Ulnar Collateral Ligament Injury gamekeeper’s thumb skier’s thumb
Two bones, the proximal phalanx of the thumb and the 1st metacarpal, come together at the base of the thumb to form the metacarpophalangeal (MCP) joint.  Ligaments on either side of the joint help to stabilize the thumb.  They are known as the radial and ulnar collateral ligaments.
Injuries to the ulnar collateral ligament can result in pain and sometimes instability at the base of the thumb.  This can make it painful and difficult to grip objects. 
Ulnar collateral ligament sprains and tears are diagnosed based on a history, physical exam, and magnetic resonance imaging (MRI).  Pain is often reported after a blow to the inside of the thumb that stretches the thumb to the outside.  A common cause is a ski pole getting caught and wrenching the thumb.
During the physical exam, the hand thumb are inspected for any deformity.  There is often swelling and tenderness over the ulnar collateral ligament at the base of the thumb.  Sprains are graded from 1 to 3.  A grade 1 sprain is when the ligament is stretched but not torn.  A grade 2 sprain is when the ligament is partially torn.  A grade 3 sprain is a complete tear of the ligament.  If the ligament has been completely torn, there may be instability.  A stress test is performed by pulling the thumb out to the side.  Too much laxity indicates a more severe tear.  Because this can be painful, it is sometimes done after injecting the area with a local anesthetic such as lidocaine.
Ligament tears cannot be visualized on x-rays.  However, they should be obtained to rule out other causes of thumb pain such as fractures.  They may also demonstrate an avulsion fracture associated with the collateral ligament injury.  This means that the ligament was stretched forcefully enough to pull off a fragment of bone.
Treatment for ulnar collateral ligament injuries of the thumb depends on the severity of the tear, the amount of instability, and the potential for healing without surgery.  Grade 1 and 2 sprains are typically treated nonsurgically.  The hand is placed into a thumb spica cast or brace that immobilizes the base of the thumb to give the UCL a chance to heal.  Immobilization is usually necessary for about 4 weeks.
If the UCL is torn badly enough that the thumb is unstable, the thumb may require surgery.  If the UCL injury is associated with a fracture (a piece of bone breaks off with the injured ligament) and the broken fragment is displaced, the thumb may need surgery.  Surgery involves repairing the torn ligament and, if necessary, pinning the broken bone.
Ulnar Collateral Ligament Injury
proximal phalanx
An injury to the base of the thumb can stretch and tear the ulnar collateral ligament.  If this is the result of one traumatic injury to the thumb, it is known as a skier’s thumb.  If it is the result of chronic damage or injuries to the thumb, it is known as gamekeeper’s thumb.
If a complete rupture of the UCL is suspected, an MRI may be obtained.  The torn ligament can be visualized on the MRI.
1st metacarpal
metacarpophalangeal (MCP) joint
grade 1
grade 2
grade 3
avulsion fracture associated with UCL injury
MRI of thumb
Another reason to to treat a UCL injury with surgery is if a Stener lesion is present.  This means that muscle tissue gets stuck between the torn ends of the ligament, preventing it from healing.  Surgery must be performed to free the muscle from the between the ends of the ligament and to repair the ligament.
Stener Lesion
radial collateral ligament (RCL)
ulnar collateral ligament (UCL)