OVERLOAD STUDY ON ADJACENT DISC AFTER ARTHRODESIS IN THORACOLUMBAR FRACTURES

Objetivo: Analizar la degeneracion del disco adyacente despues de artrodesis debido a fracturas toracolumbares. Metodos: Ochenta tres pacientes que fueron sometidos a artrodesis posterolateral en los niveles toracolumbares tuvieron las radiografias analizadas para la degeneracion de los discos adyacentes a la artrodesis. Los espacios de los discos se clasifican segun la escala de UCLA. Resultados: De los 83 pacientes evaluados, 66 eran hombres (79%) y 18 mujeres (21%), con edad promedio de 35,5 anos. El periodo medio de seguimiento fue de 40 meses. Con respecto a las fracturas, el 75% se encontro entre T12 y L2 (p < 0,001), siendo del tipo A3 en el 65% de los casos (p < 0,001). El mecanismo mas comun de lesion, representando el 50% de los casos (p < 0,001), fue la caida de altura. Solo el 6% de los discos superiores y 12% de los discos inferiores mostraron algun grado de degeneracion. Ningun paciente fue sometido a un nuevo abordaje quirurgico. Conclusion: La incidencia de la degeneracion del disco adyacente en pacientes despues de la artrodesis debido a las fracturas vario de 6% a 12% con un seguimiento promedio de 40 meses.


INTRODUCTION
Thoracolumbar fractures are the most common fractures of the axial skeleton, corresponding to around 89% of all fractures of the vertebral spine, which mainly occur between T11 and L2. 1 In recent decades, the growing number of automobile and industrial accidents has directly increased the complexity of fractures, as has the emergence of new instrumentation techniques, which have led to an increased prevalence of thoracolumbar arthrodesis. This results in complications, one of which is adjacent disc disease (ADD).
ADD is defined as an abnormal process that develops at the level above and/or below the segment where the arthrodesis was performed. 2 This process may take the form of disc degeneration (39%), instability, facet hypertrophy, disc hernia (28%), stenosis of the spinal canal (33%), vertebral fracture and scoliosis (17%). 3 Its occurrence depends on the type of fusion, due to the increased mobility of the free segments of the fusion, intra-disc metabolic changes, increased intradiscal pressure, or biomechanical changes caused by changes to the spinal column, such as loss of lumbar lordosis. 2 With the appearance of ADD, careful considerations were made on the use of fusion implants and new implants, such as dynamic stabilization and the use of artificial discs. 5 However, no conclusions have been drawn on this subject as yet i.e. whether ADD is a radiological finding, or a consequence that indicates poor clinical results. 5 Some authors still divide the definition of this disease into adjacent level degeneration, in which there are altered imaging exams with asymptomatic patients, or symptomatic patients with compatible imaging exams. 6 This article performs a retrospective radiographic analysis of 83 patients with thoracolumbar fracture submitted to posterolateral arthrodesis. It evaluates the behavior of the discs adjacent to the fusions performed, with an average follow-up of 40 months. A retrospective analysis was conducted of eighty-three records of patients seen at Hospital das Clínicas de Marília (FAMEMA), in the period 2000 to 2012, who presented thoracolumbar fractures, treated surgically and with outpatient follow-up. All the patients had undergone posterolateral arthrodesis, with fixation by pedicle screws of 2 to 5 levels.

METHODS
The radiographic images of the thoracolumbar spine (T4 to S1) on the day of the trauma, and at least 24 months after surgery, were analyzed and compared, by different doctors. The fractures were given an AO score, 7 and the discs adjacent to the arthrodesis (upper and lower), by the Scale of the University of California at Los Angeles (UCLA), which was used to measure the radiographic disc degeneration, 8 (Table 1) type of trauma and some type of associated lesion.
Inclusion criteria: minimum age of 18 years, last level of arthrodesis of the lumbar spine, minimum of 24 months since surgery.
Exclusion criteria: age below 18 years, any motor deficit, pure thoracic arthrodesis, less than 24 months since surgery, signs of degeneration in the initial radiograph, infection, and patients who have undergone removal of material for any reason.

RESULTS
Of the 83 patients evaluated, 65 were male (78.3%) and 18 female (21.7%); ages ranged from 18 to 51 years, with an average age of 35.6 years (CI 95%: 32.9 -38.1). A minimum follow-up period of 24 months and a maximum of 115 months was observed, with an average of 40 months (CI 95%: 36.3 -46.6). All the patients underwent posterolateral arthrodesis with instrumentation using pedicle screws via the posterior route, and decompression.

DISCUSSION
Fractures of the thoracolumbar spine are the most common fractures of the axial skeleton, corresponding to around 89% of all fractures of the vertebral spine, which mainly occur between T11 and L2. Two thirds of thoracolumbar fractures occur at the thoracolumbar transition between T11 and L2 (50% of fractures of the thoracic spine at level T12 and 60% of spinal fractures at level L1). The prevalence of fractures in this region is related to the reduction of stability between the thoracic segment (more rigid and stable) and the lumbar (greater flexibility and greater range of movement). These fractures are the result of falling from a height in 47% of patients, automobile accidents in 44.1% and direct trauma 8.8%. 1,4 Disc lesion associated with fracture (at the time of the trauma) should be considered, as this influences the stability and genesis of acute and chronic pain, and can lead to

CONTRIBUTIONS OF THE AUTHORS:
All the authors made individual and significant contributions to the development of the manuscript. GSR, JMSJ, ECMM and RYO performed the surgeries and followed up the patients. GSR, FZG and RST were the main contributors to writing the manuscript and the data collection. JMSJ, ECMM, RYO, ABF and RRM revised the manuscript and contributed to the intellectual concept of the study. sagittal imbalance and worsening of the quality of life (pain); 1 however, this is rarely found in the discs adjacent to the levels of arthrodesis in the immediate postoperative period.
Adjacent disc disease (ADD) after fusion of the lumbar vertebral spine is responsible for a significant percentage of revision surgeries of the spine. Although the development of degeneration of the adjacent segment can be considered a normal part of the degenerative process that occurs with aging, this phenomenon appears to be, at least in part, influenced by changes that emerge as a result of the lumbar arthrodesis. [9][10][11][12] Other studies have been conducted on the subject, which take into consideration the height of the disc and the signs of instability. Wide variation in prevalence is seen; from 5% to 43%, but the need for revision surgeries ranged from 2% to 15%, [13][14][15] in which, in the majority of cases, only decompression of the canal was performed, without increasing the level of the arthrodesis.
Biomechanical studies defend the increased prevalence of adjacent degenerative disease after arthrodesis. 9,10,16 Their authors affirm that a possible etiology of degeneration of the adjacent segment after arthrodesis is due to increased stress, or to a hypermobility. Lee and Langrana show that there is increased tension in the adjacent joints of L3-L4 and L4-L5 after lumbosacral arthrodesis. 16 A single level of lumbar arthrodesis was studied by Quinnell and Stockdale, who observed that the adjacent disc above was not affected, unlike the disc below, which suffered changes in its characteristics. 10 Tests using in vitro models were conducted by Axelsson et al., 17 who found hypermobility of the adjacent segment, thereby proving, biomechanically, that lumbar fusions produce negative consequences on the adjacent discs.
According to Ghiselli et al., 8 the incidence of ADD ranged from 0 to 6.1%, with an average of 3.9% per year, with follow-up of five to ten years. Of these patients, 83.5% and 63.9% were free of disease, respectively, and it was concluded that 16.5% and 36.1%, respectively, would require a new surgical procedure due to the adjacent disc disease.
Park et al., 2 point out the causes that most favor the development of ADD, described in Chart 1. The main factor for non-occurrence of DDA is preservation of the proximal facet, as affirmed by Wiltse et al. 18 By comparing the increased load on the joints, in the different forms of fusion, it is concluded that in posterior arthrodesis this is great; in anterior, intermediary and circumferential fusion, there is little effect. 8 ADD has been greater in posterior fusions when compared with circumferential arthrodesis and with anterior fusion. 15,19,20 In our study, the vast majority of patients were male (79%) as these are more exposed to traumas. As in the literature, the most frequent level of the fracture was T12 -L2 in 75% of cases, and the most common grade was A3, in 65%.
It should be emphasized that in our study, only patients operated on and without neurological deficits were considered. The findings in the literature also corroborate our findings, in terms of the types of trauma found in this research: falling from a height (50%), automobile accident (37%), direct trauma (10%), being run over (3%).
It is observed that the majority of patients involved in traumas are young, and that the follow-up time of this study was too short to allow for evaluation of early disc degeneration. A longer follow-up time is therefore necessary, as only 6% (above) and 12% (below) of the adjacent discs show relevant radiographic signs of degeneration. Also, although magnetic resonance is the gold standard exam for evaluation of the disc, this exam was not performed due to questions of cost.

CONCLUSION
Adjacent disc degeneration (ADD) should continue to be the object of studies, with longer follow-up times, as its incidence is still low in patients with arthrodesis following fractures without previous pathologies and low age group. Within this context, we also emphasize that preservation of the joint facet at the adjacent level above, and sagittal alignment, continue to be the main forms of prevention of ADD.