COMPUTERISED TOMOGRAPHIC MORPHOMETRIC ANALYSIS OF ATLAS AND AXIS VERTEBRAE.

Dr. Murugesan Govindarajan Mch And Dr. Jaganadh Kamalasekaran. 1. Associate professor of Neurosurgery , Dept of Neurosurgery,Coimbatore medical college,Coimbatore-18. 2. Mch Neurosurgery Resident ,Dept of Neurosurgery, Coimbatore Medical College ,Coimbatore-18. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History


ISSN: 2320-5407
Int. J. Adv. Res. 6(9), 325-331 326 With modern era of thin slice CT, it is easy to measure these dimensions with CT scan. There are so many western studies done to analyse the morphometry of atlas and axis vertebrae,but our study is to analyse the morphometric dimensions of atlas and axis and their surgical implications. CT brain with lower limit extending to C3 vertebra with multiplanar reconstruction images are used . The maximum dimension of C1 lateral mass, odontoid, body of C2, lateral mass of C2 are identified and recorded in detail. Surgery in these locations are highly complex, thus the dimensions of instrumentations and their point of entry, directions are important for surgical planning. As there is ethnical difference, we want to know the morphometric dimensions of atlas and axis in our Indian population.
Material AndMethods:-50 subjects from both sexes who took CT scan of brain or cervical spine in Coimbatore medical college for any indication (following trauma, CVA, degenerative diseases) without any abnormalities of C1 and C2 are taken for morphological analysis.
Study includes the patients of adult age group more than 20 years.
In CV junction surgery, the thickest part of C1 and C2 lateral mass is used for lateral mass screw fixation. C2 body and odontoid is used for odontoid screw fixation in case of odontoid fracture . Thickest part of spine and lamina of C2 is used for translaminar screw. Trans articular screw is used in C1-C2 subluxation.
In this study we want to define the variations in the dimensions of screw purchasable bony parts of C1 and C2 in Indian population. Vertebral artery injury is the worst complication in CV junction surgery . The relation of vertebral artery in relation to screwable C2 lateral mass is identified and measured . Spinal canal dimension is larger when compared to cord diameter.
In this study, we measured the following dimensions: C2 body -AP and TR dimension, C2 lamina length and height , total length from C2 base towards tip of dens, C2 lateral mass AP and TR dimension, the length from C2 articular surface to superior margin of vertebral artery in parasagital plane to know about the safety of screw purchase . In C1 ,lateral mass AP ,TR dimensions noted . Spinal canal and spinal cord dimensions noted . In C1, screwable area in the posterior surface of C1 lateral mass below the lamina is measured. The diameter of transverse foramina is measured. Favourable C2 anatomy for segmental fixation is identified.
The available vertical dimensions of the posterior surface of C1 lateral mass below the lamina in vertical plane for screw entry point is identified. The safe angle of screw insertion in sagittal plane and the thickness of posterior arch near the vertebral artery groove documented. The safe angle of screw trajectory in sagittal plane and the safe angle of screw trajectory in axial plane are noted .

Results:-
The dimensions related to atlas vertebra are described below and also showed in Table-1 and Table-2 The maximum AP dimension of C1 lateral mass on right side ranges from 11.6 mm to 18.5 mm and the mean is 15.64 mm. The maximum AP dimension of C1 lateral mass on left side ranges from 13.4 mm to 18.1 mm and the mean is 16.06 mm. The maximum transverse dimension of C1 lateral mass on right side ranges from 9.5 mm to 15.8 mm and the mean is 12.74 mm. The maximum transverse dimension of C1 lateral mass on left side ranges from 9.1 mm to 16.3 mm and the mean is 12.5 mm.
The average vertical dimension in the posterior surface of C1 lateral mass below posterior arch is 5.4 mm. The average height of posterior arch at the level of vertebral artery groove is 5.3 mm. The mean distance from midline to the vertebral artery groove is 14.75 mm and 14.04 mm on right and left side respectively.
The sagittal angle in the sagittal plane is measured at junction of anterior arch and the lateral mass. The measured mean sagittal angle is 13 0 on right and 10.86 0 on left side respectively. The safe cranial angle in axial plane were 11.9 0 and 12.9 0 on right and left side respectively.

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Mean canal dimension at C1 AP is 18.95 mm transverse is 27.80 mm and cord dimension is AP 8.02 mm and transverse 10.41 mm. Mean canal dimension of C2 AP is 17.43 mm and transverse 24.14 mm and cord dimension AP is 8.15 mm and transverse is 11.05 mm. The dimensions related to axis vertebra are described below and also showed in Table-2 and Table-3 The mean length of C2 base to odontoid tip is 30.38 mm. The transverse dimension of odontoid process is 10.37 mm.
Average length of C2 lamina is 23.96 mm on right and 24.09mm on left side. Average width of C2 lamina is 5.28 mm on right side and 5.51 mm on left side. Average vertical height of C2 lamina is 6.58 mm on right side and 6.72 mm on left side.
The average safe sagittal angle of C2 lateral mass is 41.     The study which we performed is a random analysis of C1,C2 bone morphometry of 50 patients who have taken CT brain with bone cuts or CT-C spine for other reasons with normal bony anatomy of C1, C2. The parameters are measured are analysed which will be useful in planning for surgeries on C1 and C2.
In our study, we showed the various dimensions of C1 and C2. Our study showed the mean height of C2 30.3 mm which is almost similar to Sinan et al. 1 study which can be utilized for odontoid screw fixation.
Odontoid fractures which need fixation requires the knowledge regarding the diameter of odontoid process. The number of screws required for fixation varies with diameters of odontoid. The average distance from midline to vertebral groove is 14.7 mm at right and 14 mm on left side which more or less similar to the mean inner distance of vertebral artery groove as enumerated by Lalitha B et al 11 .
The safety margin of C1 screw observed in our population is 5.44 mm and 5.45 mm on Rt and Lt respectively, which is more when compared to study done by Serkan et al 12 where he analyzed ideal screw entry point and projection angle for posterior lateral mass fixation of Atlas in 40 dry human adult vertebra.
The range of safe sagital angle of C1 lateral mass screw is 10 0 to 16 0 and the range of safe cranial angle of C1 lateral mass screw at axial plane is 10 0 to 16 0 . The mean safe angle of trajectory of C1 lateral mass in our populations is 13 0 in sagital plane and 11 0 in axial plane. In a study by Roch et al 13 maximum angle of medialisation from midline was calculated as 16.7 0 . Hong et al 14 reported the screw angulations to be 14.7 0 relative to axial plane..
The percentage canal cord ratio is more at C1 C2 region which gives mobility of cord during extreme position. The canal cord ratio at C1 in anteroposterior dimension is 2.3 and in the transverse dimension is 2.6 and canal cord ratio at C2 in anteroposterior dimension is 2.1 and in transverse dimension is 2.1.
The average screwable dimension of C2 lamina is 23.9mm*5.2mm*6.8mm. So it can accommodate screw length of size 20 mm and diameter 3.5 mm. Our results pertaining to the C2 lamina are comparable to various study in the literature (Soyeon et al) 15 . Ideal screw trajectory in C2 lateral mass in Indian population is 41.2 0 in sagittal angle and 27 0 degree in cranial angle. Distance of vertebral artery to the superior articular surface is 4.4mm and to the medial margin of spinal canal is 4.3 mm . The diameter of screw that can be used varies from 2.3 and 6.5 mm.

Conclusion:-
Morphometric analysis of C1 C2 vertebra shows light on the ideal dimensions of screws to be used in our population. It also shows light on the safety angle and safety margin to avoid complications related to vertebral artery and spinal cord injury.