Skull base abnormalities in osteogenesis imperfecta: a cephalometric evaluation of 54 patients and 108 control volunteers

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Object

Osteogenesis imperfecta (OI), which usually results from mutations in type I collagen genes, causes bone fragility and deformities. The head is often abnormally shaped, and changes in skull base anatomy in the form of basilar impression and basilar invagination have been reported. The authors analyzed the skull base anatomy on standardized lateral cephalograms from 54 patients with OI (Types I, III, and IV) and 108 control volunteers. They were surprised to find that the previously used diagnostic measures for basilar abnormality in patients with OI were exceeded in 6.5 to 7.4% of the controls, and hence needed to be reevaluated.

Methods

The authors calculated the distance from the odontoid process to four reference lines, including a novel one, in the controls. The normal mean distances were exceeded by more than two standard deviations (SDs) in 28.3 to 35.2%, and by more than three SDs in 13.2 to 16.6% of the patients with OI. The latter figures reliably reflect the prevalence of basilar impression. As a sign of basilar invagination the odontoid process protruded into the foramen magnum or reached the foramen magnum level in 22.2% of the patients with OI, whereas none of the controls showed this feature. Platybasia (an anterior cranial base angle > 146°) was present in 11.1% of the patients but in none of the controls.

Conclusions

Platybasia, basilar impression, and basilar invagination were often coexpressed, but each was also present as an isolated abnormality. These three abnormalities and wormian bones were predominantly found in OI Types III and IV as well as in patients exhibiting dentinal abnormality.

Abbreviations used in this paper: MR = magnetic resonance; OI = osteogenesis imperfecta; SD = standard deviation.

Article Information

Address reprint requests to: Janna Waltimo-Sirén, D.D.S., Ph.D., Department of Orthodontics, Institute of Dentistry, P.O. Box 41, FIN-00014 University of Helsinki, Finland. email: janna.waltimo@helsinki.fi.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Schematic illustrating the linear and angular variables analyzed in the lateral radiographs. 1, perpendicular distance from the tip of the odontoid process, or dens point (D), to the McRae line (foramen magnum line) running from the basion (Ba) to the opisthion (Op); 2, perpendicular distance from D to the modified Chamberlain line running from the posterior nasal spine (PNS) to the Op; 3, perpendicular distance from D to the McGregor line running from the PNS to the lowermost point of the posterior cranial base (M); 4, the D-M distance—a novel variable—measuring the perpendicular distance from D to a line parallel to the nasion (N)–sella (S) line and drawn through M; 5, anterior cranial base angle between the N-S line and the S-Ba line; 6, craniovertebral angle between the N-S line and the longitudinal axis of the odontoid process; 7, anteroposterior relation of the odontoid process to the clivus (odontoid process axis runs anterior to, posterior to, or through the Ba); and 8, wormian bones (present, not present, or not present with certainty). The higher the D is situated in relation to the reference lines 1 to 4, the greater the (positive) measured value.

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    Graph showing the distribution of the McRae and Chamberlain measures in patients with OI (open squares) and controls (closed circles). All controls displayed negative McRae measures, indicating that the entire odontoid process lay below the foramen magnum. McRae measures greater than or equal to 0 are indicative of basilar invagination. Chamberlain measures that exceed the mean by three SDs are diagnostic for basilar impression. These pathological conditions often coincide. The two measures correlate highly significantly with each other in both patients and controls (p = 0.000).

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    Graph displaying the distribution of measures for the anterior cranial base angle and craniovertebral angle in patients with OI and in controls. The anterior cranial base angle tends to be larger in patients with OI than in controls, and angles 146° or greater are indicative of platybasia. The craniovertebral angle in patients with OI may deviate from the control mean by more than three SDs in any direction. These two angles show a statistically significant positive correlation with each other in both patients (p = 0.050) and controls (p = 0.019).

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    Drawings based on lateral radiographs obtained in six patients with OI, showing marked variation in the form of the head and anatomy of the craniovertebral junction. Any part of the odontoid process above the foramen magnum line has been shaded black and indicates basilar invagination. The area of the odontoid process that lies above one or several of the reference lines (Chamberlain line, McGregor line, or the baseline for the D-M distance) has been dotted and indicates basilar impression. A: Thirty-one-year old man with OI Type IVB showing basilar invagination but no sign of basilar impression. Flexure of the anterior cranial base is normal. B: Sixteen-year-old boy with OI Type IVB demonstrating basilar impression but not invagination. Flexure of the anterior skull base is normal. C: Thirty-one-year-old woman with OI Type IVB exhibiting both basilar impression and invagination but normal flexure of the anterior skull base. D: Thirty-one-year-old woman with OI Type III/IV(B) showing neither basilar impression nor invagination but displaying platy-basia (abnormally large anterior skull base angle). E: Twenty-year-old man with OI Type III demonstrating marginal basilar invagination (odontoid process reaching the foramen magnum), basilar impression, and platybasia. F: Seventeen-year-old girl with OI Type IVB exhibiting all three abnormalities: basilar impression, basilar invagination, and platybasia.

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