In Vitro Studies for Investigating Creep of Intervertebral Discs under Axial Compression: A Review of Testing Environment and Results

Creep responses of intervertebral discs (IVDs) are essential for spinal biomechanics clarification. Yet, there still lacks a well-recognized investigation protocol for this phenomenon. Current work aims at providing researchers with an overview of the in vitro creep tests reported by previous studies, specifically specimen species, testing environment, loading regimes and major results, based on which a preliminary consensus that may guide future creep studies is proposed. Specimens used in creep studies can be simplified as a “bone–disc–bone” structure where three mathematical models can be adopted for describing IVDs’ responses. The preload of 10–50 N for 30 min or three cycles followed by 4 h-creep under constant compression is recommended for ex vivo simulation of physiological condition of long-time sitting or lying. It is worth noticing that species of specimens, environment temperature and humidity all have influences on biomechanical behaviors, and thus are summarized and compared through the literature review. All factors should be carefully set according to a guideline before tests are conducted to urge comparable results across studies. To this end, this review also provides a guideline, as mentioned before, and specific steps that might facilitate the community of biomechanics to obtain more repeatable and comparable results from both natural specimens and novel biomaterials.

The effect of vertebral body stapling on spine biomechanics and structure using a bovine model

BACKGROUND

Adolescent idiopathic scoliosis is a common condition affecting 2.5% of the general population. Vertebral body stapling was introduced as a method of fusionless growth modulation for the correction of moderate idiopathic scoliosis (Cobb angles of 20-40°), and was claimed to be more effective than bracing and less invasive than fusion. The aim of this study was to assess the effect of vertebral body stapling on the stiffness of a thoracic motion segment unit under moment controlled load, and to assess the vertebral structural damage caused by the staples.

METHODS

Thoracic spine motion segments from 6 to 8 week old calves (n=14) were tested in flexion/extension, lateral bending, and axial rotation. The segments were tested un-instrumented, then a left anterolateral intervertebral Shape Memory Alloy (SMA) staple was inserted and the test was repeated. Data were collected from the tenth load cycle of each sequence and stiffness was calculated. The staples were carefully removed and the segments were studied with micro-computed tomography to assess physical damage to the bony structure. Visual assessment of the vertebral bone structure on micro-CT was performed.

FINDINGS

There was no change in motion segment stiffness in flexion/extension nor in axial rotation. There was a reduction in stiffness in lateral bending with 30% reduction bending away from the staple and 12% reduction bending towards the staple. Micro-CT showed physeal damage in all the specimens.

INTERPRETATION

Intervertebral stapling using SMA staples cause a reduction in spine stiffness in lateral bending. They also cause damage to the endplate epiphyses.

Feasibility of the annulus fibrosus repair with in situ gelating hydrogels - A biomechanical study

The surgical standard of care for lumbar discectomy leaves the annulus fibrosus (AF) defect unrepaired, despite considerable risk for a recurrent herniation. Identification of a viable defect repair strategy has until now been elusive. The scope of this ex vivo biomechanical study was to evaluate crosslinking hydrogels as potentially promising AF defect sealants, and provide a baseline for their use in combination with collagen scaffolds that restore disc volume. This study directly compared genipin crosslinked fibrin hydrogel (FibGen) as a promising preclinical candidate against a clinically available adhesive composed of glutaraldehyde and albumin (BioGlue). Forty-two bovine coccygeal functional spine units (FSU) were randomly allocated into four groups, namely untreated (control, n = 12), repaired with either one of the tested hydrogels (BioGlue, n = 12; FibGen, n = 12), or FibGen used in combination with a collagen hydrogel scaffold (FibGen+Scaffold, n = 6). All specimens underwent a moderate mechanical testing protocol in intact, injured and repaired states. After completion of the moderate testing protocol, the samples underwent a ramp-to-failure test. Lumbar discectomy destabilized the FSU as quantified by increased torsional range of motion (28.0° (19.1, 45.1) vs. 41.39° (27.3, 84.9), p<0.001), torsional neutral zone (3.1° (1.2, 7.7) vs. 4.8° (2.1, 12.1), Z = -3.49, p < 0.001), hysteresis(24.4 J (12.8, 76.0) vs. 27.6 J (16.4, 54.4), Z = -2.61, p = 0.009), with loss of both disc height (7.0 mm (5.0, 10.5) vs 6.1 mm (4.0, 9.3), Z = -5.16, p < 0.001) and torsional stiffness (0.76 Nmdeg-1 (0.38, 1.07) vs. 0.66 Nmdeg-1 (0.38, 0.97), Z = -3.98, p < 0.001). Most FibGen repaired AF endured the entire testing procedure whereas only a minority of BioGlue repaired AF and all FibGen+Scaffold repaired AF failed (6/10 vs. 3/12 vs. 0/6 respectively, p = 0.041). Both BioGlue and FibGen+Scaffold repaired AF partially restored disc height (0.47 mm (0.07, 2.41), p = 0.048 and 1.52 mm (0.41, 2.57), p = 0.021 respectively) compared to sham treatment (0.08 mm (-0.63, 0.88)) whereas FibGen-only repaired AF had no such effect (0.04 mm (-0.73, 1.13), U = 48.0, p = 1). The AF injury model demonstrated considerable change of FSU mechanics that could be partially restored by use of an AF sealant. While inclusion of a volumetric collagen scaffold led to repair failure, use of FibGen alone demonstrated clinically relevant promise for prevention of mechanical reherniation, outperforming an FDA approved sealant in this ex vivo test series.

Examination of an in vitro methodology to evaluate the biomechanical performance of nucleus augmentation in axial compression

Intervertebral disc degeneration is one of the leading causes of back pain, but treatment options remain limited. Recently, there have been advances in the development of biomaterials for nucleus augmentation; however, the testing of such materials preclinically has proved challenging. The aim of this study was to develop methods for fabricating and testing bone-disc-bone specimens in vitro for examining the performance of nucleus augmentation procedures. Control, nucleotomy and treated intervertebral disc specimens were fabricated and tested under static load. The nucleus was removed from nucleotomy specimens using a trans-endplate approach with a bone plug used to restore bony integrity. Specimen-specific finite element models were developed to elucidate the reasons for the variations observed between control specimens. Although the computational models predicted a statistically significant difference between the healthy and nucleotomy groups, the differences found experimentally were not significantly different. This is likely due to variations in the material properties, hydration and level of annular collapse. The deformation of the bone was also found to be non-negligible. The study provides a framework for the development of testing protocols for nucleus augmentation materials and highlights the need to control disc hydration and the length of bone retained to reduce inter-specimen variability.

Spinal biomechanical properties are significantly altered with a novel embalming method

In vitro tests on the biomechanical properties of human spines are often performed using fresh frozen specimens. However, this carries the risk of pathogen transfer from specimen to the worker and the specimens can only be used for a limited amount of time. Human spinal specimens embalmed with formaldehyde carry an almost absent risk of transfer of pathogens and can be stored and used for a long time, but the tissue properties are strongly affected making this method inapplicable for biomechanical testing. In this study, a new embalming technique called Fix for Life (F4L), which claims to preserve the tissue properties, was tested. The range of motion (ROM) and stiffness of six fresh human spinal specimens was measured using a spinal motion simulator before and after F4L embalming. After F4L embalming, spinal stiffness increased in flexion-extension by 230%, in lateral bending by 284% and in axial rotation by 271%. ROM decreased by 46% in flexion-extension, 56% in lateral bending and 54% in axial rotation. In conclusion, based on this study, F4L does not maintain physiological spinal biomechanical properties, and we propose that this method should not be used for biomechanical studies. Nevertheless, the method may be an alternative to formaldehyde fixation in situations such as training and education because the effect on spinal biomechanics is less detrimental than formaldehyde and tissue color is maintained.

Biomechanics of the human intervertebral disc: A review of testing techniques and results

Many experimental testing techniques have been adopted in order to provide an understanding of the biomechanics of the human intervertebral disc (IVD). The aim of this review article is to amalgamate results from these studies to provide readers with an overview of the studies conducted and their contribution to our current understanding of the biomechanics and function of the IVD. The overview is presented in a way that should prove useful to experimentalists and computational modellers. Mechanical properties of whole IVDs can be assessed conveniently by testing 'motion segments' comprising two vertebrae and the intervening IVD and ligaments. Neural arches should be removed if load-sharing between them and the disc is of no interest, and specimens containing more than two vertebrae are required to study 'adjacent level' effects. Mechanisms of injury (including endplate fracture and disc herniation) have been studied by applying complex loading at physiologically-relevant loading rates, whereas mechanical evaluations of surgical prostheses require slower application of standardised loading protocols. Results can be strongly influenced by the testing environment, preconditioning, loading rate, specimen age and degeneration, and spinal level. Component tissues of the disc (anulus fibrosus, nucleus pulposus, and cartilage endplates) have been studied to determine their material properties, but only the anulus has been thoroughly evaluated. Animal discs can be used as a model of human discs where uniform non-degenerate specimens are required, although differences in scale, age, and anatomy can lead to problems in interpretation.

Experimental Model of Proximal Junctional Fracture after Multilevel Posterior Spinal Instrumentation

There is a high risk of proximal junctional fractures (PJF) with multilevel spinal instrumentation, especially in the osteoporotic spine. This problem is associated with significant morbidity and possibly the need for reoperation. Various techniques have been proposed in an attempt to decrease the risk of PJF but there is no experimental model described for in vitro production of PJF after multilevel instrumentation. The objective of this study is to develop an experimental model of PJF after multilevel posterior instrumentation. Initially, four porcine specimens including 4 vertebrae and instrumented at the 3 caudal vertebrae using a pedicle screw construct were subjected to different loading conditions. Loading conditions on porcine specimens involving cyclic loading along the axis of the center vertebral body line, with constrained flexion between 0° and 15° proximally, and fully constraining the specimen distally resulted in a fracture pattern most representative of a PJF seen clinically in humans, so to undergo human cadaveric testing with similar loading conditions was decided. Clinically relevant PJF were produced in all 3 human specimens. The experimental model described in this study will allow the evaluation of different parameters influencing the incidence and prevention of PJF after multilevel posterior spinal instrumentation.