Radial tunnel syndrome&Posterior Interosseous Nerve (PIN) Palsy
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
Radial tunnel syndrome and posterior Interosseous Nerve palsy are both a result ofcompression of the Posterior interosseous nerve with a more severe compression occurring in posterior interosseous nerve palsy. The posterior interosseous nerve is the motor branch of the radial nerve and arises at the lateral border of the elbow when it splits from the sensory part called the Superficial Radial nerve. The PIN enters the supinator muscle’s fibrous arch called the arcade of Froshe. There are several reasons for compression of the PIN and includes a tight fascia at the extensor Carpi Radialis Brevis muscle, dense less elastic entry at the Arcade of Froshe, the presence of fibrous bands along the course of the PIN, a arterial array of vessels branching off the radial artery called the leash of Henry vessels and at times a cystic mass or ganglion from the joint that presses on the nerve in patients with rheumatoid arthritis. With mild compression, radial tunnel syndrome demonstrates pain, weakness, and arm fatigue as a result of repetitive use or from a result of a direct traumatic event. When the compression is more severe, there is severe weakness and an inability to raise the fingers at the level of the MPJ’s of the hand, which is the hallmark of PIN palsy. A PIN palsy that has not had an injury and is severe or progressive is considered to be the result of a tumor until proven otherwise. An MRI of the forearm in this case is indicated. If there is no tumor, the MRI can confirm atrophy of the respective muscles innervated by the PIN including the extensor carpi ulnaris, extensor digitorum communis, extensor pollicus longus, extensor indicis, abductor pollicus longus and extensor pollicus brevis.
Radial tunnel syndrome can accompany tennis elbow if 5%-30% of cases and must be considered and ruled out when treating tennis elbow because a release of a tennis elbow that does not address radial tunnel syndrome will result in persistent lateral elbow and dorsal forearm pain.
Treatment
Most cases of radial tunnel syndrome can be resolved with activity modification, a home exercise program, a steroid injection and a properly conducted therapy program. The key in the therapy program is stretching of the tight fascia elements of the muscles that trap and bind the posterior interosseous nerve (PIN). About three months of supervised therapy is usually needed to fully resolve the problem. The exercise program is directed at stretching tight muscles and their respective fascia. However, if tight bands, ganglion cysts or vascular leashes which are an anatomic static change are the cause of the compression, these will usually not respond to stretching and will often require surgical release. Surgery involves a short incision over the dorsal forearm that takes about fifteen minutes to perform. Only the arm is put to sleep and the use of the hand after surgery is allowed immediately.