Musculoskeletal X-ray - General principles
Bone anatomy

Key points

  • When describing the location of a bone abnormality within a growing bone you can refer to its position in the diaphysis, metaphysis or epiphysis
  • It is also correct to use simple descriptive terms such as - shaft - proximal/distal end - cortical - medullary - articular surface

Most bones develop from cartilaginous ossification centres which form either a diaphysis (shaft) or an epiphysis (end). During bone growth the diaphysis and epiphysis are separated by the growth plate (also known as the epiphyseal line or physis) which fuses later in life. The zone adjacent to the growth plate on the diaphyseal side is called the metaphysis.

Sesamoid

A sesamoid bone is a bone that ossifies within a tendon. The largest is the patella. Sesamoid bones are also present at the first metatarsophalangeal joint (big toe) and the first metacarpophalangeal joint (thumb).

Apophysis

An apophysis is a normal developmental outgrowth of a bone which arises from a separate ossification centre, and fuses to the bone later in development. An apophysis usually does not form a direct articulation with another bone at a joint, but often forms an important insertion point for a tendon or ligament.

Occasionally an apophysis can persist into adult life and if injured may become symptomatic. The many apophyses in the body have variable appearances and are often mistaken for fractures.

Bone anatomy example - Knee

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Bone anatomy example - Knee

  • Long bones comprise diaphysis, metaphysis and epiphysis
  • The growth plate separates the metaphysis from the epiphysis until fusion in adult life
  • A sesamoid is a bone that ossifies within a tendon
  • An apophysis is a normal bony outgrowth of a bone

Bone structure

In simple terms bone is made of an outer cortex and an inner medulla. Difference in density allows for differentiation on X-rays - the cortex being denser and, therefore, whiter than the medulla.

Cortex v medulla example - Tibia and fibula

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Cortex v medulla example - Tibia and fibula

  • Cortex denser - therefore whiter
  • Medulla - note the fine trabecular pattern

Descriptive terms

Once the skeleton is fused the distinction between epiphysis, metaphysis and diaphysis becomes less clear, and is clinically less important. Less specific terms can be used to describe the location of an abnormality.

Descriptive terms example - Metacarpal

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Descriptive terms example - Metacarpal

  • The metacarpal bone shown has a base, shaft, neck and head
  • Less specific terms may be appropriate - such as proximal or distal end
  • Many bones also have proximal and distal articular surfaces
  • When describing abnormalities of an articular surfaces remember to mention whether it is proximal or distal