14 Spine Biomechanics
Preclinical evaluation is necessary to prove the safety and efficacy of new spinal implants. Besides pure mechanical testing, which is required for approval, in vitro tests are essential to investigate the mechanical functionality of an implant in its natural environment. In vitro experiments allow the measurement of the initial postoperative stability from quasistatic flexibility measurements, or long-term performance of a device (e.g., subsidence or screw loosening), from cyclic tests of an instrumented specimen. Concomitantly to the mechanical testing, finite element calculations may provide an “insight” into different structures. As a final step, animal and clinical studies may address the requirements of the biological environment in situ.
In the first decades of spine surgery, the main goal of a spinal system was to provide enough stability to allow fusion of the treated segment. Despite good or excellent clinical results regarding fusion rates and patient satisfaction, several studies have shown unintentional alterations in the segments adjacent to the fusion site, the so-called adjacent segment degeneration. This finding led to the development of motion preserving devices with the theoretical goal of a reduction or elimination of adjacent segment degeneration. Due to the high degree of innovation of new spinal implants, biomechanical spinal research needs to address this dynamic process. This requires adapting the test requirements to specific implant types in order to provide surgeons with a good basis to choose the appropriate, indication-specific implant.
14.1 Basic Biomechanical Concepts
Biomechanical tests can be performed on intact or on defect specimens. An intact specimen consisting of fresh cadaveric material without severe diseases or structural damage is often used to obtain normative values for the test setup. Injured or defect specimens are spinal segments with an existing or a created disturbance of the ligaments, bony tissues, or disks. Each testing sequence in mechanical testing of spinal segment includes some sort of preconditioning. Preconditioning is used to minimize the viscoelastic behavior of the spinal structures and to allow the implant to settle into the surrounding tissues.
14.2 Rigidity/Stability and Flexibility/Instability
The goal of a fusion implant is to provide stability to the treated segment. A fusion system reduces the motion and decreases flexibility, whereas a flexible system allows greater motions and increases flexibility in comparison to the intact specimen.
The term “stability/instability” should be used solely in the context of the in vivo environment. In this system, an instability (e.g., abnormally large intervertebral motions seen in spondylolisthesis) is thought to be caused by a dysfunction of the stabilizing structures (active, passive, and neural). 1
Jargon Simplified: Functional Spinal Unit
Two adjacent vertebrae with the intervening intervertebral disk, ligaments, and joint capsules intact represent a functional spinal unit or motion segment. It is the smallest biomechanical unit representing the overall behavior of a specific spinal region.
Jargon Simplified: Construct
A construct is considered a cadaveric or surrogate specimen instrumented with an implant or with a combination of implants.
In the field of spinal biomechanics, a right-handed coordinate system with the following axis orientation is used: The positive x axis is pointing ventrally, the positive y axis to the left lateral side, and the positive z axis cranially. The transverse plane corresponds to the x-y plane, the sagittal plane to the x-z plane, and the frontal plane to the y-z plane (Fig. 14.1).
14.3 Biomechanical Test Methods
In contrast to the strength tests required for approval of a spinal implant or implant system, biomechanical test methods evaluate the characteristics of a device in conjunction with its natural mechanical environment.
These methods can be subdivided into two main categories:
Test for fatigue to measure the mechanical durability of a construct (failure of the implant, as well as the biological structures).
Flexibility test to measure the multidirectional primary stability provided by the spinal device at the treatment site.
14.3.1 Quasistatic Testing
Two principal quasistatic test methods can be distinguished: the stiffness and the flexibility protocol. In the stiffness method, the specimen is rigidly fixed at the caudal end while the upper vertebra is moved to a predefined amount; the resultant forces and moments are measured. In the flexibility test, the caudal end is constrained and a load is applied to the cranial vertebra while the motion of the specimen is recorded. 2 This allows for a more unconstrained and therefore more physiological behavior of the specimen. Nowadays, the flexibility test protocol is considered as “gold standard” when the primary stability of a construct is measured.
Because of its nondestructive character, the flexibility test allows the comparison of varying defect situations and implant configurations within one specimen and therefore represents a sensitive indicator for an implant′s mechanical behavior in situ. The specimen can be tested in different loading modes, which can be adapted to the individual characteristics of an implant. This test method is of huge clinical importance because it indicates the potential for rapid healing and fusion.2
Jargon Simplified: Range of Motion, Neutral Zone, and Elastic Zone (Fig. 14.2)
The range of motion is the maximal deflection a specimen reaches in a single motion direction. The neutral zone describes the deflection reached with minimal resistance of the specimen. It is a good indicator for the laxity of a specimen. The elastic zone describes the elastic behavior of a specimen between the end of the neutral zone and the point of maximal deflection.
14.3.2 Spinal Loading Simulator
To physiologically simulate the motion of a spinal specimen, a custom-built apparatus, modified materials testing machines, as well as industry robots can be used. Independent of the simulator, several basic requirements should be fulfilled (Fig. 14.3):
The apparatus must enable the specimen to move freely in all six degrees of freedom.
The simulator shall be capable of simulating the six loading components separately.
Loading shall be applied either continuously or in stepwise fashion.3
14.3.3 Motion Measurement System
At the early stages of spine biomechanics, the motion of the whole specimen was often recorded by angular transducers measuring the motion of the cranial vertebra of a specimen. If the mobility at the fusion site was the parameter of interest, the specimen length was limited to a single motion segment. With the emergence of more flexible and more powerful measurement systems using ultrasonic or optical detectors, the motion of each vertebra can be recorded separately, enabling the measurement of multisegmental specimens.
14.3.4 Intradiskal Pressure
Sometimes it can be useful to measure the intradiskal pressure with a pressure transducer implanted centrally into the nucleus pulposus of the disk. This allows a comparison of different implants or defects on the exposure of the intervertebral disk of the treated segment (as long as the disk remains intact) or adjacent segments, which is thought to be a good indicator for the development of accelerated degeneration. To check the validity of the loads applied to the specimen, the data can be compared with in vivo measurements during varying activities. 4, 5
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