Since its first description, there was no uniform definition and precise location of axis ring fractures [5, 12]. Authors who used different terms for this injury usually reported variable sites and different anatomical locations in such fracture. Recently, Menon et al. [5] conducted a research and confirmed fracture lines could occur in any part of the ring (neural arch) of axis, e.g. the pedicles, pars interarticularis, facets, laminae, the cortex of vertebral body, or even foramen transversarium of C2 in axis ring fractures. And the anatomical information in their study may modify the surgical treatment in such injury [5]. Therefore, research with large-sample accurately describing the precise anatomical location and frequency of the fracture lines in these injuries is of importance.
To the best of our knowledge, this is the first multiple-center and large-sample study observing the anatomical features of axis ring fractures using three-dimensional CT. The result of this current study demonstrated: Firstly, most of fracture lines of axis ring fractures were asymmetrical, and typical “hangman’s fractures” described as bilateral pedicle or par interarticularis fractures of axis were rare; Secondly, facet joints injuries in axis ring fractures were more common than pure bony injuries, and superior articular facet injuries were most common; Thirdly, when axis ring was divided into anterior, middle, and posterior elements, the injuries of anterior element were more common than that of middle or posterior element.
Several authors tried to describe, classify, and correlate between different patterns of axis ring fractures and mechanism of injury or even treatment options [1–5, 17]. Levine and Edwards modified Effendi’s classification and published the most widely accepted, comprehensive, and consummate classification scheme for axis ring fracture [4, 9, 17]. However, Levine- Edwards’ classification was based on X-ray films of cervical spine, which supposed that most of such injuries were bilateral pars fractures or pedicle of C2 rather than fractures involving various anatomical structures of axis ring. The result of our study showed that typical bilateral pedicle or par fractures of axis were rare, and most of fracture lines were asymmetrical, which was consistent with previous or recently published studies [3, 4, 10, 11, 13, 18]. Therefore, our study reminded spine surgeons that three-dimensional CT was necessary for axis ring fractures, and using the precise anatomical structures to replace the general term “neural arch fracture” or “Hangman’s fractures” might be more proper to assist in planning the reasonable treatment [5, 11, 19].
The result of this current study showed not just the location, pattern, extent and symmetry in these fractures but also high prevalence of facet joints injuries. Menon et al. [5] conducted a similar study in which 32 patients with axis ring fractures were included, and they found that facet joint involvement were demonstrated in 50% of patients, which consist with that of our study. The high prevalence of facet joints injuries in axis ring fractures means that most of axis ring fractures may be involved with intra-articular injuries, and it also has the potential to plan the suitable treatment. However, literature is unclear about the suitable treatment for axis ring fractures involving the superior facet of axis. In axis ring fractures, listhesis of C2 over C3 vertebra is common, and if superior facet of axis injury exists, C1-C2 joints and C2-C3 disc/joints are unstable [20, 21]. So stabilization of both these joints might be necessary, and anterior cervical surgery such as anterior cervical discectomy and fusion (ACDF) of C2/3 does not play any role in such a situation [20]. Geol suggested posterior fixation and fusion at C1-C3 level could be a most suitable surgical technique [20].
Based on the features of 202 axis ring fractures: 1. most of fracture tended to be asymmetrical; 2. facet joints injuries were more common than pure bony injuries; 3. the injuries of anterior element were more common than that of middle or posterior element, we postulated that the probable mechanism for most of axis ring fractures might be hyperextension and axial load, which caused anterior element fractures on one side as primary injury, and the rotational element during the extension component of the injury vector caused contralateral middle or posterior element fracture as asymmetrical secondary injury. Our hypothesis was supported by previous studies: biomechanical studies demonstrated that the extension compression force was the primary injury vector in axis ring fractures [1, 5, 17]; In regard to asymmetry of such fracture, Duggal et al. [22] suggested that this injury was almost always asymmetrical because of the rotational element during the extension component of the injury vector.
Limitations
First, although this study is multicenter studies with large sample sizes, it is still a retrospective research. Second, fractures extending into the transverse foramen were not recorded, because transverse foramen is located outside the superior facet joint and pedicle of the axis, and its damage does not cause instability; and fracture lines running into this structure were always accompanied with superior facet joint or pedicle fracture. Third, CT with axial cuts and three-dimensional reconstructions in the frontal and sagittal plane were the only tool in this study, combination plain X-rays, MRI, and CT may give more persuasive information about axis ring fractures.