Cervical spine flexion biomechanical study in cadaver submitted to resection of vertebral body and stabilization with fibular graft

Abstracts

Este estudo apresenta e discute os resultados da análise biomecânica, radiográfica e anatômica de 20 peças de coluna cervical de cadáveres humanos, submetidas à corpectomia de C5, discectomia adjacente e estabilização com enxerto de fíbula. Os ensaios em flexão foram realizados em Máquina Universal de Testes. Nenhuma fratura ou extrusão do enxerto foi observada. A falha mecânica ocorreu na interface corpo vertebral-enxerto fibular, caracterizada por fratura dos corpos vertebrais adjacentes em 11 experimentos e afundamento da esponjosa em nove. O ligamento longitudinal posterior e o complexo ligamentar posterior não foram lesados em nenhuma das peças. Concluem que, em estudo experimental, o enxerto de fíbula é resistente e proporciona estabilidade imediata à coluna cervical quando submetido a carga em flexão.

Biomecânica; coluna cervical; enxerto fibular

ARTIGO ORIGINAL

Cervical spine flexion biomechanical study in cadaver submitted to resection of vertebral body and stabilization with fibular graft

Adriano Marchetto^I; Gilberto Luis Camanho^II; Itibagi Rocha Machado^III; Antonio Carlos Shimano^IV; José Baptista Portugal Paulin^V;Tarcísio Eloy Pessoa de Barros Filho^VI

^IMaster Degree in Orthopedic, FMUSP

^IIProfessor, IOT-FMUSP

^IIIDoctor Degree in Medicine, FMUSP; Adjunct Professor, College of Medicine, Jundiaí

^IVDoctor Degree in Mechanical Engineering, College of Medicine, Ribeirão Preto-USP, Bioengineering Laboratory.

^VProfessor Doctor, College of Medicine, Ribeirão Preto- USP

^VIAssociate Professor, FMUSP

^{Correspondence}

SUMMARY

The authors present and discuss the results of a biomechanical, radiographic and anatomical analysis of 20 specimens, obtained from human cadaver of cervical spine submitted to C₅ corpectomy, adjacent discectomy and stabilization with fibular graft. The flexion tests were carried out in the Test Universal Machine. Fracture or graft extrusion was not observed. Mechanical failure was observed in the vertebral body-fibular graft interface, characterized by fracture of the adjacent vertebral bodies in 11 experiments and depression of the cancellous bone in nine. The posterior longitudinal ligament and the posterior ligamental complex were not injured in any of the specimens. The authors concluded that, in an experimental study, the fibular graft is resistant and provides immediate stability to the cervical spine when submitted to flexion load.

Key Words: Biomechanics; cervical spine; fibular graft.

INTRODUCTION

Trauma in flexion is the most common type of cervical trauma⁽¹⁾. These lesions can cause fracture of the vertebral body, associated or not to ligamental instability, making vertebral stabilization difficult to be kept or obtained with conservative methods. Besides that, flexion trauma can result in medullar compression secondary to body fracture or disc herniation.

Degenerative, inflammatory, infectious and neoplastic processes can also cause significant medullar compression, deformity and cervical instability. The potential severity of these clinical situations demands surgical procedures in order to protect the medulla and the nerve roots from additional damage, decompress them when necessary, reestablish the physiological alignment and restore cervical stability. Failure in stabilizing the column increases the risk of angular deformities and neurological deterioration.

Removal of vertebral body is widely used in medullar compressions and anterior instabilities; the defect created by resection of the vertebral body needs a supportive structure to promote stability and reestablish the vertebral alignment⁽¹²⁾.

In the last few years different arthrodesis techniques and new implant systems were developed and different kinds and shapes of graft were used attempting to refine the cervical disorders treatment. Notwithstanding, experimental studies are scarce for us to establish which is the best technique and the kind of graft to be used in the treatment of all kinds of instability.

Biomechanical models are important to evaluate new stabilization techniques before clinical utilization. The information obtained can serve as a base for the development and refinement of different structures and fixation materials used in the cervical region.

The aim of this study was to propose a biomechanical model using human cadaver cervical spine undergoing C5 vertebral body removal and adjacent discectomy and access it practical applicability as well as to evaluate the immediate in vitro behavior of the stabilization with fibular bone grafting during simulation of the flexion mechanism in the Test Universal Machine.

MATERIAL AND METHODS

This study was performed in 20 specimens of the subaxial (C₃ - C₇) region of male human cadavers cervical spine. All corpses were adults and dead from non accidental and non related to musculoskeletal system causes. Age of death ranged from 31 to 58 years, average 46 years. All corpses were registered in Serviço de Verificação de Óbitos da Capital - SP, in the period from March to October 2000.

The specimens were obtained from fresh bodies in prone position. The spine was approached through a longitudinal incision over the spinal processes, involving skin and subcutaneous tissues, with exposure from the occipital bone to the upper third of thoracic spine. As carefully as necessary the spine specimens from C₁ —T₁ were removed, keeping intact the bony, muscular and ligamental structures. The fibular grafts were obtained from legs of seven cadavers through a posterior-lateral approach, muscle divulsion and resection of 20 cm of fibular shaft.

The specimens were packed in plastic bags and kept frozen at — 20⁰ C. This procedures aim to keep physical properties of the bone, annulus fibrosus and ligaments.

In order to build the biomechanical model, each spine was removed from the freezer and left at room temperature and humidity for 12 hours, after this being submitted to removal of soft tissues of vertebral body of the first cervical vertebra and from the vertebral body of the first thoracic vertebra, except the articular capsule and ligaments.

Following, it was performed a corpectomy of C₅ with oscillating electric saw Dyonics / Smith-Nephew with a narrow blade 2 mm thick and 1 cm wide, reference 3704, keeping intact the intervertebral joints and the posterior longitudinal ligament. All discal material of the segments C₄ — C₅ and C₅ —C₆ was properly removed leaving terminal plates adequately clean and plain for performing the cavities for mortising the graft. All remaining capsular, ligamental and muscular structures were untouched.

In order to prepare the inferior cavity of C₄ terminal plate it was used a battery operated mini drill with an abrasion drill of 5.5 x 10 mm, branded Linvatec. In an standard manner cavities of 2-3 mm deep, and wide as about one third of vertebral body width, and placed in the connection of medium to anterior third (anterior-posterior) were prepared for fitting the fibular graft.

The fibular graft was prepared in its upper and inferior extremities with an oscillating Dyonics / Smith-Nephew electric saw. The notch created was 2-3 mm long and involved one third of fibular diameter, aiming to fit better the graft to the receptor site, allowing a better locking. The size of the graft was calculated from the measurement of the remaining space after the removal of the vertebral body and the adjacent discs from each individual specimen. The graft was placed in the receptor site under pressure, and for this it was made 1 mm larger than the space obtained by the corpectomy and adjacent discectomy. The final aspect of the fibular graft in the site is displayed (Figure 1). The grafts were removed from the cortical portion of fibular shaft and each fibula gave approximately four to five grafts.

The spines were fixed in their extremities by means of two stainless steel wires of 1.5 mm perpendicularly crossed and involved in a polymetilmetacrylate mold according to Machado's technique ⁽¹⁵⁾.

All specimens with their acrylic basis underwent X-ray examination in anterior-posterior and lateral views in order to identify and exclude specimens with bony alterations suggestive of neoplasia, metastasis and fracture.

All the pieces were again packed in plastic bags and kept frozen at — 20 ⁰ C.

In order to perform the tests the specimens were defrosted at room temperature for 12 hours and following adapted to an equipment for performing flexion testing ⁽¹⁵⁾ (Figure 2).

The testing equipment fixed to the specimen was placed in an Universal Machine for mechanical assay with a 20 kN Kratos load cell from the Bioengineering Lab of Faculdade de Medicina de Ribeirão Preto — USP ⁽²⁴⁾. The Universal Machine was adapted to a Series 200 Sodmex extensiometry bridge with a reader module of the forces applied to the load cell in a 10 units to 1 Kgf scale. This bridge had a speed control device of 5 mm/min.

One specimen (number 01) was prepared only for use as a pilot test, in which the course of the Universal Machine was found to be enough to produce a mechanical failure of the piece in the flexion mechanism. In order to accommodate the system, it was established a pre-load of 2 Kgf, which produced a starting flexion of the piece. It was standardized that all the tests would start after this pre-load. The flexion provoked in the spine during the test was measured in angles by means of an analogical reader fitted to the Testing Machine.

It was recorded, at each degree of flexion of the test, the applied load and the resulting deformation degree. The relationship between the applied load (loading cell times piece length, that represents the Flexion Momentum — MF exerted over the testing piece) and de angular deformation recorded by the angle reader was expressed by means of a mathematic equation ^{(15, 16, 27)}.

The tests were stopped at the point of mechanical failure of the test represented by the sudden drop of load recording in the reader module.

After the test a new radiographic study was performed both in anterior-posterior and lateral views, in order to check the bony modifications suffered by the test body or by the graft. The following parameters were observed and recorded: displacement of the graft and fracture of the graft, vertebral body, spinous process, lamina and facet.

For investigation of inside injuries, the frozen specimens were removed from their molds and cut sagitally, using an electric saw with stainless steel blade 1 mm thick in order to avoid injuries to soft tissues.

Following it was performed a dissection were there were analyzed in an standard way the following structures: vertebral body, graft, intervertebral discs, canal and medulla, posterior longitudinal ligament, zigoapophiseal joints, joint capsules, yellow ligament, inter-spinous and supra-spinous ligaments.

RESULTS

In the after test anterior-posterior and lateral radiographic evaluation, it was observed fracture of vertebral body 11 specimens (Table 1). In specimens number 04, 05, 06, 11, 16, 19 and 23 the fracture occurred only in the C₆ vertebral body. In specimens number 08, 10, 17 and 21 fracture occurred in C₄ and C₆. No fractures were observed in the spinous process, lamina, facet or fibular graft of the specimens. Anterior or posterior displacement of the graft was not evidenced.

Biomechanical analysis resulted in the 20 studied samples that the Maximum Flexion Momentum was in average 17.73 + 1.74 Nm (Newton x meter). Concerning degree of deformation, the average was of 30.87º + 5.73º. Table 2 presents the Minimum MF for each specimen at the beginning of the test and the Maximum MF of each sample before failure of the system, correlating them to the fracture of adjacent to the graft vertebral bodies.

After the test, sagital section of the samples has shown fracture of the vertebral body anterior cortical in 11 samples, previously observed in the X-rays study. The nine remaining samples presented depression of the adjacent cancellous bone of vertebral bodies, closely contacting the incisions made on the fibular graft. Lesions in the posterior longitudinal, yellow, supraspinous and interespinous ligaments were not observed. Intervertebral disc lesion was observed in specimens number 07, 10, 14 and 15. (Table 3).

DISCUSSION

Corpectomy is widely accepted as treatment method of vertebral injuries presenting anterior instability or needing medullar decompression ^(6,15,22,49). Notwithstanding, the surgical techniques to restore biomechanical stability after removing the vertebral body are controversial.

Supporting grafts are used to reconstruct the segment submitted to corpectomy, reestablish the anatomical alignment and provide stability to the spine until bone healing takes place ^(3,7,23,26).

A number of sites can provide grafts, preferably the fibula, the iliac crest and ribs. In some situations, cervical stabilization can be complemented by metal implants ^(8,11), methylmetacrylate⁽²⁸⁾ or hydroxyapatite⁽³⁰⁾.

The iliac graft is the most used in the anterior cervical spine. Fitted into different shapes ^(3,7,21,26), it has been used since the fifties with good clinical results. However, it is not very much efficient to provide mechanical support after corpectomy. Complications as fractures and extrusion of the graft and loss of vertebral alignment with the development of late kyphotic deformities are also frequently observed⁽²³⁾.

Considering this, we chose the fibular graft to stabilize the proposed model. We believe that the mechanical support provided to the region submitted to corpectomy is better when compared to other types of available grafts and its anatomical characteristics of shape and size facilitate adaptation at the receptor site.

Assessing in the laboratory the immediate mechanical resistance of several kinds of graft, some authors concluded that the fibula is approximately four times more resistant than the anterior and posterior iliac crest graft, and that the rib graft is the weakest ^(25,30). Conversely, a statistically significant difference was not observed between the fibula, rib and iliac crest resistance after axial compression tests⁽⁴⁾.

In this study, we used a human cadaver model under controlled conditions of anterior instability created by means of C₅ corpectomy and adjacent discectomy, aiming to evaluate the biomechanical applicability in order to use it in studies analyzing different kinds of stabilization.

To test de proposed model we used the fibula cortical graft, filling the defect created by the vertebral body withdrawal. We evaluated its functional behavior, the system mechanical resistance, the attained immediate stabilization, as well as the capacity of the graft to resist fracture and extrusion.

In the first phase of our investigation we noticed that during the biomechanical test, none of the fibular grafts evidenced macroscopic fracture, and this was confirmed in the second phase by the radiographic and anatomical study. This was attributed to the fibula basically consists in cortical bone, being able to provide immediate structural support and high endurance to deforming forces ^(13,14,17).

During the tests the fibular graft presented stable behavior and displacement or extrusion were not observed. We believe that the notch made in the superior and inferior part of the graft allows it to lock in the adjacent bodies, preventing displacement during the flexion mechanism when the anterior structures are compressed, pushing the graft outwards.

Terminal plates of superior and inferior vertebrae should be carefully removed, preserving to the most anterior portion of the vertebral body, that will pose a mechanical barrier to graft extrusion. Excessive removal of subcondral bone may allow the graft to depress into adjacent vertebral bodies, and thus must be avoided ^{(5, 9)}.

We emphasize that bone resection of the receptor site must be the most economical as possible, aiming only to create the minimum space necessary to the adaptation of the notch and we believe that the limit of 2-3 mm depth in the subchondral bone provides a good anatomical basis for the graft to lock, not increasing the risk of extrusion.

The literature reports complications related to fibular graft displacement and extrusion which contributes to increase post-operative morbidity⁽²⁹⁾. We stress, however, that the techniques of making and fixating grafts are different and thus it is difficult to compare the results.

We think that the technique to make the notch in the graft and its placement under pressure, preserving to the most the anterior cortical and the cancellous bone of the adjacent vertebral bodies can significantly contribute to avoid this kind complication.

The tests were carried out in flexion because we believe that this movement is the most similar to the trauma mechanism being thus able to evaluate the cervical spine under similar conditions to those observed in the clinic; conversely, in the literature most of the tests were performed with axial compression load ^(4,19,20,22).

Biomechanical analysis of the tests, due to angular deformation and different length of the spines, was carried out using the Flexion Momentum ⁽¹⁸⁾, a resultant used to detect the load variation during angulation. Each deformation angle of the specimen presents its own Flexion Momentum, calculated by means of a mathematic equation developed by bioengineers^(15,16,27).

Although it is known that the structures composing the cervical spine are viscoelastic and this property and bone resistance tend to decrease with ageing^(2,15), when analyzing our results it is observed that deformation which biomechanically corresponds to elasticity presented an homogeneous behavior. A possible explanation is that the age of the specimens was similar.

The Maximum Mean Flexion Momentum (MFMM) also presented an homogeneous behavior in the twenty tested specimens. The relatively low mean age and the use of spines from male cadavers contributed to a lower variability of the results, agreeing with ⁽²⁾.

The experimental study ⁽¹⁵⁾ performed in 1993 caused flexion of normal cervical spine of human cadavers to the collapse point demonstrates that the group of males with ages ranging 35 to 40 years presented a MFMM of 19.78 Nm while the group aged between 55 and 60 years old presented a MFMM of 16.21 Nm. Our results present a MFMM of 17.73 Nm, thus allowing us to say that the model with fibular grafting stabilization presents an endurance that is similar to normal spine, since both studies were performed at the same laboratory and used the same methodology.

The post-test X-rays study allowed us to confirm the absence of fracture or migration of the fibular graft. Samples number 04, 05, 06, 08, 10, 11, 16, 17, 19, 21 and 23 presented fracture of anterior cortical of vertebral bodies, adjacent to the graft, suggesting that the vertebral body is less resistant when compared to the fibular graft. In samples number 08, 10, 17 and 21 the fracture occurred in C₄ and C₆ simultaneously while the remaining samples presented fracture only in C₆ . We could not establish a direct relationship of the applied loads to the observed damage. Some specimens that presented depression of cancellous bone needed a higher load to cause mechanical failure (a bigger Maximum Fexion Momentum) than specimens presenting with fracture of anterior cortical of vertebral bodies of C₄ and C₆ (Table 2). This should be additionally studied due to need of a more detailed biomechanical analysis. It was not observed any avulsion fracture of spinous processes, lamina or facet in any of the assays.

In samples number 07, 09, 12, 13, 14, 15, 18, 20 and 22, we observed depression of the superior or inferior cancellous bone, in close contact with the notch made on the graft. We believe that depression of the cancellous bone took place due to the compression of the graft notch and was responsible for the mechanical failure before the fracture of anterior cortical of the vertebral bodies. Some authors ⁽³⁷⁾ suggest that some constructions with fibular grafting may fail in vertebral body-fibular graft interface due to cortical bone of fibular graft is more rigid than the cancellous bone of adjacent vertebral bodies.

Our anatomical analysis did not evidence injuries of the posterior longitudinal ligament and posterior ligamental complexes, though the tests were effected until failure. We believe that failure occurred in the transition body-graft before posterior ligamental injury took place.

Application of flexion deforming forces, makes the cervical spine to suffer anterior compression and posterior distraction. The posterior ligaments have their tension increased during cervical flexion favoring rupture, what was not observed in any assay. In the twenty samples tested, only the anterior structures failed.

The intervertebral discs are also submitted to the anterior compressive force during the flexion mechanism. We observed disc injuries in samples number 07, 10, 14 and 15 (Table 3), probably due to increased internal pressure exerted by the applied load. These injuries were joined by flattening of the adjacent terminal plates without disc protrusion inwards the medullar canal, agreeing with the study ⁽²⁰⁾, that states that the vertebral body is less resistant to compression than the normal disc and that the pulp nucleus under pressure provokes terminal plate protrusion into the center of the vertebral body until bone failure. It is important to highlight that these samples also presented cancellous bone depression in contact with the notch in the bone, making difficult to evaluate whether only one or both structures were responsible for the mechanical failure and which one.

Biomechanical models are important for evaluation of cervical stabilizations, however its correlation to clinics is not yet totally established. Choice of appropriated stabilization technique and type of bone grafting is of fundamental importance for therapeutic success. The challenge that remains is to simulate in a precise way the functional cervical complex and the load conditions observed in vivo aiming to obtain more efficacious biomechanical evaluation.

CONCLUSION

It is concluded that the tested biomechanical model was shown to be adequate to the proposed objective, being useful for analysis of different types of cervical stabilizations, and that fibular cortical bone graft, used in the proposed model, proved to be resistant, giving immediate stability to cervical spine when submitted to flexion load.

REFERÊNCIAS BIBLIOGRÁFICAS

*Work performed at Bioengineering Laboratory, College of Medicine, Ribeirão Preto, University of São Paulo

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Publication Dates

• Publication in this collection
  02 Sept 2005
• Date of issue
  June 2002

History

• Accepted
  28 Mar 2002
• Received
  10 July 2001