David Volkheimer

Limitations of current in vitro test protocols for investigation of instrumented adjacent segment biomechanics: critical analysis of the literature.

PurposeAccelerated degenerative changes at intervertebral levels adjacent to a spinal fusion, the so-called adjacent segment degeneration (ASD), have been reported in many clinical studies. Even though the pathogenesis of ASD is still widely unknown, biomechanical in vitro approaches have often been used to investigate the impact of spinal instrumentation on the adjacent segments. The goal of this review is (1) to summarize the results of these studies with respect to the applied protocol and loads and (2) to discuss if the assumptions made for the different protocols match the patients’ postoperative situation.MethodsA systematic MEDLINE search was performed using the keywords “adjacent”, “in vitro” and “spine” in combination. This revealed a total of 247 articles of which 33 met the inclusion criteria. In addition, a mechanical model was developed to evaluate the effects of the current in vitro biomechanical test protocols on the changes in the adjacent segments resulting from different stiffnesses of the “treated” segment.ResultsThe surgical treatments reported in biomechanical in vitro studies investigating ASD can be categorized into fusion procedures, total disc replacement (TDR), and dynamic implants. Three different test protocols (i.e. flexibility, stiffness, hybrid) with different loading scenarios (e.g. pure moment or eccentric load) are used in current biomechanical in vitro studies investigating ASD. According to the findings with the mechanical model, we found that the results for fusion procedures highly depend on the test protocol and method of load application, whereas for TDR and dynamic implants, most studies did not find significant changes in the adjacent segments, independent of which test protocol was used.ConclusionsThe three test protocols mainly differ in the assumption on the postoperative motion behavior of the patients, which is the main reason for the conflicting findings. However, the protocols have never been validated using in vivo kinematic data. In a parallel review on in vivo kinematics by Malakoutian et al., it was found that the assumption that the patients move exactly the same after fusion implemented with the stiffness- and hybrid protocol does not match the patients’ behavior. They showed that the motion of the whole lumbar spine rather tends to decrease in most studies, which could be predicted by the flexibility protocol. However, when the flexibility protocol is used with the “gold standard” pure moment, the difference in the kinematic changes between the cranial and caudal adjacent segment cannot be reproduced, putting the validity of current in vitro protocols into question.

Increase or decrease in stability after nucleotomy? Conflicting in vitro and in vivo results in the sheep model

Nucleotomy is a common surgical procedure to treat disc herniations. The potential occurrence of segmental instability after surgery, however, is suspected to necessitate re-operation and fusion. Although in vitro studies support the theory of destabilization after nucleotomy, a prior, in-house animal study contrarily revealed an increase in stability after surgery. To identify which structural compartment of the motion segment is decisive for increased stability after nucleotomy in vivo, the flexibilities of ovine motion segments were measured after different stepwise reductions at the anterior and posterior spinal column. Different test groups were used in which nucleotomy had been performed during surgery in vivo and under isolated in vitro conditions, respectively. In accordance with expectations, in vitro nucleotomy on ovine motion segments significantly increased flexibility. By contrast, nucleotomy significantly decreased flexibility 12 weeks after surgery. After removal of the posterior structures, however, the differences in flexibility diminished. The present results thus suggest that it might not exclusively be the trauma to the intervertebral disc during surgery which is decisive for post-operative stability, but rather adaptive mechanisms in the posterior structures. Therefore, care should be taken to minimize the damage to the posterior structures in the course of the surgical approach, which more likely compromises stability.

Comparison between different methods for biomechanical assessment of ex vivo fracture callus stiffness in small animal bone healing studies.

For ex vivo measurements of fracture callus stiffness in small animals, different test methods, such as torsion or bending tests, are established. Each method provides advantages and disadvantages, and it is still debated which of those is most sensitive to experimental conditions (i.e. specimen alignment, directional dependency, asymmetric behavior). The aim of this study was to experimentally compare six different testing methods regarding their robustness against experimental errors. Therefore, standardized specimens were created by selective laser sintering (SLS), mimicking size, directional behavior, and embedding variations of respective rat long bone specimens. For the latter, five different geometries were created which show shifted or tilted specimen alignments. The mechanical tests included three-point bending, four-point bending, cantilever bending, axial compression, constrained torsion, and unconstrained torsion. All three different bending tests showed the same principal behavior. They were highly dependent on the rotational direction of the maximum fracture callus expansion relative to the loading direction (creating experimental errors of more than 60%), however small angular deviations (<15°) were negligible. Differences in the experimental results between the bending tests originate in their respective location of maximal bending moment induction. Compared to four-point bending, three-point bending is easier to apply on small rat and mouse bones under realistic testing conditions and yields robust measurements, provided low variation of the callus shape among the tested specimens. Axial compressive testing was highly sensitive to embedding variations, and therefore cannot be recommended. Although it is experimentally difficult to realize, unconstrained torsion testing was found to be the most robust method, since it was independent of both rotational alignment and embedding uncertainties. Constrained torsional testing showed small errors (up to 16.8%, compared to corresponding alignment under unconstrained torsion) due to a parallel offset between the specimens’ axis of gravity and the torsional axis of rotation.

Spinal fusion without instrumentation – Experimental animal study

Background The number and cost of instrumented spinal fusion surgeries have increased rapidly, primarily for the treatment of lumbar segmental instabilities. However, what if the organism itself is able to restore segmental stability over time? This large‐animal study using sheep aimed to investigate whether the reparative response after destabilization via facetectomy and nucleotomy without instrumentation can effectively fuse the spinal segment comparable to instrumented standard fusion surgery. Methods The following four surgical interventions were investigated: dorsal fixation via internal fixator, ventral fixation via cage as well as facetectomy and nucleotomy without additional instrumentation. Six months postoperatively, the animals were sacrificed, and the lumbar spines were used for biomechanical tests. Findings Spinal stability was restored to the destabilized spinal segments at six months postoperatively and was comparable to the results of conventional surgery via screws and cages. Iatrogenic hypomobilization caused significant reductions in facet joint space and intervertebral disc height of segments at index and adjacent level. Restabilized segments after iatrogenic hypermobilzation also significantly decreased facet joint space and disc height at index level, but revealed no influence on adjacent segments. Interpretation These findings in the sheep model question the necessity of costly instrumentation and suggest the alternative possibility of stimulating the reparative capacity of the body in human lumbar spine fusion surgery. HighlightsAnimal study on fusion after spinal destabilization with and without instrumentationSpinal stability was restored six months postoperatively without instrumentation.Results were comparable to conventional surgery via screws and cages.Findings challenge the necessity of costly instrumentation.

Is intervertebral disc degeneration related to segmental instability?: An evaluation with two different grading systems based on clinical imaging

Background Several in vitro studies investigated how degeneration affects spinal motion. However, no consensus has emerged from these studies. Purpose To investigate how degeneration grading systems influence the kinematic output of spinal specimens. Material and Methods Flexibility testing was performed with ten human T12-S1 specimens. Degeneration was graded using two different classifications, one based on X-ray and the other one on magnetic resonance imaging (MRI). Intersegmental rotation (expressed by range of motion [ROM] and neutral zone [NZ]) was determined in all principal motion directions. Further, shear translation was measured during flexion/extension motion. Results The X-ray grading system yielded systematically lesser degeneration. In flexion/extension, only small differences in ROM and NZ were found between moderately degenerated motion segments, with only NZ for the MRI grading reaching statistical significance. In axial rotation, a significant increase in NZ for moderately degenerated segments was found for both grading systems, whereas the difference in ROM was significant only for the MRI scheme. Generally, the relative increases were more pronounced for the MRI classification compared to the X-ray grading scheme. In lateral bending, only relatively small differences between the degeneration groups were found. When evaluating shear translations, a non-significant increase was found for moderately degenerated segments. Motion segment segments tended to regain stability as degeneration progressed without reaching the level of statistical significance. Conclusion We found a fair agreement between the grading schemes which, nonetheless, yielded similar degeneration-related effects on intersegmental kinematics. However, as the trends were more pronounced using the Pfirrmann classification, this grading scheme appears superior for degeneration assessment.

Is pelvic fixation the only option to provide additional stability to the sacral anchorage in long lumbar instrumentation? A comparative biomechanical study of new techniques

Background: Supplementary iliac screws have the highest potential to protect S1‐pedicle‐screws from loosening in long fusion constructs. However, this technique bridges the iliosacral joint with potential disadvantages for the patient. This study aimed to evaluate if two different established fixation techniques can be used in addition to pedicle screws as alternative to iliac screws, and if these two techniques can provide similar stability when S1‐pedicle‐screws are loosened. Methods: Flexibility testing with pure moments of 7.5 Nm was performed with six human osteopenic/osteoporotic L4‐pelvis specimens. The following conditions were investigated: 1. Intact; 2. Destabilization L5/S1; 3. Fixation with rigid L4‐S1 pedicle‐screw‐system; 4. Condition 3‐ loosening of S1‐screws; 5. Condition 4‐ L5‐S2‐lamina‐hooks; 6. Condition 4‐ L5/S1‐translaminar‐screws; 7. Condition 4‐ S2‐ala‐ilium screws. Findings: Application of compressive L5‐S2‐lamina‐hooks or L5/S1‐translaminar‐screws next to pedicle screws in L5 and S1 was feasible in all specimens. L4‐S1‐pedicle‐screw‐instrumentation reduced the Range of Motion significantly compared to the destabilized condition. After simulation of S1 screw loosening, lamina hooks only reduced the Range of Motion in flexion/extension significantly. L5/S1‐translaminar‐screws had a higher stabilizing effect in lateral bending and axial rotation, but the effect of both systems was smaller than with an instrumentation extension to the os ilium. Interpretation: In long lumbar pedicle screw instrumentations including L5/S1, additional ilium screws have the highest potential to protect the S1‐anchorage. Additional L5/S1‐translaminar‐screws can increase stability of the lumbosacral junction without bridging the iliosacral joint, whereas lamina hooks showed no significant biomechanical benefit. HighlightsPlacing L5‐S2‐lamina‐hooks or L5/S1‐translaminar‐screws next to pedicle screws in L5 and S1 was feasible.L4‐S1‐pedicle‐screw‐instrumentation reduced the Range of Motion significantly.Supplemental lamina hooks only reduced the Range of Motion in flexion/extension significantly.L5/S1‐translaminar‐screws insignificantly re‐stabilized the junction in all motion directions.An extension of the construct to the os ilium had the strongest stabilizing effect.

In Vitro Testing of Cadaveric Specimens

Abstract In vitro testing is employed to investigate various aspects of the biomechanical response of spinal specimens as a whole as well as of its individual components and for the preclinical assessment of novel implants and surgical techniques. The golden standard for in vitro testing is to use fresh/fresh frozen human specimens; the suitability of embalmed and animal specimens should be checked with respect to the specific research question. A biomechanical investigation conducted on a spinal specimen should aim to replicate the complex loading and constraint conditions acting in vivo, which are determined by the combination of gravity and the action of the trunk muscles. However, a simplified loading environment, such as pure moments with or without a compressive follower load, is preferred in most cases because the exact loads applied are not known. This chapter describes various test approaches that have been developed to simulate such loading conditions, as well as other relevant aspects such as the measurement of the intradiscal pressure, the simulation of a repetitive loading scenario such as the spinal loads acting in daily activities, as well as the assessment of fluid exchange between the spine and the surrounding environment.

Basic Biomechanics of the Lumbar Spine

Abstract As the lowest section of the mobile human spine, the lumbar spine’s key role lies in its ability to support the upper body by transmitting forces and bending moments to the sacrum, which is connected to the pelvis via both sacroiliac joints. Like all regions of the spine, the lumbar spine protects the spinal cord and nerve roots from damage by providing a protective sheath. Mechanical stability, which can be defined as the ability of the vertebrae to maintain their relationship and to limit their relative displacement during physiologic loads and postures, is required to fulfill this task and to prevent premature mechanical and biologic deterioration of its structures. A complex interaction between the active (muscles), passive (osteoligamentous spine), and neural components is necessary to prevent instability. In the following section, the basic anatomy and physiology of the osteoligamentous lumbar spine is described, and its differences to the other sections of the spine (cervical and thoracic spine) are pointed out. Because the lumbar spine must carry much heavier loads than the cervical and thoracic sections, subsequent focus is placed on loading of the lumbar spine.