Pronator Syndrome
Normal Elbow and Forearm Anatomy
The elbow and forearm form a functional unit whose primary role is to accurately position the hand in space. The elbow has a long lever arm and moves the forearm and hand 130 degrees covering much of the functional space used in our activities of daily living and in this sense the elbow is considered the most important single joint of the upper limb, second only to the thumb carpometacarpal joint. The functional space is defined as the space directly in front of a person’s body that lies below eye level and above the belly button. Because the elbow joint covers the entire functional space by virtue of its arc of motion, there has never been a satisfactory position to fuse an elbow joint if it were ever destroyed by injury or disease.
The elbow anatomy is best understood by thinking of it in three layers. The first layer is the bony layer, the second layer is the ligament layer and the third layer is the muscular layer within which the arteries and nerves travel.
Elbow & Forearm Elbow Joint Front View & Side View
The first layer is comprised of three bones that unite to form the elbow joint. The elbow joint is located at the lower end of the arm bone (humerus), and the proximal end (towards the shoulder) of the radius and ulna which are the two forearm bones. These three bones allow the elbow to bend (Flex) and straighten (Extend) in a hinged type motion and the forearm to rotate the hand in a palm up (supination) and palm down position (pronation). The joint surfaces of the elbow are covered by hyaline cartilage which forms a cushioned layer that allows for smooth gliding and painless elbow motion.
Elbow & Forearm Ligament LayerElbow Joint Ligament Layer
Lateral Elbow Ligament View Medial Elbow Ligament View
The second layer in the elbow and forearm is the capsular ligament layer made up of collagen tissue. The ligament layer in the forearm is called the interosseous membrane. The collagen tissue forming the elbow joint capsule is normally very thin except on the inside (medial) and outside (Lateral) sides of the elbow where thickenings in the collagen substance form the ligaments of the elbow. These ligaments provide passive stability to the elbow joint keeping the humerus, radius and ulna together, allowing them to function as a unit
Foerarm Flexor CompartmentForearm Flexor & Mobile Wad Compartments
Elbow Flexor Muscles Elbow Extensor Muscles
The third layer of the elbow and forearm is the muscle layer and is formed by the various muscle groups that pass through the elbow and forearm either originating (beginning) or ending in the elbow and forearm. The muscles of the arm that pass through the elbow include the triceps muscle in the back of the arm and the brachialis and biceps muscles in the front of the arm. The forearm muscles that cross the elbow joint are divided into three compartments with each compartment containing several muscles that work to move the elbow, forearm, wrist and hand in specific directions. The three compartments are: 1. the extensor compartment 2. the flexor compartment and 3. the mobile wad of Henry compartment. The median, ulnar, and radial nerves pass through the elbow and forearm within the muscle layers and innervate the muscles within these compartments as well as, the muscles of the hand.
Pathology
Pronator syndrome is the compression of the median nerve around the elbow and palmar side of the forearm. Because it is the same nerve that causes carpal tunnel syndrome, there is overlap in the symptoms making the diagnosis difficult, at times. The median nerve is one of the five nerves that arise from the brachial plexus and travel down the arm to innervate the forearm and hand muscles. The median nerve is responsible for most of the flexor mass muscles and also the muscles on the palmar side of the thumb. It is also responsible for the sensation of the thumb, index, long and thumb side of the ring finger. When this nerve is compressed, the sensation of these 3 ½ fingers is progressively lost and the muscle girth of the forearm muscles decrease due to muscle atrophy. There are four places where the median nerve is compressed in pronator syndrome. Depending on where the compression occurs will depend upon whether the sensation and muscle strength is affected.
Pronator syndrome can also present as a dynamic condition that only occurs with repetitive forearm activities involving wrist flexion, extension and forearm rotation. There are four sites of median nerve compression in pronator syndrome secondary to anatomic variants including fibrous bands, vascular plexus of veins or arterial malformations, and thickenings of the normal fascia in forearm muscles. The four sites of median nerve compression in pronator syndrome are:
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1. A bony or cartilaginous spur called a supracondylar process creates a tunnel above and inside the elbow that has a restricted diameter and can cause both sensory and motor deficits in the forearm and hand.
2. The pronator muscle, for which this syndrome is named, is where the median nerve travels under as it leaves the elbow and is where a fascial bands that constrict the median nerve can be found. This constriction will produce similar symptoms as in the compression produced by the supracondylar process.
3. The bicipitalaponneurosisis a normal fascial band on the volar side of the elbow which the biceps tendon contributes some fibers to is another structure the median nerve travels under and on rare occasions can compress the median nerve producing similar sensory and motor symptoms.
4. As the median nerve travels down the arm, it will divide into the anterior interosseous nerve (AIN) and the remainder of the median nerve that travels down the arm to innervate the hand. As the AIN splits from the median nerve proper, it travels under a fibrous arch of the muscles called the flexor digitorum superficialis (FDS) arch which is the fourth area of median compression in pronator syndrome. Since the AIN is mostly a motor nerve to the forearm muscles, the forearm muscles atrophy and develop weakness resulting in difficulty bending the last joint of the thumb and index finger without affecting sensation to the hand. The physical exam and EMG/NCS nerve study are the only two studies necessary to make a diagnosis. Many patients with pronator syndrome have a false negative nerve conduction study and the experience of the surgeon will dictate if surgery will be required.
Treatment
Pronator syndrome like carpal tunnel syndrome is initially treated conservatively with elimination of the offending repetitive activity, therapy, anti-inflammatory medication and a resting wrist splint. If these measures fail, or if the forearm and thenar atrophy (loss of muscle girth in the forearm and thumb) is severe or progressive, surgery is the best option. Surgery for pronator syndrome like carpal tunnel surgery is done to release the offending structures. The presence of a supracondylar process (bony spur on the inside of the arm bone) is rare, can typically be seen with a plain x-ray with the arm rotated internally 40 degrees and will have both sensory and motor findings. When present, the incision must be above the elbow. However, the majority of cases of pronator syndrome are the result of pronator muscle compression,FDS sheath edge effect or the bicipital aponeurosis in order of frequency. These anatomic tight areas are all released though a small single forearmincision. After surgery, a soft dressing is applied and motion exercises are begun immediately. Full recovery is expected within one month after surgery if severe muscle atrophy is not present.