Long-term follow-up of patients with infantile idiopathic scoliosis: is the rib vertebra angle difference (RVAD) a reliable indicator of evolution?

PurposeSince its original description by Mehta, the rib vertebra angle difference (RVAD) and, in particular, a threshold of 20 degrees have become an accepted and widely utilised prognostic indicator in the assessment of patients presenting with an infantile idiopathic scoliosis (IIS). However, uncertainty in the utility of the RVAD in the prognosis of IIS remains. The aims of this study were to investigate the prognostic significance of the RVAD and to describe the changes in RVAD over long-term follow-up of patients with progressive and resolving IIS.MethodsThis was a retrospective analysis of patients presenting with IIS at a tertiary spinal deformity unit in the UK. Serial patient radiographs were reviewed and a logistic regression model using the patients index RVAD was created to predict the likelihood of curve progression.ResultsAt both index presentation and over long-term follow-up, patients with a progressive curve had significantly greater mean Cobb angle and RVAD measurements than those with resolving curves. The RVAD and Cobb were found to correlate positively in both groups, reflecting the underlying costovertebral pathoanatomy. The logistic regression model demonstrated that the optimal RVAD threshold in predicting IIS progression was lower at 17.1 degrees than the 20 degrees cut-off previously advocated.ConclusionThis study describes the utility of the RVAD in predicting IIS evolution. From this analysis, we would advise caution in predicting outcomes based on the index RVAD at presentation.Level of evidenceII.