Stress response or stress reaction is the early result of bone failing to withstand a repetitive, cumulative loading force.
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Terminology
Stress response of bone is synonymously used with 'stress reaction' and is an early form of osseous 'stress injury', which also encompasses a stress fracture 1,2.
Pathology
A stress response is the early form of osseous stress injury, leading to morphologic alterations in the bony structure and will eventually lead to a stress or insufficiency fracture, if the causative repetitive loading force continues and is not omitted 2.
Etiology
A stress response can occur either as a typical overuse injury on the normal bone (fatigue) or in a normal activity affecting weakened bone (insufficiency). In an overuse situation, the weight-bearing parts of bone are affected, whereas in insufficiency related stress reactions the trabecular bone tends to be involved 1,2.
Location
Stress injuries are more common in the foot and the lower leg though can occur anywhere.
High-risk locations are the femoral neck, anterior cortex of the tibia, medial malleolus, navicular bone, talus, proximal 2nd and 5th metatarsal, patella, great toe sesamoid 3.
Radiographic features
Imaging findings may be found in compact bone or trabecular bone.
Plain radiograph
May show a subtle cortical thickening or periosteal alterations or signs of callus formation in later stages, early usually does not show anything.
CT
There might be may mild density changes in the trabecular bone due to effacement of the fatty marrow, often better appreciated on dual-energy CT.
MRI
A stress response is characterized by a bone marrow edema like appearance with no obvious fracture cleft or cortical defect and with possible endosteal and/or periosteal edema. A stress reaction might be difficult to differentiate from a stress fracture and in that case, should be called stress injury 1.
Signal characteristics
- T1: mildly hypointense, with effacement but not a replacement of the fatty marrow
- T2FS/PDFS: hyperintense
An MRI grading scheme for stress reactions or stress injuries originally proposed for tibial stress injuries 5, the first three grades can be applied for a stress response:
- grade 1: periosteal edema without bone marrow changes
- grade 2: bone marrow edema or changes seen on fat-saturated T2 weighted images
- grade 3: bone marrow changes, also clearly visible on T1 weighted images
- grade 4: fracture line present on T1 weighted and T2 weighted images
Nuclear medicine
- focal radiotracer uptake on bone scintigraphy
Treatment and prognosis
Treatment typically includes activity modification, restricted impact activities e.g. weight-bearing. Non-steroidal anti-inflammatory drugs should be avoided as these may impair bone healing 3.
Differential diagnosis
- stress fracture: no fracture line or cleft
- osteomyelitis: marrow replacement and not marrow effacement
- osteitis: difficult to differentiate
- osteoid osteoma
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