Looser zones, also known as cortical infractions, Milkman lines or pseudofractures, are wide, transverse lucencies with sclerotic borders traversing partway through a bone, usually perpendicular to the involved cortex, and are associated most frequently with osteomalacia and rickets.
Given that these lesions are a type of insufficiency fracture, they are not themselves diagnostic of osteomalacia. Osteomalacia is the strongly favored diagnosis when these are bilaterally symmetric and in a classic location such as the axillary border of the scapulae, ribs, or posterior ulnae. Other frequently involved sites include the superior and inferior pubic rami, and proximal medial femora. True fractures may occur through these weakened sites.
The term pseudofracture is a misnomer, as they are considered a type of insufficiency fracture. Typically, the fractures have sclerotic irregular margins and are often symmetrical.
Looser zones contain regions of demineralized osteoid, frequently with superimposed osteitis fibrosa cystica due to the presence of hyperparathyroidism.
- renal osteodystrophy
- fibrous dysplasia
- Paget disease of bone
- X-linked hypophosphatemia
- osteogenesis imperfecta
Looser zones occur in the same locations as insufficiency fractures in weight-bearing bones:
- pubic rami
- medial femoral neck
- medial proximal femoral shaft (c.f. bisphosphonate-related fractures that occur on the lateral cortex of the proximal femoral shaft)
In non-wieghting-bearing bones, they often occur along nutrient foramina and represent true pseudofractures:
- lateral scapula
- posterior proximal ulna
- iliac wing
History and etymology
Looser zones are named after Emil Looser, a Swiss surgeon, working in Zurich (1877-1936) .
Louis Arthur Milkman (1895-1951) was an American radiologist who described the findings in seminal papers in 1930 and 1934 .