The forearm is the region of the body spanning from the elbow to the wrist. It consists of several muscles and an extensive neurovascular network encased in three compartments. These include the volar compartment, dorsal compartment, and the mobile wad. The mobile wad consists of three muscles: the brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis. The dorsal compartment contains nine muscles, four superficial groups and five deep.
The dorsal compartment of the forearm is also known as the extensor compartment since the muscles primarily function to extend the wrist and digits of the hand. The volar compartment is known as the flexor compartment since the muscles primarily function to flex the wrist and digits. The mobile wad contains the brachioradialis which is the strongest flexor of the forearm.
The dorsolateral somite cells migrate to the upper limb around the fourth week to form the muscles of the upper limb. Many transcription factors are activated at various stages, allowing the limb buds to develop. Connective tissue derived from the lateral plate of the mesoderm divides the forearm into extensor (dorsal) and flexor (volar) compartments.
The forearm has a vast vascular network. The brachial artery divides at the elbow giving rise to the ulnar and radial arteries which reside along the ulnar and radial aspects of the volar forearm compartment — the radial artery which supplies the posterolateral forearm courses lateral to the flexor carpi radialis. The ulnar artery supplies the anteromedial aspect of the forearm and gives rise to the anterior and posterior interosseous arteries which course in the deep volar and dorsal compartments respectively bringing supply to deeper structures of the forearm.
Three primary nerves supply the majority of muscles in the forearm. The radial nerve and its branches supply the mobile wad of Henry and the dorsal compartment. The ulnar nerve and median nerve, and their respective branches supply the volar compartment. The radial nerve provides innervation to the anconeus, brachioradialis, and extensor carpi radialis longus. The posterior interosseous nerve, a branch of the radial nerve provides innervation to the rest of the muscles in the dorsal compartment of the forearm. The ulnar nerve provides innervation to the flexor carpi ulnaris and ulnar half of the flexor digitorum profundus. The median nerve provides innervation to the palmaris longus, flexor carpi radialis, pronator teres, and flexor digitorum superficialis. The anterior interosseous nerve, a branch of the median nerves supplies flexor pollicis longus, radial half of the flexor digitorum profundus, and pronator quadratus.
The dorsal compartment of the forearm contains a superficial and deep muscle group. The superficial group includes the extensor digitorum communis, extensor digiti minimi, extensor carpi ulnaris, and anconeus. These muscles are relatively subcutaneous as there is usually not much subcutaneous fat on the dorsal aspect of the forearm. The primary function of these muscles is digit and wrist extension, with the extensor carpi ulnaris assisting in ulnar deviation of the wrist during wrist extension. The deep group contains the supinator which supinates the forearm. It also contains the abductor pollicis longus and extensor pollicis brevis and the extensor pollicis longus which function to extend and abduct the thumb. The deep dorsal compartment also contains the extensor indices whose tendon lays radial to the extensor digitorium tendon to the index finger, and it assists in index finger extension. Similarly, the tendon of the extensor digiti minimi of the superficial dorsal compartment travels ulnar to the extensor digitorium tendon the small finger, and assists in small finger extension.
The mobile wad contains three muscles. The brachioradialis originates over the proximal two-thirds of the lateral supracondylar ridge of the humerus, while both extensor carpi radialis longus and extensor carpi radialis brevis originate over the distal lateral supracondylar ridge. The brachioradialis inserts just proximal the radial styloid. The extensor carpi radialis longus and extensor carpi radialis brevis insert on the second and third metacarpal base respectively.
The volar compartment consists of a deep and superficial layer. The superficial volar compartment contains the pronator teres, the flexor carpi radialis, the flexor carpi ulnaris, and the flexor digitorum superficialis. The palmaris longus is also present in the superficial compartment but is often absent in many people. All these muscles originate as part of the common flexor origin at the medial epicondyle. The deep volar compartment of the contains the flexor digitorum profundus, flexor pollicis longus, and pronator quadratus. The carpal tunnel consists of nine tendons and the median nerve. It includes four tendons of the flexor digitorum profundus, four tendons of the flexor digitorum superficialis and one tendon from flexor pollicis longus.
One of the most common variations in forearm anatomy is the presence of the palmaris longus. It is present in most people but can be absent in up to 26% of individuals. Additionally, when present, the palmaris longus itself can have many variations. It usually has a tendinous proximal and long tendinous distal end, and a spindle-shaped muscle belly. Cadaveric studies have shown several variations such as a reversed palmaris longus coexisting with an additional abductor digiti minimi muscle, digastric palmaris longus with intermediate muscle belly and duplication.
Martin-Gruber connection, which occurs in approximately 15 to 18% of people is just one example of the many neurovascular variations in the forearm. It involves the median nerve connecting to the ulnar nerve during its course under the flexor digitorum superficialis. This cross-connection allows for median nerve innervation to some intrinsic muscles of the hand that are usually supplied by the ulnar nerve.
Compartment syndrome of the forearm is a relatively common clinical problem, and prompt surgical intervention can be limb saving. Orthopedic, vascular and general surgeons should be familiar with surgical decompression of the forearm.A standard OR table with a hand table is preferred. Starting on the volar forearm, an incision should begin just proximal to the wrist crease and extend down just distal to the ulnar aspect of the elbow flexion crease. Go through the subcutaneous tissue and the fascia into the volar compartment. Make a small nick in the fascia, and slide Mayo scissors down the length of the forearm compartment releasing the compartment. Assess the deep and superficial muscles. Now mark the second radial sided volar incision. This incision is a middorsal straight line incision that begins 3 cm proximal to the wrist crease and extends down to the radial aspect of the elbow flexion crease. After going through the subcutaneous tissue and fascia along the entire length of the radial sided incision, finger dissect and release the dorsal compartment and mobile wad. Examine all the muscles in both compartments. All nonviable necrotic tissue in any fasciotomy should be debrided. However, after the fascial releases, muscles will bulge out and even some slightly devitalized appearing muscle may recover over time. After adequate release on all three compartments has been achieved many dressing options can be explored. There are reports of good outcomes with negative pressure wound therapy, tissue expanders, wet to dry dressings. The patient should be monitored and a repeat surgical irrigation and debridement in 1 to 3 days may be required prior to definitive closure.
Compartment syndrome is primarily a clinical diagnosis. Stryker monitoring can is an option when the clinical exam is unequivocal, or the patient is intubated and sedated. A delta pressure of less than 30 is more specific to compartment syndrome than an absolute compartment pressure of greater than 30. Delta pressure is the difference between diastolic blood pressure and compartment pressures measured by the stryker monitor. The history usually includes a swollen forearm following some form of trauma or a vascular insult. Multiple bone fractures, especially when on the ipsilateral extremity has a strong association with compartment syndrome. However, isolated fractures such as both-bone forearm fractures in adults and supracondylar humerus fractures, especially those with associated vascular injuries, in kids are frequent causes of compartment syndrome. The flexor digitorum profundus and flexor pollicis longus are often injured most severely secondary to their deep location. Tense and non-compressible compartments are more specific to compartment syndrome than a softly swollen forearm. During the physical exam, it is vital to assess all three compartments, as the pathology can be limited to just one compartment. The most sensitive exam finding is a pain with passive stretch or extension of the fingers or wrist. Pulselessness, paresthesias, and loss of motor function are late findings, at which point ischemic insults have already taken place.
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