RESUMO
Objectives:To observe the anatomical location and mechanism of axis ring fractures (ARF) using 3-D CT scans, and propose a new classification for such fractures.Methods:By reviewing prospectively maintained database collecting ARF from 7 medical centers in China, 202 patients were included in this study. According to anatomical location, ARFs were classified into axis arthrosis fracture (AAF) and axis bony damage (ABD). The axis ring was divided into anterior, middle, and posterior rings, based on the border of the pars interarticularis (or pedicle) of axis. According to the features of ARF and previous study, a new classification was proposed based on the anatomical features of different fracture patterns, which was divided into three types and six subtypes (A1, A2, B1, B2, C1 and C2). The incidence of AAF and ABD and their distribution in different location of axis ring and the new classification, were observed.Results:In 202 patients with ARF, 501 anatomical structures were involved. 288 AAFs were found in 178 patients (288/501, 57%), while 213 ABDs were found in 149 patients (213/501, 43%). In anterior ring, 304 structures (304/501, 61%) were involved in injury, with 225 AAF and 79 ABD. In middle ring, 99 structures (99/501, 20%) were involved in injury, and all of them were ABD. In posterior ring, 98 structures (98/501, 19%) were involved in injury, with 63 AAF and 35 ABD. The anterior ring injuries (61%) were more common than middle (20%) or posterior ring (19%). In anterior ring, AAF (84%) were morecommon than ABD (16%); In middle ring, all the injuries were ABD; In posterior ring, AAFs (64%) were more common than ABD (36%). Type A fractures were featured with pedicle fractures and were identified in 30 patients (30/202, 15%). Type A1 fractures were bilateral pedicle fracture lines symmetrically or asymmetrically and identified in 12 (6%) patients; Type A2 fractures were pedicle fracture lineson one side and inferior articular facet injuries or lamina fractures on the otherside and identified in 18 (9%) patients. Type B fractures were featured with superior articular facet injuries or posterior wall of C2 body fractures on one side and identified in 136 patients (67%). Type B1 fractures were superior articular facet injuries or posterior wall of C2 body fractures on one side and pedicle fracture on the other side and identified in 57 (28%) patients; Type B2 fractures were superior articular facet injuries or posterior wall of C2 body fractures on one side and inferior articular facet injuries or lamina fractures on the otherside and identified in 79 (39%) patients. Type C fractures were featured with bilateral superior articular facet injuries or posterior wall of C2 body fractures and identified in 36 patients (18%). Type C1 fractures were bilateral superior articular facet injuries or posterior wall of C2 body fractures symmetrically and identified in 22 (11%) patients; Type C2 fractures were bilateral superior articular facet injuries or posterior wall of C2 body fractures asymmetrically and identified in 14 (7%) patients.Conclusion:ARF could occur in different anatomical locations, and most of these fractures were caused by hyperextension and axial load on superior articular facet on one or two sides. The new CT classification of ARF with three types and six subtypes might provide all fracture patterns, which could be useful for the choice of proper diagnosis and treatment for such fractures.