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The elbow anatomy of a child with achondroplasia has distinctive characteristics. In this context, I found a very interesting study:  "Deformities of the elbow in achondroplasia", published by  Hiroshi Kitoh et al, 2002, Nagoya University, Japan. It is quite enlightening and I reviewed several x-ray images from healthy children to compare with the ones from this study.

"Deformities of the elbow are particularly significant diagnostically in young children with achondroplasia. Bailey JA. (Elbow and other upper limb deformities in achondroplasia. Clin Orthop 1971;80:75-8)  reviewed the clinical and radiological findings in 41 patients and described various deformities of the upper limb including:

-loss of extension,

-limited supination or pronation,

-a prominent radial head,

-a short ulna,

-cubitus varus.

A deficit in extension was the most common, being present in 38 of the 41 patients (92.7%). Only three patients had full extension. Bailey also observe  that loss of extension increased with age. The number of elbows with this deformity in our series was lower (68.3%) and is probably a reflection of the relative youth of our patients. Even the 13 elbows without an extension deficit may represent a mild deformity since most normal adolescents and infants have some hyperextension of the joint.
A healthy elbow of a child. Credits: Wikiradiography.
Deformities of the elbow in achondroplasia at
Deformities of the elbow in achondroplasia. Lateral radiograph of the elbow of an eight-year-old girl with achondroplasia. Posterior bowing of the distal humerus and posterior dislocation of the head of the radius are seen. Credits: Hiroshi Kitoh et al, 2002.

Posterior bowing of the distal humerus
was a consistent radiological abnormality and when greater than 20 ° resulted in loss of extension.
And in  this study there is a quite relevant note:  Restriction of elbow extension, even when shoulder function is normal, may cause functional impairment of a shortened arm.

Recently, surgical lengthening of the humerus has been performed to improve the function of the arm and the activities of daily living in patients with bilateral short arms. When lengthening the humerus of a patient with achondroplasia the flexion deformity should be taken into account and should be corrected simultaneously.

Bailey  stated that deformities of the head of the radius, with or without dislocation, were normally the cause of loss of extension. Our study provides further evidence to link dislocation with more severe loss of full extension. The hypoplastic capitellum seen in achondroplasia suggests that the dislocation is congenital.

A short ulna in the forearms and a longer fibula in the lower limbs are common radiological findings probably as a result of unequal growth rates. Overgrowth of the radius, when associated with a reduced length of the ulna, may result in congenital dislocation of the head.

The significantly smaller defect in extension in the presence of posterior bowing in those patients without dislocation suggests that the soft tissue structures around the elbow may be lax, as is seen in other joints in achondroplasia."

The elbow deformity has  similarities  to what happens in the genu varus in achondroplasia.

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