Purpose: Ligament laxity is a common feature of trisomy 21 and is incriminated in most of the orthopaedic disorders observed. Early diagnosis and management is essential. C1-C2 instability is a recognised manifestation in trisomy 21 and is associated, at least theoretically, with significant risk of cord complications. The purpose of this work was to provide a descriptive analysis of the C1-C2 joint in trisomy 21 and to analyse instability factors in order to determine the tolerable C1-C2 distance.

Material and methods: Within the framework of a French national epidemiology survey of trisomy 21, we focused on the C1-C2 joint. A total of 472 children with trisomy 21 were identified; 458 who were examined were included in this study. Careful history taking and a detailed physical examination with neurological tests (search for even minimal signs of neurological disorders) was conducted. The Carter and Wilkinson method was used to assess joint laxity. The same specialist searched for other orthopaedic disorders. Patients were divided into two groups depending on the presence or absence of neurological signs. Two groups were also distinguished according to the presence or absence of generalised laxity (Carter and Wilkinson). Lateral x-rays centred on C1-C2 were performed by the same technician on the same machine with the patient in a neutral position, hyperflexion and hyperextension. The same technique inspired by the method described by Singer et al. and modified for simplification was used in all cases. The same observer interpreted the images using a single-blinded protocol to search for congenital malformations and signs of degeneration, measure the C1-C2 distance the minimal sagittal diameter and the C1-C2 angle (not reported in the literature and described for this study). These measures were then compared with data in the literature as available and correlated by age, gender, presence of neurological signs and joint laxity. Seven patients were excluded from the study due to insufficient cooperation for the x-rays and nine because of incomplete clinical or radiological data. The statistical analysis was performed on data from 442 patients. Quantitative variables were compared with the Pearson test and parameteric ANOVA was used to search for correlations of quantitative and qualitative variables. Significance was set at p< 0.05.

Results: Mean patient age was 13.8 years. There were 184 girls and 258 boys. Minor neurological anomalies were found in 42% of the patients. There were no cases of major motor deficit. Generalised laxity as defined by Carter and Wilkinson was observed in 24% of patients. Other orthopaedic problems, basically of the foot, were found in 85%. The radiograms revealed a very wide range of measures were thus expressed as means. The C1-C2 distance was greater than 4 mm in 34 patients on the flexion films (limit established in the literature for instability in trisomy 21). The maximal C1-C2 distance in the neutral position was 8 mm, 9.6 mm in flexion. The lowest minimal sagittal distance was 8 mm in flexion and 10 mm in the neutral position (the lower limit reported in the literature before considering the cord to be threatened in 14 mm). The greatest variability was found for the C1-C2 angle. Ligament laxity and atlantoaxial distance were inversely proportional to patient age, but there was no significant correlation between atlantoaxial instability (C1-C2 distance > 4 mm) and gender or generalised hyperlaxity. There was no significant correlation between C1-C2 instability or laxity and neurological signs.

Discussion and conclusion: Compared with earlier publications, our series offers the advantage of a large unselected population providing epidemiological data on trisomy 21. A standard radiography protocol was used. The large majority of the radiographic measures reported in the literature do not take into account the magnification effect nor position variability between patients. Our findings confirm certain data in the literature and also provide new information suggesting it could be useful to revisit certain pathogenic hypotheses about C1-C2 instability and its neurological consequences in trisomy 21. Two important observations were the absence of a correlation between general laxity and C1-C2 instability and the absence of correlation between C1-C2 instability and the presence of neurological signs.