Ulnar collateral ligament of the elbow

Key points

The ulnar collateral ligament is a thick triangular band on the inner side of the elbow, running from the humerus (upper arm bone) to the ulna¹.
A 2018 review says it has been shown to be the elbow’s main stabiliser against valgus stress (a sideways push on the elbow that opens its inner side)².
One account puts the pull on the inner elbow in overhead throwing at 290 newtons (a unit of force), above the 260.9 newtons at which its anterior (front) bundle failed in a cadaver model².
A 2022 review describes throwers’ injuries as typically a chronic wearing-thin, and contact athletes’ injuries as usually acute tears of otherwise normal ligament³.
In an ultrasound study of 185 professional pitchers in one club, the ligament thickened over the season and thinned again over the off-season⁴.

Function

One ligament, several names

The same structure at the elbow goes by four names². They are the medial ulnar collateral ligament, the ulnar collateral ligament, the medial collateral ligament and the anterior bundle². This page calls it the ligament. A ligament is a tough band of tissue that holds bones together at a joint.

It is a thick triangular band with a front and a back portion, joined by a thinner middle portion¹. Each portion has its own path:

The front portion starts on the front of the medial epicondyle of the humerus¹. It runs obliquely forward to the inner margin of the coronoid process of the ulna¹. The medial epicondyle is the inner bony bump at the elbow end of the upper arm bone. The ulna is the forearm bone on the little-finger side, and the coronoid process is a bony lip at its front.
The back portion is also triangular¹. It runs from the lower back part of the medial epicondyle to the inner margin of the olecranon, the bony point of the elbow¹.
A transverse band spans the notch that separates the coronoid process from the olecranon¹.

Engraving of the left elbow joint seen from the front and inner side. The fan-shaped ulnar collateral ligament runs from the medial epicondyle of the humerus to the coronoid process and olecranon of the ulna, the anterior ligament covers the front of the joint, and the annular ligament wraps the head of the radius. — Elbow joint ligaments from the inner sideHenry Vandyke Carter, in Henry Gray, Anatomy of the Human Body (1918), Fig. 329. Public domain, via Wikimedia Commons. (source, Public domain)

A 2018 review describes a complex of three ligaments: the medial ulnar collateral ligament proper, the posterior bundle and the transverse ligament². In 28 cadaver elbows, the ligament had anterior and posterior bundles and only occasionally a transverse bundle⁵. A cadaver is a body donated for study.

How the front bundle is built

The anterior bundle starts on the front and underside of the medial epicondyle². It ends on the sublime tubercle of the ulna, a tubercle being a small, rounded bony bump². Cadaver studies have measured it:

What was measured	What was found
Length of the ligament	Averaged 53.9 mm in a study of 10 cadaver specimens and 100 bone specimens⁶
Attachment on the ulna	Ran 29.2 mm along a ridge of bone extending down from the sublime tubercle, in the same study⁶
Area of its two ends	Averaged 45.5 mm² at its start and 127.8 mm² at its end in 13 cadaver elbows, widening toward the ulna⁷
Change in length with bend	Lengthened by 2.2 mm between 0° and 90° of flexion (bending) in 10 cadaver elbows⁸

Bundles from 34 cadaver specimens were pulled in tension in one study⁹. The anterior bundle was stiffer and stronger than the posterior bundle, and its fibres of collagen (a strong, rope-like protein) were more strongly aligned⁹.

What it holds steady

A 2018 review says the ligament has been shown to be the elbow’s main stabiliser against valgus stress². Valgus is an angle at a joint where the lower part points away from the body’s midline. Dynamic stabilisers such as the flexor-pronator muscles come later in that account². These are the forearm muscles that start at the inner side of the elbow.

A cutting study of 28 cadaver elbows reported which part does this work at each angle of flexion⁵:

The anterior bundle was made of a front band and a back band that tightened in turn as the elbow bent and straightened⁵.
At 30°, 60° and 90° of flexion the front band was the main restraint to valgus rotation, and at 120° it shared that role⁵.
The back band shared the main restraint at 120°, and was a secondary restraint at 30° and 90°⁵.
The posterior bundle was a secondary restraint to valgus rotation at 30° of flexion only⁵.

The transverse ligament is generally thought not to add significantly to elbow stability². Its direct attachment onto the anterior bundle may give it a role². In 12 embalmed cadaver elbows held at 30° of flexion, cutting it made the inner joint space significantly wider under valgus stress¹⁰. The authors took this to suggest that the transverse bundle may be involved in valgus stability¹⁰.

Breakdown

The load of a throw

The ligament is commonly injured in overhead throwing athletes². This happens when a valgus load is placed on the elbow in the late cocking and early acceleration phases of the throw². Studies have put numbers on that load and on the ligament’s strength^11,12. Torque, a turning force, is given in newton-metres (N·m) and force in newtons (N).

What was measured	What was found
Varus torque the elbow generated in a pitch	64 N·m in 26 highly skilled, healthy adult pitchers, by motion analysis¹¹
Valgus torque across 81 professional pitchers	A mean of 60 N·m, with individual pitchers’ means from 41 to 94 N·m²
Force at which the anterior ligament failed	An average of 260 N, tested with bone left at both ends¹²

Varus is an angle at a joint where the lower part points toward the body’s midline. One account puts the load on the inner elbow in overhead throwing at 290 N of pulling force². That is more than the 260.9 N at which the anterior bundle failed in a cadaver model². Earlier studies indicated that throwing asks more of the ligament than its failure strength, which suggests that muscle forces must contribute¹³.

Throwers and contact athletes

A 2022 review separates two patterns of injury³. In overhead throwers it is typically a chronic wearing-thin of the ligament from repeated valgus stress³. In contact athletes it is usually an acute tear or avulsion of otherwise normal ligament³. An avulsion is a tear in which a tendon or ligament pulls away from the bone it attaches to.

NCAA surveillance of 25 college sports recorded 109 of these injuries from 2009-10 to 2013-14¹⁴. Of those, 76.1% came from contact and 23.9% from throwing¹⁴. Men’s wrestling and men’s baseball had the highest rates¹⁴. Throwing athletes had a significantly higher share of severe injuries than contact athletes¹⁴.

Reports from outside throwing describe other loads^15,16:

Gymnastics. One report says gymnasts load the arm along its length again and again with the elbow nearly straight¹⁵. Among five female collegiate gymnasts injured over three years, MRI showed four avulsions at the lower attachment and one at the upper¹⁵. MRI is a scan that shows soft tissue in detail.
Dislocation. In 31 patients with an elbow dislocation and no fracture, surgery found the medial ligaments of every elbow completely ruptured or avulsed from the epicondyle¹⁶. A dislocation is when a bone is forced fully out of its normal place at a joint.

The UCL tear page covers how the injury is defined, who it affects and what studies of surgery found. A systematic review of 18 studies in baseball found the injuries mostly affected professional or collegiate pitchers¹⁷.

Changes seen in elbows without symptoms

On MRI of 426 skeletally mature baseball players, the grade of ligament abnormality did not differ between elbows with and without symptoms¹⁸. About 30% of elbows in both groups showed high-grade injury¹⁸.

Ultrasound, a scan made with sound waves, was used on 26 major league pitchers without symptoms¹⁹. Dark spots in the ligament were seen in 69% of pitching arms against 12% of the other arms¹⁹.

Recovery

How the ligament changes with throwing

Ultrasound studies have compared pitchers’ throwing arms with their other arms, and one season with the next^4,20.

Thicker in the throwing arm. Preseason scans over 10 years covered 368 professional pitchers without symptoms²⁰. The ligament averaged 6.15 mm thick in the throwing elbow and 4.82 mm in the other²⁰.
By high school. Among 71 uninjured school baseball players without symptoms, it was thicker in the throwing arm in the high school players only²¹.
Through the season. In 185 professional pitchers in one club, the ligament became thicker and the elbow more lax to valgus stress over the season⁴. Both changes reversed over the off-season⁴. Laxity is looseness in a joint or ligament.
Within one outing. In 25 high school baseball players who each threw 100 pitches, the inner joint space under valgus stress had widened significantly after 60 pitches²². In 15 adult male pitchers without symptoms, neither the joint gap nor the ligament’s thickness or length differed after up to 110 fastballs²³.

Among the 368 professional pitchers scanned without symptoms, 12 later injured the ligament²⁰. Their baseline thickness and joint opening were higher than the others’, but not significantly so²⁰.

Blood supply along the ligament

In 18 cadaver elbows injected with ink, the upper part of the ligament had a dense blood supply and the lower part had little²⁴. The upper part appeared to get blood from an artery of the flexor-pronator muscles, and possibly from the bone of the medial epicondyle²⁴. The authors suggested this difference may be a factor in how well injuries at each site heal²⁴.

The graft as tissue

In reconstruction, surgery rebuilds a torn ligament using a graft, a piece of tissue moved from one place to rebuild a damaged part. The palmaris longus tendon, a thin tendon of the forearm, is the graft used most often to rebuild this ligament¹². In one laboratory study it was similar to the anterior ligament, at 357 N against 260 N¹².

One ultrasound study scanned eight professional pitchers at least two years after reconstruction, and eight matched healthy pitchers²⁵. Joint space at rest, joint space under stress and laxity did not differ significantly between the groups²⁵. The rebuilt ligament was thicker, as grafts typically are²⁵. The UCL tear page covers outcomes of surgery and return to sport.

Connections

The ligament joins the humerus to the ulna on the inner side of the elbow joint¹. It lies against the triceps brachii, the flexor carpi ulnaris and the ulnar nerve¹. Part of the flexor digitorum superficialis, a forearm muscle that bends the fingers, arises from it¹.

A 2020 review names the flexor carpi ulnaris and flexor digitorum superficialis as the dynamic stabilisers of the inner elbow²⁶. The flexor carpi ulnaris is a forearm muscle that bends the wrist toward the little finger. In eight elbow specimens, loading either muscle significantly relieved strain on the ligament, the flexor carpi ulnaris most²⁷. The common flexor tendon page covers these muscles and their shared attachment.

Related exercise pages: push-up. In nine healthy men doing push-ups, the valgus torque at the elbow, which the medial ligaments oppose, rose significantly with the hands placed further up²⁸. It rose by 42% in a one-handed push-up that simulated a fall²⁸.