Section 12.2: Acute & Chronic Musculoskeletal Injuries

Acute & Chronic Musculoskeletal Injuries: Osteoporosis, Gout, Septic Arthritis, and Low Back Pain

Musculoskeletal conditions are highly prevalent in the USMLE Step 3 exam, requiring clinicians to master diagnostic pathways, treatment ladders, and screening guidelines. Key concepts include osteoporosis prevention and management, the medical treatment of acute and chronic gout, the diagnosis of septic arthritis, and the evaluation of low back pain.

Osteoporosis Screening and Treatment

Osteoporosis is characterized by reduced bone mass and microarchitectural deterioration, leading to increased bone fragility and fracture risk. The United States Preventive Services Task Force (USPSTF) recommends screening for osteoporosis using dual-energy X-ray absorptiometry (DEXA) in women aged 65 and older, and in younger postmenopausal women whose 10-year fracture risk (determined by the FRAX tool) is equivalent to a 65-year-old white woman.

Screening results are reported as T-scores:

• Normal: T-score of -1.0 or higher.
• Osteopenia: T-score between -1.0 and -2.5.
• Osteoporosis: T-score of -2.5 or lower.
• Severe Osteoporosis: T-score of -2.5 or lower with a history of one or more fragility fractures.

Non-pharmacological management includes regular weight-bearing exercise, smoking cessation, limiting alcohol intake, and fall prevention. Patients should ensure adequate daily intake of calcium (1200 mg total daily through diet and supplements) and Vitamin D3 (800 to 1000 IU daily).

Pharmacological treatment is indicated for postmenopausal women and men aged 50 and older with a history of hip or vertebral fracture, a T-score of -2.5 or lower, or a T-score between -1.0 and -2.5 with a 10-year probability of a hip fracture >= 3% or a major osteoporotic fracture >= 20% (using the FRAX calculator).

First-line therapy consists of oral bisphosphonates (e.g., alendronate) or intravenous zoledronic acid. Oral bisphosphonates are contraindicated in esophageal disorders (e.g., strictures, achalasia), inability to stand upright for 30 minutes, or GFR <35 mL/min. Key adverse effects include pill esophagitis, atypical femur fractures, and jaw osteonecrosis.

Alternative agents include:

• Denosumab: A monoclonal antibody against RANKL that prevents osteoclast maturation. It is safe in renal impairment but requires continuous therapy, as discontinuation leads to rapid bone loss and rebound vertebral fractures.
• Teriparatide: Recombinant human parathyroid hormone (anabolic agent). Reserved for severe osteoporosis (T-score < -3.0 or multiple fractures); use is limited to 2 years due to osteosarcoma risk (contraindicated in Paget's disease or prior bone radiation).
• Raloxifene: A selective estrogen receptor modulator (SERM) with estrogen agonist effects in bone and antagonist effects in breast/uterine tissue. It is useful in postmenopausal women with osteoporosis who have a high breast cancer risk, but it increases venous thromboembolism (VTE) risk and hot flashes. Raloxifene does not reduce coronary heart disease risk and may increase fatal stroke risk in high-risk patients.

Acute Gout Management and Prophylaxis

Gout is an inflammatory arthritis caused by monosodium urate (MSU) crystal deposition in joints, triggered by rapid changes in serum uric acid. Diagnosis is established by arthrocentesis and synovial fluid analysis showing needle-shaped, negatively birefringent crystals under polarized light. Uric acid levels can be normal during an acute attack.

Acute management targets pain and inflammation. First-line options include:

• NSAIDs (e.g., indomethacin): Avoid in renal impairment, peptic ulcer disease, or heart failure.
• Colchicine: Inhibits microtubule polymerization; most effective within 36 hours. Side effects include severe diarrhea. Avoid in severe renal or hepatic impairment. Concomitant use of colchicine with strong CYP3A4 inhibitors (e.g., clarithromycin) or P-glycoprotein inhibitors is contraindicated due to the risk of fatal toxicity.
• Corticosteroids: Preferred when NSAIDs and colchicine are contraindicated. Intra-articular injection is preferred for 1–2 joints; otherwise, use systemic prednisone.

Urate-Lowering Therapy (ULT) is indicated for >= 2 attacks/year, tophi, or renal disease. Do not start ULT during an acute flare, but continue it if already established. First-line is allopurinol (xanthine oxidase inhibitor). Screen patients of Asian or African descent for the HLA-B*5801 allele to prevent SJS/DRESS. Always co-prescribe low-dose colchicine or NSAIDs for 3-6 months to prevent mobilization flares.

Septic Arthritis Diagnosis

Septic arthritis presents as acute monoarticular joint pain, swelling, warmth, fever, and restricted range of motion (knee in >50% of cases). Urgent arthrocentesis is mandatory before antibiotics. Synovial fluid shows WBC >50,000/mm³ (>75% polymorphonuclear cells).

Pathogens:

• Staphylococcus aureus is the most common cause overall.
• Neisseria gonorrhoeae occurs in young, sexually active adults, often presenting as disseminated gonococcal infection (triad of tenosynovitis, migratory polyarthralgias, and pustular lesions). Synovial fluid cultures are often negative; confirm via mucosal NAAT.
• Pseudomonas aeruginosa is common in intravenous drug users.

Management requires joint drainage (arthroscopy, arthrotomy, or needle aspiration) and empiric IV vancomycin plus ceftriaxone.

Acute and Chronic Low Back Pain Evaluation

Low back pain is classified as acute (<6 weeks), subacute (6-12 weeks), or chronic (>12 weeks). Assess for red flags:

• Malignancy/Infection: Cancer history, unexplained weight loss, night pain, fever, IV drug use, or immunosuppression.
• Cauda Equina Syndrome: Saddle anesthesia, urinary retention, bowel incontinence, or progressive motor weakness.

Without red flags, diagnose mechanical back pain; imaging is not indicated. Treat conservatively with NSAIDs, heat, and activity (avoid bed rest). Spinal MRI is indicated for cauda equina, epidural abscess, or cord compression. Plain films and inflammatory markers (ESR, CRP) are used for suspected malignancy or infection without neurological deficits. Radiculopathy presents with a positive straight leg raise and is managed conservatively for 6 weeks unless progressive deficits occur.