15.3 Musculoskeletal Diagnosis

The current FNP-BC Test Content Outline scores Musculoskeletal as one of the 13 body systems and lists MSK drug agents (NSAIDs, urate-lowering therapy, osteoporosis medicines, and related classes) separately. Domain II still comes first: name the joint disease, decide who needs a radiograph, and recognize the child whose limp is not “growing pain.”

OA, RA, gout, and the septic joint

Osteoarthritis is mechanical cartilage failure: Heberden nodes at the DIP, Bouchard nodes at the PIP, squaring of the first carpometacarpal joint, and bony crepitus. Labs are normal. Rheumatoid arthritis is a systemic autoimmune synovitis. Prolonged morning stiffness, squeeze tenderness of the MCPs, and a positive anti-CCP antibody move you to early rheumatology — do not wait for erosions. Gout is intracellular monosodium-urate needles that are negatively birefringent on polarized microscopy. A high serum uric acid supports the story but does not diagnose an acute monoarthritis by itself, and a normal uric acid does not exclude an attack. Start anti-inflammatory treatment for the flare (colchicine, NSAID, or steroid if no infection) and delay starting allopurinol until you have a plan, not during an undiagnosed septic joint.

Septic arthritis is the item you cannot miss. A single hot joint plus fever, inability to bear weight, or immunosuppression is tap and emergency care. Do not inject corticosteroid into a joint that might be infected. Gonococcal disease can look migratory with pustules in a sexually active adult; still, a closed, red, immobile knee in a febrile patient is not an office “gout trial.”

Back pain: nonspecific, radicular, and red flags

Most acute back pain is nonspecific mechanical pain. No imaging is indicated in the first 6 weeks if there are no red flags and the neurologic exam is nonfocal. Stay active, use heat, and teach that bed rest delays recovery. Radicular pain follows a dermatome, may have a positive straight-leg raise, and still does not need early MRI in a neurologically stable patient — most disc herniations improve. Image sooner if progressive motor loss appears.

Red flags change the destination:

A 45-year-old who lifted a box, has unilateral lumbar pain, and can toe- and heel-walk goes home with activity advice. A 62-year-old with prostate cancer and night pain that wakes him does not.

Shoulder, knee, and ankle

Subacromial impingement / tendinopathy produces a painful arc and positive Neer or Hawkins testing with preserved strength. Rotator-cuff tear adds true weakness (drop-arm, lag signs), night pain, and — especially after a fall in an older adult — an inability to lift the arm. Adhesive capsulitis (frozen shoulder) is global loss of active and passive range, especially external rotation; diabetes is the classic comorbidity. Frozen shoulder is a motion diagnosis; a tear is a strength diagnosis. Refer traumatic weakness and locked shoulders; start structured motion and physical therapy for frozen shoulder and uncomplicated impingement.

Ottawa knee rules — obtain a radiograph after acute knee injury if any one is present: age 55 or older, isolated patellar tenderness, tenderness of the fibular head, inability to flex to 90 degrees, or inability to bear weight four steps both immediately and in the clinic (limping is still “unable” if they cannot take four steps).

Ottawa ankle and midfoot rules — ankle radiographs if there is pain in the malleolar zone and either bone tenderness at the posterior edge or tip of the lateral or medial malleolus or inability to take four steps immediately and in the clinic. Foot radiographs if there is pain in the midfoot zone and either tenderness at the navicular or the base of the fifth metatarsal or the same inability to bear weight. The rules exist to reduce unnecessary films, not to skip films when a rule is positive.

Pediatric limp, fibromyalgia, and osteoporosis

A child who will not walk owns a hip until you prove otherwise. Slipped capital femoral epiphysis (SCFE) is the obese adolescent (often 10–16 years) with a limp or referred knee pain and limited internal rotation. Do not force range of motion. Make the child non-weight-bearing and obtain urgent hip radiographs (AP and frog-leg when safe); this is urgent orthopedics. Legg-Calvé-Perthes is idiopathic avascular necrosis of the femoral head, typically in children about 4–8 years old with a painless or mildly painful limp. Transient synovitis is a post-viral, well-appearing child who can often still walk and who has bland labs. Septic hip is the toxic, febrile, non-weight-bearing child. Kocher criteria (conceptual) stack probability: non-weight-bearing, ESR ≥40 mm/h, fever >38.5°C, and WBC >12,000/µL; CRP is often added. Four positives is not a “watch overnight” primary-care plan — it is emergency orthopedics and a joint aspiration. Never send a toxic non-walker home labeled transient synovitis.

Fibromyalgia is a diagnosis of pattern after a limited workup: widespread pain on both sides of the body, above and below the waist, lasting at least three months, with fatigue, unrefreshing sleep, and cognitive fog. Tender-point counts are no longer required. Check a CBC, TSH, and a targeted inflammatory marker when the story is mixed; do not order an autoimmune panel on every tired adult with pain. It is not a diagnosis of exclusion that requires an MRI of every body part. It is also not “all in her head” — name it, then treat with education, sleep, graded exercise, and selected neuromodulators in later chapters.

Osteoporosis screening is a hard FNP number. DXA is indicated for women 65 years and older and for younger postmenopausal women whose 10-year fracture risk is at least that of a 65-year-old white woman without additional risk factors — FRAX is the usual calculator for that comparison. Screen earlier for prior fragility fracture, long-term glucocorticoids, rheumatoid arthritis, very low body weight, or other secondary causes. USPSTF does not give a parallel “screen all men at 65” A-grade mandate; men are individualized by age, prior fracture, hypogonadism, and steroids. A T-score of −2.5 or lower at the lumbar spine, total hip, or femoral neck is osteoporosis; −1.0 to −2.5 is osteopenia (low bone mass). A hip or vertebral fragility fracture also diagnoses osteoporosis regardless of T-score. FRAX reports 10-year probabilities of hip fracture and of major osteoporotic fracture; U.S. treatment conversations often use thresholds around a 3% hip risk or a 20% major-fracture risk, but the exam point is that FRAX stratifies risk — it is not a lab you order instead of a DXA when a DXA is indicated.