Estudo mecânico de implante para fixação do segmento lombossacro da coluna vertebral Mechanical study of implant for lumbossacral spinal fixation

Foram estudados, do ponto de vista mecânico, a rigidez e os pontos críticos de um implante para fixação interna da coluna lombossacra. Aplicou-se o implante sobre um modelo de madeira simulando o segmento lombossacro da coluna. Realizamos sete ensaios de flexo-compressão, sete de rigidez axial, sete de rigidez radial e um ensaio destrutivo. Os resultados demonstraram que o implante foi eficiente e seguro para uso em seres humanos.


INTRODUCTION
We developed an implant composed by a shaft with "U" shape, that works as a fixation barr of pedicular screws which allows the assemblage of tridimensional model we called A-S1 (Implant a Series 1).
The objective of this work is, using biomechanical simulations in standardized model of lumbossacral spine submitted to efforts of flexocompression, determine the axial and radial stability, the limits of elasticity and resistence, as well as the critical points of the proposed implant for internal fixation of the lumbossacral segment.

MATERIAL AND METHODS
The fixation device is composed of the following parts: (Fig. 1) 1. Shaft of stainless steel ASTM EF 138, with hardness of 38 HRC and diameter section of 6,4 mm (1/4 inch) bented in "U", 110 cm long.2. Four pedicular screws with spongiosus screw thread (sew type) 3. Four fixation staples, comprised of two counterplaced clinches, which are the elements of union between the shaft and the screw, forming a tridimensional model.4. The barr keeps the distance between the "arms" of the shaft, making it a rigid retangle.
SPINE TEMPLATE (Fig. 2) Based on work of TOLEDO (1989), we used a model of lumbossacral spine, constructed with two segments of wood ipê, hard enough and homogeneous to permit placement of screws and bearing the loads without alterations.

The experimental model aimed the evaluation of the effect of eccentric (non-axial) compression loads over lumbossacral vertebral bodies (L5-S1) stabilized by the fixator, respecting the geometry of the model (shape, angles and distances) and the local biomechanical characteristics.
In this work we accepted, as initial parameter for the analysis of the axial and radial hardness, 100 Kgf of maximum load for flexion-extension what corresponds to patients with 80 Kg of weight in orthostatic position.O implante foi montado paralelo na parte posterior do sistema, contraposto a uma articulação taça-esfera, mantendo 20 mm de espaçamento entre os discos.

ENSAIOS DE FLEXOCOMPRESSÃO
The implant was mounted parallel in the posterior part of the system, opposed to one articulation cup-spheric, keeping 20 mm of space between the disks.
The pedicular screws were placed with a distance between them, as it occurs "in vivo", equivalent to the distance between the pedicles of same vertebral body.
The flexo-compression tests were performed in a universal machine for mechanical tests, Kratos K5002.The speed of application of the compressive load was 20 mm/min for the tests to determine axial and radial rigidity and of 10 mm/min for the destructive.

TESTS FOR DETERMINATION OF AXIAL RIGIDITY
Seven tests of flexo-compression were performed, with use of continuous load from zero to 100 Kgf to the model, to obtain the respective diagrams of strength of axial compression-deformation (Fig. 4 and 5).

TEST FOR DETERMINATION OF THE RADIAL RIGIDITY
Seven tests of flexo-compression were performed, with use of sequential progressive loads from zero (initial position) to 100 100 Kgf, with increments of 20 Kgf (fig.6).

We analyzed the axial rigidity of the implant in the diagrams load-deformity obtained in the tests of flexor-compression (Fig 5).
In the diagram of load-destruction of the destructive test (Fig. 9), the following points were found: a -limit of elasticity b -limit of resistance to flexo-compression The limit of elasticity correspond to the end of the linear phase of the diagram and represents the maximum load and deformation, that the implant can support without presenting any damage ou permanent deformation (residual) after decompression, according to SOUZA (1982) and CHIAVERINI (1986).The radial values were taken with pachimeter digital Mitutoyo Digimatic Caliper 500-21 5 (0,01 mm), with load from zero to 20, 40, 60, 80 e 100 Kgf (flexocompression).These data were transferred to computerized tables and calculation of spatial movement was performed.
The maximum distance between pointers (lateral movement), was observed after sequential application of loads.The "maximum lateral movement" represents the bigger radial sliding between the segments, responsible for the problem "in vivo".

STATISTICAL METHODS
The data obtained in the mechanical tests were statistically analysed.

RESULTS
The results of the tests are presented in Tables 1 to 6.The form for data collection and the values were transferred to computerized tables, and calculations were performed.The average axial displacement observed after 100 Kgf. of flexo-compression was 3,52 mm (Table . 1 e 2), what correspond to 7% of wedge shaping of the vertebral body (50 mm) in the adopted model.Shortenings of 7% are within the tolerated limits which do not harm the spinal cord nor posterior ligaments of the spine, responsible by the segment stability.A Tabela 3 mostra as cargas necessárias para obtermos 1 mm de deformação axial, e nos permite avaliar a movimentação máxima que pode ocorrer em pacientes, conhecendo seu peso corpóreo.

Table 3 shows the necessary loads to obtain 1 mm of axial deformity, and permits evaluate the maximum movement that could occur in patients, once his/her body weight is known
In Table 4 we observe lateral movement of 2,68 mm under of flexo-compression load of 100 kgf.In practice values around 1 mm are used as ideal for systems of internal fixation.The values obtained are superior but not sufficient to compromise mechanically its application.
In the results of the destructive test we see a failure of fixation, with sliding between the shaft and staple with 185 Kgf in the limit of elasticity (Table 5)

Tab. 5-Determination of elasticity limit of the implant under destructive test of flexo-compression (resistence in Kgf and N and deformity in mm=
10 -3 m and %) With 260 Kgf, limit of resistance, occurred a deformity of the pedicular screws in the transition between the screw threads (Table 6).

DISCUSSION
With better knowledge of the spine biomechanics and adequate materials there was great development of the implants for lumbosacral internal fixation (HANLEY, PHILLIPS, KOSTUYK, 1991) 8 .The devices from HARRINGTON (1962) 9 have limited use in the degenerative pathologies, since there are difficulties to implant the distal staple in the sacrum.Another problem of the distension shafts is the retification of the lumbar lordosis (posterior distension) The Luque's shaft has broader use, mainly in the scoliosis, kifosis, fractures, tumors and degenerative spondilolistesis.However, they are not efficient to counteract the strengths of compression, extension and rotation.BARROS F o ( 1987 ) 1 , using the technique of Harrington-Luque, with base in a former report ( BASILE, 1985 )  2 , verified that this was na efficient method when associated to segmentar fixation with sublaminar strings of Luque.ROY-CAMILLE (1986)  16 developed the first practical method of fixation with pedicular screw and one plaque.In this method, if occurs bone reabsortion or compression under the plaque, movements will occur between the plaque orifices and the screws, causing aging of the material followed by rupture of them.STEFEE et al. (1986)  18 to solve the problem placed screws perpendicular to the plaque, however, as the plaque is rigid, not always succeed to place the screws in perpendicular position, what caused a flexion load in the screw, with consequent rupture.KRAG (1991)  11 studied several internal fixation systems ( Wiltse, Zielke, -Cotrell-Dubousse, Puno and other models) employing na intermediate part between the screw and the barr or fix the screw directly to the longitudinal barr, but none of them, takes into consideration the lack of alignment of the longitudinal axis between the screws and its inclination in relation to the barr.A biomechanical analysis of the pedicular systems and the techniques of Galveston and Luque were presented by PUNO et al.(1991)  15 .The mechanical tests could show that the rigidity of the pedicular fixation was between the quoted techniques.BLUMENTHAL & GILL (1993) 3 evaluated a group of 470 patients that underwent a lumbar arthrodesis and instrumentation with implant of Wiltse.They used multiple configurations of the Wiltse system in 95% of the surgeries and placed the pedicular screw to the shaft with an intermediate staple.Complications were present in 29 patients (6,17%).We can conclude, considering these data, that with tri-dimensional assemblage that use intermediate staple, the number of complications with implant is very low.Comparing this implant with others we can say that: enough rigidity to support the mechanical loads of the spine, as much as in the systems of LUQUE et al (1982)  12 ., de HARRINGTON (BARROS FILHO, 1987) 1 of ROY-CAMILLE et al. (1986)  16 , of STEFEE et al. (1986)  18 , of WILTSE (1991) 21 and of PUNO et al. (1991)  15 .The average axial displacement after application of 100 Kgf in flexo-compression was 3,52 mm (Table 2), what corresponds to 7% of reduction of height (50 mm of wedging) of the vertebral body in the adopted model.This is a simulation of the stabilized spine only for implant, without the participation of biological structures (bone contact, tension of soft tissues, etc), that in a model reflects more extreme situations than what occurs in real life.Shortenings of 7% are within corpóreo.
tolerable limits that cause no damage to the nervous structures or to posterior ligaments of the spine responsible for the stabilization of the segment.
The Table 3 depicts the necessary loads to obtain 1 mm of axial deformity, and permits evaluate the maximum movement that could occur in patients, once his/her weight is known.
The radial rigidity is evaluated by the maximum lateral sliding seen between the disks of the model.It represents the risk of sliding between the vertebral bodies.
On Table 4 we see a lateral sliding of 2,68 mm under a load of 100 Kgf.
The axial and radial deformities are due to the flexion of the perpendicular screws mainly in transition region between the usual and spongiosus screw thread.
On Tables 5 and 6 we see the results from the destructive tests which have detected a failure of fixation, a sliding being observed between the shaft and staple with 185 Kgf, in the limit of the elasticity.With 260 Kgf, on the limit of resistance, a deformation occurred in the pedicular screws at the transition between the screw threads.The results obtained in the tests with placement of implants are sufficient to permit deambulation without orthesis, once there are no additional external loads.
As advantages, we have the simplicity of the four components of the implant that can be placed with usual instruments, with several alternatives of use and that can be manufactures in Brazil with low cost, avoiding importation of similar devices.

CONCLUSIONS
1.The implant must be versatile, easy to be placed and with structure rigid enough to support the lumbossacral mechanical forces.After mounted should resist loads of at least 185 Kgf.

The internal fixator is an important complement in the technique
of spine arthrodesis, but does not replace or avoid the use of bone graft, according to the usual procedure.3. The objective of the implant is the firm fixation of the lumbossacral spine, to have an early mobilization of the patient without the use of cast or orthesis.