Weak Ligaments and Sloping Joints: A New Hypothesis for Development of Congenital Atlantoaxial Dislocation and Basilar Invagination

Abstract

Objective: Developmental bony craniovertebral junction (CVJ) anomalies seem to have a genetic basis and also abnormal joint morphology causing atlantoaxial dislocation (AAD) and basilar invagination (BI). Methods: DNA extracted polymerase chain reaction single-stranded conformation polymorphism (SSCP) performed for mutation screening of FBN1 gene (n = 50 cases+ 50 age/sex-matched normal; total: 100). Samples with a deviated pattern of bands in SSCP were sequenced to detect the type of variation. Computed tomography (CT) scans of 100 patients (15-45 years old) compared with an equal number of age/sex-matched controls (21.9 +/- 8.2 years). Joint parameters studied: sagittal joint inclination (SI), craniocervical tilt (CO), coronal joint inclination (CI). Results: Thirty-nine samples (78%) showed sequence variants. Exon 25, 26, 27, and 28 showed variable patterns of DNA bands in SSCP, which on sequencing gives various types of DNA sequence variations in intronic region of the FBN1 gene in 14%, 14%, 6%, and 44% respectively. CT radiology: SI and CCT correlated with both BI and AAD (p < 0.01). The mean SI value in controls: 83.35 degrees +/- 8.65 degrees, and in patients with BI and AAD:129 degrees +/- 24.05 degrees. Mean CCT in controls: 60.2 degrees +/- 9.2 degrees, and in patients with BI and AAD: 86.0 degrees +/- 18.1 degrees. Mean CI in controls:110.3 degrees +/- 4.23 degrees, and in cases: 125.15 degrees +/- 16.4 degrees. Conclusion: The study showed mutations in FBN1 gene (reported in Marfan syndrome). There is also an alteration of joint morphology, correlating with AAD and 131 severity. Hence, we propose a double-hit hypothesis: the presence of weak ligaments (due to FB1 gene alterations) and abnormal joint morphology may contribute to AAD and BI.