What is the differential diagnosis for a patient with bilateral lower extremity pain, predominantly in the right knee, and a limping gait, who has a history of knee replacement and cervical fusion?

Differential Diagnosis for Bilateral Lower Extremity Pain with Limping Gait

The differential diagnosis for this patient with bilateral lower extremity pain (right > left), predominantly knee pain, limping gait, and history of knee replacement and cervical fusion should prioritize: (1) prosthetic knee complications including periprosthetic joint infection, aseptic loosening, or extensor mechanism failure; (2) adjacent segment degeneration from the cervical fusion causing referred pain or gait disturbance; (3) peripheral arterial disease; and (4) lumbar spinal stenosis or radiculopathy causing neurogenic claudication.

Primary Considerations Related to Knee Replacement

Prosthetic Knee Complications

• Periprosthetic joint infection must be ruled out first, as this represents a potential surgical emergency requiring prompt specialist evaluation 1
• Aseptic loosening of the prosthetic components can cause progressive pain and altered gait mechanics 1
• Extensor mechanism deficiency (quadriceps or patellar tendon disruption) would cause significant functional impairment and limping 1
• Instability from component malposition or ligamentous insufficiency can manifest as pain and altered gait 2

Adjacent Joint Pathology

• Contralateral knee osteoarthritis is common, as the unoperated knee often bears increased load after unilateral replacement 2, 3
• Progressive functional decline with difficulty walking is a significant indicator of clinically relevant joint pathology 3
• The American College of Radiology recommends focusing on functional limitations and pain rather than incidental findings like crepitus alone 3

Cervical Fusion-Related Complications

Adjacent Segment Degeneration

• Adjacent segment disease occurs in 12-27% of patients following anterior cervical fusion, with higher rates in fusion patients compared to disc replacement 4
• Cervical pathology can cause referred pain to the lower extremities through myelopathy or radiculopathy 3
• Cervical myelopathy from adjacent segment degeneration can cause gait disturbance, leg weakness, and bilateral lower extremity symptoms 5, 4

Pseudarthrosis Complications

• Cervical pseudarthrosis occurs in approximately 10-20% of anterior cervical fusions and is associated with persistent neck and arm pain 2
• Symptomatic pseudarthrosis may cause neurological symptoms including lower extremity dysfunction if spinal cord compression develops 2
• 67-70% of patients with documented pseudarthrosis remain symptomatic and may require revision surgery 2

Vascular Considerations

Peripheral Arterial Disease

• Critical limb ischemia presents with limb pain at rest or with walking, often worse when supine and improved with leg dependency 2
• Risk factors include diabetes, smoking, and advanced age—all should be assessed in this patient 2
• Bilateral symptoms with limping gait (claudication) warrant ankle-brachial index measurement 2
• The absence of classic claudication does not exclude PAD, as many patients have atypical presentations 2

Neurogenic Causes

Lumbar Spinal Stenosis

• Neurogenic claudication from lumbar stenosis causes bilateral leg pain with walking, relieved by rest or forward flexion 3
• The American College of Radiology advises against attributing symptoms solely to knee pathology without considering referred pain from hip or lumbar spine, especially if knee radiographs are unremarkable 3
• Lumbar pathology can coexist with cervical fusion and knee replacement in elderly patients 3

Critical Pitfalls to Avoid

• Do not assume all symptoms originate from the knee replacement without evaluating the cervical spine, vascular status, and lumbar spine 3
• Do not overlook infection in a patient with prosthetic hardware—this requires urgent evaluation with inflammatory markers, joint aspiration if indicated 1
• Do not attribute symptoms to incidental radiographic findings like asymptomatic meniscal tears or mild degenerative changes without correlating with clinical presentation 3
• Do not delay vascular assessment if rest pain, tissue loss, or severe claudication is present, as these represent potential vascular emergencies 2

Diagnostic Approach

Immediate Evaluation

• Assess for infection signs: fever, warmth, erythema, elevated inflammatory markers (ESR, CRP) 1
• Vascular examination: palpate pulses, measure ankle-brachial index if claudication suspected 2
• Neurological examination: assess for myelopathy signs (hyperreflexia, Hoffman's sign, clonus), radiculopathy, or peripheral neuropathy 5, 4

Imaging Strategy

• Standing knee radiographs (AP, lateral, Merchant views) to assess prosthetic position, loosening, or contralateral arthritis 2
• Cervical spine imaging (flexion-extension radiographs or CT) to evaluate fusion status and adjacent segment degeneration 2, 4
• MRI without contrast if radiographs are unremarkable but pain persists, as it is more sensitive for detecting underlying pathology 3
• Lumbar spine imaging if neurogenic claudication suspected 3