15.3 Femur & Hip

Anatomy of the Femur and Hip

The femur is the longest bone in the body. Proximally, identify the rounded head with its central pit, the fovea capitis; the neck; the large greater trochanter laterally; the smaller lesser trochanter posteromedially; and the intertrochanteric crest (posterior) and intertrochanteric line (anterior) connecting them. The shaft (body) ends distally in the medial and lateral condyles. The hip joint is a ball-and-socket articulation between the femoral head and the acetabulum, a cup formed by the fused ilium, ischium, and pubis. Two geometric facts drive positioning: the femoral neck forms roughly a 120-130 degree angle with the shaft, and it is anteverted (rotated anteriorly) about 15-20 degrees. This anteversion is why a routine AP hip requires internal rotation of the leg.

Femur

The AP femur places the leg with the foot internally rotated about 5 degrees (unless trauma), CR perpendicular to the midfemur. Because of length, the projection must include the joint nearest the pathology: image the hip for proximal-third injuries and the knee for distal-third injuries; ideally both joints are demonstrated over two images. The lateral femur for the distal portion is a mediolateral with the affected side down and the knee flexed. For a proximal femur with a suspected fracture, do not attempt a mediolateral roll; use a cross-table (axiolateral) approach instead.

Hip: Routine

The AP hip centers the CR perpendicular to the femoral neck, about 2.5 inches distal to the midpoint of a line between the ASIS and the pubic symphysis (the neck lies 1-2 inches medial to the ASIS). The foot is internally rotated 15-20 degrees to overcome femoral anteversion and place the femoral neck in profile without foreshortening. The routine lateral is the modified Cleaves (frog-leg): the knee and hip are flexed and the femur is abducted 40-45 degrees from vertical with the plantar surface toward the opposite limb, demonstrating the femoral head, neck, and trochanters in a lateral profile.

Hip: Trauma

When a proximal femur or hip fracture is suspected, the frog-leg is contraindicated because its required abduction and external rotation could displace an unstable fracture. The safe substitute is the axiolateral (Danelius-Miller) inferosuperior projection: the unaffected leg is elevated out of the beam, the image receptor is placed vertically against the iliac crest and parallel to the femoral neck, and a horizontal central ray is directed perpendicular to the femoral neck and receptor. Because the injured limb is never moved, this projection safely demonstrates the femoral head, neck, and trochanters. A modified axiolateral (Clements-Nakayama), with the CR angled 15 degrees posteriorly, is used when both limbs are restricted.

Hip Projection Comparison

Acetabulum and Additional Projections

The acetabulum is evaluated with the AP oblique (Judet) method: 45-degree posterior obliques demonstrate the acetabular rim and columns. The internal (affected-side-down) oblique shows the iliopubic (anterior) column and posterior rim, while the external (affected-side-up) oblique shows the ilioischial (posterior) column and anterior rim. These are ordered after complex acetabular trauma. For centering, recall that the femoral head lies about 1.5 to 2 inches distal to the midpoint of a line drawn between the ASIS and the pubic symphysis, and roughly 1 to 2 inches medial to the ASIS; localizing it reliably prevents cutting off the acetabulum or proximal shaft.

Technical Factors and Evaluation Criteria

Unlike distal extremities, the hip and proximal femur use a grid and higher kVp (about 75 to 90) because of tissue thickness, with a large focal spot common for the AP hip. A correctly positioned AP hip shows the femoral neck in full profile without foreshortening, the greater trochanter in profile laterally, and the lesser trochanter barely visible, confirming the 15-20 degree internal rotation was achieved. If the lesser trochanter is prominent and the neck foreshortened, the limb was externally rotated. Respiration is suspended for the exposure.

Worked Trauma Scenario

An elderly patient falls and presents with a shortened, externally rotated leg and hip pain, the classic look of a femoral neck fracture. The technologist takes an AP hip in the as-presented position (no forced internal rotation) and adds a Danelius-Miller axiolateral using a horizontal beam, never a frog-leg. This pair confirms the fracture and its displacement while protecting the patient. Always include the hip joint on a proximal femur study and the knee joint on a distal femur study.

Common Traps

The single most important safety concept in this section is the trauma rule: never rotate a limb with a suspected fracture. If the stem says fracture, the answer is the Danelius-Miller axiolateral, not the frog-leg or an AP with internal rotation. A second trap is forgetting that the 15-20 degree internal rotation on a routine AP hip exists specifically to counteract anteversion; if you see the femoral neck foreshortened with the lesser trochanter prominent, the foot was externally rotated. Finally, remember to include the correct joint on a femur study rather than centering only to the shaft.