Biomechanical Behavior of a New Nucleus Prosthesis Made of Knitted Titanium Filaments

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

Background

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.

Methods

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.

Results

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.

Conclusions

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.

Mechanical characterization and design of biomaterials for nucleus pulposus replacement and regeneration

Biomaterials for nucleus pulposus (NP) replacement and regeneration have great potential to restore normal biomechanics in degenerated intervertebral discs following nucleotomy. Mechanical characterizations are essential for assessing the efficacy of biomaterial implants for clinical applications. While traditional compression tests are crucial to quantify various modulus values, relaxation behaviors and fatigue resistance, rheological measurements should also be conducted to investigate the viscoelastic properties, injectability, and overall stability upon deformation. To recapitulate the physiological in vivo environment, the use of spinal models is necessary to evaluate the risk of implant extrusion and the restoration of biomechanics under different loading conditions. When designing devices for NP replacement, injectable materials are ideal to fully fill the nucleus cavity and prevent implant migration. In addition to achieving biocompatibility and desirable mechanical characteristics, biomaterial implants should be optimized to avoid implant extrusion or re-herniation post-operatively. This review discusses the most commonly used testing protocols for assessing mechanical properties of biomaterial implants and serves as reference material for enabling researchers to characterize NP implants through a unified approach whereby newly developed biomaterials may be compared and contrasted to existing devices.

How does a novel knitted titanium nucleus prosthesis change the kinematics of a cervical spine segment? A biomechanical cadaveric study

BACKGROUND

Total disc replacement is a possible treatment alternative for patients with degenerative disc disease, especially in the cervical spine. The aim is to restore the physiological flexibility and biomechanical behavior. A new approach based on these requirements is the novel nucleus prosthesis made of knitted titanium wires.

METHODS

The biomechanical functionalities of eight human cervical (C4-C7) spine segments were investigated. The range of motion was quantified using an ultra-sound based motion analysis system. Moreover, X-rays in full flexion and extension of the segment were taken to define the center of rotation before and after implantation of the nucleus prosthesis as well as during and after complex cyclic loading.

FINDINGS

The mean range of motion of the index segment (C5/6) in flexion/extension showed a significant reduction of range of motion from 9.7° (SD 4.33) to 6.0° (SD 3.97) after implantation (P = 0.037). Lateral bending and axial rotation were not significantly reduced after implanting and during cyclic loading in our testing. During cyclic loading the mean range of motion for flexion/extension increased to 7.2° (SD 3.67). The center of rotation remained physiological in the ap-plane and moved cranially in the cc-plane (-27% to -5% in cc height) during the testing.

INTERPRETATION

The biomechanical behavior of the nucleus implant might lower the risk for adjacent joint disorders and restore native function of the index segment. Further in vivo research is needed for other factors, like long-term effects and patient's satisfaction.

Characterisation of porous knitted titanium for replacement of intervertebral disc nucleus pulposus

Effective restoration of human intervertebral disc degeneration is challenged by numerous limitations of the currently available spinal fusion and arthroplasty treatment strategies. Consequently, use of artificial biomaterial implant is gaining attention as a potential therapeutic strategy. Our study is aimed at investigating and characterizing a novel knitted titanium (Ti6Al4V) implant for the replacement of nucleus pulposus to treat early stages of chronic intervertebral disc degeneration. Specific knitted geometry of the scaffold with a porosity of 67.67 ± 0.824% was used to overcome tissue integration failures. Furthermore, to improve the wear resistance without impairing original mechanical strength, electro-polishing step was employed. Electro-polishing treatment changed a surface roughness from 15.22 ± 3.28 to 4.35 ± 0.87 µm without affecting its wettability which remained at 81.03 ± 8.5°. Subsequently, cellular responses of human mesenchymal stem cells (SCP1 cell line) and human primary chondrocytes were investigated which showed positive responses in terms of adherence and viability. Surface wettability was further enhanced to super hydrophilic nature by oxygen plasma treatment, which eventually caused substantial increase in the proliferation of SCP1 cells and primary chondrocytes. Our study implies that owing to scaffolds physicochemical and biocompatible properties, it could improve the clinical performance of nucleus pulposus replacement.

A pedicle screw system and a lamina hook system provide similar primary and long-term stability: a biomechanical in vitro study with quasi-static and dynamic loading conditions

PURPOSE

For the stabilization of the thoracolumbar spine area, various stabilization techniques have been developed in recent decades. The aim of these techniques is to immobilize the treated segment to repositioning or correct the spine and guaranty long-term stability to achieve a reliable fusion. The aim of this study was to simulate in an in vitro experiment the postoperative long-term situation in elderly osteoporotic patients to compare two different stabilization principles; a pedicle screw system and a lamina hook system.

METHODS

Two comparable groups with respect to age and bone mineral density with each n = 6 fresh-frozen human, bi-segmental thoracolumbar spine specimens (T11-L1) were used. Antero-posterior and lateral radiographs were taken before the test, to assess the spinal status. Then the intact specimens were biomechanically characterized with pure moments in the three anatomical planes in different states in terms of range of motion and neutral zone. After implantation of either, a pedicle screw system or a lamina hook system, the primary stability was determined under the same conditions. Subsequently the specimens were cyclically loaded under complex loading, using a custom-made set-up in a dynamic materials testing machine with increasing moments from 3 to 66 Nm until 100,000 cycles or until one of the three defined "failure" criteria was reached. (1) A failure of a bony structure. (2) Exceeding of the threefold ROM of the primary stability after implantation in flexion plus extension. (3) Reaching of the ROM based on the intact state before implantation both in flexion plus extension.

RESULTS

The results showed that the ROM was strongly reduced after instrumentation similar for both implant systems in all motion planes. The highest stabilization was found in flexion/extension. During cyclic loading with increasing moments, the ROM increased continuously for both systems. The number of load cycles until one of the failure criteria was reached varied only slightly between the two groups. In the pedicle screw group 30,000 (median) loading cycles (range 5000-80,000) with a corresponding moment of 24 Nm (range 9-54) could be reached. In the lamina hook group 32,500 load cycles (range 20,000-45,000) could be achieved with a corresponding moment of 25.5 Nm (range 18-33). There was a slight trend that the pedicle screw system is influenced more by bone mineral density.

CONCLUSION

Both implant systems provide similar primary stability and similar long-term stability. In the pedicle screw group, there was a stronger correlation between bone mineral density and the reached number of load cycles.

Nucleus pulposus replacement and regeneration/repair technologies: present status and future prospects

Degenerative disc disease is implicated in the pathogenesis of many painful conditions of the back, chief among which is low back pain. Acute and/or chronic low back pain (A/CLBP) afflicts a large number of people, thus making it a major healthcare issue with concomitant cost ramifications. When conservative treatments for A/CLBP, such as bed rest, anti-inflammatory medications, and physical therapy, prove to be ineffectual, surgical options are recommended. The most popular of these is discectomy followed by fusion. Although there are many reports of good to excellent outcomes with this method, there are concerns, such as long-term adverse biomechanical consequences to adjacent functional spinal unit(s). A surgical option that has been attracting much attention recently is replacement or regeneration/repair of the nucleus pulposus, an approach that holds the prospect of not compromising either mobility or function and causing no adjacent-level injury. There is a sizeable body of literature highlighting this option, comprising in vitro biomechanical studies, finite element analyses, animal-model studies, and limited clinical evaluations. This work is a review of this body of literature and is organized into four parts, with the focus being on replacement technologies, regeneration/repair technologies, and detailed expositions on 14 areas for future study. This review ends with a summary of the salient points made.