First let's remember the names of the two joints we are talking about. The Metacarpal Phalangeal or MP joint is the middle joint of the thumb and the Interphalangeal or IP joint is the distal joint of the thumb.

There are four collateral ligaments in the thumb: two ulnar collateral ligaments (or UCLs) and two radial collateral ligaments (RCLs).

Ulnar Collateral Ligament (UCL) of the Metacarpal Phalangeal (MP) Joint

The first ulnar collateral ligament is attached to the medial side of the MP joint, right next to the web of the thumb. Its function is to hold the two bones at the medial side of the thumb together so that it does not side-flex more than a tiny bit. This is one of the most commonly injured ligaments in the thumb.

Chronic sprains of this structure are often referred to as gamekeeper's thumb—a term coined in the 1950s, when many Scottish gamekeepers were observed to suffer from this injury. Acute UCL injuries at the MP joint are also known as skier's thumb, since you can sustain this type of damage by falling down with your hand strapped to a ski pole. When this ligament is injured it is painful to grasp or pinch something and the thumb begins to feel unstable.

Radial Collateral Ligament of the MP Joint

The radial collateral ligament of the MP joint, located at the lateral side of the thumb, may also get injured. However, these injuries are significantly less frequent than UCL injuries, because the RCL is not as easily stressed.

When either of the ligaments at the MP joint are injured, using the thumb becomes painful. The area around the joint may swell. As a result, the person tends to use the thumb less, and those muscles begin to weaken and atrophy. In addition, since the ligaments are no longer doing their job of keeping the bones firmly in place, the thumb may become unstable. Over time, the MP joint can degenerate and develop osteoarthritis.

Ulnar Collateral Ligament of the Interphalangeal (IP) Joint

The ulnar collateral ligament of the IP joint of the thumb is a very small but important structure that holds together the medial side of this distal thumb joint. It limits valgus (or outward) motion at the joint.

The radial collateral ligament of the IP joint holds together the lateral side of the distal thumb joint. It limits varus (or inward) motion.

Take a moment now to explore the range of motion at the IP joint. First use the joint as a hinge, moving into flexion and extension. You should have roughly 80 to 90 degrees of motion in flexion and 20-40 degrees in extension.

Then hold your distal thumb on either side of your thumbnail, and try to bend it in a side-to-side direction; you'll notice that barely any movement is possible. This is thanks to those two important collateral ligaments; together, they help to protect the joint from injury by limiting lateral movement.

Now flex your MP joint, it also should have 80-90 degrees of motion. However, a normal MP joint has practically no extension. Again, there is very little, side-to-side movement if the collateral ligaments are doing their job. The ulnar collateral ligament at the MP joint is one of the most frequently injured ligaments in the thumb.