The posterior elbow capsule spans the olecranon fossa, blending with the overlying triceps expansion, and attaches to the posterior margins of the olecranon, the coronoid fossa, and the lateral and medial epicondyles. It is thin and loose in extension, allowing full elbow extension, and becomes taut in flexion. The posterior capsule houses the posterior fat pad, which is elevated off the olecranon fossa floor by joint effusion.
Provides the posterior joint capsule containment, limits hyperextension minimally, and its reflection of the posterior fat pad on radiograph (the posterior fat pad sign) is an indirect indicator of joint effusion and associated intraarticular pathology.
The posterior elbow fat pad sign on lateral radiograph (elevation of the posterior fat pad from the olecranon fossa floor) indicates intraarticular fluid and is highly sensitive for occult radial head fracture in children and adults with a mechanism of injury consistent with elbow fracture. Posterior capsule contracture from prolonged immobilisation produces elbow flexion contracture limiting extension. Posterior capsule excision (posterior capsulectomy) as part of elbow stiffness management restores extension. Valgus extension overload in throwing athletes produces posterior olecranon impingement from shear forces on the posterior capsule.
Elevation of the posterior elbow fat pad from the olecranon fossa on lateral radiograph indicates intraarticular haemarthrosis displacing the fat pad; in the setting of acute elbow trauma with normal bony radiograph, this sign indicates occult radial head fracture that is managed with early mobilisation rather than immobilisation.
