The effect of posterior compression of the facet joints for initial stability and sagittal profile in the treatment of thoracolumbar fractures: a biomechanical study.

PURPOSE: Surgical treatment of thoracolumbar A3-fractures usually comprises posterior fixation-in neutral position or distraction-potentially followed by subsequent anterior support. We hypothesized that additional posterior compression in circumferential stabilization may increase stability by locking the facets, and better restore the sagittal profile. METHODS: Burst fractures Type A3 were created in six fresh frozen cadaver spine segments (T12-L2). Testing was performed in a custom-made spinal loading simulator. Loads were applied as pure bending moments of ± 3.75 Nm in all six movement axes. We checked range of motion, neutral zone and Cobb's angle over the injured/treated segment within the following conditions: Intact, fractured, instrumented in neutral alignment, instrumented in distraction, with cage left in posterior distraction, with cage with posterior compression. RESULTS: We found that both types of instrumentation with cage stabilized the segment compared to the fractured state in all motion planes. For flexion/extension and lateral bending, flexibility was decreased even compared to the intact state, however, not in axial rotation, being the most critical movement axis. Additional posterior compression in the presence of a cage significantly decreased flexibility in axial rotation, thus achieving stability comparable to the intact state even in this movement axis. In addition, posterior compression with cage significantly increased lordosis compared to the distracted state. CONCLUSION: Among different surgical modifications tested, circumferential fixation with final posterior compression as the last step resulted in superior stability and improved sagittal alignment. Thus, posterior compression as the last step is recommended in these pathologies.

Bone-Preserving Decompression Procedures Have a Minor Effect on the Flexibility of the Lumbar Spine.

OBJECTIVE: To mitigate the risk of iatrogenic instability, new posterior decompression techniques able to preserve musculoskeletal structures have been introduced but never extensively investigated from a biomechanical point of view. This study was aimed to investigate the impact on spinal flexibility caused by a unilateral laminotomy for bilateral decompression, in comparison to the intact condition and a laminectomy with preservation of a bony bridge at the vertebral arch. Secondary aims were to investigate the biomechanical effects of two-level decompression and the quantification of the restoration of stability after posterior fixation. METHODS: A universal spine tester was used to measure the flexibility of six L2-L5 human spine specimens in intact conditions and after decompression and fixation surgeries. An incremental damage protocol was applied : 1) unilateral laminotomy for bilateral decompression at L3-L4; 2) on three specimens, the unilateral laminotomy was extended to L4-L5; 3) laminectomy with preservation of a bony bridge at the vertebral arch (at L3-L4 in the first three specimens and at L4-L5 in the rest); and 4) pedicle screw fixation at the involved levels. RESULTS: Unilateral laminotomy for bilateral decompression had a minor influence on the lumbar flexibility. In flexion-extension, the median range of motion increased by 8%. The bone-preserving laminectomy did not cause major changes in spinal flexibility. Two-level decompression approximately induced a twofold destabilization compared to the single-level treatment, with greater effect on the lower level. Posterior fixation reduced the flexibility to values lower than in the intact conditions in all cases. CONCLUSION: In vitro testing of human lumbar specimens revealed that unilateral laminotomy for bilateral decompression and bone-preserving laminectomy induced a minor destabilization at the operated level. In absence of other pathological factors (e.g., clinical instability, spondylolisthesis), both techniques appear to be safe from a biomechanical point of view.

Uncertainty analysis of material properties and morphology parameters in numerical models regarding the motion of lumbar vertebral segments.

The kinematics of a spinal motion segment is determined by the material properties of the soft-tissue and the morphology. The material properties can vary within subjects and between vertebral levels, leading to a wide possible range of motion of a spinal segment independently on its morphology. The goal of this numerical study was to identify the most influential material parameters concerning the kinematics of a spinal motion segment and their plausible ranges. Then, a method was tested to deduce the material properties automatically, based on a given ROM and morphology. A fully parametric finite element model of the morphology and material properties of a lumbar spinal motion segment was developed. The impact of uncertainty of twelve spinal material parameters, as well as the size of the gap between the articular surfaces of the facet joints was examined. The simulation results were compared to our own in vitro data. The flexibility of a lumbar segment was especially influenced by the properties of the anterior annulus region, the facet gap size and the interspinous ligament. The high degree of uncertainty in the material properties and facet gap size published in the literature can lead to a wide scatter in the motion of a spinal segment, with a range of 6°-17° in the intact condition in flexion/extension, from 5°-22° in lateral bending and from 3°-14° in axial rotation. Statistical analysis of the variability might help to estimate the sensitivity and total uncertainty propagated through biomechanical simulations, affecting the reliability of the predictions.

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.

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.

Two-piece ALIF cage optimizes the bone-implant interface in a 360° setting.

INTRODUCTION: Clinical research has documented that cage subsidence and the loss of balance correction is a significant complication related to spinal fusion. Subsidence is a multifactorial complication, where implant design is one important element. The aim of the study is to compare the rigidity and bone-implant relative motion of segments treated with either a conventional one-piece ALIF cage versus a two-piece ALIF cage, which adapts in situ and permits 7°-21° of lordosis. METHODS: Seven lumbosacral (L3-S1) human cadaver specimens were tested in a universal spine tester in the intact condition, the specimens instrumented with a two-piece ALIF cage (Statur®-L, FBC Device, Denmark) and a one-piece ALIF cage (Pezo™-A, Ulrich GmbH & Co. KG., Germany), both supplemented with a pedicle screw system using pure moments of ±7.5 Nm in three principal motion directions. For assessment of the bone-implant interface, fluoroscopic videos were captured during motion and 3D motion was measured using an optical motion capturing system. RESULTS: Significantly less motion at the implant-endplate interface was found for the two-piece cage (1.0° ± 0.6°) in comparison to the one-piece cage (4.2° ± 1.7°) in flexion/extension. No significant differences in segment rigidity were found between the one-piece and two-piece cages in the 360° setup, while both configurations significantly reduced the range of motion compared to the intact condition (p < 0.05). CONCLUSION: In comparison to the traditional one-piece ALIF cages, the two-piece cage concept reduced the relative motion at the bone-implant interface without compromising stability.

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 7.5Nm 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.

A pedicle screw system and a lamina hook system provide similar primary and long-term stability: a biomechanical in vitro study with quasi-static and dynamic loading conditions.

PURPOSE: For the stabilization of the thoracolumbar spine area, various stabilization techniques have been developed in recent decades. The aim of these techniques is to immobilize the treated segment to repositioning or correct the spine and guaranty long-term stability to achieve a reliable fusion. The aim of this study was to simulate in an in vitro experiment the postoperative long-term situation in elderly osteoporotic patients to compare two different stabilization principles; a pedicle screw system and a lamina hook system. METHODS: Two comparable groups with respect to age and bone mineral density with each n = 6 fresh-frozen human, bi-segmental thoracolumbar spine specimens (T11-L1) were used. Antero-posterior and lateral radiographs were taken before the test, to assess the spinal status. Then the intact specimens were biomechanically characterized with pure moments in the three anatomical planes in different states in terms of range of motion and neutral zone. After implantation of either, a pedicle screw system or a lamina hook system, the primary stability was determined under the same conditions. Subsequently the specimens were cyclically loaded under complex loading, using a custom-made set-up in a dynamic materials testing machine with increasing moments from 3 to 66 Nm until 100,000 cycles or until one of the three defined "failure" criteria was reached. (1) A failure of a bony structure. (2) Exceeding of the threefold ROM of the primary stability after implantation in flexion plus extension. (3) Reaching of the ROM based on the intact state before implantation both in flexion plus extension. RESULTS: The results showed that the ROM was strongly reduced after instrumentation similar for both implant systems in all motion planes. The highest stabilization was found in flexion/extension. During cyclic loading with increasing moments, the ROM increased continuously for both systems. The number of load cycles until one of the failure criteria was reached varied only slightly between the two groups. In the pedicle screw group 30,000 (median) loading cycles (range 5000-80,000) with a corresponding moment of 24 Nm (range 9-54) could be reached. In the lamina hook group 32,500 load cycles (range 20,000-45,000) could be achieved with a corresponding moment of 25.5 Nm (range 18-33). There was a slight trend that the pedicle screw system is influenced more by bone mineral density. CONCLUSION: Both implant systems provide similar primary stability and similar long-term stability. In the pedicle screw group, there was a stronger correlation between bone mineral density and the reached number of load cycles.

Do in vivo kinematic studies provide insight into adjacent segment degeneration? A qualitative systematic literature review.

PURPOSE: While much evidence suggests that adjacent segment degeneration is merely a manifestation of the natural degenerative process unrelated to any spine fusion, a significant body of literature supports the notion that it is a process due in part to the altered biomechanics adjacent to fused spine segments. The purpose of this study was to review and critically analyze the published literature that investigated the in vivo kinematics of the adjacent segments and entire lumbar spine in patients receiving spinal fusion or motion-preserving devices. METHODS: A systematic review of the PubMed database was conducted, initially identifying 697 studies of which 39 addressed the in vivo kinematics of the segments adjacent to spinal implants or non-instrumented fusion of the lumbar spine. RESULTS: Twenty-nine articles studied fusion, of which three reported a decrease in range of motion of the caudal adjacent segment post-fusion. Examining the rostral adjacent segment, twelve studies observed no change, nine studies found a significant increase, and three studies reported a significant decrease in sagittal plane range of motion. Of the six studies that analyzed motion for the entire lumbar spine as a unit, five studies showed a significant decrease and one study reported no change in global lumbar spine motion. Kinematics of the segment rostral to a total disc replacement was investigated in six studies: four found no change and the results for the other two showed dependence on treatment level. Fifteen studies of non-fusion posterior implants analyzed the motion of the adjacent segment with two studies noting an increase in motion at the rostral level. CONCLUSIONS: There appears to be no overall kinematic changes at the rostral or caudal levels adjacent to a fusion, but some patients (~20-30%) develop excessive kinematic changes (i.e., instability) at the rostral adjacent level. The overall lumbar ROM after fusion appears to decrease after a spinal fusion.

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

PURPOSE: Accelerated 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. METHODS: A 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. RESULTS: The 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. CONCLUSIONS: The 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.

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.

Pedicle screw loosening: a clinically relevant complication?

PURPOSE: Literature studies showed a very wide range of pedicle screw loosening rates after thoracolumbar stabilization, ranging from less than 1 to 15 % in non-osteoporotic patients treated with rigid systems and even higher in osteoporotic subjects or patients treated with dynamic systems. Firstly, this paper aims to investigate how much this complication is affecting the success rate of pedicle screw fixation, in both non-osteoporotic and osteoporotic patients, and to discuss the biomechanical reasons which may be related to the variability of the rates found in the literature. The secondary aim was to summarize and discuss the published definitions and conventions about screw loosening from a clinical and radiological point of view. METHODS: Narrative literature review. RESULTS: Screw loosening appears to be a minor problem for fixation and fusion of healthy, non-osteoporotic bone. Pedicle screw fixation in osteoporotic bone is believed to be at risk of loosening, but clinical data are actually scarce. Both expandable and augmented screws may be a viable option to reduce the risk of loosening, but clinical evidence is missing. Posterior motion-preserving implants seems to have a significant risk of screw loosening. Standardization appears to be lacking regarding the radiological assessment. Marked differences in the loosening rates found based either on planar radiography or on CT scanning were observed. CONCLUSIONS: Reported loosening rates primarily depended on the protocol used for the clinical examination during follow-up and on the conventions used for the radiological assessment. Aiming to a better comparability of published data, we recommend the authors of clinical studies to describe which criteria were used to assess a loosened screw, as well as the protocol of the clinical follow-up examination. Low-dose CT should be used for the assessment of screw loosening whenever possible.

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.