CT based evaluation of odontoid morphology in the Indian population

Anterior fixation using two 3.5 mm screws is typically recommended for type II odontoid fractures. However, it is unsuitable in patients with an odontoid diameter of <9.0 mm. There is no data regarding the morphology of odontoid process in the Indian population. The aim of our study was to: a) Measure the external diameters of odontoid process in the Indian population using CT scan and thus determine the feasibility of two 3.5 mm screw fixation in them. b) Determine if any correlation exists between body height (Ht) and weight (Wt) and external odontoid diameters.

CT images of odontoid process of 100 consecutive patients were analyzed. Antero- posterior (AP) and transverse (TD), outer diameters of the odontoid process were measured from the base and at 1 mm interval upwards on axial CT images.

The mean AP and mean TD were 11.52 mm and 9.85 mm, respectively. Fifty-five (55%) of the patients had at least one TD <9.0 mm. Five (5%) patients had at least one TD <7.4 mm. None of the patients had any diameter <5.5 mm. Body Ht correlated significantly with mean AP and mean TD of the odontoid process (AP: r = 0.276, P = 0.013; TD: r = 0.359, P = 0.001), whereas body Wt correlated significantly only with mean TD (AP: r = 0.162, P = 0.15; TD: r = 0.297, P = 0.007).

More than half of the study population (55%) was unsuitable for two 3.5 mm screw fixation for type II odontoid fracture. Two 2.7 mm screws can be safely used in 95% of the population. A 4.5 mm Herbert screw can be safely used in the entire population. We recommend two 2.7 mm screws or a 4.5 mm Herbert screw for fixation of these fractures in the Indian population. Body height showed a significant correlation with external odontoid diameters, whereas weight showed significant correlation only with TD of the odontoid process.