R ecent studies of the fibroblast growth factor receptor 3 (FGFR3) gene have established that achondroplasia and hypochondroplasia are allelic disorders of different mutations.To determine whether the genotype could be distinguished on the basis of the phenotype, we analysed height, arm span, and skeletal radiographs from 23 patients with achondroplasia and the G380R mutation of FGFR3 and eight with hypochondroplasia and the N540K mutation. Both conditions share the classical pathological features of micromelic short stature, reduced or unchanged interpedicular distances in the lumbar spine, disproportionately long fibulae, and squared and shortened pelvic ilia. These were significantly more severe in the G380R patients than in the N540K patients.Our About 60% of patients with HCH have been reported to have an asparagine-to-lysine substitution at residue 540 (N540K) of FGFR3. 10,11 Although the genotypic background of the remaining cases of HCH has not been clarified, the N540K substitution seems to be responsible for a significant proportion of cases of this condition. 12-14In spite of a long history of observations suggesting allelism between ACH and HCH, 15 it has been claimed that the clinical and radiological features of ACH and HCH overlap. 16,17 We have compared the phenotype of genotyped populations to clarify whether or not each of these two genotypes has a distinctive phenotype. Patients and MethodsBlood samples were collected from patients clinically diagnosed as having ACH and HCH, and the FGFR3 genotype was determined as previously described. 18 There were 23 patients with ACH and the G380R substitution, 11 boys and 12 girls with a mean age of 10.0 years (5 to 18). Of the eight HCH patients with the N540K substitution, there were five boys and three girls with a mean age of 11.6 years (7 to 18), all of whom had sporadic mutations. We also assessed 30 genetically normal individuals as a control group. There were 17 boys and 13 girls with a mean age of 11.1 years (3 to 17). We analysed height, arm span and skeletal radiographs. Height was evaluated by the height standard deviation score which was obtained from standard growth curves for the Japanese population. Arm span was examined by calculating the span to height ratio percentage.We studied three of the pathological features of ACH and HCH. [19][20][21] We assessed the ratio of the interpedicular distances at the first and fourth lumbar vertebrae (L1/L4 ratio, Fig. 1a) irrespective of whether they were reduced or not.We also determined the ratio of the length of the fibula to that of the tibia (F/T ratio, Fig. 1b) to ascertain whether the

Nine patients with achondroplasia and one patient with Apert syndrome underwent the surgical lengthening of both humerus and simultaneous correction of both associated bone deformity. An unilateral external fixator was applied to the lateral aspect of the humerus with four half-pins and percutaneous predrilling osteotomy was performed at the apex of flexion deformity of the bone. During the waiting period before distraction, the flexion deformity of the distal humerus was corrected using an additional external fixator. Slow gradual distraction was subsequently carried out at a rate of 0. 25 mm every 6 hours. The average lengthening was 8 cm (range 7.5 to 9 cm), the overall treatment time 312 days (range 192 to 406 days), and the average healing index 39.0 days/cm. The average correction of the elbow flexion deformity was 20 degrees. We believe this treatment is useful to improve the function of the arms and the activity of daily living for the patients with bilateral short humeri.