The posterior bundle of the elbow MCL (ulnar collateral ligament) is a fan-shaped thickening of the posteromedial capsule running from the medial epicondyle to the medial margin of the olecranon. It is taut in flexion and adds medial stability mainly beyond about 90 degrees, complementing the anterior bundle. It forms the floor of the cubital tunnel, so it lies directly beneath the ulnar nerve.
| Origin | Posterior inferior medial epicondyle |
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| Insertion | Medial olecranon border |
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Provides medial elbow stability in deeper flexion and limits posteromedial opening of the joint.
Posteromedial elbow impingement from valgus extension overload in overhead athletes involves the posterior MCL bundle, medial olecranon, and olecranon fossa osteophytes. The moving valgus stress test provokes posteromedial pain from 120-70 degrees of flexion. Arthroscopic debridement of posteromedial osteophytes combined with UCL reconstruction (if unstable) addresses this combined pathology. Because the posterior bundle is taut in flexion, its contracture limits elbow flexion in the stiff elbow and it may be released during contracture surgery, with care for the overlying ulnar nerve.
Valgus extension overload producing medial olecranon and posterior bundle stress fractures and osteophytes in throwers managed with arthroscopic debridement.
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