Orthopaedic Surgery - Atypical Stress Fractures

Basics

Atypical stress fractures most commonly involve the subtrochanteric region or femoral shaft and are strongly associated with prolonged bisphosphonate therapy.

These fractures occur distal to the lesser trochanter and proximal to the supracondylar region of the femur.

They generally result from a low-energy mechanism, such as a fall from standing height, and some patients report no preceding traumatic event at all.

A characteristic radiographic feature is localized thickening or beaking of the lateral femoral cortex.

The fracture is usually transverse or short oblique, is generally noncomminuted, and may occasionally demonstrate a medial cortical spike.

Epidemiology

The reported incidence of atypical femoral fractures is approximately 50–130 cases per 100,000 patient-years.

The true incidence may be lower because epidemiologic studies sometimes have difficulty consistently identifying fractures that meet formal atypical femoral fracture criteria.

These fractures remain substantially less common than typical osteoporotic hip fractures.

They appear to occur more frequently in women and individuals of Asian ethnicity.

Risk Factors

The most important risk factor is long-term bisphosphonate use.

The risk increases with longer duration of treatment and becomes more significant after approximately 4 years of continuous therapy, although atypical fractures have been reported after as little as 1.5 years of treatment.

Other recognized risk factors include Asian ethnicity, prolonged glucocorticoid therapy, autoimmune disease, obesity with a BMI greater than 30 kg/m², age younger than approximately 70 years, and earlier menopause.

Etiology and Pathophysiology

The principal proposed mechanism is prolonged suppression of normal bone remodeling caused by bisphosphonates.

The femur is exposed to repetitive mechanical loading during everyday activities. This produces microscopic damage, particularly along the lateral cortex, which normally undergoes continuous remodeling and repair.

Bisphosphonate therapy can markedly reduce bone turnover. When remodeling is excessively suppressed, microdamage may accumulate rather than being repaired.

Over time, small cortical cracks may propagate and eventually develop into an incomplete or complete atypical fracture.

Mechanical Factors

The lateral femoral cortex experiences considerable tensile stress during weight-bearing.

Certain anatomical characteristics can increase these stresses, including lateral femoral bowing and varus alignment of the proximal femur.

These mechanical factors may concentrate forces along the lateral cortex and increase susceptibility to fracture.

Genetic Factors

A genetic predisposition has also been reported in some patients.

The precise genetic contribution remains incompletely understood, but inherited differences in bone remodeling or femoral geometry may influence susceptibility.

Associated Conditions

Atypical femoral fractures are frequently encountered in patients with osteoporosis receiving prolonged bisphosphonate treatment.

They may also occur in individuals with autoimmune disorders who have required prolonged glucocorticoid therapy, which can adversely affect bone quality and healing.

Diagnosis

Signs and Symptoms

Some patients are completely asymptomatic before sustaining a complete fracture.

Others experience characteristic prodromal pain for weeks or months before the fracture becomes complete.

The pain is often described as a dull or aching discomfort involving the groin or thigh, particularly during weight-bearing.

Symptoms may be vague or sharply localized.

History

The clinical history should specifically assess the duration of bisphosphonate use.

Although the greatest concern traditionally involves treatment extending beyond 4–5 years, atypical fractures can occur earlier.

A history of long-term glucocorticoid use should also be documented.

Patients should be questioned about preceding thigh or groin pain and whether symptoms worsen with walking or prolonged standing.

Physical Examination

Patients with an incomplete fracture may have localized tenderness along the thigh and pain during weight-bearing.

Hip range of motion may reproduce discomfort, particularly internal rotation.

A complete displaced fracture usually results in inability to bear weight.

Visible deformity and shortening of the affected limb may also be present.

Imaging

Plain Radiographs

Initial evaluation should include an anteroposterior radiograph of the pelvis together with AP and lateral views of the hip, entire femur, and knee.

It is important to image the entire femur because the lesion may occur anywhere from the subtrochanteric region to the distal shaft.

Early Radiographic Findings

Before a complete fracture develops, radiographs may show focal periosteal reaction, cortical thickening, or beaking along the lateral femoral cortex.

These changes may represent an impending atypical fracture.

A transverse radiolucent line extending inward from the lateral cortex may indicate an incomplete fracture.

Complete Fracture Pattern

Complete atypical femoral fractures are typically transverse or short oblique.

The fracture pattern is usually simple and demonstrates little or no comminution.

A characteristic medial cortical spike may occasionally be present.

These features help distinguish atypical fractures from typical high-energy femoral shaft fractures.

Contralateral Femur Imaging

Atypical femoral fractures are frequently bilateral or sequential.

Therefore, radiographs of the contralateral femur should routinely be obtained when an atypical fracture is identified.

The opposite femur should be examined for cortical thickening, beaking, periosteal reaction, or a transverse radiolucent line.

MRI

MRI is generally unnecessary when a complete fracture is clearly visible on radiographs.

However, it is the preferred imaging study for detecting an occult or incomplete atypical fracture when plain radiographs are normal or equivocal but clinical suspicion remains high.

Typical findings include bone marrow edema and a linear low-signal fracture line.

MRI is particularly useful in patients receiving long-term bisphosphonates who present with unexplained thigh or groin pain.

Bone Scintigraphy

Bone scintigraphy may detect increased metabolic activity at an impending fracture site.

However, it is less specific than MRI and is not generally required when a complete fracture is already visible radiographically.

It may be useful when MRI cannot be performed.

CT

CT is also usually unnecessary for complete atypical fractures.

It has lower sensitivity than MRI for detecting early stress reactions and incomplete fractures.

Its use is therefore generally reserved for selected cases in which detailed cortical anatomy is required.

Dual-Energy X-Ray Absorptiometry

Dual-energy X-ray absorptiometry may occasionally identify cortical abnormalities such as periosteal reaction or lateral cortical beaking during monitoring of patients receiving prolonged bisphosphonate therapy.

Its primary purpose remains assessment of bone mineral density rather than diagnosis of atypical fracture.

Differential Diagnosis

The differential diagnosis includes insufficiency fracture, intertrochanteric or peritrochanteric fracture, typical osteoporotic hip fracture, and femoral neck stress fracture.

Musculoskeletal conditions such as overuse tendinopathy may also produce thigh or groin pain and can mimic prodromal symptoms.

Treatment

General Measures

Long-term bisphosphonate therapy should be reassessed in patients at increased risk of atypical femoral fracture.

A drug holiday may be considered in appropriate patients after balancing the risk of atypical fracture against the continuing risk of osteoporotic fragility fracture.

Patients receiving bisphosphonates for approximately 3–5 years or longer should be monitored for symptoms suggestive of an atypical femoral fracture.

Monitoring for Prodromal Symptoms

Patients on prolonged bisphosphonate treatment should be questioned about new groin or thigh pain, particularly pain occurring during weight-bearing.

When such symptoms develop, imaging should be obtained promptly.

Early identification of an incomplete lesion may allow prophylactic treatment before displacement occurs.

Laboratory Evaluation

Bone turnover markers may occasionally be assessed when excessive suppression of remodeling is suspected.

Potential investigations include serum or urine N-telopeptide or C-telopeptide levels and bone-specific alkaline phosphatase.

These tests may provide information about bone turnover but do not establish the diagnosis of an atypical fracture.

Weight-Bearing Restrictions

Patients with prodromal pain and radiographic or imaging evidence of an incomplete fracture should reduce loading of the affected limb.

Partial or non-weight-bearing may be recommended while definitive treatment is arranged.

The objective is to reduce the risk of progression to a complete displaced fracture.

Surgical Management

Intramedullary Nailing

Intramedullary nailing is the preferred treatment for most complete atypical femoral fractures.

It is also commonly recommended for incomplete fractures that are painful or demonstrate features indicating a high risk of progression.

The intramedullary device provides load-sharing fixation along the length of the femur and allows early mobilization.

Incomplete Fractures

Incomplete atypical fractures have a substantial risk of progressing to complete displaced fractures.

This risk is particularly high when a visible radiolucent fracture line, lateral cortical beaking, and persistent prodromal pain are present.

Observation alone may therefore be inappropriate in high-risk lesions.

Prophylactic Intramedullary Nailing

Prophylactic intramedullary fixation may be recommended for patients with an incomplete atypical lesion before a displaced fracture occurs.

Important indications include lateral cortical beaking combined with persistent thigh or groin pain.

Preventive fixation can reduce the likelihood of sudden complete fracture and may allow a more predictable recovery.

Plate Fixation

Intramedullary nailing may be technically difficult in patients with substantial femoral bowing.

In such situations, lateral plate fixation may be considered.

The choice of fixation should take femoral geometry, fracture location, and surgeon experience into account.

Contralateral Femur Management

Management of the opposite femur is controversial.

Routine prophylactic fixation of an asymptomatic contralateral femur is not recommended for every patient.

However, prophylactic intramedullary nailing may be considered when multiple high-risk features are present.

High-Risk Contralateral Features

Factors that may support prophylactic fixation of the opposite femur include Asian ethnicity, prodromal thigh pain, varus proximal femoral geometry, marked femoral bowing, and radiographic abnormalities such as cortical beaking or a transverse radiolucent line.

The decision should be individualized according to overall fracture risk and symptoms.

Follow-Up

Patients require close follow-up after both operative and nonoperative management.

Serial radiographs are used to assess healing, implant position, and possible progression of incomplete lesions.

The contralateral femur should also be monitored because a second atypical fracture may develop later.

Prognosis

Intramedullary fixation generally permits relatively early or immediate protected weight-bearing and facilitates return to function.

However, atypical femoral fractures often heal more slowly than typical traumatic femoral fractures.

They have a higher incidence of delayed union, nonunion, and revision surgery compared with more typical hip or femoral fractures.

Delayed Healing

Suppressed bone remodeling and the chronic stress-fracture nature of the injury may contribute to prolonged healing.

Radiographic union can therefore take considerable time even after technically successful fixation.

Patients should be counseled that recovery may be slower than after a conventional femoral fracture.

Complications

An incomplete atypical fracture may progress to a complete displaced fracture, particularly if weight-bearing continues despite prodromal symptoms.

Other important complications include delayed union, nonunion, implant-related problems, and the development of an atypical fracture in the contralateral femur.

Contralateral Fracture

Patients who sustain one atypical femoral fracture have an increased risk of developing a similar lesion on the opposite side.

Persistent or new contralateral thigh pain should therefore be investigated promptly.

Long-term surveillance of both femora may be appropriate in high-risk individuals.

Patient Teaching

Patients receiving long-term bisphosphonate therapy should understand that atypical femoral fractures are uncommon but recognized complications of prolonged treatment.

They should be advised to report new groin or thigh pain during weight-bearing, even if there has been no trauma.

Early Warning Signs

Patients should understand that an atypical fracture may initially produce no symptoms.

When symptoms do occur, vague aching pain in the thigh or groin may precede complete fracture by weeks or months.

Radiographs may show lateral cortical thickening or beaking before the fracture becomes complete.

Importance of Early Evaluation

Early recognition of an impending atypical femoral fracture can prevent progression to a displaced injury.

Patients with significant prodromal symptoms, radiographic abnormalities, or other high-risk findings should undergo prompt orthopedic evaluation.

Patients with a complete atypical fracture, or those with an incomplete lesion at high risk of progression, commonly require surgical stabilization.