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Landmarks of vertebral artery groove of the first cervical (Atlas) vertebra for posterior cranio-cervical approach Gamal Hamed El-Sayed Hassanein Anatomy Department, Faculty of Medicine –North Jeddah Branch, King Abdulaziz University Abstract: Background & Objectives: The vertebral artery groove on the superior surface of the atlas vertebra (C1) is an impression of the third part of the vertebral artery. In the previous location; it is liable to injury during posterior surgical approach in cranio-cervical region. The aim of this study was to determine relevant anatomical landmarks of the vertebral artery groove (VAG) on the posterior arch of the atlas vertebra for identifying a safe zone to avoid the potential injury of the vertebral artery during these procedures. Methods: Thirty six dry atlas vertebrae (24 male and 12 female) were used in this study. The vertebral artery groove (VAG) was identified on the superior surface of each specimen and five parameters of the groove were measured bilaterally (right and left) using a Vernier sliding caliper. Results : Starting from the posterior midline, the distance to the medial most edge of the VAG on the inner cortex was 12.19 ± 3.0 mm (males) and 11.32 ± 2.88 mm (females) and the distance to the medial most edge of the VAG on the outer cortex was 16.84 ± 3.58 mm (males) and 15.21 ± 3.66 mm (females). The distance to the lateral most edge of the VAG on the inner cortex was 17.99 ± 3.81 mm (males) and 16.78 ± 3.53 mm (females) and the distance to the lateral most edge of the VAG on the outer cortex was 19.15 ± 3.64 mm (males) and 17.83 ± 3.53 mm (females). The groove thickness was 4.28 ± 1.14 mm (males) and 3.41 ± 0.86 mm (females). Conclusion: Dissection of the posterior arch of atlas (C1) vertebra on the posterior and superior aspects requires a great caution and should remain within about 10 mm and 6mm lateral to the midline respectively. The placement of screws via the posterior arch of atlas appears unfeasible and carries a great risk of injuring the vertebral artery. Introduction The vertebral artery groove lies on the anterior portion of the cephalad surface of the posterior ring of atlas. It extends horizontally from the medial border of the transverse foramen to the medial edge of the posterior ring [1]. Craniovertebral junction surgery requires knowledge regarding the neurovascular and musculoskeletal anatomy of this region. The course of vertebral artery on C1 lamina complicates surgical procedures in this area [2]. The injury to the artery during surgery can lead to catastrophic intra-operative bleeding and compromise to the blood flow can lead to unpredictable neurological deficits, which will depend on the adequacy of blood flow from the contralateral vertebral artery [3]. Moreover, the increasing use of C1 (atlas) lateral mass screw placements for atlantoaxial fixation requires a detailed anatomy of the lateral mass and the relation of the entry zone to the vertebral artery [4]. Due to the importance of a detailed anatomical knowledge of vertebral artery for orthopedicians and neurosurgeons during posterior craniovertebral approaches, the present study was carried out to determine relevant anatomical landmarks of the vertebral artery groove on the posterior arch of the atlas vertebra for providing a safe zone to avoid the potential injury of the vertebral artery during these procedures. Material and methods Thirty six dry atlas vertebrae (24 male and 12 female) were used in this study. The specimens were collected from Departments of Anatomy, Faculties of medicine, Universities of Zagazig and King AbdulAziz. The selected vertebrae were grossly normal and did not show any acquired pathology or congenital abnormality. The vertebral artery groove (VAG) was identified on the superior surface of each specimen and five parameters of the groove were measured, as previously described [1]; [5]; [6]. The following 4 linear parameters, starting from the posterior midline, were measured: (D1) and (D2) extended to the medial most edge of the VAG on the inner and outer cortexes respectively. (D3) and (D4) extended to the lateral most edge of the VAG on the inner and outer cortexes respectively (Fig.1). Figure (1): A photograph of the superior surface of the atlas vertebra showing the linear parameters of VAG relative to the posterior midline. (D1) & (D2) extend to the medial most edge of the groove on inner and outer cortexes respectively. (D3) & (D4) extend to the lateral most edge of the groove on inner and outer cortexes respectively. The groove thickness (T) was taken as the height between the upper and lower surfaces of the lateral part of the posterior arch of atlas vertebra in its thinnest part (Fig.2). Figure (2): A photograph of the posterior aspect of the atlas vertebra showing the thickness (T) of the lateral part of its posterior arch at the vertebral artery groove (double-headed arrow). All measurements were taken bilaterally (right and left) using a Vernier sliding caliper; accurate to 0.1mm. The range, mean and standard deviation of each parameter were calculated by the Microsoft office Excel computer program. A student-t test was used to compare each parameter on right and left sides and between male and female specimens to determine significant differences (P ≤ 0.05). Results: (D1) was 12.19 ± 3.0 mm (males) and 11.32 ± 2.88 mm (females) and (D2) was 16.84 ± 3.58 mm (males) and 15.21 ± 3.66 mm (females). (D3) was 17.99 ± 3.81 mm (males) and 16.78 ± 3.53 mm (females) and (D4) was 19.15 ± 3.64 mm (males) and 17.83 ± 3.53 mm (females) (table 1). Table (1): The parameters of VAG relative to posterior midline in the present study ( in mm): Parameters D1/R D1/L D2/R D2/L D3/R D3/L D4/R D4/L Male Range 5.9618.25 6.3619.59 10.5622.5 10.9722.89 11.5923.6 12.6524.26 12.7124.11 13.8625.21 Mean ± SD 11.74 ± 2.95 12.64 ±3.06 16.56±3.62 17.11±3.6 17.44± 3.78 18.53±3.86 18.74 ± 3.71 19.58± 3.6 Female Mean Range (Bilateral) 7.5712.19 ± 16.53 3.0 8.2217.85 9.6516.84 ± 20.45 3.58 10.2921.25 10.2617.99 ± 21.31 3.81 11.3721.75 11.4519.15 ± 22.55 3.64 12.3723.26 Mean ± SD 10.85 ± 2.85 11.79±2.94 Mean (Bilateral) 11.32 ± 2.88 14.97±3.72 15.44±3.74 15.21 ± 3.66 16.37±3.69 16.78 ± 17.18 ±3.48 3.53 17.46±3.67 18.19±3.51 17.83 ± 3.53 The VAG thickness (T) was 4.28 ± 1.14 mm (males) and 3.41 ± 0.86 mm (females) (table 2). Table (2): The thickness of VAG in the present study ( in mm): Parameters T/R T/L Mean (Bilateral) Male Range 2.25-6.86 2.35-6.95 4.28 ± 1.14 Mean ± SD 4.21±1.12 4.36±1.17 Female Range 1.88-4.85 2.25-4.98 3.41 ± 0.86 Mean ± SD 3.33±0.88 3.49±0.87 Statistical study: The linear parameters & thickness of VAG were higher in males and on the left side, however, these differences between the right and left sides and between male and female specimens, based on the student-t test, were not statistically significant (P ≥ 0.05). Discussion The vertebral artery makes a loop after its exit from foramen transversarium of C1, then occupies a vertebral artery groove over the surface of the posterior arch of atlas and in this location it is vulnerable to injury during a posterior midline approach [3]. To define a safe zone from the posterior midline to avoid injuring the vertebral artery, Ebraheim et al [1] , described a mean distance of 10.4 ± 1.7 mm (males) and 8.9 ± 0.8 mm (females); with a minimum of 8 mm for both genders, from the posterior midline to the medial most edge of VAG on the inner cortex, and described a mean distance of 19.2 ± 3.2 mm (males) and 16.5 ± 1.0 (females); with a minimum of 12 mm for both genders, from the posterior midline to the medial most edge of VAG on the outer cortex. The previous authors suggested, according to these finding, that dissection on the posterior aspect of the posterior ring should remain within 12 mm lateral to the midline, and that dissection on the superior aspect of the posterior ring should remain within 8 mm of the midline. In the present study, the mean distance from the posterior midline to the medial most edge of VAG on the inner cortex was 12.19 ± 3.0 mm (males) and 11.32 ± 2.88 mm (females), with a minimum of 5.96 mm in both genders and on the outer cortex was 16.84 ± 3.58 mm (males) and 15.21 ± 3.66 mm (females), with a minimum of 9.65 mm in both genders. These findings, although slightly lower in value, are in line with those of Ebraheim et al [1]. However, higher values were reported for distances from the midline to the medialmost edge of the vertebral artery groove; on the outer cortex 15.03 ± 1.22 mm [2] , approximately 12 mm and 16 on the inner and outer and cortexes respectively [5] ; 14.3 mm to 19.7 mm (average 18.2 mm) on the outer cortex [3]. From the findings of the present study, to avoid injury of vertebral artery , it suggested that dissection of the posterior aspect of the posterior arch requires a great caution and should remain within about 10 mm lateral to the midline, and that dissection on the superior aspect of the posterior ring should remain within about 6 mm of the midline. Apart from the distances of VAG from the posterior midline, the groove thickness is another important parameter of the VAG that gains its surgical importance after the increasing popularity of cranio-vertebral fixation processes via the posterior arch of the atlas vertebra [7]; [8]; [9]. In the present study, the mean thickness of VAG was 4.28 mm (males) and 3.41 mm (females). In agreement with the findings of the present study, Tan et al [7] and Ma et al [8] reported a thickness of 4.25 mm and 4.58 mm respectively. Moreover, Ebraheim et al [1] And Lee et al [9] described a mean thickness of the vertebral groove of 4.1 mm (males) and 3.5 mm (females). According to Tan et al [7] the posterior arch at the position of VAG is regarded as a vertebral pedicle through the screw fixation via the posterior arch of atlas could be achieved. However, Lee et al [9 ] reported that a minimum thickness of 5mm is required to safely pass a 3.5 mm screw via the posterior lateral arch of atlas, without violating any of the cortical margins. In the present study, all specimens were less than 5mm and this indicates that the screw placement via the posterior arch described by Tan et al [7] to be unfeasible and carries a great risk of injuring the vertebral artery. References [1] Ebraheim NA, Xu R, Ahmad M, Heck B. The quantitative anatomy of the vertebral artery groove of the atlas and its relation to the posterior atlantoaxial approach. Spine 1998; 23(3): 320-323. [2] Naderi S, Çakmakçi H, Acar F,Arman C, Metrol T, Arda MN. Anatomical and computed tomographic analysis of C1 vertebra. Clinical Neurology and Neurosurgery 2003; 105 (4): 245- 248 [3] Cacciola F, Phalke U, Goel A.Vertebral artery in relationship to C1-C2 vertebrae: An anatomical study. Neurology India 2004;52(2): 178 - 184 [4] Gupta T. Cadaveric morphometric anatomy of C-1 vertebra in relation to lateral mass screw placement. Surg Radiol Anat 2008 ; 30 : 589 - 593 [5] Gupta T. Quantitative anatomy of vertebral artery groove on the posterior arch of atlas in relation to spinal surgical procedures. Surg Radiol Anat 2008; 30: 239–242 [6] Carvalho MF; Rocha RT ; Monteiro JTS; Pereira CU; Leite RF; Defino HLA.Vertebral artery groove anatomy. Acta Ortop Bras 2009; 17(1): 50-54 [7] Tan M ; Wang H ; Wang W; Zhang G; Yi P; Li Z; Wei H ; Yang F. Morphometric evaluation of screw fixation in atlas via posterior arch and lateral mass. Spine 2003 ; 28 (9) : 888-895 [8] Ma X-Y; Yin Q-S; Wu Z-H; Xia H; Lu J-F ; Zhong S-Z .Anatomic considerations for the pedicle screw placement in the first cervical vertebra. Spine 2005 ; 30 (13) : 1519-1523 [9] Lee MJ; Cassinelli E ; Riew D. The feasibility of inserting atlas lateral mass screws via the posterior arch. Spine 2006 ; 31 (24) : 2798- 2801 الملخص العربي )معالم أخدود الشريان الفقاري للفقرة العنقية األولى (األطلس للنهج القحفي العنقي الخلفي جمال حامد السيد حسانين جامعة الملك عبد العزيز, كلية الطب فرع شمال جدة, قسم التشريح ان أخدود الشريان الفقاري علي السطح العلوي لفقرة األطلس (العنقية األولى) هو بصمة للجزء الثالث من وفي هذا الموضع يكون الشريان الفقاري معرضا ً لإل صابة أثناء النهج الجراحي الخلفي, الشريان الفقاري ولقد كان الهدف من هذه الدراسة هو تحديد معالم تشريحية ألخدود الشريان الفقاري علي, للمنطقة القحفية العنقية القوس الخلفي لفقرة األطلس ذات الصلة بهذا النهج لتعريف منطقة آمنة لتجنب اإلصابة المحتملة للشريان الفقاري .أثناء هذه اإلجراءات وتم تحديد موضع, ) مؤنث24 مذكر و42 ( استخدم في هذه الدراسة ستة وثالثون فقرة أطلس جافة تجويف الشريان الفقاري علي السطح العلوي لكل عينة وتم قياس خمسة معلمات لألخدود علي الجانبين (يمين ويسار) باستخدام فرجار فيرنيير المنزلق ,و أظهرت الدراسة أن المسافة من خط المنتصف الخلفي إلي أقصى 11.32 ± 2.88مم (ذكور) و 12.19 ± 3.0الحافة األنسية ألخدود الشريان الفقاري علي القشرة الداخلية هي مم (إناث) ,والمسافة من خط المنتصف الخلفي إلي أقصى الحافة األنسية ألخدود الشريان الفقاري علي القشرة مم (إناث) ,أما المسافة من خط المنتصف 15.21 ± 3.66مم (ذكور) و 16.84 ± 3.58الخارجية هي مم 17.99 ± 3.81الخلفي إلي أقصى الحافة الوحشية ألخدود الشريان الفقاري علي القشرة الداخلية هي مم (إناث) ,والمسافة من خط المنتصف الخلفي إلي أقصى الحافة الوحشية ألخدود (16.78 ± 3.53ذكور) و مم (إناث) ,بينما 17.83 ± 3.53مم (ذكور) و 19.15 ± 3.64الشريان الفقاري علي القشرة الخارجية هي مم (إناث) 3.41 ± 0.86.مم (ذكور) و 4.28 ± 1.14كانت سماكة األخدود هي االستنتاج :يجب الحذر عند تشريح القوس الخلفي لفقرة األطلس ( العنقية األولى) من الجهتين الخلفية والعلوية وأال يتجاوز ذلك 21مم و 6مم علي التعاقب من خط المنتصف ,كما أن وضع المسامير عن طريق القوس الخلفي لفقرة األطلس يبدو غير مجديا ويحمل خطرا كبيرا نحو اصابة الشريان الفقاري.