Nucleus Replacement Devices Research Articles

Comparison of four in vitro test methods to assess nucleus pulposus replacement device expulsion risk.

Nucleus replacement devices (NRDs) are not routinely used in clinic, predominantly due to the risk of device expulsion. Rigorous in vitro testing may enable failure mechanisms to be identified prior to clinical trials; however, current testing standards do not specify a particular expulsion test. Multiple methods have therefore been developed, complicating comparisons between NRD designs. Thus, this study assessed the effectiveness of four previously reported expulsion testing protocols; hula-hoop (Protocol 1), adapted hula-hoop (Protocol 2), eccentric cycling (Protocol 3), and ramp to failure (Protocol 4), applied to two NRDs, one preformed and one in situ curing. Nucleus material was removed from 40 bovine tail intervertebral disks. A NRD was inserted posteriorly into each cavity and the disks were subjected to one of four expulsion protocols. NRD response was dependent on both the NRD design and the loading protocol. Protocol 1 resulted in higher migration and earlier failure rates compared to Protocol 2 in both NRDs. The preformed NRD was more likely to migrate when protocols incorporated rotation. The NRDs had equal migration (60%) and expulsion (60%) rates when using unilateral bending and ramp testing. Combining the results of multiple tests revealed complimentary information regarding the NRD response. Adapted hula-hoop (Protocol 2) and ramp to failure (Protocol 4), combined with fluoroscopic analysis, revealed complimentary insights regarding migration and failure risk. Therefore, when adopting the surgical approach and animal model used in this study, it is recommended that NRD performance be assessed using both a cyclic and ramp loading protocol.

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.

Long-Term Outcomes Following Lumbar Nucleus Replacement.

Nucleus replacement devices are designed to replace the native pain-generating lumbar nucleus while preserving the annulus fibrosus, endplates, and natural motion. The DASCOR Disc Arthroplasty Device seemed to perform well clinically but was discontinued in 2009. While there are no commercially available NRDs today, the potential advantages of using such devices have prompted a renewed interest in further developing the technology and assessing long-term outcomes for the DASCOR device. A retrospective review of hospital records from a single institution was performed to identify all patients that underwent implantation of a DASCOR device between 2006 and 2009. Clinical outcome and imaging data were gathered to assess device performance of the DASCOR device over an extended period of time. Clinical data were assessed using a visual analog scale for back pain (VAS), Oswestry Disability Index (ODI) scores, and analgesic medication use score (ANS). Magnetic resonance images were systematically evaluated at the index and adjacent disc levels: disc height, Modic changes, Pfirrmann grade, and any implant-associated findings. Twenty-six patients received DASCOR implants. Follow-up data were available for 22 patients. The mean age at time of implantation was 34.9 years. Long-term data were available for 14 patients (mean, 11.5 years) with a mean improvement in VAS of 4.0 +/- 2.8 (P < 0.0001), ODI 33.4 +/- 18.4 (P < 0.0001), and ANS 0.5 (P = 0.07) compared to baseline values, while 46% developed radiological adjacent segment degeneration on MRI and 54% did not. In total, 6 patients underwent revision surgery at the index level and 3 underwent surgery at adjacent lumbar levels. While any conclusions should be interpreted with caution, there was a subgroup of patients that had excellent clinical and radiological outcomes. Additional studies on this device or other similar devices would add to our knowledge regarding ideal treatment of discogenic low back pain in younger patients. Treatment of discogenic low back pain. 4.

Biomechanical assessment and fatigue characteristics of an articulating nucleus implant

BackgroundExtrusion is a known complication of lumbar nucleus replacement devices. Despite this fact, this complication has not been well studied in an in vitro cadaveric model under fatigue-loading conditions. MethodsLumbar constructs (with treated and control levels) were tested in intact, postdisectomy, and postnucleus implant conditions under compression, torsion, and bending for initial biomechanical assessment. Constructs were then tested for 100(k) cycles under fatigue loading to assess extrusion risk. Potential adverse effects to vertebral and endplate fractures were assessed using gross dissection and macroscopic and micro-computed tomography evaluation techniques. ResultsBased on the initial biomechanical assessment, implantation of the nucleus device significantly increased disc height compared with the discectomy condition, and there were no significant differences between the intact and implanted conditions for range of motion or stiffness. All constructs completed the 100(k) cycles with no extrusions. There was evidence of implant shift toward the right lateral annulus on postfatigue images. Postfatigue dissection and imaging showed no evidence of macroscopic endplate or trabecular fractures. ConclusionUsing a 2-level lumbar in vitro construct, the biomechanical function of the treated level with an articulating nucleus implant was similar to intact. In vitro fatigue testing showed no implant extrusion and macroscopic changes to the bony structure or cartilaginous endplates when comparing treated and intact levels.

Intervertebral Disc and Nucleus Replacement Devices and Instrumentations

Intervertebral Disc and Nucleus Replacement Devices and Instrumentations

Compression Failure in Lumbar Functional Spinal Units After Nucleus Augmentation with an Injectable In Situ Cured Hydrogel

Nucleus replacement devices are intended to replace or augment the pathologic intervertebral disc, particularly in the early stages of the degenerative cascade. In order to evaluate the clinical viability of these devices, it is necessary to conduct tests simulating potential failure modes in the physiologic setting in addition to bench-top mechanical device testing and traditional in vitro flexibility evaluation.

DAMPING PROPERTIES OF THE NUCLEUS PULPOSUS

Background: The nucleus pulposus is extremely deformable and it is not uncommon to observe strain amplitudes as large as 12.5% in physiological loading conditions. It has been shown that the nucleus pulposus contributes to the damping properties of the intervertebral disc. The quantification of the damping properties of the nucleus pulposus under physiological large deformations is then a key aspect for its mechanical characterization and for the design of nucleus replacement devices. Methods: A specific mechanical device has been developed to encapsulate nucleus pulposus tissues into a deformable and permeable device, while quantifying its water content. The specific damping capacity was defined by dividing the energy loss by the work input. With this device and definition, the specific damping capacity of the bovine coccygeal nucleus pulposus was quantified in large compressive deformations (12.5%) and for frequencies ranging between 10−2 and 101 Hz. Findings: It is found that the specific damping capacity of the nucleus pulposus of the bovine coccygeal ranged between 18 and 36%. The lowest values of specific damping capacity are found for frequencies corresponding to the dynamics of loads in all day activities such as walking (0.1 to 1 Hz). Interpretation: The nucleus pulposus contributes to dissipate energy under physiological large deformations. However, it seems that the nucleus pulposus is designed so that damping is minimal for frequencies corresponding to moderate daily activities.

Damping properties of the nucleus pulposus

BackgroundThe nucleus pulposus is extremely deformable and it is not uncommon to observe strain amplitudes as large as 12.5% in physiological loading conditions. It has been shown that the nucleus pulposus contributes to the damping properties of the intervertebral disc. The quantification of the damping properties of the nucleus pulposus under physiological large deformations is then a key aspect for its mechanical characterization and for the design of nucleus replacement devices. MethodsA specific mechanical device has been developed to encapsulate nucleus pulposus tissues into a deformable and permeable device, while quantifying its water content. The specific damping capacity was defined by dividing the energy loss by the work input. With this device and definition, the specific damping capacity of the bovine coccygeal nucleus pulposus was quantified in large compressive deformations (12.5%) and for frequencies ranging between 10−2 and 101Hz. FindingsIt is found that the specific damping capacity of the nucleus pulposus of the bovine coccygeal ranged between 18 and 36%. The lowest values of specific damping capacity are found for frequencies corresponding to the dynamics of loads in all day activities such as walking (0.1 to 1Hz). InterpretationThe nucleus pulposus contributes to dissipate energy under physiological large deformations. However, it seems that the nucleus pulposus is designed so that damping is minimal for frequencies corresponding to moderate daily activities.

Lessons Learned After 9 Years' Clinical Experience with 3 Different Nucleus Replacement Devices

Nucleus replacement devices have been developed with the goal of treating moderate forms of degenerative disk disease, trying to fill the gap between discectomy and fusion. This is a retrospective analysis of a nonrandomized, single-center clinical series of 125 patients, presenting with moderate forms of degenerative disk disease and treated with nuclear replacement devices. Eighty patients were treated with prosthetic disk nucleus (Ray Medica) disk prosthesis, 26 patients with percutaneous nucleus replacement (TranS1), and 19 patients using the NUBAC (Pioneer) devices. The surgical techniques for each device were performed following the prosthesis indications. Patients were followed up to 9 years postoperatively, and their complications were recorded. After 9 years' follow-up, the overall retrieval incidence was 48.8% (61/125). Of these, 15 (57.7%) had PNR failures, 8 (42.1%) experienced NUBAC retrievals, and 38 (47.5%) had prosthetic disk nucleus flaws. The failures included significant loss of disk height at the operated level, displacement, silicon inside the spinal canal, and migration. All patients underwent fusion as a retrieval surgery. The retrieval rate in our series was very high. It shows that endplate reactions occur in a high percentage of patients over time, resulting in subsidence and mechanical back pain. Device expulsion was another cause of pain requiring revision surgery.

A Long-term Clinical Experience with Three Different Nucleus Replacement Devices - Lessons Learned after 9-Years Follow-up

The nucleus replacement devices have been developed for treating moderate forms of degenerative disc disease, trying to fill the gap between discectomy and fusion. The surgical goals are pain relief, maintenance of the disc height and flexibility at the index and adjacent levels.

Silicone annulus for mechanically testing nucleus replacement implants for the lumbar intervertebral disc

Nucleus replacement implants are designed to replace the nucleus pulposus of the intervertebral disc, while preserving the integrity of the annulus fibrosus. The aim of this study was to design and manufacture a geometrically accurate annulus for use in mechanical testing lumbar nucleus replacement devices. The key dimensions for the annulus were determined from measurements made on magnetic resonance images and from cadaver lumbar discs. Cluster analysis enabled three size ranges to be defined. The smallest of these was chosen for designing the annulus. Solid modelling software was used to design an annulus with the required dimensions and reasonable curvatures. A mould was made using rapid prototyping and used to manufacture a synthetic annulus from silicone rubber. The stiffness of the silicone annulus was found to be in the range 718?907 N/mm and so mimicked the mechanical properties of the natural annulus. The silicone annulus can be used in the pre-clinical testing of nucleus replacement implants.

Minimally invasive spine technology and minimally invasive spine surgery: a historical review

The trend of using smaller operative corridors is seen in various surgical specialties. Neurosurgery has also recently embraced minimal access spine technique, and it has rapidly evolved over the past 2 decades. There has been a progression from needle access, small incisions with adaptation of the microscope, and automated percutaneous procedures to endoscopically and laparoscopically assisted procedures. More recently, new muscle-sparing technology has come into use with tubular access. This has now been adapted to the percutaneous placement of spinal instrumentation, including intervertebral spacers, rods, pedicle screws, facet screws, nucleus replacement devices, and artificial discs. New technologies involving hybrid procedures for the treatment of complex spine trauma are now on the horizon. Surgical corridors have been developed utilizing the interspinous space for X-STOP placement to treat lumbar stenosis in a minimally invasive fashion. The direct lateral retroperitoneal corridor has allowed for minimally invasive access to the anterior spine. In this report the authors present a chronological, historical perspective of minimal access spine technique and minimally invasive technologies in the lumbar, thoracic, and cervical spine from 1967 through 2009. Due to a low rate of complications, minimal soft tissue trauma, and reduced blood loss, more spine procedures are being performed in this manner. Spine surgery now entails shorter hospital stays and often is carried out on an outpatient basis. With education, training, and further research, more of our traditional open surgical management will be augmented or replaced by these technologies and approaches in the future.

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.

Nucleus replacement technologies

Nucleus replacement offers a less invasive alternative to traditional fusion or total disc replacement techniques in the treatment of symptomatic lumbar degenerative disc disease (DDD). The authors discuss the classification of nucleus replacement devices as well as their potential indications. The authors review the history and evolution of nucleus replacement devices emphasizing several that are actively in US Investigational Device Exemption pilot feasibility trials. Nucleus replacement devices can be functionally categorized as elastomeric and mechanical. A classification scheme is discussed. Nucleus replacement remains investigational, but early clinical results have been encouraging. Further clinical investigation with well-designed prospective, randomized pivotal trials is needed to determine the efficacy of nucleus replacement in the treatment of lumbar DDD, as well as its ideal indications.