AGK-BRAF is associated with distant metastasis and younger age in pediatric papillary thyroid carcinoma.

BACKGROUND: The incidence of thyroid carcinoma has increased in most populations, including pediatric patients. The increase is almost exclusively due to an increase in the incidence of papillary thyroid carcinoma (PTC). Genetic alterations leading to mitogen-activated protein kinase (MAPK) pathway activation are highly prevalent in PTC, with BRAF V600E mutation being the most common event in adult PTC. Although a lower prevalence of BRAF V600E had been reported among pediatric patients, a higher prevalence of BRAF fusion has been identified in both radiation-exposed and sporadic pediatric PTC. However, little is known about the prognostic implications of BRAF fusions in pediatric PTC. PROCEDURE: In this study, we investigated the prevalence of BRAF alterations (AGK-BRAF fusion and BRAF V600E mutation) in a large set of predominantly sporadic pediatric PTC cases and correlate with clinicopathological features. Somatic AGK-BRAF fusion was investigated by RT-PCR and confirmed by FISH break-apart. The BRAF V600E mutation was screened using Sanger sequencing. RESULTS: AGK-BRAF fusion, found in 19% of pediatric PTC patients, was associated with distant metastasis and younger age. Conversely, the BRAF V600E, found in 15% of pediatric PTC patients, was correlated with older age and larger tumor size. CONCLUSION: Collectively, our results advance knowledge concerning genetic bases of pediatric thyroid carcinoma, with potential implications for diagnosis, prognosis, and therapeutic approaches.

Effects of Glycine on Collagen, PDGF, and EGF Expression in Model of Oral Mucositis.

Oral mucositis is frequently a toxic effect of chemotherapeutic and/or radiotherapeutic treatment, resulting from complex multifaceted biological events involving DNA damage. The clinical manifestations have a negative impact on the life quality of cancer patients. Preventive measures and curative treatment of mucositis are still not well established. The glycine has anti-inflammatory, immunomodulatory, and cytoprotective actions, being a potential therapeutic in mucositis. The objective was to evaluate the effects of glycine on the expression of collagen and growth factors, platelet and epidermal in a hamster model oral mucositis. The mucositis was induced by the protocol of Sonis. There were 40 hamsters used, divided into two groups: Group I-control; Group II-supplemented with 5% intraperitoneal glycine, 2.0 mg/g diluted in hepes. Histopathological sections were used to perform the immune-histochemical method, the evaluation of collagen expression, and the growth factors: Epidermal growth factor (EGF) and platelet (PDGF). It was observed that the group supplemented with glycine experienced higher amounts of collagen expression and predominance type of collagen I. The glycine group presented lower immunoexpression of the growth factors, EGF and PDGF. The group supplemented with glycine showed a marked healing process of the oral mucosite, demonstrated by the predominance of collagen type I and reduction of growth factors, EGF and PDGF.

Aspectos clínicos, epidemiológicos e sobrevida de crianças com neuroblastoma: 21 anos de experiência do instituto de oncologia pediátrica, são paulo / Clinical and epidemiological characteristics and survival outcomes of children with neuroblastoma: 21 years of experience at the instituto de oncologia pediátrica, in são paulo, brazil

RESUMO Objetivo: Descrever as características clínicas e epidemiológicas e a sobrevida de crianças com neuroblastoma (NB) atendidas em centro de oncologia pediátrica, no período de 1991 a 2012. Métodos: Estudo retrospectivo, com análise de dados clínicos e epidemiológicos de 258 pacientes com NB atendidos em centro de oncologia pediátrica, no período de 1991 a 2012, por meio de consulta a prontuários médicos. Resultados: A idade média das crianças foi de 40,5±46,4 meses, e a mediana, de 28,9 meses (intervalo interquartil 42,2); relação masculino:feminino 1,3:1, sendo 1% dos pacientes assintomáticos. As manifestações mais frequentes foram: febre (25%), dor abdominal (22%), massa abdominal (19%) e dor óssea (19%). O tempo médio do início dos sintomas até a realização do diagnóstico foi de 3,0±4,8 meses. A localização do tumor mais frequente foi o abdome (63%). As metástases ocorreram na medula óssea (37%) e nos ossos (33%). A sobrevida global (SG) e a sobrevida livre de eventos (SLE) em cinco anos foram de 62 e 52%, respectivamente. A principal causa de óbito foi a progressão da doença (72%). Conclusões: As características clínicas das crianças com NB são variáveis e, em sua maioria, inespecíficas, o que torna o reconhecimento clínico difícil e, em geral, tardio. Em crianças com idade inferior a 5 anos, massa abdominal e/ou dor óssea, irritabilidade e febre de origem indeterminada, o diagnóstico de NB deve ser considerado.

ABSTRACT Objective: To describe the clinical and epidemiological characteristics and survival outcomes of children with neuroblastoma (NB) treated at a pediatric oncology center from 1991 to 2012. Methods: A retrospective study with clinical and epidemiological data from 258 patients with neuroblastoma treated at a pediatric oncology center from 1991 to 2012, using medical records. Results: The average age of the children at diagnosis was 40.5±46.4 months with a median age of 28.9 months (interquartile range 42.2). The male:female ratio was 1.3:1, and 1% of the patients were asymptomatic. The most frequent manifestations were: fever (25%), abdominal pain (22%), abdominal mass (19%), and bone pain (19%). The mean time from symptom onset to diagnosis was 3.0±4.8 months. The most common location of the tumor was the abdomen (63%). Metastases occurred in the bone marrow (37%) and in the bone (33%). Overall survival (OS) and event-free survival (EFS) in five years were 62 and 52%, respectively. The main cause of death was the progression of the disease (72%). Conclusions: The clinical features of children with neuroblastoma are variable and mostly nonspecific, which makes clinical recognition difficult and, in general, too late. In children less than 5 years old, with an abdominal mass and/or bone pain, irritability, and a fever from an unknown cause, neuroblastoma should be considered as a possible diagnosis.

CLINICAL AND EPIDEMIOLOGICAL CHARACTERISTICS AND SURVIVAL OUTCOMES OF CHILDREN WITH NEUROBLASTOMA: 21 YEARS OF EXPERIENCE AT THE INSTITUTO DE ONCOLOGIA PEDIÁTRICA, IN SÃO PAULO, BRAZIL. / ASPECTOS CLÍNICOS, EPIDEMIOLÓGICOS E SOBREVIDA DE CRIANÇAS COM NEUROBLASTOMA: 21 ANOS DE EXPERIÊNCIA DO INSTITUTO DE ONCOLOGIA PEDIÁTRICA, SÃO PAULO.

OBJECTIVE: To describe the clinical and epidemiological characteristics and survival outcomes of children with neuroblastoma (NB) treated at a pediatric oncology center from 1991 to 2012. METHODS: A retrospective study with clinical and epidemiological data from 258 patients with neuroblastoma treated at a pediatric oncology center from 1991 to 2012, using medical records. RESULTS: The average age of the children at diagnosis was 40.5±46.4 months with a median age of 28.9 months (interquartile range 42.2). The male:female ratio was 1.3:1, and 1% of the patients were asymptomatic. The most frequent manifestations were: fever (25%), abdominal pain (22%), abdominal mass (19%), and bone pain (19%). The mean time from symptom onset to diagnosis was 3.0±4.8 months. The most common location of the tumor was the abdomen (63%). Metastases occurred in the bone marrow (37%) and in the bone (33%). Overall survival (OS) and event-free survival (EFS) in five years were 62 and 52%, respectively. The main cause of death was the progression of the disease (72%). CONCLUSIONS: The clinical features of children with neuroblastoma are variable and mostly nonspecific, which makes clinical recognition difficult and, in general, too late. In children less than 5 years old, with an abdominal mass and/or bone pain, irritability, and a fever from an unknown cause, neuroblastoma should be considered as a possible diagnosis.

OBJETIVO: Descrever as características clínicas e epidemiológicas e a sobrevida de crianças com neuroblastoma (NB) atendidas em centro de oncologia pediátrica, no período de 1991 a 2012. MÉTODOS: Estudo retrospectivo, com análise de dados clínicos e epidemiológicos de 258 pacientes com NB atendidos em centro de oncologia pediátrica, no período de 1991 a 2012, por meio de consulta a prontuários médicos. RESULTADOS: A idade média das crianças foi de 40,5±46,4 meses, e a mediana, de 28,9 meses (intervalo interquartil 42,2); relação masculino:feminino 1,3:1, sendo 1% dos pacientes assintomáticos. As manifestações mais frequentes foram: febre (25%), dor abdominal (22%), massa abdominal (19%) e dor óssea (19%). O tempo médio do início dos sintomas até a realização do diagnóstico foi de 3,0±4,8 meses. A localização do tumor mais frequente foi o abdome (63%). As metástases ocorreram na medula óssea (37%) e nos ossos (33%). A sobrevida global (SG) e a sobrevida livre de eventos (SLE) em cinco anos foram de 62 e 52%, respectivamente. A principal causa de óbito foi a progressão da doença (72%). CONCLUSÕES: As características clínicas das crianças com NB são variáveis e, em sua maioria, inespecíficas, o que torna o reconhecimento clínico difícil e, em geral, tardio. Em crianças com idade inferior a 5 anos, massa abdominal e/ou dor óssea, irritabilidade e febre de origem indeterminada, o diagnóstico de NB deve ser considerado.

Tissue Microarray Analysis Applied to Bone Diagenesis.

Taphonomic processes affecting bone post mortem are important in forensic, archaeological and palaeontological investigations. In this study, the application of tissue microarray (TMA) analysis to a sample of femoral bone specimens from 20 exhumed individuals of known period of burial and age at death is described. TMA allows multiplexing of subsamples, permitting standardized comparative analysis of adjacent sections in 3-D and of representative cross-sections of a large number of specimens. Standard hematoxylin and eosin, periodic acid-Schiff and silver methenamine, and picrosirius red staining, and CD31 and CD34 immunohistochemistry were applied to TMA sections. Osteocyte and osteocyte lacuna counts, percent bone matrix loss, and fungal spheroid element counts could be measured and collagen fibre bundles observed in all specimens. Decalcification with 7% nitric acid proceeded more rapidly than with 0.5 M EDTA and may offer better preservation of histological and cellular structure. No endothelial cells could be detected using CD31 and CD34 immunohistochemistry. Correlation between osteocytes per lacuna and age at death may reflect reported age-related responses to microdamage. Methodological limitations and caveats, and results of the TMA analysis of post mortem diagenesis in bone are discussed, and implications for DNA survival and recovery considered.

Synovial C-Shaped Tibial Footprint of the Anterior Cruciate Ligament.

BACKGROUND: Although numerous anatomic studies about the anterior cruciate ligament (ACL) structure and attachments have been performed, these studies have not reached consensus on the ACL footprint. PURPOSE: To investigate the existing controversy regarding the morphology of the tibial ACL insertion (footprint) and confirm histologically that the tibial ACL footprint is not completely filled with ligament tissue. STUDY DESIGN: Descriptive laboratory study. METHODS: The tibial ACL footprint was dissected from 20 different fresh-frozen cadaveric knees (all males; mean age, 68.8 ± 5.4 years [range, 55-80 years]; mean weight, 78 ± 6.6 kg [range, 45-93 kg]). Two knees, 1 with severe osteoarthritis and 1 with previous knee surgery, were excluded. The tibial ACL insertion was observed, and this area was longitudinally divided into 4 parallel slices (0%-25%, 25-50%, 50%-75%, and 75%-100%), embedded in paraffin wax, and stained with hematoxylin-eosin, alcian blue, and picrosirius-polarization. The specimens were measured using a microscope to determine the distances from the anterior to the posterior border of the ACL ligament tibial insertion and the distance from the posterior border to the end of the ligament fibers of the ACL ligament tibial insertions. RESULTS: The 18 evaluated knee specimens confirmed the finding of a C-shaped tibial insertion of the ACL. The measurements showed that the ligament (vertical parallel collagen fibers) occupied only 30.8% of the complete insertion. The remaining area was filled with synovial tissue, demonstrating histologically the "C" shape. CONCLUSION: This study confirms macroscopically the C-shaped tibial insertion of the ACL and shows histologically that synovial tissue is an indirect insertion filling the major part of the footprint. CLINICAL RELEVANCE: This anatomic study suggests a different shape of the ACL tibial footprint, which may be useful for new perspectives regarding ACL reconstruction surgery research.

Correlation between selected angiogenic markers and prognosis in pediatric adrenocortical tumors: Angiogenic markers and prognosis in pediatric ACTs.

BACKGROUND/PURPOSE: Pediatric adrenocortical tumor (ACT) remains a challenging disease. Tumor weight and disease stage are still the most used indicators to prognosis and guidance of clinical decisions. Histology has not added meaningful data for risk stratification and management. ACT is metabolically active, highly vascularized, locally invasive and has the propensity to produce distant metastasis. Our objective was to correlate the expression of vascular endothelial growth factor (VEGF) and intratumoral microvessel density (MVD) with clinical and prognostic aspects in pediatric ACT. PROCEDURE: In 27 tumors, immunohistochemical expression of VEGF, CD105 (endoglin) and CD34 was analyzed. MVD was determined by CD34 and CD105 antibodies. MVD and VEGF expression was correlated with clinical characteristics and outcome. Normal pediatric glands were used as controls. RESULTS: Endoglin MVD was significantly higher and CD34 MVD was significantly lower in ACT than control. The VEGF expression did not differ between groups. Cytoplasmic staining for endoglin was correlated with hypertension in ACT. Endoglin MVD greater than 1 mv/field, CD34 MVD less than 32 mv/field and VEGF expression levels above 4.8% were associated with clinical and biological indicators of poor prognosis. CONCLUSIONS: Endoglin and CD34 MVD values are potential histological markers to refine the histologic classification of pediatric ACT.

Facial reconstruction surgery 10 years after treatment for hemangiopericytoma: planning considerations and clinical outcomes.

This paper describes the surgical correction of ankylosis of the right temporomandibular joint and reconstruction of the middle and lower thirds of the right side of the face in a young man who had undergone treatment for hemangiopericytoma 10 years earlier, which led to serious functional, esthetic, and psychological problems and diminished his quality of life. The results of reconstruction surgery with alloplastic material (Medopor®) and surgical planning with fibrotic tissue due to radiation and the risk of osteoradionecrosis are discussed.

Fifteen Years' Experience of the Brazilian Osteosarcoma Treatment Group (BOTG): A Contribution from an Emerging Country.

PURPOSE: Little information is available regarding the tumor features, prognostic factors, and treatment results in children and adolescents and young adults (AYAs) with osteosarcoma diagnosed in developing countries. We reviewed the results of three observational cohorts of osteosarcoma patients treated in an emerging country. METHODS: A total of 604 patients below the age of 30 years with high-grade osteosarcoma were prospectively enrolled in the Brazilian Osteosarcoma Treatment Group (BOTG) studies III, IV, and V. Gender, age, time from onset of symptoms to diagnosis, primary tumor site, presence or absence of metastases at diagnosis, tumor size, type of surgery (limb-sparing or amputation), treatment protocol, and histological response were correlated with survival. RESULTS: The estimated 5-year overall survival and event-free survival (EFS) rates for the 553 eligible patients were 49% and 39% respectively; of the 390 non-metastatic patients included in the total, overall- and event-free survival were 59% and 48% respectively. Metastases at diagnosis, primary tumor site, type of surgery, and histological response were significant predictors of overall survival and EFS in univariate and multivariate analysis, whereas tumor size and treatment protocol lost prognostic significance in multivariate analysis. CONCLUSION: We report on the outcome of three consecutive studies for the treatment of osteosarcoma carried out in Brazil over 15 years. Although the survival rates presented are below those reported in current literature, it represents the result of a favorable experience gathered from the national collaborative work.

Effects of low-level laser therapy on collagen expression and neutrophil infiltrate in 5-fluorouracil-induced oral mucositis in hamsters.

BACKGROUND AND OBJECTIVES: Several studies have suggested that low-level laser therapy (LLLT) can ameliorate oral mucositis; however, the mechanisms involved are not well understood. The aim of this study was to investigate the mechanisms of action of LLLT on chemotherapy-induced oral mucositis, as related to effects on collagen expression and inflammation. MATERIALS AND METHODS: A hamster cheek pouch model of oral mucositis was used with all animals receiving intraperitoneal 5-fluorouracil, followed by surface irritation. Animals were randomly allocated into three groups, and treated with an InGaAIP diode laser at a wavelength of 660 nm and output power of 35 or 100 mW laser, or no laser. Clinical severity of mucositis was assessed at four time-points by a blinded examiner. Buccal pouch tissue was harvested from a subgroup of animals in each group at four time-points. Collagen was qualitatively and quantitatively evaluated after picrosirius staining. The density of the neutrophil infiltrate was also scored. RESULTS: Peak clinical severity of mucositis was reduced in the 35 mW laser group as compared to the 100 mW and control groups. The reduced peak clinical severity of mucositis in the 35 mW laser group was accompanied by a decrease in the number of neutrophils and an increase in the proportion of mature collagen as compared to the other two groups. The total quantity of collagen was significantly higher in the control (no laser) group at the day 11 time-point, as compared to the 35 mW laser group, consistent with a more prolonged inflammatory response in the control group. CONCLUSION: This study supports two mechanisms of action for LLLT in reducing mucositis severity. The increase in collagen organization in response to the 35 mW laser indicates that LLLT promotes wound healing. In addition, LLLT also appears to have an anti-inflammatory effect, as evidenced by the reduction in neutrophil infiltrate.

Cyclooxygenase-2 and vascular endothelial growth factor expression in 5-fluorouracil-induced oral mucositis in hamsters: evaluation of two low-intensity laser protocols.

GOAL OF WORK: The aim of this study was to investigate the mechanisms whereby low-intensity laser therapy may affect the severity of oral mucositis. MATERIALS AND METHODS: A hamster cheek pouch model of oral mucositis was used with all animals receiving intraperitoneal 5-fluorouracil followed by surface irritation. Animals were randomly allocated into three groups and treated with a 35 mW laser, 100 mW laser, or no laser. Clinical severity of mucositis was assessed at four time-points by a blinded examiner. Buccal pouch tissue was harvested from a subgroup of animals in each group at four time-points. This tissue was used for immunohistochemistry for cyclooxygenase-2 (COX-2), vascular endothelial growth factor (VEGF), and factor VIII (marker of microvessel density) and the resulting staining was quantified. MAIN RESULTS: Peak severity of mucositis was reduced in the 35 mW laser group as compared to the 100 mW laser and control groups. This reduced peak clinical severity of mucositis in the 35 mW laser group was accompanied by a significantly lower level of COX-2 staining. The 100 mW laser did not have an effect on the severity of clinical mucositis, but was associated with a decrease in VEGF levels at the later time-points, as compared to the other groups. There was no clear relationship of VEGF levels or microvessel density to clinical mucositis severity. CONCLUSION: The tissue response to laser therapy appears to vary by dose. Low-intensity laser therapy appears to reduce the severity of mucositis, at least in part, by reducing COX-2 levels and associated inhibition of the inflammatory response.

Correlation of c-erbB-2 oncogene and p53 tumor suppressor gene with malignant transformation of hydatidiform mole.

AIM: Considering the roles of c-erB-2 and p53 oncoproteins in tumor progression, we aimed to evaluate their expression in hydatidiform moles, and the possible predictive value of this immunoexpression in postmolar follow-up. METHODS: Group I comprised 35 patients with progression to gestational trophoblastic tumor, and group II included 32 patients with progression to spontaneous remission. Immunohistochemical tests were performed by streptavidin-peroxidase method. c-erbB-2 immunoexpression was evaluated according to quantitative and semiquantitative criteria; p53 according to percentage of cells with stained nuclei. Data were analyzed by Student t-test, Mann-Whitney test, ROC curve and logistic regression analysis. RESULTS: c-erbB-2 and p-53 expressions were significantly increased in group I. Quantitative and semiquantitative analysis of c-erb-2 showed that its expression may be associated with mole hydatidiform progression to gestational trophoblastic tumor. Taking into account cells with complete membranous delineation we proposed a cut-off value of 10.8%. Similarly, considering the percentage of cells presenting nuclei marked by p53 we suggested a cut-off value of 40.1% for the prediction of malignant transformation of mole hydatidiform. CONCLUSIONS: c-erbB-2 and p53 immunoexpression in hydatidiform mole are usually increased with malignant transformation. In addition to beta-fraction of human chorionic gonadotropin, they could possibly help the establishment of a therapeutic protocol.

Clinicopathologic study and Ki-67 proliferative marker evaluation in human osteosarcomas

BACKGROUND: Osteosarcoma is a relatively uncommon malignant neoplasm and little information has been reported on its cell proliferation using Ki-67. OBJECTIVES: Evaluate histological, clinical and immunohistochemical parameters using Ki-67 labeling index (LI), correlate one to another and with follow-up. MATERIAL AND METHOD: Fifty-seven patients with available clinicopathological data submitted to the study of cell proliferation as determined by Ki-67 expression measured by immunohistochemistry (IHC) staining using formalin-fixed paraffin embedded sections. In each sample, positive cells were quantified on at least a thousand nuclei and expressed as Ki-67 LI according to median value. RESULTS AND DISCUSSION: Non-significant correlations were observed in metastatic and non-metastatic cases when variables as surgery, tumor size, death and relapse were compared with Ki-67 LI values (cut-off of 45 percent). In the group of non-metastatic cases there was a direct correlation between higher values of Ki-67 index and better overall survival. Metastatic patients overall survival curve and LI high and low Ki-67 did not show significant differences. CONCLUSION: Based on our results the Ki-67 LI could be useful as a prognostic marker in patients without metastasis at diagnosis.

INTRODUÇÃO: Osteossarcoma é uma neoplasia rara do tecido ósseo, tendo sido publicados poucos trabalhos na literatura que avaliaram o índice de proliferação celular Ki-67 (IPC-Ki-67) nesse tipo de sarcoma. OBJETIVO: Avaliação e comparação dos parâmetros clínicos, histológicos e de imunoexpressão do Ki-67 com a evolução clínica dos pacientes. MATERIAL E MÉTODO: Blocos de parafina de 57 casos de osteossarcomas, cujos pacientes apresentavam dados clinicopatológicos fidedignos, foram submetidos a estudo da proliferação celular através da técnica de imuno-histoquímica. Em cada amostra o IPC foi determinado pela contagem de núcleos marcados em pelo menos mil células neoplásicas. Em cada grupo de pacientes o IPC-Ki-67 foi considerado alto ou baixo, tendo como referência a mediana dos valores do Ki-67. RESULTADOS E DISCUSSÃO: Não houve correlação estatística significativa entre os valores do IPC-Ki-67 (valor de corte = 45 por cento) dos osteossarcomas dos pacientes com ou sem metástases ao diagnóstico, em relação às variáveis óbito, recaída tumoral, tamanho do tumor e tipo de cirurgia. No grupo de pacientes sem metástases houve correlação direta entre o tempo de sobrevida e o índice de proliferação celular (isto é, o IPC-Ki-67 > 45 por cento correlacionou-se com maior tempo de sobrevida). Nos casos de pacientes com metástases ao diagnóstico, os valores do IPC-Ki-67 não apresentaram correlação significativa com a sobrevida. CONCLUSÃO: Com base nos nossos resultados, o IPC-Ki-67 pode ser utilizado para avaliação prognóstica em pacientes com osteossarcoma sem metástases ao diagnóstico.

Osteocondroma acetabular / Acetabular osteochondroma

Apresentamos o relato de um caso de osteocondroma solitário acetabular, tumor ósseo comum em diversas regiões do esqueleto porém raro nesta localização. O tratamento realizado foi a ressecção do tumor por uma via posterior ao quadril e o paciente evoluiu com resolução do quadro clínico.

Estudo das terminações nervosas dos discos intervertebrais da coluna lombar de humanos / Study of nerve endings of intervertebral discs in the human lumbar spine

Os autores estudaram a terminação nervosa existente na coluna lombar de humanos, utilizando cinco colunas de cad veres de adultos jovens. Foram retirados os discos intervertebrais de L1 até L5, num total de 25 discos. O material colhido foi fixado em formalina a 10 por cento. Os discos assim tratados foram a seguir divididos no plano transverso e subdivididos em quatro partes no plano sagital, sendo identificada cada parte como: anterior, posterior e laterais. Foram a seguir feitos cortes finos de 4μm corados pelo metodo imunohistoqu¡mico estrepto-avidina-biotina-peroxidase para o anticorpo policlonal antiproteina S100. Foi avaliada de maneira interativa toda a circunferencia do disco intervertebral, com aumento de 400 vezes, com auxilio de um sistema de an lise digital de imagem. Os autores encontraram terminaçäes nervosas em toda a superficie externa e camada superficial do ânulo fibroso. Não foram encontradas terminações nervosas na camada interna do unulo fibroso e no n£cleo pulposo. Houve diferença estatisticamente significante entre o numero de terminações nervosas nas regiões laterais e posterior do disco intervertebral