The provocative radiographic traction test for diagnosing craniocervical dissociation: a cadaveric biomechanical study and reappraisal of the pathogenesis of instability.

Abstract

Craniocervical dissociation is a rare but serious condition, and missed injuries have been associated with poor neurologic outcomes and deterioration. A fluoroscopic traction test is employed to interrogate the craniocervical ligaments when clinical and imaging findings are equivocal. However, no specific protocol or known parameters with respect to traction or force applied have been established. This study sought to define the parameters of the radiographic traction test with sequential sectioning of the primary ligamentous restraints under controlled distraction of the craniocervical junction in a biomechanical model. This is a cadaveric biomechanical study. A custom loading apparatus applied traction forces in six specimens (O-C3) and the following ligaments were sectioned: alar, tectorial membrane, and occiput-C1 capsules to simulate varying degrees of craniocervical dissociation. Traction was applied 0 to 20 lb with fluoroscopy. Digital image analysis quantified the relative displacements of C0-C1, average craniocervical excursion, and under what load could a 2-mm craniocervical displacement be reproducibly recorded. A weight-distance table was produced and showed a marked loss of stability with sectioning of the ligaments and across all specimens in a similar pattern. Minimal translation was noted with sectioning of two of three ligaments in any order (<1-2 mm). All specimens exhibited a firm restraint to dissociation until the last of the three stabilizers was sectioned. Thus an "all-or-none" restraint to instability is present. All specimens failed at a weight of 5-10 lb (>2 mm). The current knowledge base of craniocervical injuries is very limited. This study shows that the key restraints to craniocervical instability are the alar ligaments, tectorial membrane, and the atlantooccipital joint capsules. Dissociation requires the complete incompetence of all three. The craniocervical traction test reliably demonstrates instability and requires no more than 5-10 lb of traction to perform.