RESULTADOS DO USO DE CAGE EM PEEK NO TRATAMENTO DA INVAGINAÇÃO BASILAR PELA TÉCNICA DE GOEL

Objetivo: Analise do uso do implante tipo cage em poli-eter-eter-cetona (PEEK) no realinhamento e distracao facetaria atlantoaxial da invaginacao basilar pela tecnica de Goel. Metodo: Analise estatistica descritiva retrospectiva de estado neurologico, dor, presenca de fusao ossea e subsidencia com o uso do cage em PEEK em 8 articulacoes atlantoaxiais de 4 pacientes portadores de invaginacao basilar, todos tratados com distracao, realinhamento atlantoaxial e artrodese posterior C1-C2 pela tecnica de Goel, modificada pela utilizacao do cage em PEEK. Resultados: Todos os pacientes apresentaram melhora na escala de avaliacao neurologica de Nurick e na Escala Visual Analogica (EVA) de dor. Nao ocorreu caso de subsidencia, migracao ou dano a arteria vertebral decorrente da colocacao do cage. Cem por cento das articulacoes evoluiram com fusao ossea, avaliada por radiografia dinâmica e tomografia computadorizada. Dois pacientes evoluiram com dor neuropatica no dermatomo de C2 e em um paciente houve lesao unilateral da arteria vertebral durante a instrumentacao de C2, tratada com insercao do parafuso pedicular para controle do sangramento. Conclusao: Os resultados da reducao vertical da invaginacao basilar pela tecnica de Goel com a utilizacao de cage em PEEK mostrou ser eficaz e segura, porem ainda sao necessarios estudos para confirmar essa utilizacao.


INTRODUCTION
Pathologies of the craniocervical junction represent a great challenge for spine surgeons, due to the anatomical singularity of the region, which contains extremely important neurovascular structures, as well as the complex visualization and interpretation of the osteoligamentary structures and biomechanical peculiarities of this segment of the cervical spine.Numerous pathologies can lead to degeneration of the craniocervical junction, such as congenital malformations, trauma, metabolic diseases, rheumatoid arthritis, and primary osteoarthritis, among others.Congenital basilar invagination is a rare abnormality in which the odontoid process is in a higher position than normal, projecting into the interior of the foramen magnum, which can cause neurological symptoms, limiting the space available for the brainstem and spinal cord.In patients with classic basilar invagination, classified by Goel as group A, there appears to be significant atlantoaxial instability, and the respective facets have an oblique arrangement, instead of their usual horizontal orientation.It is considered that the atlantoaxial joint is "active", leading to cranial migration and flexion of the odontoid process. 1 A wide range of surgical techniques has been described for decompression and stabilization of the craniocervical junction in patients with basilar invagination.Traditionally, the proposed treatment in symptomatic cases is transoral resection of the dens of the axis, with posterior stabilization through occipitocervical fusion. 2 In recent years, the technique described by Goel et al. 1 has gained increasing importance in the treatment of this pathology.This technique consists of indirect decompression of the foramen magnum by reducing the odontoid process through the opening of the joints of the lateral masses of C1 and C2, and stabilizing these through instrumentation with lateral mass screws in the atlas, and pedicle screws in the axis..4 Distraction of the facet joints and decortication of their joint surfaces, besides promoting reduction of the atlantoaxial joint, provide an additional bed for the bone fusion.(Figure 1) Originally, the technique of Goel used a stainless steel cage to maintain the craniovertebral realignment, together with autologous graft of the posterior iliac crest. 1 In our cases, we used a polyetheretherketone (PEEK) cage, as we believe it has some theoretical advantages: lower subsidence rate, higher consolidation rate, and better visualization in imaging examinations during postoperative follow-up.The aim of this work is to carry out a retrospective clinical and radiological examination of the results of atlantoaxial facet distraction using PEEK cage in the treatment of basilar invagination.

MATeRIAlS AND MeThODS
This study is a retrospective analysis of 8 cages in four patients with Goel Group A basilar invagination.All the patients were submitted to indirect decompression by atlantoaxial facet distraction and posterior fusion, using the technique of Goel. 1 As a technical modification, we used a PEEK cage as intraarticular C1-C2 spacer, and polyaxial screws and titanium rods for the monosegment fusion.All the patients were operated by the same surgical team during the years 2013 and 2014, and all signed the informed consent form.In all patients, the Visual Analog Scale (VAS) 5 and the Nurick myelopathy scale 6 were adopted as postoperative evaluation.These scales were also used in the postoperative follow-up.
As this is purely a review article of patient records and description of a new surgical technique, it did not require submission to the ethics committee.
Three patients were female and one was male.The mean age was 50.25 ± 8.01 years (39 to 58 years); the mean follow-up time of these patients was 16 ± 6.48 months (9 to 22 months).The patients were followed up on an outpatient basis, with evaluation at 3, 6 and 9 months after surgery through a neurological examination and control computed tomography at 6 months after surgery.Descriptive statistical analysis was carried out using the software STATA 11.1 (StataCorp, Texas, USA), considering the nominal and numerical variables.

Surgical Technique
The patient was positioned in ventral decubitus, with the head of the table elevated degrees after installation of cranial traction with 1/8 of body weight.Longitudinal access was performed along the midline, with subperiosteal exposure of the craniocervical junction.The ganglion of C2 was identified and sectioned bilaterally to allow wide exposure of the atlantoaxial facet joints.In many cases of basilar invagination, it is common to find malformations, such as occipitalization of the atlas and more rostral localization of the facet of C1, which complicates the procedure.The atlantoaxial facet joint capsules are excised, and the joint surfaces are exposed.In order to reduce the invagination through facet distraction, progressively larger osteotomies are introduced and turned 90 degrees, to provide sufficient space for the introduction of the tests using PEEK cages of 5x11x14mm or 5x12.5x14mmused in conventional subaxial cervical fusion surgeries 7 containing spongy bone taken from the patient's posterior iliac crest.(Figure 2)..9 The surgical wound is closed by planes, without drainage, and the patient is advised to use a Philadelphia type collar for 12 weeks.

ReSUlTS
There was no case of infection of the surgical wound, worsening of the deficit, or death.All patients improved by at least one degree in the neurological evaluation by the Nurick scale. 6he evaluation of the position of the odontoid process before and

A B
after surgery showed satisfactory reduction.There was no case of subsidence, migration, or damage to the vertebral artery when placing the cage in all eight joints of four patients operated on.
(Table 1) Dynamic radiographs of the craniocervical junction and computed tomography showed bone fusion in 100% of the joints.
The criteria for fusion that we used was the presence of a bone bridge in coronal and/or sagittal sections, and the absence of mobility in dynamic radiographs performed 6 months after surgery.(Figure 3) As a complication, two patients had intense neuropathic pain in the dermatome of C2, which persisted for 3 months in one case and 15 days in another.After this period, there was complete resolution of pain, with persistence only of local hypoesthesia.In one case, there was unilateral lesion of the vertebral artery during the instrumentation of C2, which was treated with insertion of a pedicle screw to control the bleeding.The patient awakened and remained for 3 months without neurological changes related to the lesion of the vertebral artery, and after this period, developed unilateral amaurosis ipsilateral to the lesion of the vertebral artery.However, magnetic angioresonance showed a pervious vertebrobasilar system, ruling out the lesion of the artery as a causal factor.The patient remains under etiological investigation.

DISCUSSION
The projection of the odontoid process into the foramen magnum above the lines of McRae 10 , according to Chamberlain 11 confirms the presence of basilar invagination.In the presence of symptoms, the treatment traditionally proposed is transoral resection of the odontoid process, associated with posterior occiptocervical fusion.However, this procedure has well-established risks, the most feared one being the occurrence infection following transoral decompression, 3 added to the high morbidity of occiptocervical fusion, which promotes a significant loss of flexion-extension and lateral rotation of the head and neck. 12,13Successful occipitocervical fusion has been reported by some authors. 14.2 The use of shorter constructions with smaller rods and longer bolts in C1 and C2 provides biomechanical advantages in relation to longer instrumentations and smaller screws involving the occiput, which is much thinner. 8n recent years, the classification of basilar invagination in Group A or Group B, and the technique of atlantoaxial facet realignment and distraction proposal by Goel et al. 1 has gained increasing numbers of followers.The technique spares the patient the morbidity associated with the transoral approach, and performing a monosegment fusion by the posterior approach minimizes the loss of range of movement of the cervical spine, while at the same time, provides sufficient biomechanical stability to the craniocervical junction. 15,16longside the advantages already mentioned, exposure of the facet joints of C1 and C3 promotes additional bone bed for the consolidation of the fusion, and maintenance of indirect decompression and reduction of the odontoid process.The original description of this technique uses stainless steel cages, but we believe in the theoretical advantages of PEEK cage, therefore we decided to use it.
Although the scientific literature on the use of the PEEK cage in the craniocervical junction is scarce, 15,16 by analogy with its use in conventional anterior arthrodesis of the subaxial cervical spine, we can believe in some potential advantages of this material in surgery of the craniocervical joint, which is what prompted us to use the PEEK cage.In the initial cases performed by the technique of Goel, a Harms basket type cage was used.However, one patient in this first series, after a year of improvement in neurological symptoms, evolved with reappearance of the symptoms.The patient's computed tomography showed significant subsidence, and the patient was submitted to review by transoral decompression. 17In anterior intersomatic cervical fusion surgeries, PEEK cages presented some advantages over metal cages.It is known that onset of subsidence is related to the preparation of the terminal plates, the size of the contact surface of the implant, the amount of distraction applied during surgery, and the quality of the patient's bone. 18,19owever, the most important variable related to the presence of subsidence is the material from which the cage is manufactured. 20he elasticity module of the PEEK cage is similar to that of bone, a characteristic that is attributed to the lower rate of subsidence than metallic cages, and the lower likelihood of stress shielding.A frame cage design enables a large amount of bone graft to be installed in its interior, and is another factor that can assist in bone fusion, 21 facts that were observed in our cases.Another special feature of cage in PEEK is the higher radioluminescence of the material and the generation of less artifact, which facilitates the confirmation of bone consolidation in plain radiographs and control computed tomography scans during the follow-up of patients. 21Unlike the stainless steel cage initially used by Goel 22 , the PEEK cage is compatible with the performance of postoperative magnetic resonance imaging, which enables the evaluation of syringomyelia, Chiari malformation, and changes in spinal cord signal, abnormalities commonly associated with basilar invagination.This is another potential advantage of the use of a PEEK cage over metallic cages.(Figure 4).

A B
The occurrence of neuropathic pain in 50% of our patients, which we attributed to the resection of the C2 ganglion for visualization and exposure of the atlantoaxial joints, was a relevant clinical complication, and does not coincide with the proportion of this complication in other, larger series described in literature. 16,23n the other hand, our small series does not allow us to establish the real incidence of this complication.

CONClUSION
Vertical reduction of the basilar invagination by the Goel technique with the use of a PEEK cage proved to be safe and effective, allowing adequate postoperative follow up with imaging exams.Long-term multicenter studies are needed to confirm this.
All authors declare no potential conflict of interest concerning this article.

Figure 1 .Figure 2 .
Figure 1.Sagittal section of computed tomography before (A) and after (b) indirect decompression by atlantoaxial facet distraction with PEEK cage.The lines of McGregor, Chamberlaim and McRae prove caudal migration and realignment of the odontoid process.A B

Figure 3 .
Figure 3. Computed tomography 6 months after surgery confirms consolidation with the presence of a bilateral bone bridge and adequate positioning of the PEEK cage in the atlantoaxial interfacet space.Coronal section (A) and (B), sagittal section (C).

Table 1 .
Clinical and radiological results.