Medial Knee Lump and Pain: Diagnostic Approach
A palpable lump on the medial knee with pain is most commonly caused by medial compartment osteoarthritis with effusion, medial meniscal pathology, or medial plica syndrome, and requires initial evaluation with weight-bearing radiographs followed by MRI if radiographs are non-diagnostic. 1
Most Common Structural Causes
Medial compartment osteoarthritis is the most frequent cause because 70-80% of joint load passes through this compartment during gait, making it the predominant site of degenerative changes 1. Physical examination routinely reveals knee effusions in OA patients, and the severity of synovitis/effusion correlates significantly with frequent knee pain 1. The palpable "lump" you feel is typically joint effusion combined with osteophyte formation at the medial joint line 1.
Medial meniscal tears cause localized medial joint line tenderness and swelling, presenting as a palpable fullness 1. Joint line tenderness has 83% sensitivity and 83% specificity for meniscal tears, while the McMurray test (knee rotation with extension) has 61% sensitivity and 84% specificity 2.
Medial plica syndrome causes anterior-medial knee pain and swelling, presenting as a palpable band or thickened fold 1, 3. This is an inflamed synovial fold that becomes symptomatic with repetitive flexion-extension activities 4. The main complaint is intermittent, dull, aching pain medial to the patella above the joint line, worsening with activity 4.
Critical Physical Examination Findings
Palpation-induced tenderness over the medial joint line that matches the quality and location of pain during activity is highly specific for intra-articular pathology 1.
Inspect for erythema, warmth, and effusion—joint effusions are uncommon with isolated tendinopathy and suggest intra-articular pathology requiring further workup 1. The presence of a true effusion makes osteoarthritis, meniscal pathology, or inflammatory arthritis more likely than soft tissue causes 1.
Algorithmic Diagnostic Approach
Step 1: Obtain anteroposterior and lateral weight-bearing knee radiographs first to exclude fractures, osteoarthritis, osteophytes, and loose bodies 1, 5. Weight-bearing views are essential as they demonstrate joint space narrowing that may not be apparent on non-weight-bearing films 6.
Step 2: If radiographs show osteoarthritis and symptoms match the radiographic findings, no further imaging is needed initially 6. Proceed with conservative management including exercise therapy, weight loss if overweight, and education 2.
Step 3: If radiographs are normal or show only effusion but pain and the palpable lump persist, proceed to MRI without IV contrast 6, 1. MRI can identify MCL bursitis, meniscal tears, bone marrow lesions, synovitis, and medial plica pathology 1.
Step 4: If radiographs show osteoarthritis but symptoms are unexplained by the degree of radiographic changes, MRI without IV contrast is indicated to identify pain generators such as bone marrow lesions, synovitis, or meniscal tears 6.
Common Pitfalls to Avoid
Do not order MRI without obtaining radiographs first—approximately 20% of patients with chronic knee pain inappropriately receive MRI without radiographs within the prior year 7. This wastes resources and delays appropriate diagnosis 7.
Be aware that MRI findings do not always correlate with symptoms, especially in patients over 70 years where bilateral structural abnormalities can exist with unilateral symptoms 6. Meniscal tears are often incidental findings in older patients and may not be the pain source 6.
Joint effusions suggest intra-articular pathology, not isolated tendinopathy or bursitis 1. If you palpate a true effusion, focus your differential on osteoarthritis, meniscal tears, inflammatory arthritis, or plica syndrome rather than soft tissue causes 1.
Initial Management Considerations
For osteoarthritis predominantly involving the medial tibiofemoral compartment, bracing can be useful nonoperative treatment 1. First-line management consists of exercise therapy, weight loss if overweight, and self-management education programs 2.
For medial plica syndrome, treatment includes physiotherapy, activity modification, and rest 4. If conservative treatment fails after 4-6 weeks, corticosteroid injections may be considered 4. Conservative treatment is more appropriate in young patients with short symptom duration 4.
For meniscal tears, conservative management with exercise therapy for 4-6 weeks is appropriate for most tears 2. Surgery is only indicated for severe traumatic bucket-handle tears with displaced meniscal tissue, not for degenerative tears even with mechanical symptoms 2.