.0, posterior 60.0; postoperative: anterior 92.9 vs posterior 84.9, P < .005), and WOSI (preoperative: anterior 55.95, posterior 60.95; postoperative: anterior 92% of normal vs posterior 84%, P < .005). Conclusion: This study outlines clear distinctions between anterior and posterior shoulder instability in terms of presentation and clinical findings. Patients with anterior instability present primarily with an identifiable mechanism of injury and complaints of instability, whereas most patients with classic posterior instability have no identifiable mechanism of injury and their primary symptom is pain. Anterior instability outcomes in this matched cohort were superior in all domains versus posterior instability after arthroscopic stabilization, which further highlights the differences between anterior and posterior instability.