Posterior rod system of the cervical spine: a new implant allowing optimal screw insertion

Spinous Process Screw Fixation: A Salvage Technique in Subaxial Cervical Spinal Instrumentation

OBJECTIVE

We sought to evaluate the feasibility and efficacy of spinous process screws in subaxial cervical fixation.

METHODS

A retrospective study was conducted on 7 patients receiving posterior cervical instrumentation for subaxial fracture dislocation or atlantoaxial dislocation from 2014 to 2015. In addition, a biomechanical test was performed on 7 fresh-frozen cadaveric spine samples (from the occiput to C7) from healthy human subjects with no history of spinal trauma. And lastly, the potential trajectories of cervical spinous process screws of 100 adults were measured through axial computed tomography in order to establish the general rule for applicability of this technique.

RESULTS

Rigid fixation and solid fusion were achieved in all 7 patients included in the study. Biomechanical test results revealed no significant difference between bilateral lateral mass screw fixation and the hybrid lateral mass and spinous process screw fixation constructs. Measurement of the potential trajectory suggested that spinous process screw fixation was a viable technique in the subaxial cervical spine in the general population.

CONCLUSIONS

The spinous process screw fixation technique in the subaxial cervical spine is a viable and effective salvage option for patients in whom conventional posterior fixations were not sufficient and salvage, supplementary techniques were required.

Avoiding iatrogenic injuries to the vertebral artery: A morphometric study of the vertebral artery-free dissection area

OBJECTIVE

To determine the area of a safety window that excludes the vertebral artery for the safe access of the occipital condyle screws during occipitocervical fixation.

METHODS

This study included 138 cervical computed tomography angiograms. Six measurements per side were made in each imaging study. These measurements are from the vertebral artery to (A) the mastoid process, (B) the mastoid incisura, (C) the posterior condylar fossa, (D) the occipital condyle in its midline, and (E) the medial border of the condyle. We also measured from the tip of the mastoid process to the lower border of the occipital condyle on its lateral side (F).

RESULTS

A total of 276 areas from 138 individuals were included, of which 51.4 % were men. The mean age was 54.2 ± 18.63 years. The mean variable measurements (mm) for all the population were 21 ± 4, 16 ± 3, 6 ± 2, 3 ± 2, 2 ± 1 and 35 ± 4 for variables A-F, respectively. We found significant differences between sex when we compared measurements A (p = 0.003), C (p = 0.001), D (p = 0.000) and F (p = 0.000). The incidence rate of dominance for the vertebral artery was 18.8 % and 30.4 % for right and left respectively.

CONCLUSION

Women had significantly smaller measures than men. This could indicate a higher risk of iatrogenic injury secondary to a smaller vertebral artery-free area. Results may guide surgeons in the pre-surgical planning aiming to reduce the risk of iatrogenic injuries to the vertebral artery.

History and advances in spinal neurosurgery

Insight into the historic contributions made to modern-day spine surgery provides context for understanding the monumental accomplishments comprising current techniques, technology, and clinical success. Only during the last century did surgical growth occur in the treatment of spinal disorders. With that growth came a renaissance of innovation, particularly with the evolution of spinal instrumentation and fixation techniques. In this article, the authors capture some of the key milestones that have led to the field of spine surgery today, with an emphasis on the historical advances related to instrumentation, navigation, minimally invasive surgery, robotics, and neurosurgical training.

Misplaced Cervical Screws Requiring Reoperation

STUDY DESIGN

A multicenter, retrospective case series.

OBJECTIVE

In the past several years, screw fixation of the cervical spine has become commonplace. For the most part, this is a safe, low-risk procedure. While rare, screw backout or misplaced screws can lead to morbidity and increased costs. We report our experiences with this uncommon complication.

METHODS

A multicenter, retrospective case series was undertaken at 23 institutions in the United States. Patients were included who underwent cervical spine surgery from January 1, 2005, to December 31, 2011, and had misplacement of screws requiring reoperation. Institutional review board approval was obtained at all participating institutions, and detailed records were sent to a central data center.

RESULTS

A total of 12 903 patients met the inclusion criteria and were analyzed. There were 11 instances of screw backout requiring reoperation, for an incidence of 0.085%. There were 7 posterior procedures. Importantly, there were no changes in the health-related quality-of-life metrics due to this complication. There were no new neurologic deficits; a patient most often presented with pain, and misplacement was diagnosed on plain X-ray or computed tomography scan. The most common location for screw backout was C6 (36%).

CONCLUSIONS

This study represents the largest series to tabulate the incidence of misplacement of screws following cervical spine surgery, which led to revision procedures. The data suggest this is a rare event, despite the widespread use of cervical fixation. Patients suffering this complication can require revision, but do not usually suffer neurologic sequelae. These patients have increased cost of care. Meticulous technique and thorough knowledge of the relevant anatomy are the best means of preventing this complication.

Complications of occipital screw placement for occipitocervical fusion in children

OBJECT

Occipitocervical stabilization in the pediatric age group remains a challenge because of the regional anatomy, poor occipital bone purchase, and, in some instances, significant thinning of the occipital bone. Multiple bicortical fixation points to the occipital bone may be required to increase construct rigidity. The authors evaluated the complications of bicortical occipital screw placement in children with occipital fusion constructs.

METHODS

The records of 20 consecutive pediatric patients who had undergone occipitocervical fusion between September 1, 2007, and November 30, 2010, at Texas Children's Hospital were reviewed.

RESULTS

The patients consisted of 10 girls and 10 boys, ranging in age from 10 months to 16 years (mean ± SD, 7.7 ± 5.1 years). Two patients were lost to follow-up, 2 died for reasons unrelated to the surgery, and the remaining patients had at least 3 months of follow-up (mean 14 ± 11.8 months) with evaluation via dynamic radiography and CT. Four patients experienced 8 complications: 2 CSF leaks, 2 vigorous venous bleedings, worsening of quadriparesis, wound infection, radiographic pseudarthrosis, and transient dysphagia. Among 114 screws, there were 2 cases of intraoperative dural venous sinus injury and 2 cases of intraoperative CSF leakage, without clinical sequelae from these complications. Only 1 case of radiographic pseudarthrosis was identified in a patient with skeletal dysplasia and a prior failed C1-2 posterior arthrodesis. There were no difficulties with wound healing because of prominent occipital instrumentation, and there was only 1 wound infection.

CONCLUSIONS

Data in this report confirm that including bicortical occipital screw placement in occipitocervical constructs in children may result in a high fusion rate but at the cost of a notable complication rate.

Comparison of lateral mass screw fixation technique and hartshill rectangle technique in the treatment of sub-axial cervical spine fractures

ABSTRACT

INTRODUCTION

Cervical injury in a polytrauma patient is one of the most critical injuries. The aim of this study was to compare the lateral mass screw technique with the Hartshill rectangle technique for treatment of such cases.

MATERIALS AND METHODS

This prospective study consisted of 40 patients. Both groups were followed for three years clinically and radiologically.

RESULTS

In the lateral mass technique, there were no cases of vertebral artery injury, radiculopathy, screw pullout, dural tears, residual kyphosis or persistent pain. In the Hartshill technique 3 patients experienced intraoperative dural tears, 1 case of wire breakage at the six months follow up, 6 patients with persistent neck pain and 1 with worsening neurological status. One hundred per cent fusion was achieved in both groups. There was significant radiation exposure in the lateral mass group. Post-operative immobilisation was required only in the Hartshill.

CONCLUSION

Lateral mass screw technique is definitely a relatively better procedure. But Hartshill rectangle still stands out in certain practical situations.

KEY WORDS

lateral mass, Hartshill.

Computed tomography–based determination of a safe trajectory for placement of transarticular facet screws in the subaxial cervical spine

Object

Placement of transarticular facet screws is one option for stabilization of the subaxial cervical spine. Small clinical series and biomechanical data support their role as a substitute for other posterior stabilization techniques; however, the application of transarticular facet screws in the subaxial cervical spine has not been widely adopted, possibly because of surgeon unfamiliarity with the trajectory. In this study, the authors' objective is to define insertion points and angles of safe trajectory for transarticular facet screw placement in the subaxial cervical spine.

Methods

Thirty fine-cut CT scans of cervical spines were reconstructed in the multiplanar mode and evaluated for safe transarticular screw placement in the subaxial cervical spine (C2–3, C3–4, C4–5, C5–6, C6–7). As in placement of lateral mass screws, the vertebral artery and exiting nerve root were bypassed posterolaterally. The entry point was set 1 mm medial and 1 mm caudal to the center of the lateral mass. From this entry point, the sagittal angulation was set to traverse the facet joint plane approximately perpendicularly. For the axial angulation, the exit point was set posterolaterally to the transverse process. After ideal insertion angles and screw lengths were identified, the trajectory was simulated on CT scans of 20 different cervical spines to confirm safe screw placement.

Results

The mean optimal mediolateral insertion angles (± SD) were as follows: 23° ± 5° at C2–3; 24° ± 4° at C3–4; 25° ± 5° at C4–5; 25° ± 4° at C5–6; and 33° ± 6° at C6–7. The mean sagittal insertion angles measured to the sagittal projection of the facet joint space were as follows: 77° ± 10° at C2–3; 77° ± 10° at C3–4; 80° ± 11° at C4–5; 81°± 8° at C5–6; and 100° ± 11° at C6–7. The mean trajectory lengths were 15 ± 2 mm at C2–3; 14 ± 1 mm at C3–4; 15 ± 1 mm at C4–5; 16 ± 2 mm at C5–6; and 23 ± 4 mm at C6–7. Simulation of these insertion angles on 20 different cervical spine CTs yielded a safe trajectory in 85%–95% of spines for C2–3, C3–4, C4–5, C5–6, and C6–7.

Conclusions

The calculated optimal insertion angles and lengths for each level may guide the safe placement of subaxial cervical transfacet screws.

The biomechanical effect of transverse connectors use in a pre- and postlaminectomy model of the posterior cervical spine: an in vitro cadaveric study

STUDY DESIGN

An in vitro biomechanical study investigating the effect of transverse connectors on posterior cervical stabilization system in a laminectomy model.

OBJECTIVE

To evaluate the optimal design, number, and location of the transverse connectors in stabilizing long segment posterior instrumentation in the cervical spine.

SUMMARY OF BACKGROUND DATA

In the cervical spine, lateral mass screw (LMS) fixation is used for providing stability after decompression. Transverse connectors have been used to augment segmental posterior instrumentation. However, in the cervical region the optimal design, number, and the location of transverse connectors is not known.

METHODS

Seven fresh human cervicothoracic cadaveric spines (C2-T1) were tested by applying ±1.5 Nm moments in flexion (F), extension (E), lateral bending (LB), and axial rotation (AR). After testing the intact condition, LMS/rods were placed and then were tested with two different transverse connectors (top-loading connector [TL] and the head-to-head [HH] connector) in multiple levels, pre- and postlaminectomy (PL).

RESULTS

LMS significantly reduced segmental motion by 77.2% in F, 75.6% in E, 86.6% in LB, and 86.1% in AR prelaminectomy and by 75.4% in F, 76% in E, 80.6% in LB, and 76.4% in AR postlaminectomy compared to intact (P < 0.05). Only in AR, PL constructs with HH connectors at C3 & C7, TL connectors at C4-C5 & C5-C6, and at C3-C4 & C6-C7 significantly reduced the range of motion by 12.9%, 11.9%, and 11.9%, respectively, compared to PL LMS (P < 0.05). No statistical significance was observed between TL connector and HH connector in all loading directions.

CONCLUSION

The biomechanical advantage of transverse connectors is significant in AR, when using two connectors at the proximal and distal ends, compared to one connector. In a clinical setting, this data may guide surgeons on transverse connector configurations to consider during posterior cervical instrumentation.

Posterior instrumentation for occipitocervical fusion

Since 1995, 29 consecutive patients with craniocervical spine instability due to several pathologies were managed with posterior occipitocervical instrumentation and fusion. Laminectomy was additionally performed in nineteen patients. The patients were divided in two groups: Group A which included patients managed with screw-rod instrumentation, and Group B which included patients managed with hook-and-screw-rod instrumentation. The patients were evaluated clinically and radiographically using the following parameters: spine anatomy and reconstruction, sagittal profile, neurologic status, functional level, pain relief, complications and status of arthrodesis. The follow-up was performed immediately postoperatively and at 2, 6, 12 months after surgery, and thereafter once a year. Fusion was achieved in all but one patient. One case of infection was the only surgery related complication. Neurological improvement and considerable pain relief occurred in the majority of patients postoperatively. There were neither intraoperative complications nor surgery related deaths. However, the overall death rate was 37.5% in group A, and 7.7% in group B. There were no instrument related failures. The reduction level was acceptable and was maintained until the latest follow-up in all of the patients. No statistical difference between the outcomes of screw-rod and hook-and-screw-rod instrumentation was detected. Laminectomy did not influence the outcome in either group. Screw-rod and hook-and-screw-rod occipitocervical fusion instrumentations are both considered as safe and effective methods of treatment of craniocervical instability.

[Treatment of occipitocervical instability by SUMMIT]

The occipitocervical junction with its complex anatomy and biomechanics represents unique anatomical structure difficult for operative treatment. Biomechanics forces in this region need to be resisted with rigid metallic construction to facilitate bone fusion. Surgeon must be careful about close relations of the bone, vascular and neurologic structures and must have good skills and knowledge to avoid serious complications during operations. Before 2004 for instability treatment in this region of spine wires and pin construction for fixation of bone grafts were used. In our Institute rod constructions with screws have been used to get more rigid construction since 2004. In this paper we will present our early results in occipitocervical instability treatment by SUMMIT instrumentation.

Placement of occipital condyle screws for occipitocervical fixation in a pediatric patient with occipitocervical instability after decompression for Chiari malformation

In cadaveric studies and recently in one adult patient the occipital condyle has been studied as an option to allow bone purchase by fixation devices. In the current case the authors describe the use of occipital condyle screws in a child undergoing occipitocervical fixation. To the best of the authors' knowledge this case is the first reported instance of this technique in a pediatric patient. This girl had a history of posterior fossa decompression for Chiari malformation Type I when she was 22 months of age. When she was 6 years old she presented with neck pain on flexion and extension of her head. Magnetic resonance imaging in flexion and extension revealed occipitocervical instability. She underwent an occiput to C-2 posterior arthrodesis with bilateral screw placement in the occipital condyles, C-2 lamina, and C-1 lateral masses. Postoperatively, she was neurologically intact. Computed tomography demonstrated a stable construct, and her cervical pain had resolved on follow-up.

Craniocervical fixation with occipital condyle screws: biomechanical analysis of a novel technique

STUDY DESIGN

A human cadaveric biomechanical study comparing craniocervical fixation techniques.

OBJECTIVE

To quantitatively compare the biomechanical stability of a new technique for occipitocervical fixation using the occipital condyles with an established method for craniocervical spine fusion.

SUMMARY OF BACKGROUND DATA

Stabilization of the occipitocervical junction remains a challenge. The occiput does not easily accommodate instrumentation because of access and spatial constraints. In fact, the area available for the implant fixation is limited and can be restricted further when a suboccipital craniectomy has been performed, posing a challenge to current fixation techniques. Occipital screws are also associated with the potential for intracranial complications.

METHODS

Six fresh frozen cadaveric specimens occiput-C4 were tested intact, after destabilization and after fixation as follows: (1) occipital plate with C1 lateral mass screws and C2 pars screws and (2) occipital condyle screws with C1 lateral mass screws and C2 pars screws. Specimens were loaded in a custom spine testing apparatus and subjected to the following tests, all performed under 50-N unconstrained axial preload: flexion, extension, lateral bending, and axial rotation at 1.5 Nm. The constructs were statistically compared with a one-way analysis of variance and compared with the intact condition.

RESULTS

Motions were reduced by approximately 80% compared with the intact condition for both configurations under all motions. There were no statistically significant differences in the range of motion (ROM) between the 2 instrumentation conditions. The mean values indicated decreased ROM with the novel occipital condyle screw construct in comparison with the standard occipital plate and rod system.

CONCLUSION

Craniocervical stabilization using occipital condyle screws as the sole cephalad fixation point is biomechanically equivalent with regard to the modes tested (ROM and stiffness) to the standard occipital plate construct.

Feasibility of occipital condyle screw placement for occipitocervical fixation: a cadaveric study and description of a novel technique

STUDY DESIGN

Occipital-cervical (OC) stabilization using occipital condyle fixation with a polyaxial screw-rod construct is described.

OBJECTIVES

To describe a novel technique and initial radiographic results for posterior OC fixation using the occipital condyles for cranial fixation.

SUMMARY OF BACKGROUND DATA

Stabilization of the OC junction remains a challenge. Owing to the regional anatomy and the poor occipital bone purchase, multiple attachment points to the occipital bone are required to increase construct rigidity. To address these issues, we propose a novel OC fixation technique using polyaxial occipital condyle screws for cranial purchase.

METHODS

The OC junction was exposed posteriorly in silicone-injected cadaver heads. Polyaxial titanium screws (3.5 mm) were inserted bicortically solely into the occipital condyles; C1 lateral masses and C2 pedicles, or transarticularly through C1-C2, followed by fixation to a 3-mm rod. Drilling was guided by anatomic landmarks and fluoroscopy. Computerized tomography scans were obtained. Condylar screw angles and lengths were analyzed with respect to historical morphometric condyle measurements and with respect to neurovascular structures.

RESULTS

The condylar entry point was 4 to 5-mm lateral to the foramen magnum on the axial plane, and 1 to 2-mm rostral to the atlantooccipital joint. The mean angle of medialization was 17 degrees (range: 12 to 22 degrees). In the sagittal plane, the maximal superior screw angulation was 5 degrees. The mean screw length to obtain bicortical purchase was 22 mm (range: 20 to 24 mm). The hypoglossal canal was uninterrupted during its full course. The jugular bulb, carotid, and vertebral arteries were not injured by condyle screw placement. No fractures were identified.

CONCLUSION

Condyle screws can be placed without injury to neurovascular structures. OC junction fixation using polyaxial occipital condyle screws is feasible and can be considered a salvage technique or an alternative where other fixation techniques are not available.

SURGICAL TREATMENT OF OCCIPITOCERVICAL INSTABILITY

OBJECTIVE

Instability of the occipitocervical junction can be a challenging surgical problem because of the unique anatomic and biomechanical characteristics of this region. We review the causes of instability and the development of surgical techniques to stabilize the occipitocervical junction.

METHODS

Occipitocervical instrumentation has advanced significantly, and modern modular screw-based constructs allow for rigid short-segment fixation of unstable elements while providing the stability needed to achieve successful fusion in nearly 100% of patients. This article reviews the preoperative planning, the variety of instrumentation and surgical strategies, as well as the postoperative care of these patients.

RESULTS

Current constructs use occipital plates that are rigidly fixed to the thick midline keel of the occipital bone, polyaxial screws that can be placed in many different trajectories, and rods that are bent to approximate the acute occipitocervical angle. These modular constructs provide a variety of methods to achieve fixation in the atlantoaxial complex, including transarticular screws or C1 lateral mass screws in combination with C2 pars, C2 pedicle, or C2 translaminar trajectories.

CONCLUSION

Surgical techniques for occipitocervical instrumentation and fusion are technically challenging and require meticulous preoperative planning and a thorough understanding of the regional anatomy, instrumentation, and constructs. Modern screw-based techniques for occipitocervical fusion have established clinical success and demonstrated biomechanical stability, with fusion rates approaching 100%.

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.

Occipitocervical fusion with rigid internal fixation: long-term follow-up data in 69 patients

Object.

Instability of the occipitocervical junction may result from degenerative disease, infection, tumor, and trauma. Surgical stabilization involving screw fixation and rigid implants has been found to be biomechanically superior to wire-based implants. To evaluate the long-term results in a large and diverse patient population, the authors prospectively studied a consecutive group of 69 patients.

Methods.

All patients underwent occipitocervical fusion in which rigid posterior instrumentation included either plates or rods and screws. Patients ranged in age from 11 to 90 years (mean 51.4 years); there were 34 female and 35 male patients. The mean follow-up duration was 37 months (range 6–66 months). Fifty-seven (83%) of the 69 patients had long-standing occipitocervical anomalies, whereas the remainder presented with acute instability. Basilar invagination was present in 20 patients.

Results.

Correction of a severe cervical kyphotic deformity was accomplished in six patients. There were no fatalities or medical complications associated with the procedures. During the follow-up period, 87% of the patients exhibited improvement in their myelopathic symptoms; in 13% the symptoms were unchanged. Complications were minimal. Stability was demonstrated on flexion/extension studies in all cases. There were no treatment-related deaths, although four patients died within the follow-up period, all due to progression of metastatic disease.

Conclusions.

The authors found that rigid internal fixation of the occipitocervical complex was safe, effective, and technically possible for spine surgeons familiar with occipital bone anatomy and lateral mass fixation.

The cervicothoracic junction

The cervicothoracic junction (CTJ) represents a unique region in the spine because of its biomechanical properties. It is predisposed to various traumatic injuries, tumor, and iatrogenic instability. It is also a difficult region to access anteriorly because of the vital structures ventral to the CTJ. The development of new surgical techniques and new instrumentation has allowed better access and fixation to the CTJ.

Neurological and functional outcome after unstable cervicothoracic junction injury treated by posterior reduction and synthesis

BACKGROUND CONTEXT

Unstable lesions of the cervicothoracic junction present a severe clinical problem for diagnosis, treatment, and prognosis.

PURPOSE

The objective of the present study was to evaluate the neurological and functional outcomes following surgical treatments which combine in all cases posterior reduction and stabilization.

STUDY DESIGN

Retrospective clinical and radiological study.

PATIENT SAMPLES

Between September 1996 and September 2003, 30 patients underwent surgery for unstable fracture at the cervicothoracic junction. This group included 23 patients who sustained a motor vehicle accident, 5 who had fallen from a height, 1 case of ballistic trauma, and 1 person injured by diving in shallow water. There were 22 male and 8 female patients aged between 18 and 80, with an average age of 49. In 18 cases the lesion level was vertebra C7, in 5 cases vertebra T1, in 2 cases vertebra T2, and in 5 cases vertebra T3. Neurologically, on initial clinical examination 16 patients were classified Frankel A, 6 Frankel B, 2 Frankel C, and 6 Frankel D. Surgically, all the patients underwent posterior reduction and synthesis. Posterior stabilization was performed using rods and screws 3 times, plate-screw fixation 25 times, and rods and screws at the thoracic level linked to plate-screw at the cervical level 2 times. Spinal cord compression of more than two levels was associated with 25 cases. In these 25 cases, spinal cord decompression was associated with reduction and stabilization.

OUTCOME MEASURES

Clinical outcome using neurological scale of Frankel, radiological outcomes using computed tomographic (CT) scans and plain X-ray evaluations.

METHODS

Follow-up periods ranged from 11 to 48 months, with an average of 18 months. Seven patients died as a result of cardiopulmonary insufficiency within 4 months postoperative. Twenty-eight CT scans with sagittal and frontal slides were examined to evaluate postoperative reduction and to control screw placement.

RESULTS

The observed reductions were satisfactory in 27 cases. In one case, reduction was satisfactory in the sagittal plane but lateral translation persisted in the frontal plane. Two mechanical failures with delayed mobilization of implants occurred. Bony fusion was recorded in all cases on CT scan evaluation. Complete or partial neurological recovery was observed in only 10 of 14 patients. The initial neurological status of these 14 patients was Frankel B, C, or D.

CONCLUSION

The surgical procedure was chosen according to the particularity of the anatomical region and the possibility of associated medullar decompression. Insertion of pedicle screws in the upper thoracic portion in T1, T2, and T3 requires a careful technique and knowledge of the posterior projection points of the pedicles and their orientation in space. The high rate of fusion observed in these patients justified posterior reduction and stabilization. The high death rate and the low rate of neurological recovery in this group of patients emphasizes the severe prognosis of unstable injuries of the cervicothoracic junction. Considering the few mechanical failures observed at the last examination, the choice of the posterior approach was appropriate as the one stage procedure. Plate synthesis is preferable in fractures that do not require extension of synthesis beyond T2, whereas screws and rods systems are more appropriate for superior thoracic injuries. Despite early diagnosis and surgical treatment, the presence of neurological or pulmonary lesions resulted in increased mortality of the operated patients.

The evolution of spinal instrumentation for the management of occipital cervical and cervicothoracic junctional injuries

STUDY DESIGN/SETTING

Independent computer-based literature review of articles pertaining to instrumentation and fusion of junctional injuries of the cervical spine.

OBJECTIVE

To review and discuss the evolution of instrumentation techniques and systems used in the treatment of cervical spine junctional injuries.

SUMMARY OF BACKGROUND DATA

Instrumentation of junctional injuries of the cervical spine has been limited historically by failure to achieve rigid internal fixation in multiple planes. The evolution of these techniques has required increased insight into the morphology and unique biomechanics of the structures to be instrumented.

METHODS

Computer-based literature search of Ovid and PubMed databases.

RESULTS

Extensive literature search yielded insights into the evolution of systems initially based on onlay bone graft combined with wiring techniques. Such techniques have come to include systems incorporating rigid, longitudinal struts that accommodate multiplanar screws placed in the lateral masses, pedicles, transarticular regions, and occipital bone.

CONCLUSIONS

Despite a rapid evolution of techniques and instrumentation technologies, it remains incumbent on the physician to provide the patient with a surgical procedure that balances the likelihood of a favorable outcome with the risk inherent in the implementation of the procedure.

Indications for surgery and stabilization techniques of the occipito-cervical junction

Occipito-cervical (OC) instrumentation and fusion is indicated in traumatic atlanto-occipital dissociation and type III Anderson-Montesano occipital condyle fractures. The goals of surgery are to stabilize the mechanically compromised OC junction, correct deformity or displacement, and decompress compromised neural structures. The goals of instrumentation are to provide immediate stability, improve fusion rate, diminish the need for postoperative external immobilization, and decrease rehabilitation time. To successfully instrument the occipito-cervical spine, a working knowledge of the anatomy of the occipital-cervical junction is imperative. A wide variety of stabilization techniques and instrumentation systems are currently available, each with its own advantages and disadvantages. With familiarity of the constraints and benefits of the available instrumentation systems, the individual fixation needs of a clinical situation can be fulfilled and successful patient outcomes can be achieved.

Factors associated with good outcome using lateral mass plate fixation

The immediate and long-term outcomes of 70 consecutive patients who underwent subaxial lateral mass fixation between June 1996 and June 2001 were reviewed. Intraoperative fluoroscopy and somatosensory evoked potential (SEP) monitoring were used in all patients. Immediate postoperative computed tomography (CT) was performed to determine screw trajectory and placement. Follow-up ranged from 2 to 7 years. Postoperative CT showed 206 (58%) of 356 screws had unicorticate and 42% bicorticate purchase. Furthermore, 96 (27%) screws had suboptimal trajectory, but only 5 of these screws minimally penetrated the foramen transversarium without resultant vascular or neurological sequelae. A sudden unilateral intraoperative SEP amplitude decrease during screw placement in 2 patients resolved with screw removal and alteration of screw trajectory. The overall fusion success rate was 91.5% and screw pull-out developed in 2 patients. The recommended drilling technique and trajectory (15 degrees - 25 degrees rostral in the sagittal plane, 20 degrees - 30 degrees lateral in the axial plane), supplemented bone grafting, and intraoperative SEP monitoring are all associated with good screw placement, fusion, and neurological outcome and are recommended for all lateral mass fusion procedures.

Biomechanical analysis of transpedicular screw fixation in the subaxial cervical spine

STUDY DESIGN

An in vitro biomechanical study to compare 2 different dorsal screw fixation techniques in the cervical spine with respect to primary stability and stability after cyclic loading.

OBJECTIVES

To investigate if the biomechanical stability is better in pedicle screw or in lateral mass fixation.

SUMMARY OF BACKGROUND DATA

In patients with poor bone quality who require multisegmental fixations, the current dorsal stabilization procedures in the subaxial cervical spine using lateral mass screws are often insufficient. Cervical pedicle screw fixation has been suggested as an alternative procedure, but there are still limited data available on the biomechanical differences between pedicle screw and lateral mass fixation.

METHODS

A severe multilevel discoligamentous instability was created in 8 human cervical spine specimens (C2-C7). Dorsal stabilization was performed with the assistance of computer navigation (SurgiGate, Medivison, Switzerland) using either lateral mass or pedicle screw fixation. In the first part of the study, primary stability was measured by means of a multidirectional flexibility test. Then, specimens were divided into 2 groups, randomized for bone mineral density. Cyclic loading was applied with sinusoidal loads in flexion/extension (1000 cycles, +/-1.5 Nm, 0.1 Hz). Mechanical behavior of the specimens was determined by a flexibility test before and after the application of cyclic loads. Data analysis was performed by calculating the ranges of motion, and statistical differences were determined with the t test for group comparison.

RESULTS

Pedicle screw fixation showed a significantly higher stability in lateral bending (pedicle screw range of motion 0.86 +/- 0.31 degrees; lateral mass range of motion 1.43 +/- 0.62 degrees; P = 0.037). No significant differences were seen in flexion/extension and axial rotation. After cyclic loading, the decrease in stability was less with pedicle screw fixation in all load directions. Differences in the decrease of stability were statistically significant in flexion/extension (pedicle screw 95.4 +/- 9.4%; lateral mass 70.5 +/- 9.8%; P = 0.010) and lateral bending (pedicle screw 105.3 +/- 5.0%; lateral mass 84.2 +/- 13.6%; P = 0.046), whereas there was no significant difference in axial rotation.

CONCLUSIONS

The major finding of the current study was the higher stability of pedicle screws over lateral mass fixation with respect to primary stability and stability after cyclic loading. From a biomechanical point of view the use of pedicle screws in the subaxial cervical spine seems justified in patients with poor bone quality and need for multisegmental fixation.

Posterior cervicothoracic instrumentation in spine tumors

STUDY DESIGN

We retrospectively review 32 patients who underwent posterior fixation for cervicothoracic junctional tumors. All patients possessed unstable or potential after surgery unstable spines as a result of either their tumors or the surgery performed. We examined cervicothoracic spine stability, maintenance of alignment, and associated complications.

OBJECTIVES

To review our experience with 3 different posterior osteosynthesis systems applied to the cervico-thoracic junction for spinal tumors. Our review includes surgical outcomes and complications. The evolution through 3 different systems between 1994 and 1997 reflects our attempts to improve accuracy in light of variable facet and pedicle interspaces. Our goal is not to compare the efficacy of the systems but to assess the efficiency of cervicothoracic facet and transpedicular screw and plate or rod fixation. However, we will comment on why the evolution occurred. The 3 different systems share a similar characteristic. Each system employs posterior cervical facet screw fixation and thoracic trans-pedicular screw fixation.

SUMMARY OF BACKGROUND DATA

Spinal disorders involving the cervicothoracic junction and specific instrumentation to this region have been sparsely described in the literature.

METHODS

Between June 1994 and June 2000, 32 patients underwent surgery for spinal tumors involving the cervicothoracic junction at our institution. There were 27 males and 5 females. The ages ranged from 17 to 72 years with a mean age of 52 years. A total of 32 cervicothoracic instrumentations were performed. We used the R. Roy-Camille thoracolumbar plate in 20 patients, the cervico-thoracic plate in 8, and the Agora rod system in 4. In all, 96 lateral mass screws were implanted from C4 to C6, 54 into C7, and 180 pedicle screws from T1 to T8. Nineteen patients had lung cancer with vertebral body invasion (Pancoast tumors), 11 had metastasis to the cervicothoracic junction, 1 had a chondrosarcoma, and 1 had myeloma. In a first group consisting of 19 patients, a combination of anterior and extended posterior surgical approaches allowed complete en bloc resection of the tumors, including all invaded vertebrae. Four total vertebrectomies and 15 partial vertebrectomies were performed. A second group of 13 patients had only posterior palliative stabilizing procedures with laminectomy and cervicothoracic fixation.

RESULTS

The follow-up period varied from 3 to 54 months, average 15 months. The average duration of survival for patients who underwent partial or total vertebrectomy was 16 months (range 3-54 months). The average duration of survival for patients who underwent palliative decompression and stabilization was 11 months (range 5-19 months). No changes in the sagittal alignment occurred during the immediate postoperative period for 30 patients. However, 2 mechanical failures occurred. Two patients experienced a clinically significant early increase in thoracic kyphosis and required revision of the posterior instrumentation. A 21-month minimum follow-up was available for 6 patients, in whom all implants were stable. We noted no screw, plate, or rod breakage in this series. No neurologic complications, including root impingement or spinal cord injury, or vertebral artery injury occurred related to screw insertion into either the thoracic pedicles (180 screws) or the cervical lateral masses (96 screws in C4-C5-C6 and 54 screws in C7).

CONCLUSIONS

Posterior plate or rod and screw fixation is a good method of treatment for cervicothoracic instability in spine tumors. Facet screw fixation in the cervical spine with Roy-Camille drilling technique and transpedicular screw fixation in the thoracic spine provides an efficacious means by which to stabilize the cervicothoracic junction. This stabilization technique was effective even in cases of high postoperative instability, such as with partial or total vertebrectomy. This screw-type stabilization is clinically effective and well documented. The evolution through 3 different systems reflects our attempts to improve accuracy in light of variable facet and pedicle interspaces. Importantly, posterior instrumentation will not interfere with subsequent laminectomy or more extreme surgical procedures.

Two asymmetric contoured plate-rods for occipito-cervical fusion

The author presents a retrospective clinical study addressing the outcome after posterior stabilisation of the occipital-cervical spine using a new cranio-spinal implant. The range of surgical methods for operative treatment of occipito-cervical instability remains wide, and it is still a demanding technique that frequently requires improvisation by the surgeon. No previous studies have been published of occipito-cervical reconstructions using two contoured asymmetrical occipital plates interdigitating in the midline at the occiput and allowing various methods of cervical fixation, by means of different hooks, a claw device or screws. Nine patients with severe occipito-cervical instability and/or subaxial malalignment underwent reconstructive surgery with the new implants between 1998 and 2001. Seven patients suffered from rheumatoid arthritis (RA) including cranial settling. Two patients had widespread cervical metastases. All patients suffering RA were treated by preoperative cervical traction for up to 28 days, and intraoperative traction, to try to restore the malalignment. Traction was also used, to diminish pain and to improve neurological symptoms. The lowest vertebra fused was T3. All patients were immobilised with an external orthosis or brace for 6 weeks or 3 months. A solid fusion was achieved in all patients. None of the patients deteriorated postoperatively. No serious complications occurred. One occipital screw broke and one hook loosened, needing a re-fixation. The simplicity of applying these cranio-cervical implants makes them practical for every orthopaedic or neurosurgeon with a special interest in cervical spine surgery.

Stability analysis of craniovertebral junction fixation techniques

BACKGROUND

Craniovertebral arthrodesis in the upper cervical spine is challenging because of the high degree of mobility afforded by this region. A novel method for achieving atlantoaxial fixation with use of polyaxial screws inserted bilaterally into the lateral masses of C1 and transpedicularly into C2 with longitudinal rod connection has recently been introduced. The question remains as to whether this technique provides adequate stability when extended cephalad to include the occiput. The purpose of this study was to determine the primary stability afforded by this novel construct and compare its stability with the current standard of bilateral longitudinal plates combined with C1-C2 transarticular screws.

METHODS

We used ten fresh-frozen human cadaveric cervical spines (C0-C4). Pure moment loads were applied to the occiput, and C4 was constrained during the testing protocol. We evaluated four conditions: (1) intact, (2) destabilized by means of complete odontoidectomy, (3) stabilization with longitudinal plates with C1-C2 transarticular screw fixation, and (4) stabilization with a posterior rod system with C1 lateral mass screws and C2 pedicle screws. Rigid-body three-dimensional rotations were detected by stereophotogrammetry by means of a three-camera system with use of marker triads. The range of motion data (C0-C2) for each fixation scenario was calculated, and a statistical analysis was performed.

RESULTS

Destabilization of the specimen significantly increased C0-C2 motion in both flexion-extension and lateral bending (p < 0.05). Both fixation constructs significantly reduced motion in the destabilized spine by over 90% for all motions tested (p < 0.05). No significant differences were detected between the two constructs in any of the three rotational planes.

CONCLUSIONS

Both hardware systems provide equivalent construct stability in the immediate postoperative period when it is critical for the eventual success of a craniovertebral arthrodesis. On the basis of this work, we believe that the decision to use either construct should be determined by clinical rather than biomechanical concerns.

History of instrumentation for stabilization of the subaxial cervical spine

In the past several decades methods have been developed to stabilize the subaxial cervical spine both posteriorly and anteriorly. Methods of posterior stabilization have progressed from interspinous wiring, through facet wiring and sublaminar wiring, to the lateral mass screws with plates and rods that are in use today. Plates for anterior stabilization have evolved from rigid plates requiring bicortical screws through those used with unicortical locking screws, to dynamic load-sharing plates used with variable angle screws. The original description of spinous process wiring was published by Hadra in 1891. In 1942 Rogers described the interspinous wiring method used for trauma-induced cervical instability, which was modified by Bohlman in 1985 (triple wiring technique). Luque rods with sublaminar wires were introduced in the late 1970s to address multilevel and occipitocervical instability. Facet wiring was developed in 1977 by Callahan to address the problem of stabilization when laminae are not present. Wiring remained the method used until Roy-Camille introduced the lateral mass screw-plate construct in the 1980s. The first plate for anterior stabilization was designed by Orozco and Llovet in 1970 and was later refined by Caspar; this was a rigid plate with bicortical screws. Morscher devised unicortical locking screws in the 1980s. The latest concept of dynamic load-sharing plates with variable angle screws was developed in 2000. In this article historical landmarks in surgical methods for the stabilization of the subaxial cervical spine are reviewed.

Evolution of posterior cervical and occipitocervical fusion and instrumentation

The past several decades have been the setting for a remarkable evolution of spinal instrumentation technology. The advancements that have been made have allowed previously complex disorders of the cervical spine, the atlantoaxial articulation, and the occipitocervical junction to be managed more effectively with direct methods of internal fixation and arthrodesis. This has resulted in improvements in patient outcomes and fusion success rates. The improved strength of instrumentation constructs allows minimal, if any, external bracing, obviating the need for a halo orthosis in many cases. In this paper the authors review key events that have occurred in neuroimaging, biomechanical testing, and the development of fusion and instrumentation constructs.

Biomechanical comparison of expandable cages for vertebral body replacement in the cervical spine

OBJECT

Recently, expandable cages for vertebral body replacement in the cervical spine have been developed. The purpose of this study was to compare the biomechanical properties of expandable cages with those of a tricortical iliac crest graft and a nonexpandable cage.

METHODS

Forty human cervical spines (C3-5) were tested in flexion, extension, axial rotation, and lateral bending. First all motion segments were evaluated intact. After corpectomy of C4 the spines were divided into five groups of eight and the following stabilization techniques were used: 1) autologous iliac crest bone graft; 2) mesh titanium cage; 3) anterior distraction device; 4) Synex-C titanium; and 5) Synex-C PEEK. Additionally, anterior plating and anterior plating plus posterior screw/rod fixation were applied. Stiffness, range of motion, and neutral and elastic zones were determined. In comparison with the intact motion segment all implants significantly increased stiffness in flexion and bending, but decreased stiffness in extension. There were no biomechanical differences between the nonexpandable and expandable cages. Furthermore, there were no biomechanical differences between the tricortical iliac crest graft and the cages, except for Synex-C in rotation. Additional anterior plating significantly increased biomechanical stiffness in all test modes; particularly in rotation mode, combined anterior-posterior stabilization increased stiffness by up to 102% compared with anterior plating alone.

CONCLUSIONS

In comparison to a tricortical iliac crest bone graft and a nonexpandable cage, expandable cages have no biomechanical advantages. Due to the low extension and rotational stiffness, none of the implants can be recommended as a stand-alone device. Additional anterior plating increased biomechanical stability adequately. Therefore, additional posterior stabilization should only be considered in cases of severe rotational instability of the cervical spine.

Biomechanical evaluation of new posterior occipitocervical instrumentation system

Twelve fresh-frozen cadaveric occipitocervical specimens were randomized based on dual energy xray absorptiometry analysis of bone mineral density. The specimens were subjected to physiologic loads in a device that applied pure unconstrained flexion and extension, lateral bending, and axial rotational moments. The spines were tested intact and after major injury simulating transoral decompression of the dens. Biomechanical testing using pure moments with physiologic loads (< 1.5 N-m) was used to compare stability of posterior occipitocervical plates and screws, loop and cable construct, and new cervical rod and screw system. The injury created significantly less stiffness and greater range of motion and neutral zone at C1-C2 in flexion and extension and lateral bending and greater range of motion and neutral zone in axial rotation than the intact state. In lateral bending, the new rod construct had significantly lower mean values for range of motion than the loop and the plate construct. In axial rotation, the rod construct had a significantly higher mean value for stiffness than the other two devices and a significantly lower mean value for range of motion than the loop. The new rod-based instrumentation system for occipitocervical fixation is biomechanically equivalent or superior to a plate and screw construct and a rod and cable system.

Development of a model for occipital fixation--validation of an analogue bone material

Several implant systems may be used to fuse the skull to the upper cervical spine (occipitocervical fusion). Current biomechanical evaluation is restricted by the limitations of human cadaveric specimens. This paper describes the design and validation of a synthetic testing model of the occipital bone. Data from thickness measurement and pull-out strength testing of a series of human cadaveric skulls was used in the design of a high-density rigid polyurethane foam model. The synthetic occipital model demonstrated repeatable and consistent morphological and biomechanical properties. The model provides a standardized environment for evaluation of occipital implants.

Biomechanical evaluation of occipital fixation

Many studies have shown a positive correlation among screw pullout strength, screw insertional torque, bone thickness, and areal bone mineral density (BMD) measured by dual-energy X-ray absorptiometry. Variations are significant in the anatomy of the occipital bone. But no studies have correlated these variables with respect to the two locations commonly used for plate fixation to the occiput. The purpose of this study was to determine the thickness and quality of the occipital bone and to correlate these variables with the insertional torque of screws and the pullout strength of plates secured into two different locations on the occiput. The occiputs of 12 adult human fresh frozen cadaveric specimens were used. The specimens were analyzed by dual-energy X-ray absorptiometry. Direct thickness measurements of the occiput were performed. Areal and volumetric BMD were measured. A simple pelvic reconstruction plate (3.2 mm) was fixed to the occiput either laterally or at the midline with bicortical 4-mm cancellous screws. Torque was recorded at the time of insertion of each screw. Axial pullout tests were performed on all specimens. The peak load, failure load, stiffness, and energy to failure were recorded for each construct. Statistical analysis showed that the average thickness of occipital bone is greater in the midline than laterally. Occipital bone is thicker and screw torque is greater close to the inion. There is a positive correlation between bone thickness, areal BMD as measured by dual-energy X-ray absorptiometry, screw insertional torque, and strength of fixation. A plate fixed in the midline region of the occiput provides more rigid fixation than a plate fixed laterally. Areal BMD correlates better than volumetric BMD with bone thickness and is a reliable predictor of the strength of occipital fixation.

Cervical deformity: rationale for selecting the appropriate fusion technique (anterior, posterior, and 360 degree)

This article examines cervical deformities and their treatments, such as iatrogenic deformities, posttraumatic deformities, ankylosing spondylitis, rheumatoid arthritis, degenerative subaxial spondylolisthesis, myopathy, infectious spondylitis, and tumors. Congenital scoliosis and kyphosis and torticollis and rotatory atlanto-axial subluxation also are discussed.