A comparison of three screw types for unicortical fixation in the lateral mass of the cervical spine

Sublaminar polyester bands as a salvage fixation method in the cervical spine: novel application in two patients

Pedicle and lateral mass screws are the most common means of rigid fixation in posterior cervical spine fusions. Various other techniques such as translaminar screw placement, paravertebral foramen screw fixation, sublaminar and spinous process wiring, cement augmentation, and others have been developed for primary fixation or as salvage methods. Use of these techniques can be limited by a prior history of osteotomies, poor bone density, destruction of the bone-screw interface, and unfavorable vascular and osseous anatomy.Here, the authors report on the novel application of cervical sublaminar polyester bands as an adjunct salvage method or additional fixation point used with traditional methods in the revision of prior constructs. While sublaminar polyester bands have been used for decades in pediatric scoliosis surgery in the thoracolumbar spine, they have yet to be utilized as a method of fixation in the cervical spine. In both cases described here, sublaminar banding proved crucial for fixation points where traditional fixation techniques would have been less than ideal. Further study is required to determine the full application of sublaminar polyester bands in the cervical spine as well as its outcomes.

Biomechanical analysis of the durability of a modified S1 vertebrae transpedicular screws insertion technique

BACKGROUND

One of the most important elements of the transpedicular screw implantation technique, which enables a strong screw-bone interface, is the precise choice of the site of screw insertion and the screw's trajectory. Due to the complex biomechanics of the lumbosacral interface and different shape of the sacrum, fixation of this segment remains a challenge for surgeons. Because of this, Kubaszewski et al. proposed a modified technique in which the entry point for screw insertion in the S1 vertebra is changed.

METHODS

Six human cadaver specimens of the S1 vertebrae were examined. Two transpedicular screws were inserted into the body of each examined vertebra using two implantation methods with different screw entry points and trajectories. The screws were subjected to cyclic preloading, followed by the pull-out test. The ultimate pull-out force, displacement, stiffness, and failure energy were measured.

FINDINGS

The average pull-out force obtained for the standard method of implantation was 498 N (SD 201), whereas for the modified technique, it was 1308 N (SD 581). Displacement of the inserted screws in the new method was 36% higher than in the case of the standard method. This method is also characterized by the greater stiffness of the obtained interface and greater failure energy than the normally used technique.

INTERPRETATION

The obtained results demonstrate that the use of the new technique of implantation significantly increases the strength of the obtained screw-bone interface. It should also increase the success rate of the performed fixations and increase the safety of such fixations in clinical practice.

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.

Morphological study of the posterior osseous structures of subaxial cervical spine in a population from northeastern China

Background

Laminar screws and lateral mass screws have been increasingly used in the treatment of cervical diseases. The purpose of this study is to determine the morphological characteristics of the posterior anatomical structures of the subaxial cervical vertebrae in a northeastern Chinese population.

Methods

Sixty-one consecutive patients underwent cervical spine computed tomography (CT). We analyzed a total of 610 axial images and 61 sagittal images. The following parameters were measured: lamina outer width (LOW), lamina inner width (LIW), lamina axis length (LAL), lamina transverse angle (LTA), lateral mass longitudinal diameter (LMLD), lateral mass transverse diameter (LMTD), sagittal spinous process length (SSPL), axial spinous process length (ASPL), spinal canal transverse diameter (SCTD), spinal canal longitudinal diameter (SCLD), osseous spinal canal area (OSCA), and Pavlov ratio (PR). The participants were classified into male and female groups and developmental canal stenosis (DCS; PR ≤0.75) and non-DCS (NDCS; PR >0.75) groups.

Results

Significant differences were observed among the different vertebral levels for almost all evaluated parameters, except for LTA and OSCA. Statistical differences were found between the right and left sides in all parameters, except for LIW and LOW. All linear parameters, except for SCLD and the angular parameter LTA, significantly differed between the sexes. Significant differences were found between the DCS and NDCS groups in terms of all parameters, except for SCTD.

Conclusions

Various measurements of the posterior structures of subaxial cervical vertebrae differed between the left and right sides, females and males, and the DCS and NDCS groups. Different techniques for lateral mass screw insertion should be used according to different vertebral level. Only C7 laminar may be able to safely accommodate a 2.5-mm translaminar screw. The study data can help doctors to make better surgical decisions and develop more appropriate implants for northeastern Chinese patients.

Is polymethyl methacrylate a viable option for salvaging lateral mass screw failure in the subaxial cervical spine?

Study Design Biomechanical analysis of lateral mass screw pullout strength. Objective We compare the pullout strength of our bone cement-revised lateral mass screw with the standard lateral mass screw. Methods In cadaveric cervical spines, we simulated lateral mass screw "cutouts" unilaterally from C3 to C7. We salvaged fixation in the cutout side with polymethyl methacrylate (PMMA) or Cortoss cement (Orthovita, Malvern, Pennsylvania, United States), allowed the cement to harden, and then drilled and placed lateral mass screws back into the cement-augmented lateral masses. On the contralateral side, we placed standard lateral mass screws into the native, or normal lateral, masses and then compared pullout strength of the cement-augmented side to the standard lateral mass screw. For pullout testing, each augmentation group was fixed to a servohydraulic load frame and a specially designed pullout fixture was attached to each lateral mass screw head. Results Quick-mix PMMA-salvaged lateral mass screws required greater force to fail when compared with native lateral mass screws. Cortoss cement and PMMA standard-mix cement-augmented screws demonstrated less strength of fixation when compared with control-side lateral mass screws. Attempts at a second round of cement salvage of the same lateral masses led to more variations in load to failure, but quick-mix PMMA again demonstrated greater load to failure when compared with the nonaugmented control lateral mass screws. Conclusion Quick-mix PMMA cement revision equips the spinal surgeon with a much needed salvage option for a failed lateral mass screw in the subaxial cervical spine.

Feasibility of bilateral crossing c7 intralaminar screws: a cadaveric study

Objective

When the pedicle screw insertion technique is failed or not applicable, C7 intralaminar screw insertion method has been used as an alternative or salvage fixation method recently. However, profound understanding of anatomy is required for safe application of the bilaterally crossing laminar screw at C7 in clinic. In this cadaveric study, we evaluated the anatomic feasibility of the bilateral crossing intralaminar screw insertion and especially focused on determination of proper screw entry point.

Methods

The C7 vertebrae from 18 adult specimens were studied. Morphometric measurements of the mid-laminar height, the minimum laminar thickness, the maximal screw length, and spino-laminar angle were performed and cross-sectioned vertically at the screw entry point (spino-laminar junction). The sectioned surface was equally divided into 3 parts and maximal thickness and surface area of the parts were measured. All measurements were obtained bilaterally.

Results

The mean mid-laminar height was 13.7 mm, mean minimal laminar thickness was 6.6 mm, mean maximal screw length was 24.6 mm, and mean spinolaminar angle was 50.8±4.7°. Based on the measured laminar thickness, the feasibility of 3.5 mm diameter intralaminar screw application was 83.3% (30 sides laminae out of total 36) when assuming a tolerance of 1 mm on each side. Cross-sectional measurement results showed that the mean maximal thickness of upper, middle, and lower thirds was 5.0 mm, 7.5 mm, and 7.3 mm, respectively, and mean surface area for each part was 21.2 mm², 46.8 mm², and 34.7 mm², respectively. Fourteen (38.9%) sides of laminae would be feasible for 3.5 mm intralaminar screw insertion when upper thirds of C7 spino-laminar junction is the screw entry point. In case of middle and lower thirds of C7 spino-laminar junction, 32 (88.9%) and 28 (77.8%) sides of laminae were feasible for 3.5 mm screw insertion, respectively.

Conclusion

The vertical cross-sectioned area of middle thirds at C7 spinolaminar junction was the largest area and 3.5 mm screw can be accommodated with 77.8% of feasibility when lower thirds were the screw entry point. Thus, selection of middle and lower thirds for each side of screw entry point in spino-laminar junction would be the safest way to place bilateral crossing laminar screw within the entire lamina. This anatomic study result will help surgeons to place the screw safely and accurately.

[Surgical treatment of early-onset scoliosis with the StarLock implant system]

OBJECTIVE

The problem of early-onset scoliosis is the progression of spine curvature. With the StarLock instrumentation a corrected spinal growth can be achieved. Distraction is necessary once or twice a year.

INDICATIONS

Idiopathic, congenital and neuromuscular scoliosis.

CONTRAINDICATIONS

Ostoeoporosis. Arthrogryposis. Kyphosis.

SURGICAL TECHNIQUE

The use of a C-arm (anterior-posterior and lateral view) has to be possible. Pedicle screws are placed at the proximal and distal end of the curvature. Through distraction of the rods which are screwed to each other via parallel connectors, correction of the scoliosis can be achieved.

POSTOPERATIVE MANAGEMENT

Mobilization should be started 1 day postoperatively using an individual corset for 6 months. Distraction has to be done once or twice a year.

RESULTS

From March 2003 to October 2005, 14 children with early- onset scoliosis were treated with the StarLock instrumentation. After the first operation, the Cobb angle improved from 65 degrees (42-80 degrees ) to 26.5 degrees (18-45 degrees ) in the thoracic spine and from 22 degrees (18-55 degrees ) to 15 degrees (10-32 degrees ) in the lumbar spine. Lenghtenings were done every 6.8 months (5-9 months). The follow-up time was 4.5 years (3-6 years).

COMPLICATIONS

three rods and two screws broke. Infections and neurologic deficits were not observed.

Lateral mass screw fixation in children

PURPOSE

The safety and feasibility of posterior screw fixation of the cervical spine in children has not been well documented in the orthopedic literature. We performed a retrospective review of our experience using posterior cervical screw fixation in children.

METHODS

The medical records and radiologic records of 36 children at a mean age of 10 years (range 3-16 years) were reviewed. Diagnoses included: ten instability, 11 deformity, seven trauma, five tumor, and three congenital abnormalities. Operative reports and postoperative computed tomography (CT) scans were reviewed to determine the technical feasibility of screw placement, any screw-related complications, and to assess for correct screw position. In this series, there were no neurologic complications, no vertebral artery injuries, and no screw-related complications.

RESULTS

Thirty patients (141 screws) had screws evaluated postoperatively and were shown to be completely contained on postoperative CT scans. There were no revisions due to screw failure or dislodgement. There were no vascular or neurologic complications.

CONCLUSIONS

Posterior screw fixation in the pediatric population may be done safely and greatly enhances fixation strength for a variety of disorders requiring instrumentation and fusion.

Using lamina screws as a salvage technique at C-7: computed tomography and biomechanical analysis using cadaveric vertebrae

Object

Transpedicular instrumentation at C-7 has been well accepted, but salvage techniques are limited. Lamina screws have been shown to be a biomechanically sound salvage technique in the proximal thoracic spine, but have not been evaluated in the lower cervical spine. The following study evaluates the anatomical feasibility of lamina screws at C-7 as well as their bone-screw interface strength as a salvage technique.

Methods

Nine fresh-frozen C-7 cadaveric specimens were scanned for bone mineral density using dual energy x-ray absorptiometry. Prior to testing, all specimens were imaged using CT to obtain 1-mm axial sections. Caliper measurements of both pedicle width and laminar thickness were obtained. On the right side, pedicle screws were first inserted and then pulled out. Salvage intralaminar screws were inserted into the left lamina from the right spinous process/lamina junction and then pulled out. All screws were placed by experienced cervical spine surgeons under direct fluoroscopic visualization. Pedicle and lamina screws were 4.35- and 3.5-mm in diameter, respectively. Screws sizes were chosen based on direct and radiographic measurements of the respective anatomical regions. Insertional torque (IT) was measured in pounds per inch. Tensile loading to failure was performed in-line with the screw axis at a rate of 0.25 mm/sec using a MiniBionix II system with data recorded in Newtons.

Results

Using lamina screws as a salvage technique generated mean pullout forces (778.9 ± 161.4 N) similar to that of the index pedicle screws (805.3 ± 261.7 N; p = 0.796). However, mean lamina screw peak IT (5.2 ± 2.0 lbs/in) was significantly lower than mean index pedicle screw peak IT (9.1 ± 3.6 lbs/in; p = 0.012). Bone mineral density was strongly correlated with pedicle screw pullout strength (r = 0.95) but less with lamina screw pullout strength (r = 0.04). The mean lamina width measured using calipers (5.7 ± 1.0 mm) was significantly different from the CTmeasured mean lamina width (5.1 ± 0.8 mm; p = 0.003). Similarly, the mean pedicle width recorded with calipers (6.6 ± 1.1 mm) was significantly different from the CT-measured mean pedicle width (6.2 ± 1.3 mm; p = 0.014). The mean laminar width measured on CT at the thinnest point ranged from 3.8 to 6.8 mm, allowing a 3.5-mm screw to be placed without difficulty.

Conclusions

These results suggest that using lamina screws as a salvage technique at C-7 provides similar fixation strength as the index pedicle screw. The C-7 lamina appears to have an ideal anatomical width for the insertion of 3.5-mm screws commonly used for cervical fusions. Therefore, if the transpedicular screw fails, using intralaminar screws appear to be a biomechanically sound salvage technique.

Biomechanical pullout strength and stability of the cervical artificial pedicle screw

STUDY DESIGN

This study used cadaveric specimens to compare the biomechanical performance of artificial pedicle screws (APS) versus lateral mass screws (LMS).

OBJECTIVE

The goal of this study was to biomechanically characterize APS range-of-motion and pullout strength in surgical instances that preclude LMS insertion.

SUMMARY OF BACKGROUND DATA

Posterior approaches used in instances of ventral spine tumors often necessitate complete facetectomy, thereby removing fixation points for LMS and requiring longer constructs with fewer segmental fixations to span the resected levels. Recently, APS were developed to overcome this obstacle. Although APS have been used successfully in clinical cases, they have yet to be biomechanically validated.

METHODS

Seven fresh-frozen cervical spine segments (C2-C7) were harvested from human donors (F = 1, M = 6; 65 +/- 5 years old, range: 50-72 years old). Nondestructive range-of-motion tests were conducted on each specimen in its intact and surgically destabilized states, and after each of 3 different APS and LMS surgical stabilizations. After nondestructive bending tests, a final pullout test of APS and LMS at the C4 level was performed for each specimen.

RESULTS

The pullout strength of the APS was twice as strong on average as that for the LMS (503.4 +/- 338.3 vs.- 254.3 +/- 142.3 N); this difference approached but was not statistically significant (P = 0.07). There was no significant difference in specimen stiffness between the APS- and LMS-instrumented configurations in all ranges of primary and off-axis motions (P > 0.05). However, all fixation methods increased specimen stiffness in comparison with the intact conditions (1.7-36.5 times increase; P < 0.05).

CONCLUSION

Our results demonstrate that APS provide comparable stability to LMS and can therefore be considered a viable alternative in surgical scenarios requiring the complete removal of lateral masses. Moreover, APS may provide some enhanced strength in the face of destructive pullout forces.

A biomechanical evaluation of three revision screw strategies for failed lateral mass fixation

STUDY DESIGN

This is a biomechanical study evaluating 3 revision strategies for failed cervical lateral mass screw fixation.

OBJECTIVE

Our primary objective was to compare, following a Magerl trajectory screw failure in the subaxial cervical spine, the pullout strength of (1) a revision screw in the same trajectory, (2) a Roy-Camille trajectory, and (3) pedicle screw fixation. We additionally analyzed the contributions of bone mineral density (BMD) and peak insertional torque to pullout strength.

SUMMARY OF BACKGROUND DATA

Biomechanical studies that have examined revision screw strategies for lateral mass fixation have found either unsatisfactory or highly variable performance.

METHODS

Fresh frozen cervical spinal segments were harvested and BMD testing performed. Bicortical (3.5-mm Vertex) lateral mass screws were placed in a Magerl trajectory in 57 fresh frozen human subaxial cervical vertebrae. All screws were then stripped and revision screws (4.0-mm Vertex) placed using either the same screw path or conversion to a Roy-Camille trajectory. In line pullout testing was performed on each of the revision screws (57 in Magerl revision group, 55 in Roy-Camille). Specimens that had not fractured during testing then had cervical pedicle screws (3.5-mm Vertex) placed and in-line pullout testing repeated (64 pedicles were instrumented) The pullout failure results of the Magerl revision, Roy-Camille revision, and pedicle screw revision groups were compared.

RESULTS

No significant difference was noted in insertional torque (0.28-Nm Magerl, 0.35 Nm Roy-Camille, P > 0.05) or pullout (382-N Magerl, 351 N Roy-Camille, P > 0.05) between the Magerl and Roy-Camille revision groups. Pedicle screw revision had greater pullout strength (566 N) when compared with either the Magerl (382 N) or Roy-Camille (351 N) revision groups (P < 0.01) but also had a 20% pedicle wall breech rate by visual inspection. Insertional torque and pullout strength increased with increased BMD and were significantly correlated in all 3 revision groups (P < 0.05). Similarly, increased BMD was associated with increased pullout strength as demonstrated by the significant positive correlation (P < 0.05).

CONCLUSION

Conversion of a stripped lateral mass screw to an alternate trajectory appears to offer no biomechanical advantage over placement of an increased diameter salvage screw using the same trajectory. Pedicle screw fixation provides superior biomechanical fixation but was associated with a significant breech rate.

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.

A Comparative Biomechanical Analysis of Spinal Instability and Instrumentation of the Cervicothoracic Junction

OBJECTIVE

Stabilization of the cervicothoracic junction is challenging but commonly required in patients with traumatic, neoplastic, congenital, and postlaminectomy conditions. Although extensive research has been performed on stabilization of the cervical spine, there remains a paucity of published data on instrumentation at the cervicothoracic junction. Using 2-column, 3-column, and corpectomy instability models, a biomechanical analysis was performed on the effects of increasing the number of posterior segmental fixation points and/or anterior column reconstruction at the cervicothoracic junction.

METHODS

Multidirectional flexibility testing was performed utilizing a 6-degree-of-freedom spine simulator and 7 fresh-frozen human cadaveric spines (occiput-T6). After intact spine analysis, each specimen was destabilized and reconstructed as follows: (1) C7/T1 2-column injury with posterior instrumentation; (2) C7/T1 3-column injury with posterior instrumentation; (3) C7/T1 3-column injury with anterior interbody cage/plate and posterior instrumentation; and (4) C7/T1 3-column injury plus C7 corpectomy with anterior cage/plate and posterior instrumentation. All reconstruction groups were tested with posterior instrumentation (screws connected by dual-diameter rods) from C5-T1, C5-T2, and C5-T3.

RESULTS

For 2-column injuries, there were no statistically significant differences in flexibility (P>0.05), although there was a trend toward reduced flexibility with increasing levels of thoracic fixation. For 3-column injuries, posterior fixation alone resulted in excessive flexibility in flexion/extension even with instrumentation to T3 (P<0.05). With the addition of anterior column instrumentation, there were no observed differences in flexion/extension and lateral bending. For axial rotation, instrumentation to T1 alone demonstrated increased motion relative to the intact spine (P<0.05). The 3-column injury with corpectomy model demonstrated similar flexibility properties to the 3-column injury model.

CONCLUSIONS

With 3-column instability posterior segmental fixation alone from C5-T3 was inadequate, and the addition of anterior instrumentation restored flexibility to the intact condition. There was a strong trend toward reduced flexibility with increasing levels of thoracic fixation in all instability models.

Biomechanical Evaluation of the Pullout Strength of Cervical Screws

OBJECTIVE

In the process of anterior cervical fusion, little is known about the biomechanics of anterior cervical screw pullout. In this study, three different aspects of cervical screw fixation were evaluated: self-tapping (ST) versus self-drilling (SD) screws, the effect of screw geometry (length, diameter, thread pitch), and the use of rescue screws.

METHODS

Nine screws consisting of different diameters, lengths, and thread pitch (cancellous and cortical) were tested in peak pullout force in an artificial bone model using an MTS 858 Mini Bionix test system. Rescue screws (4.5 mm) were then inserted in the failed holes of 4.0-mm screws and extracted to determine their holding strength.

RESULTS

Length of screws and thread pitch both had a significant effect on the pullout force. Each 1 mm of increased screw length translates to 16 N of increased force to pullout in the foam bone model. Pullout strength did not vary significantly according to screw diameter or between SD and ST screws. However, the SD screw has an advantage because it can decrease the length of surgery. A decrease in pullout force of between 43% and 70% was found when using rescue screws.

CONCLUSIONS

In situations in which the use of rescue/salvage screws is required, the surgeon should anticipate a significant decrease in the holding force compared with the original screw. Future directions for research include an evaluation of pullout force for screw and plate constructs.

Posterior fixation of subaxial cervical spine fractures in patients with ankylosing spondylitis

Cervical spine fractures in patients with ankylosing spondylitis are serious and potentially lethal injuries with high complication rates. Treatment obstacles include long lever arms that generate large forces on any fixation device, osteoporosis, and, usually, kyphotic deformity. The Olerud Cervical Fixation System (OC), with cervical pedicle screws and rods, offers an opportunity to create a biomechanically stable posterior fixation in these complicated cases. The present study is a retrospective chart review and a radiological follow-up of patients with this diagnosis, treated at our department between 1995 and 2000. Nineteen patients (two women) with a mean age of 60 years (32-78 years) were included. The fracture levels were predominantly C5-C6 (five patients) and C6-C7 (five patients). All patients were treated with a long posterior fixation with the OC, and in four patients this was combined with an anterior plate fixation. One patient with severe lordosis also received a short posterior plate fixation. The patients' notes and plain radiographs have been reviewed. Five patients died during the post-operative follow-up period; the others had a mean follow-up time of 24 months (10-55 months). Eleven patients had no neurological deficits preoperatively. One of them developed moderate weakness in his right arm, postoperatively, due to a misplaced pedicle screw in the right pedicle of C5. However, after extraction of the screw he almost totally recovered in 6 months. Eight patients had neurological deficits. Two were paraplegic; two had motor weakness combined with sensory deficiency, and four had a sensory deficiency. Two of the patients with neurological deficits improved postoperatively, but the others were unchanged. Peroperative problems were recorded in five patients; one C6 pedicle was perforated, and two patients had pedicles on one or more levels that the surgeon was not able to probe. In one of the latter patients, transfacet screws were chosen, instead, for one of the levels. Extensive peroperative bleeding was encountered in two patients. One deep-wound infection was noted, postoperatively, and required surgical drainage, but no patients have been re-operated due to loosening of the instrument or to healing problems. In conclusion, the results of the present study indicate that the OC--and possibly other similar long-fixation systems that allow using both pedicle screws and lateral mass screws rigidly connected to a rod--is suited for treating subaxial cervical spine fractures in patients with ankylosing spondylitis, allowing high healing rates.

Biomechanical evaluation of cervical lateral mass fixation: a comparison of the Roy-Camille and Magerl screw techniques

Object. The purpose of this study was to assess human cervical spine pullout force after lateral mass fixation involving two different techniques: the Roy-Camille and the Magerl techniques. Although such comparisons have been conducted previously, because of the heterogeneity of results and the importance of this procedure in clinical practice, it is essential to have data derived from a prospective and randomized biomechanical study involving a sufficient sample of human cervical spines. The authors also evaluated the influence of the sex, the vertebral level, the bone mineral density (BMD), the length of bone purchase, and the thickness of the anterior cortical purchase.

Methods. Twenty-one adult cervical spines were harvested from fresh human cadavers. Computerized tomography was performed before and after placing 3.5-mm titanium lateral mass screws from C-3 to C-6. Pullout forces were evaluated using a material testing machine. The load was applied until the pullout of the screw was observed. A total of 152 pullout tests were available, 76 for each type of screw fixation. The statistical analysis was mainly performed using the Kaplan—Meier survival method.

The mean pullout force was 266 ± 124 N for the Roy-Camille technique and 231 ± 94 N for the Magerl technique (p < 0.025). For the C3–4 specimen group, Roy-Camille screws were demonstrated to exert a significantly higher resistance to pullout forces (299 ± 114 N) compared with Magerl screws (242 ± 97 N), whereas no difference was found between the two techniques for the C5–6 specimen group (Roy-Camille 236 ± 122 N and Magerl 220 ± 86 N). Independent of the procedure, pullout strengths were greater at the C3–4 level (271 ± 114 N) than the C5–6 level (228 ± 105 N) (p < 0.05).

No significant correlation between the cancellous BMD, the thickness of the anterior cortical purchase, the length of bone purchase, and maximal pullout forces was found for either technique.

Conclusions. The difference between pullout forces associated with the Roy-Camille and the Magerl techniques was not as significant as has been previously suggested in the literature. It was interesting to note the influence of the vertebral level: Roy-Camille screws demonstrated greater pullout strength (23%) at the C3–4 vertebral level than Magerl screws but no significant difference between the techniques was observed at C5–6.