What is a Skyline X-ray View of the Knee?
A skyline X-ray (also called an axial or tangential view) is a specialized radiographic projection that images the patellofemoral joint from above, showing the relationship between the patella and the femoral trochlear groove to assess patellar tracking, tilt, subluxation, and cartilage damage. 1, 2
Technical Positioning
- The skyline view is obtained with the knee in flexion, typically at 30 degrees, which provides the most reproducible measurements and optimal detection of patellofemoral abnormalities 3
- The X-ray beam is directed tangentially across the anterior surface of the knee, parallel to the patellofemoral joint surface 4
- Weight-bearing skyline views provide superior assessment of patellofemoral kinematics compared to non-weight-bearing views, as they capture true functional patellar tracking under load 1, 2, 5
- When standard positioning is not possible due to limited flexion (such as after acute patellar dislocation), the technique can be adapted based on the degree of flexion the patient can achieve 4
Clinical Applications
The skyline view is the primary radiographic method for evaluating:
- Patellar tilt or subluxation - demonstrates the degree of lateral or medial displacement of the patella relative to the femoral trochlea 1, 2, 5
- Patellar tracking abnormalities - assesses whether the patella moves properly within the trochlear groove during knee flexion 5, 6
- Osteochondral fractures - particularly avulsion fractures at the medial patellar facet following acute dislocation 4
- Patellofemoral osteoarthritis - the skyline view has greater sensitivity and specificity for detecting cartilage damage at the patellofemoral joint compared to lateral views 1, 6
- Post-surgical complications - evaluates patellar component positioning and complications after total knee arthroplasty 1, 5
Key Measurements Obtained
The skyline view allows measurement of specific patellofemoral parameters including:
- Patellofemoral angle - assesses patellar tilt 7
- Congruence angle - assesses patellar lateralization 7
- Trochlear depth - evaluates the femoral trochlear groove morphology 7
Clinical Recommendations
- For osteoarthritis evaluation, the EULAR guidelines recommend weight-bearing patellofemoral views (skyline) as part of routine knee radiography to optimize detection of OA features 1
- The American College of Radiology recommends that routine knee evaluation should include standing AP and lateral views plus an axial tangential view (skyline) of the patellofemoral joint 1, 2
- The skyline view should be performed as part of trauma knee radiographic series when patellar dislocation is clinically suspected, as it can reveal fractures not visible on standard AP and lateral views 4
- For detecting radiographic knee osteoarthritis, adding either a skyline or lateral view to an AP view yields high sensitivity (94-97%), though the skyline view provides more reproducible assessment of the patellofemoral joint space 8
Important Caveats
- Skyline views obtained at flexion angles less than 30 degrees are often technically inadequate and may miss patellofemoral abnormalities 7, 3
- Skyline views have limitations in screening for early maltracking because they cannot reliably image the patellofemoral joint in near-extension (less than 30 degrees flexion), where many tracking abnormalities are most apparent 7
- Technical quality can be problematic - skyline views are more frequently excluded from analysis due to unsatisfactory imaging of the patellofemoral joint compared to other knee views 8
- Surgical decisions should never be based on skyline views alone, as conventional radiography has limitations in accuracy; CT or MRI should be obtained when radiographic findings are equivocal 7, 1