Biomechanical comparison between C-7 lateral mass and pedicle screws in subaxial cervical constructs

C7 distal fixation anchor and its influence on sagittal profile in posterior cervical fusion; a retrospective analysis of 44 cases

Study design

Retrospective.

Purpose

In multilevel posterior cervical fusion, whether to stop distal fixation at C7 or T1, remains a matter of debate. We aimed to assess clinical feasibility of C7 as distal fixation point and sought to compare complication rates and radiological outcome between lateral mass screws and pedicle screws at C7.

Overview of literature

Current literature remains inconclusive regarding need for thoracic extension of instrumentation in multilevel posterior cervical fusion.

Methods

We did a retrospective review of 44 consecutive patients who underwent posterior instrumented cervical decompression and fusion for degenerative cervical myelopathy with C7 as distal fixation point, and a minimum follow-up period of two years. We had two groups of patients based on C7 instrumentation.

Group 1

Lateral mass screw fixation.

Group 2

Pedicle screw fixation.

Primary outcome

Post-operative clinico-radiological evaluation of whole study population Secondary outcome: Comparison of complication rates and radiological outcome between groups 1 and 2.

Results

Mean age was 58.06 ± 14.4 years with average follow-up duration of 35.4 ± 4.5 months. There were 18 patients in Group 1 and 26 patients in Group 2. Mean pre-operative mJOA score was 10.51 and post-operative mJOA score was 15.74 with mean recovery rate (RR) 69.82%, of which 30 patients (70.23%) had good recovery and 14 patients (29.77%) had fair recovery at final follow up. The two groups didn't show any significant difference in complication rates and outcome.

Conclusion

C7 as distal fixation anchor is safe and effective in maintaining cervical sagittal balance following multilevel posterior cervical fusion. C7 lateral mass screws are found to be equally efficacious as pedicle screws in preventing worsening of sagittal profile.

Comparison of Morphometric Measurements of Traditional Posterior Cervical Screw and Paravertebral Foramen Screw in Chinese Population

STUDY DESIGN

A morphometric measurement study.

OBJECTIVE

To measure the length and angle parameters of the screw paths of pedicle screws (PS), lateral mass screws (LMS), and paravertebral foramen screws (PVFS) of subaxial cervical spine in Chinese population.

SUMMARY OF BACKGROUND DATA

Aramomi proposed a novel internal fixation technology, named PVFS, as an alternative to LMS and PS for subaxial cervical vertebrae (C3-C7).

METHODS

This study measured the length and medial angles of screw paths on the three-dimensional reconstruction model of cervical computerized tomography data of 50 patients (25 men and 25 women) in our hospital from January 2018 to June 2018.

RESULTS

In general, the optimum length and medial angle of the PVFS in Chinese population were 10.65 mm and 21.12° at C3; 10.12 mm, 22.62° at C4; 9.82 mm, 23.66° at C5; 9.19 mm, 24.13° at C6; and 9.10 mm, 27.54° at C7. The optimum axial length and medial angle of PS in Chinese population were 30.94 mm, 33.92° at C3; 30.50 mm, 34.95° at C4; 31.92 mm, 33.42° at C5; 30.50 mm, 31.94° at C6; and 29.87 mm, 31.01° at C7. The optimum lengths of LMS paths in Chinese population were C3, 14.84 mm; C4, 15.33 mm; C5, 15.44 mm; C6, 14.74 mm; and C7, 14.06 mm.

CONCLUSION

Although the length of PVFS is limited, it still can be used as an effective substitute for LMS and PS. The PVFS does not have the risk of directly injuring the vertebral artery, its safety angle of insertion is larger than that in PS, and it has higher surgical safety.Level of Evidence: 3.

Risk Factor Analysis of Facet Fusion Following Cervical Lateral Mass Screw Fixation with a Minimum 1-Year Follow-up: Assessment of Maximal Insertional Screw Torque and Incidence of Loosening

Posterior stabilization is a common surgical procedure, which aims for rigid stabilization by facet fusion. Facet non-union has a potential risk of the screw loosening and malalignment. Although some authors have reported the influencing factors about screw loosening in the lumbar spine, there are few reports about the risk factor contributing to the facet non-union in the cervical spine. In all, 22 patients (78 facets and 122 screws) with degenerative cervical kyphosis or spondylolisthesis who underwent decompression and lateral mass screw (LMS) fixation were analyzed. Age, gender, smoking, bone mineral density (BMD), the degree of facet decortication with bone packing, and screw loosening were investigated as risk factors contributing to the facet non-union at each segmental fused level. Facet fusion rate was 85.9% (67/78 facets) and the incidence of loosening was 4.9% (6/122 screws, 4 patients). Insufficient facet decortication with bone packing is a significant risk factor of facet non-union (p <0.05, odds ratio: 26.5). All six loosened screws were associated with bony non-union of the facet and were located in the uppermost or lowermost vertebrae. Comparing loosened screws and stable screws, the average maximal insertional screw torque (MIT) was 9.8 cNm and 39.5 cNm, respectively (p <0.05). Additionally, the length of the stable screws was significantly longer versus the loosened screws (p <0.05). Lower MIT and shorter screw length located near the ends of the lateral mass may predict loosening, which can lead to facet non-union. Sufficient facet decortication with bone packing is one of the important factors contributing to the facet fusion.

Comparison of Three Different Options for C7 Posterior Vertebral Anchor in the Indian Population-Lateral Mass, Pedicle, and Lamina: A Computed Tomography-Based Morphometric Analysis

Study Design

Radiological cohort study.

Purpose

The options of posteriorly stabilizing C7 vertebra include using lateral mass, pedicle or lamina, as bony anchors. The current study is a computed tomography (CT)-based morphometric analysis of C7 vertebra of 100 Indian patients and discusses the feasibility of these different techniques.

Overview of Literature

C7 is a peculiar vertebra with unique anatomy, which poses challenges for each of these fixation modalities. There are no reports available in the literature, which discuss and compare the feasibility of diverse posterior C7 fixation techniques in Indian population.

Methods

We included 100 consecutive cervical spine CT scans of Indian patients performed between July 2016 and September 2016. We excluded CT scans with any significant congenital anomaly or other pathological lesions of C7 and patients with non-Indian ethnicity. Regarding screw placement, we assessed and studied various dimensions of the C7 lateral mass, pedicles, and laminae in relevant sections.

Results

The mean age of our patients was 49.5±16.1 years. We included 56 male and 44 female patients. The mean anteroposterior and mediolateral dimensions of the lateral mass were 11.38±1.76 and 12.91±1.82 mm, respectively. The mean length of the lateral mass screw (Magerl technique) was 12.17±1.9 mm; 92% of patients could accommodate a lateral mass screw at least 10-mm long (unicortical), whereas 48% could accommodate a screw (unicortical) longer than 12 mm. Foramen transversarium was found in 30.5% of lateral masses. The mean outer and inner cortical widths of the pedicles were 6.5±0.71 mm and 3.72±0.61 mm, respectively. Approximately 58% of pedicles could accommodate 3.5-mm screws (based on the inner cortical pedicle width). The outer cortical and inner cortical widths of the laminae were 6.21±1.2 mm and 3.23±0.9 mm, respectively. Subsequently, 37% of the laminae could accommodate 3.5-mm screws. The mean angle of intralaminar screw trajectory was 50.7°±5.1°, and the mean length of the intralaminar screw was 32.6±3.05 mm. In addition, 96.4% and 60.7% of male patients could accommodate lateral mass screws longer than 10 mm and 12 mm, respectively. However, only 86.4% and 31.8% of female patients could accommodate 10- and 12-mm long lateral mass screws, respectively. Furthermore, 75% of male patients and 36% of female patients had pedicles that could accommodate 3.5-mm diameter screws, and 48.2% of male patients had laminae that could accommodate 3.5-mm screws; however, only 22.7% of female patients could accommodate 3.5-mm laminar screws.

Conclusions

Based on our CT-guided morphometric analysis, 92% and 48% of Indian patients could accommodate at least 10- and 12-mm long lateral mass screws, and 58% of pedicles and 37% of laminae could accommodate 3.5-mm screws. Thus, lateral mass screws (between 10- and 12-mm long) seem to be the safest feasible option for C7 fixation. In case of the need for an alternative mode of stabilization (pedicle or intralaminar screw), particularly in female patients, careful preoperative planning with a CT scan is of utmost importance.

Does pedicle screw fixation of the subaxial cervical spine provide adequate stabilization in a multilevel vertebral body fracture model? An in vitro biomechanical study

BACKGROUND

Cervical vertebral body fractures generally are treated through an anterior-posterior approach. Cervical pedicle screws offer an alternative to circumferential fixation. This biomechanical study quantifies whether cervical pedicle screws alone can restore the stability of a three-column vertebral body fracture, making standard 360° reconstruction unnecessary.

METHODS

Range of motion (2.0 Nm) in flexion-extension, lateral bending, and axial rotation was tested on 10 cadaveric specimens (five/group) at C2-T1 with a spine kinematics simulator. Specimens were tested for flexibility of intact when a fatigue protocol with instrumentation was used to evaluate construct longevity. For a C4-6 fracture, spines were instrumented with 360° reconstruction (corpectomy spacer + plate + lateral mass screws) (Group 1) or cervical pedicle screw reconstruction (C3 and C7 only) (Group 2).

FINDINGS

Results are expressed as percentage of intact (100%). In Group 1, 360° reconstruction resulted in decreased motion during flexion-extension, lateral bending, and axial rotation, to 21.5%, 14.1%, and 48.6%, respectively, following 18,000 cycles of flexion-extension testing. In Group 2, cervical pedicle screw reconstruction led to reduced motion after cyclic flexion-extension testing, to 38.4%, 12.3%, and 51.1% during flexion-extension, lateral bending, and axial rotation, respectively.

INTERPRETATION

The 360° stabilization procedure provided the greatest initial stability. Cervical pedicle screw reconstruction resulted in less change in motion following cyclic loading with less variation from specimen to specimen, possibly caused by loosening of the shorter lateral mass screws. Cervical pedicle screw stabilization may be a viable alternative to 360° reconstruction for restoring multilevel vertebral body fracture.

Safety and efficacy of lateral mass screws at C7 in the treatment of cervical degenerative disease

Background:

To evaluate the safety and efficacy of lateral mass screws at C7 in the treatment of cervical degenerative disease.

Methods:

Patients with cervical degenerative disease who underwent posterior cervical fusion and fixation from 2009 to 2015 were included in the study. All complications were captured. Postoperative X-ray and computed tomography (CT) confirmed fusion at 6 and 12 months after surgery. X-ray and CT confirmed screw loosening, misplacement, pull-out, breakage, or rod breakage.

Results:

Seventy-two patients underwent cervical laminectomy and fixation with lateral mass screws at C7 and had at least 1 year follow-up. One patient had C3 screw pull-out; revision was not required. There were no complications related to the C7 screws, and all were in the lateral mass.

Conclusions:

Lateral mass screws are as safe and effective as pedicle screws at C7 in long-segment posterior cervical fixation, have a lower rate of perioperative complications than pedicle screws, and are technically easier to place.

Quantitative Anatomy of C7 Vertebra in Southern Chinese for Insertion of Lateral Mass Screws and Pedicle Screws

Study Design

Retrospective study.

Purpose

To analyze the quantitative anatomy of C7 vertebra for insertion of lateral mass screws and pedicle screws in Southern Chinese patients.

Overview of Literature

C7 lateral mass is smaller when compared to other subaxial cervical levels, which limits the length of lateral mass screws that can be used. Some studies have suggested pedicle screws for better fixation. But, this option is limited by the narrow pedicle width.

Methods

We have obtained computed tomography (CT) cervical spine data in 0.625 mm slices from our radiology department. The patients were adults. CTs were from May to August, 2015. The lateral mass screw length was measured using Margerl's technique and pedicle width and pedicle screw trajectory were determined in three-dimensional reformated images.

Results

CT scans of cervical spines of 94 patients were obtained and 188 lateral masses and pedicles of C7 vertebrae were measured. The mean lateral mass screw length was 13.2 mm (standard deviation [SD] 1.6 mm), mean outer pedicle width was 5.9 mm (SD 1.0 mm) and mean pedicle screw trajectory was 29.4 degrees (SD 3.6 degrees). Most (91.0%) of the pedicles had an outer diameter ≥4.5 mm.

Conclusions

The mean lateral mass screw length was longer when compared with other similar studies, while the mean outer pedicle width was narrower. Nearly 10% of the pedicles were unable to accommodate 3.5 mm screws. These findings favor the use of lateral mass screws to provide a safe and stable fixation for C7 vertebrae in Southern Chinese patients, while the final choice of fixation method should only be confirmed after careful preoperative planning with CT scan.

FREE-HAND PLACEMENT OF C7 PEDICLE SCREWS: A CADAVERIC STUDY

Objective : To evaluate the accuracy of free-hand pedicle screws placement at the seventh cervical vertebra. Methods : The authors have exposed the cervicothoracic junction of 9 adult cadavers (7 male and 2 female) preserved in formalin from the Faculty of Medicine of the Universidad Andina Néstor Cáceres Velásquez, city of Juliaca, Puno - Peru, locating the C7 vertebra based on anatomical parameters. According to previous publications, the entry point for the C7 pedicle was determined as 3-4mm lateral and 5-6mm superior to the center of the lateral mass, and the pedicle was drilled manually and instrumented with 3.5mm screws. After the screws placement, the C7 vertebrae were removed for radiographic analysis. Results : The authors were able to adequately locate the C7 entry point in 12 pedicles (66.6% accuracy), finding a great variability both laterally (2-5mm) and cranially (3-10mm). The angulation in the coronal plane was correct in 13 pedicles (72.3%), despite the incorrect location of the entry point. Angle values in the coronal plane ranged from 38 to 62 degrees. In the sagittal plane angulation, 2 screws were placed in the C6-C7 disc. The midtransversal diameter of the 18 pedicles ranged from 4 to 7mm. Conclusions : The location of the entry point for placement of C7 pedicle screws with pure free-hand technique is very variable due to anatomical differences and the authors recommend some type of guidance for increased safety and accuracy.

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.