Facet Loads Research Articles

Cervical Spine Arthroplasty: Kinematics of Cervical Arthroplasty

Cervical disc arthroplasty is a relatively young surgical concept in the history of surgical interventions designed to treat pathologic conditions of the cervical spine. The introduction of these devices has stimulated novel studies aimed at understanding motion in the cervical spine, and driven investigators to examine the consequences that result from surgical alteration of pathologic structures. The study of “cervical kinematics” has evolved from basic analysis of flexion/extension radiographs to complex, computer-assisted modeling that aides investigators in understanding concepts such as center of rotation, functional spinal unit translation, and coupled motion. In recent years, kinematic studies have contributed to our understanding of adjacent level degeneration and index-level facet loading. We review the young science of cervical arthroplasty kinematics.

Biomechanics of a Novel Minimally Invasive Lumbar Interspinous Spacer

To study the alteration to normal biomechanics after insertion of a lumbar interspinous spacer (ISS) in vitro by nondestructive cadaveric flexibility testing. Seven human cadaveric specimens were studied before and after ISS placement at L1-L2. Angular range of motion, lax zone, stiff zone, sagittal instantaneous axis of rotation (IAR), foraminal height, and facet loads were compared between conditions. Flexion, extension, lateral bending, and axial rotation were induced using pure moments (7.5 Nm maximum) while recording motion optoelectronically. The IAR was measured during loading with a 400 N compressive follower. Foraminal height changes were calculated using rigid body methods. Facet loads were assessed from surface strain and neural network analysis. After ISS insertion, range of motion and stiff zone during extension were significantly reduced (P < .01). Foraminal height was significantly reduced from flexion to extension in both normal and ISS-implanted conditions; there was significantly less reduction in foraminal height during extension with the ISS in place. The ISS reduced the mean facet load by 30% during flexion (P < .02) and 69% during extension (P < .015). The IAR after ISS implantation was less than 1 mm from the normal position (P > .18). The primary biomechanical effect of the ISS was reduced extension with associated reduced facet loads and smaller decrease in foraminal height. The ISS had little effect on sagittal IAR or on motion or facet loads in other directions.

Biomechanical Changes of the Lumbar Segment after Total Disc Replacement : Charite®, Prodisc® and Maverick® Using Finite Element Model Study

The purpose of this study was to analyze the biomechanical effects of three different constrained types of an artificial disc on the implanted and adjacent segments in the lumbar spine using a finite element model (FEM). The created intact model was validated by comparing the flexion-extension response without pre-load with the corresponding results obtained from the published experimental studies. The validated intact lumbar model was tested after implantation of three artificial discs at L4-5. Each implanted model was subjected to a combination of 400 N follower load and 5 Nm of flexion/extension moments. ABAQUS version 6.5 (ABAQUS Inc., Providence, RI, USA) and FEMAP version 8.20 (Electronic Data Systems Corp., Plano, TX, USA) were used for meshing and analysis of geometry of the intact and implanted models. Under the flexion load, the intersegmental rotation angles of all the implanted models were similar to that of the intact model, but under the extension load, the values were greater than that of the intact model. The facet contact loads of three implanted models were greater than the loads observed with the intact model. Under the flexion load, three types of the implanted model at the L4-5 level showed the intersegmental rotation angle similar to the one measured with the intact model. Under the extension load, all of the artificial disc implanted models demonstrated an increased extension rotational angle at the operated level (L4-5), resulting in an increase under the facet contact load when compared with the adjacent segments. The increased facet load may lead to facet degeneration.

Simulated Whiplash Modulates Expression of the Glutamatergic System in the Spinal Cord Suggesting Spinal Plasticity Is Associated with Painful Dynamic Cervical Facet Loading

The cervical facet joint and its capsule have been reported to be injured during whiplash scenarios and are a common source of chronic neck pain from whiplash. Both the metabotropic glutamate receptor 5 (mGluR5) and the excitatory amino acid carrier 1 (EAAC1) have pivotal roles in chronic pain. In this study, spinal mGluR5 and EAAC1 were quantified following painful facet joint distraction in a rat model of facet-mediated painful loading and were evaluated for their correlation with the severity of capsule loading. Rats underwent either a dynamic C6/C7 joint distraction simulating loading experienced during whiplash (distraction; n = 12) or no distraction (sham; n = 6) to serve as control. The severity of capsular loading was quantified using strain metrics, and mechanical allodynia was assessed after surgery. Spinal cord tissue was harvested at day 7 and the expression of mGluR5 and EAAC1 were quantified using Western blot analysis. Mechanical allodynia following distraction was significantly (p < 0.001) higher than sham. Spinal expression of mGluR5 was also significantly (p < 0.05) greater following distraction relative to sham. However, spinal EAAC1 was significantly (p = 0.0003) reduced compared to sham. Further, spinal mGluR5 expression was significantly positively correlated to capsule strain (p < 0.02) and mechanical allodynia (p < 0.02). Spinal EAAC1 expression was significantly negatively related to one of the strain metrics (p < 0.003) and mechanical allodynia at day 7 (p = 0.03). These results suggest that the spinal glutamatergic system may potentiate the persistent behavioral hypersensitivity that is produced following dynamic whiplash-like joint loading; chronic whiplash pain may be alleviated by blocking mGluR5 expression and/or enhancing glutamate transport through the neuronal transporter EAAC1.

Do design variations in the artificial disc influence cervical spine biomechanics? A finite element investigation

Various ball and socket-type designs of cervical artificial discs are in use or under investigation. Many artificial disc designs claim to restore the normal kinematics of the cervical spine. What differentiates one type of design from another design is currently not well understood. In this study, authors examined various clinically relevant parameters using a finite element model of C3–C7 cervical spine to study the effects of variations of ball and socket disc designs. Four variations of ball and socket-type artificial disc were placed at the C5–C6 level in an experimentally validated finite element model. Biomechanical effects of the shape (oval vs. spherical ball) and location (inferior vs. superior ball) were studied in detail. Range of motion, facet loading, implant stresses and capsule ligament strains were computed to investigate the influence of disc designs on resulting biomechanics. Motions at the implant level tended to increase following disc replacement. No major kinematic differences were observed among the disc designs tested. However, implant stresses were substantially higher in the spherical designs when compared to the oval designs. For both spherical and oval designs, the facet loads were lower for the designs with an inferior ball component. The capsule ligament strains were lower for the oval design with an inferior ball component. Overall, the oval design with an inferior ball component, produced motion, facet loads, implant stresses and capsule ligament strains closest to the intact spine, which may be key to long-term implant survival.

P39. Midline Sparing Bilateral Laminotomy Prevents Disc Collapse as Compared to Traditional Laminectomy – A Biomechanical Finite Element Analysis

Objective: To biomechanically compare two competing surgical techniques for alleviating spinal stenosis. Summary and Background Data: Traditional laminectomy is the long standing surgical gold standard for relief of pain induced to spinal stenosis. However, loss of the posterior tension band due to this technique may result into settling of the motion segment leading to neural foraminal stenosis and recurrence of the symptoms. An alternative technique, which conserves the posterior tension band through the midline sparing bilateral laminotomies, may provide good long term outcomes. Methods: An experimentally validated finite element model of the intact L3/S1 segment was used to simulate the laminectomy and bilateral laminotomies across the L4/L5 level. Segmental Motion, intra-discal pressure (IDP), facet load and foramina opening were compared between the intact and surgical models in response to 400N of axial compression and 10Nm moments. Results: Segmental motion and IDP after L4 laminectomy increased in flexion, especially at L4/L5 disc level. The laminotomy model had biomechanics close to the intact model, especially in flexion. The distance of L4/L5 foramina in both surgical models decreased in extension, lateral bending and axial rotation compared with the intact model. However in flexion the distance in foramina opening increased in laminectomy model. Facet loads at L4/L5 after both procedures decreased in extension, lateral bending, and axial rotation. Conclusion: The midline laminotomy surgery induced much less alterations in biomechanics of spine compared to traditional laminectomy. The increase in IDP and motion for laminectomy model means that the load on the disc is higher as compared to the midline sparing which may lead to long-term disc degeneration. The findings of the current study show that midline sparing laminotomies may prevent disc collapse due to reduced disc loading without sacrificing the opening of formina, as compared to the traditional laminectomy.

In Situ Contact Analysis of the Prosthesis Components of Prodisc-L in Lumbar Spine Following Total Disc Replacement

A three-dimensional, nonlinear finite element analysis was performed to predict the in situ contact interaction of prosthesis components of the Prodisc-L in a multisegmental lumbar model following total disc replacement (TDR). Efforts were made to investigate how the TDR implant contact characteristics could affect the 3-dimensional kinematics, facet loads of the lumbar spine following TDR. Although spinal motion analyses of human lumbar cadaveric models after Prodisc TDR have been widely studied, the interaction of the disc prosthesis, particularly its in situ contact mechanics, is never known. A validated intact multisegmental lumbar finite element model L2-L4 was altered to accommodate the TDR prosthesis through anterior approach. At L3-L4 disc space, the Prodisc-L of 6 degrees lordosis angle was implanted centrally. The model was subjected to compressive preload and pure moments to create flexion, extension, lateral bending, and axial rotation motion in physiologic range. The contact interaction between the superior component of Prodisc-L and the UHMWPE inlay were assessed in terms of contact region (CR), contact area (CA), and contact pressure (CP). Parameters of range of motion (ROM) and facet loading transfer were simultaneously analyzed and compared with those of the intact model. The predicted contact area was 3.5 times larger in flexion than that observed in extension, whereas the maximum contact pressure in the disc articulation was very similar with 15.1 MPa for flexion and 14.5 MPa for extension. Joint surface incongruence was developed in extension motion. The implanted model exhibited a 91.4% increase in ROM accompanied by a 150.6% rising in facet force during extension, while the flexion motion showed the least effects of TDR. In lateral bending and axial rotation, the abnormal joint "lift off" was not seen. The in situ function of the TDR prosthesis was highly dependent on how well the device could incorporate itself into the mechanical environment in the disc space, which has been determined by the rest of the spinal structures, including the retained disc anulus, articular facets, ligaments, vertebrae, and muscular stabilizers. The different contact interaction of the artificial disc components revealed here could be attributed to the violation of this mechanical environment which, in turn, may bring adverse effects to those spinal elements.

Four Year History Of Recalcitrant Back Pain In A Teenager

HISTORY: A 15-year-old volleyball and soccer player has had back pain since 2004 with recent recurrence. She first came to see us in July, 2007. At that time she had right lower back pain and buttock pain since November, 2005. In August, 2007 she was diagnosed with bilateral L5 spondylolysis and treated with a Boston Overlap brace, activity modification, and physical therapy. Over the course of the next 7 months, the patient improved to the point where she was pain-free, and she resumed playing soccer in March, 2008. In August, 2008, the patient developed a recurrence of right sided back pain and buttock pain with playing volleyball. Additionally, she developed subjective right leg weakness and numbness and tingling radiating down the leg. PHYSICAL EXAMINIATION: Examination in clinic revealed tenderness over the right L4-L5 paraspinal region and the superior portion of the right SI joint. Patient had antalgic gait. Patient was capable of heel and toe walking. Patient had pain on the right lumbar region with facet loading maneuvers. Sensory exam revealed diminished sensation to light touch on right lateral and medial thigh, right lateral and medial lower leg, and right lateral and medial foot. Strength was 5/5 in the distribution of L3-S1. DTR's were +1 at patella and achilles bilaterally. Figure 4 stretch caused pain in right buttock. Slumped straight leg raise on right caused right buttock pain. DIFFERENTIAL DIAGNOSIS: 1. Recurrence or incomplete healing of bilateral spondylolysis 2. Disk bulge or neural foramina narrowing 3. Piriformis inflammation with sciatic nerve entrapment TEST AND RESULTS: MRI lumbar spine 8/08 possible left-sided spondylolysis with suggested cortical thickening of right pedicle disc height and neural formanina L1-S1 normal Bone scan 10/08 ongoing changes consistent with healing in the left pars of L4 and bilateral pars of L5 CT scan lumbar spine normal pars without evidence of fracture finding suggestive of disc bulge at L2-L3 and L3-L4 WORKING DIAGNOSIS: Bilateral pars fractures with resolution and then disc herniation. TREATMENT AND OUTCOMES 1. Medrol dose pack 2. Repeat MRI 3. With-holding from volleyball at this time

The effect of spinous process resection on facet loading; comparison of conventional approach and muscle-preserving interlaminar decompression

The effect of spinous process resection on facet loading; comparison of conventional approach and muscle-preserving interlaminar decompression

Total Disc Replacement Positioning Affects Facet Contact Forces and Vertebral Body Strains

A validated nonlinear three-dimensional finite element (FE) model of a single lumbar motion segment (L3-L4) was used to evaluate the effects of total disc replacement (TDR). The model was implanted with a fixed-bearing TDR (ProDisc-L) at 2 surgically relevant positions and exercised about the 3 anatomic axes. Facet forces, range of motion (RoM), and vertebral body strains were evaluated. The objective of the current study was to evaluate how TDR implantation and positioning affects facet joint forces and vertebral body strains. We hypothesized that facet contact forces (FCFs) would increase with TDR to compensate for the loss of periprosthetic load-bearing structures, and that vertebral body strains would increase in the region around the metallic footplates. TDR has the potential to replace fusion as the gold standard for the treatment of painful degenerative disc disease. However, complications after TDR include index level facet arthrosis and implant subsidence. Alterations in facet and vertebral body loading after TDR and their dependence on implant positioning are not fully understood. An FEM of L3-L4 was created and validated using RoM, disc pressure, and bony strains from previously published data. A TDR was incorporated into the L3-L4 spine model. All models were subjected to a compressive follower load of 500 N and moments of 7.5 Nm about the 3 anatomic axes. Overall RoM and FCFs tended to increase with TDR. FCFs increased by an order of magnitude during flexion. Posterior placement of the device resulted in an unloading of the facets during extension. Areas of strain maxima were observed in the anterior portion of the vertebral body during flexion after TDR. The area of initial bone resorption signal under the metal footplate was greater when the device was anteriorly placed. The current study predicted a decrease in segmental rotational stiffness resulting from TDR. This resulted from the removal of load bearing soft tissue structures, and caused increased loading in the facets. Additionally, vertebral body strains were generally higher after TDR, and tended to increase with decreased rotational stiffness. Posterior placement of the device provided a more physiologic load transfer to the vertebral body.

The use of surface strain data and a neural networks solution method to determine lumbar facet joint loads during in vitro spine testing

A new method for determining facet loads during in vitro spine loading using strain gauges and a neural networks solution method was investigated. A test showed that the new solution method was more robust than and as accurate as a previously presented graphical solution method for computing facet loads using surface strain. The technique was subsequently utilized to assess facet loads at L1–L2 during flexibility testing [7.5 N m pure moments in flexion (FL), extension (EX), right and left axial rotation (AR), and right and left lateral bending (LB)], and stiffness testing (FL-EX with 400 N compressive follower load) of six human lumbar spine segments (T12–L2). In contrast to other techniques, such as thin film sensors or pressure-sensitive film, the strain-gauge method leaves the facet joint capsule intact during data collection, presumably allowing more natural load transmission. During flexibility tests, the mean (±standard deviation) calculated facet loads (in N) were 46.1±41.3 (FL), 51.5±39.0 (EX), 70.3±43.2 (AR-contralateral side), 31.3±33.4 (AR-ipsilateral side), 30.6±29.1 (LB-contralateral side), and 32.0±44.4 (LB-ipsilateral side). During stiffness tests, the calculated facet loads were 45.5±40.4 (upright), 46.6±41.9 (full FL), and 75.4±39.0 (full EX), corresponding to an equivalent of 11.4%, 11.6%, and 18.8% of the compressive follower load (upright, full FL and EX, respectively). The error associated with this technique, which was below 11 N for loads up to 125 N, is comparable to that reported with other techniques. The new method shows promise for assessing facet load during in vitro spine testing, an important parameter when evaluating new implant systems and surgical techniques.

A Biomechanical Study of Artificial Cervical Discs Using Computer Simulation

A virtual simulation model of the subaxial cervical spine was used to study the biomechanical effects of various disc prosthesis designs. To study the biomechanics of different design features of cervical disc arthroplasty devices. Disc arthroplasty is an alternative approach to cervical fusion surgery for restoring and maintaining motion at a diseased spinal segment. Different types of cervical disc arthroplasty devices exist and vary based on their placement and degrees of motion offered. A virtual dynamic model of the subaxial cervical spine was used to study 3 different prosthetic disc designs (PDD): (1) PDD-I: The center of rotation of a spherical joint located at the mid C5-C6 disc, (2) PDD-II: The center of rotation of a spherical joint located 6.5 mm below the mid C5-C6 disc, and (3) PDD-III: The center of rotation of a spherical joint in a plane located at the C5-C6 disc level. A constrained spherical joint placed at the disc level (PDD-I) significantly increased facet loads during extension. Lowering the rotational axis of the spherical joint towards the subjacent body (PDD-II) caused a marginal increase in facet loading during flexion, extension, and lateral bending. Lastly, unconstraining the spherical joint to move freely in a plane (PDD-III) minimized facet load build up during all loading modes. The simulation model showed the impact simple design changes may have on cervical disc dynamics. The predicted facet loads calculated from computer model have to be validated in the experimental study.

Can extra-articular strains be used to measure facet contact forces in the lumbar spine? An in-vitro biomechanical study

Experimental measurement of the load-bearing patterns of the facet joints in the lumbar spine remains a challenge, thereby limiting the assessment of facet joint function under various surgical conditions and the validation of computational models. The extra-articular strain (EAS) technique, a non-invasive measurement of the contact load, has been used for unilateral facet joints but does not incorporate strain coupling, i.e. ipsilateral EASs due to forces on the contralateral facet joint. The objectives of the present study were to establish a bilateral model for facet contact force measurement using the EAS technique and to determine its effectiveness in measuring these facet joint contact forces during three-dimensional flexibility tests in the lumbar spine. Specific goals were to assess the accuracy and repeatability of the technique and to assess the effect of soft-tissue artefacts. In the accuracy and repeatability tests, ten uniaxial strain gauges were bonded to the external surface of the inferior facets of L3 of ten fresh lumbar spine specimens. Two pressure-sensitive sensors (Tekscan) were inserted into the joints after the capsules were cut. Facet contact forces were measured with the EAS and Tekscan techniques for each specimen in flexion, extension, axial rotation, and lateral bending under a +/- 7.5 N m pure moment. Four of the ten specimens were tested five times in axial rotation and extension for repeatability. These same specimens were disarticulated and known forces were applied across the facet joint using a manual probe (direct accuracy) and a materials-testing system (disarticulated accuracy). In soft-tissue artefact tests, a separate set of six lumbar spine specimens was used to document the virtual facet joint contact forces during a flexibility test following removal of the superior facet processes. Linear strain coupling was observed in all specimens. The average peak facet joint contact forces during flexibility testing was greatest in axial rotation (71 +/- 25 N), followed by extension (27 +/- 35 N) and lateral bending (25 +/- 28 N), and they were most repeatable in axial rotation (coefficient of variation, 5 per cent). The EAS accuracy was about 20 per cent in the direct accuracy assessment and about 30 per cent in the disarticulated accuracy test. The latter was very similar to the Tekscan accuracy in the same test. Virtual facet loads (r.m.s.) were small in axial rotation (12 N) and lateral bending (20 N), but relatively large in flexion (34 N) and extension (35 N). The results suggested that the bilateral EAS model could be used to determine the facet joint contact forces in axial rotation but may result in considerable error in flexion, extension, and lateral bending.

The Effect of Dynamic Posterior Stabilization on Facet Joint Contact Forces

Facet contact forces in the lumbar spine were measured during flexibility tests using thin film electroresistive sensors in intact cadaveric spine specimens and in injured specimens stabilized with a dynamic posterior system. The purpose of this study was to investigate the effect of the Dynesys system on the loading in the facet joints. The Dynesys, a posterior nonfusion device, aims to preserve intersegmental kinematics and reduce facet loads. Recent biomechanical evidence showed that overall motion is less with the Dynesys than in the intact spine, but no studies have shown its effect on facet loads. Ten human cadaveric lumbar spine specimens (L2-L5) were tested by applying a pure moment of +/-7.5 N m in 3 directions of loading with and without a follower preload of 600 N. Test conditions included an intact specimen and an injured specimen stabilized with 3 Dynesys spacer lengths. Bilateral facet contact forces were measured during flexibility tests using thin film electroresistive sensors (Tekscan 6900). Implanting the Dynesys significantly increased peak facet contact forces in flexion (from 3 N to 22 N per side) and lateral bending (from 14 N to 24 N per side), but had no significant effect on the magnitude of the peak forces in extension and axial rotation. Peak facet loads were significantly lower with the long spacer compared with the short spacer in flexion and lateral bending. Implantation of the Dynesys did not affect peak facet contact forces in extension or axial rotation compared with an intact specimen, but did alter these loads in flexion and lateral bending. The spacer length affected the compression of the posterior elements, with a shorter spacer typically producing greater facets loads than a longer one.

Comparison of Biomechanical Function at Ideal and Varied Surgical Placement for Two Lumbar Artificial Disc Implant Designs

Finite element model. To estimate the effect of lumbar mobile-core and fixed-core artificial disc design and placement on the loading of the facet joints, and stresses on the polyethylene core. Although both mobile-core and fixed-core lumbar artificial disc designs have been used clinically, the effect of their design and the effect of placement within the disc space on the structural element loading, and in particular the facets and the implant itself, have not been investigated. A 3D nonlinear finite element model of an intact ligamentous L4-L5 motion segment was developed and validated in all 6 df based on previous experiments conducted on human cadavers. Facet loading of a mobile-core TDR and a fixed-core TDR were estimated with 4 different prosthesis placements for 3 different ranges of motion. Placing the mobile-core TDR anywhere within the disc space reduced facet loading by more than 50%, while the fixed-core TDR increased facet loading by more than 10% when compared with the intact disc in axial rotation. For central (ideal) placement, the mobile- and fixed-core implants were subjected to compressive stresses on the order of 3 MPa and 24 MPa, respectively. The mobile-core stresses were not affected by implant placement, while the fixed-core stresses increased by up to 40%. A mobile-core artificial disc design is less sensitive to placement, and unloads the facet joints, compared with a fixed-core design. The decreased core stress may result in a reduced potential for wear in a mobile-core prosthesis compared with a fixed-core prosthesis, which may increase the functional longevity of the device.

Gradual degradation of red-emitting high-power diode laser bars

The authors analyze early stages of gradual degradation in highly reliable 650nm emitting high-power diode laser arrays with continuous wave emission powers of 2.5W (facet load of 4mW∕μm). In all cases the edges of the metallized emitter stripes are identified as the starting points of gradual degradation. The magnitude of the observed degradation signatures, however, is highly correlated with the bar-specific packaging-induced strain at each emitter. We find a bar-specific effect, namely, the presence of packaging-induced strain, to be the driving force of gradual degradation. Our findings point to the significance of proper strain management in advanced device structures.

Anatomic Facet Replacement System (AFRS) Restoration of Lumbar Segment Mechanics to Intact: A Finite Element Study and In Vitro Cadaver Investigation

Background Many decompression procedures involve complete or partial facetectomy. Spinal fusion usually stabilizes the motion segment after complete facetectomies. However, problems with fusion, such as adjacent-level degeneration, have increased interest in motionpreservation technologies. Facet arthroplasty may become an important posterior motion-preservation device, but its biomechanical literature is sparse. Methods We conducted an in vitro investigation and finite element study to compare the biomechanical effects of an artificial facet system to the intact spine. In the in vitro study, we tested human osteo-ligamentous segments (L3-S1) in intact, injured, and artificial facet–repaired conditions. For the finite element study, we used a 3-dimensional ligamentous L3-S1 segment model. We simulated destabilization in the intact model by removing the facets across the L4-L5 functional unit, then repaired it with appropriately sized facet implants and compared the ranges of motion, facet loads, disc pressures, and device loads. We also analyzed a finite element model with a rigid posterior pedicle-rod fixation system. We subjected the cadaveric specimens and the models to 400 N of follower load plus a 10 Nm moment in extension, flexion, bending, and rotation. We used a novel technique to apply the follower load in the finite element models such that preload induced minimal vertebral rotation during the range of motion. Results The predicted ranges of motion for the intact and implanted models were consistent with cadaver data. After destabilization and facet replacement, the artificial facet system restored motion in all loading modes to intact values. The implant facet loads were similar to intact facet loads in extension and axial rotation, but less in lateral bending. The intradiscal pressure at the implanted level for the facet replacement device was similar to the intact pressure, whereas with the rigid system the intradiscal pressure was up to 70% less than the intact pressure. The maximum von-Mises stress predicted in the facet replacement construct was 85 MPa in extension at the bone–pedicle screw interface, compared with 174 MPa in the rigid system. Contact stresses at implant mating surfaces were minimal. Conclusions The artificial facet system replicated natural facet kinematics. The cadaveric ranges of motion and the predicted finite element–based data indicated that the implant can “restore” the normal function of the segment after artificial facet replacement. Clinical Relevance Compared to rigid posterior pedicle-rod fixation, the artificial facet system restored the intact mechanics at the implanted level and may prevent adjacent-level degeneration.

Clinical Predictors of Success and Failure for Lumbar Facet Radiofrequency Denervation

To determine the clinical factors associated with the success and failure of radiofrequency denervation of the lumbar facet joints. Clinical data were garnered from 3 academic medical centers on 192 patients with low back pain who underwent radiofrequency denervation after a positive response to diagnostic blocks. Success was defined as >/=50% pain relief lasting at least 6 months. Factors evaluated for their association with outcome included duration of pain, opioid use, symptom location, paraspinal tenderness, pain exacerbated by extension/rotation (ie, facet loading), MRI abnormalities, diabetes, smoking, scoliosis, obesity, prior surgery and levels treated. The only factor associated with a successful outcome was paraspinal tenderness. Variables that correlated with treatment failure were 'facet loading,' long duration of pain, and previous back surgery. It is counterproductive to use 'facet loading' as the sole basis for choosing patients for facet interventions. In patients at high risk for treatment failure, taking additional steps to reduce the rate of false-positive screening blocks may improve outcomes.

Anatomic Facet Replacement System (AFRS) Restoration of Lumbar Segment Mechanics to Intact: A Finite Element Study and In Vitro Cadaver Investigation.

BackgroundMany decompression procedures involve complete or partial facetectomy. Spinal fusion usually stabilizes the motion segment after complete facetectomies. However, problems with fusion, such as adjacent-level degeneration, have increased interest in motion- preservation technologies. Facet arthroplasty may become an important posterior motion-preservation device, but its biomechanical literature is sparse.MethodsWe conducted an in vitro investigation and finite element study to compare the biomechanical effects of an artificial facet system to the intact spine. In the in vitro study, we tested human osteo-ligamentous segments (L3-S1) in intact, injured, and artificial facet–repaired conditions. For the finite element study, we used a 3-dimensional ligamentous L3-S1 segment model. We simulated destabilization in the intact model by removing the facets across the L4-L5 functional unit, then repaired it with appropriately sized facet implants and compared the ranges of motion, facet loads, disc pressures, and device loads. We also analyzed a finite element model with a rigid posterior pedicle-rod fixation system. We subjected the cadaveric specimens and the models to 400 N of follower load plus a 10 Nm moment in extension, flexion, bending, and rotation. We used a novel technique to apply the follower load in the finite element models such that preload induced minimal vertebral rotation during the range of motion.ResultsThe predicted ranges of motion for the intact and implanted models were consistent with cadaver data. After destabilization and facet replacement, the artificial facet system restored motion in all loading modes to intact values. The implant facet loads were similar to intact facet loads in extension and axial rotation, but less in lateral bending. The intradiscal pressure at the implanted level for the facet replacement device was similar to the intact pressure, whereas with the rigid system the intradiscal pressure was up to 70% less than the intact pressure. The maximum von-Mises stress predicted in the facet replacement construct was 85 MPa in extension at the bone–pedicle screw interface, compared with 174 MPa in the rigid system. Contact stresses at implant mating surfaces were minimal.ConclusionsThe artificial facet system replicated natural facet kinematics. The cadaveric ranges of motion and the predicted finite element–based data indicated that the implant can “restore” the normal function of the segment after artificial facet replacement.Clinical RelevanceCompared to rigid posterior pedicle-rod fixation, the artificial facet system restored the intact mechanics at the implanted level and may prevent adjacent-level degeneration.

Hybrid multidirectional test method to evaluate spinal adjacent-level effects

Several clinical studies have documented long-term adjacent-level effects of spinal fusion, due to stress concentration and motion loss at the fused segment. Non-fusion motion preservation devices are designed to eliminate or slow down such adverse effects. Therefore, appropriate biomechanical evaluation of the adjacent-level effects in spine is important and timely. Although many biomechanical studies are available and have provided some understanding of the adjacent-level effects, results have large variation and are conflicting, mostly due to the use of inappropriate and ill-defined methods. A new test method especially designed to study spinal adjacent-level effects is needed. The proposed Hybrid method uses unconstrained pure moment to provide rotation-input for multi-directional testing. The new method has four steps: (1) Intact spine specimen with entire mobile region is used. The specimen is prepared to measure various biomechanical parameters, e.g., disc pressures, ligament strains, and facet loads. (2) Appropriate unconstrained pure moment is applied to the intact specimen and total range of motion is determined. (3) Unconstrained pure moment is applied to the spinal construct (specimen with an implant) until the total range of motion of the construct equals that of the intact. (4) Statistical comparison of the biomechanical parameters between the construct and intact quantifies the adjacent-level effects. The uniqueness of the proposed method, to study the adjacent level effects due to fusion and non-fusion devices, is that it applies the needed rotation-input to the spine specimen, using available methodology with minimal modification. Previous studies have lacked appropriate and well-defined methodologies to evaluate spinal adjacent-level effects. The proposed method uses well-known methodology and yields high quality, and laboratory-independent results for the fusion and non-fusion devices.