Musculoskeletal Disorders - Comprehensive Overview

Musculoskeletal disorders encompass a diverse group of conditions affecting bones, joints, muscles, tendons, ligaments, and connective tissues. They are the leading cause of disability worldwide, affecting more than 1.7 billion people. This article provides comprehensive coverage of major musculoskeletal and rheumatic conditions.

Osteoarthritis (OA)

Parameter	Detail
Definition	Degenerative joint disease characterized by progressive loss of articular cartilage, bone remodeling (osteophytes, subchondral sclerosis), and synovial inflammation
Prevalence	Most common joint disorder; 10% of men and 18% of women >60 years
Risk factors	Age, obesity, joint trauma/repetitive use, genetic predisposition, female sex, malalignment, metabolic syndrome
Most commonly affected joints	Knees, hips, hands (DIP, PIP, CMC), cervical/lumbar spine; typically spares MCP, wrists, elbows, ankles

OA vs RA

Feature	Osteoarthritis	Rheumatoid Arthritis
Pathophysiology	Degenerative (mechanical + biochemical)	Autoimmune (inflammatory synovitis)
Joint distribution	Asymmetric, weight-bearing (knees, hips, hands DIP/PIP/CMC)	Symmetric, small joints (MCP, PIP, wrists, MTP)
Morning stiffness	<30 minutes	>60 minutes
Stiffness after rest (gelling)	Brief (<15 minutes)	Prolonged
Inflammatory signs	Mild	Prominent (warmth, swelling, erythema)
Extra-articular features	None	Nodules, vasculitis, serositis, sicca
Rheumatoid factor / anti-CCP	Negative	Positive (70-80%)
Radiographic findings	Joint space narrowing, osteophytes, subchondral sclerosis, cysts	Erosions (marginal), juxta-articular osteopenia, symmetric joint space narrowing
Laboratory	Normal	Elevated ESR, CRP

OA Treatment

Modality	Intervention	Evidence
Non-pharmacologic	Exercise (aerobic, strengthening, aquatic), weight loss (if overweight/obese), physical therapy, occupational therapy, assistive devices (cane, walker), bracing (knee)	Strong
Topical	Topical NSAIDs (diclofenac), capsaicin	Strong (knee OA)
Oral analgesics	Acetaminophen, NSAIDs (naproxen, ibuprofen, diclofenac, celecoxib)	Strong (NSAIDs > acetaminophen)
Intra-articular	Corticosteroid injections, hyaluronic acid (viscosupplementation - controversial benefit)	Moderate (steroids); inconclusive (hyaluronic acid)
Disease-modifying	No DMOADs approved (several in trials)	–
Surgical	Total joint arthroplasty (hip, knee, shoulder) for severe, refractory OA	Strong
Emerging	Nerve growth factor (NGF) inhibitors (tanezumab) - limited availability	Moderate

Rheumatoid Arthritis (RA)

Parameter	Detail
Definition	Chronic, systemic autoimmune inflammatory disease primarily affecting synovial joints
Prevalence	0.5-1% of population; female:male 3:1
Peak age of onset	30-60 years
Pathophysiology	Autoimmune reaction (citrullinated proteins) -> synovial inflammation (T-cells, B-cells, macrophages, cytokines: TNF, IL-6, IL-1) -> pannus formation -> cartilage/bone erosion -> systemic inflammation

RA Diagnostic Criteria (ACR/EULAR 2010)

Domain	Criteria	Points
Joint involvement	1 large joint / 2-10 large / 1-3 small / 4-10 small / >10 (at least 1 small)	0-5
Serology	Negative RF and negative anti-CCP / Low-positive RF or anti-CCP / High-positive RF or anti-CCP	0-3
Acute phase reactants	Normal CRP and normal ESR / Abnormal CRP or abnormal ESR	0-1
Duration of symptoms	<6 weeks / >=6 weeks	0-1

Interpretation: Total score >=6/10 = definite RA.

RA Extra-Articular Manifestations

System	Manifestation
Skin	Rheumatoid nodules (20%, seropositive, pressure points), vasculitis, palmar erythema, pyoderma gangrenosum
Ocular	Keratoconjunctivitis sicca (secondary Sjogren), episcleritis, scleritis (can be severe)
Pulmonary	Interstitial lung disease (most common), pleuritis, pleural effusion, rheumatoid nodules (lung), obliterative bronchiolitis
Cardiac	Pericarditis, myocarditis, accelerated atherosclerosis (increased CV risk), valvular disease
Hematologic	Anemia of chronic disease, Felty syndrome (RA + splenomegaly + neutropenia), large granular lymphocyte leukemia
Neurologic	Compressive neuropathies (carpal tunnel, cervical myelopathy from atlantoaxial subluxation), mononeuritis multiplex (vasculitis)
Vascular	Vasculitis, accelerated atherosclerosis

RA Treatment

Drug Class	Examples	Mechanism	Onset	Key Monitoring
NSAIDs	Naproxen, ibuprofen, diclofenac, celecoxib	COX inhibition	Rapid	GI, renal, CV
Corticosteroids	Prednisone, methylprednisolone	Broad anti-inflammatory	Rapid	Multiple side effects with chronic use
Conventional DMARDs	Methotrexate (anchor drug), leflunomide, sulfasalazine, hydroxychloroquine	Multiple mechanisms	4-12 weeks	Methotrexate: LFTs, CBC, renal; hydroxychloroquine: retinal exam q1-5 years
TNF inhibitors	Adalimumab, etanercept, infliximab, certolizumab, golimumab	Neutralize TNF-alpha	2-12 weeks	TB screening (PPD/IGRA), HBV reactivation, injection site/infusion reactions
Anti-IL-6	Tocilizumab (IV/SC), sarilumab (SC)	Block IL-6 receptor	2-8 weeks	Neutropenia, increased LFTs, perforation risk
Anti-CD20 (B-cell)	Rituximab (IV)	B-cell depletion	4-12 weeks	Infusion reactions, HBV reactivation, PML (rare)
CTLA4-Ig (co-stimulation blocker)	Abatacept (IV/SC)	Block T-cell co-stimulation	4-12 weeks	TB screening, infection risk
JAK inhibitors	Tofacitinib, baricitinib, upadacitinib	Block JAK-STAT signaling	2-8 weeks	Zoster (VZV) risk, thrombosis, TB screening
Targeted synthetic DMARDs	–	–	–	–

RA Treatment Principles

Principle	Detail
Early diagnosis and treatment	Window of opportunity (first 3-6 months) for better outcomes
Treat-to-target	Aim for remission or at least low disease activity (DAS28, CDAI, SDAI)
Start with conventional DMARDs	Methotrexate is first-line (unless contraindicated)
Escalate to biologics if inadequate response	Add TNF inhibitor or other biologic if MTX + another csDMARD fails
Combination therapy	MTX + biologic is more effective than either alone
Regular monitoring	Disease activity, labs, side effects, functional status

Gout

Parameter	Detail
Definition	Inflammatory arthritis caused by deposition of monosodium urate crystals in joints and soft tissues due to hyperuricemia
Prevalence	4% of adults in US; male > female (3:1); increases with age
Pathophysiology	Hyperuricemia (overproduction 10% or underexcretion 90%) -> urate crystallization in joints -> NLRP3 inflammasome activation -> IL-1 beta release -> neutrophil influx -> acute inflammatory arthritis

Gout Phases

Phase	Description	Serum Urate	Management
Asymptomatic hyperuricemia	Elevated urate without gout flares	>6.8 mg/dL	No treatment unless very high or CKD; address underlying causes
Acute gout flare	Acute onset (peaks at 12-24h), monoarticular (1st MTP - podagra 50%), excruciating pain, swelling, erythema	Often elevated (may be normal during flare)	NSAIDs (indomethacin, naproxen), colchicine, corticosteroids (oral/IA)
Intercritical period	Symptom-free between flares	Variable	Consider ULT if frequent flares, tophi, CKD, or urate >9 mg/dL
Chronic tophaceous gout	Tophi (urate deposits), chronic arthritis, joint damage	Consistently elevated	ULT to target urate <6 mg/dL (<5 mg/dL for tophi resolution)

Urate-Lowering Therapy (ULT)

Drug	Mechanism	Dosing	Dose Adjustment	Key Points
Allopurinol	Xanthine oxidase inhibitor	Start 100 mg/day, titrate up to 800 mg/day (max)	Renal impairment (reduce starting dose)	First-line ULT; monitor for allopurinol hypersensitivity syndrome (rare, serious, especially in HLA-B*5801 carriers)
Febuxostat	Xanthine oxidase inhibitor	40-80 mg/day	Hepatic impairment caution	Second-line; higher CV mortality signal vs allopurinol (CARES trial)
Probenecid	Uricosuric (inhibits URAT1)	250-500 mg BID, max 2 g/day	Avoid if CrCl <30 mL/min	Avoid in overproducers (urate >800 mg/24h), nephrolithiasis risk, requires adequate hydration
Pegloticase	Recombinant uricase (IV)	8 mg IV q2weeks	–	For severe, refractory, tophaceous gout; high risk of infusion reactions, anaphylaxis; co-administer with methotrexate

Systemic Lupus Erythematosus (SLE)

Parameter	Detail
Definition	Chronic autoimmune disease with multisystem organ involvement, characterized by autoantibodies against nuclear antigens
Prevalence	20-150 per 100,000; female:male 9:1; more common in African American, Hispanic, Asian populations
Peak age of onset	15-45 years (reproductive years)

SLE Classification Criteria (SLICC 2012 or ACR/EULAR 2019)

Domain	Criteria
Constitutional	Fever, fatigue, weight loss
Mucocutaneous	Malar rash, discoid rash, photosensitivity, oral/nasal ulcers, alopecia
Musculoskeletal	Arthritis (non-erosive, >=2 joints)
Serositis	Pleuritis (pleuritic pain, pleural rub, pleural effusion), pericarditis
Renal	Proteinuria (>0.5 g/day), cellular casts, glomerulonephritis (biopsy)
Neurologic	Seizures, psychosis, mononeuritis multiplex, myelopathy, neuropathy, acute confusional state
Hematologic	Hemolytic anemia, leukopenia (<4000), lymphopenia (<1000), thrombocytopenia (<100,000)
Immunologic	ANA (+), anti-dsDNA, anti-Smith, antiphospholipid antibodies, low complement (C3, C4, CH50), direct Coombs test

Diagnosis: Requires at least one clinical and one immunologic criterion, or biopsy-proven lupus nephritis with ANA or anti-dsDNA.

SLE Treatment

Manifestation	Treatment
Mild (rash, arthritis, serositis)	Hydroxychloroquine (all patients), NSAIDs, low-dose corticosteroids, topical corticosteroids
Moderate-severe (organ-threatening)	High-dose corticosteroids, mycophenolate mofetil, cyclophosphamide, azathioprine, methotrexate
Lupus nephritis (class III/IV)	Induction: mycophenolate mofetil or cyclophosphamide + corticosteroids; Maintenance: mycophenolate mofetil or azathioprine
Severe neuropsychiatric SLE	High-dose corticosteroids + cyclophosphamide or rituximab
Refractory	Belimumab (anti-BAFF/BLyS), rituximab (off-label), anifrolumab (anti-IFNAR1)
Antiphospholipid syndrome	Anticoagulation (warfarin, DOACs - caution with DOACs in APS)

Ankylosing Spondylitis (AS)

Parameter	Detail
Definition	Chronic inflammatory arthritis primarily affecting the axial skeleton (sacroiliac joints and spine)
Prevalence	0.1-0.5%; male:female 2-3:1
Genetics	HLA-B27 positive in >90% of AS patients (5-8% of HLA-B27 carriers develop AS)
Pathophysiology	HLA-B27 misfolding, IL-23/IL-17 axis, TNF-alpha, new bone formation (entheseal new bone, syndesmophytes, bamboo spine)

AS Clinical Features

Feature	Description
Inflammatory back pain	Age <40, insidious onset, morning stiffness >30 min, improves with exercise, worsens with rest, nocturnal pain (awakening in second half of night)
Sacroiliitis	Buttock pain (may alternate), SI joint tenderness
Enthesitis	Inflammation at tendon/ligament insertion sites: Achilles tendon, plantar fascia, greater trochanter
Dactylitis	Sausage digit (finger or toe)
Extra-articular manifestations	Uveitis (anterior, 30-40%), psoriasis (10%), IBD (5-10%), aortic insufficiency, conduction abnormalities, apical lung fibrosis, amyloidosis
Spinal mobility limitation	Modified Schober test (decreased lumbar flexion), chest expansion reduced, occiput-to-wall distance increased
Radiographic findings	Sacroiliitis (MRI: bone marrow edema, erosions; X-ray: sclerosis, erosions, ankylosis); spine: syndesmophytes, shiny corners (Romanus lesions), bamboo spine

AS Treatment

Drug Class	Examples	Role	Notes
NSAIDs	Indomethacin, naproxen, meloxicam, diclofenac	First-line symptomatic treatment	Used continuously or on-demand
Physical therapy	Exercise (stretching, strengthening, postural training)	Essential, adjunctive	Improves function and reduces stiffness
TNF inhibitors	Adalimumab, etanercept, infliximab, certolizumab, golimumab	First-line biologic for active disease despite NSAIDs	Highly effective for axial disease, uveitis, enthesitis
IL-17 inhibitors	Secukinumab, ixekizumab	Second-line biologic (after TNFi) or first-line alternative	Effective for axial and peripheral disease; caution in IBD (may worsen)
JAK inhibitors	Tofacitinib, upadacitinib	Oral alternative	Emerging evidence
Sulfasalazine	–	For peripheral arthritis only (no effect on axial disease)	Used for peripheral arthritis when biologics not indicated
Corticosteroids	Systemic: limited role; IA SI joint injection	For localized inflammation	Not disease-modifying

Fibromyalgia

Parameter	Detail
Definition	Chronic widespread pain syndrome associated with fatigue, sleep disturbance, cognitive dysfunction, and mood symptoms
Prevalence	2-4% of population; female:male 2-3:1
Pathophysiology	Central sensitization (amplified pain processing in CNS), altered pain modulation, small fiber neuropathy, neurotransmitter imbalances (low serotonin/norepinephrine, high glutamate), sleep dysfunction

Fibromyalgia Diagnostic Criteria (ACR 2016)

Domain	Criteria
Widespread pain index (WPI)	Number of painful body regions (0-19)
Symptom severity scale (SSS)	Fatigue, waking unrefreshed, cognitive symptoms, somatic symptom count (0-12)
Diagnosis	WPI >=7 + SSS >=5, OR WPI 4-6 + SSS >=9; symptoms present for >=3 months; no other disorder explaining pain

Fibromyalgia Treatment

Category	Intervention	Evidence
Exercise	Aerobic exercise, strength training, stretching, tai chi, yoga	Strong
Psychological	CBT, mindfulness, acceptance and commitment therapy (ACT)	Strong
Pharmacologic: first-line	Duloxetine (SNRI), milnacipran (SNRI), pregabalin (gabapentinoid)	Strong (FDA-approved)
Pharmacologic: second-line	Amitriptyline (TCA), cyclobenzaprine (muscle relaxant), gabapentin, tramadol	Moderate
Pharmacologic: limited	NSAIDs, acetaminophen, opioids (avoid; not effective, risk of sensitization/tolerance/dependence)	Not recommended
Complementary	Acupuncture, massage, relaxation techniques, biofeedback	Moderate
Sleep management	Sleep hygiene, low-dose amitriptyline, trazodone, cyclobenzaprine	Moderate

Back Pain

Type	Duration	Causes	Prognosis
Acute	<4 weeks	Mechanical (sprain/strain 85%), disc herniation (5-10%), compression fracture (older adults)	90% resolve within 6 weeks
Subacute	4-12 weeks	Same + ongoing mechanical or nociceptive factors	Variable
Chronic	>12 weeks	Multifactorial: mechanical, neuropathic, central sensitization, psychosocial (yellow flags: fear avoidance, catastrophizing, work dissatisfaction, depression)	Poorer prognosis; risk of disability

Red Flags for Serious Back Pain Causes

Condition	Red Flag
Cauda equina syndrome	Saddle anesthesia, bowel/bladder incontinence or retention, bilateral leg weakness
Spinal infection (osteomyelitis, discitis, epidural abscess)	Fever, IV drug use, recent infection, immunocompromised, severe pain, elevated ESR/CRP
Malignancy (metastasis, multiple myeloma)	Age >50 or <20, history of cancer, unexplained weight loss, night pain, pain at multiple sites, failure to improve with conservative care
Vertebral compression fracture	Older age, osteoporosis, corticosteroid use, trauma (may be minor), focal midline tenderness
Inflammatory back pain	Age <40, insidious onset, morning stiffness >30 min, improvement with exercise, nocturnal pain

Back Pain Treatment

Intervention	Acute	Chronic
Patient education	Stay active, avoid bed rest (only 1-2 days if severe), good prognosis	Pain neuroscience education, activity pacing, self-management strategies
Physical therapy	Supervised exercise for chronic/subacute	Exercise therapy (motor control, stretching, strengthening, aerobic)
NSAIDs	First-line analgesic	Limited evidence for chronic use
Muscle relaxants	Short-term (3-7 days) for acute muscle spasm	Not recommended for chronic
Acetaminophen	Weak evidence (not better than placebo)	Not recommended
Opioids	Avoid (limited efficacy, high risk)	Contraindicated (no evidence for chronic, risk of OUD)
Gabapentinoids	Not effective for acute	Not effective for back pain without radiculopathy
Antidepressants	Not indicated	Duloxetine (moderate benefit)
Interventional	Not indicated (unless radicular)	Epidural steroid injections (for radicular pain, limited), medial branch blocks/radiofrequency ablation (for facetogenic pain)
Surgery	Cauda equina: emergent decompression; progressive motor deficit	For radiculopathy (discectomy) or spinal stenosis (laminectomy) after failed conservative care (6-12 weeks); fusion for instability/fracture/deformity
Spinal manipulation	For acute and subacute	Moderate evidence
CBT	Not indicated	Strong evidence for chronic pain

Carpal Tunnel Syndrome (CTS)

Parameter	Detail
Definition	Compression of median nerve at the wrist within the carpal tunnel
Prevalence	3-6% of adults; female > male (3:1); peak age 40-60 years
Risk factors	Repetitive hand/wrist use, obesity, pregnancy, diabetes, hypothyroidism, RA, acromegaly, amyloidosis, wrist fracture/anatomy

CTS Clinical Features

Symptom/Sign	Description
Paresthesias (classic)	Numbness, tingling in median nerve distribution (thumb, index, middle, and radial half of ring finger)
Nocturnal symptoms	Pain/numbness waking patient, relieved by shaking hand (flick sign)
Thenar weakness	Late finding: atrophy of thenar eminence, weakness of thumb abduction/opposition
Tinel sign	Tapping over median nerve at wrist reproduces paresthesias
Phalen maneuver	Wrist flexion for 60 seconds reproduces symptoms
Durkan compression test	Direct compression over carpal tunnel reproduces symptoms

CTS Diagnostic Categories

Category	Clinical Findings	NCS Findings	Management
Mild	Intermittent paresthesias, no weakness/atrophy, normal hand function	Sensory nerve conduction slowing (mild-moderate prolongation of sensory latency)	Night splinting (wrist in neutral position), activity modification
Moderate	Frequent/persistent paresthesias, occasional nocturnal awakening	Sensory + motor latency prolongation, decreased SNAP amplitude	Night splinting, consider corticosteroid injection
Severe	Constant paresthesias, thenar weakness/atrophy, functional impairment	Absent SNAP, prolonged/distal CMAP latency, decreased CMAP amplitude, fibrillations on EMG	Carpal tunnel release (open or endoscopic)

Paget Disease of Bone

Parameter	Detail
Definition	Chronic disorder of bone remodeling characterized by excessive osteoclastic bone resorption followed by compensatory osteoblastic new bone formation, resulting in enlarged, deformed, and weakened bone
Prevalence	1-2% of population >55 years; more common in European descent
Monostotic vs Polyostotic	Monostotic (single bone, 10-20%), Polyostotic (multiple bones, 80-90%)
Affected sites	Pelvis (most common), spine (lumbar), femur, tibia, skull, humerus

Paget Disease Treatment

Drug	Regimen	Indication	Notes
Bisphosphonates (first-line)	Zoledronic acid 5 mg IV (single dose); alendronate 40 mg PO daily x6 months	Pain, deformity, fracture risk, hypercalcemia, pre-surgery, high bone turnover markers	Zoledronic acid preferred (superior efficacy, longer remission)
Calcitonin	Salmon calcitonin SC/IM or nasal	Alternative when bisphosphonates contraindicated	Less effective; used for bone pain
NSAIDs	Various	Symptomatic pain relief	Not disease-modifying
Surgery	Joint replacement (if secondary OA), osteotomy (for deformity), fracture fixation	Complications	Perform after bisphosphonate treatment