Normal Femoropatellar Joint Space Size on Radiography
The femoropatellar (patellofemoral) joint space is the cartilage-lined gap between the posterior patellar surface and the anterior femoral trochlea. Accurate measurement of this space on dedicated knee radiographs is essential for evaluating chondral integrity and identifying early degenerative or inflammatory joint disease.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Sunrise | >5 mm |
Clinical Significance
A femoropatellar joint space of greater than 5 mm is considered normal on the sunrise (axial/skyline) view. Narrowing below this threshold is a reliable radiographic indicator of cartilage loss and should prompt further evaluation. Joint space narrowing in the patellofemoral compartment often precedes or coexists with tibiofemoral disease, and its recognition can alter surgical planning or conservative management strategies.
Isolated patellofemoral narrowing is a common and often underappreciated finding. Key pitfalls include suboptimal knee flexion angle on the sunrise view, which can artificially widen or narrow the apparent joint space, and failure to assess the joint space in both medial and lateral facets separately, as lateral compartment narrowing is more frequent.
- Patellofemoral osteoarthritis — progressive cartilage loss with subchondral sclerosis and osteophyte formation
- Rheumatoid arthritis — uniform joint space narrowing often with periarticular osteopenia
- Chondromalacia patellae — early cartilage softening, may show subtle narrowing
- Crystalline arthropathy (CPPD) — chondrocalcinosis with secondary joint space reduction
- Post-traumatic arthritis — asymmetric narrowing following patellar fracture or trochlear injury
Reference: Moeller T. Normal Findings in Radiography. TFL. (2000).
Imaging Notes
The femoropatellar joint space is best assessed on the sunrise (axial/skyline) view, obtained with the knee flexed typically between 30° and 45°. The central X-ray beam is directed along the patellofemoral joint. Consistent flexion angle is critical — greater flexion reduces the measurable joint space and may mimic narrowing. Both medial and lateral facets should be evaluated independently, as pathology is frequently asymmetric.
Standard anteroposterior and lateral knee projections are inadequate for reliable patellofemoral joint space measurement and should not substitute for the dedicated axial projection. When radiographic findings are equivocal, MRI provides superior assessment of cartilage thickness and morphology.