An anatomically shaped medial meniscus prosthesis is able to partially restore the contact mechanics of the meniscectomized knee joint.

PURPOSE: The aim of this study was to determine whether a flexible medial meniscus prosthesis is more capable of sharing loads with the direct tibiofemoral cartilage contact than the stiffer first-generation prosthesis. Additionally, the effect of the prosthesis on the tibial pressure distribution after total meniscectomy was investigated. METHODS: In an artificial knee joint, the relative amounts of load transferred through both meniscus prostheses and the direct tibiofemoral contact were assessed with pressure-sensitive sensors. Additionally, six cadaveric knee joints were loaded in a physiological environment. Tibial contact pressures were measured with an intact native meniscus, after total meniscectomy and after implantation of the second-generation meniscus prosthesis. RESULTS: Whereas the first generation of the meniscus prosthesis transferred virtually all the load from femur to tibia, the second-generation prosthesis allowed for load sharing with the direct tibiofemoral contact. No differences in load sharing were found between the native meniscus and the second-generation meniscus prosthesis. The prosthesis decreased peak and mean pressures on the medial tibial cartilage compared to meniscectomy. No significant differences in pressure were found between the native meniscus and the meniscus prosthesis. CONCLUSIONS: The second-generation meniscus prosthesis presented in this study can share loads with the direct tibiofemoral contact, a characteristic that the first-generation prosthesis did not have. The flexible meniscus prosthesis significantly reduces the contact pressures on the medial tibial plateau after total meniscectomy. Although the biomechanical performance of the native meniscus could not be reproduced completely, the meniscus prosthesis may have the potential to relieve post-meniscectomy pain symptoms.

Biomechanical effects of a titanium intervertebral cage as a stand-alone device, and in combination with locking plates in the canine caudal cervical spine.

OBJECTIVE: To evaluate the change in ex vivo biomechanical properties of the canine cervical spine, due to an intervertebral cage, both as a stand-alone device and in combination with plates. STUDY DESIGN: Experimental ex vivo study. ANIMALS: Cervical spinal segments (C5-C7) from eight canine cadavers. METHODS: The range of motion (ROM) and elastic zone stiffness (EZS) of the spines were determined with a four-point bending device in flexion/extension, lateral bending, and axial rotation for four conditions: native, discectomy, cage (at C6-C7), and cage with plates (at C6-C7). The disc height index (DHI) for each condition was determined using radiography. RESULTS: Discectomy resulted in overall increased ROM (p < .01) and EZS (p < .05) and decreased DHI (p < .005) when compared to the native condition. Placement of the cage increased DHI (p < .001) and restored total ROM during flexion/extension, lateral bending and axial rotation, and EZS during flexion/extension to the level of the native spine. Application of the plates further reduced the total ROM during flexion/extension (p < .001) and lateral bending (p < .001), but restored ROM in extension and EZS during lateral bending. No implant failure, subsidence, or significant cage migration occurred during loading. CONCLUSION: An anchorless intervertebral cage used as a stand-alone device was able to restore the disc height and spinal stability to the level of the native cervical spine, whereas the addition of plates further reduced the spinal unit mobility. CLINICAL SIGNIFICANCE: This study implies that the intervertebral cage may be used as a stand-alone device in the spinal unit fixation in the canine cervical spine.

Mechanical stress regulates bone regulatory gene expression independent of estrogen and vitamin D deficiency in rats.

Mechanical stress determines bone mass and structure. It is not known whether mechanical loading affects expression of bone regulatory genes in a combined deficiency of estrogen and vitamin D. We studied the effect of mechanical loading on the messenger RNA (mRNA) expression of bone regulatory genes during vitamin D and/or estrogen deficiency. We performed a single bout in vivo axial loading with 14 N peak load, 2 Hz frequency and 360 cycles in right ulnae of nineteen weeks old female control Wistar rats with or without ovariectomy (OVX), vitamin D deficiency and the combination of OVX and vitamin D deficiency (N = 10/group). Total bone RNA was isolated 6 hours after loading, and mRNA expression was detected of Mepe, Fgf23, Dmp1, Phex, Sost, Col1a1, Cyp27b1, Vdr, and Esr1. Serum levels of 25(OH)D, 1,25(OH)2 D and estradiol were also measured at this time point. The effect of loading, vitamin D and estrogen deficiency and their interaction on bone gene expression was tested using a mixed effect model analysis. Mechanical loading significantly increased the mRNA expression of Mepe, and Sost, whereas it decreased the mRNA expression of Fgf23 and Esr1. Mechanical loading showed a significant interaction with vitamin D deficiency with regard to mRNA expression of Vdr and Esr1. Mechanical loading affected gene expression of Mepe, Fgf23, Sost, and Esr1 independently of vitamin D or estrogen, indicating that mechanical loading may affect bone turnover even during vitamin D deficiency and after menopause.

Comparison of polyetheretherketone versus silicon nitride intervertebral spinal spacers in a caprine model.

Polyetheretherketone (PEEK) is commonly used as a spinal spacer for intervertebral fusion surgery. Unfortunately, PEEK is bioinert and does not effectively osseointegrate into living bone. In contrast, comparable spacers made of silicon nitride (Si3 N4 ) possess a surface nanostructure and chemistry that encourage appositional bone healing. This observational study was designed to compare the outcomes of these two biomaterials when implanted as spacers in an adult caprine model. Lumbar interbody fusion surgeries were performed at two adjacent levels in eight adult goats using implants of PEEK and Si3 N4 . At six-months after surgery, the operative and adjacent spinal segments were extracted and measured for bone fusion, bone volume, bone-implant contact (BIC) and soft-tissue implant contact (SIC) ratios, and biodynamic stability. The null hypothesis was that no differences in these parameters would be apparent between the two groups. Fusion was observed in seven of eight implants in each group with greater bone formation in the Si3 N4 group (52.6%) versus PEEK (27.9%; p = 0.2). There were no significant differences in BIC ratios between PEEK and Si3 N4 , and the biodynamic stability of the two groups was also comparable. The results suggest that Si3 N4 spacers are not inferior to PEEK and they may be more effective in promoting arthrodesis. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 00B: 000-000, 2018. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 688-699, 2019.

The Influence of Radiotherapy on the Mechanical Properties of Silicone Breast Implants.

BACKGROUND: Silicone breast implants have been used for decades for cosmetic breast augmentation or reconstruction after mastectomy. In selected cases, postmastectomy adjuvant radiotherapy is given with the breast implants in situ. Previous clinical studies have shown that radiotherapy may lead to complications such as capsular contracture and infection and that removal of the implant may be required. Yet, the effect of radiotherapy on silicone breast implants themselves is unknown. The aim of this study was to investigate if irradiation of breast implants influences their mechanical properties. METHODS: This was an ex vivo study on 32 ready-to-use silicone breast implants (Mentor and Silimed). Half of the implants of each brand were irradiated with 1 × 60 Gy, the other half were not irradiated. Tensile, mechanical hysteresis, and rheology tests were performed. Differences in mechanical properties between the irradiated and nonirradiated implants were determined. RESULTS: No significant differences were found in tensile strength, mechanical hysteresis, and rheological properties between irradiated and nonirradiated implants. CONCLUSIONS: Breast implants' mechanical properties for these 2 brands were not significantly affected after single-dose irradiation in an ex vivo setting.

Changes in Intervertebral Disk Mechanical Behavior During Early Degeneration.

Intervertebral disk (IVD) degeneration is commonly described by loss of height and hydration. However, in the first stage of IVD degeneration, this loss has not yet occurred. In the current study, we use an ex vivo degeneration model to analyze the changes in IVDs mechanical behavior in the first phase of degeneration. We characterize these changes by stretched-exponential fitting, and suggest the fitted parameters as markers for early degeneration. Enzymatic degeneration of healthy lumbar caprine IVDs was induced by injecting 100 µL of Chondroïtinase ABC (Cabc) into the nucleus. A no-intervention and phosphate buffered saline (PBS) injected group were used as controls. IVDs were cultured in a bioreactor for 20 days under diurnal, simulated-physiological loading (SPL) conditions. Disk deformation was continuously monitored. Changes in disk height recovery behavior were quantified using stretched-exponential fitting. Disk height, histological sections, and water- and glycosaminoglycan (GAG)-content measurements were used as gold standards for the degenerative state. Cabc injection caused significant GAG loss from the nucleus and had detrimental effects on poro-elastic mechanical properties of the IVDs. These were progressive over time, with a propensity toward more linear recovery behavior. On histological sections, both PBS and Cabc injected IVDs showed moderate degeneration. A small GAG loss yields changes in IVD recovery behavior, which can be quantified with stretched-exponential fitting. Parameters changed significantly compared to control. Studies on disk degeneration and biomaterial engineering for degenerative disk disease (DDD) could benefit from focusing on IVD biomechanical behavior rather than height and water-content, as a marker for early disk degeneration.

Osmosis and viscoelasticity both contribute to time-dependent behaviour of the intervertebral disc under compressive load: A caprine in vitro study.

The mechanical behaviour of the intervertebral disc highly depends on the content and transport of interstitial fluid. It is unknown, however, to what extent the time-dependent behaviour can be attributed to osmosis. Here we investigate the effect of both mechanical and osmotic loading on water content, nucleus pressure and disc height. Eight goat intervertebral discs, immersed in physiological saline, were subjected to a compressive force with a pressure needle inserted in the nucleus. The loading protocol was: 10 N (6 h); 150 N (42 h); 10 N (24 h). Half-way the 150 N-phase (24 h), we eliminated the osmotic gradient by adding 26% poly-ethylene glycol to the surrounding fluid. For 62 additional discs, we determined the water content of both nucleus and annulus after 6, 24, 48, or 72 h. The compressive load was initially counterbalanced by the hydrostatic pressure in the nucleus. The load forced 4.3% of the water out of the nucleus, which reduced nucleus pressure by 44(±6)%. Reduction of the osmotic gradient disturbed the equilibrium disc height, and a significant loss of annulus water content was found. Remarkably, pressure and water content of the nucleus pulposus remained unchanged. This shows that annulus water content is important in the response to axial loading. After unloading, in the absence of an osmotic gradient, there was substantial viscoelastic recovery of 53(±11)% of the disc height, without a change in water content. However, for restoration of the nucleus pressure and for full restoration of disc height, restoration of the osmotic gradient was needed.

Coupled motions in human and porcine thoracic and lumbar spines.

Coupled motions, i.e., motions along axes other than the loaded axis, have been reported to occur in the human spine, and are likely to be influenced by inclined local axes due to the sagittal plane spine curvature. Furthermore, the role of facet joints in such motions is as yet unclear. Therefore, this study aimed at assessing coupled motions in multiple spine sections in vitro, before and after removal of posterior elements. Six elderly human and 6 young porcine spines were sectioned in four segments (high thoracic, mid thoracic, low thoracic and lumbar), each consisting of four vertebrae and three intervertebral discs. Segments were loaded along each of the three axes, and three-dimensional rotations of the middle segment were quantified. Subsequently, posterior elements were removed and the protocol was repeated. To avoid mixed loading between Axial Rotation (AR) and Lateral Bending (LB), in contrast to other studies, local axes at the vertebrae were defined as aligned with the loading device prior to each load application. Expressed as a percentage of motion in the loaded direction, coupled motions were on average larger in human (22.7%, SD = 2.2%) than in porcine (11.9%, SD = 1.2%) spines (p < .001). Largest coupled motions were obtained in AR loading of the lumbar spine segments, with mean magnitudes averaged over coupling axes for human L2-L3 joints of 48.9% (SD = 13.2%), including somewhat more LB (56.4%, SD = 18.6) than FE (41.4%, SD = 14.1%) coupling. For porcine L3-L4 joints average coupling in AR loading was 29.3% (SD = 8.2%). In human segments removal of posterior elements only had substantial effects in the lumbar spine segments, where posterior element removal decreased coupled motion during AR loading, averaged over LB and FE coupling, from 48.9% (SD = 13.2%) to 27.7% (SD = 6.1%), mainly through increased motion in the loaded direction. The present results indicate that coupled motions were largest in the lumbar spine. In human spines, posterior elements only contributed to coupled motions in lumbar axial rotation loading.

3D-printing zirconia implants; a dream or a reality? An in-vitro study evaluating the dimensional accuracy, surface topography and mechanical properties of printed zirconia implant and discs.

PURPOSE: The aim of this study was to evaluate the dimensional accuracy, surface topography of a custom designed, 3D-printed zirconia dental implant and the mechanical properties of printed zirconia discs. MATERIALS AND METHODS: A custom designed implant was 3D-printed in zirconia using digital light processing technique (DLP). The dimensional accuracy was assessed using the digital-subtraction technique. The mechanical properties were evaluated using biaxial flexure strength test. Three different build angles were adopted to print the specimens for the mechanical test; 0°(Vertical), 45° (Oblique) and 90°(Horizontal) angles. The surface topography, crystallographic phase structure and surface roughness were evaluated using scanning electron microscopy analysis (SEM), X-ray diffractometer and confocal microscopy respectively. RESULTS: The printed implant was dimensionally accurate with a root mean square (RMSE) value of 0.1mm. The Weibull analysis revealed a statistically significant higher characteristic strength (1006.6MPa) of 0° printed specimens compared to the other two groups and no significant difference between 45° (892.2MPa) and 90° (866.7MPa) build angles. SEM analysis revealed cracks, micro-porosities and interconnected pores ranging in size from 196nm to 3.3µm. The mean Ra (arithmetic mean roughness) value of 1.59µm (±0.41) and Rq (root mean squared roughness) value of 1.94µm (±0.47) was found. A crystallographic phase of primarily tetragonal zirconia typical of sintered Yttria tetragonal stabilized zirconia (Y-TZP) was detected. CONCLUSIONS: DLP prove to be efficient for printing customized zirconia dental implants with sufficient dimensional accuracy. The mechanical properties showed flexure strength close to those of conventionally produced ceramics. Optimization of the 3D-printing process parameters is still needed to improve the microstructure of the printed objects.

A Novel Spinal Implant for Fusionless Scoliosis Correction: A Biomechanical Analysis of the Motion Preserving Properties of a Posterior Periapical Concave Distraction Device.

STUDY DESIGN: Biomechanical study. OBJECTIVE: Recently, a posterior concave periapical distraction device for fusionless scoliosis correction was introduced. The goal of this study was to quantify the effect of the periapical distraction device on spinal range of motion (ROM) in comparison with traditional rigid pedicle screw-rod instrumentation. METHODS: Using a spinal motion simulator, 6 human spines were loaded with 4 N m and 6 porcine spines with 2 N m to induce flexion-extension (FE), lateral bending (LB), and axial rotation (AR). ROM was measured in 3 conditions: untreated, periapical distraction device, and rigid pedicle screw-rod instrumentation. RESULTS: The periapical distraction device caused a significant (P < .05) decrease in ROM of FE (human, -40.0% and porcine, -55.9%) and LB (human, -18.2% and porcine, -17.9%) as compared to the untreated spine, while ROM of AR remained unaffected. In comparison, rigid instrumentation caused a significantly (P < .05) larger decrease in ROM of FE (human, -80.9% and porcine, -94.0%), LB (human, -75.0% and porcine, -92.2%), and AR (human, -71.3% and porcine, -86.9%). CONCLUSIONS: Although no destructive forces were applied, no device failures were observed. Spinal ROM was significantly less constrained by the periapical distraction device compared to rigid pedicle screw-rod instrumentation. Therefore, provided that scoliosis correction is achieved, a more physiological spinal motion is expected after scoliosis correction with the posterior concave periapical distraction device.

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.

Intradiscal pressure measurements: A challenge or a routine?

Intradiscal pressure (IDP) is an essential biomechanical parameter and has been the subject of numerous in vivo and in vitro investigations. Although currently available sensors differ in size and measurement principles, no data exist regarding inter-sensor reliability in measuring IDP. Moreover, although discs of various species vary significantly in size and mechanics, the possible effects of sensor insertion on the IDP have never been investigated. The present in vitro study aimed to address these issues. The synchronized signals of two differently sized pressure transducers (Ø1.33 and Ø0.36 mm) obtained during the measurements in two species (bovine and caprine) and their influence on the measured pressure were compared. First, the discs were subjected to three loading periods, and the pressure was measured simultaneously to assess the inter-sensor reliability. In the second test, the effect of the sensor size was evaluated by alternatingly inserting one transducer into the disc while recording the resulting pressure change with the second transducer. Although both sensors yielded similar pressure values (ICC: consistency: 0.964-0.999; absolute agreement: 0.845-0.996) when used simultaneously, the sensor size was determined to influence the measured pressure during the insertion tests. The magnitude of the effect differed between species; it was insignificant in the bovine specimens but significant in the caprine specimens, with a pressure increase of 0.31-0.64 MPa (median: 0.43 MPa) obtained when the larger sensor was inserted. The results suggest that sensor selection for IDP measurements requires special attention and can be crucial for species with smaller disc sizes.

The poro-elastic behaviour of the intervertebral disc: A new perspective on diurnal fluid flow.

Diurnal disc height changes, due to fluid in- and outflow, are in equilibrium while daytime spinal loading is twice as long as night time rest. A direction-dependent permeability of the endplates, favouring inflow over outflow, reportedly explains this; however, fluid flow through the annulus fibrosus should be considered. This study investigates the fluid flow of entire intervertebral discs. Caprine discs were preloaded in saline for 24h under four levels of static load. Under sustained load, we modulated the disc׳s swelling pressure by exchanging saline for demineralised water (inflow) and back to saline (outflow), both for 24h. We measured disc height creep and used stretched exponential models to determine time-constants. During inflow disc height increased in relation to applied load, and during outflow disc height decreased to preload levels. When comparing in- and outflow phases, there was no difference in creep, and time-constants were similar indicating no direction-dependent resistance to fluid flow in the entire intervertebral disc. Results provoked a new hypothesis for diurnal fluid flow: in vitro time-constants for loading are shorter than for unloading and in vivo daytime loading is twice as long as night time unloading, i.e. in diurnal loading the intervertebral disc is closer to loading equilibrium than to unloading equilibrium. Per definition, fluid flow is slower close to equilibrium than far from equilibrium; therefore, as diurnal loading occurs closer to loading equilibrium, fluid inflow during night time unloading can balance fluid outflow during daytime loading, despite a longer time-constant.

Range of Motion in Segmental Versus Nonsegmental Ultrahigh Molecular Weight Polyethylene Sublaminar Wire Growth Guidance Type Constructs for Early-Onset Scoliosis Correction.

STUDY DESIGN: An in vitro biomechanical study in porcine thoracic spine segments comparing range of motion (ROM) in segmental versus multiple nonsegmental ultrahigh molecular weight polyethylene (UHMWPE) sublaminar wire constructs. OBJECTIVE: To determine the effect of varying instrumentation (wire) density in an UHMWPE sublaminar wire construct for patients with early-onset scoliosis (EOS) to find an optimal wire density, which allows maximum growth whereas still providing adequate correction and fixation. SUMMARY OF BACKGROUND DATA: UHMWPE sublaminar wires in a segmental construct did not negatively affect longitudinal spinal growth during a 24-week period in an ovine model; application in growth guidance system for EOS may therefore be feasible. To avoid ectopic bone formation as much as possible, a reduction of instrumented levels, without affecting spinal stabilization, is desirable. METHODS: ROM of 9 porcine thoracic spines (T6-T14) was determined in flexion/extension (FE), lateral bending (LB), and axial rotation up to ±â€Š4 Nm. Tests were performed for the uninstrumented spine in a segmental construct with UHMWPE sublaminar wires and dual pedicle screws at the most caudal level, and in four nonsegmental constructs that were attained by stepwise removal of the most caudal wire. RESULTS: Segmental instrumentation led to a decrease in total ROM by approximately 70% for both FE and LB. A stepwise increase in ROM with decreasing number of consecutively instrumented levels was most clearly observed in LB. However, consistent significant but also relevant substantial differences in ROM for both FE and LB were noted only when comparing two and one consecutively instrumented end levels (P < 0.05). CONCLUSION: A construct with two consecutive end levels instrumented with UHMWPE sublaminar wires seems to provide the best balance between spinal stabilization and minimizing the number of instrumented levels and thereby surgical exposure, which is crucial for allowing longitudinal growth. LEVEL OF EVIDENCE: N/A.

The effects of single-level instrumented lumbar laminectomy on adjacent spinal biomechanics.

Study Design Biomechanical study. Objective Posterior instrumentation is used to stabilize the spine after a lumbar laminectomy. However, the effects on the adjacent segmental stability are unknown. Therefore, we studied the range of motion (ROM) and stiffness of treated lumbar spinal segments and cranial segments after a laminectomy and after posterior instrumentation in flexion and extension (FE), lateral bending (LB), and axial rotation (AR). These outcomes might help to better understand adjacent segment disease (ASD), which is reported cranial to the level on which posterior instrumentation is applied. Methods We obtained 12 cadaveric human lumbar spines. Spines were axially loaded with 250 N for 1 hour. Thereafter, 10 consecutive load cycles (4 Nm) were applied in FE, LB, and AR. Subsequently, a laminectomy was performed either at L2 or at L4. Thereafter, load-deformation tests were repeated, after similar preloading. Finally, posterior instrumentation was added to the level treated with a laminectomy before testing was repeated. The ROM and stiffness of the treated, the cranial adjacent, and the control segments were calculated from the load-displacement data. Repeated-measures analyses of variance used the spinal level as the between-subject factor and a laminectomy or instrumentation as the within-subject factors. Results After the laminectomy, the ROM increased (+19.4%) and the stiffness decreased (-18.0%) in AR. The ROM in AR of the adjacent segments also increased (+11.0%). The ROM of treated segments after instrumentation decreased in FE (-74.3%), LB (-71.6%), and AR (-59.8%). In the adjacent segments after instrumentation, only the ROM in LB was changed (-12.9%). Conclusions The present findings do not substantiate a biomechanical pathway toward or explanation for ASD.

How Does Spinal Release and Ponte Osteotomy Improve Spinal Flexibility? The Law of Diminishing Returns.

STUDY DESIGN: Experimental study. OBJECTIVES: To evaluate the effect of stepwise resection of posterior spinal ligaments, facet joints, and ribs on thoracic spinal flexibility. SUMMARY OF BACKGROUND DATA: Posterior spinal ligaments, facet joints and ribs are removed to increase spinal flexibility in corrective spinal surgery for deformities such as adolescent idiopathic scoliosis (AIS). Reported clinical results vary and biomechanical substantiation is lacking. METHODS: Ten fresh-frozen human cadaveric thoracic spinal specimens (T6-T11) were studied. A spinal motion simulator applied a pure moment of ±2.5 Nm in flexion, extension, lateral bending (LB) and axial rotation (AR). Range of motion (ROM) was measured for the intact spine and measured again after stepwise resection of the supra/interspinous ligament (SIL), inferior facet, flaval ligament, superior facet, and rib heads. RESULTS: SIL resection increased ROM in flexion (10.2%) and AR (3.1%). Successive inferior facetectomy increased ROM in flexion (4.1%), LB (3.8%) and AR (7.7%), and flavectomy in flexion (9.1%) and AR (2.5%). Sequential superior facetectomy only increased ROM in flexion (6.3%). Rib removal provided an additional increase in flexion (6.3%), LB (4.5%) and AR (13.0%). Extension ROM increased by 10.5% after the combined removal of the SIL, inferior facet and flaval ligament. CONCLUSIONS: Posterior spinal releases in these non-scoliotic spines led to an incremental increase in spinal flexibility, but each sequential step had less effect. As compared to SIL resection with inferior facetectomy, additional superior facetectomy did not improve flexibility in AR and LB and only 6.3% in flexion. The data presented from this in vitro study should be interpreted with care, as no representative cadaveric spine model for AIS was available, However, the results presented here at least question the benefits of performing routine complete facetectomies (i.e. Ponte osteotomies) to increase spinal flexibility in scoliosis surgery.

Intradiscal pressure depends on recent loading and correlates with disc height and compressive stiffness.

PURPOSE: Intervertebral discs exhibit time-dependent deformation (creep), which could influence the relation between applied stress and intradiscal pressure. This study investigates the effect of prolonged dynamic loading on intradiscal pressure, disc height and compressive stiffness, and examines their mutual relationships. METHODS: Fifteen caprine lumbar discs with 5 mm of vertebral bone on either side were compressed by 1 Hz sinusoidal load for 4.5 h. After preload, 'High' (130 ± 20 N) or 'Low' (50 ± 10 N) loads were alternated every half hour. Continuous intradiscal pressure measurement was performed with a pressure transducer needle. RESULTS: Each disc showed a linear relationship between axial compression and intradiscal pressure (R (2) > 0.91). The intercept of linear regression analysis declined over time, but the gradient remained constant. Disc height changes were correlated to intradiscal pressure changes (R (2) > 0.98): both decreased during High loading, and increased during Low loading. In contrast, compressive stiffness increased during High loading, and was inversely related to intradiscal pressure and disc height. CONCLUSIONS: Intradiscal pressure is influenced by recent loading due to fluid flow. The correlations found in this study suggest that intradiscal pressure is important for disc height and axial compliance. These findings are relevant for mechanobiology studies, nucleus replacements, finite element models, and ex vivo organ culture systems.

Single level lumbar laminectomy alters segmental biomechanical behavior without affecting adjacent segments.

BACKGROUND: Degenerative lumbar spinal stenosis causes neurological symptoms due to neural compression. Lumbar laminectomy is a commonly used treatment for symptomatic degenerative spinal stenosis. However, it is unknown if and to what extent single level laminectomy affects the range of motion and stiffness of treated and adjacent segments. An increase in range of motion and a decrease in stiffness are possible predictors of post-operative spondylolisthesis or spinal failure. METHODS: Twelve cadaveric human lumbar spines were obtained. After preloading, spines were tested in flexion-extension, lateral bending, and axial rotation. Subsequently, single level lumbar laminectomy analogous to clinical practice was performed at level lumbar 2 or 4. Thereafter, load-deformation tests were repeated. The range of motion and stiffness of treated and adjacent segments were calculated before and after laminectomy. Untreated segments were used as control group. Effects of laminectomy on stiffness and range of motion were tested, separately for treated, adjacent and control segments, using repeated measures analysis of variance. FINDINGS: Range of motion at the level of laminectomy increased significantly for flexion and extension (7.3%), lateral bending (7.5%), and axial rotation (12.2%). Range of motion of adjacent segments was only significantly affected in lateral bending (-7.7%). Stiffness was not affected by laminectomy. INTERPRETATION: The increase in range of motion of 7-12% does not seem to indicate the use of additional instrumentation to stabilize the lumbar spine. If instrumentation is still considered in a patient, its primary focus should be on re-stabilizing only the treated segment level.

Which factors prognosticate rotational instability following lumbar laminectomy?

PURPOSE: Reduced strength and stiffness of lumbar spinal motion segments following laminectomy may lead to instability. Factors that predict shear biomechanical properties of the lumbar spine were previously published. The purpose of the present study was to predict spinal torsion biomechanical properties with and without laminectomy from a total of 21 imaging parameters. METHOD: Radiographs and MRI of ten human cadaveric lumbar spines (mean age 75.5, range 59-88 years) were obtained to quantify geometry and degeneration of the motion segments. Additionally, dual X-ray absorptiometry (DXA) scans were performed to measure bone mineral content and density. Facet-sparing lumbar laminectomy was performed either on L2 or L4. Spinal motion segments were dissected (L2-L3 and L4-L5) and tested in torsion, under 1,600 N axial compression. Torsion moment to failure (TMF), early torsion stiffness (ETS, at 20-40 % TMF) and late torsion stiffness (LTS, at 60-80 % TMF) were determined and bivariate correlations with all parameters were established. For dichotomized parameters, independent-sample t tests were used. RESULTS: Univariate analyses showed that a range of geometric characteristics and disc and bone quality parameters were associated with torsion biomechanical properties of lumbar segments. Multivariate models showed that ETS, LTS and TMF could be predicted for segments without laminectomy (r (2) values 0.693, 0.610 and 0.452, respectively) and with laminectomy (r (2) values 0.952, 0.871 and 0.932, respectively), with DXA-derived measures of bone quality and quantity as the main predictors. CONCLUSIONS: Vertebral bone content and geometry, i.e. intervertebral disc width, frontal area and facet joint tropism, were found to be strong predictors of ETS, LTS and TMF following laminectomy, suggesting that these variables could predict the possible development of post-operative rotational instability following lumbar laminectomy. Proposed diagnostic parameters might aid surgical decision-making when deciding upon the use of instrumentation techniques.

Modelling creep behaviour of the human intervertebral disc.

The mechanical behaviour of an intervertebral disc is time dependent. In literature different constitutive equations have been used to describe creep. It is unsure whether these different approaches yield valid predictions. In this study, we compared the validity of different equations for the prediction of creep behaviour. To this end, human thoracic discs were preloaded at 0.1 MPa for 12h, compressed (0.8 MPa) for 24h and finally unloaded (0.1 MPa) for 24h. A Kohlrausch-Williams-Watts (KWW) model and a Double-Voight (DV) model were fitted to the creep data. Model parameters were calculated for test durations of 4, 8, 12, 16, 20 and 24h. Both models described the measured data well, but parameters were highly sensitive to test duration. The estimated time constant varied with test duration from 3.6 to 17h. When extrapolating beyond test duration, the DV model under-estimated and the KWW model over-estimated creep. The 24h experiment was still too short for an accurate determination of the parameters. Therefore, parameters obtained in this paper can be used to describe normal behaviour, but are not suitable for extrapolation beyond the test duration.