Q. How do you tell the difference between a disc and a ligament injury?

A. Disc and ligament injuries present differently in the following areas: nerve-specific weakness, asymmetrical limitation of movement, distal reference of pain, one-sided pain and reflex changes.

The answer to this question is complex. It's a physician's role to accurately diagnose which type of injury is present. The more knowledge you have, the better - but it's not necessary to be able to make such a diagnosis. Below are some areas you should be cognizant of when someone comes in with neck or low back pain.

Different movements of the arm and hand are controlled by different nerves. For example, resisted extension is enervated by the C7 nerve, resisted flexion primarily by the C6 nerve and resisted medial rotation by the C5 nerve. Therefore, if a nerve root is compressed by a protruding disc at the C7 level, the triceps will be weakened. If the C6 nerve is compressed by a disc, the biceps will be weakened, and so forth. If there is only a ligament injury, the muscles will not be weakened.

Distal reference means pain is referred peripherally. For instance, the pain from a nerve compression in the neck usually is felt in the lower arm and hand. With a ligament injury, the pain usually is felt more proximally, which in the case described, would mean the pain is located near the neck in the shoulder, upper arm or upper back.

Asymmetrical limitation means the person can move in one direction but not in the opposite direction. For example, the person can rotate the neck to the right, but is very limited in rotating the neck to the left; or the person can bend forward at the waist easily, but can't bend back. When a disc is affected, the pain does not switch from the right to the left; it stays on one side. In cases of ligament injury, it's common for the pain to switch from one side to the other. Pain also will come and go with a ligament injury, but not with a disc injury.

Reflexes in the arm or leg diminish with disc injuries but do not with ligament injuries. Similarly, disc and ligament injuries cause different types of numbness and disturbances in sensation.

As with most diagnoses, a practitioner needs to know the complete range of a patient's symptoms in order to make sound conclusions about their injury and appropriate treatment. In the case of disc and ligament injuries, it is easy to mistake one for the other in the absence of the appropriate knowledge. For more information, see www.benbenjamin.com, Neck and Low Back Series.