Evaluation for Radicular Pain from Lumbar Spine Pathology
This patient requires immediate evaluation for lumbar spine pathology causing L5 radiculopathy, as knee pain radiating to the great toe is a classic presentation of nerve root compression, not a primary knee disorder. The ACL examination findings are likely incidental or secondary to altered biomechanics from the underlying neurological issue.
Critical Diagnostic Approach
Primary Concern: L5 Nerve Root Compression
Pain radiating from the knee to the great toe follows the L5 dermatome distribution, which is pathognomonic for lumbar spine pathology rather than intra-articular knee disease 1.
Lumbar spine pathology must be considered when knee radiographs are unremarkable and clinical evidence suggests spinal origin, as referred pain from the lower back commonly presents as knee symptoms 1, 2, 3.
The American College of Radiology explicitly states that in patients with chronic knee pain, referred pain from the lower back must be considered, especially if knee radiographs are unremarkable 1.
Essential Clinical Examination Elements
Perform straight leg raise test to assess for nerve root tension, which would be positive with L5 radiculopathy 3.
Examine for motor weakness in ankle dorsiflexion and great toe extension (L5 myotome), as weakness in these distributions confirms nerve root involvement 3.
Test sensation along the lateral leg and dorsum of the foot, as L5 dermatome sensory changes would support radicular etiology 3.
Assess deep tendon reflexes, noting that L5 radiculopathy typically does not affect reflexes but may show subtle changes 3.
Imaging Algorithm
Step 1: Initial Knee Radiographs
- Obtain standard knee radiographs (anteroposterior, lateral, and tangential patellar views) to exclude fracture or significant osteoarthritis 1, 2.
- These serve primarily to rule out knee pathology rather than establish the diagnosis 1.
Step 2: Lumbar Spine Imaging (Primary Diagnostic Study)
- If knee radiographs are unremarkable and clinical evidence suggests lumbar spine pathology, image the lumbar spine 1, 3.
- MRI of the lumbar spine without IV contrast is the preferred study to evaluate for disc herniation, spinal stenosis, or other causes of L5 nerve root compression 3.
Step 3: Consider Hip Evaluation
- Hip pathology must be evaluated when knee imaging is normal, as hip disease commonly refers pain to the knee 1, 2, 3.
- However, hip pathology typically does not cause radiation to the great toe, making this less likely than lumbar pathology in this presentation 2, 3.
Why the ACL Examination May Be Misleading
Altered gait mechanics from L5 radiculopathy can create secondary knee instability that may mimic ACL pathology through compensatory movement patterns 3.
Patients with chronic radicular pain often develop quadriceps weakness and altered neuromuscular control, which can produce false-positive findings on ligamentous testing 3.
The presence of neurological symptoms (radiation to great toe) supersedes mechanical knee findings in determining the primary pathology 1, 3.
Common Diagnostic Pitfalls to Avoid
Do not pursue knee MRI or arthroscopy based solely on ACL examination findings when radicular symptoms are present 1, 3.
Approximately 20% of patients with chronic knee pain inappropriately receive knee MRI without recent radiographs or consideration of referred pain sources 1.
Not all structural abnormalities found on imaging are symptomatic, particularly in patients over 45 years where meniscal tears and degenerative changes are often incidental 1, 3.
Failure to recognize referred pain patterns leads to unnecessary knee interventions that will not address the underlying neurological problem 2, 3.
Treatment Implications
If lumbar spine imaging confirms L5 radiculopathy, treatment should focus on the spine pathology through conservative management (physical therapy, NSAIDs, neuropathic pain medications) or surgical decompression if indicated 3.
Any ACL instability should be reassessed after addressing the neurological component, as apparent knee instability may resolve with treatment of the radiculopathy 3.
Knee-directed interventions (including ACL reconstruction) should be deferred until the radicular component is fully evaluated and treated 1, 3.