Nucleus Replacement Device Research Articles

Quantifying internal intervertebral disc strains to assess nucleus replacement device designs: a digital volume correlation and ultra-high-resolution MRI study.

Introduction: Nucleus replacement has been proposed as a treatment to restore biomechanics and relieve pain in degenerate intervertebral discs (IVDs). Multiple nucleus replacement devices (NRDs) have been developed, however, none are currently used routinely in clinic. A better understanding of the interactions between NRDs and surrounding tissues may provide insight into the causes of implant failure and provide target properties for future NRD designs. The aim of this study was to non-invasively quantify 3D strains within the IVD through three stages of nucleus replacement surgery: intact, post-nuclectomy, and post-treatment. Methods: Digital volume correlation (DVC) combined with 9.4T MRI was used to measure strains in seven human cadaveric specimens (42 ± 18years) when axially compressed to 1kN. Nucleus material was removed from each specimen creating a cavity that was filled with a hydrogel-based NRD. Results: Nucleus removal led to loss of disc height (12.6 ± 4.4%, p = 0.004) which was restored post-treatment (within 5.3 ± 3.1% of the intact state, p > 0.05). Nuclectomy led to increased circumferential strains in the lateral annulus region compared to the intact state (-4.0 ± 3.4% vs. 1.7 ± 6.0%, p = 0.013), and increased maximum shear strains in the posterior annulus region (14.6 ± 1.7% vs. 19.4 ± 2.6%, p = 0.021). In both cases, the NRD was able to restore these strain values to their intact levels (p ≥ 0.192). Discussion: The ability of the NRD to restore IVD biomechanics and some strain types to intact state levels supports nucleus replacement surgery as a viable treatment option. The DVC-MRI method used in the present study could serve as a useful tool to assess future NRD designs to help improve performance in future clinical trials.

Finite Element Study of a Lumbar Intervertebral Disc Nucleus Replacement Device.

Nucleus replacement technologies are a minimally invasive alternative to spinal fusion and total disc replacement that have the potential to reduce pain and restore motion for patients with degenerative disc disease. Finite element modeling can be used to determine the biomechanics associated with nucleus replacement technologies. The current study focuses on a new nucleus replacement device designed as a conforming silicone implant with an internal void. A validated finite element model of the human lumbar L3–L4 motion segment was developed and used to investigate the influence of the nucleus replacement device on spine biomechanics. In addition, the effect of device design changes on biomechanics was determined. A 3D, L3–L4 finite element model was constructed from medical imaging data. Models were created with the normal intact nucleus, the nucleus replacement device, and a solid silicone implant. Probabilistic analysis was performed on the normal model to provide quantitative validation metrics. Sensitivity analysis was performed on the silicone Shore A durometer of the device. Models were loaded under axial compression followed by flexion/extension, lateral bending, or axial rotation. Compressive displacement, endplate stresses, reaction moment, and annulus stresses were determined and compared between the different models. The novel nucleus replacement device resulted in similar compressive displacement, endplate stress, and annulus stress and slightly higher reaction moment compared with the normal nucleus. The solid implant resulted in decreased displacement, increased endplate stress, decreased annulus stress, and decreased reaction moment compared with the novel device. With increasing silicone durometer, compressive displacement decreased, endplate stress increased, reaction moment increased, and annulus stress decreased. Finite element analysis was used to show that the novel nucleus replacement device results in similar biomechanics compared with the normal intact nucleus.

5:27 PM, Abstract No. 80 - Finite element modeling of a disc nucleus replacement device

A new intervertebral disc (IVD) nucleus replacement device is being developed, consisting of an inner and outer chamber. The outer chamber is filled with silicone and reinforced with a textile annular band, and the inner chamber is filled with gas. The purpose of this study was to utilize finite element modeling (FEM) to guide device design through characterizing spine biomechanics.

An In Vitro Assessment of Wear Particulate Generated From NUBAC

In vitro biotribological wear particulate investigation. To characterize poly-ether-ether-ketone (PEEK)-OPTIMA wear particulate generated from a series of wear tests used to evaluate the wear resistance and long-term biodurability of NUBAC, a PEEK-on-PEEK articulating nucleus replacement device, and compare with wear particulate associated with hip and knee total joint arthroplasties. The use of PEEK in spinal arthroplasty represents a unique application of this material. Clinically, osteolysis, osteolytic changes, and adverse reactions to metal ions have been documented in spinal arthroplasty. Therefore, it is critically important to analyze the PEEK wear particulate to evaluate its resultant biologic reactivity. Historically, scanning electron microscopy (SEM) has been used for wear debris characterization. Light scattering, specifically laser diffraction, has also been successfully used. The combined use of both techniques may provide a more comprehensive analysis than either method alone. Proteinacious serum containing PEEK wear debris generated from four groups of devices from separate wear tests underwent enzymatic and acid digestion. The particulate was analyzed using laser diffraction in duplicate, followed by SEM analysis. Laser diffraction analysis demonstrated a relatively large mean particle diameter on the basis of particle volume (16.5-40.0 µm) as compared with particle number (0.9-2.2 µm). For all groups, more than 50% of debris was larger than 5.0 µm. SEM analysis revealed a mean particle size consistent with the number-based laser diffraction results. The morphology of the wear particulate appeared to be similar for all the groups analyzed. The analysis of the particles generated from an articulating PEEK-on-PEEK nucleus replacement device shows debris within size ranges typical of other total joint arthroplasty implants, with relatively round morphology, along with the results suggesting a reduced particle load. These attributes tend to diminish the potential of these PEEK particles to elicit an inflammatory response.

A Kangaroo Spine Lumbar Motion Segment Model: Biomechanical Analysis of a Novel <i>In Situ</i> Curing Nucleus Replacement Device

This in vitro study compared the effects of nucleotomy alone, with nucleotomy then implantation with a novel nucleus replacement device (D3 device) in a single segment kangaroo spine model. This study utilised dynamic biaxial biomechanical testing of intact, nucleotomy and nucleus replacement implant conditions to evaluate the kinematic behaviour of the single segment kangaroo lumbar spine. Studies have examined the biomechanical efficacy of invasive treatments such as Total Disc Replacement and Intervertebral Fusion for the treatment of chronic low back pain, however no studies to date have investigated the biomechanical effects of a novel elastomeric compressive load sharing nucleus replacement device. Kangaroo lumbar spine motion segments with all musculature, ligamentous tissue and posterior elements removed, were tested in intact state prior to undergoing nucleotomy or nucleotomy then nucleus implantation using the D3 device. All specimens were tested in flexion-extension and lateral-bending; Range of motion (ROM), Neutral Zone (NZ), Hysteresis (H), and Elastic Stiffness (ES) were evaluated. Nucleotomised motion segments demonstrated a 30% to 90% increase in ROM, NZ, H, but not ES for all Flexion-Extension testing conditions and in Lateral Bending test conditions when compared to intact state. Implantation of the nucleus replacement device demonstrated no significant difference when compared to intact state except for H during Lateral Bending testing conditions when compared to the intact state. Therefore, there was a significant increase in ROM, NZ, and H after Nucleotomy during Flexion-Extension motions and an increase in ROM alone during lateral bending motions in the single segment kangaroo spine model. These changes return to that of the intact state with the placement of a novel nucleus replacement device. Our data suggest that the D3 device tested can restore the kinematic changes of a degenerated disc represented by the nucleotomised single motion segment.

Nucleus Replacement Device Failure

Case report and biomechanical study. The objectives of this study were to report on a single case of a failed nucleus replacement device and to test the biomechanical properties of the failed device. The use of spine arthroplasty techniques in the treatment of degenerative disc disease is becoming a popular alternative to spinal fusion and discectomy. Nucleus replacement is an emerging surgical treatment that is in the early stages of development. A 36-year-old woman presented to our institution with excruciating low back pain 15 months after receiving a prosthetic disc nucleus (PDN; Raymedica, Inc.) at L5-S1 as part of an IDE clinical trial. A computed tomography scan showed subsidence of the PDN into the endplates and asymmetric collapse of the L5-S1 disc space. The patient underwent surgery for removal of the device and fusion of L5-S1. After removal, the nucleus replacement device underwent micro-computed tomography imaging and was tested in unconfined and confined compression. The density of the inner core of the PDN was estimated to be 105 g/cm. Compression testing revealed that the stiffness of the PDN was grossly elevated in comparison to previously published values for human lumbar nuclei and other candidate nucleus replacement hydrogels. The linear-region modulus values were 0.94 MPa for unconfined compression and 32.4 MPa for confined compression. The PDN device excised from this patient failed to reproduce the function of a healthy nucleus. Because preoperative mechanical values were not available for this device, it is difficult to know if the PDN was abnormally stiff at implantation or if it became increasingly stiff after implantation. Whether this was a result of manufacturing, the patient's biologic response to the PDN, or some yet unknown contraindication to PDN placement in this specific patient is unclear.

An In Vitro Biotribological Assessment of NUBAC, a Polyetheretherketone-on-Polyetheretherketone Articulating Nucleus Replacement Device

In vitro biotribological investigation. To evaluate the wear resistance and long-term biodurability of NUBAC, a PEEK-on-PEEK articulating nucleus replacement device, using a series of wear tests with different motion profiles, test frequencies, and environmental conditions. Wear resistance is critical for any disc arthroplasty device, and osteolysis remains a clinical concern. The use of PEEK for an articulating load bearing nucleus replacement device represents a unique application of this material. NUBAC has an inner ball/socket articulation for motion, similar to total disc replacements. American Society for Testing and Materials and International Organization for Standardization have recommended wear testing methodologies for total disc replacements, however, they have not been clinically validated. Therefore, a series of wear tests were performed to characterize the wear properties of the device. Four groups of devices were evaluated. Group 1 consisted of +/-7.5 degrees flexion/extension to 10 million cycles (Mc) followed by +/-7.5 degrees lateral bending to 10 Mc, alternated to 40 Mc. Groups 2 to 4 consisted of International Organization for Standardization motion and load profiles to 10 Mc, except Group 3 incorporated frequency shifting to ensure a nonrepetitive load and motion profile. Group 4 underwent simulated aging. All studies incorporated a load magnitude of 225 to 1024 N. The average wear rates were determined using linear regression analysis with significant differences between groups determined (analysis of variance). A wear-in period was observed from 0 to 1 Mc. Wear rates were therefore calculated from 1 Mc. The wear rate for Group 1 was significantly less than Groups 2 to 4 through 10 Mc. From 1 to 5 Mc, the wear rate for Group 1 was significantly less than all groups, with Groups 2 to 4 not significantly different from each other. The wear rates for Groups 2 to 4 were seen to decrease after 5 Mc with only Group 3 significantly different than Group 1. The Group 1 wear rate was consistent throughout the test duration of 40 Mc. The experimental wear rates compare well with the reported wear rates of other material combinations used in nucleus replacement and total disc arthroplasty. Overall, wear rates were relatively low and consistent, suggesting long-term durability, a critical requirement of disc arthroplasty devices.

Fill of the Nucleus Cavity Affects Mechanical Stability in Compression, Bending, and Torsion of a Spine Segment, Which Has Undergone Nucleus Replacement

Axial loading, rotation, and bending were applied to human cadaveric lumbar segments to investigate the changes in disc mechanics with denucleation and incremental delivery of a novel hydrogel nucleus replacement. The purpose of this study was to investigate the effect of nucleus implant injection pressure/volume relationships on the quasi-static mechanical behavior of the human cadaveric lumbar intervertebral disc to determine if intact biomechanics could be reproduced with nucleus-implanted discs. Previous studies have shown that volumetric filling of the nucleus cavity with a compliant nucleus replacement device will affect compressive stiffness of the implanted intervertebral disc, but data regarding restoration of mechanics through cavity pressurization are lacking. A total of 12 intact lumbar anterior column units were loaded in series in axial loading, axial rotation, lateral bending, and flexion/extension (FE). Each segment was fully denucleated and implanted with a hydrogel nucleus replacement using pressurization between 12 psi and 40 psi. Range of motion (ROM), neutral zone (NZ), energy dissipation (HYS), disc height (DH), and stiffness were compared among the intact, denucleated, and implanted conditions. Denucleation significantly destabilized the segments compared to intact controls as shown by increased ROM, NZ, and HYS, and decreased DH and stiffness through the NZ. As the nucleus cavity was repressurized with increasing volumes of hydrogel implant, the segments were stabilized and DH was restored to the intact level. No significant differences from intact were observed in any loading direction for ROM, NZ, or DH after the segments were implanted with the nucleus replacement device using inflation pressures between 20 psi and 40 psi. Compliant nucleus replacement using inflation pressures of 20 to 40 psi resulted in restoration of intact mechanics. Mechanical function was dependent on the volume of implant injected into the nucleus cavity.

Effect of Nucleus Replacement Device Properties on Lumbar Spine Mechanics

A validated nonlinear three-dimensional finite element model of a single lumbar motion segment (L3-L4) was used to evaluate a range of moduli for ideally conforming nucleus replacement devices. The objective of the current study was to determine the biomechanical effects of nucleus replacement technology for a variety of implant moduli. We hypothesized that there would be an optimal modulus for a nucleus replacement that would provide loading in the surrounding bone and anulus similar to the intact state. Nucleus pulposus replacements are interventional therapies that restore stiffness and height to mildly degenerated intervertebral discs. Currently a wide variety of nucleus replacement technologies with a large range of mechanical properties are undergoing preclinical testing. A finite element model of L3-L4 was created and validated using range of motion, disc pressure, and bony strains from previously published data. The intact model was altered by changing the mechanical properties of the nucleus pulposus to represent a wide range of nucleus replacement technologies (E = 0.1, 1, 4, and 100 MPa). All of the models were exercised in compression, flexion, extension, lateral bending, and axial rotation. Vertebral body strain, peak anulus fibrosus shear strain, initial bone remodeling stimulus, range of motion, and center of rotation were analyzed. A nucleus replacement modulus of 1 and 4 MPa resulted in vertebral body strains similar to the intact model. The softest device indicated increased loading in the AF and bone resorption adjacent to the implant. Areas of strain maxima and bone formation were observed adjacent to the implant for the stiffest device. The current study predicted an optimal nucleus replacement of 1 to 4 MPa. An overly stiff implant could result in subsidence, which would preclude the benefit of disc height increase or restoration. Conversely, an overly soft implant could accelerate a degenerative cascade in the anulus.

Mechanical and Biomechanical Characterization of a Polyurethane Nucleus Replacement Device Injected and Cured In Situ Within a Balloon

Background The DASCOR device has recently been introduced as an innovative nucleus replacement alternative for the treatment of low-back pain caused by degenerative intervertebral disc disease. The purpose of this study was to characterize, through a series of preclinical mechanical bench and biomechanical tests, the effectiveness of this device. Methods A number of samples were created using similar preparation methods in order to characterize the nucleus replacement device in multiple mechanical bench tests, using ASTM-guided protocols, where appropriate. Mechanical bench testing included static testing to characterize the device's compressive, shear properties, and fatigue testing to determine the device's compressive fatigue strength, wear, and durability. Biomechanical testing, using human cadaveric lumbar spines, was also conducted to determine the ability of the device to restore multidirectional segmental flexibility and to determine its resulting endplate contact stress. Results The static compressive and shear moduli of the nucleus replacement device were determined to be between 4.2–5.6 MPa and 1.4–1.9 MPa, respectively. Similarly, the ultimate compressive and shear strength were 12,400 N and 6,993 N, respectively. The maximum axial compressive fatigue strength of the tested device that was able to withstand a runout without failure was determined to be approximately 3 MPa. The wear assessment determined that the device is durable and yielded minimal wear rates of 0.29mg/Mc. Finally, the biomechanical testing demonstrated that the device can restore the multidirectional segmental flexibility to a level seen in the intact condition while concurrently producing a uniform endplate contact stress. Conclusions The results of the present study provided a mechanical justification supporting the clinical use of the nucleus replacement device and also help explain and support the positive clinical results obtained from two European studies and one US pilot study. Clinical Relevance Nucleus replacement devices are rapidly emerging to address specific conditions of degenerative disc disease. Preclinical testing of such devices is paramount in order to potentially ensure successful clinical outcomes post implantation

Novel Synthetic Total Disk Model for Mechanical Testing of Nucleus Replacement Devices

We are presenting a novel, non-biologic model of the healthy human annulus. This lumbar Total Disc Model (TDM) ideally would be both biofidelic and sufficiently robust to withstand long-term fatigue testing without breaking down or tearing apart. Mechanical validation testing was performed to confirm that the compressive and torsional properties were similar to literature values of denucleated human lumbar discs in two-body constructs. Long-term fatigue tests were performed to establish the durability of the model. We have reported data for both our empty TDM and the TDM filled with a representative nucleus replacement device (NRD). The silicone model is geometrically equivalent to the healthy human lumbar disc, including the discoid cavity present following a total nuclectomy, the annulus fibrosis with micro-annulotomy, and the cartilaginous endplates. The pressure transmitted through the center of the disc went from negligible when the TDM was empty to over 40% when the TDM was filled. The compression stiffness was 992±15 N/mm for the empty TDM and 1583±136 N/mm for the filled TDM. The torsional stiffness was 0.505±0.024 Nm/° for the empty and 0.550±0.056 Nm/° for the filled TDM. Lastly, the only mechanical damage suffered by either empty or filled TDMs during dynamic testing came from debonding from the endplates at higher torque levels. No damage was seen during dynamic compression testing. After determining the appropriate geometry for the TDM, validation testing was performed to ensure that the load sharing, compressive, and torsional properties were similar to the native human disc. The silicone model was durable enough to avoid tearing or mechanical failure at physiologic loads. This study demonstrated that a silicone total disc model was developed with appropriate properties necessary for determining the mechanical degradation properties of a NRD and will not mechanical fail at physiologic loads.

Mechanical and biomechanical characterization of a polyurethane nucleus replacement device injected and cured in situ within a balloon.

BackgroundThe DASCOR device has recently been introduced as an innovative nucleus replacement alternative for the treatment of low-back pain caused by degenerative intervertebral disc disease. The purpose of this study was to characterize, through a series of preclinical mechanical bench and biomechanical tests, the effectiveness of this device.MethodsA number of samples were created using similar preparation methods in order to characterize the nucleus replacement device in multiple mechanical bench tests, using ASTM-guided protocols, where appropriate. Mechanical bench testing included static testing to characterize the device's compressive, shear properties, and fatigue testing to determine the device's compressive fatigue strength, wear, and durability. Biomechanical testing, using human cadaveric lumbar spines, was also conducted to determine the ability of the device to restore multidirectional segmental flexibility and to determine its resulting endplate contact stress.ResultsThe static compressive and shear moduli of the nucleus replacement device were determined to be between 4.2–5.6 MPa and 1.4–1.9 MPa, respectively. Similarly, the ultimate compressive and shear strength were 12,400 N and 6,993 N, respectively. The maximum axial compressive fatigue strength of the tested device that was able to withstand a runout without failure was determined to be approximately 3 MPa. The wear assessment determined that the device is durable and yielded minimal wear rates of 0.29mg/Mc. Finally, the biomechanical testing demonstrated that the device can restore the multidirectional segmental flexibility to a level seen in the intact condition while concurrently producing a uniform endplate contact stress.ConclusionsThe results of the present study provided a mechanical justification supporting the clinical use of the nucleus replacement device and also help explain and support the positive clinical results obtained from two European studies and one US pilot study.Clinical RelevanceNucleus replacement devices are rapidly emerging to address specific conditions of degenerative disc disease. Preclinical testing of such devices is paramount in order to potentially ensure successful clinical outcomes post implantation

P61. Preliminary Results of Prospective PDN-SOLO Study: A Report of One Center of International Multicenter Study

BACKGROUND CONTEXT: The prosthetic disc nucleus (PDN) device, unlike total disc replacement, is a nucleus replacement device. The PDN-SOLO device, which is an evolved form from paired devices, expands in an anteroposterior dimension as well as height when hydrated.