Musculoskeletal, Skin, and Connective Tissue

Osteoblast: Builds bone by secreting collagen and catalyzing mineralization in alkaline environment via ALP. - Differentiates from mesenchymal stem cells in periosteum. - Osteoblastic activity measured by bone ALP, osteocalcin, propeptides of type I procollagen. Osteoclast: Dissolves bone by secreting H+ and collagenases. - Differentiates from a fusion of monocyte/macrophage lineage precursors.- RANK receptors on osteoclasts are stimulated by RANKL (RANK ligand, secreted by osteoblasts). RANK receptors blocked by osteoprotegerin (RANKL decoy receptor) → ↓ osteoclast activity.- Urinary deoxypyridinoline (cross-links collagen fibers) is marker. Parathyroid hormone: At low, intermittent levels, exerts anabolic effects (building bone) on osteoblasts and osteoclasts (indirect). - Chronically ↑ PTH levels (1° hyperparathyroidism) cause catabolic effects (osteitis fibrosa cystica). Estrogen: Inhibits apoptosis in bone-forming osteoblasts and induces apoptosis in bone-resorbing osteoclasts.- Causes closure of epiphyseal plate during puberty.- Estrogen deficiency (surgical or postmenopaual) → ↑ cycles of remodeling and bone resorption → ↑ risk of osteoporosis.

0-15°: Supraspinatus (suprascapular nerve) 15-100°: Deltoid (axillary nerve) >90°: Trapezius (accessory nerve) >100°: Serratus anterior (long thoracic nerve nerve)

Medial epicondylitis (golfer's elbow): Repetitive flexion (forehand shots) or idiopathic  Lateral epicondylitis (tennis elbow): Repetitive extension (backhand shots) or idiopathic

Greenstick fracture- Incomplete fracture extending partway through width of bone following bending stress.- Bone fails on tension side, compression side intact (compare to torus fracture). Torus (buckle) fracture:- Axial force applied to immature bone → cortex buckles on compression side and fractures.- Tension side (other side of cortex) remains intact.

Causes of injury: - Fractured surgical neck of humerus- Anterior dislocation of humerus Presentation:- Flattened deltoid- Loss of arm abduction at shoulder (>15°)- Loss of sensation over deltoid muscle and lateral arm

Causes of injury:- Upper trunk compression Presentation:- Loss of forearm flexion and supination- Loss of sensation over lateral forearm

Causes 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

Causes of injury:- Supracondylar fracture of humerus (proximal lesion)- Carpal tunnel syndrome and wrist laceration (distal lesion) Presentation:- "Ape hand" and "Pope's blessing"- Loss of wrist flexion, flexion of lateral fingers, thumb opposition, lumbricals of 2nd and 3rd digits- Loss of sensation over thenar eminence and dorsal and palmar aspects of lateral 31/2 fingers with proximal lesion

Causes of injury:- Fracture of medial epicondyle of humerus "funny bone" (proximal lesion)- Fractured hook of hamate (distal lesion) from fall on outstretched hand Presentation:- "Ulnar claw" on digit extension- Radial deviation of wrist upon flexion (proximal lesion)- Loss of wrist flexion, flexion of medial fingers, abduction and adduction of fingers (interossei), actions of medial 2 lumbrical muscles- Loss of sensation over medial 11/2 fingers including hypothenar eminence

Branches off immediately after passing through carpal tunnel. Motor innervation.(vs. palmar cutaneous branch, which branches off before carpal tunnel and provides sensory information) Causes of injury:- Superficial laceration of palm Presentation:- "Ape hand"- Loss of thenar muscle group: opposition, abduction, and flexion of thumb- No loss of sensation

Anterior talofibular ligament - most common ankle sprain overall, classified as low ankle sprain- Due to overinversion/supination of foot Anterior tibiofibular ligament - most common high ankle sprain

Noninflammatory thickening of abductor pollicus longus and extensor pollicis brevis tendons. - Characterized by pain or tenderness at radial styloid. - ⊕ Finkelstein test (pain at radial styloid with active or passive stretch of thumb tendons).

Idiopathic avascular necrosis of femoral head. - Commonly presents between 5-7 years with insidious onset of hip pain that may cause child to limp.- More common in males (4:1 ratio). - Initial x-ray often normal.

- Classically presents in an obese ~12-year-old child with hip/knee pain and altered gait. - Increased axial force on femoral head → epiphysis displaces relative to femoral neck (like a scoop of ice cream slipping off a cone). - Diagnosed via x-ray. Treatment: surgery

Overuse injury caused by repetitive strain and chronic avulsion of the secondary ossification center of proximal tibial tubercle. - Occurs in adolescents after growth spurt.- Common in running and jumping athletes. - Presents with progressive anterior knee pain.

Common elbow injury in children <5 years. - Caused by a sudden pull on the arm → immature annular ligament slips over head of radius. - Injured arm held in flexed and pronated position.

Erythema nodosum --> Sarcoidosis Erythema infectiosum --> Parvovirus B19 Erythema migrans --> Early-stage Lyme disease Erythema multiforme --> Herpes simplex, mycoplasma, adverse drug reactions Erythema toxicum --> Harmless rash that appears in half of all newborns

Encompasses four bony diseases associated with advanced chronic renal failure:1. Hyperparathyroid bone disease2. Osteomalacia3. Mixed uremic osteodystrophy4. Aplastic bone

Trabecular (spongy) and cortical bone lose mass and interconnections despite normal bone mineralization and lab values (serum Ca2+ and PO43-). - Most commonly due to ↑ bone resorption related to ↓ estrogen levels and old age. - Can be 2° to drugs (eg, steroids, alcohol, anticonvulsants, anticoagulants, thyroid replacement therapy) or other medical conditions (hyperparathyroidism, hyperthyroidism, multiple myeloma, malabsorption syndromes). Diagnosed by bone mineral density measurement by DEXA (dual-energy X-ray absorptiometry) at the lumbar spine, total hip, and femoral neck, with a T-score of ≤ -2.5 or by a fragility fracture (eg, fall from standing height, minimal trauma) at hip or vertebra.- One time screening recommended in women ≥65 years old. Prophylaxis: regular weight-bearing exercise and adequate Ca2+ and vitamin D intake throughout adulthood. Treatment: bisphosphonates, teriparatide, SERMs, rarely calcitonin; denosumab (monoclonal antibody against RANKL). Vertebral compression fractures – acute back pain, loss of height, kyphosis. Also can present with fractures of femoral neck, distal radius (Colles fracture).

Location: Vertebrae Characteristics: - Similar histology to osteoid osteoma.- Larger size (>2 cm). - Pain unresponsive to NSAIDs.

- 20-40 years old. Location: Epiphysis of long bones (often knee region).- Arises most commonly at distal femur and proximal tibia. Characteristics:- Locally aggressive benign tumor.- Neoplastic mononuclear cells that express RANKL and reactive multinucleated giant (osteoclast-like) cells.- "Soap bubble" appearance on x-ray.

- Most common benign bone tumor.- Males <25 years old. Location: Metaphysis of long bones (appendix like). Characteristics:- Lateral bony projection of growth plate (continuous with marrow space) covered by cartilaginous cap.- Rarely transforms to chondrosarcoma.

Location: Medulla of small bones of hand and feet. Characteristics: Benign tumor of cartilage.

- Middle age. - Location: Surface of facial bones. - Associated with Gardner syndrome.

Location: Medulla of pelvis and central skeleton. Characteristics: Tumor of malignant chondrocytes.

- Most common in Caucasians.- Generally boys <15 years old. Location: Diaphysis of long bones (especially femur), pelvic flat bones. Characteristics:- Anaplastic small blue cells of neuroectodermal origin (resemble lymphocytes).- Differentiate from conditions with similar morphology (eg, lymphoma, chronic osteomyelitis) by testing for t(11;22) (fusion protein EWS-FLI1).- "Onion skin" periostal reaction in bone.- Aggressive with early metastases, but responsive to chemotherapy.

Childhood arthritis seen in <12 year olds. - Usually presents with daily spiking fevers, salmon-pink macular rash, uveitis, and arthritis (commonly 2+ joints).- Frequently presents with leukocytosis, thrombocytosis, anemia, ↑ ESR, ↑ CRP. Treatment: NSAIDs, steroids, methotrexate, TNF inhibitors

Seronegative spondyloarthritis. - Associated with skin psoriasis and nail lesions. - Asymmetric and patchy involvement. - Dactylitis and "pencil-in-cup" deformity of DIP on x-ray. - Seen in fewer than 1/3 patients with psoriasis.

Seronegative spondyloarthritis, formerly known as Reiter syndrome.- >80% carry HLA B27 allele - Preceding infection by 2-6 weeks- Genitourinary infection: Chlamydia trachomatis- Enteritis: Shigella, Yersinia, Campylobacter, Salmonella, Clostridium difficile - Musculoskeletal: Asymmetric oligoarthritis, enthesitis, dactylitis- Ocular: Conjunctivitis, anterior uveitis- Genital: Urethritis, cervicitis, prostatitis- Dermal: Keratoderma blennorrhagicum (skin rash on palms and soles), circinate balanitis- Oral ulcers Manifestations are caused by immune complexes → joint aspirates are sterile! (reactive, not infectious, arthritis)

1° or 2° autoimmune disorder (most commonly in SLE). Diagnose based on clinical criteria including history of thrombosis (arterial or venous) or spontaneous abortion along with laboratory findings of lupus anticoagulant, anticardiolipin, anti-β2 glycoprotein antibodies. Treat with systemic anticoagulation. - Anticardiolipin antibodies can cause false-positive VDRL/RPR- Lupus anticoagulant can cause prolonged PTT that is not corrected by the addition of normal platelet-free plasma.

Features of SLE, systemic sclerosis, and/or polymyositis. - Associated with anti-U1 RNP antibodies (speckled ANA).

Shoulder muscles that form the rotator cuff:- Supraspinatus (suprascapular nerve) – abducts arm initially (before the action of the deltoid); most common rotator cuff injury (trauma or degeneration and impingement → tendinopathy or tear), assessed by "empty/full can" test.- Infraspinatus (suprascapular nerve) – externally rotates arm; pitching injury.- Teres minor (axillary nerve) – adducts and externally rotates arm.- Subscapularis (upper and lower subscapular nerves) – internally rotates and adducts arm. Innervated primarily by C5-C6.

Upper row (from thumb to pinky): Trapezium, trapezoid, capitate, hamateLower row: Scaphoid, lunate, triquetrum, pisiform - Scaphoid (palpable in anatomic snuff box) is the most commonly fractured carpal bone, typically due to a fall on an outstretched hand.- Complications of proximal scaphoid fractures include avascular necrosis and nonunion due to retrograde blood supply. Fracture not always seen on initial x-ray. - Dislocation of lunate may cause acute carpal tunnel syndrome.

Also called boxer's fracture. - Common fracture causd by direct blow with closed fist (eg, from punching a wall or individual). - Most commonly seen in 4th and 5th metacarpals.

Entrapment of median nerve in carpal tunnel (between transverse carpal ligament and carpal bones) - Nerve compression → paresthesia, pain, and numbness in distribution of median nerve. - Thenar eminence atrophies but sensation spared, because palmar cutaneous branch enters hand external to carpal tunnel. - Suggested by ⊕ Tinel sign (percussion of wrist causes tingling) and Phalen maneuver (90° flexion of wrist causes tingling). - Associated with pregnancy (due to edema), rheumatoid arthritis, hypothyroidism, diabetes, acromegaly, dialysis-related amyloidosis; may be associated with repetitive use.

Traction or tear of upper trunk: C5-C6 roots Causes:- Infants – lateral traction on neck during delivery- Adults – trauma Deficits:- Nerves affected: Axillary, suprascapular, musculocutaneous- Deltoid, supraspinatus – Abduction (arm hangs by side)- Infraspinatus – Lateral rotation (arm medially rotated) - Biceps brachii – Flexion, supination (arm extended and pronated)- Sensory loss on lateral forearm to base of thumb (loss of musculocutaneous nerve)

Traction or tear of lower trunk: C8-T1 root Causes:- Infants – upward force on arm during delivery- Adults – trauma (eg, grabbing a tree branch to break a fall) Deficit:- Intrinsic hand muscles: lumbricals, interossei, thenar, hypothenar – Total claw hand

Compression of lower trunk and subclavian vessels Causes: - Cervical rib- Pancoast tumor Deficit: Same as Klumpke palsy – atrophy of instrinsic hand muscles; ischemia, pain, and edema due to vascular compression

Lesion of long thoracic nerver, roots C5-C7 Causes:- Axillary node dissection after mastectomy- Stab wounds Deficit: - Serratus anterior – Inability to anchor scapula to thoracic cage → cannot abduct arm above horizontal position

Sensory – scrotum/labia majora, medial thighMotor – cremaster Cause: Laparascopic injury - ↓ anterior thigh sensation beneath inguinal ligament- Absent cremasteric reflex

Sensory – suprapubic regionMotor – transversus abdominis and internal oblique Cause: Abdominal surgery - Burning or tingling pain in surgical incision site radiating to inguinal and suprapubic region

Sensory – anterior and lateral thigh Cause: Tight clothing, obesity, pregnancy, pelvic procedures - ↓ thigh sensation (anterior and lateral)

Sensory – medial thighMotor – obturator externus, adductor magnus, adductor longus, adductor brevis, gracilis, pectineus Cause: Pelvic surgery - ↓ thigh sensation (medial) and adduction

Sensory – anterior thigh, medial legMotor – quadriceps, iliacus, pectineus, sartorius Cause: Pelvic fracture - ↓ thigh flexion and leg extension

Motor – semitendinosus, semimembranosus, biceps femoris, adductor magnus Cause: Herniated disc, posterior hip dislocation - Splits into common peroneal and tibial nerves

Superficial peroneal nerve:- Sensory – dorsum of foot (except webspace between hallux and 2nd digit)- Motor – peroneus longus and brevis Deep peroneal nerve:- Sensory – webspace between hallux and 2nd digit- Motor – tibialis anterior Cause: Trauma or compression of lateral aspect of leg, fibular neck fracture - Loss of sensation on dorsum of foot- Foot drop – inverted and plantarflexed at rest, loss of eversion and dorsiflexion; "steppage gait"

Sensory – sole of footMotor – biceps femoris (long head), triceps surae, plantaris, popliteus, flexor muscles of foot Cause: Knee trauma, Baker cyst (proximal lesion); tarsal tunnel syndrome (distal lesion) - Inability to curl toes and loss of sensation on sole- In proximal lesions, foot everted at rest with loss of inversion and plantarflexion

Motor – gluteus maximus Cause: Posterior hip dislocation - Difficulty climbing stairs, rising from seated position- Loss of hip extension

Sensory – perineumMotor – external urethral and anal sphincters Cause: Stretch injury during childbirth - ↓ sensation in perineum and genital area- Can cause fecal or urinary incontinence- Can be blocked with local anesthetic during childbirth and using ischial spine as a landmark for injection

Also called shin splints. - Common cause of shin pain and diffuse tenderness in runners and military recruits. - Caused by bone resorption that outpaces bone formation in tibial cortex.