Preosteoarthritic static posterior humeral subluxation in young adults (posterior shoulder instability type C1): associative factors, diagnosis, and treatment

Type C1 posterior shoulder instability represents a preosteoarthritic condition characterized by chronic, static posterior subluxation of the humeral head. Its multifactorial etiology encompasses increased glenoid retroversion, altered posterior acromial morphology, reduced humeral retrotorsion, abnormal scapulothoracic alignment, and muscular imbalances. The diagnosis relies on a comprehensive clinical history, targeted physical examination, and advanced imaging modalities that accurately detect posterior humeral head decentering, glenoid rim deficiencies, reduced posterior acromial coverage, and labral hypertrophy. Although treatment strategies range from conservative management to various surgical interventions, there has been no introduction of a procedure that reliably recenters the humeral head and halt the progression of osteoarthritis. This review aims to provide guidance in the accurate identification of C1 posterior shoulder instability by outlining its diagnostic criteria and discussing current treatment strategies.