Musculoskeletal

Knee pain: meniscal, ligamentous, and patellofemoral

Clinical Overview and When to Suspect Knee Pain Syndromes

— Non-contact pivot/deceleration with a "pop" and immediate effusion within 0–4 hours → suspect ACL tear (hemarthrosis in ~70%).

— Twisting on a planted foot, delayed effusion (next day), catching/locking → meniscal tear.

— Valgus blow to lateral knee (football, skiing) → MCL injury; varus blow → LCL.

— Dashboard injury / fall on flexed knee with plantarflexed foot → PCL tear.

— Anterior knee pain with stairs, prolonged sitting ("theater sign"), squatting, no trauma → PFPS.

Board pearl: Effusion timing distinguishes the big three traumatic causes — immediate (<2 hr) tense effusion = ACL/fracture/patellar dislocation (hemarthrosis); next-day effusion = meniscal tear (synovial fluid); no effusion + anterior pain = PFPS.

Step 3 management: Most non-locked, neurovascularly intact knee complaints in primary care can begin with relative rest, NSAIDs, structured physical therapy, and 4–6 week reassessment before advanced imaging — a high-yield value-based-care principle tested on Step 3 ambulatory vignettes.

Knee pain accounts for ~2% of primary care visits; in adults <50 the differential is dominated by patellofemoral pain syndrome (PFPS), meniscal tears, and ligamentous injuries (ACL, MCL, LCL, PCL); in adults >50, add osteoarthritis and degenerative meniscal tears.

Mechanism-based triage is the Step 3 starting point:

Red flags requiring urgent workup: inability to bear weight 4 steps, fever, severe atraumatic effusion (consider septic arthritis, crystal arthropathy), neurovascular deficit (suspect knee dislocation — popliteal artery injury until proven otherwise), or pediatric night pain (osteosarcoma).

Step 3 framing emphasizes the outpatient diagnostic algorithm: history + exam → selective imaging → conservative trial → escalate to MRI/ortho if mechanical symptoms persist >6 weeks or red flags emerge.

Presentation Patterns and Key History

— Mechanism: contact vs non-contact, direction of force, foot position, deceleration/pivot.

— Timing of swelling: minutes (hemarthrosis), hours-to-next-day (meniscal/synovial), recurrent (degenerative or inflammatory).

— Mechanical symptoms: true locking (inability to fully extend, suggests displaced meniscal fragment or loose body) vs pseudo-locking (pain-mediated guarding in PFPS).

— Instability: "giving way" with cutting/pivoting → ACL; giving way on stairs or uneven ground → patellar instability or quadriceps weakness in PFPS.

— Location: anterior/peripatellar (PFPS, patellar tendinopathy), medial joint line (medial meniscus, MCL), lateral joint line (lateral meniscus, LCL, IT band), posterior (Baker cyst, PCL, hamstring).

— Young female runner, anterior knee pain worse descending stairs, pain after prolonged sitting → PFPS (theater/movie sign).

— Middle-aged patient, squatted to garden, felt pop, next-day swelling, medial joint line pain, catching → degenerative medial meniscus tear.

— College athlete, non-contact pivot, audible pop, immediate swelling, knee gave way → ACL tear.

— Skier with valgus blow, medial pain, no effusion, stable to anterior drawer → MCL sprain.

— Passenger in MVA with dashboard impact, posterior knee pain, posterior sag → PCL injury.

Key distinction: True locking = mechanical block to extension, lasts seconds-minutes, often unlocks with manipulation → strongly suggests meniscal flap or loose body and warrants MRI. Pseudo-locking = pain-limited motion, resolves with rest and NSAIDs → consistent with PFPS or effusion-related guarding and does not by itself require MRI.

Anchor the history on five questions that drive the differential efficiently:

Pattern recognition shortcuts:

Ask about prior knee surgery, gout/pseudogout, IBD/psoriasis, tick exposure (Lyme), recent GU/GI infection (reactive arthritis), and anticoagulation (spontaneous hemarthrosis).

Physical Exam Findings

— ACL: Lachman test (30° flexion, anterior translation of tibia) is most sensitive (~85%); anterior drawer less sensitive acutely; pivot shift most specific but requires relaxation.

— PCL: posterior drawer, posterior sag sign (tibia sags posteriorly at 90° flexion), quadriceps active test.

— MCL: valgus stress at 30° (isolated MCL); valgus laxity in full extension suggests combined injury.

— LCL: varus stress at 30°.

— Meniscus: joint line tenderness (most sensitive single finding, ~75%), McMurray test (flexion + rotation + extension, palpable click or pain), Thessaly test (weight-bearing rotation at 20° flexion, sensitivity ~90% in trained hands), Apley compression.

— Patellofemoral: patellar grind/Clarke test, patellar apprehension (lateral glide → fear of dislocation), squat test reproducing pain, J-sign (lateral tracking on terminal extension).

Board pearl: A patient who "walks in" after a knee injury can still have had a spontaneously reduced knee dislocation — multiligamentous laxity (≥2 ligaments) on exam mandates vascular evaluation regardless of pulses, because intimal injury can present delayed.

Step 3 management: Document a focused neurovascular exam before and after any reduction or splinting — a recurring patient-safety item.

Inspection and gait: antalgic gait, quadriceps atrophy (chronic), genu varum/valgum, Q-angle >15° (women) / >10° (men) predisposes to PFPS, patellar alta (high-riding patella) on lateral inspection.

Effusion assessment: ballottement for large effusions, bulge sign for small (sweep medial gutter, tap lateral, watch fluid wave).

Range of motion: loss of full extension suggests displaced meniscal tear or locked knee; flexion contracture suggests chronic effusion.

Provocative tests by structure:

Always perform neurovascular exam: dorsalis pedis/posterior tibial pulses, peroneal nerve (dorsiflexion, first webspace sensation), saphenous nerve. Asymmetric pulses or ABI <0.9 after knee trauma → emergent vascular imaging (CT angiography) to rule out popliteal injury from occult knee dislocation.

Diagnostic Workup — Initial Imaging and Decision Rules

— Age ≥55

— Isolated patellar tenderness

— Tenderness at fibular head

— Inability to flex to 90°

— Inability to bear weight 4 steps both immediately and in the ED/clinic

— Segond fracture (lateral tibial plateau avulsion) → pathognomonic for ACL tear (~75–100% association); order MRI.

— Reverse Segond (medial tibial avulsion) → PCL tear.

— Lipohemarthrosis (fat-fluid level on horizontal-beam lateral) → intra-articular fracture.

— Patella alta/baja, bipartite patella (often incidental, superolateral pole), fabella.

— Osteoarthritis: joint space narrowing (medial >lateral typically), osteophytes, subchondral sclerosis, cysts.

CCS pearl: In a CCS case with atraumatic monoarticular knee swelling and fever, immediately order arthrocentesis before antibiotics (when feasible) and IV access — delaying tap to image first costs you points.

Ottawa Knee Rules determine need for plain radiographs after acute knee injury — order x-rays if any one is present:

Sensitivity ~99% for clinically significant fracture; validated for outpatient use and reduces unnecessary imaging — a value-based care point Step 3 loves.

Standard radiographic series: AP, lateral, sunrise/Merchant (patellofemoral joint), and tunnel/notch view for loose bodies or OCD lesions. Add weight-bearing PA flexed (Rosenberg) view in patients >40 to detect joint space narrowing of OA.

Findings to recognize:

Labs are not routine for mechanical knee pain. Order arthrocentesis with cell count, crystals, Gram stain, culture for atraumatic effusion, fever, or suspicion of septic arthritis / crystal disease — WBC >50,000 with neutrophil predominance suggests septic until cultures return. Synovial fluid in hemarthrosis is grossly bloody; fat globules suggest occult fracture.

Ultrasound can identify effusion, Baker cyst, quadriceps/patellar tendon tears at point of care.

Diagnostic Workup — Advanced and Confirmatory Studies

— Indications: persistent mechanical symptoms >6 weeks despite conservative therapy, suspected ACL/PCL tear, true locking, large hemarthrosis without fracture, failed PT for PFPS with atypical features, preoperative planning.

— Bone bruise pattern of lateral femoral condyle + posterolateral tibial plateau is classic for ACL pivot-shift injury.

— Bucket-handle meniscal tear: "double PCL sign" or "double anterior horn sign" — surgically urgent if locked.

— Bone scan or MRI for suspected stress fracture in runners with focal tibial/femoral tenderness.

— EMG/NCS if peroneal neuropathy suspected after fibular head injury or prolonged immobilization.

— Inflammatory workup (CBC, ESR, CRP, RF, anti-CCP, ANA, uric acid, Lyme serology) only when systemic features, polyarticular involvement, or atraumatic recurrent effusions.

Board pearl: A degenerative meniscal tear on MRI in a 60-year-old with OA does not mandate arthroscopy — multiple RCTs (e.g., FIDELITY, MeTeOR) show arthroscopic partial meniscectomy offers no benefit over PT for degenerative tears without true mechanical locking.

Step 3 management: Order MRI selectively; let symptoms and exam — not patient request — drive imaging.

MRI without contrast is the gold standard for soft-tissue knee injury — meniscus, cruciate and collateral ligaments, articular cartilage, bone marrow edema, occult fractures.

MR arthrography improves detection of recurrent or postoperative meniscal tears and chondral defects.

CT is reserved for complex tibial plateau or patellar fractures for surgical planning; CT angiography for suspected vascular injury after knee dislocation.

Diagnostic arthroscopy is rarely first-line but remains the definitive standard for intra-articular pathology and is often combined with therapeutic intervention.

Avoid early MRI in classic PFPS, isolated MCL grade I–II sprains, and uncomplicated degenerative meniscal tears in patients >40 with OA — management does not change and adds cost. The Choosing Wisely / American Academy of Family Physicians recommendation supports conservative therapy first.

Special studies:

Risk Stratification and First-Line Management Logic

— Surgical urgent (refer same week): locked knee with displaced meniscal fragment, complete ACL/PCL in active athlete, multiligamentous injury, suspected septic joint (→ ED), unstable fracture, neurovascular compromise.

— Surgical elective (refer within 4–6 weeks if persistent): symptomatic meniscal tear failing PT, ACL tear in pivoting athlete, recurrent patellar dislocation, advanced OA failing conservative care.

— Conservative: PFPS, MCL grade I–II, LCL grade I–II, degenerative meniscal tear without locking, mild-moderate OA, patellar/quadriceps tendinopathy.

— Early relative rest with progressive loading, not strict immobilization (avoids stiffness/atrophy).

— Ice 15–20 min q2–4h first 48–72 h.

— Compression sleeve for swelling.

— NSAIDs short course (see chunk 7).

— Structured physical therapy within 1–2 weeks: quadriceps (especially VMO), hip abductor/external rotator strengthening (critical for PFPS), hamstring flexibility, core stability, neuromuscular retraining.

Key distinction: Grade I MCL (pain, no laxity) → brace + early PT, return in 1–2 weeks. Grade II (laxity with firm endpoint) → hinged brace, 3–6 weeks. Grade III (no endpoint) → hinged brace 6–8 weeks; isolated MCL III still often non-operative — surgery reserved for combined injuries or chronic instability.

Step 3 management: Document a 6-week conservative trial with PT compliance before authorizing MRI or orthopedic referral for non-emergent cases — payers and guidelines align here.

Triage framework for outpatient knee pain:

Conservative bundle (the "PRICE-MEAT" evolution — now favoring POLICE: Protection, Optimal Loading, Ice, Compression, Elevation):

PFPS-specific: hip-focused PT outperforms knee-only PT; address foot pronation with orthotics if present; patellar taping (McConnell) as adjunct; avoid prolonged knee flexion activities.

Return-to-play criteria: full painless ROM, ≥90% strength symmetry, sport-specific functional testing (hop tests for ACL rehab), no effusion.

Pharmacotherapy — First-Line Regimens

— Ibuprofen 400–600 mg PO q6–8h PRN (max 2400 mg/day outpatient), naproxen 250–500 mg BID, or meloxicam 7.5–15 mg daily.

— Use lowest effective dose, shortest duration (typically 7–14 days for acute injury).

— Topical diclofenac 1% gel (4 g QID to knee) preferred in elderly, CKD, GI/cardiovascular risk — comparable efficacy to oral NSAIDs for knee OA with far fewer systemic adverse effects (AAOS and ACR strong recommendation).

— Screen for contraindications: CKD (eGFR <30), active PUD, heart failure, uncontrolled HTN, anticoagulation, third-trimester pregnancy.

— Add PPI if age >65, prior GI bleed, or concurrent antiplatelet/anticoagulant therapy.

— Corticosteroid injection (triamcinolone 40 mg + lidocaine) — short-term relief (4–8 weeks); limit to ≤3–4/year per joint; concerns for accelerated cartilage loss with frequent use.

— Hyaluronic acid: ACR conditionally recommends against for knee OA (inconsistent evidence); some patients benefit.

— PRP: emerging evidence, not standard, often not covered by insurance.

— Duloxetine 30–60 mg daily — FDA-approved for chronic musculoskeletal pain/OA; useful when comorbid depression or NSAIDs contraindicated.

— Topical capsaicin for chronic OA pain.

— Glucosamine/chondroitin: ACR recommends against; modest placebo-level effect.

Board pearl: In a 70-year-old with knee OA, HTN, CKD stage 3, and on warfarin, the correct first-line pharmacotherapy is topical diclofenac + acetaminophen, not oral NSAIDs — a high-yield Step 3 polypharmacy/elderly question.

Step 3 management: Always pair pharmacotherapy with PT — drugs alone underperform combined therapy in every guideline.

NSAIDs are first-line analgesics for mechanical knee pain and OA:

Acetaminophen 500–1000 mg q6h (max 3 g/day in elderly, 4 g otherwise) — modest efficacy in OA, useful when NSAIDs contraindicated; avoid in significant hepatic disease or heavy alcohol use.

Avoid opioids for acute or chronic knee pain — CDC 2022 guideline reaffirms non-opioid first-line; if used for severe acute injury, limit to ≤3 days and document.

Intra-articular therapies (for OA or persistent inflammatory pain):

Adjuncts:

Procedures and Surgical Management

— Lateral suprapatellar or lateral midpatellar approach; 18g needle; send fluid for cell count, crystals, Gram stain, culture.

— Therapeutic drainage of large effusion can relieve symptoms and improve ROM for PT.

— Indicated for OA flares, inflammatory arthritis, persistent effusion failing oral therapy.

— Avoid if prosthetic joint (infection risk) or suspected septic arthritis.

— Delay knee arthroplasty ≥3 months after intra-articular steroid to reduce PJI risk.

— ACL reconstruction: young, active, pivoting-sport athletes, recurrent instability; typically delayed 3–6 weeks post-injury for "prehab" (regain ROM, reduce effusion) to lower arthrofibrosis risk. Sedentary older adults can often manage non-operatively with PT and bracing.

— Meniscal repair vs partial meniscectomy: repair preferred in young patients, peripheral (red-red zone) tears, vertical/longitudinal patterns; partial meniscectomy for complex/degenerative tears with mechanical symptoms; leave alone asymptomatic degenerative tears.

— PCL: most isolated PCL injuries managed non-operatively with quad-focused rehab; reconstruction for combined injuries or persistent functional instability.

— MCL/LCL: surgery rare for isolated injuries; combined ligamentous injury → reconstruction.

— Patellar stabilization (MPFL reconstruction): recurrent patellar dislocations.

— Knee arthroplasty (TKA/UKA): advanced OA failing 3–6 months of optimized conservative care, with functional limitation and radiographic K-L grade 3–4.

— VTE prophylaxis post-TKA: aspirin 81 mg BID, DOAC, or LMWH for 10–14 days (up to 35 days).

— Hold DMARDs per ACR/AAHKS perioperative guideline (methotrexate may continue; biologics held 1 dosing cycle).

— Optimize: A1c <7.5–8%, BMI <40, smoking cessation 4+ weeks pre-op, MRSA screening.

CCS pearl: In CCS, after aspirating a knee, always reassess pain and ROM and document — and send fluid studies before starting empiric antibiotics in suspected septic joint.

Arthrocentesis (therapeutic + diagnostic):

Intra-articular corticosteroid injection:

Surgical referral indications:

Perioperative Step 3 points:

Special Populations — Elderly and Renal/Hepatic Impairment

— Avoid attributing all pain to OA — screen for crystal disease (CPPD especially), pseudoseptic flares, occult fracture in osteoporotic patients (low-energy mechanism), and referred hip pain (groin pain, internal rotation loss).

— Falls assessment is mandatory: knee pain limits mobility and increases fall risk; offer multifactorial fall intervention (vitamin D if deficient, home safety, gait/balance PT).

— Beers criteria flags chronic NSAID use — favor topical diclofenac, acetaminophen ≤3 g/day, duloxetine.

— CKD (eGFR <30): avoid systemic NSAIDs; topical NSAIDs are acceptable (minimal systemic absorption ~5–10%); acetaminophen preferred; corticosteroid injections safe.

— Hepatic impairment: limit acetaminophen to 2 g/day; avoid NSAIDs in cirrhosis (risk of variceal bleed, hepatorenal syndrome); duloxetine contraindicated in significant hepatic disease.

— Anticoagulated patients: avoid NSAIDs (bleeding); intra-articular injections safe if INR <3.0 or DOAC at trough; do not routinely hold anticoagulation for arthrocentesis.

Board pearl: A 1-lb weight loss reduces knee joint loading by ~4 lb per step — a high-yield counseling number; 5–10% body weight loss produces clinically meaningful pain and function improvement in knee OA, comparable to NSAIDs.

Step 3 management: In elderly polypharmacy patients, deprescribe before adding analgesics — review for duplicative or interacting agents (e.g., concurrent NSAID + SSRI + anticoagulant raises bleed risk substantially).

Older adults (≥65) dominate the chronic knee pain population via osteoarthritis and degenerative meniscal tears; presentation often blends mechanical and inflammatory features.

Pharmacology in elderly:

Surgical decisions: chronological age alone does not contraindicate TKA; functional status, comorbidity burden, and goals of care drive shared decision-making. UKA may be appropriate for isolated compartment disease.

Rehabilitation considerations: home-based PT, assistive devices (cane in contralateral hand offloads ~25% knee force), weight loss (every 1 lb lost = 4 lb less knee load).

Special Populations — Pediatrics, Adolescents, Pregnancy, and Athletes

— Osgood-Schlatter disease: tibial tubercle pain/swelling in active adolescents (boys 12–15, girls 10–13); self-limited; activity modification, ice, stretching, NSAIDs; resolves with growth plate closure.

— Sinding-Larsen-Johansson: inferior pole patella traction apophysitis, similar age group.

— Patellofemoral pain: extremely common in adolescent female athletes; hip strengthening PT.

— Osteochondritis dissecans (OCD): subchondral bone/cartilage lesion (often medial femoral condyle); vague activity-related pain, effusion, catching; MRI for staging; stable lesions in skeletally immature heal with activity restriction.

— Slipped capital femoral epiphysis (SCFE) can present as referred knee pain in obese adolescents — always examine the hip.

— Septic arthritis and juvenile idiopathic arthritis in younger children with atraumatic swelling.

— Bone tumors (osteosarcoma at distal femur/proximal tibia): night pain, palpable mass, constitutional symptoms → urgent imaging.

— Patellar dislocation common in adolescents with trochlear dysplasia, ligamentous laxity, high Q-angle.

— Avoid NSAIDs after 20 weeks (oligohydramnios, premature ductus closure); contraindicated in third trimester.

— Acetaminophen first-line analgesic.

— Relaxin-mediated ligamentous laxity increases risk of mechanical pain; PT, supportive footwear, weight management.

— Female athletes have 2–8× higher ACL injury rate (anatomic, hormonal, neuromuscular factors); neuromuscular training programs (FIFA 11+, PEP) reduce ACL injury by ~50% and are tested USPSTF-style preventive recommendations.

— Concussion and second-impact concerns separate from knee but think holistically about return-to-play clearance.

Key distinction: In any child or adolescent with knee pain, always examine the hip — SCFE and Legg-Calvé-Perthes classically present as referred knee pain, and missing them is a board favorite and a real-world malpractice scenario.

Step 3 management: Recommend ACL injury-prevention programs to coaches/parents of adolescent female athletes — Step 3 rewards preventive counseling at every visit.

Pediatric/adolescent knee pain demands a different differential:

Pregnancy:

Athletes — Step 3 longitudinal care:

Complications and Adverse Outcomes

— Develops in ~50% of patients within 10–20 years after ACL or meniscal injury, regardless of surgical vs nonsurgical management.

— Driven by altered biomechanics, cartilage injury, and meniscal loss; meniscectomy accelerates OA more than meniscal repair or intact meniscus.

— Untreated ACL deficiency → recurrent giving-way episodes, secondary meniscal/chondral damage, "ACL-deficient knee" cascade.

— Recurrent patellar dislocation → progressive trochlear damage, chronic PFPS.

Board pearl: A patient with knee dislocation and normal distal pulses still requires ABI — if <0.9, get CTA; >0.9 with normal exam allows serial observation. Missing popliteal injury is a classic litigation and Step 3 patient-safety vignette.

Step 3 management: Counsel ACL/meniscal injury patients about long-term PTOA risk during initial encounter — informed expectations are an ethics/safety point.

Post-traumatic osteoarthritis (PTOA):

Chronic instability:

Arthrofibrosis: stiffness post-surgery or prolonged immobilization; risk reduced by early ROM and prehab before ACL reconstruction.

DVT/PE: post-knee surgery (especially TKA) — incidence reduced by prophylaxis; have low threshold for D-dimer/duplex US in post-op calf pain or swelling beyond expected.

Compartment syndrome: after high-energy trauma, tibial plateau fracture, or prolonged surgery — pain out of proportion, pain with passive stretch, paresthesia → emergent fasciotomy.

Popliteal artery injury: associated with knee dislocation (any direction, especially posterior); up to 30% incidence; can present delayed with intimal flap thrombosis hours-days later → mandatory ABI + CT angiography even with palpable pulses.

Peroneal nerve injury: from fibular head trauma, LCL/PCL combined injuries, or tight casts/braces → foot drop, lateral leg numbness.

Septic arthritis post-injection: rare (~1:10,000–50,000) but emergent; presents with severe pain, swelling, fever within days of injection.

Complex regional pain syndrome (CRPS): disproportionate burning pain, allodynia, autonomic changes (color, temperature) — early recognition and multidisciplinary referral key.

Quadriceps atrophy from disuse: as early as 1 week of immobilization; rehab focus.

When to Escalate Care — Consult, Inpatient, and Emergent Triage

— Suspected or confirmed knee dislocation (even spontaneously reduced) — vascular evaluation.

— Open fracture, compartment syndrome, neurovascular compromise.

— Septic arthritis suspicion (fever, severe atraumatic effusion, immunocompromise) — needs emergent arthrocentesis, IV antibiotics, orthopedic washout.

— High-energy trauma with multiple injuries.

— Locked knee (displaced bucket-handle meniscus) — surgical reduction may preserve meniscal tissue if performed early.

— Complete ACL/PCL in active patient, especially pivoting athletes.

— Multiligamentous injury (≥2 ligaments).

— Patellar tendon or quadriceps tendon rupture (inability to extend, palpable defect, patella alta/baja on x-ray) — surgical repair within 2 weeks for best outcomes.

— Displaced patellar or tibial plateau fracture.

— Persistent mechanical symptoms or instability despite PT.

— Failed conservative therapy for symptomatic meniscal tear or PFPS with structural cause.

— Advanced OA considering joint replacement.

— Recurrent atraumatic effusions, polyarticular involvement, morning stiffness >1 hour, elevated inflammatory markers, suspected gout/CPPD/RA/spondyloarthritis.

— Septic joint (IV antibiotics + surgical drainage).

— Surgical fixation of fractures.

— Postoperative monitoring after TKA, complex reconstruction.

— Pain crisis with inability to ambulate safely at home.

CCS pearl: In the CCS septic knee case, the correct ordered sequence is: IV access → arthrocentesis with fluid studies → empiric antibiotics (vancomycin + ceftriaxone) → orthopedic consult for surgical washout → admit. Skipping the tap or delaying surgery loses points.

Step 3 management: Always document why you escalated or didn't — Step 3 vignettes test the rationale for referral level, not just the choice.

Emergency department / immediate transfer:

Urgent orthopedic referral (within 1 week):

Routine orthopedic referral (4–6 weeks):

Rheumatology referral:

Inpatient admission criteria:

Key Differentials — Same-Category (Mechanical/Structural)

Key distinction: Pes anserine bursitis is medial and distal to the joint line; medial meniscus tear is at the joint line. This 2–3 cm difference is a classic exam question separator.

Step 3 management: Always consider Baker cyst rupture in the patient with new calf pain and known knee pathology before treating as DVT — but get the duplex anyway, they coexist.

Patellar tendinopathy ("jumper's knee"): anterior pain at inferior pole of patella; common in basketball/volleyball; tender on palpation; treat with eccentric quad strengthening (decline squats), activity modification, avoid corticosteroid injection (tendon rupture risk).

Quadriceps tendinopathy: pain at superior pole of patella; older patients; similar rehab approach.

IT band syndrome: lateral knee pain in runners/cyclists, worse with downhill or after specific distance; Ober test positive; PT focused on hip abductor strengthening and IT band stretching; avoid lateral compartment misdiagnosis as meniscus.

Pes anserine bursitis: medial knee pain ~5 cm distal to joint line at insertion of sartorius/gracilis/semitendinosus; tender to palpation; common in overweight middle-aged women, runners, diabetics; treat with NSAIDs, ice, PT; corticosteroid injection if refractory.

Prepatellar bursitis ("housemaid's knee"): anterior swelling over patella from chronic kneeling; rule out septic bursitis (warmth, fever, cellulitis) with aspiration if concerning.

Baker (popliteal) cyst: posterior knee fullness, often secondary to underlying meniscal tear or OA; can rupture and mimic DVT ("pseudothrombophlebitis") — order duplex US.

Plica syndrome: snapping medial plica band, anteromedial pain, often confused with meniscal tear; MRI helpful; PT first-line.

Loose bodies / osteochondral fragments: catching, locking, intermittent symptoms; visible on x-ray or MRI; arthroscopic removal.

Stress fracture: tibial plateau or femoral condyle in runners or osteoporotic patients; focal bony tenderness; MRI or bone scan; protected weight-bearing.

Osteonecrosis of the knee (SONK): sudden severe medial knee pain in older women, often without trauma; MRI shows subchondral edema/lesion; can progress to collapse.

Key Differentials — Other-Category (Systemic/Referred)

— Gout: monoarticular, rapid onset, erythematous, exquisitely tender; knee is second-most common joint after first MTP; synovial fluid: negatively birefringent needle-shaped urate crystals; treat acute with NSAIDs, colchicine, or steroids; urate-lowering therapy (allopurinol) for chronic management.

— CPPD (pseudogout): older adults, knee is most common joint; rhomboid, weakly positive birefringent crystals; chondrocalcinosis on x-ray (linear calcification in cartilage); treat similarly to gout acute flares.

— Hip pathology (OA, SCFE in adolescents, AVN) → anterior/medial knee pain; check internal rotation of hip.

— Lumbar radiculopathy (L3-L4): anterior thigh/knee pain with neuro findings.

Board pearl: A monoarticular hot knee in a sexually active young adult with tenosynovitis (wrist/ankle) and pustular skin lesions = disseminated gonococcal infection until proven otherwise — treat with ceftriaxone IV and test for chlamydia co-infection.

Step 3 management: "Atraumatic knee swelling + fever" always gets arthrocentesis first — never empirically treat as gout without ruling out infection.

Crystal arthropathies:

Septic arthritis: monoarticular, hot swollen joint, fever; risk factors (IVDU, immunosuppression, prosthetic joint, recent injection); synovial WBC often >50,000 with PMN predominance; S. aureus most common; gonococcal in young sexually active patients (tenosynovitis-dermatitis-polyarthralgia triad).

Rheumatoid arthritis / autoimmune: symmetric polyarthritis, morning stiffness >1 hour, RF/anti-CCP positive; knee involvement common.

Spondyloarthropathies: psoriatic arthritis, reactive arthritis, IBD-associated arthritis — asymmetric large joint involvement, enthesitis, dactylitis.

Lyme arthritis: late manifestation, often large effusions of knee in endemic areas; serology + Western blot; treat with doxycycline 28 days (or ceftriaxone if refractory).

Referred pain:

Vascular: DVT, popliteal aneurysm (pulsatile mass), peripheral arterial disease with claudication.

Hematologic: hemophilic arthropathy (recurrent hemarthrosis), sickle cell (avascular necrosis).

Malignancy: primary bone tumors (osteosarcoma in adolescents), metastatic disease (breast, prostate, lung, renal), multiple myeloma.

Secondary Prevention and Long-Term Plan

— Target 5–10% body weight loss; refer to structured programs, dietitian, or anti-obesity pharmacotherapy (GLP-1 agonists increasingly used) when BMI ≥30.

— Each pound lost = ~4 pounds less knee joint load per step.

— Quadriceps and hip strengthening maintenance program 3×/week long-term.

— Low-impact aerobic activity: cycling, swimming, elliptical, walking — preserve cardiovascular health without joint overload.

— Avoid prolonged immobility; "motion is lotion."

— For PFPS: avoid deep squats, prolonged stair descent, kneeling.

— For OA: limit high-impact running on hard surfaces; consider supportive footwear; cane in contralateral hand.

— FIFA 11+, PEP neuromuscular training programs reduce ACL injury ~50% in female athletes — recommend at preventive visits.

— Proper footwear, gradual training progression (10% rule), cross-training.

— Unloader brace for unicompartmental OA.

— Patellar stabilizing brace for recurrent dislocations.

— Functional ACL brace post-reconstruction during sport return.

— Vitamin D screen/supplement, calcium intake, DEXA per USPSTF (women ≥65, men with risk factors).

— Treat osteoporosis to reduce fracture risk.

Board pearl: A 5% weight loss combined with exercise in knee OA produces pain/function improvements comparable to NSAIDs without their adverse effects — counsel at every visit.

Step 3 management: Build long-term self-management plans (written exercise prescription, weight goal, follow-up cadence) rather than only treating flares — Step 3 rewards longitudinal thinking.

Weight management is the single most effective long-term intervention for knee OA, PFPS, and degenerative meniscal disease:

Exercise and strengthening:

Activity modification:

Injury prevention programs for athletes:

Bracing:

Bone health and fall prevention in older adults:

Vaccinations if surgery planned: ensure influenza, pneumococcal, COVID-19 up to date; vaccinate ≥2 weeks pre-elective surgery when feasible.

Smoking cessation: smoking impairs ligament/tendon healing, increases TKA complication and PJI risk.

Follow-Up, Monitoring, and Counseling

— Acute soft-tissue injury managed conservatively: 2 weeks to assess PT initiation and pain control; 6 weeks for full reassessment and decision on imaging/referral.

— Post-injection (corticosteroid): 4–6 weeks to evaluate response.

— Post-arthroscopy: 2 weeks wound check, 6 weeks functional assessment, 3 months return-to-sport evaluation.

— Post-TKA: 2 weeks, 6 weeks, 3 months, 1 year, then annually for implant surveillance.

— Chronic OA: every 3–6 months for medication review, weight, function, and quality of life.

— Pain (0–10 scale, WOMAC or KOOS for chronic disease).

— Functional status: timed up-and-go, stair climb, walking distance.

— Effusion, ROM (goniometer), strength symmetry.

— Medication adverse effects: BP, renal function (NSAIDs), hepatic function (acetaminophen, duloxetine).

— Adherence to PT — single biggest predictor of outcome.

— Typical course 6–12 weeks, 2–3 sessions/week with home exercise program.

— Discharge to independent home program with structured progression.

— Set realistic expectations: PFPS may take 6–12 weeks to improve; ACL rehab 9–12 months to full sport return; OA is chronic and managed, not cured.

— Discuss return-to-activity criteria explicitly: symmetric strength (≥90%), painless full ROM, sport-specific testing.

— Sleep, mood, and chronic pain interact — screen with PHQ-9 in persistent pain.

— Communicate with PT, orthopedics, rheumatology via shared notes.

— Document return-to-work restrictions for occupational cases.

CCS pearl: In the ambulatory CCS case, always schedule the follow-up visit before ending the encounter — leaving without "Schedule follow-up in 2 weeks" loses points.

Step 3 management: Use validated functional outcome scales (KOOS, WOMAC) for chronic knee disease to track objective improvement and justify continued therapy or escalation.

Follow-up cadence:

Monitoring parameters:

PT prescription specifics:

Counseling pearls:

Care coordination:

Ethical, Legal, and Patient Safety Considerations

— Explicitly discuss risk of infection (~1:10,000–50,000 for intra-articular injection), bleeding, post-injection flare, accelerated cartilage damage with repeated steroids, and the 3-month delay required before TKA after intra-articular steroid (PJI risk).

— For arthroscopy/TKA: discuss alternatives including conservative care, expected functional improvement, lifespan of implant (15–20 years), revision risk, and DVT/PE risk.

— Document shared decision-making, particularly for elective procedures where conservative options remain reasonable.

— Do not order MRI before a trial of conservative therapy for non-traumatic knee pain.

— Do not perform arthroscopy for degenerative meniscal tears in knee OA absent true mechanical locking — multiple RCTs show no benefit; doing so may constitute non-evidence-based practice.

— Avoid opioids for chronic mechanical knee pain.

— In pediatric knee injuries inconsistent with stated mechanism, consider non-accidental trauma; bilateral injuries, bucket-handle marks, or multiple healing fractures trigger mandatory child-protective services reporting.

— In athletes, balance return-to-play pressure from coaches/parents/patient with medical judgment; document refusals to clear when criteria unmet.

— Post-operative discharge is a high-risk safety window: ensure anticoagulation prescription filled, PT scheduled, follow-up appointment, clear red-flag instructions (calf pain, chest pain/dyspnea, fever, wound drainage, increasing pain).

— Medication reconciliation: confirm NSAID stopped if on DOAC, antiplatelets resumed at appropriate interval, opioids tapered.

— Counsel patients not to drive while on opioids or with right-knee braces/immobilizers; document specific restrictions.

Board pearl: Document the 3-month gap between intra-articular knee steroid and elective TKA in the chart — failing to disclose this to surgical patients is both a safety and informed-consent failure.

Step 3 management: Standardize post-op discharge checklists (medications, PT, follow-up, red flags) to reduce transition-of-care errors — a Step 3 systems-based practice favorite.

Informed consent for procedures and injections:

Avoiding low-value care (Choosing Wisely):

Mandatory reporting and safety:

Transitions of care:

Driving and work clearance:

Equity considerations: TKA is underutilized in women, racial/ethnic minorities, and low-income patients; recognize and address referral disparities.

High-Yield Associations and Rapid-Fire Clinical Facts

Board pearl: When a knee question stem mentions "felt a pop, immediate swelling, knee gave way" → think ACL; "twisting, next-day swelling, catching" → meniscus; "stairs, prolonged sitting" → PFPS. These three openers cover the majority of Step 3 knee vignettes.

Ottawa Knee Rules: 5 criteria; sensitivity ~99% for fracture; reduces unnecessary x-rays.

Segond fracture = lateral tibial plateau avulsion → ACL tear.

Reverse Segond = medial tibial avulsion → PCL tear.

Lachman test: most sensitive for ACL; pivot shift: most specific.

Posterior sag sign: pathognomonic for PCL injury.

Bucket-handle meniscal tear: "double PCL sign" on MRI; causes true locking; needs early surgery.

Unhappy triad (O'Donoghue): ACL + MCL + medial meniscus (some texts say lateral — medial is more commonly associated with the classic triad).

Joint line tenderness is the single most sensitive sign for meniscal tear.

Theater sign / movie-goer sign = anterior knee pain after prolonged sitting → PFPS.

Q-angle: men <14°, women <17°; elevated angle predisposes to patellar maltracking.

Patellar dislocation: almost always lateral; medial patellofemoral ligament (MPFL) tears.

VMO weakness central to PFPS pathophysiology — hip-focused PT outperforms knee-only.

Chondrocalcinosis on x-ray = CPPD; rhomboid weakly positively birefringent crystals.

Gout crystals: negatively birefringent, needle-shaped, yellow when parallel to compensator axis.

Septic joint synovial WBC typically >50,000, PMN >75%; gonococcal often lower (10,000–60,000).

S. aureus is most common septic arthritis organism overall; N. gonorrhoeae in young sexually active.

Lyme arthritis: large knee effusions, late stage; doxycycline 28 days.

Popliteal cyst rupture mimics DVT → "pseudothrombophlebitis"; get duplex US.

Knee dislocation → mandatory vascular assessment (ABI, +/- CTA) even with palpable pulses.

Peroneal nerve injury: foot drop after fibular head trauma or tight cast.

APM for degenerative meniscal tears in OA: no benefit vs PT (FIDELITY, MeTeOR trials).

5–10% weight loss = clinically meaningful improvement in knee OA.

TKA after intra-articular steroid: wait ≥3 months to reduce PJI.

ACL injury prevention programs (FIFA 11+, PEP): reduce risk ~50% in female athletes.

Topical NSAIDs = first-line in elderly knee OA with comorbidities.

Board Question Stem Patterns

Board pearl: Step 3 question stems frequently pair the right diagnosis with the wrong next step as the most attractive distractor — choose the lowest-acuity correct intervention unless red flags are present (locked knee, neurovascular compromise, septic features, age red flags).

Stem 1 — Classic ACL: "22-year-old soccer player plants foot to pivot, hears a pop, knee swells within 1 hour, unable to continue play." → ACL tear; best initial test is Lachman; confirm with MRI; refer to ortho for reconstruction discussion.

Stem 2 — Meniscal tear: "55-year-old gardener squatted, felt twinge, next-day swelling, intermittent catching for 3 weeks, medial joint line tenderness, positive McMurray." → medial meniscus tear; given age + OA-likely degenerative pattern, best next step is PT trial 6 weeks, not MRI or arthroscopy.

Stem 3 — PFPS: "26-year-old female runner with anterior knee pain worse on stairs and after sitting through a movie." → PFPS; best treatment is hip and quadriceps strengthening PT; MRI not indicated.

Stem 4 — MCL: "Skier struck on lateral aspect of knee, medial pain, no effusion, valgus laxity at 30° with firm endpoint, stable in extension." → Grade II MCL sprain; hinged brace + PT, no surgery, no MRI.

Stem 5 — Knee dislocation: "MVA, severely deformed knee reduced in field, palpable distal pulses, ABI 0.85." → CT angiography; concern for popliteal artery injury.

Stem 6 — Septic vs gout: "67-year-old man with red hot swollen knee, fever 38.5°C, no trauma." → arthrocentesis first, send for cell count, crystals, Gram stain, culture; empiric vancomycin + ceftriaxone if WBC >50K pending cultures.

Stem 7 — Elderly OA pharmacology: "72-year-old with knee OA, on warfarin, eGFR 35." → topical diclofenac + acetaminophen, not oral NSAIDs.

Stem 8 — Pediatric referred pain: "13-year-old obese boy with 3 weeks of vague right knee pain, no trauma, limited internal rotation of hip." → SCFE; order AP pelvis + frog-leg lateral; non-weight-bearing; ortho consult.

Stem 9 — Patellar tendon rupture: "55-year-old man fell, unable to extend knee, patella alta on x-ray, palpable defect inferior to patella." → surgical repair within 2 weeks.

Stem 10 — Baker cyst rupture: "Patient with known knee OA develops sudden calf pain and swelling." → duplex US to rule out DVT; if negative, consider ruptured Baker cyst.

One-Line Recap

Approach knee pain by mechanism and effusion timing — non-contact pivot with immediate hemarthrosis = ACL, twisting injury with next-day swelling and joint line tenderness = meniscus, anterior pain on stairs and after sitting = patellofemoral pain syndrome — and apply Ottawa Knee Rules to triage imaging, give nearly everyone a 6-week trial of physical therapy and short-course NSAIDs first, escalate to MRI and orthopedics only for true mechanical locking, instability, multiligamentous injury, or failed conservative care, while always ruling out septic joint with arthrocentesis in any atraumatic hot swollen knee and ruling out popliteal artery injury in any suspected knee dislocation.

Board pearl: When in doubt on Step 3, choose physical therapy + NSAIDs + 6-week follow-up unless the stem gives you a locked knee, a hot joint, neurovascular compromise, or an extensor mechanism rupture — these are the four scenarios that genuinely require immediate escalation.

Diagnostic anchors: mechanism + effusion timing + joint line tenderness + Lachman + McMurray + patellar grind cover ~90% of knee vignettes; Ottawa Rules guide x-ray; MRI only for persistent mechanical symptoms or red flags.

First-line therapy is conservative: relative rest, ice, compression, hip- and quad-focused PT, topical or oral NSAIDs short-course, weight loss 5–10% for OA, activity modification — with reassessment at 2 and 6 weeks.

Red flags demanding escalation: locked knee (bucket-handle meniscus), suspected knee dislocation (ABI + CTA), septic joint (tap first, then empiric antibiotics + ortho washout), extensor mechanism rupture (surgical repair within 2 weeks), pediatric night pain or referred hip pain (SCFE, malignancy).

Step 3 longitudinal mindset: emphasize injury-prevention programs (FIFA 11+, PEP) in young female athletes, schedule structured follow-up, prefer topical NSAIDs and acetaminophen in elderly with CKD/anticoagulation, delay TKA ≥3 months after intra-articular steroid, deliver value-based care by avoiding low-yield MRI and arthroscopy in degenerative meniscal tears without locking.