Subtrochanteric Fractures of the Femur

Anatomy

The adult femoral shaft has an asymmetric anterior bow with an average radius of curvature between 109 and 120 cm. The average proximal neck-shaft angle is 129 degrees in men and 133 degrees in women, although significant variability exists. The femoral neck and head are anteverted approximately 13 degrees and offset anteriorly relative to the central axis of the femoral shaft. The lesser trochanter is the attachment for the iliacus and psoas major; these are strong deforming forces if the lesser trochanter remains intact. Additionally, an intact lesser trochanter represents a significant contributor to fracture stability if an anatomic reduction is accomplished.

The subtrochanteric region of the femoral shaft is almost completely encased in a muscular envelope that provides significant blood supply to the fracture region and is important with regard to fracture healing. These same muscular attachments that surround the proximal femur are primarily responsible for the commonly observed deformity patterns after fracture in the subtrochanteric region. Because of these strong deforming forces, reduction is difficult, especially in young patients. The integrity of the trochanters influences the deformities observed. For fractures below the lesser trochanter, the proximal segment is typically flexed, abducted, and externally rotated by the hip abductors, external rotators, and iliopsoas muscles. The adductors typically medialize the distal shaft component. For subtrochanteric fractures with an associated fracture of the lesser trochanter, the deformity pattern of the proximal segment may actually be less severe because the flexion and external rotation of the psoas will be absent. Given these multiple and large muscular forces, reduction with pure axial traction is frequently unsuccessful. Generally a combination of positioning, bumps, externally applied forces, and simultaneous control of both the proximal and distal fracture segments is necessary to reduce subtrochanteric fractures ( Fig. 57.1 ).

The relevant blood supply to the femoral shaft is from the primary nutrient vessel(s) combined with contributions from the multiple periosteal vessels. The nutrient artery enters in the region of the linea aspera in the proximal half or third of the femur dictating that the linea aspera should not be exposed or stripped of its muscular attachments. The femoral head blood supply involves branches from the medial femoral circumflex vessel that are in close proximity to nail insertion locations. These branches are located posterior to and in close proximity to the piriformis fossa entry portal and injury can occur with nail entry preparation ( Fig. 57.2 ). However, avascular necrosis of the femoral head after antegrade piriformis entry nailing in adults is rarely reported.

Biomechanics

The femur is subjected to high compressive, tensile, and torsional forces with normal activities. Hip joint forces in adults were measured by Paul and found to range from approximately four times body weight for slow walking to nearly seven times in rapid walking. The subtrochanteric region is subjected to high mechanical stresses as a result of a combination of body weight, velocity of activity, and the multiple muscles that exert a deforming force on the proximal femur. Because the angle of the applied force is nearly perpendicular to the axis of the femoral neck, there is considerable bending induced in the subtrochanteric region of the femur.

Using simple femur modeling it has been calculated that the highest stresses in compression occur at the base of the medial subtrochanteric region (≈8.6 × 10 ⁶ N/m ² ), and in tension, just below the greater trochanter (≈6.3 × 10 ⁶ N/m ² ). A more detailed model using a finite element analysis method that included the effect of muscular forces confirmed this location for maximal stresses but also demonstrated that the tensor fasciae latae significantly reduces the overall load on the shaft of the femur. Both the joint load and abductor muscles apply bending moments that effectively cause the femur to want to bow laterally while the tensor fasciae latae counteracts these moments.

Since the mechanical axis is situated medial to the anatomic axis of the femur in the subtrochanteric region, axial loading injuries are expected to produce compressive fracture patterns medially and tensile patterns laterally.

Fixation of subtrochanteric fractures requires an understanding of the biomechanical impact of the commonly used implants (mechanobiology) and their relationship to stable and unstable fracture patterns. Intramedullary (IM) implants are mechanically well suited for femoral fractures given their central location. Current nail designs have multiple interlocking options that have resistance to deformation in response to axial and torsional loads. Some manufacturers permit the use of three screws through the proximal nail, which results in a stiffer construct compared with two screws. Additionally, it has been shown that the use of two crossed screws is superior to a pair of parallel screws in the proximal segment.

There is significant support in biomechanical studies for the use of cephalomedullary nails. Forward et al. have demonstrated superior biomechanical results with a cephalomedullary nail compared with a proximal femoral plate and a 95-degree blade plate.

The age-independent average radius of curvature of an adult femur has been estimated to range between 109 and 120 cm in a North American population. Other populations, however, will have differing characteristics that need to be considered when choosing a fixation method. A Chinese study using a three-dimensional (3-D) model and computer-aided design technology showed that in their population males had a radius of curvature of 102 ± 21 cm, whereas females had a radius of curvature of 87 ± 17 cm. Furthermore they noted a positive correlation between height and radius of curvature, which had not been identified by others. In virtually all nails manufactured, there is a mismatch between the radius of curvature of the nail and femur. Most femoral nails are significantly straighter than the average femur and have an average radius of curvature ranging from 150 to 300 cm. This can affect final sagittal plane alignment, entry portal location, and entry bursting strains. A more posterior starting point in the posterior third of the greater trochanter is associated with anterior cortical impingement or perforation of the nail at the distal tip of the nail in the distal femur. This problem has been decreased substantially with newer nail designs using a smaller radius of curvature. During antegrade piriformis entry femoral nailing, it has been demonstrated that anterior misplacement of the nail entry site increased the potential for femoral bursting during nail insertion. Similarly, the starting point for a trochanteric nail has the potential to affect the capacity to achieve an anatomic reduction. An excessively lateral entry point will result in varus malalignment. The starting point should be at the tip of the trochanter or slightly medial, based on a cadaver study.

Proximal femoral plates, applied laterally, are eccentric relative to the mechanical axis of the femur compared with nails and are therefore associated with decreased bending stiffness after fixation. Implant design characteristics have a significant influence on the mechanical stability after fixation. Implants with a proximal fixed angle are thought to offer a mechanical and fixation advantage. The 95-degree angled blade plate has been shown to produce superior torsional stability compared with other plate constructs, including locking plates. The newer locking implants that use angled locked screws into the femoral head have higher stiffness in axial bending and have the least irreversible deformation. Biomechanical studies have shown that newer designs of proximal femoral locking plate have improved stiffness when compared with older designs that were recognized to have poor clinical outcomes. Forward et al. have shown that a contemporary proximal femoral locking plate has similar biomechanical characteristics to a blade plate, and Kim and colleagues demonstrated that the inverted contralateral distal femur locking plate (LCP-DF) has better biomechanical properties than a dynamic condylar screw (DCS). Also, the “kickstand” screw confers additional axial stiffness to a construct. An early biomechanical study showed plates were stiffer in torsion than antegrade interlocking nails but similar in bending stiffness. However, nails were found to be stronger in combined compression and bending to failure. A further study showed that a cephalomedullary nail can withstand more loading cycles and failed at higher loads compared with various plate constructs.

Incidence and Mechanism of Injury

There is an asymmetric age- and gender-related bimodal distribution of fractures, with high-energy injuries occurring in young men and low-energy injuries occurring in elderly women. The majority of these fractures occur in elderly patients. Approximately half of patients are elderly and are injured by low-energy falls, approximately 25% are young patients with high-energy mechanisms, and approximately 25% are pathologic fractures. The vast majority of the nonpathologic fractures are unstable patterns with associated posteromedial comminution.

Approximately 10 years ago, a form of “atypical” insufficiency fracture in the subtrochanteric region or femoral shaft was described. This fracture has been associated with the use of alendronate, which causes suppression of bone remodeling. Management of this fracture variant has proven difficult, with outcomes generally being poorer than typical subtrochanteric fractures. Management of this variant is discussed in Chapter 58 .

Fracture in the subtrochanteric region can also occur as a complication of screw fixation for femoral neck fracture ( Fig. 57.3 ). Placement of screws weakens the tension side cortex of the proximal femur. Because these stresses are highest at the level of the inferior margin of the greater trochanter, it is recommended that the screw entry site be situated above this level. The screw configuration may also influence the incidence of subtrochanteric fracture after cannulated screw fixation. If a triangular screw configuration is planned for stabilization of a femoral neck fracture, an apex-distal configuration may minimize this risk.

Similarly, subtrochanteric fractures may occur after other prior proximal femoral surgery that violate the lateral cortex of the femur. Core decompression for avascular necrosis and free fibular grafting have both been complicated by infrequent subtrochanteric fracture. Current recommendations to prevent this include keeping the lateral cortical defect above the level of the inferior margin of the lesser trochanter. A further question arises with respect to management of the lateral cortical defect that remains after implant removal in this area, in particular the dynamic hip screw (DHS), which leaves a substantial defect. Some surgeons recommend application of bone graft to this defect after implant removal.

Physical Examination

A comprehensive and methodical examination of all extremities, the spine, and the pelvis should be performed to ensure that associated injuries are not missed. A careful visual inspection of the entire circumference of the hip and thigh should be performed to look for open wounds and closed degloving injuries. A closed degloving or Morel Lavallee lesion may be extensive but not obvious without a thorough examination, and often repeated examination. This additional lesion may affect the treatment of a subtrochanteric fracture. Surgery undertaken through such a lesion is known to have a higher risk of infection. Any skin disruption should be considered a potential open fracture and further evaluated. The knee should be examined for any associated ligamentous injury. The vascular examination of the extremity is determined with palpation of the distal pulses and confirmed with Doppler examination if necessary. Evaluation and documentation of femoral and sciatic nerve function are essential.

Associated injuries are more common in young patients with high-energy mechanisms. Ipsilateral noncontiguous femur fractures, acetabular and pelvic injuries, spine fractures, and other ipsilateral fractures are seen. Associated abdominal, thoracic, and head trauma related to the mechanism of injury requires a careful evaluation by a specialized team of physicians.

Radiographic Studies

The radiographic evaluation begins with full-length anterior-posterior (AP) and lateral radiographs of the entire femur from the hip to the knee. Additionally, biplanar hip radiographs and an AP pelvis radiograph are necessary to fully evaluate the extent of injury. Traction radiographs are extremely helpful for delineating subtle fracture lines, for understanding the fracture pattern, and for preoperative planning. The radiographs should be used to comprehensively evaluate the fracture pattern, bone quality, and the presence of bone loss. In particular, proximal extension into the trochanteric region and distally into the femoral shaft should be assessed. Contralateral hip and femur radiographs may be helpful for preoperative planning and can help determine the femoral length, canal diameter, femoral bow, and femoral neck anteversion; however, they are not routinely undertaken. Radiographs should be scrutinized for the presence of osteopenia, metastases, and cortical irregularities in the femur or at the site of fracture.

Computed tomography (CT) can be valuable in complex fracture patterns, fractures with proximal extension, and fractures with significant rotation of the proximal segment such that visualization is poor. In the setting of multiple trauma, a pan-scan is frequently undertaken and will include the femur; this can be very useful. Often a CT of the abdomen or pelvis is obtained for other reasons and is available to assess the hip and proximal femur. Alternatively the CT can be extended distally to include the proximal femur. Valuable information revealed by CT scanning includes proximal extension into the piriformis fossa, the presence of an associated femoral neck or intertrochanteric fracture, and the presence of nondisplaced fracture lines that may influence the surgical approach or implant selection ( Fig. 57.4 ). Additional radiographic studies for evaluation of the osseous injury are unnecessary. However, additional radionucleotide studies or magnetic resonance imaging (MRI) may be indicated if a pathologic fracture is suspected.

Classification

Subtrochanteric femoral fractures have been classified by the anatomic location, fracture morphology, number of fragments, degree of comminution, and combinations thereof. Early classification systems did little to influence treatment but did identify fracture patterns of particular difficulty. The Arbeitsgemeinschaft für Osteosynthesefragen (AO) introduced a comprehensive classification that included description of the fracture morphology and the degree of comminution but did not include a means of describing fractures with extension into the trochanteric region.

The Russell-Taylor classification ( Fig. 57.5 ) was introduced with the important considerations in this system being the integrity of the lesser trochanter and proximal extension into the region of the greater trochanter and the piriformis fossa. The integrity of the lesser trochanter is a reasonable surrogate for posteromedial support of the subtrochanteric region. The presence or absence of involvement of the posteromedial proximal femur is important when considering implant choice and fixation stability at the completion of surgery. Additionally, fracture extension into the region of the starting point for IM implants complicates treatment, changes the surgical strategy, and may affect the implant selection.

Similar to many fracture classifications, the AO and the Russell-Taylor classification systems (and others) have been shown to have poor reproducibility. A literature review by Damany and colleagues from 1966 to 2003 involving 2725 subtrochanteric fractures found 16 classifications, none of which showed value in determining treatment or outcomes.

Management

The decision for operative management requires a thorough understanding of the associated injuries, the condition of the patient, the fracture pattern, the biomechanics of the subtrochanteric region of the femur, and the available implant options. Although nonoperative treatment has had a role in these fractures in the past, operative treatment is advocated to allow for patient mobilization, restoration of anatomy, and maximization of function ( Table 57.1 ).