The elbow complex has both medial and lateral collateral ligaments that are formed by specialized thickening of the medial and lateral elbow joint capsules. The medial collateral ligament is more commonly referred to as the ulnar collateral ligament (UCL) given the location of the ulna medially to the radius in the proximal forearm (although there is also a lateral UCL that originates on the lateral humeral epicondyle as well).
The UCL consists of three different fibrous structures which all originate from the anteroinferior surface of the medial epicondyle:
- Anterior bundle – clinically and experimentally, this is the major portion of the UCL. The origin of the anterior bundle is located at the lateral projection of the anteroinferior margins of the medial epicondyle, which coincides with the flexion/extension axis of rotation. The width of the anterior bundle is approximately 4 to 5 mm.
- Posterior bundle – extends as a thickening of the posterior capsule and is only well defined at around 90 degrees of flexion. This bundle inserts along the midportion of the medial margin of the semilunar notch. The posterior bundle is slightly thicher than the anterior bundle at 5 to 6 mm at the midportion of the fan-shaped posterior bundle.
- Transverse bundle (Cooper ligament) – offers much less stability
The ulnar nerve runs along the posterior aspect of the medial epicondyle but is not intertwined in the fibers of the UCL.