Mechanical assay of a pedicular fixation system with transversal rod

Abstracts

Foi realizado estudo biomecânico de um protótipo de sistema de fixação vertebral que utiliza o pedículo vertebral como ponto de ancoragem dos implantes, associado a hastes que conectam transversalmente os parafusos pediculares. De modo diferente dos sistemas convencionais, nos quais os parafusos são conectados no sentido longitudinal, no sistema desenvolvido as hastes são conectadas transversalmente aos parafusos. Foram realizados ensaios mecânicos (flexo-compressão, flexão lateral e torção) utilizando-se corpos de prova de madeira em máquina universal de testes, com a finalidade de comparar a resistência desse sistema de fixação com o sistema convencionalmente utilizado. Os testes biomecânicos mostraram que o sistema desenvolvido apresentou menor resistência nos ensaios mecânicos realizados, quando comparado com o sistema de fixação convencional.

Coluna vertebral; Fixação pedicular

ARTIGO ORIGINAL

Mechanical assay of a pedicular fixation system with transversal rod

Helton Luiz Aparecido Defino^I; Antônio Carlos Shimano^II

^IAssociate Professor of Departamento de Cirurgia, Ortopedia e Traumatologia of Faculdade de Medicina de Ribeirão Preto- USP

^IIMechanical Engineer of Laboratório de Bioengenharia of Faculdade de Medicina de Ribeirão Preto- USP - E.Mail.: [e-mail]

SUMMARY

A biomechanical study of a spine fixation prototype was performed. This system uses vertebral pedicle as anchor point of the implants in association with transversally connecting rods. This is different form the usual systems where longitudinally placed rods connect the pedicular screws. Mechanical assays were performed (flexo-compression, lateral flexion, torsion) using probe wood pieces in an universal testing machine, aiming to compare the resistance of this system of fixation to the conventionally used ones.

Biomechanical tests showed that the system was less resistant to the mechanical assays when compared to conventional systems.

Key Words: Spine, Pedicular fixation

INTRODUCTION

Systems using vertebral pedicle as insertion point for screws have been largely used in treatment of surgical treatment of spine pathologies and the growing acceptance of this kind of fixation widened the indications of the method.^(1,2,4)

The use of this method reaching several vertebras presents a technical difficulty to perform regarding screw alignment since it is not easy to implant several pedicular screws in a longitudinal alignment. And mal-alignment of the screws makes difficult to fit to the rods and this becomes even more difficult as more rigid rods are used.^(3,7)

These difficulties have been noticed since the beginnings of development of fixation devices using pedicular screws, and poliaxial screws were developed to solve them.

Aiming to solve this difficulty during pedicular fixation we developed a fixation system where fitting the rods do not depend on the pedicular screws alignment. (Figures 1 and 2), and the objective of this work is to present a prototype of this system of vertebral fixation and the result of the mechanical assays performed with the system.

MATERIAL AND METHODS

The study was performed using wood proof bodies (mahogany) aiming to simulate a situation of corporectomy and also to eliminate some variables that would be introduced if an heterogeneous group of vertebra was used.

Proof bodies consisted in two cylindrical wood pieces, 50 mm diameter and 90 cm long, to what fixation system screws were applied. The screws used were 35 mm long and had 6 mm diameter being applied forming an 60 degree angle among them, and a distance of 10 mm from the border of the wood piece. The wood pieces were fixated keeping a distance of 40 mm between their bases (Figure 3).

Mechanical stability of the developed fixation system was evaluated by means of comparison to a routinely used conventional system, forming two experimental groups: I, the conventional system and II, the newly developed system (Figure 3).

In group I, screws were connected by means of 6 mm rods applied longitudinally as in routinely used fixations.

In group II, the screws of each piece were transversally linked by means of a cylindrical rod of 6 mm diameter, and these bars connected to each other by means of 2 longitudinal bars. At the beginning of the tests only one bar was used, but it was abandoned due to lack of resistance.

Mechanical assays were performed using an universal testing machine, and the proof bodies underwent flexo-compression, lateral flexion and torsion tests.

Speed of load application was 1 mm/minute in lateral flexion test and torsion, and 0.08 mm/minute in the flexo-compression test.

The established limit for the mechanical assay was 0.16 radians for torsion test, 1 mm of deformation for flexo-compression test and 3 mm of deformation for lateral flexion test.

Three mechanical assays were performed for each group, and mean values were considered. Results of groups I and II were compared by statistical analysis, and t test of Student with a significance level of 5% was used.

The parameters used for evaluation of each mechanical assay were the applied load, the rigidity and the absorbed energy. Load was expressed by the last value measured for the deformation determined for each test (1 mm for flexo-compression, 3 mm for lateral flexion and 0.16 radians for torsion).

Rigidity was calculated by means of the tangent of the angle of the curve (applied load X deformation).

Resiliency represents the absorbed energy in the elastic phase, and was calculated by the area under the curve (applied load x deformation).

RESULTS

Results of the different mechanical assays (flexo-compression, lateral flexion and torsion) are presented in figures 4, 5, 6,7,8 and 9, and in tables ^I, ^II and ^III.

In the flexo-compression assay it was observed that the group I mounts were mechanically more resistant. Average load necessary for 1 mm deformation was 207.83 N in group I and 78.24 N in group II. Mean rigidity in group I was 207 830 N/m and 78240 N/m in group II. Average absorbed energy was 0.104 J in group I and 0.039 J in group II.

Values found in group I in flexo-compression assay were in average 62% above those of group II, and the difference was statistically significant (p<0,05).

In lateral flexion assay the values observed in group I were also superior to those found in group II in about 52%, and the difference was statistically significant (p<0,05). Values observed for flexor momentum were 2.69N.m in group I and 1.29 Nm in group II. Flexural rigidity was 897.67 J/m in group I and 429.675 J/m in group II. Absorbed energy was 4x10^-3 J.m in group I and 1.9xx10^-3 J.m in group II.

In torsion assays it was observed for torsor momentum values of 2,17 N.m in group I and 1.65 N.m in group II. Torsional rigidity was 18.75 N.m/rad in group I and 12.50 N.m/ rad in group II. The absorbed energy was 0.132J in group I and 0.095 J in group II. Values of torsion assays were also superior in group I, and a statistically significant difference was found (p<0,05). The values of torsor momentum were 4% superior in group I and those of torsional rigidity 33%, and absorbed energy 28% in relation to group II.

CONCLUSION

The pedicular system using transversal rods was less resistant than the conventional longitudinal rods system.

DISCUSSION

In the last decade hundreds of different systems for fixation of spine were developed using the pedicle as anchoring place for implants and is interesting to see how in all of these systems pedicular screws are connected by longitudinal rods.^(2,5) We found in the literature, except a system developed by Kluger (1999)⁽⁵⁾, no system for spine fixation based in transpedicular screws using transversal connection of the screws.

To this moment it is not yet known the ideal rigidity of an implant used in fixation of the spine; clinical and experimental investigations indicate that the increase in mechanical stability of the system accelerates the bone healing and reduces incidence of pseudoarthrosis. Vertebral implants are developed based on this evidences.⁽⁶⁾

Multi axis biomechanical assays aiming to test and compare vertebral fixation systems have several limitations related to the material used in performing the tests ^(3,8), and the model we used, nevertheless the limitations it has, simulates a corporectomy situation and, as a matter of fact, we only aimed to compare the systems using transversal and longitudinal rods.

The results observed demonstrate that the transversal rod connection system is less resistant from a mechanical point of view, however we do not know to what extent is this relevant from a clinical standpoint, since it is still unknown the ideal rigidity for vertebral implants. This smaller mechanical resistance of the transversal rod system was not found by Egger (1999)⁽¹⁾, and we believe that improving the connection between screw and rod could improve the stability of this system. Data are still missing for us to state that this system would be inadequate from a biomechanical standpoint, being possible only to say that from a mechanical point of view it is inferior to the conventional system.

Our most important objective in this report is to divulge the idea of this kind of pedicular fixation for maybe serving as a start point for development of new developments. This system was not used in patients, and the results of this ''in vitro'' study confirm the importance of its use before clinical application of these implants, objecting to find weak points in the system and having the opportunity to improve it before clinical use. Even though we didn't use this system in patients we agree with Egger et al. (1999)⁽¹⁾ on the stressed disadvantages, which would be the high profile and difficulty for approximation of paravertebral muscles after its application.

We are still far away from an ideal surgical solution for the problems of spine, since when compared to reconstructive surgeries of large joints such as hip and knee, that allow maintenance of movements, we are still performing vertebral arthrodesis and eliminating movements. Considering the possibilities of fixation systems currently used, there are clear evidences that poliaxial screws can importantly contribute to solve problems related to screw alignment and their use with rods allowing lateral adjusts can even more help solving this problems. It would be very difficult at this moment to abandon the use of longitudinal rods connecting the pedicular screws systems, however we believe that it is valid to divulge the idea of using transversal rods that could eventually be improved in the future or help in future development of alternative systems for vertebral fixation.

REFERÊNCIAS

Trabalho recebido em 25/05/2000. Aprovado em 25/09/2000

Work performed with grant from CNPq.

• 1. Egger,W. ; Klugger,P. ; Claes,L. ; Wilke,H.J. Characteristics of an extended internal fixation system for polysegmental transpedicular reduction and stabilization of the thoracic, lumbar and lumbosacral spine. Eur. Spine J.,8:61-69, 1999.
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• 7. Thalgott, J.S. & Aebi, M. Manual of internal fixation of the spine. Lippincott Raven, Philadelphia.1996.pag.305.
• 8. Wilke,H.J. ; Krischak, S.T.; Wenger,K.H., Claes, L.E. Load displacement properties of the thoracolumbar spine: experimental results and comparison to know human data. Eur.Spine J.,6:129-137,1991.

Publication Dates

• Publication in this collection
  13 Oct 2005
• Date of issue
  Dec 2001

History

• Received
  25 May 2000
• Accepted
  25 Sept 2000