Elastic resistance of the spine: Why does motion preservation surgery almost fail?

Abstract

Single metamere motility should not be interpreted merely as a movement on the 3 planes but also, and above all, as elastic resistance to dynamic stress on these 3 planes. In the light of this consideration, the aim of motion preservation is to neutralize excessive movements while preserving the physiological biomechanical properties of the metamere involved to interrupt the progression of degenerative processes and to prevent adjacent segment disease. Despite the fact that a myriad of devices have been developed with the purpose of achieving dynamic neutralization of the spine, there are now some doubts regarding the true efficacy of these devices.

Key Words: Elastic resistance; Disc prosthesis; Dynamic implant; Interspinous device; Biomechanics

Core tip: Elastic resistance of the spinal motor unit is a biomechanical property often underestimated but crucial for the stability of the spine. The biomechanics of dynamic implants take into account only the motility of the devices and not the elastic resistance. Is it possible that this is the reason why dynamic implants almost fail?

INTRODUCTION

In April 2009 and March 2011, two earthquakes in Abruzzo and Japan destroyed 65% of the buildings in reinforced concrete. In April 2011, the results of the Chi-Quadrato DIMS research project which evaluated the effects of high magnitude earthquakes on buildings built in wood was published. Wooden buildings proved to be those with the highest resistance to the mechanical movements of an earthquake owing to the physical properties of wood, elastic resistance to load-bearing and twisting. For this reason, in May 2011, 500 houses built in wood were delivered to the homeless Abruzzo population. The overall characteristics of the vertebral column are the same as those of wood, namely elastic resistance to movement, twisting potential and elastic resistance to load bearing. These aspects reflect the three main functional characteristics of the spine: motility in all 3 spatial planes, passive and active resistance to the axial load and elastic resistance to excessive degrees of movement. In the light of this, we can assert that motility at the level of a single metamere should not be interpreted merely as movement on the 3 planes but also, and above all, as elastic resistance to dynamic stress on these 3 planes. In fact, metameric movement depends on an active motility, involving the intervertebral disc, the articular masses and the muscular structures, and a passive motility, involving the disc, ligamentous system and articular capsules[1-15]. In the light of this, the aim of motion preservation is to neutralize excessive movements while preserving the physiological biomechanical properties of the metamere involved in interrupting the progression of the degenerative process and to prevent adjacent segment disease (ASD). This procedure was firstly named “dynamic stabilization” but nowadays the term “dynamic neutralization” (intended as neutralization of excessive degrees of movement) seems to be more appropriate[16]. The numerous devices developed to achieve a dynamic neutralization of the spine have been divided into anterior (aimed at restoring or maintaining disc height and motion by total disc replacement) and posterior (aimed at restoring or maintaining articular movement or posterior tension band). These devices comprise total disc prosthesis, posterior interspinous or interlaminar systems, systems with pedicular screws and prosthesis of the facets or posterior ligaments. Despite the good intentions of dynamic neutralization, there are now some doubts regarding the efficacy of these devices.

DISC PROSTHESIS

The aim is to restore active movement in flexion-extension, rotation and lateral bending of the damaged disc. They provide excellent restoration of movement in all 3 planes but poor elastic resistance to movement, also due to removal of the anterior longitudinal ligament (ALL)[17]. Disc prosthesis allows good movement of the metamere but with a greater range of motion (ROM) in comparison to a normal disc, especially in rotation. This causes over-loading of the facet joints. This is the result of an underestimation of the properties of the disc whose principal characteristic is elastic resistance.Nowadays, the materials and design of disc prostheses are not able to completely guarantee the biomechanical characteristics of a healthy disc and the physiological role of the nucleus pulposus during segmental motion[4,5,18-22]. Moreover, the surgical technique used for disc prosthesis insertion markedly reduces the elastic resistance of the metamere involved due to the elastic properties of the disc and the tension of the ALL. Hence, the results are very good in terms of movement but poor in terms of elastic resistance; this feature causes an acceleration of the degeneration of the spinal motor unit which often ends in heterotopic ossification of the prosthesis[23-37]. Moreover, recent studies have shown that the incidence of the ASD, ASD is not influenced by the use of disc prosthesis or by the interbody cage. This feature explains the controversy regarding prevention of ASD[38-43].

INTERSPINOUS-INTERLAMINAR SYSTEMS

The aim is to neutralize excessive movement in flexion and extension associated with distraction of the metamere to opening of the foramens. It provides fair control of flexion-extension but no control of active movements or passive resistance in rotation and lateral bending. Moreover, the insertion of the device in distraction causes an anterior overloading of the already damaged disc with a change in the 80-20 rule of the spine loading. This biomechanical aspect accelerates and does not prevent degenerative processes of the metamere, with the possibility of developing spondylolisthesis[44-53]. In a recent work, Ayturk et al[2], reviewing the spinal literature concerning the postoperative status of interspinous devices (ID) followed over an average of 23.0-42.9 postoperative months, revealed that ID were burdened by an 11.6%-38.0% complication rate, 4.6%-85.0% reoperation rate and a 66.7%-77.0% incidence of poor outcomes. Last but not least, the devices implanted have a very high cost. In the light of the above, with high maximal complication rates (38%), reoperation rates (85%), poor outcomes (77%) and high costs, the utilization and implantation of ID remains extremely controversial[54]. In my opinion, this sums up the real situation about ID: high costs and poor outcome.

PEDICULAR DYNAMIC SYSTEMS

The aim is to dynamically neutralize excessive movement and prevent ASD. They provide excellent control of movement in flexion-extension and lateral bending but minimal control in axial rotation. We can say that the intended functions of a motion preservation system are maintenance of the intervertebral ROM to reduce intradiscal pressure and reduce facet joint forces[55]. In this regard, metameric movement in axial rotation plays an essential role as the biomechanical vector of force that solicits the facet joints and the disc[56]. To be able to control this movement, the implant should have physical and mechanical characteristics. Biomechanically, the maintenance of the natural intervertebral motion, which especially includes the elastic resistance to torsion, can only be achieved if the elastic modulus of the longitudinal rod is high, but to considerably reduce the intradiscal pressure, high implant stiffness is required and in order to reduce facet joint forces, a rigid connection between longitudinal rod and pedicle screw is necessary. For these reasons the intended functions of a motion preservation system must have a contradictory implant stiffness[57,58]. In fact, a dynamic system based on screws and elastic rods have to have some particular properties in order to maintain the biomechanical characteristics of the spinal motor unit, namely rigidity and flexibility that are not compatible with one other. Actually, since rigid systems guarantee rigidity and dynamic systems guarantee flexibility, the result is: (1) in the case of rigid systems, a complete loss of the ROM; (2) in the case of dynamic systems, an overload of the disc and the articular facets. Preservation of these structures relies exclusively on the control of the elastic twisting movements. Since the only way to control these forces is the rigid connection of the rods and since this connection does not preserve the physiological movement of the spinal motor unit, no dynamic pedicle system able to control such rotation movements currently exists[57-63]. The screw insertion technique via the transverse process, such as the DYNamic NEutralization SYStem (DyNeSys) system, permits a rotational axis of the metamere posterior with respect to the physiological one, generating a movement with a fulcrum of metameric rotation different to that of the other metameres. Screw insertion via the articular process, such as the Flex system, modifies the rotational axis that becomes more posterior than the DyNeSys and the physiological one. Both of these systems increase overloading of the facet joints with biomechanical variations[64]; (3) normally, after distraction of the metamere, intradiscal pressure values are markedly reduced for rigid implants. The effect on intradiscal pressure is the same as in a dynamic implant; meaning that the mechanical effects of a dynamic implant on discs are similar to those of a rigid fixation device, except after distraction. In the light of this, as dynamic pedicle systems are incompatible with a distraction implanting, a dynamic implant does not necessarily reduce axial spinal loads compared to an uninstrumented spine[57,58]; (4) pedicle-screw-based dynamic implants strongly reduce posterior disc bulging during extension since the presence of a dynamic rod controls the movements in compression on the posterior elements. However, in contrast to the intact spine, based on a posterior shift of the core, insertion of such an implant increases posterior disc bulging during flexion. The reason for this is that the dynamic rods, secured by the screw in the pedicle, prevent normal displacement of the nucleus pulposus within the disc during normal movements in flexion. This implies an increased tension on the fibers of the annulus which can lead to a higher risk of a recurrence of a bulging disc[65-67]; and (5) the use of a dynamic system as a hybrid implant due to prevention of ASD has shown that there are no clinical benefits in the disc with initial degeneration[41,68,69]. This is probably attributable to the fact that the biomechanics of the hybrid system do not control the hypermobility that is at the base of the initial degeneration of the adjacent disc. So if the disc is already degenerated, the hybrid system does not seem to protect against the progression of degeneration[70].

TOTAL FACET REPLACEMENT

The aim is to dynamically neutralize hyper-movement of the facet joints and to restore the articular ROM. It provides excellent control of movement in flexion-extension and lateral bending and good control of movement in rotation and isindicated in cases of moderate disc degeneration, facet pain and arthrosis of the articular masses[71,72].

TOTAL POSTERIOR ELEMENT REPLACEMENT TOPS

This procedure allows at least 85% of the ROM in the sagittal plane and mimics the flexibility of the metamere in lateral bending.In axial rotation, it mimics the biomechanical behavior of the posterior complex[30,71,73].

These last two devices, in my opinion, mimic the physiological motility of the posterior elements of the spinal motor unit in an attempt to restore the biomechanical characteristics of the facet joints and posterior ligamentous system.

CONCLUSION

On the basis of this analysis, we can assert that dynamic neutralization systems seem to be very promising although long-term results are lacking for many of them. However, the certainty is that the future of vertebral stabilization will be dynamic systems instead of rigid ones. Unfortunately, to date, none of the dynamic systems used alone is capable of controlling movements on all 3 planes of motion of the functional motor unit. Moreover, motion preservation technology should take into account not only movement but also, and above all, the elastic resistance properties of the metamere involved. In the light of the above considerations, the future of dynamic neutralization will be the control of all the components of the motor unit. Only in this way will it be possible to preserve both motion and the biomechanical properties of the metamere, guaranteeing a degree of vertebral motion and elasticity as physiological as possible.