Injury or disease may damage the rotator cuff, causing instability of the glenohumeral joint. The supraspinatus tendon is the most commonly torn part of the rotator cuff. Acute tears are uncommon in young persons.

The palmaris brevis, innervated by the ulnar nerve, wrinkles the skin of the hypothenar eminence and deepens the hollow of the palm, assisting the palmar grip. The muscle actually covers and protects the ulnar artery and the ulnar nerve,
which innervates it. The muscle is not by definition in the hypothenar compartment. The palmaris longus, on the other hand, flexes the hand at the wrist and tightens the palmar aponeurosis

Lumbricals 1–2 and palmar interossei 1–3 are unipennate muscles. Lumbricals 3–4 and dorsal interossei 1–4 are bipennate muscles. The deltoid is multipennate

During development, dorsal cells organize as the epimere and ventral cells organize as the hypomere. Dorsal rami innervate muscles derived from the epimere, whereas ventral rami innervate muscles derived from the hypomere. Myoblasts of the epimere form the extensor muscles of the vertebral column, and those of the hypomere give rise to muscles of the limbs and body wall. Somites and somitomeres form the musculature of the limbs.

The flexor pollicis brevis is located medial to the abductor pollicis brevis. It flexes the thumb
at the carpometacarpal and metacarpophalangeal joints and assists in opposition. Its tendon
typically contains a sesamoid bone. It is innervated by the recurrent branch of the median
nerve

The acromioclavicular joint is a plane-type synovial joint and is strengthened by the AC ligament and the coracoclavicular ligament, which is composed of the conoid and trapezoid ligaments. It is supplied by the suprascapular and thoracoacromial arteries and is innervated by the supraclavicular, lateral pectoral, and axillary
nerves. When dislocated, it is referred to as a “separated shoulder”

The sternoclavicular joint, which does not dislocate easily, is a saddle-type synovial joint
but functions as a ball-and-socket joint. It is the articulation of the sternal end of the clavicle with
the manubrium of the sternum. The joint is supplied by the internal thoracic and suprascapular
arteries and is innervated by branches of the medial supraclavicular nerve and the nerve to
the subclavius

Damage to the inferior trunks of the brachial plexus (C8–T1) affects the short muscles of the hand, resulting in “clawhand.” The patient might have grabbed a tree limb to catch himself while falling to cause this injury. “Clawhand” may also be caused by an injury to the ulnar nerve.

The radial nerve gives a superficial branch that innervates skin on the dorsum of the hand.
The radial nerve itself innervates the brachioradialis and extensor carpi radialis longus. It then
gives a deep branch that innervates the extensor carpi radialis brevis and the supinator before
continuing as the posterior interosseous nerve, which innervates the extensor digitorum, ex
tensor digiti minimi, extensor carpi ulnaris, abductor pollicis longus, extensor pollicis brevis,
extensor pollicis longus, and extensor indicis

The posterior portion of the deltoid causes extension of the arm at the glenohumeral joint. The deltoid (as a whole, but especially the central part) causes abduction, whereas the pectoralis major and latissimus dorsi cause adduction. The subscapularis causes medial rotation, whereas the infraspinatus causes lateral rotation.

The pectoralis major (clavicular head) and deltoid (anterior part) flex the arm at the glenohumeral joint. The coracobrachialis and the biceps brachii assist.

The triangle of auscultation, a good place to examine lung sounds, is bounded by the superior horizontal border of the latissimus dorsi, the medial border of the scapula, and the inferolateral border of the trapezius. The 6th and 7th ribs and the 6th intercostal space is subcutaneous.

The proximal and distal radioulnar joints are pivot-type synovial joints. The radiocarpal (wrist) joint is a condyloid type of synovial joint. Intercarpal joints are plane-type synovial joints. Metacarpophalangeal joints are condyloid types of synovial joints. Interphalangeal joints are hinge-type synovial joints.

The ulnar nerve is often injured where it passes posterior to the medial epicondyle of the humerus. The patient experiences loss of sensation in the medial part of the palm as well as in the medial 11⁄2 digits. Most intrinsic hand muscles are paralyzed, and the patient loses the ability to adduct the hand at the wrist. Patients cannot make a fist since they are unable to flex the 4th and 5th digits at the DIP joints. The resulting deformity is known as “clawhand”.

Amelia is the complete absence of one or more extremities while meromelia is the partial absence of one or more extremities. All segments of extremities are present but abnormally short in micromelia. In phocomelia, long bones are absent, and small hands or feet are attached to the trunk by short, irregular bones. In cleft hand (lobster claw deformity), the third metacarpal is absent and digits 1–2 and 4–5 are fused.

In an upper brachial plexus injury causing Erb-Duchenne palsy, one would expect damage to C5–C6, resulting in “waiter’s tip position” (adducted shoulder, medially rotated arm, and extended elbow). This results from paralysis of the deltoid, biceps, brachialis, and brachioradialis. The lateral aspect of the upper limb also experiences loss of sensation.

When the median nerve is injured, the patient often exhibits the “hand of benediction.” When the patient tries to make a fist, digits 2 and 3 remain partially extended because flexion of the PIP joints is lost in digits 1–3 and weakened in digits 4–5. Flexion of the DIP joints is lost in digits 2–3 but maintained in digits 4–5 (since the ulnar nerve controls the medial part of the flexor digitorum profundus). Flexion of the MCP joints of digits 2-3 will also be affected due to a loss of the lumbricals 1 and 2.

The carpal tunnel contains the median nerve, the four tendons of the flexor digitorum superficialis, the four tendons of the flexor digitorum profundus, and the tendon of the flexor pollicis longus

The deep branch of the ulnar nerve innervates the adductor pollicis, abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi, lumbricals 3 and 4, dorsal interossei 1–4, and palmar interossei 1–3. The median nerve innervates lumbricals 1 and 2

All carpometacarpal and intermetacarpal joints are the plane-type synovial joints except for the carpometacarpal joint of the thumb, which is a saddle joint.

The elbow is a hinge type of synovial joint, strengthened by radial and ulnar collateral ligaments. It is supplied by arteries derived from the anastomosis around the elbow and is innervated by the musculocutaneous, radial, and ulnar nerves. It is surrounded by the intratendinous olecranon bursa, the subtendinous olecranon bursa, and the subcutaneous olecranon bursa.

With paralysis of the latissimus dorsi, the patient is unable to raise the trunk as necessary for climbing. The cause could be injury to the thoracodorsal nerve (C6–C7–C8).

The clavicle varies more in shape than most other long bones and is thicker and more curved in manual workers. The right clavicle is stronger than the left and is usually shorter. The clavicle can also be pierced by a branch of the supraclavicular nerve. The clavicle is a long bone with no medullary cavity. It consists of spongy (cancellous) bone with a shell of compact bone.

Syndactyly involves abnormal fusion of fingers and toes. Cleft hand (lobster claw deformity) consists of an abnormal cleft between the 2nd and 4th metacarpal bones, with the 3rd metacarpal and phalangeal bones being absent and with digits 1–2 and 4–5 being fused. Polydactyly involves extra fingers or toes, while ectrodactyly involves the absence of a digit. Mutations in HOXA13 result in hand-foot-genital syndrome, where carpals and short digits are fused and the genitalia have altered structures.

Partial or complete absence of one or more muscles is rather common. One of the bestknown examples is total or partial absence of the pectoralis major (Poland anomaly). Similarly, the palmaris longus, serratus anterior, and quadratus femoris may be partially or entirely absent.

Injury to the dorsal scapular nerve (C4–C5) can paralyze the rhomboids, causing the scapula on one side to be located farther from the midline than that on the normal side.

The recurrent branch of the median nerve innervates the abductor pollicis brevis, flexor
pollicis brevis, and opponens pollicis, but the deep branch of the ulnar nerve innervates adductor pollicis

The deltoid atrophies when the axillary nerve (C5–C6) is damaged. Therefore, the rounded contour of the shoulder often disappears.
A loss of sensation may occur on the lateral side of the proximal part of the arm.

Fractures of the scapula typically involve the protruding subcutaneous acromion. The remainder of the scapula is well protected by muscles and the thoracic wall itself.

The surgical neck of the humerus is in direct contact with the axillary nerve, the radial nerve runs in the radial groove, the distal end of the humerus is in direct contact with the median nerve, and the medial epicondyle is in contact with the ulnar nerve.

Damage to the long thoracic nerve results in “winging” of the scapula.

The ulnar nerve gives rise to articular branches that innervate the elbow joint and muscular branches that innervate the flexor carpi  ulnaris and medial half of the flexor digitorum profundus. The palmar cutaneous branch innervates the skin of the medial part of the palm, and the dorsal cutaneous branch innervates the posterior surface of the medial part of the hand and
digits. The deep branch innervates the hypothenar muscles, adductor pollicis, interossei, and the 3rd and 4th lumbricals

A knife wound to the axilla would damage the musculocutaneous nerve and result in paralysis of the coracobrachialis, biceps, and brachialis. Therefore, flexion of the elbow joint and supination of the forearm would be weakened. The patient would also lose sensation on the lateral surface of the forearm.