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INTRODUCTION
The atlas is located at the junction of the skull and spine, serving as a connecting link in the occipital-atlantoaxial complex. It is adjacent to the “vital knot”, medulla oblongata. The vertebral artery enters the cranium through the transverse foramen of the atlas. Therefore, atlas fractures are life-threatening injuries. Adult atlas fractures account for 25 % of upper cervical spine injuries, 2 % to 13 % of cervical spine fractures, and 1 % to 2 % of spinal fractures (Hadley et al., 1988; Kakarla et al., 2010). Many researchers have investigated the classification and treatment of atlas fractures, whereas rare studies have examined the predilection sites of atlas fractures from the morphological point of view. Therefore, this study aimed to analyze the bone microarchitecture of different regions of the atlas by using micro-CT, in order to explain the mechanism of the predilection sites of atlas fractures.
MATERIAL AND METHOD
Specimens and tools. A total of 28 dry specimens of intact atlas, whose gender and age were unknown, provided by the Department of Human Anatomy, Inner Mongolia Medical University were randomly selected. Specimens with defects and deformities were excluded. Hiscan XM Micro CT (Hounsfield, Suzhou, China) and its analyzer software (Hiscan Analyzer) were used with the following parameters: slice thickness 0.05 mm, slice gap 0.05 mm, single exposure time 50 ms, tube voltage 60 KV, current 134 uA, matrix 2000 x 1600, field of view 10 x 8 cm, and pixel size 0.05 x 0.05. The acquired DICOM images were processed and stored by the Lenovo P320 workstation provided by Haysfield (Suzhou, China).
Methods: The 28 atlas specimens were numbered, scanned with Hiscan XM Micro CT, and divided into a region from the anterior arch midpoint (AAM) to the la teral masses (LM), a LM region, and a region from the LM to the posterior arch midpoint (PAM) (Fig 1). The following parameters were measured for each of the three regions using the built-in Hiscan Analyzer software: total volume (TV), bone volume (BV), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), trabecular number (Tb.N), trabecular connectivity (Conn), and trabecular structure model index (SMI). The definition and significance of each parameter are shown in Table I. The reconstructed trabeculae of the atlas are shown in Figure 2. Each parameter was measured 3 times and the average was recorded.
Statistical analysis. Data were analyzed using SPSS 13.0. Normally distributed measurement data were expressed as mean ± standard deviation ( ± s) and analyzed by analysis of variance. An a = 0.05 (P < 0.05) was considered significant.
RESULTS
As shown in Table II, trabecular thickness was all measured to be 0.11 ± 0.00 mm for AAM to LM, LM, and LM to PAM. Trabecular separation: AAM to LM > LM to PAM > LM. Trabecular number: LM > LM to PAM > AAM to LM. Connectivity: LM > LM to PAM > AAM to LM. Structure model index: LM > LM to PAM > AAM to LM.
DISCUSSION
Strengths of micro-CT in evaluating atlas microarchitecture. The study of trabecular architecture currently is based on imaging. Traditionally, the imaging of trabeculae is performed by sectioning (Wan et al., 2007), which, however, is associated with large bone damage, difficult analysis, long time requirement, and low accuracy. With the rapid development of computer and imaging technology, micro-CT is able to offer high resolution (up to micron level), image quality, and accuracy with a short time requirement for the structural analysis of trabeculae. Clear, separable trabecular images obtained by micro-CT can be used for two-dimensional image analysis or three-dimensional model analysis to facilitate the observation of trabecular changes and analyze the state of bone tissue (Cui et al., 2012). For example, microCT has been used to analyze the anatomy of the trabecular bone of the jaw by Jin et al. (2011) to examine the bone mineral density and trabecular number, thickness, and morphology around the dental implant in Beagle dogs by Zhang et al. (2017) and to determine the effect of a Chinese medicine, Qiangguyin, on the trabecular number, thickness, and morphology of bone callus in ovariectomized rats by Yao et al. (2016).
Predilection sites of atlas fractures. The atlas consists of an anterior arch, a posterior arch, and two lateral masses. According to Landells and Van Peteghem’s classification (Landells & Van Peteghem, 1988). Type I fracture involves the anterior or posterior arch alone. Type II fracture involves fractures of both the anterior and posterior arches. Type III fracture is a fracture of the lateral mass with or without a fracture of the arch. Of the 33 cases of fresh unstable atlas fractures collected by Li et al. (2008) for surgical strategy and efficacy analysis, 3 (9 %) were Type I, 21 (64 %) were Type II, and 9 (27 %) were Type II; and anterior and/or posterior arch fractures accounted for 73 %. Of the 35 cases of atlas fractures reviewed by Landells & Van Peteghem. Sixteen (46 %) were Type I, 13 (37 %) were Type II, and 6 (17 %) were Type III; and anterior and/or posterior arch fractures accounted for 83 %. It can be seen that atlas fractures most frequently involve the anterior and/or posterior arch, followed by the lateral masses.
Differences in bone microarchitecture between different regions of the atlas. The trabeculae of the atlas are interconnected to form an irregular reticular arrangement. Trabecular thickness was all measured to be 0.11 ± 0.00 mm for AAM to LM, LM, and LM to PAM. Trabecular separation: AAM to LM > LM to PAM > LM. Trabecular number: LM > LM to PAM > AAM to LM. Connectivity: LM > LM to PAM > AAM to LM. Structure model index: LM > LM to PAM > AAM to LM.
Clinical significance of microarchitecture measurements in atlas fractures. Changes in trabecular architecture are also an important factor in osteoporosis, which, in turn, can easily lead to fracture. Greater trabecular thickness, number, and connectivity and smaller trabecular separation contribute to lower fracture rates.
Outlook and limitations. In this study, the microarchitecture of the atlas was analyzed using micro-CT with high resolution, image quality, and accuracy. However, as the gender and age of the specimens were unknown, the differences in the degree of osteoporosis due to gender and age cannot be excluded. Moreover, this study did not explore the differences in atlas fracture sites caused by external forces in different directions, but simply compared the bone microarchitecture of different regions of the atlas to identify the predilection sites of atlas fractures based on trabecular number, thickness, separation, and connectivity. In future research, it is therefore important to consider gender, age, and external forces in different directions when examining the predilection sites of atlas fractures.
CONCLUSION
In this study, bone microarchitecture of different regions of the atlas was examined in 28 dry atlas specimens using micro-CT. A lower trabecular number and connectivity and higher trabecular separation were seen in the anterior and posterior arches, in which higher fracture rates were reported. By contrast, a higher trabecular number and connectivity and lower trabecular separation were seen in the lateral masses, in which lower fracture rates were reported. In summary, atlas fractures most frequently involve the anterior arch, followed by the posterior arch, and least frequently involve the lateral masses. This is consistent with clinical observations. This study explains the predilection sites of atlas fractures from the morphological point of view.
