Timothy S. Achor

Bony Anatomy

The subtrochanteric region of the femur includes the area from the lesser trochanter to a point 8 cm distally. However, any fracture occurring in the diaphysis of the proximal one-third of the femur can generally be referred to as subtrochanteric (Fig. 19-1).
The anatomy of the proximal femur is complex from both bone and soft tissue standpoints.
There are multiple deforming forces acting on the subtrochanteric region that result in a classic pattern of proximal fragment flexion with abduction and external rotation and distal fragment adduction and shortening. Failure to correct varus and flexion moments at the fracture often lead to malunion, nonunion, or implant failure.
The cortical bone in this region is particularly dense, as the proximal femur sees tremendous forces. Comminution, especially medially, is not uncommon and can pose a challenge to restoring coronal alignment.
There is no true spongy “metaphyseal bone” in the proximal femur.
The isthmus of the femur is typically located at the junction of the proximal and middle thirds.

Radiographic Anatomy

The piriformis fossa is in line with the medullary canal (anatomic axis) of the femur on the AP projection. In cases of medial subtrochanteric comminution, a piriformis nail offers a favorable mechanical advantage to preventing varus malalignment but requires adequate intraoperative reduction techniques during reaming and nail insertion.
AP and lateral radiographs of the proximal femur will reveal the injury with predictable displacement patterns.
Lateral projection images are usually obtained in a cross-table manner with contralateral hip and knee flexion.
Typical displacement pattern of the proximal segment includes flexion, abduction, and external rotation due to the muscle pull of the iliopsoas, gluteus medius/minimus, and external rotators, respectively (Fig. 19-2).
Distally, there can be an adduction deformity (Fig. 19-3).
Low-energy subtrochanteric fractures with a history of bisphosphonate use must raise suspicion of a pathologic fracture.
Radiographic findings associated with bisphosphonate fractures include cortical thickening, lateral cortical beaking, and a relatively transverse pattern (Fig. 19-4).

As proximal femur fractures are typically high-energy injuries, a screening AP pelvis radiograph is indicated to rule out any associated injuries (Fig. 19-5).
Plain films radiographs are typically sufficient for diagnosis and preoperative planning; however, CT scan can be helpful in identifying occult fracture lines, and 3-D reconstructions can be utilized as well.
Full-length orthogonal images of the femur are critical to rule out associated segmental injuries (Figs. 19-6 and 19-7).
Traction views can be helpful to determine the exact level of the fracture. This is important in determining the appropriate implant (nail vs. plate) and proximal interlocking bolt options (standard vs. reconstruction/cephalomedullary) (Figs. 19-8 and 19-9).
Any suggestion on plain films or clinical suspicion of an ipsilateral femoral neck fracture warrants additional advanced imaging studies (i.e., CT scan) (Figs. 19-10 and 19-11).
Again, medial comminution associated with subtrochanteric fractures is common and must be noted; these fractures are at risk for varus malreduction/displacement due to loss of medial support.