What musculoskeletal problems can an unequal patellar reflex indicate?

Unequal Patellar Reflex: Neurological Pathology, Not Musculoskeletal

An unequal patellar reflex does not indicate a musculoskeletal problem—it indicates neurological pathology, specifically nerve root compression or radiculopathy affecting the L2-L4 nerve roots that mediate the patellar tendon reflex.

Understanding the Patellar Reflex

The patellar tendon reflex (PTR) is a deep tendon reflex that tests the integrity of the L2-L4 nerve roots and the femoral nerve pathway. Asymmetry between right and left reflexes is a pathological neurological finding, not a musculoskeletal one.

Key Neurological Causes of Unequal Reflexes

• Lumbar radiculopathy from nerve root compression (disc herniation, spinal stenosis, or degenerative disease) is the primary cause of diminished or absent patellar reflexes on one side 1
• Upper motor neuron lesions can cause increased reflexes on the affected side, while lower motor neuron disorders cause diminished or absent reflexes 1
• Asymmetric reflexes suggest lateralizing pathology requiring neurological evaluation rather than musculoskeletal treatment

Important Clinical Confounders

Total Knee Arthroplasty Effect

Patients who have undergone unilateral total knee arthroplasty (TKA) demonstrate a PTR that is only 55.1% of their contralateral uninvolved side, which can confound the diagnosis of lumbar radiculopathy 2. This is a critical pitfall when evaluating patients with both knee replacement history and suspected spine pathology.

• This diminished reflex persists at least 6 months postoperatively 2
• Patients with severe knee arthritis planned for TKA maintain normal reflexes (96.03% of contralateral side) until after surgery 2
• Clinicians must account for this when interpreting reflex asymmetry in post-TKA patients to avoid misdiagnosing or missing true radiculopathy 2

Differential Diagnosis Algorithm

When encountering unequal patellar reflexes, systematically evaluate:

1. History of knee surgery: Document any prior TKA, which independently diminishes the reflex 2

2. Pattern of reflex change:

   • Diminished/absent reflex = lower motor neuron pathology (radiculopathy, peripheral neuropathy) 1
   • Increased reflex with abnormal plantar response = upper motor neuron pathology (spinal cord lesion, stroke) 1

3. Associated symptoms:

   • Radiating leg pain, numbness, or weakness suggests nerve root compression 1
   • Back pain worse with sitting and relieved when supine suggests spinal stenosis or disc disease 1

4. Sensory examination: Test for dermatomal sensory deficits corresponding to L2-L4 distribution 1

5. Motor examination: Assess for quadriceps weakness, which accompanies L3-L4 radiculopathy 1

What This Is NOT

Unequal patellar reflexes do not indicate:

• Patellofemoral pain syndrome (which presents with anterior knee pain, not reflex changes) 3, 4
• Patellar hypermobility or instability (which involves excessive lateral patellar movement on examination, not reflex asymmetry) 5
• Primary muscle disorders (though muscular dystrophies can have diminished reflexes bilaterally, not unilaterally) 1

Clinical Action Required

Unequal patellar reflexes warrant neurological evaluation, not orthopedic musculoskeletal treatment. Consider MRI of the lumbar spine to evaluate for nerve root compression if clinical suspicion for radiculopathy exists, after accounting for the confounding effect of prior knee arthroplasty 1, 2.