Laminar screw fixation in the subaxial cervical spine: A report on three cases

Although laminar screw fixation is often used at the C2 and C7 levels, only few previous case reports have presented the use of laminar screws at the C3-C6 levels. Here, we report a novel fixation method involving the use of practical laminar screws in the subaxial spine. We used laminar screws in the subaxial cervical spine in two cases to prevent vertebral artery injury and in one case to minimize exposure of the lamina. This laminar screw technique was successful in all three cases with adequate spinal rigidity, which was achieved without complications. The use of laminar screws in the subaxial cervical spine is a useful option for posterior fusion of the cervical spine.     

The feasibility of laminar screw placement in the subaxial spine: analysis using 215 three-dimensional computed tomography scans and simulation software

Background contextThere have been several reports describing the usage of subaxial cervical laminar screws. However, the anatomic feasibility of placing such screws has not been thoroughly evaluated yet.PurposeTo determine the feasibility of the laminar screw placement in the subaxial cervical spine using a large number of computed tomography (CT) scans and three-dimensional screw trajectory software.Study designThree-dimensional simulation study of screw placement.Patient sampleComputed tomography scans of 215 consecutive patients were examined, for a total of 430 screws at each level of the subaxial cervical spine.Outcome measuresSuccessful screw placement without laminar cortical breach, facet joint violation, and collision between two screws in the same level.MethodsWe simulated the placement of 4.0-mm subaxial (C3–C7) cervical laminar screws. Unilateral and bilateral screw placement was simulated, and their success rates were evaluated at each level of the subaxial cervical spine. This study was not supported by any financial sources. One of the authors received royalties for a posterior cervical fixation system, which is not the topic of this article and is not used or mentioned in this article.ResultsThe success rate of unilateral screw placement was the highest at C7 (91.4%), followed by C6 (31.9%), C3 (30.2%), C4 (6.3%), and C5 (4.0%). It was significantly higher (p<.001) in men than in women at C6 and C7 but not at the other levels. The success rate of bilateral screw placement was the highest at C7 (68.8%), followed by C3 (13.5%), C6 (8.8%), C4 (1.9%), and C5 (0.9%). It was significantly higher in men (83.5%) than in women (52.0%) at C7 (p<.001) but not at the other levels.ConclusionsThe relatively high success rate at C7, particularly of unilateral placement, suggests that laminar screw placement can be a sound alternative method for fixation at this level. However, careful preoperative CT scan evaluation and patient selection are required, particularly for bilateral fixation in women. At C3 and C6, unilateral screw placement can be considered in approximately 30% of patients after careful selection using preoperative CT scans. At C4 and C5, neither unilateral nor bilateral screw fixation is recommended for most patients.     

Lamina-guided lateral mass screw placement in the sub-axial cervical spine

Lateral mass (LM) screws are commonly used in posterior instrumentation of the cervical spine because of their perceived safety over pedicle screws. A possible complication of cervical LM screw placement is vertebral artery injury or impingement. Several screw trajectories have been described to overcome the risks of neurovascular injury; however, each of these techniques relies on the surgeon’s visual estimation of the trajectory angle. As the reliability hereof is poorly investigated, alignment with a constant anatomical reference plane, such as the cervical lamina, may be advantageous. The aim of this investigation was to determine whether alignment of the LM screw trajectory parallel to the ipsilateral cervical lamina reliably avoids vertebral artery violation in the sub-axial cervical spine. 80 digital cervical spine CT were analysed (40 female, 40 male). Exclusion criteria were severe degeneration, malformations, tumour, vertebral body fractures and an age of less than 18 or greater than 80 years. Mean age of all subjects was 39.5 years (range 18–78); 399 subaxial cervical vertebrae (C3–C7) were included in the study. Measurements were performed on the axial CT view of C3–C7. A virtual screw trajectory with parallel alignment to the ipsilateral lamina was placed through the LM. Potential violation of the transverse foramen was assessed and the LM width available for screw purchase measured. There was no virtual violation of the vertebral artery of C3–C7 with lamina-guided LM screw placement. LM width available for screw purchase using this technique ranged from 5.2 to 7.4 mm. The sub-axial cervical lamina is a safe reference plane for LM screw placement. LM screws placed parallel to the ipsilateral lamina find sufficient LM width and are highly unlikely to injure the vertebral artery, even in bi-cortical placement. Placing LM screws parallel to the lamina appears favourable over conventional techniques.     

改良枢椎椎板螺钉置钉方法的临床应用

目的设计一种改良的枢椎椎板螺钉置钉方法并应用于临床,使螺钉置入更简单、更准确。方法改良枢椎椎板螺钉置钉方法的进钉点仍为棘突和椎板的交界处,但在椎板中份用磨钻磨除少许皮质开窗,用刮匙刮除松质骨,保留椎板内板,用手钻沿着枢椎椎板钻孔,在直视下通过开窗处,再继续向前钻入,探针探查钉道并攻丝后置入直径3.5~4.0 mm、长度24~30 mm螺钉。临床应用26例,其中男17例,女9例,平均年龄56.7岁。结果共置入42枚枢椎椎板螺钉,螺钉平均长度为27.1 mm(24~30 mm),术中无需透视,术中和术后无明显技术相关的并发症发生。术后即刻或随访CT示椎板螺钉位置正确,均位于椎板间。全组病例平均随访13.4个月,未发生螺钉松动、脱出或断裂。结论改良的枢椎椎板螺钉置入技术操作简单,无需透视,并保证了螺钉处于正确位置。     

导向器结合钉道内壁探查法在下颈椎椎弓根螺钉置入中应用研究

目的:探讨导向器结合钉道内壁探查法置入下颈椎弓根螺钉的技巧并评价其准确性及安全性。方法:2014年1月至2016年10月采用自行设计带角度仪导向器结合钉道内壁探查法对11例患者置入下颈椎椎弓根螺钉,其中男7例,女4例;年龄32~63岁,平均48.1岁;脊髓型颈椎病4例,颈椎骨折脱位4例,无骨折脱位颈脊髓损伤1例,寰枢椎骨折脱位2例。术前CT测量出目标椎弓根直径、理想进钉点及矢状面夹角和横断面内倾角。术中使用自行设计带角度仪导向器严格控制进钉角度,结合探查钉道内壁作为安全置钉标志。术后观察患者有无脊髓及椎动脉损伤表现,复查CT评价椎弓根螺钉位置,并将螺钉准确性进行分级,计算螺钉优良率。结果:11例患者共置入71枚下颈椎弓根螺钉,均未出现手术所导致的脊髓及神经根损伤症状,亦无椎动脉损伤症状。术后CT评估螺钉位置并进行分级:0级52枚,1级13枚,2级4枚,3级2枚。螺钉位置良好率91%。6枚误置螺钉,内壁穿破4枚,2枚穿破外壁。结论:钉道内壁探查法置入下颈椎弓根螺钉安全可靠,但有一定的学习曲线。术中探查出钉道内壁及使用角度仪导向器控制进钉角度是置钉关键。     

Wright's Technique with the Addition of Visualized Axial Cortical Windows in Odontoid Fractures

This study sought to investigate and evaluate a modified axial translaminar screw fixation for treating odontoid fractures. We performed a retrospective study at Wenzhou Medical University Affiliated Second Hospital between March 2016 and June 2018. We retrospectively collected and analyzed the medical records of 23 cases with odontoid fractures. All patients were identified as type II odontoid fractures without neurological deficiency and serious diseases following the classification of Anderson. The average age, gender ratio, and body mass index (BMI) were 54.3 ± 11.1 years, 12 men to 11 women, and 22.6 ± 2.4 kg/m², respectively. Patients in this study accepted screw fixation using our modified axial translaminar screw fixation combined with atlas pedicle or lateral mass screw fixation. Within the technique, a small cortical “window” was dug in the middle of the axial contralateral lamina, such that the screws in the lamina were visualized to prevent incorrectly implanting the posterior spinal canal through the visualized “window.” A total of 46 bone screws were accurately inserted into the axial lamina without using fluoroscopy. The length of all translaminar screws ranged between 26 and 30 mm, while the diameter was 3.5 mm. During the follow‐up survey, the visual analog scale (VAS) and neck disability index (NDI) were measured. We provide a simple modification of Wright''s elegant technique with the addition of “visualized windows” at the middle of the axial lamina. In all patients, screws were inserted accurately without bony breach and the screw angle was 56.1 ± 3.0°. Mean operative time was 102 ± 28 min with an average blood loss of 50 ± 25 mL. Postoperative hemoglobin and mean length of hospital stay were 12.0 ± 1.4 g/dL and 10.4 ± 3.4 days, respectively. The average follow‐up time of all cases was 14.7 months and no internal fixation displacement, loosening, or breakage was found. All patients with odontoid fractures reported being satisfied with the treatment during the recheck period and good clinical outcomes were observed. At 1, 6, and 12 months, NDI and VAS showed that the symptoms of neck pain and limitations of functional disability improved significantly during follow‐up. Our results suggest that the modified translaminar screw fixation technique can efficiently treat Anderson type II odontoid fracture, followed by the benefits of less soft tissue dissection, simple operation, no fluoroscopy, and accurate placement of screws.     

Morphological Study of Subaxial Cervical Pedicles by Using Three-Dimensional Computed Tomography Reconstruction Image

Malpositioning of cervical screws risks neurovascular injury. A cervical screw fixation system can provide proper rigidity, alignment correction, and high rates of fusion afforded by high pullout biomechanical strength. The objective is to assess the dimensions and axis of the C3–C7 cervical pedicles. A 1-mm slice thickness computed tomography (CT) scan of the cervical spine of 30 patients (15 males, 15 females) were analyzed and reconstructed in three-dimensions using Mimics^® 10.01 software. We measured pedicle axis length (PAL), pedicle and lateral mass length (PL-LM), pedicle length (PL), outer pedicle width (OPW), and pedicle transverse angle (PTA) from the axial image and outer pedicle height (OPH) and pedicle sagittal angle (PSA) from the sagittal image. The OPH and OPW at all subaxial cervical spines were suitable for insertion of 3.5 mm cervical pedicle screws. PSA was directed cranially at C3 to C5 (13.84, 7.09, and 2.71) and directed caudally at C6 and C7 (–4.55, –6.94). PTA was greatest at C5 and smallest at C7. The respective difference between the left and right side for nearly all parameters was not statistically significant (except for C6 PL and C7 OPH). Females had a significantly smaller OPH and OPW than males at nearly all levels. The PTA was not significantly different between the sexes. Cervical pedicle screw fixation in the Thai population can be safely performed and guidelines for insertion at each vertebra documented. Appropriate preoperative planning is necessary to achieve safe and accurate placement of the screws.     

Translaminar screws of the axis—an alternative technique for rigid screw fixation in upper cervical spine instability

C2 pedicle screws or transarticular atlantoaxial screws are technically demanding and carry an increased risk of vertebral artery injury. In up to 20% of cases, pedicle and transarticular screw placement is not possible due to a high-riding vertebral artery or very small C2 pedicles in addition to other anatomical variations. Translaminar screws have been reported to rigidly capture posterior elements of C2 and therefore appear to be a suitable alternative. We present our first experiences and clinical results with this new method in two neurosurgical spine centers. Twenty-seven adult patients were treated between 2007 and 2010 in two neurosurgical spine departments with C2 translaminar screw fixation for upper cervical spine instability of various origins (e.g., trauma, tumor, dens pseudarthrosis). Eight patients were men and 19 were women. Mean age was 68.9 years. In most cases, translaminar screws were used because of contraindications for pedicle or transarticular screws as a salvage technique. All patients were clinically assessed and had CT scans postoperatively to verify correct screw placement. Follow-up was performed with reexamination on an ambulatory basis. Mean follow-up was 7.6 months for all patients. In 27 patients, 52 translaminar screws were placed. There were no intraoperative complications. Postoperatively, we identified four screw malpositions using a new accuracy grading scale. One screw had to be revised because of violation of the spinal canal >4 mm. None of the patients had additional neurological deficits postoperatively, and all showed stable cervical conditions at follow-up. Two patients died due to causes not associated with the stabilization technique. The fusion rate for patients with C1/C2 fixation is 92.9%. Translaminar screws can be used at least as an additional technique for cases of upper cervical spine instability when pedicle screw placement is contraindicated or not possible. The current data suggest comparable biomechanical stability and fusion rates of translaminar screws to other well-known posterior fixation procedures. In addition, translaminar screw placement is technically less demanding and reduces the risk of vertebral artery injury.     

Deviation analysis of C2 translaminar screw placement assisted by a novel rapid prototyping drill template: a cadaveric study

Purpose

The goal of this study is to evaluate the accuracy of patient-specific CT-based rapid prototype drill templates for C2 translaminar screw insertion.

Methods

Volumetric CT scanning was performed in 32 cadaveric cervical spines. Using computer software, the authors constructed drill templates that fit onto the posterior surface of the C2 vertebrae with drill guides to match the slope of the patient’s lamina. Thirty-two physical templates were created from the computer models using a rapid prototyping machine. The drill templates were used to guide drilling of the lamina and post-operative CT images were obtained. The entry point and direction of the planned and inserted screws were measured and compared.

Results

Sixty-four C2 translaminar screws were placed without violating the cortical bone of a single lamina. The bilateral average transverse angle of intended and actual screw for C2TLS was 56.60 ± 2.22°, 56.38 ± 2.51°, 56.65 ± 2.24°, 56.39 ± 2.45°. The bilateral mean coronal angle of the planned and actual screw for C2TLS was 0°, 0°, −0.07 ± 0.32°, 0.12 ± 0.57°. The average displacement of the entry point of the superior and inferior C2TLS in the x, y, z axis was 0.27 ± 0.85, 0.49 ± 1.46, −0.28 ± 0.69, 0.43 ± 0.88, 0.38 ± 1.51, 0.23 ± 0.64 mm.

Conclusion

The small deviations seen are likely due to human error in the form of small variations in the surgical technique and use of software to design the prototype. This technology improves the safety profile of this fixation technique and should be further studied in clinical applications.     

Ball Tip Technique for S2AI Screw Placement in Sacropelvic Fixation: A Comparative Study with Conventional Freehand Technique

ObjectiveTo evaluate the efficiency of the ball tip technique for S2AI screw placement and introduce this technique.MethodsSixty‐three patients who underwent pelvic fixation with S2AI screws were retrospectively reviewed. They were 29 males and 34 females with an average age of 59.6 ± 12.5 years. Among these patients, 35 patients (14 males and 21 females with an average age of 58.8 ± 11.3 years) received ball tip technique and 28 patients (15 males and 13 females with an average age of 63.7 ± 12.6 years) received conventional freehand technique. Ball tip technique was used in ball tip technique group. After a pedicle probe just penetrated the sacroiliac joint, a ball‐tipped probe consisting of a ball shaped metal tip with a flexible shaft was malleted to make a guide track within ilium. This ball‐tipped probe could bend automatically away from the cortex and forward through the cancellous bone when the tip met the cortical lamina of ilium, which can avoid penetration. After repeating the procedures, a guide hole was gradually formed. S2AI screw was inserted along the guide hole after tapping. In the conventional freehand group, S2AI screw was placed according to the conventional method. Postoperative computed tomography (CT) was used to assess the accuracy of screws. The time cost of screw insertion and screw‐related complications were recorded. Independent t‐test was used to compare the time cost between ball tip group and conventional freehand group. A chi‐square test was used to compare the accuracies of the ball tip group with the conventional group.ResultsThere were 35 patients (70 S2AI screws) in ball tip group and 28 patients (56 S2AI screws) in conventional freehand group. No screw‐related complication occurred in all patients. Time costs were 9.8 ± 4.5 mins in ball tip group and 20.2.0 + 8.6 mins in conventional freehand group, respectively (P < 0.05). Four screws penetrated iliac cortex in the ball tip group vs 10 screws in conventional freehand group (5.7% vs 17.9%) (P < 0.05).ConclusionsThe ball tip technique enhances the accuracy of screw placement and has less time cost compared with conventional freehand technique.     

组合枢椎椎板螺钉及经枢椎椎板关节突螺钉固定技术的临床应用

目的探讨组合枢椎椎板螺钉及经枢椎椎板关节突螺钉(TLFS)固定技术治疗颈椎损伤的可行性和应用价值。方法自2005年10月～2008年10月,采用枢椎椎板螺钉及经枢椎TLFS固定技术治疗颈椎损伤36例。结果本组获随访13~32个月,平均18个月。30例行自体髂骨植骨,6例行异体骨植骨,获得了良好的骨性愈合。术后CT示枢椎椎板螺钉及经枢椎TLFS位置良好,未侵犯椎管和脊髓。结论枢椎椎板螺钉及经枢椎TLFS固定技术不受C2横突孔中椎动脉的位置和C2椎弓根大小限制,避免了螺钉置入过程中损伤椎动脉的风险,所以该方法可作为传统枢椎后路螺钉固定技术的补充。     

Morphometric subaxial lateral mass evaluation allows for preoperative optimal screw trajectory planning

Purpose

Lateral mass (LM) fixation has become a standard in cervical spine instability treatment; however, maximal biomechanical stability combined with low morbidity remains a challenge. We evaluated our own patient cohort for bicortical screw placement and complication rates and investigated optimal screw trajectories with preoperative multiplanar computed tomography (CT) scans.

Methods

Fifty-five patients were retrospectively evaluated after LM fixation at various subaxial cervical spine levels with a modified Magerl technique. Postoperative CTs and clinical records were used to determine LM anatomy, screw lengths, bicortical screw percentages, and complication rates. Additionally, 3D CT subaxial cervical spine data sets from 45 additional subjects with clinical indications for cervical spine imaging were evaluated. Subject LM geometries (thickness) were evaluated at different sagittal angulations (strict sagittal, 20°, 30° and the optimal angulation) for the optimal screw trajectories at the C3–C7 segments.

Results

In total, 284 LM screws were placed, with a mean screw length of 16 mm and an 88 % bicortical bone purchase. Additionally, a 3.8 % malplacement rate was observed. LM thickness varied substantially between each subaxial cervical level and at each of the investigated angulations. The optimal angulation, at which LM thickness was maximal, increased continuously from C3 (14°) to C7 (38°). This increase permitted 8 % (C3) to 39 % (C7) gains in screw length compared with the strict sagittal plane assessments.

Conclusions

The optimal LM trajectory varied for each subaxial segment. The knowledge of LM geometry allows for safe, long and even bicortical screw placements using preoperative sagittal CT imaging evaluations.     

Accuracy of pedicle screw insertion in the cervical spine for internal fixation using frameless stereotactic guidance

OBJECT: Although transpedicular fixation is a biomechanically superior technique, it is not routinely used in the cervical spine. The risk of neurovascular injury in this region is considered high because the diameter of cervical pedicles is very small and their angle of insertion into the vertebral body varies. This study was conducted to analyze the clinical accuracy of stereotactically guided transpedicular screw insertion into the cervical spine. METHODS: Twenty-seven patients underwent posterior stabilization of the cervical spine for degenerative instability resulting from myelopathy, fracture/dislocation, tumor, rheumatoid arthritis, and pyogenic spondylitis. Fixation included 1-6 motion segments (mean 2.2 segments). Transpedicular screws (3.5-mm diameter) were placed using 1 of 2 computer-assisted guidance systems and lateral fluoroscopic control. The intraoperative mean deviation of frameless stereotaxy was < 1.9 mm for all procedures. RESULTS: No neurovascular complications resulted from screw insertion. Postoperative computed tomography (CT) scans revealed satisfactory positioning in 104 (90%) of 116 cervical pedicles and in all 12 thoracic pedicles. A noncritical lateral or inferior cortical breach was seen with 7 screws (6%). Critical malplacement (4%) was always lateral: 5 screws encroached into the vertebral artery foramen by 40-60% of its diameter; Doppler sonographic controls revealed no vascular compromise. Screw malplacement was mostly due to a small pedicle diameter that required a steep trajectory angle, which could not be achieved because of anatomical limitation in the exposure of the surgical field. CONCLUSIONS: Despite the use of frameless stereotaxy, there remains some risk of critical transpedicular screw malpositioning in the subaxial cervical spine. Results may be improved by the use of intraoperative CT scanning and navigated percutaneous screw insertion, which allow optimization of the transpedicular trajectory.     

枢椎棘突螺钉与椎弓根螺钉的解剖学比较

目的：比较枢椎棘突螺钉和椎弓根螺钉的技术难度和相关解剖学参数,探讨枢椎棘突螺钉固定的可行性和安全性。方法：自2010年2月至7月,选取10具颈椎标本,男5具,女5具,年龄45～76岁,平均60.5岁。将标本俯卧,颈部置于中立位。从C1-C3剔除颈部后侧所有的软组织,以清楚地暴露枢椎侧块和峡部。枢椎椎体左右侧任意选择进行棘突螺钉和椎弓根螺钉固定,各10枚螺钉,置入直径为4.0mm的皮质骨螺钉。枢椎棘突螺钉以枢椎棘突螺钉的进钉点选择为棘突的基底部、棘突和椎板的交界处,进钉角度水平置钉,螺钉由对侧棘突基底部穿出,形成双层皮质固定;枢椎椎弓根螺钉进钉点为枢椎下关节突根部中点,钉道方向与矢状面夹角15°～20°,与横断面夹角约30°。螺钉置入后,使用多层螺旋CT扫描机对标本进行扫描重建。测量螺钉在骨内的实际深度,记录椎弓根螺钉和棘突螺钉置钉失败、穿破椎弓根、进入椎管或置入横突孔的螺钉数目。结果：枢椎棘突螺钉和椎弓根螺钉的置入均无明显的技术困难。棘突螺钉未见螺钉置入椎管和劈裂棘突,但椎弓根螺钉有1枚螺钉突出椎弓根外侧皮质,侵犯横突孔。枢椎棘突螺钉的平均钉道长度为（21.4±1.4）mm,稍短于枢椎椎弓根螺钉的（23.7±1.0）mm,但两者间差异无统计学意义（t=-4.387,P〉0.05）。结论：枢椎棘突基底部具有螺钉固定的可行性,枢椎棘突螺钉较椎弓根螺钉固定相对安全、简单。     

Biomechanical comparison of pedicle screws versus spinous process screws in C2 vertebra: A cadaveric study

Background:

Biomechanical studies have shown C2 pedicle screw to be the most robust in insertional torque and pullout strength. However, C2 pedicle screw placement is still technically challenging. Smaller C2 pedicles or medial localization of the vertebral artery may preclude safe C2 pedicle screw placement in some patients. The purpose of this study was to compare the pullout strength of spinous process screws with pedicle screws in the C2.

Materials and Methods:

Eight fresh human cadaveric cervical spine specimens (C2) were harvested and subsequently frozen to −20°C. After being thawed to room temperature, each specimen was debrided of remaining soft tissue and labeled. A customs jig as used to clamp each specimen for screw insertion firmly. Screws were inserted into the vertebral body pairs on each side using one of two methods. The pedicle screws were inserted in usual manner as in previous biomechanical studies. The starting point for spinous process screw insertion was located at the junction of the lamina and the spinous process and the direction of the screw was about 0° caudally in the sagittal plane and about 0° medially in the axial plane. Each vertebrae was held in a customs jig, which was attached to material testing machine (Material Testing System Inc., Changchun, China). A coupling device that fit around the head of the screw was used to pull out each screw at a loading rate of 2 mm/min. The uniaxial load to failure was recorded in Newton''st dependent test (for paired samples) was used to test for significance.

Results:

The mean load to failure was 387 N for the special protection scheme and 465 N for the protection scheme without significant difference (t = −0.862, P = 0.403). In all but three instances (38%), the spinous process pullout values exceeded the values for the pedicle screws. The working distances for the spinous process screws was little shorter than pedicle screws in each C2 specimen.

Conclusion:

Spinous process screws provide comparable pullout strength to pedicle screws of the C2. Spinous process screws may provide an alternative to pedicle screws fixation, especially with unusual anatomy or stripped screws.     

Computed tomography morphometric analysis of C2 translaminar screw fixation of Wright’s technique and a modified technique in the pediatric cervical spine

Purpose

To establish reference data on the dimensions of C2 lamina to guide the use of translaminar screws with Wright’s technique and a modified technique for pediatric patients in different age groups.

Methods

113 pediatric patients were divided into six age groups, and their cervical vertebrae were studied on CT scans. Laminar height, width, length and screw angle were measured. Statistical analysis was performed using Student t tests, Pearson’s correlation and linear regression analysis.

Results

Mean laminar height was 10.95 ± 2.81 mm, and mean width was 6.01 ± 0.90 mm. For Wright’s technique, mean laminar length was 30.65 ± 3 mm, and the screw angle was 56.02° ± 3.62°. For the modified technique, mean laminar length was 22.07 ± 2.38 mm, and the screw angle was 67.40° ± 3.39°. 95.6 % (108/113) of the children could insert a screw into the lamina (laminar width ≥4.5 mm), 72.6 % (82/113) could accept bilateral translaminar screws (laminar width ≥4.5 mm and laminar height ≥9 mm).

Conclusion

Our investigation provides insight into the anatomy of C2 lamina in six pediatric age groups. Compared to adults, the benefits of C2 translaminar screws fixation are more obvious in the pediatric spine which has a large C2 lamina. Compared to Wright’s technique, the modified technique should insert a screw with bigger insert angle and shorter screw length.     

Subaxial cervical and cervicothoracic fixation techniques--indications, techniques, and outcomes

The subaxial and cervicothoracic junction is a relatively difficult area for spine surgeons to navigate. Because of different transitional stressors at the junction of the smaller cervical vertebrae and the larger thoracic segments, proximity to neurovascular structures, and complex anatomy, extreme care and precision must be assumed during fixation in these regions. Lateral mass screws, pedicle screws, and translaminar screws are currently the standard of choice in the subaxial cervical and upper thoracic spine. This article addresses the relevant surgical anatomy, pitfalls, and pearls associated with each of these fixation techniques.     

Biomechanical comparison of unicortical versus bicortical C1 lateral mass screw fixation

STUDY DESIGN: Biomechanical study of pullout strength of unicortical versus bicortical C1 lateral mass screws using a cadaveric cervical spine model. OBJECTIVE: To compare pullout strength of unicortical versus bicortical C1 lateral mass screws. SUMMARY OF BACKGROUND DATA: The internal carotid artery and hypoglossal nerve lie over the anterior aspect of the lateral mass of the atlas and are at risk from bicortical C1 lateral mass screws. Unicortical screws would reduce the risk of injury to these neurovascular structures; however, no data are available on the relative strength of unicortical versus bicortical C1 lateral mass screws. METHODS: Fifteen cadaveric cervical spine specimens underwent axial pullout testing of C1 lateral mass screws. A unicortical C1 lateral mass screw was placed on 1 side with a contralateral bicortical screw. RESULTS: The mean pullout strengths of the unicortical screws and bicortical screws were 588 N (range, 212 to 1234 N) and 807 N (range, 163 to 1460 N), respectively (P=0.008). CONCLUSIONS: Bicortical C1 lateral mass screws were significantly stronger than unicortical screws; however, the mean pullout strength of both the unicortical and bicortical C1 screws were greater than previously reported values for subaxial lateral mass screws. On the basis of these data, the clinical necessity for using bicortical screw fixation in all patients must be questioned. If similar strength can be achieved using unicortical C1 lateral mass screw to that currently accepted in the subaxial spine, bicortical screws might not be justified for the C1 lateral mass. However, the ability to extrapolate C1-C2 data to subaxial spine data is uncertain because of the difference in normal physiologic loading at these levels.     

Modification of Wright's technique for placement of bilateral crossing C2 translaminar screws: technical note

BACKGROUND CONTEXT: Several relatively new screw techniques have been described that rigidly capture the posterior elements of C2. The previously described procedures of axis fixation are technically demanding and place the vertebral artery at some risk. A novel and less technically demanding technique of obtaining C-2 translaminar screws has been recently described. Although the risk of vertebral artery injury has been essentially eliminated, the authors recognize that neurologic injury from breakthrough of the inner cortex of the lamina by the drill or screw is still a possibility. PURPOSE: Describe and illustrate a modified C2 translaminar technique and review the results of patients who have undergone the surgery. The current modification of the C2 translaminar screw technique was designed to reduce the risk of inadvertent screw placement within the spinal canal. STUDY DESIGN/SETTING: A techniques paper combined with a retrospective clinical review of patients undergoing the surgery. PATIENT SAMPLE: Patients undergoing posterior instrumented fusion surgery of the cervical spine, which incorporates C2 posterior elements using the translaminar technique. OUTCOME MEASURES: Radiographic analysis of the fusion construct incorporating the C2 translaminar screws. METHODS: We have modified the previously described technique of C-2 translaminar screw placement with the addition of "exit" cortical windows to assure bicortical, intralaminar screw placement. RESULTS: The results of the first six patients with an average follow-up of 12 months demonstrated this method to be safe and effective in fixating the axis. CONCLUSIONS: We have made a simple modification of Wright's elegant technique with the addition of "exit" windows at the facet-laminar junctions. This gives us the assurance that the C2 screw has not entered the spinal canal by directly visualizing the tip of the screw exiting the outer cortices of the lamina before leaving the operating room.     

The ipsilateral lamina–pedicle angle: can it be used to guide pedicle screw placement in the sub-axial cervical spine?

Pedicle screws in the sub-axial spine are infrequently used because of concerns over their safety and difficulty in placement, despite their superior pullout strength. In the sub-axial cervical vertebrae, we have observed that the lamina appears to project at right angles to the ipsilateral pedicle axis. The aim of this investigation was to confirm the lamina orientation as a reliable landmark for pedicle screw placement. 80 digital cervical spine CT were analysed. The angle formed by the ipsilateral outer lamina cortex to the pedicle axis was recorded. A total of 398 vertebrae were analysed from patients with a mean age of 39.5 years (range 18–78). Average axial lamina–pedicle angle ranged from 96.6° at C3 to 87.2° at C7 in males, and from 95.6° to 87.5° in females. The angle formed by the posterior cortex of the lamina and the ipsilateral pedicle shows a high level of consistency for sub-axial cervical vertebrae ranging from 96° at C3 to 87° at C7. Although the angle is not exactly 90° at all levels as hypothesised, the orientation of the lamina, nevertheless, forms a useful reference plane for insertion of pedicle screws in the sub-axial cervical spine.