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The calcaneal apophysis is the posterior secondary ossification center of the calcaneus that forms during increased weight bearing in early childhood and contributes to the formation of the calcaneal tuberosity 1.
As a non-articular secondary center it is referred to as an apophysis rather than an epiphysis, with the latter being typically associated with the articular ends of long bones.
The calcaneal apophysis forms a temporary cartilaginous synchondrosis joint with the calcaneal body, which closes through ossification around puberty.
The calcaneal (Achilles) tendon inserts onto the inferior part of calcaneal apophysis.
The lateral radiograph allows visualization of stage of ossification and fusion of apophysis to body of calcaneus:
appears as a crescent-shaped thin radiopaque band
Overcomes limitations of superimposition of medial and lateral processes of calcaneal tuberosity experienced by lateral radiographs 2.
commencement of ossification in apophysis can be difficult to assess in MRI due to similar signal intensities between bone and tendon
after fusion of the apophysis, low signal intensity bands have been reported parallel to the apophyseal line/scar, which may be mistaken for a fracture 3
The calcaneal apophysis commences ossification approximately 2 years earlier in females at a mean age of 5 years and is fully fused from 10-14 years 2. The apophysis can form from one or multiple secondary centers 2,3, largely in the middle or inferior zone of the calcaneal tuberosity and enlarges first superiorly and inferiorly, then medially and laterally to form first the medial process and then lateral process. Fusion commences in the middle zone and lateral process, progresses to the medial process and inferior zone, and finishes in the superior zone 2.
Apophyseal growth may occur via two main mechanisms:
appositional growth through an extension of existing ossification centers, or
fusion of additional secondary ossification centers such as in the lateral and/or medial processes or superior zone 2
This variation in ossification patterns may be misdiagnosed as fractures in the pediatric foot.