Tangential Patella Settegast is a radiography positioning projection of the Lower Extremity. Prone, knee flexed to at least 90 degrees (Bontrager) or until the patella is perpendicular to the IR (Merrill's), CR tangential to the patellofemoral joint. 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 15-20 degrees from the long axis of the lower leg, tangential to the patellofemoral joint space. Image-quality criteria include patella in profile, with the articulating surfaces of the patellofemoral joint, patellofemoral joint space open and uniform. A sample technique is 70 kVp with DR 2 or CR 4 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Patellar articular surface in tangential view.
- Patellofemoral articulation.
- Femoral groove.
- Anterior femoral condyles.
Patient preparation
- WARNING: do not attempt until a patellar fracture has been ruled out by other projections (Bontrager), or a transverse patellar fracture by a lateral image (Merrill's): acute flexion can displace fragments. Merrill's also says not to attempt it if the patient is in pain.
- Verify patient identity using two identifiers.
- Remove shoes and any radiopaque items from the leg.
- Position patient prone on table with knee forward.
- Flex the knee slowly: to a minimum of 90 degrees (Bontrager), or as much as possible or until the patella is perpendicular to the IR, if the condition permits (Merrill's).
- Place the IR under the knee, centered to the patellofemoral joint.
Position & centering point
Tangential to the patellofemoral joint, the beam skimming the underside of the patella.
Central ray
Angled 15-20 degrees from the long axis of the lower leg, tangential to the patellofemoral joint space.
Exposure / technique
- kVp
- 70
- mAs
- DR 2 / CR 4
- SID
- 40" (102 cm)
- Notes
- Tangential projection shows the patellofemoral joint articulating surfaces. Merrill's adds that vertical patellar fractures are shown. Sample technique: DR 2 / CR 4 mAs at 70 kVp, 40 in SID, for a 12 cm part. Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Patella in profile, with the articulating surfaces of the patellofemoral joint.
- Patellofemoral joint space open and uniform.
- Collimation 4 x 4 inches around patella.
Common errors
- 1 Knee not flexed to a minimum of 90 degrees (Bontrager).
- 2 Acute flexion draws the patella into the intercondylar sulcus, reducing diagnostic value (Bontrager).
- 3 CR not tangential to the patellofemoral joint space.
Clinical indications
- Patellofemoral joint articulating surfaces (Merrill's).