Computed tomography morphometriC analysis of the vertebrae C 7 and t 1

Objective: The anatomical study of the vertebrae C7 and T1 of the cervicothoracic junction aimed to evaluate quantitatively, by axial computerized tomography (CT), the linear and angular dimensions of the anatomical laminae of the vertebrae of the cervicothoracic junction C7 and T1 in adults over 18 years. Methods: We retrospectively analyzed 49 CT of the cervical and thoracic spine (C7 and T1) of individuals over 18 years, of both sexes. We also evaluated the length and thickness of the laminae, as well as spinolaminar angle in axial sections of C7 and T1 at the point of least thickness between the inner cortical layers. The variables were correlated with age groups and sex of the individuals. Statistical analysis was performed using the t test and the results were considered significant when p<0.05. Results: After analyzing tomographic measurements of 49 patients, it was found that men had greater laminae thickness than women, both in C7 and T1, with 71% of C7 laminae and 92% of T1 laminae thicker than 5mm, and 97% of C7 laminae and 100% of T1 thicker than 4mm. The mean spinolaminar angle was 56.40 degrees in C7 and 57.31 degrees in T1. Conclusion: This study brings important anatomical information about the cervicothoracic junction C7 and T1 in the Brazilian population, showing that fixation of C7 and T1 with intralaminar screws is anatomically possible.


A B A B B A INTRODUCTION
The cervico-thoracic region is an area of transition between the relatively mobile cervical spine and the more rigid thoracic spine.This characteristic means that there is more stress on the commonly used posterior approach fixation implants. 13][4] The most common techniques are those that use screws that can be fixed to the lateral masses or to the pedicles; 3,5 however there are limitations, due to the complication rates reported. 6,7][10][11][12] Panjabi et al. [9][10][11] detailed most of the bony structures of the spine, but not those of the laminae.Zindrick et al. 12 quantified the height and thickness of the laminae, the interlaminar distance, and the epidural space of the thoracic spine in their study of factors that interfere with the penetration of wires into the spinal canal.The authors evaluated the spines of 13 cadavers and reported average values of 18-24mm for the laminar height (superior-inferior diameter) and 7-8mm for the laminar thickness (anterior-posterior diameter).
Another quantitative anatomical study of the spinal laminae was conducted by Xu et al. 8 from the perspective of the use of intra-laminar screws.The authors evaluated all the spinal laminae from C2 to L5 in 37 cadavers.However, this study has several limitations that should be highlighted: the laminar length was measured from the middle of the spinous process to the medial margin of the facet joint, which does not correspond to the diameter of the screw, given that it is placed by means of the contralateral lamina; the laminar thickness was measured at its midpoint, with no reference to the thinnest part of the lamina; and the number of cases evaluated was relatively small.
There is no study in the national literature that has quantitatively analyzed the linear and angular parameters of the spine at the cervicothoracic transition in the adult Brazilian population.Thus, the objective of this study was to quantitatively evaluate the linear and angular dimensions of the laminae of C7 and T1 in adults older than 18 years of age in order to provide useful information for making decisions about surgery.

MATERIAlS AND METhODS
We retrospectively analyzed 54 computed tomographies performed during routine outpatient and emergency treatment at the complex of the Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo (HC FMUSP).This study was approved by the Scientific Commission of the Orthopedics Institute of HC FMUSP.The Ethics Commission for the Analysis of Research Projects (CAPPesq) waives approvals for this type of study.
Adult patients aged over 18 years of age were selected, and data on the age and the sex of the patients were collected.Cases presenting fractures of the C7 and T1 cervicothoracic transition, with or without fixation, or spinal abnormalities, such as deformities, neoplasias in the C7-T1 vertebrae, or rheumatologic diseases, were excluded.Of the 54 tomographies analyzed, five were excluded from the study because one of more of the exclusion criteria applied.
The linear and angular dimensions of the cervicothoracic spinal laminae of C7 and T1 were evaluated in axial CT sections where the lamina was the thinnest.The thickness of laminae C7 and T1 (measurement A) was measured in millimeters (mm).Measurements were obtained at the point of least thickness between the internal cortical layers, as shown in Figure 1.Specific measurements were taken for each side of the lamina.The length of laminae C7 and T1 (measurement B) was measured from the same axial section.The measurements were taken from the cortex opposite the lamina to the limit of the length visible in that section, as we can see in Figure 2. The spine-lamina angle (measurement C) was drawn to both sides, with one line on the inside of the laminae of C7 and T1 parallel to the cortices of these laminae and the other line passing through the center of the spinous process and the vertebral body dividing the vertebra into two hemivertebrae.(Figure 3) The morphometric analysis was performed using the iSite PACS program from Philips Healthcare Informatics ® .
The statistical analysis was conducted using SPSS 13.0 for Windows.Average, standard deviation, and minimum and maximum values were obtained.The values obtained for each of the variables were correlated with the age groups and the sex of the individuals.Comparisons were made using the student's t test and the results were considered to be significant when p< 0.05.

RESUlTS
Fifty-four tomographies were selected, 5 of which were excluded: 3 due to fractures and 2 because of prior fixation in the region.The remaining 49 tomographies, of 37 men and 12 women, were analyzed.The average patient age was 39.7±17.16years (ranging from 19 to 81).The average thickness of the laminae of C7 was 5.78±1.16mm(ranging from 3.5 to 9.2).(Table 1) The thickness measured in the men was greater than that in the women (5.99±1.17mm vs. 5.13±0.85mm,p<0.01).Seventy-one percent    (71%) of the vertebrae measured had a thickness greater than 5mm, 97% greater than 4mm, and 99% greater than 3.5mm.(Figure 4) The average length of the C7 laminae was 26.03±3.40mm(ranging from 18.2 to 35mm).The length measured in the men was greater than that in the women (26.85±3.37mm vs. 23.51±1.97p<0.01).
The average spine-lamina angle of C7 was greater in the women.The average thickness of the laminae of T1 was 6.76±1.04mm(ranging from 4.6 to 9.5mm).The thickness of the T1 laminae was greater in the men (6.97±1.03 vs. 6.13±0.82mm,p<0.01).Of the T1 vertebrae analyzed, 92% had a thickness greater than 5mm and 100% greater than 4mm.(Figure 5) The average length of the T1 laminae was 25.94±3.22mm(ranging from 19.4 to 33.1mm).There was a statistically significant difference between the sexes, with the greater length measured in the men (26.61±3.0993 vs. 23.86±2.74p<0.01).The measurement of the spine-lamina angle of T1 was greater in the women.(Table 2) There was no difference between the measurements of the right and left sides.

DISCUSSION
The change from cervical lordosis to thoracic kyphosis in C7, in the region of cervicothoracic transition, results in a load transfer to the posterior aspect. 13This creates an increase of stress in this transitional region, which in combination with the anatomical changes in this region, leads to technical difficulties for the passage of screws and motivates the search for fixation options for this region.
Lateral mass screws are more secure and easier to implant. 14However, the lateral mass of C7 is frequently not very thick and does not permit the use of long screws.These characteristics make the pullout resistance of lateral mass screws low as compared to transpedicle or translaminar screws. 15On the other hand, transpedicle fixation permits the use of longer screws, generally between 25 and 30mm.Synthesis with transpedicle screws is more stable than fixation in the lateral masses. 15he principal challenge in using transpedicle screws in the cervical region is in the technical difficulty of insertion, due to the size of the pedicles and the potentially serious complications, such as lesion of the vertebral arteries, of the spinal cord, and of the nerve roots. 5The intraoperative use of radioscopy can assist with the placement of these screw. 16However, positioning the radioscope to obtain suitable images may be difficult in some cases.The anatomical variations in the dimensions and morphology of the pedicles of C7 and T1 make surgical techniques based exclusively on bone repairs for the point of screw insertion potentially dangerous. 1Even under direct visualization, cortical ruptures were identified in 8-13% of the patients. 3,4ne of the extremely important factors for the correct execution of any surgical procedure is precise knowledge of the anatomy of the region to be operated.In this context, there are no studies in the Brazilian literature that have focused on evaluating the characteristics of the laminae of C7 and T1.Other studies of laminar measurements were conducted by Xu et al., 8 but because they were conducted before the introduction of intralaminar screws, the measurements were taken differently, making comparison between the studies impossible.
The customary use of intralaminar screws in the C2 lamina for C1-C2 fusion, by means of the technique developed by Wright,17 is motivated by the technical difficulties of passing screws into the pars (Magerl) and the pedicle (Harms) and also by the complication rates of these procedures. 7However, there are few case reports in the literature with the use of translaminar screws in C7 and T1, the major advantage of which is to avoid vascular lesions, especially when the vertebral artery passes through the transverse foramen of C7, in addition to its being technically easier to perform. 18,19he results obtained show a significant difference between the sexes in the length and width of the laminae, but not in the angulation of the laminae.The thickness of the lamina in C7 was greater than 4mm in 97% and 5mm in 71%, which theoretically makes the passage of an intralaminar screw of greater diameter and length than usually used in the lateral mass of C7 (of 3.5 mm) possible.In T1, the thickness was greater than 4mm in 100% of the cases and greater than 5mm in 92%, which theoretically makes possible the use of a slightly thicker screw in the lamina, avoiding the risk of

Figure 2 .
Figure 2. Measurement of the laminar length.

Figure 3 .
Figure 3. Measurement of the spine-lamina angle (A right, B left).

Figure 5 .
Figure 5. Histogram of the thickness of the laminae of T1.

Figure 4 . 1 COMPUTEd
Figure 4. Histogram of the thickness of the laminae of C7.

Table 1 .
Measurements of the C7 vertebrae.

Table 2 .
Measurements of the laminae of T1.