What are early stress-related changes in the femoral head?

Early Stress-Related Changes in the Femoral Head

Early stress-related changes in the femoral head manifest as bone marrow edema on MRI (appearing as T1 hypointense and T2 hyperintense signal) that may progress to subchondral fracture lines, representing the spectrum from stress reaction to complete stress fracture. 1

Imaging Characteristics of Early Changes

MRI Findings (Most Sensitive Modality)

• Bone marrow edema is the earliest detectable change, appearing as T1 hypointense and T2 hyperintense signal in the femoral head before any fracture line develops 1
• Linear T1 and T2 hypointense signal represents actual fracture lines when stress progresses beyond the edema-only stage 1
• MRI demonstrates stress abnormalities as early as bone scintigraphy but with significantly greater specificity 1
• Hip joint effusion on initial MRI is a critical prognostic indicator—patients with effusion have 8 times the risk of fracture progression requiring surgical intervention compared to those without effusion 2

Radiographic Progression

• Initial radiographs are typically negative in early stress changes, with sensitivity of only 15-35% 1
• Follow-up radiographs at 10-14 days show increased sensitivity (30-70%) as overt bone reaction develops 1
• In trabecular bone involvement, stress changes progress from patchy areas of increased density to linear sclerosis oriented perpendicular to trabeculae 1

Anatomic Distribution Patterns

Age-Related Differences

• Young patients (<40 years): Stress fractures occur more mediolaterally and anterosuperiorly, often associated with acetabular retroversion (41.7% of cases) 3
• Middle-aged and older patients (≥40 years): Fractures are more laterally located and associated with acetabular dysplasia (38.9% of cases) 3
• Contact stress distribution in retroversion concentrates on mediolateral and superior aspects of the femoral head 3

High-Risk Locations

• Femoral head stress fractures are classified as high-risk due to increased rates of delayed union, nonunion, displacement, and avascular necrosis if not recognized promptly 1
• Lateral "tension-type" femoral neck stress fractures are inherently unstable and prone to displacement 1
• Medial "compression-type" femoral neck stress fractures are considered low-risk 1

Clinical Implications and Diagnostic Approach

When to Suspect Femoral Head Stress Changes

• Insidious hip or groin pain that may culminate in sudden severe pain, particularly in young active adults or athletes 4
• Pain may also manifest in the back, buttock, or thigh rather than isolated groin pain 1
• History of repetitive mechanical loading or increased activity level 1

Diagnostic Algorithm

1. Initial radiographs are appropriate first-line imaging despite low sensitivity 1
2. MRI without contrast is the preferred second-line study after negative radiographs to prevent delayed diagnosis—it does not use ionizing radiation and has excellent sensitivity 1
3. Immediate MRI is indicated in "need-to-know" situations or high-risk patients (athletes, osteoporosis, bisphosphonate therapy) where delayed diagnosis could lead to fracture completion 1
4. CT is not typically first- or second-line but may have an adjunctive role when other modalities are equivocal 1

Critical Pitfalls to Avoid

• Do not rely on clinical examination alone—hip examination tests have good sensitivity but poor specificity and cannot reliably distinguish hip-related pain from other causes 1
• Do not delay MRI in high-risk scenarios—femoral head stress fractures have devastating consequences if diagnosis is delayed 1, 4
• Do not dismiss isolated bone marrow edema without a fracture line—all patients with isolated edema in one study resolved with nonoperative treatment, but this represents the earliest stage of stress injury 2
• Recognize that hip effusion on initial MRI is a red flag—85.2% of patients requiring surgery had effusion on initial imaging versus only 26.3% of those who resolved nonoperatively 2

Biomechanical Context

• Elevated contact stress from reduced load-bearing capacity of stressed bone can lead to progressive deformation and eventual osteoarthritis 5
• Stress effects are magnified by larger lesion size and more lateral location of the stress injury 5
• Revascularization and repair processes begin at the periphery as early as 1-2 weeks but deformation continues with variable tissue responses including fibroblastic ingrowth, tissue resorption, and mixed fibro-osseous synthesis 6