USMLE MUSCULOSKELETAL PEARLS

Question 1

Erb-Duchenne palsy (“waiter’s tip”)

Answer 1

Traction or tear of the upper trunk of the brachial plexus (C5 and C6 roots); follows blow to shoulder or trauma during delivery. Findings: limb hangs by side (paralysis of abductors), medially rotated (paralysis of lateral rotators), forearm is pronated (loss of biceps).

Question 2

Rotator cuff muscles

Answer 2

Shou lder muscles that form the rotator cuff: • Supraspinatus (suprascapu lar nerve)- abducts arm initia lly (before the action of the deltoid); most common rotator cuff in jury (trauma or degeneration and impingement - tendinopathy or tear [arrow in rJ]), assessed by “empty/fu ll can” test. • l nfraspinatus (suprascapu lar nerve)- externally rotates arm; pitch ing injury. • t eres m inor (axillary nerve)- adducts and externally rotates arm. • Subscapularis (upper and lower subscapu lar nerves)-internally rotates and adducts arm. Innervated primarily by C5-C6.

Question 3

Arm abduction via the different degrees

Answer 3

Question 4

Bones of the wrist

Answer 4

Scaphoid, Lunate, Triquetrum, P is iform, Ilamate, Capitate, Trapezoid, T ( ong To Pinky, H ere Comes T he Thumb). Scaphoid (pa lpable in anatom ic snuff box [lJ) is the most commonly fractured carpa l bone, typically due to a fa ll on an outst retched hand. Complications of proxima l scaphoid frac tures include avascu la r necrosis and nonunion due to retrograde blood supply from a branch of the radia l artery .. Fracture not always seen on initia l x-ray. Dis location of lunate may cause acute carpal tunnel syndrome.

Question 5

Axillary nerve lesion

Answer 5

Axillary (CS-C6)

Cause of Injury

Fractured surgical neck of humerus

Anterior dislocation of humerus

Presentation

Flattened deltoid

Loss of arm abduction at shoulder >15 degree

Loss of sensation over deltoid muscle and lateral arm

Question 6

Musculocutaneous nerve lesion

Answer 6

Musculoskeletal nerve ( C5-C7)

Cause of Injury

Upper trunk compression

Presentation

⬇ biceps (C5-C6) or triceps (C7) reflex

weakness of forearm flexion and supination

loss of sensation over lateral forearm

Question 7

Radial nerve lesion

Answer 7

Radial nerve (C5-C7)

Cause of injury

Compression of axilla eg: due to crutches or sleeping with arm over chair (Saturday night palsy)

Midshaft fracture of humerus

Repetitive pronation/supination of forearm eg: due to screwdriver use (“finger drop”)

Presentation

Wrist drop: loss of elbow, wrist and finger extension

⬇ grip strength (wrist extension necessary for maximal action of flexors)

Loss of sensation over posterior arm/forearm and dorsal hand

Question 8

Median nerve lesion

Answer 8

Median nerve (C5-T1)

Proximal lesion

Cause of injury

Supracondylar fracture of the humerus

Presentation

“Ape hand” (loss of opposability of thumb)

Thenar atrophy

Sensory loss to dorsal and palmar aspects of lateral 3.5 fingers and thenar eminence

Distal lesion

Cause of injury

Carpal tunnel syndrome and wrist laceration

Dislocated lunate

Presentation

“Pope’s blessing” (loss of flexion of lumbricals of index and middle fingers)

loss of wrist flexion

loss of flexion of lateral fingers

deviation of wrist upon wrist flexion

Sensory loss to dorsal and palmar aspects of lateral 3.5 fingers