Supinator ligament syndrome is a nerve compression syndrome. This is a nerve impairment that progresses slowly and recurs again and again due to slight pressure.
A prerequisite for the development of supinator ligament syndrome is the presence of natural bottlenecks within the extremities (arm or leg). One such bottleneck is located below the elbow joint under the supinator muscle. This muscle is responsible for rotating the forearm and hand into a position in which we can look into the open palm from above.
The supinator muscle that functions in this way has nothing to do with the affected nerve. It merely indicates the localization of the nerve compression in supinator ligament syndrome.
Illustration of the supinator muscle at the elbow joint © SciePro | AdobeStock
The radial nerve and supinator ligament syndrome
Supinator ligament syndrome affects a branch of theradial nerve(radial nerve). This nerve controls all the muscles that extend the wrist, the four long fingers and the thumb.
At the level of the elbow joint crease, the radial nerve divides into two branches:
- a superficial branch(superficial ramus) and
- a deep branch(ramus profundus).
The superficial branch mainly contains nerve fibers that are responsible for the sensitivity
- on the back of the forearm,
- on the back of the hand and
- on the back of the fingers
back of the fingers. This means that this nerve branch sends touch or pressure on the back of the hand to the brain as electrical stimuli. The brain processes the stimuli so that we can feel the pressure.
Three nerves that run in the arm. The radial nerve is affected by supinator ligament syndrome © öogo3in1 | AdobeStock
The lower-lying second branch is responsible for motor function. It transmits electrical signals from the spinal cord to the muscles in the forearm. These muscles have the function of extending the wrist, fingers and thumb. It therefore contains no sensory nerve fibers, but motor fibers.
A small motor nerve branch is a regular anatomical exception. It leaves the superficial, sensitive radial nerve branch and leads to the wrist extensor muscle(extensor carpi radialis brevis muscle).
This exception is of great importance in the classification of the neurological deficits of supinator ligament syndrome. It is also important for the surgeon.
Impairment of this nerve branch causes motor deficits (paralysis) in supinator ligament syndrome. Sensory disturbances or pain do not occur.
Structure of a nerve
Each peripheral nerve consists of several grouped nerve bundles that are enclosed and held together by a relatively coarse sheath structure, the epineurium. Blood vessels run between the nerve bundles.
The nerve bundles in turn consist of thousands of individual nerve fibers. Each of these nerve fibers is surrounded by an insulating layer, comparable to the rubber coating of a copper wire. This insulating layer prevents electrical impulses from jumping from one nerve fiber to the next. It also influences the speed of transmission of this nerve stimulus.
The nerve fiber is a cylindrical cell process whose actual cell is located in the spinal cord. The nerve cell process can be up to 2 meters long within a peripheral nerve.
Cross-section of a nerve. Thousands of nerve fibers are combined into nerve bundles that are surrounded by the epineurium © crevis | AdobeStock
Development of the supinatorlogeny syndrome
The fatty insulating layer around each individual nerve fiber is highly sensitive to pressure. Repeated pressure effects on a nerve can damage this insulating layer. This reduces the speed of excitation propagation along the nerve fiber.
This unfavorable influence then leads to the neurological deficits of supinator ligament syndrome. Measurements of the nerve conduction velocity can detect the damage.
Supinator ligament syndrome affects the ramus profundus, i.e. the deep branch of the radial nerve (radial nerve). There is no pain, but a motor failure (paralysis).
The four long fingers can be lifted less and less in supinator ligament syndrome. The same applies to the thumb: its extension from the back of the hand becomes weaker and weaker.
It is striking that, in contrast, the lifting of the back of the hand remains quite strong. This can be explained by the aforementioned nerve branch from the superficial branch of the radial nerve to the wrist extensor muscle.
The pressure compression of the deep palmar branch(ramus profundus) under the supinator muscle in the supinator ligament therefore takes place at a point where the small motor muscle branch for the thumb-side wrist extensor is already pulled away.
In supinator ligament syndrome, this results in the characteristic picture of long finger and thumb extensor weakness with preserved wrist extensor ability.
The paralysis without accompanying pain that is characteristic of supinator ligament syndrome is easy for neurologists to recognize. Using electroneurography (measurement of nerve conduction velocity), he can confirm that the transmission of the electrical stimulus for finger extension is slowed down.
In addition, probing the muscles on the forearm supplied by the deep branch of the radial nerve reveals changes in the potentials that can be recorded. Electromyography is responsible for this. It measures the electrical muscle activity.
Imaging procedures such as sonography(ultrasound) or MRI(magnetic resonance imaging) cannot reliably diagnose supinator ligament syndrome. However, in individual cases they can detect or rule out a lipoma (fatty tissue tumor) located under the supinator muscle.
A lipoma can develop under the supinator muscle for reasons that are as yet unknown. This can also cause the nerve compression symptoms of supinator ligament syndrome.
Supinator ligament syndrome can only be treated surgically.
The compression site is a small crescent-shaped, tendinous structure from which muscle fibers of the supinator muscle pull away. It crosses the course of the deep branch of the radial nerve. When pressure is applied, the edge of this tendon presses into the nerve.
The aim of supinator ligament syndrome surgery is to give the insulating layer around each nerve fiber the chance to recover completely. To do this, the surgeon must notch and cut the tendinous structure that runs across the nerve.
At this point, several small vessels cross the course of the nerve at the same time. The nerve branch itself only has the diameter of a pencil lead. Great care is therefore required during supinator ligament syndrome surgery to avoid damaging the small nerve itself.
Even more care must be taken to safely expose and maintain the continuity of the even thinner nerve branch above the beginning of the supinator muscle canal, which runs to the thumb-side wrist extensor.
If this is not done, this wrist extensor will also fail!
Supinator ligament syndrome surgery is therefore demanding. It does not require microscopic magnification. However, the minimally invasive keyhole technique is not an option as a surgical method. There is too great a risk of damaging the nerve branch that has just been exposed. This is so thin that an attempt to reconstruct it may not be successful.
The skin incision and choice of approach must therefore be large enough to safely identify and protect the nerve structures in question.
There is no effective drug treatment for so-called bottleneck syndromes of peripheral nerves.
The local infiltration of cortisone can at best have a temporary effect, but cannot solve the problem itself.
Postoperative improvement in motor deficits does not occur immediately. The result is more likely to be seen after weeks or months, depending on the extent of damage before the supinator ligament syndrome operation.
However, there should be no deterioration after supinator ligament syndrome surgery due to damage to anatomical features of the nerve distribution. However, the patient must be informed about the risk of at least a potential loss of wrist extensor function.
If a lipoma is found under the supinator muscle, the planned procedure must also be extended. However, this is rarely the case.
If the wrist extensor function is preserved, there is no need to worry about the tendons of the long finger and thumb extensors overstretching due to constant drooping. In contrast to radial nerve tears caused by injury, no wrist splint is required.
Physiotherapy or individual training controlled by the head must focus on the extension movements of the long fingers and thumb.
Follow-up checks are also recommended after the operation. Electromyography should be carried out once or twice for this purpose.
Supinator ligament syndrome is a slowly progressive nerve compression of the deep branch of the radial nerve. The compression site is about 3 cm below the elbow joint crease. There is no pain, but there is increasing paralysis when lifting (stretching) the long fingers and thumb.
In contrast to this paralysis, the ability to lift the wrist is retained.
Decompression surgery is necessary, but requires great care in order to preserve the wrist extensor function.
A minimally invasive procedure is currently not recommended due to the specific anatomical situation described.
