Musculoskeletal disorders encompass conditions affecting bones, joints, muscles, tendons, ligaments, and the spine. They are the leading cause of disability worldwide, affecting over 1.7 billion people. The burden is driven by aging populations, increasing obesity, and sedentary lifestyles.

Comparison of healthy and osteoporotic vertebrae — Comparison of healthy (left) and osteoporotic (right) vertebral bodies. Osteoporosis causes progressive bone loss, leading to fragility fractures of the spine, hip, and wrist. Source: Wikimedia Commons.

Osteoarthritis

Osteoarthritis is the most common joint disorder, characterized by progressive degeneration of articular cartilage, subchondral bone remodeling, osteophyte formation, and synovial inflammation. It results from mechanical stress on a background of genetic, metabolic, and age-related susceptibility.

Knee joint anatomy from Gray's Anatomy — Anatomy of the knee joint showing the femur, tibia, patella, and cruciate ligaments. The knee is one of the most commonly affected joints in osteoarthritis. Source: Gray's Anatomy (1918).

Pathophysiology. Cartilage degradation exceeds synthesis. Matrix metalloproteinases (MMPs) and aggrecanases cleave cartilage matrix components. Chondrocytes undergo phenotypic changes and apoptosis. Subchondral bone becomes sclerotic, and marginal osteophytes form as the joint attempts to stabilize.

Clinical presentation. Pain is the dominant symptom — initially weight-bearing and activity-related, progressing to rest pain and night pain. Morning stiffness lasts less than 30 minutes. Joints commonly affected: knees, hips, hands (DIP, PIP, and CMC-1 joints), and spine. Crepitus, bony enlargement (Heberden and Bouchard nodes in the hands), and restricted range of motion are typical.

Diagnosis. Clinical diagnosis based on age >45, typical symptoms, and characteristic joint distribution. Radiographs show joint space narrowing, osteophytes, subchondral sclerosis, and cysts. MRI detects early cartilage changes and associated pathology (meniscal tears, ligamentous injury).

Management. Core treatment includes weight loss, exercise (strengthening, aerobic, aquatic), and patient education. Pharmacologic therapy: acetaminophen, topical NSAIDs, oral NSAIDs (with gastroprotection), and intra-articular corticosteroids. Hyaluronic acid injections have limited efficacy. Total joint arthroplasty (knee, hip) is definitive for end-stage disease.

⚠ Clinical Correlation

OA is a degenerative non-inflammatory (or mildly inflammatory) joint disease affecting DIP, PIP, CMC-1, knees, hips, and lumbar spine. Pain is mechanical (worse with activity, better with rest). Morning stiffness <30 minutes. Rheumatoid arthritis is a systemic inflammatory autoimmune disease affecting MCP, PIP, wrist, and MTP joints symmetrically. Pain is inflammatory (worse at rest, morning stiffness >60 minutes, improves with activity). Extra-articular manifestations (rheumatoid nodules, interstitial lung disease, vasculitis) are common in RA.

Rheumatoid Arthritis

RA is a chronic, systemic autoimmune disease characterized by inflammatory arthritis and extra-articular involvement. The primary target is the synovium, which becomes hyperplastic, infiltrated by immune cells, and invasive, leading to cartilage and bone erosion.

Pathophysiology. The trigger is unknown, but genetic susceptibility (HLA-DRB1 shared epitope) and environmental factors (smoking, periodontitis) are implicated. Autoantibodies (rheumatoid factor, anti-CCP) develop years before clinical onset. Pro-inflammatory cytokines — TNF-alpha, IL-6, IL-17, and GM-CSF — drive synovitis and joint destruction.

Clinical presentation. Insidious onset of symmetric inflammatory polyarthritis affecting the small joints of the hands and feet (MCP, PIP, wrists, MTP). Morning stiffness >60 minutes. Extra-articular: rheumatoid nodules, interstitial lung disease, pericarditis, vasculitis, and secondary Sjögren syndrome.

Diagnosis. The 2010 ACR/EULAR classification criteria combine joint involvement, serology (RF, anti-CCP), acute-phase reactants (ESR, CRP), and symptom duration. Imaging (X-ray, ultrasound, MRI) detects erosions and synovitis.

Treatment. Early, aggressive treatment prevents joint damage. Methotrexate is first-line. Combination therapy with other conventional DMARDs (sulfasalazine, leflunomide) or biologic DMARDs (TNF inhibitors, abatacept, rituximab, tocilizumab) and targeted synthetic DMARDs (JAK inhibitors: tofacitinib, baricitinib, upadacitinib) is used for inadequate response to methotrexate. Glucocorticoids provide rapid symptom control but are minimized due to toxicity.

Osteoporosis

Osteoporosis is a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration, leading to increased fracture risk. It is most common in postmenopausal women but affects older men and younger individuals with secondary causes.

Pathophysiology. Peak bone mass is achieved by age 30. After menopause, estrogen deficiency accelerates bone resorption by osteoclasts. In aging, both bone formation (osteoblast activity) declines and resorption increases. The RANKL/RANK/OPG pathway is central to osteoclast regulation.

Diagnosis. Dual-energy X-ray absorptiometry (DXA) measures bone mineral density at the hip and spine. T-score compares BMD to young adult mean: normal (T-score >−1.0), osteopenia (−1.0 to −2.5), osteoporosis (≤−2.5). FRAX estimates 10-year fracture probability.

Prevention and treatment. Calcium and vitamin D supplementation, weight-bearing exercise, and fall prevention. Pharmacotherapy: bisphosphonates (alendronate, risedronate, zoledronic acid), denosumab (RANKL inhibitor), teriparatide (PTH analog), romosozumab (sclerostin inhibitor), and raloxifene (SERM). Sequential therapy (anabolic followed by antiresorptive) is emerging for high-risk patients.

Back Pain

Low back pain is the leading cause of years lived with disability globally. Most is non-specific (mechanical) and self-limiting. Red flags (trauma, fever, weight loss, neurological deficits, age >50, cancer history, IV drug use) warrant urgent investigation for serious pathology (fracture, infection, malignancy, cauda equina syndrome).

Specific causes. Lumbar disc herniation causes radicular pain (sciatica) with or without neurological deficits. Lumbar spinal stenosis causes neurogenic claudication (leg pain with walking, relieved by sitting or flexing forward). Spondylolisthesis, vertebral compression fracture (osteoporotic or traumatic), and facet joint arthropathy are other causes.

Management. Non-specific back pain is managed with education, activity maintenance, and NSAIDs. Physical therapy, spinal manipulation, and cognitive behavioral therapy are effective. Epidural corticosteroid injections provide short-term relief for radicular pain. Surgery (discectomy, laminectomy, fusion) is reserved for neurological compromise or refractory pain.

Gout and Crystal Arthropathies

Gout is caused by monosodium urate crystal deposition in joints and soft tissues due to hyperuricemia. Acute gout presents with rapid-onset, excruciating monoarthritis — typically the first metatarsophalangeal joint (podagra). Diagnosis is confirmed by polarized light microscopy showing negatively birefringent needle-shaped crystals. Acute flares are treated with NSAIDs, colchicine, or corticosteroids. Long-term urate-lowering therapy (allopurinol, febuxostat, uricosurics) is indicated for recurrent flares, tophi, or nephrolithiasis.

Soft Tissue Disorders

Tendinopathy. Chronic tendon pain without acute inflammation. Common sites: rotator cuff (supraspinatus), lateral elbow (tennis elbow), patellar tendon, and Achilles tendon. Management includes eccentric strengthening exercises, activity modification, and NSAIDs.

Bursitis. Inflammation of bursa sacs. Olecranon, prepatellar, trochanteric, and subacromial bursitis are common. Treatment is rest, NSAIDs, aspiration if infected, and corticosteroid injection.

★ Key Concept

Prolonged immobilization from musculoskeletal conditions causes systemic deconditioning: muscle atrophy (loss of 5% muscle mass per week), joint contractures, bone demineralization, thromboembolism, pressure ulcers, atelectasis, pneumonia, constipation, and depression. Early mobilization, whether through physical therapy, occupational therapy, or simple activity, prevents these complications. For patients with fractures, early weight-bearing when stable and early range-of-motion exercises preserve function.

Summary

Musculoskeletal disorders range from degenerative conditions (osteoarthritis, osteoporosis) to systemic inflammatory diseases (rheumatoid arthritis) to acute injuries (fractures). OA and RA, though both affecting joints, have fundamentally different pathophysiology and management. Osteoporosis is preventable and treatable but remains underdiagnosed until fracture occurs. Rheumatoid arthritis requires early aggressive therapy to prevent irreversible joint damage. Across all conditions, physical activity, weight management, and smoking cessation are foundational interventions that reduce disability and improve quality of life.