Tangential Patella Settegast is a radiography positioning projection of the Lower Extremity. Prone, knee acutely flexed at least 90 degrees, CR tangential to the patellar underside. SID 40". The centering point is located tangential to the patellofemoral joint, the beam skimming the underside of the patella.. The central ray is angled 15-45 degrees from the long axis of the lower leg (about 30 degrees nominal), skimming tangent to the patellofemoral joint.. Image-quality criteria include patellar articular surface and facets clearly visualized., patellofemoral joint space open and uniform.. Standard exposure ranges from 60 to 70 kVp, 4 to 7 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Patellar articular surface in tangential view.
- Medial and lateral patellar facets.
- Patellofemoral articulation.
- Patellar articular cartilage.
- Femoral groove.
- Anterior femoral condyles.
Patient preparation
- CONTRAINDICATED until transverse patellar fracture is excluded: this projection requires acute flexion.
- Verify patient identity using two identifiers.
- Remove shoes and any radiopaque items from the leg.
- Position patient prone on table with knee forward.
- Flex knee 45 to 90 degrees, adjusting degree as tolerated.
- Position IR under patella tangential to articular surface.
Position & centering point
Tangential to the patellofemoral joint, the beam skimming the underside of the patella.
Central ray
Angled 15-45 degrees from the long axis of the lower leg (about 30 degrees nominal), skimming tangent to the patellofemoral joint.
Exposure / technique
- kVp
- 60–70
- mAs
- 4–7
- SID
- 40" (102 cm)
- Notes
- Tangential projection shows the patellar facets and the patellofemoral joint space.
Image-quality criteria
- Patellar articular surface and facets clearly visualized.
- Patellofemoral joint space open and uniform.
- No superimposition of femur over patellar facets.
- Patellar outline complete with smooth margins.
- Articular cartilage surface visible.
- Collimation 4 x 4 inches around patella.
Common errors / ARRT traps
- 1 Insufficient knee flexion prevents tangential patella visualization.
- 2 Excessive flexion foreshortens patellar surface.
- 3 CR not perpendicular to patellar underside distorts facets.
- 4 IR not positioned tangential to patella surface.
Clinical indications
- Patellar chondromalacia and cartilage damage assessment.
- Patellar facet arthritis evaluation.
- Osteochondritis dissecans of patella.
- Patellofemoral joint space and patellar articular surface assessment.
Aligned to the current ARRT Radiography Content Specifications.