Osteogenesis imperfecta is one of the most common inherited disorders of connective tissue (2
). Many classification systems have been proposed for this disease, which reflects difficulties in categorizing this disease entity due to its diverse phenotypes, varying severity, and poor correlation between genotype and phenotype (1
). A series of distinct subtypes of OI has been proposed (3
), which would have been classified as type IV according to the Sillence classification. Of these, OI type V is characterized by bone fragility, ossification of the interosseous membrane of the forearm, radial head dislocation, hyperplastic callus formation, heredity suggesting an autosomal dominant pattern and no evidence of mutation in type I collagen genes (3
From the experience of these cases we agree that OI type V is a distinct disease entity, which could be differentiated from the other types of OI by clinical and radiological examinations. Histologic findings, if available, may be confirmatory, and a negative result of type I collagen mutation analysis might be helpful for confirming the diagnosis, although neither specific nor sensitive.
Previous report on OI type V did not describe clinical manifestation of the affected family members other than the index cases (3
). Index cases of the current study were the most severely affected cases. One patient (Case 2) required walking aid due to both hip joint ankylosis, but he seemed to be the most seriously affected, showing multiple ectopic ossifications besides the forearm interosseous membrane (). All the other patients did not have any serious functional disability interfering with daily life. Therefore, OI type V may be considered a relatively mild type of OI with varying severity.
Ossification of interosseous membrane of the forearm seems to be a pathognomonic feature of OI type V. All patients included in this study showed interosseous membrane ossifications, which were symmetrical and advancing with age. But interestingly there was no significant correlation between severity of ossification and range of the forearm rotation except for Grade III which resulted in complete ankylosis. Moreover, either radial head excision or deformity correction of the forearm bones could not improve the forearm rotation. These findings suggest that an unknown pathology within the interosseous membrane or its surrounding soft tissue, rather than ossification itself, attribute to limitation of the forearm rotation, which needs to be further investigated.
Radial head dislocation is one of the most characteristic phenotypes of this disease. OI type V should be included in the differential diagnosis of radial head dislocation in children in this context. Fassier et al. (11
) reported that the incidence and direction of radial head dislocation in OI type V differ significantly from that of other types. In our series, all the patients over 10 yr-old were found to have dislocated radial heads, whereas the three affected children had radial head in situ when firstly presented under age 5 yr, suggesting that radial head dislocation in OI type V was developmental rather than congenital. Pathomechanism of the radial head dislocation has not been fully elucidated. Ossification of the interosseous membrane and angular deformity of the forearm bones seem to contribute to its development.
Radial head excision can be considered if symptomatic or for cosmetic purpose, although the timing of operation is controversial. The authors believe that a dislocated radial head in OI type V may be safely excised before skeletal maturity if clinically indicated as in the case of congenital radial head dislocation (12
Bisphosphonate treatment for osteogenesis imperfecta patients has been well established as a standard treatment (9
). Recently, researchers have reported that intravenous pamidronate therapy has similar effect in OI type V as in the other OI types (14
). We treated 4 children in this series with either oral alendronate or cyclic intravenous pamidronate, and all showed biochemical and radiological responses as expected. However, in patients with relatively mild symptoms, its clinical benefit should be weighed against its potential complications, as in other types of OI.
The present study is the first report of OI type V in Asian population. Affected family members shared some phenotypes with other types of OI, although clinical, radiological, and histological characteristics distinguished this disease from other OI types.