Patella Anatomy and Projections

Study Notes

The patella is a flat, triangular bone, approximately 2 inches in diameter.
The apex is the inferior border of the patella.
The base is the superior border of the patella.
The outer/anterior surface is rough and convex.
The inner/posterior surface is smooth and oval shaped, articulating with the femur.
The patella protects the anterior aspect of the knee joint.
The patella increases the leverage of the quadriceps femoris muscle.
When the leg is extended, the patella is loose and movable, sitting more superiorly.
When the leg is flexed, the patella locks into position, sitting more distally.

PA
- Used for evaluating patellar fractures, especially before the knee joint is flexed for other projections.
- The patient is prone with the leg fully extended.
- A true PA view typically requires 5 degrees of internal rotation.
- The CR is perpendicular and directed to the mid-patella, around the mid-popliteal crease.
Lateral (Mediolateral)
- Used for evaluating patellar fractures in conjunction with the PA view, and for abnormalities in the patellofemoral and femorotibial joints.
- The patient is in lateral recumbent position with the affected side down.
- The knee is flexed only 5-10 degrees.
- The CR is perpendicular and directed at the mid-patellofemoral joint.
Tangential
- These projections are otherwise known as axial, sunrise, or skyline projections.
- Used for evaluating patellar subluxation and other abnormalities of the patella and patellofemoral joint.
- Common tangential projections include the Merchant, Inferosuperior, Hughston, Settegast, and Hobbs methods.

Requires no grid but utilizes an air gap.
SID is 48-72 inches.
The patient is supine with knees flexed 40 degrees over the end of the table, resting on a leg support.
The IR is placed against the legs approximately 6 inches below the knees, perpendicular to the x-ray beam.
The CR is angled 30 degrees caudad from the horizontal plane, directed at the point midway between the patellae.

SID is 40-48 inches, no grid.
The patient is supine with legs together.
Adequate support is placed under the knees for 40-45 degree knee flexion (legs relaxed).
The IR rests on the midthighs, tilted to be perpendicular to the CR.
The CR is directed inferosuperiorly at 10-15 degrees from the lower legs (starting at horizontal) to be tangential to the patellofemoral joint.

The patient can be prone or seated.
The knee is flexed to a minimum of 90 degrees.
The patient should hold onto gauze or tape to maintain position, especially in the prone position.
The CR is directed tangential to the patellofemoral joint space (15-20 degrees from the lower leg, starting at horizontal).
Minimum SID is 40 inches.