Objectives: Hereditary multiple exostosis (HME) often involves forearm deformities. The aim of this study was to present the clinical results of 37 children who underwent ulnar lengthening with two different types of unilateral external fixators, and to investigate the risk factors of complications. Methods: We evaluated 37 children with forearm deformities caused by HME treated in our hospital from January 2008 to July 2019. The surgical procedures included resection of exostosis, osteotomy of

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... the ulna, and gradual lengthening of the ulna with a single arm external fixator. According to the type of external fixator they received, the children were divided into two groups: group A received monorail fixators, and group B received multi-joint fixators. Radiographic and functional parameters were assessed. Complications were recorded. Results: All patients were followed up for an average of 4.6 years (3.0 to 6.5). In both group A and group B, the ulna shortening (US), radial articular angle (RAA), carpal slip (CS), elbow flexion, forearm pronation, supination, and Mayo Elbow Performance Score (MEPS) values improved significantly from preoperatively to postoperatively ( p < 0.05). However, ulnar deviation was observed in 4 cases in group B and no cases in group A. According to logistic regression, the difference was only related to age ( p <0.05) and the type of external fixator ( p <0.05). Conclusions: Ulnar lengthening with unilateral external fixation is a safe and effective procedure for the treatment of HME. Regarding complications, deviation of the ulna axis was more likely to occur in older children with multi-joint external fixators.