How should I document the patellofemoral joint space on imaging and physical examination?

How to Document the Patellofemoral Joint Space

For imaging documentation, obtain standing anteroposterior (AP), lateral, and tangential axial (skyline) views of the patellofemoral joint, with the skyline view being the most critical for assessing the patellofemoral joint space. 1, 2

Imaging Documentation

Required Radiographic Views

The patellofemoral joint requires specific views that differ from standard knee radiographs:

• Tangential patellar (skyline) view: This is the gold standard for visualizing the patellofemoral joint space 1, 2
• Weight-bearing views are essential - standing radiographs optimize detection of joint space narrowing 1
• Lateral view of the affected knee
• At least one frontal projection (AP, Rosenberg, or tunnel view)

The skyline view consistently outperforms lateral views for patellofemoral assessment, with higher sensitivity (52.8% vs 30%) for detecting symptomatic disease and better reproducibility (kappa >0.8 for osteophytes, >0.6 for joint space narrowing) 3.

Measuring Joint Space Width

Document the minimum joint space width (mJSW) separately for medial and lateral patellofemoral compartments:

• Use a mm-graded ruler on standing skyline radiographs
• Joint space <5 mm indicates narrowing in middle-aged adults with knee pain 4
• Measurements show excellent reproducibility (ICC 0.92-0.96 for test-retest) 5
• Measure the interbone distance using the dorsal bony landmark of the patella 4

Important caveat: Radiographic measurements correlate with but are not concordant with MRI cartilage thickness, particularly at extremes - radiographs overestimate in normal joints and underestimate in severely narrowed joints 5.

Physical Examination Documentation

Document these specific findings in sequential order:

Standing Position

• Limb alignment (varus/valgus deformity)
• Gait analysis noting antalgic patterns
• Muscle atrophy (particularly vastus medialis oblique)

Sitting Position

• Patellar tracking during active knee extension
• Q-angle measurement
• Patellar tilt and subluxation

Supine Position

• Hyperpression test: Document pain with manual compression of patella against femoral trochlea 6
• Patellar mobility (medial/lateral glide)
• Range of motion with crepitus notation
• Effusion presence

Prone Position

• Hamstring and quadriceps flexibility
• Femoral anteversion assessment

7, 6

Advanced Imaging Indications

MRI without contrast is indicated when:

• Radiographs are normal but pain persists
• Evaluating patellofemoral cartilage loss and bone marrow lesions (BMLs)
• Assessing for subchondral insufficiency fractures
• Quantifying synovitis >2mm in Hoffa's fat pad

MRI accurately depicts cartilage abnormalities, BMLs (which correlate with active knee pain), and soft tissue pathology that radiographs cannot visualize 1.

CT is reserved for:

• Calculating tibial tubercle-trochlear groove distance
• Evaluating trochlear morphology in recurrent subluxation/dislocation
• Assessing rotational alignment of femoral components (post-arthroplasty)

1, 8

Documentation Pitfalls to Avoid

• Don't rely on lateral views alone - they miss significant patellofemoral pathology that skyline views detect 3
• Don't obtain non-weight-bearing films - they underestimate joint space narrowing 1
• Don't skip the axial view - it's essential for documenting patellar tilt, subluxation, and compartment-specific narrowing 8
• Don't assume radiographic joint space equals cartilage thickness - correlation exists but concordance does not, especially in advanced disease 5