Should I be concerned about medial subluxation of the femur?

Should You Be Concerned About Medial Subluxation of the Femur?

Yes, medial subluxation of the femur warrants clinical concern and requires evaluation to determine the underlying cause, as it can indicate significant pathology including ligamentous injury, malalignment, or prosthetic complications that may progress without appropriate management.

Context Matters: Where Is This Occurring?

The clinical significance of medial femoral subluxation depends critically on the anatomical location and clinical context:

In the Native Knee

• Medial subluxation at the tibiofemoral joint is uncommon in native knees but can occur with severe ligamentous injury or after corrective osteotomy procedures 1
• After opening wedge high tibial osteotomy, the proximal tibia commonly shifts medially relative to the distal femur, which correlates with correction loss and may represent a biomechanical concern 1
• This medial shift can be quantified as a percentage of tibial plateau width and should be monitored on follow-up radiographs 1

In Total Knee Arthroplasty

• Posteromedial subluxation of the femur is a recognized complication after posterior cruciate-retaining total knee arthroplasty, occurring in approximately 0.2% of cases 2
• This typically develops years after the index surgery (mean 4.8 years) and is primarily caused by tightness of the posterior cruciate ligament 2
• Progressive subluxation in this context often requires revision surgery, as it represents mechanical failure of the arthroplasty 2

At the Patellofemoral Joint

• Medial subluxation of the patella is extremely rare compared to lateral patellar subluxation 3, 4
• Lateral patellar subluxation is the common pathologic pattern and can be assessed on weight-bearing axial radiographs 4
• CT is the preferred modality for detailed assessment of rotational malalignment when evaluating patellofemoral tracking abnormalities 4

Diagnostic Approach

Initial Imaging

• Obtain weight-bearing radiographs including anteroposterior, lateral, and appropriate oblique views to assess alignment and subluxation 3
• Weight-bearing films are essential as they reveal dynamic abnormalities including joint malalignment and subluxation that may not be apparent on non-weight-bearing studies 3

Advanced Imaging When Indicated

• MRI without contrast should be performed if radiographs are normal but clinical suspicion remains high, as it can detect early bone and joint abnormalities with good sensitivity 3
• MRI is particularly valuable for identifying associated soft tissue injuries, ligamentous pathology, and bone marrow edema patterns 3
• CT may be preferred for precise measurement of bony alignment and rotational abnormalities, especially in post-arthroplasty patients 3, 4

Clinical Implications and Management

Risk of Progression

• Subluxation patterns can progress to more severe complications if not addressed 1, 2
• In post-arthroplasty patients, progressive subluxation often requires revision surgery to address the underlying mechanical problem 2
• Correction loss after osteotomy procedures correlates with the degree of medial tibial shift 1

Associated Pathology to Evaluate

• Ligamentous integrity: Assess for posterior cruciate ligament tightness in TKA patients or collateral ligament injury in native knees 2
• Meniscal pathology: Meniscal subluxation is strongly associated with joint space narrowing and osteoarthritis progression 5
• Alignment abnormalities: Evaluate the mechanical axis and component positioning on full-length standing radiographs 3

Common Pitfalls to Avoid

• Do not delay advanced imaging if clinical suspicion is high despite normal radiographs, as early bone and joint abnormalities may only be visible on MRI 3
• Do not obtain non-weight-bearing radiographs alone when evaluating subluxation, as dynamic malalignment may be missed 3
• Do not assume all subluxation is benign: Progressive patterns, especially in post-surgical patients, often indicate mechanical failure requiring intervention 2
• Do not confuse medial and lateral patellar subluxation: Lateral patellar subluxation is the common pathologic pattern, while medial patellar subluxation is rare 3, 4