Residency · Residency · Diagnostic Radiology

Stress Fractures and Insufficiency Fractures

Overview

Definitions

A stress fracture (fatigue fracture) occurs when abnormal stress is applied to normal bone and is typically seen in athletes or military recruits. An insufficiency fracture occurs when normal physiologic stress is applied to abnormal (weakened) bone, typically in the setting of osteoporosis or prior radiation. Both represent a spectrum of bone failure from repetitive loading that exceeds the bone's capacity for remodeling.

Pathophysiology

Repetitive microtrauma exceeds the rate of bone remodeling and repair, producing a progression from stress reaction (periosteal edema and bone marrow edema) to a cortical fracture line. Without treatment modification, the injury may progress to a complete fracture.

Stress Fractures in Athletes

Common Locations

The metatarsals, specifically the 2nd and 3rd metatarsal shafts, are the most common overall stress fracture location in runners and are known as "march fractures." The tibia is the next most common location, typically involving the posteromedial cortex of the mid-to-distal shaft. Importantly, anterior cortex stress fractures of the mid-tibia (the "dreaded black line") are high-risk for non-union. Femoral neck stress fractures are classified by location: compression-side fractures (inferior cortex, medial) are lower risk and may be managed conservatively, while tension-side fractures (superior cortex, lateral) are high-risk for completion or displacement and typically require surgical fixation. The navicular is a high-risk location, with fractures occurring through the central third of the tarsal navicular, which is prone to non-union due to its watershed vascular area. The fifth metatarsal base (Jones fracture zone) at the metadiaphyseal junction is another high-risk location for non-union. Pars interarticularis stress fractures cause spondylolysis and are common in gymnasts, football linemen, and divers. Sacral stress fractures present as vertical parasagittal fractures and may occur in distance runners.

High-Risk vs. Low-Risk Classification

High-risk stress fractures occur on the tension side, in areas of poor blood supply, and have high non-union rates. They include femoral neck tension-side, anterior tibia, navicular, 5th metatarsal base, patella (transverse), and medial malleolus fractures. Low-risk stress fractures occur on the compression side and heal well. They include metatarsal shaft (2nd and 3rd), tibial shaft posteromedial, calcaneus, femoral shaft, and pubic rami fractures.

Insufficiency Fractures

Common Locations

The sacrum is a common location, producing the "H-shaped" or "Honda sign" on bone scan with bilateral parasagittal and transverse components. The pubic rami (superior and inferior, often bilateral), supra-acetabular region, femoral neck (subcapital insufficiency fracture in the elderly), vertebral body (compression fractures including wedge, biconcave, and crush types), and proximal tibia (subchondral insufficiency fracture mimicking meniscal pathology) are other common sites.

Risk Factors

Osteoporosis is the most common risk factor. Other risk factors include post-radiation therapy (pelvis, sacrum), rheumatoid arthritis or chronic steroid use, Paget disease, and renal osteodystrophy.

Subchondral Insufficiency Fracture of the Knee (SIFK)

Previously called spontaneous osteonecrosis of the knee (SONK or SPONK), this entity is now recognized as a subchondral insufficiency fracture. The medial femoral condyle weight-bearing surface is the most common location. On MRI it appears as subchondral low T1 signal with surrounding bone marrow edema and may show a subchondral fracture line. It must be distinguished from osteonecrosis (AVN) and osteoarthritis.

Imaging Modalities

Radiography

Radiography is the initial imaging modality but has low sensitivity in early stress injury, at approximately 15% in the first 2-3 weeks. Early findings include subtle periosteal reaction, endosteal thickening, and faint lucency. Later findings include a visible fracture line, callus formation, and cortical thickening. Radiographs may remain negative throughout the course in cancellous bone stress fractures.

MRI (Gold Standard)

MRI is the most sensitive and specific modality for stress fractures. In stress reaction (the early stage), bone marrow edema (T2/STIR hyperintense, T1 hypointense) and periosteal edema are present without a visible fracture line. In an established stress fracture, a linear T1 hypointense fracture line is visible within the zone of bone marrow edema.

Fredericson Classification (Tibial Stress Fractures)

The Fredericson classification for tibial stress fractures grades injuries as follows: Grade 1 shows periosteal edema only; Grade 2 shows bone marrow edema on T2 with a normal T1; Grade 3 shows bone marrow edema on both T2 and T1; Grade 4a shows intracortical signal abnormality; and Grade 4b shows a visible fracture line.

Bone Scintigraphy

Bone scintigraphy was historically used and has high sensitivity but lower specificity than MRI. It shows fusiform uptake at the fracture site and is positive in all three phases of a three-phase bone scan. It has been largely replaced by MRI due to MRI's superior anatomic detail and specificity.

CT

CT is useful for cortical stress fractures (anterior tibial cortex, navicular, pars interarticularis) and shows the fracture line directly. However, it is less sensitive than MRI for early stress reaction.

Special Considerations

Female Athlete Triad / Relative Energy Deficiency in Sport (RED-S)

The female athlete triad consists of low energy availability, menstrual dysfunction, and decreased bone mineral density. It significantly increases stress fracture risk and should be considered in any female athlete with recurrent stress fractures. The updated terminology, RED-S (Relative Energy Deficiency in Sport), encompasses broader physiologic consequences beyond the classic triad.

Medial Tibial Stress Syndrome (Shin Splints)

Medial tibial stress syndrome is a traction periostitis along the posteromedial tibial cortex. On MRI it shows periosteal edema without focal bone marrow edema or a fracture line. The key distinction from a stress fracture is that shin splints produce elongated periosteal edema extending over more than one-third of the tibial length without focal marrow signal change.

<image>An MRI panel of the proximal femur showing the spectrum of femoral neck stress fracture. Image 1: Coronal T2 fat-saturated MRI showing bone marrow edema in the femoral neck without a fracture line (stress reaction, Fredericson Grade 2-3). Image 2: Coronal T1 MRI showing a linear hypointense fracture line on the compression side (inferomedial) of the femoral neck within the edema (low-risk). Image 3: Coronal T2 fat-saturated MRI showing a fracture line on the tension side (superolateral) of the femoral neck (high-risk, requiring surgical fixation). Arrows label the edema zone, fracture lines, and indicate compression vs. tension sides.</image>

<image>A composite image showing sacral insufficiency fractures. Panel 1: A bone scan (posterior view of the pelvis) demonstrating the classic "H-sign" or "Honda sign" with increased uptake along bilateral parasagittal sacral ala and a connecting transverse component. Panel 2: Coronal T2 fat-saturated MRI of the sacrum showing bilateral parasagittal fracture lines with surrounding bone marrow edema. Panel 3: Axial CT of the sacrum showing bilateral vertical fracture lines through the sacral ala. Each panel is labeled.</image>

<image>An MRI comparison panel distinguishing medial tibial stress syndrome from tibial stress fracture. Left: Axial T2 fat-saturated MRI showing periosteal edema along the posteromedial tibial cortex without bone marrow edema (shin splints). Right: Axial and sagittal T2 fat-saturated MRI of the tibia showing focal bone marrow edema and a linear fracture line in the posteromedial cortex (stress fracture). A sagittal T1 image shows the corresponding hypointense fracture line. Labels differentiate the two entities.</image>

Clinical Pearls

Tension-side femoral neck stress fractures (superolateral cortex) are high-risk for displacement and typically require urgent surgical fixation, while compression-side fractures (inferomedial) can often be managed conservatively. The anterior tibial cortex stress fracture ("dreaded black line") is high-risk for non-union and may require surgical intervention; it should not be confused with the much more common posteromedial tibial stress fracture. Sacral insufficiency fractures are commonly missed on CT and may be overlooked on radiographs; MRI is far more sensitive, and the "Honda sign" on bone scan is classic. Subchondral insufficiency fracture of the medial femoral condyle (formerly SONK/SPONK) should be considered in older patients with sudden onset medial knee pain and bone marrow edema on MRI. Recurrent stress fractures in a young female athlete should prompt evaluation for RED-S (Relative Energy Deficiency in Sport), including bone mineral density assessment. Radiographs are often negative in the first 2-3 weeks of a stress fracture; if clinical suspicion is high and radiographs are normal, MRI should be obtained directly.

References

• Fredericson M, et al. "Tibial Stress Reaction in Runners: Correlation of Clinical Symptoms and Scintigraphy with a New MRI Grading System." American Journal of Sports Medicine, 1995
• Nattiv A, et al. "The Female Athlete Triad." Medicine & Science in Sports & Exercise, 2007
• ACR Appropriateness Criteria: Stress/Insufficiency Fracture, 2019
• Mountjoy M, et al. "The IOC Consensus Statement on Relative Energy Deficiency in Sport (RED-S)." British Journal of Sports Medicine, 2014

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