The Forgotten Rotator Cuff Muscle

The Forgotten Rotator Cuff Muscle, Part 1

By Ben Benjamin, PhD

The teres minor is the weakest of the four rotator cuff muscles. It is the brother of the infraspinatus because they mostly do the same thing; lateral rotation. It gets a lot of use when you turn the steering wheel of your car or you reach high up on a shelf to get something. The teres minor works more at slightly different angles from the infraspinatus and assists in adducting the arm.

Try this. Put your hand on top of your head with your thumb facing down toward the floor and put your other hand on the ulna side of your wrist for resistance and then push up with the bottom hand. You are now engaging a number of muscles but more of the teres minor than with any other motion. The teres minor originates at the dorsal surface of the axillary border of the scapula, that's the lateral most border.

Its fibers run obliquely upward and laterally; the upper fibers end at the teres minor tendon which is inserted into the inferior facet of the greater tubercle of the humerus, that's the little bump at the most lateral edge of the greater tubercle. The tendon of the teres minor passes across the gleno-humeral joint, and joins with the posterior part of the joint capsule. The infraspinatus and teres minor attach to the head of the humerus; and form posterior part of the rotator cuff.

They help hold the humeral head into the glenoid cavity of the scapula. They work along with the posterior deltoid muscle to laterally rotate the humerus, as well as perform adduction and assist in extension of the arm behind you. Sometimes the teres minor is actually fused with the infraspinatus.

Now that you know a little bit about this muscle and how it functions, in my next article we will talk about how to assess and treat this little known, weak muscle.

Let's continue looking at the teres minor muscle – tendon unit; how to assess it and treat it.

The arm is overhead, the elbow bent at about 15 degrees, hand facing anteriorly above the forehead. The practitioner then gently grasps the clients wrist on the ulna side. The client pushes up as you apply an equal and opposite force down. Remember with resisted tests no movement through space should occur. It often takes very little pressure to induce the pain when a teres minor injury is present so start out with very little pressure.

In this next test, we combine lateral rotation with adduction (with the hand at the Elbow). Since the teres minor is both lateral rotator and an adductor of the shoulder, this test performs both at the same time. The client bends the elbow at 90 degrees and brings it a few inches away from the body. The practitioner places one hand on the distal dorsal forearm just proximal to the wrist and the other hand on the medial elbow. Now, ask the client to push laterally at their wrist as the try to bring their elbow toward their ribs into adduction.

When the teres minor is injured one or both of these two tests are painful. The overhead push test is most likely to be painful. Differentiating a teres minor from an infraspinatus injury is a very tricky piece of assessment. The first thing to notice is weakness as well as pain. When the body is injured, it's very difficult if not impossible to maintain strength, especially for a little muscle like the teres minor.

To get this injury, people are usually very athletically active. In most cases, the infraspinatus is the one that gets injured when lateral rotation is involved. In order to engage the teres minor you need to do something either very strenuous or something that pulls you to the medial aspect of your hand (that's toward your little finger).

One way to get this injury is by doing a handstand where you have to balance yourself using all parts of the shoulder in subtle ways. In order to balance yourself side to side you have to use the teres minor and if you lose your balance slightly you'll grip with this part of the rotator cuff complex.

So, if your client has pain in one or both of the tests described here, it is highly likely that there is a strain of the teres minor. In part three, we will look at palpation and treatment.