Feasibility and limitations of C1 lateral mass screw placement in patients of atlas assimilation

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Abstract

Background

C1 lateral mass is a common place for screw fixation in normal anatomy; whereas there is no research about whether screw placement is suitable in patients of C1 assimilation (C1A).

Objective

To study the feasibility and limitations of C1 lateral mass screw placement in patients with C1A.

Patients and methods

From April 2008 to March 2009, C1 lateral mass of 17 C1A patients with atlantoaxial instability (AAI) or dislocation (AAD) was observed and measured using CT reconstruction; and factors determining C1 lateral mass screw placement were studied before and during the operation.

Results

A screw of 3.5 mm in diameter could be virtually inserted in 31 C1 lateral masses of total 17 C1A patients with maximal length of the screw 18.1 ± 2.7 mm; but the entry point of screw had to be modified in the posterior part of inferior facet of C1 instead of posterior middle wall of C1 lateral mass. Clinically, abnormal course of vertebral artery in 6 of 30 (20%) and abundance of venous plexi prevented the proper exposure of C1 lateral mass and screw placement. Hypoglossal canal also had potential risk of injury during screw placement.

Conclusion

In patients of C1A, when C1 lateral mass screw placement is programmed, factors limit its use should be well studied, and CT angiography is essential.

Section snippets

Patient population

In a period from April 2008 to March 2009, a consecutive group of 17 C1A patients with original computer tomographic (CT) data was prospectively collected, to study the morphological changes of C1 lateral mass and factors influencing C1 lateral mass screw placement. All 17 patients had AAD or AAI confirmed by reconstructed CT, and/or dynamic MR, in which ventral compression of spinal cord increased in flexion. Six of them had also Chiari's malformation and/or syringomyelia. 17 patients were

Morphological observation

In all 17 patients, C1A was diagnosed by reconstructed CT scan and characterized by C1 lateral mass and condyle fusion. C1 anterior and posterior arch fusion with foramen magnum was present in all but 3 patients. Four patients had Klippel–Feil syndrome, with C2–C3 fusion in 2, C2–C4 fusion in 1, and C4–C5 fusion in 2 patients. All patients had congenital basilar invagination, of which 14 had AAD, the mean atlantodens interval was 6.2 mm, and other 3 patients had AAI, in which ventral compression

Discussion

In the presence of atlas assimilation, occipitocervical fixation for stabilization of atlantoaxial dislocation seems to be a reasonable strategy of surgical treatment. Although direct screw implantation into the C1 lateral mass in the presence of atlas assimilation is relatively difficult and dangerous, the screw purchase is much stronger in the thick and large lateral mass than screw implantation in the relatively thin occipital squama [16], [18]. Goel et al. [2] first reported the use of C1

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