Posterior shoulder instability.

Posterior shoulder instability (PSI) is defined by dynamic, recurrent and symptomatic partial or total loss of posterior joint contact. Anatomic risk factors comprise ligament hyperlaxity, glenoid retroversion or dysplasia, and high horizontal acromial morphology. Associated anatomic lesions comprise labrum lesions, posterior glenoid erosion and/or fracture, and anterior humeral head notching. We distinguish two subcategories of PSI: functional and structural, respectively without and with anatomic lesions. In both categories, there may be anatomic risk factors. Clinically, functional PSI involves reproducible asymptomatic voluntary subluxation or sometimes reproducible involuntary subluxation. Functional PSI implicates impaired external-rotation rotator cuff and scapular stabilizer muscle activity. Treatment is non-operative, by rehabilitation and shoulder pace maker. Structural shoulder instability involves anatomic lesions, often due to iterative microtrauma; pain is the most frequent symptom. It usually concerns young athletic subjects, but the clinical forms of structural and of anterior shoulder instability are not superimposable. Treatment may be surgical; arthroscopic labrum repair is effective in the absence of significant bone lesions, whereas otherwise posterior bone block is the treatment of choice. Surgical treatment of scapular features underlying structural PSI is improved by 3D preoperative planning, cutting guides and dedicated internal fixation systems. LEVEL OF EVIDENCE: expert opinion.