The upper extremity (UE) is comprised of its associated muscles, nerves, and vessels, organized into anatomical compartments. The muscles cross joints to provide tone, maintain dynamic joint stability, and perform dynamic functions of the entire extremity. In addition, the arterties and veins provide nourishment and remove waste, and the nerves provide the motor and sensory innervations.
Structure and Function
The muscles of the shoulder originate from both the axial skeleton and the scapula, inserting onto the humerus. The muscles that originate from the anterior compartment of the brachium act to flex the forearm while the muscles with their origins from the posterior compartment of the brachium act to extend the forearm. Much like the brachium, the anterior muscles of the forearm act to flex the wrist joint, and the posterior muscles act to extend the wrist joint.
The hand is comprised of eleven separate compartments. These are the four dorsal interossei, three volar interossei, the thenar, the hypothenar, the adductor, and the mid-palm compartments, respectively. The dorsal interossei act to abduct the digits, and the volar interossei act to adduct the digits. The thenar and adductor compartment muscles exert activity on the thumb, while the muscles of the hypothenar compartment act on the small finger.
The embryology of the muscles in the upper extremity is a step-wise process. The first step is where myogenic progenitor cells present in somites give rise to the primary myotomes. The second wave of myogenic progenitors arises from dermomyotomes that give rise embryonic myoblasts. The myoblasts then proliferate, fuse and ultimately give rise to the primary muscle fibers.
Blood Supply and Lymphatics
The anatomical components of the UE receive vascular supply via the subclavian artery and its branches. It first transitions into the axillary artery to supply the shoulder. The axillary artery then transitions into the brachial artery to supply the brachium. The brachial artery splits distally into the radial and ulnar arteries to supply the forearm and hand. The deoxygenated blood of the upper extremity drains via the cephalic, basilic, and the brachial veins, which then flow into the subclavian vein and ultimately to the heart. The extracellular fluid is cleared by the lymphatic system. The right upper extremity feeds the right lymphatic duct, and the left upper extremity empties into the thoracic duct.
Innervation to the upper extremity derives from the brachial plexus. It is comprised of the ventral rami from C5 to T1 nerve roots. The anterior muscles of the upper extremity receive innervation from the musculocutaneous, the median and the ulnar nerves. The posterior muscles are innervated by the axillary and the radial nerves.
The musculocutaneous nerve is composed of the C5 and C6 nerve roots, the median nerve arises from the C5 to T1 nerve roots, and the ulnar nerve is composed of the C8 and T1 nerve roots. The axillary nerve forms from the C5 and C6 nerve roots, and the radial nerve is composed of the C5 to T1 nerve roots.
The collateral nerves of the brachial plexus are listed as follows:
The dorsal scapular nerve.
The long thoracic nerve.
The suprascapular nerve.
The lateral pectoral nerve.
The medial pectoral nerve.
The upper subscapular nerve.
The lower subscapular nerve.
The thoracodorsal nerve.
The medial brachial cutaneous nerve.
The medial antebrachial cutaneous nerve.
Anterior Axioappendicular Muscles
Function: flexion, adduction, medial rotation of the humerus.
Origin: clavicular head: medial clavicle, anterior sternum, costal cartilages of ribs 1 to 6 and external oblique aponeurosis.
Insertion: the lateral edge of the intertrabecular groove of the humerus.
Many physiological variants may manifest in the UE. These variations are often a cause for concern because they may lead to a misdiagnosis. The following are a few that may occur.
Under normal anatomical pretenses, the extensor indicis (EI) arises from the dorsal aspect of the distal part of the ulna and interosseous membrane. It inserts into the expansion hood of the index finger. It may present as a double tendon, and lead to clinical symptoms within the hand.
The flexor digitorum superficialis (FDS) typically originates from the medial epicondyle, coronoid process, and the anterior aspect of the radius. It inserts at the base of the middle phalanx of the second, third, fourth, and the fifth phalanges. There have been reports of the FDS with two muscles bellies, and it may lead to volar forearm compression and pain.
The extensor digitorum brevis manus is a variant muscle located in the dorsum of the hand. It leads to a variation of the fourth extensor compartment within the hand.
Surgical considerations need to evaluate anatomical variations of the UE.
The double tendon of the EI leads to an increased volume of the fourth dorsal tunnel that may lead to clinical symptoms. Surgical excision to equalize the thickness of the medial slips of the two tendons relieves mechanical stress and may correct the symptomatology.
If the FDS has two muscle bellies, then the increased mass may lead to clinical symptoms in the anterior forearm. Many previously reported cases received surgical treatment but depending on the level of symptoms, it may be treated non-operatively.
The extensor digitorum brevis manus anatomical variant can be treated with either decompression or an excision surgery. Recent studies have shown that the excisional method of complete removal of the anatomical variant proved to be beneficial because it is the most effective and has improved outcomes compared with decompression.
Compartment syndrome is an emergent clinical condition that is well-documented in the literature. It is a clinical diagnosis that is often difficult to determine because it must be assessed in a timely manner. Although the lower leg and the forearm have the most common incidence, it may occur in any muscle compartment in the body.
Even though compartment syndrome of the hand is rare, it is important to have a high index of suspicion. The hallmark of diagnosis is pain with passive stretching, yet one may see pain and swelling as well. Determining the pressure in the compartment is a key in the diagnosis. Pressures above 30 mmHg warrant an emergent surgery. Failure to act may lead to debilitating consequences.
Neuropathies are a common manifestation in the hand compartments. Carpal tunnel syndrome (CTS) is the most common neuropathy of the hand compartments, and it affects 1 to 3 persons in 1000 per year. The specific cause is unknown, but it is a multifactorial syndrome.
CTS diagnosis is via electromyography and nerve conduction testing. Treatment begins conservatively, usually starting with a reduction in provoking factors. The next step is to implement non-steroidal anti-inflammatory medications and a nightly wrist splint. The subsequent step is to use a local glucocorticoid injection to decrease the inflammation. The final intervention is to release the carpal tunnel surgically to decompress the components.
(Click Image to Enlarge)
Hand compartments: central compartment, hypothenar compartment, interosseous compartment, midpalmar space, adductor compartment, thenar space, thenar compartment
Illustration by Emma Gregory
Kumka M, A variant extensor indicis muscle and the branching pattern of the deep radial nerve could explain hand functionality and clinical symptoms in the living patient. The Journal of the Canadian Chiropractic Association. 2015 Mar [PubMed PMID: 25729087]
Chatterton BD,Moores TS,Heinz N,Datta P,Smith KD,Thomas PB, A Conservatively Managed Anatomical Variant of the Flexor Digitorum Superficialis Muscle in the Hand. Journal of hand and microsurgery. 2016 Apr [PubMed PMID: 27616828]