Human Cadaveric Spine Specimens Research Articles

Dark-field X-ray imaging for the assessment of osteoporosis in human lumbar spine specimens.

Background: Dark-field imaging is a novel imaging modality that allows for the assessment of material interfaces by exploiting the wave character of x-ray. While it has been extensively studied in chest imaging, only little is known about the modality for imaging other tissues. Therefore, the purpose of this study was to evaluate whether a clinical X-ray dark-field scanner prototype allows for the assessment of osteoporosis. Materials and methods: In this prospective study we examined human cadaveric lumbar spine specimens (vertebral segments L2 to L4). We used a clinical prototype for dark-field radiography that yields both attenuation and dark-field images. All specimens were scanned in lateral orientation in vertical and horizontal position. All specimens were additionally imaged with CT as reference. Bone mineral density (BMD) values were derived from asynchronously calibrated quantitative CT measurements. Correlations between attenuation signal, dark-field signal and BMD were assessed using Spearman's rank correlation coefficients. The capability of the dark-field signal for the detection of osteoporosis/osteopenia was evaluated with receiver operating characteristics (ROC) curve analysis. Results: A total of 58 vertebrae from 20 human cadaveric spine specimens (mean age, 73years ±13 [standard deviation]; 11 women) were studied. The dark-field signal was positively correlated with the BMD, both in vertical (r = 0.56, p < .001) and horizontal position (r = 0.43, p < .001). Also, the dark-field signal ratio was positively correlated with BMD (r = 0.30, p = .02). No correlation was found between the signal ratio of attenuation signal and BMD (r = 0.14, p = .29). For the differentiation between specimens with and without osteoporosis/osteopenia, the area under the ROC curve (AUC) was 0.80 for the dark-field signal in vertical position. Conclusion: Dark-field imaging allows for the differentiation between spine specimens with and without osteoporosis/osteopenia and may therefore be a potential biomarker for bone stability.

Biomechanical assessment of the effect of sublaminar band tensioning on lumbar motion.

Adjacent-segment disease (ASD) proximal to lumbosacral fusion is assumed to result from increased stress and motion that extends above or below the fusion construct. Sublaminar bands (SBs) have been shown to potentially mitigate stresses in deformity constructs. A similar application of SBs in lumbar fusions is not well described yet may potentially mitigate against ASD. Eight fresh-frozen human cadaveric spine specimens were instrumented with transforaminal lumbar interbody fusion (TLIF) cages at L3-4 and L4-5, and pedicle screws from L3 to S1. Bilateral SBs were applied at L2 and tightened around the rods extending above the L3 pedicle screws. After being mounted on a testing frame, the spines were loaded at L1 to 6 Nm in all 3 planes, i.e., flexion/extension, right and left lateral bending, and right and left axial rotation. Motion and intradiscal pressures (IDPs) at L2-3 were measured for 5 conditions: intact, instrumentation (L3-S1), band tension (BT) 30%, BT 50%, and BT 100%. There was significant increase in motion at L2-3 with L3-S1 instrumentation compared with the intact spine in flexion/extension (median 8.78°, range 4.07°-10.81°, vs median 7.27°, range 1.63°-9.66°; p = 0.016). When compared with instrumentation, BT 100% reduced motion at L2-3 in flexion/extension (median 8.78°, range 4.07°-10.81°, vs median 3.61°, range 1.11°-9.39°; p < 0.001) and lateral bending (median 6.58°, range 3.67°-8.59°, vs median 5.62°, range 3.28°-6.74°; p = 0.001). BT 50% reduced motion at L2-3 only in flexion/extension when compared with instrumentation (median 8.78°, range 4.07°-10.81°, vs median 5.91°, range 2.54°-10.59°; p = 0.027). There was no significant increase of motion at L1-2 with banding when compared with instrumentation, although an increase was seen from the intact spine with BT 100% in flexion/extension (median 5.14°, range 2.47°-9.73°, vs median 7.34°, range 4.22°-9.89°; p = 0.005). BT 100% significantly reduced IDP at L2-3 from 25.07 psi (range 2.41-48.08 psi) before tensioning to 19.46 psi (range -2.35 to 29.55 psi) after tensioning (p = 0.016). In this model, the addition of L2 SBs reduced motion and IDP at L2-3 after the L3-S1 instrumentation. There was no significant increase in motion at L1-2 in response to band tensioning compared with instrumentation alone. The application of SBs may have a clinical application in reducing the incidence of ASD.

P74. During activities of daily living two-level motion preservation and hybrid constructs may be protective of adjacent segments

BACKGROUND CONTEXT In lumbar spine motion preservation, either two-level motion preservation or motion preservation device adjacent to fusion (hybrid construct), are alternatives to two-level fusion when treating a symptomatic segment next to a level that has only mild radiographic changes. By preserving motion, this strategy may theoretically protect the adjacent mobile segments. PURPOSE This work investigated the protective effect on L3-L4 of a L4-S1 hybrid construct and a L4-S1 two-level total joint replacement (TJR). STUDY DESIGN/SETTING Biomechanical cadaveric study. OUTCOME MEASURES Segmental motion. METHODS Segmental motion data were collected in human cadaveric spine specimens (T10-S1, N=8, 56±5 years) during simulation of standing flexion and the transition from standing to sitting. To determine the protective effect, we determined the change in motion at L3-L4 during activities of daily living when the segment was adjacent to a hybrid L4-S1 construct (a motion-preserving total joint replacement (TJR) at L4-L5 with fusion at L5-S1) and a two-level TJR (L4-S1). Change was calculated using displacement-control analysis. L3-L4 motion before surgery was the reference value. RESULTS Forward flexion while standing: mean L3-L4 motion with an L4-S1 hybrid construct was a tenth of a degree less than intact L3-L4 motion (Hybrid 4.8° (1.6°) vs 4.9° (1.2°), p>0.05; data is mean (standard deviation)). Adjacent to an L4-S1 TJR, the average L3-L4 motion was 1.2 degrees less than intact. This reduction in L3-L4 motion over intact was statistically significant (L4-S1 TJR 3.7° (1.2°), p=0.006). Transition from standing to sitting: L3-L4 motion for the same reduction in sacral slope (or comparable sitting posture) was not significantly different from intact with hybrid and 2-level motion preservation constructs (Intact 3.8°(1.2°), hybrid 4.0°(1.3°), L4-S1 TJR, 2.7°(1.1°), p>0.05). On average, L3-L4 motion was 0.2 degrees higher with the L4-S1 hybrid construct and 1.1 degrees lower with the two-level L4-S1 TJR. CONCLUSIONS The hybrid and two-level total joint replacement constructs did not exhibit the increased proximal segment motion during forward bending and sitting activities that may lead to junctional breakdown over time. Preserving motion using a hybrid construct or two-level total joint replacement may be an alternative surgical option to improve patient comfort and function during activities of daily living while potentially decreasing the need for adjacent level surgery. FDA DEVICE/DRUG STATUS MOTUS (Investigational/Not Approved) In lumbar spine motion preservation, either two-level motion preservation or motion preservation device adjacent to fusion (hybrid construct), are alternatives to two-level fusion when treating a symptomatic segment next to a level that has only mild radiographic changes. By preserving motion, this strategy may theoretically protect the adjacent mobile segments. This work investigated the protective effect on L3-L4 of a L4-S1 hybrid construct and a L4-S1 two-level total joint replacement (TJR). Biomechanical cadaveric study. Segmental motion. Segmental motion data were collected in human cadaveric spine specimens (T10-S1, N=8, 56±5 years) during simulation of standing flexion and the transition from standing to sitting. To determine the protective effect, we determined the change in motion at L3-L4 during activities of daily living when the segment was adjacent to a hybrid L4-S1 construct (a motion-preserving total joint replacement (TJR) at L4-L5 with fusion at L5-S1) and a two-level TJR (L4-S1). Change was calculated using displacement-control analysis. L3-L4 motion before surgery was the reference value. Forward flexion while standing: mean L3-L4 motion with an L4-S1 hybrid construct was a tenth of a degree less than intact L3-L4 motion (Hybrid 4.8° (1.6°) vs 4.9° (1.2°), p>0.05; data is mean (standard deviation)). Adjacent to an L4-S1 TJR, the average L3-L4 motion was 1.2 degrees less than intact. This reduction in L3-L4 motion over intact was statistically significant (L4-S1 TJR 3.7° (1.2°), p=0.006). Transition from standing to sitting: L3-L4 motion for the same reduction in sacral slope (or comparable sitting posture) was not significantly different from intact with hybrid and 2-level motion preservation constructs (Intact 3.8°(1.2°), hybrid 4.0°(1.3°), L4-S1 TJR, 2.7°(1.1°), p>0.05). On average, L3-L4 motion was 0.2 degrees higher with the L4-S1 hybrid construct and 1.1 degrees lower with the two-level L4-S1 TJR. The hybrid and two-level total joint replacement constructs did not exhibit the increased proximal segment motion during forward bending and sitting activities that may lead to junctional breakdown over time. Preserving motion using a hybrid construct or two-level total joint replacement may be an alternative surgical option to improve patient comfort and function during activities of daily living while potentially decreasing the need for adjacent level surgery.

37. Anatomic contributions to the stability of the L5-S1 segment under combined shear and compression: implications to isthmic/lytic spondylolisthesis

37. Anatomic contributions to the stability of the L5-S1 segment under combined shear and compression: implications to isthmic/lytic spondylolisthesis

The Feasibility of Anterior Occipital Condyle Screw for the Reconstruction of Craniovertebral Junction: A Digital Anatomical and Cadaveric Study of a Novel Technique.

BackgroundAnterior occipital condyle screw (AOCS) could be a feasible alternative technique for occipitocervical fusion for reconstruction of craniovertebral junction. This study aimed to analyze the feasibility of AOCS.MethodsThe craniovertebral junction computed tomography (CT) scans of 40 adults were enrolled and imported into Mimics software. Then, the three-dimensional reconstruction digital model of craniovertebral junction was established to determine entry point, insertion angle, and screw’s trajectory. After AOCS insertion into ten human cadaver spine specimens, CT scans were performed to verify the location between screws and important structures.ResultsThe optimal entry point was located caudally and medial to the ventral of occipital condyle. The optimal trajectory was in inclination angle (5.9°±3.4°) in the sagittal plane and divergence angle (26.7°±6.0°) in the axial plane with the screw length around 21.6±1.2mm. None of the screws invaded the hypoglossal canal and vertebral artery in any of the specimens.ConclusionAOCS fixation is a feasible, novel technique for anterior craniovertebral junction reconstruction, and it could be an effective alternative operation for anterior reconstruction with titanium mesh cage.

P12. Does posterior ligamentous complex disruption following lumbar pedicle screw and rod instrumentation affect the upper adjacent level?

<h3>BACKGROUND CONTEXT</h3> The lumbar level above the fusion segment is the most common level for the development of adjacent level instability. The posterior ligamentous complex (PLC), (eg, interspinous ligament, and supraspinous ligament) a posterior tension band that stabilizes the spine, is usually removed for laminectomy during lumbar fusion and decompression surgery. The removal of the posterior complex may jeopardize upper adjacent level stability. <h3>PURPOSE</h3> To compare mobility and principal disc strains at the upper adjacent level to L4-5 pedicel screw-rod fixation (PSR) between L4-5 PSR, L4-5 PSR+ bilateral laminotomy (PLC preservation), and L4-5 PSR+ conventional laminectomy, in cadaveric specimens. The disc strain was studied with a new optical technique of soft tissue strain analysis (3D digital image correlation (DIC) technique). <h3>STUDY DESIGN/SETTING</h3> Biomechanic testing in human cadaveric spine specimens. <h3>PATIENT SAMPLE</h3> Human cadaveric lumbar spine. <h3>OUTCOME MEASURES</h3> 1. Adjacent segment range of motion, 2. Disc surface strains. <h3>METHODS</h3> Seven human cadaveric L3-S1 specimens were instrumented with L4-5 PSR. All specimens were tested multi-directionally under pure moment loading (7.5Nm) following: 1) L4-5 PSR, 2) L4-5 PSR+bilateral L4 laminotomy while preserving PLC and 3) L4-5 PSR +L4 conventional laminectomy. DIC was performed with cameras positioned laterally (left side) to capture the change in disc principal strain (Pmax and Pmin) at upper adjacent level (L3-4) under peak load. Optical motion tracking was simultaneously performed on the right side. The disc region was divided into four similar sized quarters and included upper and lower endplates. Analysis of variance of upper adjacent level (L3-4) range of motion (ROM) and disc strain between the groups was performed. Statistically significant was set at p≤0.05. <h3>RESULTS</h3> ROM of the upper adjacent (L3-4) level was significantly greater with L4-5 PSR+laminectomy (6.26 degree) versus L4-5 PSR+ bilateral laminotomy (5.94 degree, p=0.006), and L4-5 PSR (5.20 degree, p<0.001) respectively, during flexion. During extension, left lateral bending, and axial rotation, upper adjacent level ROM was significantly greater with L4-5+ laminectomy and L4-5 PSR+ bilateral laminotomy versus L4-5 PSR (p≤0.046) but there were no significant differences between L4-5 PSR+ laminectomy and L4-5PSR+ bilateral laminotomy (p≥0.087). The intervertebral disc Pmax strains of the upper adjacent level were significantly greater with L4-5 PSR+laminectomy than L4-5 PSR during flexion (25,225 VS 18,007uE, p=0.031), extension (22,803 VS 19,356uE, p=0.020), left lateral bending (32,778 VS 24,100uE, p=0.027), right lateral bending (88,761 VS 62,869uE, p=0.024), and right axial rotation (24,747 VS 19,289uE, p=0.025). <h3>CONCLUSIONS</h3> The posterior ligamentous complex disruption during lumbar fusion surgery was associated with significant increases in upper adjacent level instability and principle (Pmax) intervertebral disc stress/strain. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.

20. Biomechanical effect of facet joint violation on adjacent segment motion and disc strain following single level lumbar fusion with transpedicular screw and rod instrumentation

<h3>BACKGROUND CONTEXT</h3> Facet joint violation (FV) is a complication occurring while placing pedicle screws at the upper-most level using an entry point and trajectory too close to the superior facet joint. The incidence of FV has been reported up to be as high as 50%. However, little is known about how FV affects superior adjacent level spinal stability and especially intervertebral disc (IVD) strain after lumbar fusion. <h3>PURPOSE</h3> To investigate the biomechanical effects of FV in terms of mobility and disc surface strains at the upper level adjacent to L4-5 pedicle screw-rod fixation (PSR) in cadaveric specimens. The disc strain was studied with a new optical technique of soft tissue strain analysis (3D digital image correlation [DIC] technique). <h3>STUDY DESIGN/SETTING</h3> Biomechanic testing in human cadaveric spine specimens. <h3>PATIENT SAMPLE</h3> A total of 14 human cadaveric L3-S1 specimens. <h3>OUTCOME MEASURES</h3> 1. Upper adjacent segment range of motion. 2. Disc surface strain/stress. <h3>METHODS</h3> A total of 14 human cadaveric L3-S1 specimens were divided into two equal groups. L4-5 pedicle-rod fixation was performed in a different fashion in each group ie, facet joint preservation (FP) and facet joint violation (FV). The violation of the facet joint was defined by denuding the facet joint capsule and removing of the external cortical surface of the superior articular process with a 3 mm Leksell-rongeur. The entry point of these pedicle screws was located at the junction between the inferolateral aspect of the facet joint. All specimens were tested multi-directionally under pure moment loading (7.5Nm). DIC was performed with cameras positioned laterally (left side) to capture the change in principal disc strains (Pmax and Pmin) at the upper adjacent level (L3-4) under peak load. Optical motion tracking was simultaneously performed on the right side. The IVD region was divided into four similar sized quarters and included upper and lower endplates. Analysis of variance of upper adjacent level range of motion (ROM) and disc strain, normalized to intact, was performed between the groups. Statistical significant was set at P≤0.05. <h3>RESULTS</h3> Normalized ROM was significantly greater with FV than FP at upper adjacent level (L3-4) during right axial rotation (1.22 vs 0.99, 23% difference, p=0.047). Normalized IVD Pmax strains at the upper adjacent level (L3-4) were significantly greater with FV than FP in the anterior quadrant during right axial rotation (1.80 vs 0.83 (ratio to intact), 97% difference, p=0.003). However, there were no significant differences in ROM and disc Pmax or Pmin strains during flexion, extension and lateral bending. <h3>CONCLUSIONS</h3> Facet violations at the uppermost level of fusion construct during pedicle screw-rod fixation were associated with adjacent level torsional instability and the increase of IVD strain. This study suggested that the preservation of uppermost facet joint of fusion segment was helpful for alleviating adjacent segment mobility and stress concentrations of the IVD above the fusion segment. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.

Comparison of three different screw trajectories in osteoporotic vertebrae: a biomechanical investigation

BackgroundPedicle screw insertion in osteoporotic patients is challenging. Achieving more screw-cortical bone purchase and invasiveness minimization, the cortical bone trajectory and the midline cortical techniques represent alternatives to traditional pedicle screws. This study compares the fatigue behavior and fixation strength of the cement-augmented traditional trajectory (TT), the cortical bone trajectory (CBT), and the midline cortical (MC).MethodsTen human cadaveric spine specimens (L1 - L5) were examined. The average age was 86.3 ± 7.2 years. CT scans were provided for preoperative planning. CBT and MC were implanted by using the patient-specific 3D-printed placement guide (MySpine®, Medacta International), TT were implanted freehand. All ten cadaveric specimens were randomized to group A (CBT vs. MC) or group B (MC vs. TT). Each screw was loaded for 10,000 cycles. The failure criterion was doubling of the initial screw displacement resulting from the compressive force (60 N) at the first cycle, the stop criterion was a doubling of the initial screw displacement. After dynamic testing, screws were pulled out axially at 5 mm/min to determine their remaining fixation strength.ResultsThe mean pull-out forces did not differ significantly. Concerning the fatigue performance, only one out of ten MC of group A failed prematurely due to loosening after 1500 cycles (L3). Five CBT already loosened during the first 500 cycles. The mean displacement was always lower in the MC. In group B, all TT showed no signs of failure or loosening. Three MC failed already after 26 cycles, 1510 cycles or 2144 cycles. The TT showed always a lower mean displacement. In the subsequent pull-out tests, the remaining mean fixation strength of the MC (449.6 ± 298.9 N) was slightly higher compared to the mean pull-out force of the CBT (401.2 ± 261.4 N). However, MC (714.5 ± 488.0 N) were inferior to TT (990.2 ± 451.9 N).ConclusionThe current study demonstrated that cement-augmented TT have the best fatigue and pull-out characteristics in osteoporotic lumbar vertebrae, followed by the MC and CBT. MC represent a promising alternative in osteoporotic bone if cement augmentation should be avoided. Using the patient-specific placement guide contributes to the improvement of screws’ biomechanical properties.

Anterior Atlantooccipital Transarticular Screw Fixation: A Cadaveric Study and Description of a Novel Technique.

Retrospective analysis of collected data and operative experiment on human cadavers. To describe a novel technique of the anterior atlantooccipital (AC) transarticular screw fixation, and to analyze the pertinent anatomy with cadaveric and radiographic assessment of the feasibility, safety, and general applicability of this technique. In some situations, the posterior AC fixation techniques may not be possible, or may require supplemental fixation, which include the congenital hypoplasia, absence of the bony elements, and even revision surgery. However, an anterior screw fixation technique may add stability to further attempts at obtaining an arthrodesis. A detailed description of the surgical technique was presented. Three-dimensional (3D) CT reconstruction of the cranioverteral region of 30 patients were performed to determine screw entry points, target points, and proposed screw trajectories. Following screw insertion in eight fresh frozen human cadaver spine specimens, dissection verified screw location relative to structures at risk. The ideal entry point is located caudal to the C1 superior facet joint in line with the medial third of the C1 superior facet. The ideal screw is directed 41.7° posteriorly in the sagittal plane and 11.6° laterally in the coronal plane with a length around 30.4 mm. The feasibility of anterior AC screw fixation was 92% (35/38 cases). There is a risk of injury to the vertebral artery and the hypoglossal nerve. Anterior AC transarticular screw fixation is feasible and can be considered as a salvage technique or an alternative for the posterior AC fixation, as well as the supplement to the anterior occipitocervical fixation. 3.

4. Lumbosacral TLIF but not ALIF increases proximal junction motion in long-segment constructs

BACKGROUND CONTEXT Proximal junctional kyphosis and lumbosacral (LS) pseudoarthrosis/instrumentation failure are relatively common complications following long instrumentation constructs to the sacrum. Methods to improve fusion rates and decrease instrumentation failure include LS anterior column support (ALIF or TLIF), iliac screw fixation, and accessory rods. The impact of these lumbosacral augmentation strategies on the proximal junction is not clear. PURPOSE The purpose of this study was to investigate the impact of various lumbosacral constructs on proximal junction biomechanics. STUDY DESIGN/SETTING Human biomechanical cadaveric study. PATIENT SAMPLE Twelve (L1-Pelvis) cadaveric spines were studied. OUTCOME MEASURES Proximal junction range of motion (ROM) at L1-2 was measured. METHODS Fourteen human cadaveric spine (L1-Ilium) specimens were prepared and potted at L1 and Ilium. Specimens were equally divided into either an L5-S1 ALIF or TLIF group. 4R conditions consisted of accessory rods spanning the L3-L4 and S1-Ilium levels. Compression (400 N) in combination with 7.5 Nm of flexion (FL), extension (EX), lateral bending (LB), or axial rotation (AR) was applied to all conditions. Specimens underwent testing in the following conditions: (1) intact, (2) L2-S1 pedicle screw fixation (PSR), (3) L2-ilium (PSR-I), (4) PSR+ALIF (ALIF-S) or TLIF (TLIF-S), (5) PSR-I + ALIF (ALIF-I) or TLIF (TLIF-I), and (6) ALIF-I + 4R or TLIF-I + 4R. Statistical comparisons were performed using one-way (RM) ANOVA (p RESULTS PSR-I did not significantly change proximal junction ROM in any direction compared to PSR-S (p>.069). However, TLIF-I and TLIF+4R resulted in significant increase in L1-2 ROM in FL and Right LB (p .095). ALIF-I did not significantly change ROM in any direction compared to ALIF+4R or PSR-I (p>.069). CONCLUSIONS In cadavers with pelvic fixation, lumbosacral TLIF results in increased ROM at the proximal junction while ALIF does not. Further augmentation of either ALIF or TLIF constructs with lumbosacral accessory rods does not impact proximal junctional ROM. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.

Iliac screws may not be necessary in long-segment constructs with L5–S1 anterior lumbar interbody fusion: cadaveric study of stability and instrumentation strain

BACKGROUND CONTEXTLumbosacral pseudoarthrosis and instrumentation failure is common with long-segment constructs. Optimizing lumbosacral construct biomechanics may help to reduce failure rates. The influence of iliac screws and interbody type on range of motion (ROM), rod strain (RS), sacral screw strain (SS) is not well-established. PURPOSEInvestigate the effects of transforaminal lumbar interbody fusion (TLIF), anterior lumbar interbody fusion (ALIF), and iliac screws on long-segment lumbosacral construct biomechanics. STUDY DESIGNBiomechanical study. PATIENT SAMPLEFourteen human cadaveric spine specimens. OUTCOME MEASURESLumbosacral ROM, RS, and SS. METHODSSpecimens were potted at L1 and the ilium. Specimens were equally divided into either an L5–S1 ALIF or TLIF group and underwent testing in the following conditions: (1) intact (2) L2–S1 pedicle screw rod fixation (PSR-S) (3) L2-ilium (PSR-I) (4) PSR-S+ALIF (ALIF-S) or TLIF (TLIF-S) (5) PSR-I + ALIF (ALIF-I) or TLIF (TLIF-I). Pure moment bending (7.5 Nm) in flexion, extension, lateral bending, axial rotation, and compressive loads (400N) were applied and ROM, SS, and RS were measured. Comparisons were performed using a one-way ANOVA (p<.05). RESULTSALIF-S and TLIF-S provided similar decreases in ROM as TLIF-I (p>.05). Compared to PSR-S, PSR-I significantly decreased SS during bending in all directions (p<.02) but increased RS in flexion and extension (p≤.02). Anterior lumbar interbody fusion-S provided similar decreases in SS as TLIF-I in all directions (p>.40) but had significantly less RS than TLIF-I in flexion, extension, compression (p<.01). TLIF-S had more SS than TLIF-I in flexion, extension, axial rotation (p<.02), while TLIF-S had less RS only in flexion (p=.03). Compared to PSR-I, ALIF-I decreased the RS (p<.02) but TLIF-I did not (p>.67). CONCLUSIONSIliac screws were protective of SS but increased RS at the lumbosacral junction. Constructs with ALIF and no iliac screws result in comparable SS as constructs with TLIF and iliac screws with significantly reduced RS. If iliac screws are utilized, ALIF but not TLIF reduces the iliac screw-induced RS. CLINICAL SIGNIFICANCEThere is a relatively high incidence of lumbosacral instrumentation failure in adult spinal deformity. Optimizing lumbosacral construct biomechanics may help to reduce failure rates. Iliac screws induce lumbosacral rod strain and may be responsible for instrumentation failure. Constructs with lumbosacral ALIF reduce iliac-screw induced rod strain and may obviate the need for fixation to the ilium.

Wednesday, September 26, 2018 7:35 AM–9:00 AM ePosters: P16. Iliac screws may not be necessary in long segment constructs with L5-S1 ALIF: cadaveric study of instrumentation strain

BACKGROUND CONTEXT Lumbosacral pseudoarthrosis and instrumentation failure is common with long segment constructs. Optimizing lumbosacral construct biomechanics may help to reduce failure rates. The influence of iliac screws and interbody type on lumbosacral stability and instrumentation bending strain is not well established. PURPOSE This study investigated the effects of TLIF, ALIF, and iliac screws on long segment lumbosacral construct biomechanics. STUDY DESIGN/SETTING In vitro biomechanical study using human cadaveric specimens. PATIENT SAMPLE 14 (L1-pelvis) cadaveric spines (5F/9M, 51.6±7.4years, DEXA 0.837 g/cm2) OUTCOME MEASURES Range of motion (ROM), sacral screw bending strain (SS), and L5-S1 rod strain (RS). METHODS Fourteen human cadaveric spine (L1-ilium) specimens were prepared and potted at L1 and ilium. Specimens were equally divided into either an L5-S1 ALIF or TLIF group. All specimens underwent testing in the following conditions: (1) Intact (2) L2-S1 pedicle screw fixation (PSF) (3) L2-ilium (PSF-I) (4) PSF+ALIF (ALIF-S) or TLIF (TLIF-S) (5) PSF-I + ALIF (ALIF-I) or TLIF (TLIF-I). Pure moment bending (7.5 Nm) in flexion (F), extension (E), lateral bending (LB), axial rotation (AR) and axial compressive (C) loads (400 N) were applied to all conditions and ROM, SS, and RS were measured. Statistical comparisons were performed using one-way ANOVA (p RESULTS ALIF-S and TLIF-S provided similar decreases in ROM as TLIF-I (p>.05). Compared to PSF, iliac screws significantly decreased SS during bending in all directions (p .16) but increased RS in flexion and extension (p .40). TLIF-S had more SS than TLIF-I in F,E,AR (p .64). CONCLUSIONS Iliac screws were most protective of SS but resulted in increased RS. Constructs with ALIF and no iliac screws provide comparable decreases in SS as constructs with TLIF and iliac screws; furthermore, RS was significantly less. If iliac screws are utilized for fixation, ALIF but not TLIF reduces the RS induced by iliac screws.

Single-Level Rigid Fixation Combined with Coflex: A Biomechanical Study.

BackgroundThe purpose of this biomechanical in vitro study was to compare the kinematics and intradiscal pressure achieved with 2 methods: L4–L5 pedicle screw-rod fixation (PSRF) with an upper L3–L4 Coflex device and L4–L5 PSRF alone. The results were used to characterize the biomechanics of the topping-off operation with a Coflex device for the lumbar motion segment adjacent to single-level rigid fixation.Material/MethodsSix human cadaveric spine specimens were biomechanically tested in vitro (6 males, 0 females). The 3-dimensional specimen motion in response to applied loads during flexibility tests was determined. Loads were applied along anatomic axes to induce flexion-extension, lateral bending, and axial rotation. All specimens were first studied with intact lumbar motion segments, then with L4–L5 PSRF alone, and finally with L4–L5 PSRF with an upper L3–L4 Coflex device. A non-paired comparison of the 3 configurations under 3 different conditions was made.ResultsPSRF, with or without a Coflex device, significantly increased the range of motion (ROM) in the upper adjacent motion segments in all directions of loading. The intradiscal pressure (IDP) changed slightly. A correlation analysis showed that the ROM and IDP are significantly positively correlated. The application of the upper motion segment of the Coflex device provided greater stability in all directions of motion than did PSRF alone, particularly for extension (p<0.05), while use of a Coflex device did not significantly decrease the IDP compared with PSRF alone (p>0.05).ConclusionsThese results suggest that L4–L5 PSRF with an L3–L4 Coflex device is more stable than L4–L5 PSRF alone. PSRF with an upper Coflex device is a promising alternative to PSRF alone. Based on these biomechanical tests, it might be considered a protective method to prevent adjacent segment degeneration (ASD), although some limitations with this in vitro study must be addressed in the future.

Effects of rod reduction on pedicle screw fixation strength in the setting of Ponte osteotomies

Background contextThe use of a rod reduction device can have deleterious consequences on pedicle screw pullout strength (POS) in the thoracic spine. However, posterior-only osteotomies in the thoracic spine are often performed to improve flexibility of the spine and offset forces of deformity correction maneuvers. PurposeTo investigate the effect on pedicle screw POS caused by the rod reduction technique in the presence of facet osteotomies in the thoracic spine. Study design/settingThe study is a biomechanical study using human cadaveric spine specimens. MethodsThoracic Ponte osteotomies were performed on 3 thoracic levels in 15 cadaveric specimens. The right rod was contoured with a 5-mm residual gap at the middle level and was reduced using a rod reduction device. On the left side (paired control), a rod with no mismatch was placed. Biomechanical testing was performed with tensile load to failure “in line” with the screw axis and POS measured in Newtons (N). ResultsAfter rod reduction, thoracic pedicle screw POS was significantly decreased (40%) compared with the control (419±426 N vs. 708±462 N, p=.002) and remained statistically significant after adjusting for bone mineral density (BMD) (p=.05). Eleven (73%) of the pedicle screws had visible pullout/failure during the reduction attempt and occurred irrespective of BMD. ConclusionsDespite thoracic Ponte osteotomies and increased flexibility of the spinal segments, the rod reduction device still significantly decreased pedicle screw POS, typically resulting in outright failure of the screw-bone interface. Therefore, rod reduction technique of any kind should be performed with caution as it frequently results in suboptimal pedicle screw fixation.

Biomechanics of the lower thoracic spine after decompression and fusion: a cadaveric analysis

Background contextFew studies have evaluated the extent of biomechanical destabilization of thoracic decompression on the upper and lower thoracic spine. The present study evaluates lower thoracic spinal stability after laminectomy, unilateral facetectomy, and unilateral costotransversectomy in thoracic spines with intact sternocostovertebral articulations. PurposeTo assess the biomechanical impact of decompression and fixation procedures on lower thoracic spine stability. Study designBiomechanical cadaveric study. MethodsSequential surgical decompression (laminectomy, unilateral facetectomy, unilateral costotransversectomy) and dorsal fixation were performed on the lower thoracic spine (T8–T9) of human cadaveric spine specimens with intact rib cages (n=10). An industrial robot was used to apply pure moments to simulate flexion-extension (FE), lateral bending (LB), and axial rotation (AR) in the intact specimens and after decompression and fixation. Global range of motion (ROM) between T1–T12 and intrinsic ROM between T7–T11 were measured for each specimen. ResultsThe decompression procedures caused no statistically significant change in either global or intrinsic ROM compared with the intact state. Instrumentation, however, reduced global motion for AR (45° vs. 30°, p=.0001), FE (24° vs. 19°, p=.02), and LB (47° vs. 36°, p=.0001) and for intrinsic motion for AR (17° vs. 4°, p=.0001), FE (8° vs. 1°, p=.0001), and LB (12° vs. 1°, p=.0001). No significant differences were identified between decompression of the upper versus lower thoracic spine, with trends toward significantly greater ROM for AR and lower ROM for LB in the lower thoracic spine. ConclusionsThe lower thoracic spine was not destabilized by sequential unilateral decompression procedures. Addition of dorsal fixation increased segment rigidity at intrinsic levels and also reduced overall ROM of the lower thoracic spine to a greater extent than did fusing the upper thoracic spine (level of the true ribs). Despite the lack of true ribs, the lower thoracic spine was not significantly different compared with the upper thoracic spine in FE and LB after decompression, although there were trends toward significance for greater AR after decompression. In certain patients, instrumentation may not be needed after unilateral decompression of the lower thoracic spine; further validation and additional clinical studies are warranted.

Biomechanical analysis of the upper thoracic spine after decompressive procedures

Background contextDecompressive procedures such as laminectomy, facetectomy, and costotransversectomy are routinely performed for various pathologies in the thoracic spine. The thoracic spine is unique, in part, because of the sternocostovertebral articulations that provide additional strength to the region relative to the cervical and lumbar spines. During decompressive surgeries, stability is compromised at a presently unknown point. PurposeTo evaluate thoracic spinal stability after common surgical decompressive procedures in thoracic spines with intact sternocostovertebral articulations. Study designBiomechanical cadaveric study. MethodsFresh-frozen human cadaveric spine specimens with intact rib cages, C7–L1 (n=9), were used. An industrial robot tested all spines in axial rotation (AR), lateral bending (LB), and flexion-extension (FE) by applying pure moments (±5 Nm). The specimens were first tested in their intact state and then tested after each of the following sequential surgical decompressive procedures at T4–T5 consisting of laminectomy; unilateral facetectomy; unilateral costotransversectomy, and subsequently instrumented fusion from T3–T7. ResultsWe found that in all three planes of motion, the sequential decompressive procedures caused no statistically significant change in motion between T3–T7 or T1–T12 when compared with intact. In comparing between intact and instrumented specimens, our study found that instrumentation reduced global range of motion (ROM) between T1–T12 by 16.3% (p=.001), 12% (p=.002), and 18.4% (p=.0004) for AR, FE, and LB, respectively. Age showed a negative correlation with motion in FE (r=−0.78, p=.01) and AR (r=−0.7, p=.04). ConclusionsThoracic spine stability was not significantly affected by sequential decompressive procedures in thoracic segments at the level of the true ribs in all three planes of motion in intact thoracic specimens. Age appeared to negatively correlate with ROM of the specimen. Our study suggests that thoracic spinal stability is maintained immediately after unilateral decompression at the level of the true ribs. These preliminary observations, however, do not depict the long-term sequelae of such procedures and warrant further investigation.

Stability of transforaminal lumbar interbody fusion in the setting of retained facets and posterior fixation using transfacet or standard pedicle screws

Background contextThe transforaminal lumbar interbody fusion (TLIF) technique supplements posterior instrumented lumbar spine fusion and has been tested with different types of screw fixation for stabilization. Transforaminal lumbar interbody fusion is usually placed through a unilateral foraminal approach after unilateral facetectomy, although extraforaminal entry allows the facet to be spared. PurposeTo characterize the biomechanics of L4–L5 lumbar motion segments instrumented with bilateral transfacet pedicle screw (TFPS) fixation versus bilateral pedicle screw-rod (PSR) fixation in the setting of intact facets and native disc or after discectomy and extraforaminal placement of a TLIF technology graft. Study designHuman cadaveric lumbar spine segments were biomechanically tested in vitro to provide a nonpaired comparison of four configurations of posterior and interbody instrumentation. MethodsFourteen human cadaveric spine specimens (T12–S1) underwent standard pure moment flexibility tests with intact L4–L5 disc and facets. Seven were studied with intact discs, after TFPS fixation, and then with TLIF and TFPS fixation. The others were studied with intact discs, after PSR fixation, and then combined with extraforaminally placed TLIF. Loads were applied about anatomic axes to induce flexion-extension, lateral bending, and axial rotation. Three-dimensional specimen motion in response to applied loads during flexibility tests was determined. A nonpaired comparison of the four configurations of posterior and interbody instrumentation was made. ResultsTransfacet pedicle screw and PSR, with or without TLIF, significantly reduced range of motion during all directions of loading. Transfacet pedicle screw provided greater stability than PSR in all directions of motion except lateral bending. In flexion, TFPS was more stable than PSR (p=.042). A TLIF device provided less stability than the intact disc in constructs with TFPS and PSR. ConclusionsThese results suggest that fixation at L4–L5 with TFPS is a promising alternative to PSR, with or without TLIF. A TLIF device was less stable than the native disc with both methods of instrumentation presumably because of a fulcrum effect from a relatively small footplate. Additional interbody support may be considered for improved biomechanics with TLIF.

A New PMHS Model for Lumbar Spine Injuries During Vertical Acceleration

Ejection from military aircraft exerts substantial loads on the lumbar spine. Fractures remain common, although the overall survivability of the event has considerably increased over recent decades. The present study was performed to develop and validate a biomechanically accurate experimental model for the high vertical acceleration loading to the lumbar spine that occurs during the catapult phase of aircraft ejection. The model consisted of a vertical drop tower with two horizontal platforms attached to a monorail using low friction linear bearings. A total of four human cadaveric spine specimens (T12-L5) were tested. Each lumbar column was attached to the lower platform through a load cell. Weights were added to the upper platform to match the thorax, head-neck, and upper extremity mass of a 50th percentile male. Both platforms were raised to the drop height and released in unison. Deceleration characteristics of the lower platform were modulated by foam at the bottom of the drop tower. The upper platform applied compressive inertial loads to the top of the specimen during deceleration. All specimens demonstrated complex bending during ejection simulations, with the pattern dependent upon the anterior-posterior location of load application. The model demonstrated adequate inter-specimen kinematic repeatability on a spinal level-by-level basis under different subfailure loading scenarios. One specimen was then exposed to additional tests of increasing acceleration to induce identifiable injury and validate the model as an injury-producing system. Multiple noncontiguous vertebral fractures were obtained at an acceleration of 21 g with 488 g/s rate of onset. This clinically relevant trauma consisted of burst fracture at L1 and wedge fracture at L4. Compression of the vertebral body approached 60% during the failure test, with -6,106 N axial force and 168 Nm flexion moment. Future applications of this model include developing a better understanding of the vertebral injury mechanism during pilot ejection and developing tolerance limits for injuries sustained under a variety of different vertical acceleration scenarios.

Comparative biomechanical investigation of a modular dynamic lumbar stabilization system and the Dynesys system

The goal of non-fusion stabilization is to reduce the mobility of the spine segment to less than that of the intact spine specimen, while retaining some residual motion. Several in vitro studies have been conducted on a dynamic system currently available for clinical use (Dynesys). Under pure moment loading, a dependency of the biomechanical performance on spacer length has been demonstrated; this variability in implant properties is removed with a modular concept incorporating a discrete flexible element. An in vitro study was performed to compare the kinematic and stabilizing properties of a modular dynamic lumbar stabilization system with those of Dynesys, under the influence of an axial preload. Six human cadaver spine specimens (L1-S1) were tested in a spine loading apparatus. Flexibility measurements were performed by applying pure bending moments of 8 Nm, about each of the three principal anatomical axes, with a simultaneously applied axial preload of 400 N. Specimens were tested intact, and following creation of a defect at L3-L4, with the Dynesys implant, with the modular implant and, after removal of the hardware, the injury state. Segmental range of motion (ROM) was reduced for flexion-extension and lateral bending with both implants. Motion in flexion was reduced to less than 20% of the intact level, in extension to approximately 40% and in lateral bending a motion reduction to less than 40% was measured. In torsion, the total ROM was not significantly different from that of the intact level. The expectations for a flexible posterior stabilizing implant are not fulfilled. The assumption that a device which is particularly compliant in bending allows substantial intersegmental motion cannot be fully supported when one considers that such devices are placed at a location far removed from the natural rotation center of the intervertebral joint.

Insertion of the Artificial Disc Replacement

Comparison of total disc replacement (TDR) with and without computer-assisted surgical navigation. To test and evaluate the accuracy of computer-assisted navigation for the lumbar spine by comparing the traditional C-arm-aided insertion of an arthroplasty device to the navigation-aided insertion of the implant. Previous studies have shown that poor placement of the CHARITE disc can be correlated to worse clinical results. Because of parallax effect, exclusive use of fluoroscopy could make placement of the artificial disc less accurate. False positioning may also lead to spondylolisthesis, disc degeneration of the adjacent segment, subsidence of the disc, and failure of the implant. Ten human cadaver spine specimens were used at 3 lumbar segments (L3-L4, L4-L5, and L5-S1). Before implantation, all artificial discs were planned for "ideal" placement on a digital computed tomography image. Fifteen lumbar intervertebral disc prostheses (Depuy, Raynham, MA) were placed using Vector Vision image guidance (BrainLAB AG, Munich, Germany), by an inexperienced TDR-surgeon. Fifteen lumbar intervertebral disc prostheses were placed with exclusive use of fluoroscopy by an experienced TDR-surgeon. After insertion, DICOM computed tomography scans were analyzed using computer software to assess placement accuracy of each disc prosthesis. The navigated placement of the disc was significantly more accurate. Only 3 navigated disc prostheses were suboptimal and none was poorly placed. Surgical computer-assisted navigation may be a useful tool in the hands of a spine surgeon to achieve more accurate placement of the disc prosthesis. Because of the parallax effect, computer-assisted navigation offers more placement accuracy than stan- dard fluoroscopy. Because the accurate placement of total disc prosthesis has been correlated with better clinical outcome, further study regarding the navigation of the TDR is essential.