Elbow Common Synovial Cavity

cavitas articularis cubiti communis

The elbow joint is a compound synovial joint in which three articulations, the humeroulnar, humeroradial, and proximal radioulnar joints, share a single synovial cavity enclosed by the elbow joint capsule. This single cavity extends from the olecranon fossa posteriorly to the coronoid fossa and radial fossa anteriorly, and circumferentially encloses the radial head within the annular ligament. The common cavity explains why inflammation or effusion in one compartment affects all three.

Region: Forearm

Clinical Relevance

Clinical Notes

The common synovial cavity of the elbow is the basis for the posterior fat pad sign on lateral elbow radiographs: effusion from any cause (fracture, synovitis, haemarthrosis) displaces both the anterior and posterior fat pads outward, with the posterior fat pad being normally hidden in the olecranon fossa and becoming visible only when displaced by fluid. A single elbow aspiration at the lateral soft spot between the radial head and olecranon accesses the entire common cavity. Corticosteroid injection into the elbow joint at this point distributes to all three articulations.

Pathology

Common Injuries & Conditions

Elbow Effusion and Fat Pad Sign

Any elbow joint pathology producing haemarthrosis or synovial effusion within the common cavity distends the capsule and displaces the posterior fat pad from the olecranon fossa, producing the pathological posterior fat pad sign on lateral radiograph that in the trauma setting indicates occult fracture until proven otherwise.