Design Requirements for Annular Closure Devices: Comparison
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Microdiscectomy procedures are the most used technique for  Lumbar disc herniation; however, the annulus fibrosus is left with a defect that without treatment may contribute to high reherniation rates and changes in the biomechanics of the lumbar spine. At present there are only a few commercial devices available for annular closure.

  • : intervertebral disc
  • lumbar IVD herniation
  • annular closure device

1. Previous Efforts at Developing an Annular Repair/Regeneration Strategy

Researchers have mostly studied the use of biodegradable or bioresorbable implants to close annular defects while aiding with tissue regeneration over a finite period. However, the extremely low cellularity of adult cartilage constitutes a serious problem when injured [1][2]. In several animal models with surgically injured IVDs, healing has been demonstrated to occur in the outer annulus but not towards the inner part of the IVD [3][4]. This is believed to be due to the outer annulus having four times the cell density of the nucleus in adult human IVDs [5] implying that the tissue may have some ability to heal, though this has not been conclusively proven [4].
Evidence suggests that annular healing proceeds in an outside to inside direction [3]. As discussed above, if any annular healing occurs at all it is hypothesized to happen slowly with tissue that has a reduced capacity to accommodate everyday biomechanical forces [6]. Should a repair/regeneration approach be selected, then degradation time of the implant should coincide with the healing process to ensure proper remodeling of the tissue [2]. These parameters are still unknown for both human and animal IVD tissue. Additionally, the mechanical properties of the implant should be adequate to promote regeneration during the patient’s everyday activities, and any change in mechanical properties due to degradation should preserve compatibility with the healing or regeneration process [2]. It is estimated that the spine undergoes approximately 100 million flexion cycles during a lifetime [7]. In the case of spinal implants, 10 million (1 × 107) cycles is considered to be the minimal life length, but 30 million cycles is considered optimal [8]. Any biodegradable/bioresorbable strategy should be able to ensure that regenerated tissue would be able to withstand the same number of cycles.
Current biodegradable/bioresorbable strategies to repair the adult human IVD are highly likely to fail before sufficient tissue regeneration occurs. From a biomechanical point of view IVD regeneration may only occur in if the following conditions are met: restoration of normal physiological range of motion, normal lordosis, spinal balance achieved, IVD height restored, normal intradiscal pressure, and normal IVD load distribution [8]. None of these are achieved by current implants, biological repair strategies, or tissue engineered approaches [9] suggesting that a permanent (non-resorbable) mechanical closure device may be the most suitable option for this application.

2. Aims and Scope

There have been many excellent reviews describing the necessary biological parameters for annular repair/regeneration [10][11][12][13][14]; suitable biomaterials [15]; tissue engineered scaffolds [9]; and the challenges and strategies [16][17] for implementation. Long et al. made an important breakthrough by identifying, quantifying, and proposing a testing paradigm for hydrogel-based strategies for AF repair [18].

3. Key Requirements for an Annular Repair Strategy

32.1. Mechanical Requirements

The AF is composed of concentric lamellae of collagen fibers embedded into a proteoglycan matrix or ground substance. In the human AF, there are approximately 15–20 lamellae, each with around 40 collagen bundles [19] alternating from 45° to 25° with an average value of 28° [20]. The highly organized lamellar structure allows it to distribute and absorb large spinal load that occurs in complex combinations of tension, compression, torsion, shear, and bending. Intradiscal pressure tensions the annular fibers and supports the endplates. It is the main contributing factor to adequate IVD height and tissue stiffness during axial compression [21]. Any reduction in the pressure reduces IVD volume through IVD height reduction; this changes the stress distribution in the IVD causing increases in stress concentrations within the IVD and leads to increased shear forces in the nucleus [21]. Therefore, AF repair devices must withstand intradiscal pressure when prone, when supine immediately after surgery, and sitting and standing pressures shortly after surgery [18]. Additionally, for an individual to continue their activities of daily living (e.g., walking, standing, climbing stairs), the range of motion must be restored in all six degrees of freedom (flexion–extension, lateral bending, and axial rotation) within an acceptable range.

32.2. Biological Requirements

In the IVD’s central regions, the nucleus and inner AF are supplied by capillaries that arise in the vertebral bodies, penetrate the subchondral bone, and terminate at the endplates [22][23]. In the outer region, cells require the blood supply in the outer AF of the IVD to receive their nutrients and metabolites. Small molecules such as glucose and oxygen then reach the cells by diffusion under gradients established by the balance between the rate of transport through the tissue to the cells and the rate of cellular demand [22][23]. Therefore, it is crucial that an annular repair strategy minimizes disruption of the blood supply in the outer annulus and does not cause significant damage or lesions to the endplates that will compromise nutrient supply [24]. Furthermore, preservation of the endplates is crucial, particularly the caudal (bottom) endplate. Caudal endplates have lower bone mineral density, are thinner than the cranial endplates, and have significantly more openings, which could contribute to the small thickness of this endplate and its susceptibility to fractures [25]. In the case of the Barricaid® device, increased prevalence of new endplate lesions and loss of surrounding bone were observed with use of the implant compared to the controls; endplate lesions close to the flexible polymer component were considerably much larger [26]. This has been suggested to be a result of the PET mesh being in contact with the adjacent vertebral body as a result of IVD height loss [27]. These lesions appeared to stabilize over time, and no vertebral fractures occurred within the five year period [26]. However, it may be sensible to recommend that surgeons place the titanium anchor in the lower vertebrae to avoid causing substantial long-term damage to the caudal endplate. Furthermore, implants that consider using a similar fixation strategy to the Barricaid® device (using the upper or lower vertebrae to secure the implant in place) may need to carefully consider the place of attachment and its biological consequences.
Additionally, the aging process and/or IVD degeneration reduces the diffusion of nutrients and metabolites causing the accumulation of lactic acid in the center of the IVD [28]. This in turns lowers pH from a healthy ~7.1 to values of 6.5–5.7 [28]. Low pH has been reported to reduce cell viability and proteoglycan and collagen synthesis in the IVD [28]. This would suggest that maintenance of a normal physiological pH would be beneficial to prevent further IVD degeneration and herniation.

32.3. Material Requirements

Any biomaterial used must be biocompatible and non-cytotoxic. Additionally, any material should ideally possess mechanical properties similar to those of the surrounding annular tissue to encourage natural load distribution throughout the intervertebral and adjacent spinal segments. If the material modulus is too high, it will not deform accordingly, and the majority of the load would be concentrated on the implant; this increases the probability of the implant fracturing and weakening the surrounding tissue [29]. On the other hand, if the material modulus is too low, the implant would not be able to support the load, putting more strain onto the adjacent tissue and establishing the possibility of fragments (or the whole implant) being expelled into the spinal canal and damaging the nerves [18][30].

32.4. Preclinical Testing

In vitro testing provides a controlled method for investigating biocompatibility and preliminary mechanical tests of a scaffold or promising annular repair materials. Both mechanical and biological in vitro testing have several challenges to overcome. IVD sizes vary across species and according to location within the spine. Most commonly used in vivo animal models range from small rodents to rats, rabbits, dogs, goats, sheep, primates [31], and more recently kangaroos [32]. One of the main issues when using animal models is that the majority of them are quadrupedal, and the few bipedal models available (primates and kangaroos) present ethical dilemmas that prevent their use in most research institutions [31]. Biomechanics are also significantly different; hence, it is hypothesized that the loads exerted on the lumbar IVDs of large animals by the surrounding structures (muscles and ligaments) may be even greater than those in human IVDs resulting from the bipedal stance due to the increased difficulty of stabilizing a horizontally aligned spine versus a vertically balanced spine [31]. However, use of animal tissues can help in understanding how aspects of testing techniques influence the results of experiments on human tissue [33].
Computational and finite element (FE) modeling have become powerful tools to evaluate performance of current and novel medical devices. The initial and long-term performance of a device could be predicted with anatomically accurate human models which can influence device design and optimization reducing the need for in vivo animal testing. However, one of the main challenges with current models is that failure mechanisms of the IVDs are quite a complex task to model due to the tissue’s complex structure. Few studies have focused on modeling annulus failure mechanics of human tissue. Early models include that of Goel et al. to evaluate delamination in the lamellae based on interlaminar shear stresses [34] and evaluation of damage in a hyperelastic anisotropic model of the annulus by Eberlein et al. [35]. Quasim et al. studied damage initiation and progression in the AF region under various cyclic loading conditions using a 3D poroelastic model [36]. However, with this model only damage to the tissue will occur in the direction of tensile principal stress. Furthermore, they did not consider the changes in the viscoelastic characteristics of the annulus [36]. More recently, Shahraki et al. used the Tsai–Wu criterion and the maximum normal stress to predict damage initiation and propagation of the AF tissue under different loading conditions [37]. Nonetheless, this model does not account for water content and porosity of the IVD. Furthermore, the annulus region was considered as a homogeneous material in terms of fiber orientation and density, but fiber orientation in the tissue changes from the inner to the outer part of AF. Additionally, viscoelasticity was not considered, and strength of the material was considered under static loading only [37].
To the authors’ knowledge there are no finite-element analyses of tissue engineered scaffolds for AF or IVD repair to date; however, Moroni et al. fabricated 3D meniscal scaffolds with porosity and architecture that mimic native tissue mechanical properties [38]. The anatomical 3D scaffolds were experimentally analyzed to tailor their mechanical properties to match those of natural menisci and numerically investigated with FE analysis to determine whether they were mechanically robust enough to be used for meniscus reconstruction [38]. While menisci and IVDs are quite different, they are both anisotropic, viscoelastic tissues, thus proving that it would be beneficial to use FE analysis to test and optimize a scaffold’s design. It could also help in reducing time and costs if it is used to test a preliminary design to determine optimal structural, mechanical, and physical properties to direct specific cell function [39].

32.5. Considerations for Clinical Translation

32.5.1. Sterilization

For any medical device, proper sterilization is crucial [4]; thus, the annular repair strategy must be sterilizable. Depending on the material used and type of repair strategy used this may prove very challenging. For example, in the case of hydrogel-based strategies sterilization must be performed carefully because these materials may be sensitive to the sterilizing agents such as heat and radiation [40]. Water content in the hydrogel can facilitate chemical bond breakdown which can result in changes of material properties, degradation and/or decomposition of the material, discoloration, embrittlement, odor generation, and promote further crosslinking or induce toxic effects [40].

32.5.2. Delivery and Attachment

As discussed in Section 1.2, current minimally invasive procedures, which are the best surgical option to treat lumbar IVD herniation, limit the amount of soft tissue dissection and minimize excessive scarring near the nerve root [35]. Thus, an AF repair device implanted during a microdiscectomy should be minimally invasive and create minimal soft tissue damage during the procedure. In this case, injectable scaffolds could allow easy filling of irregularly shaped defects [11]. One of the greatest challenges in cartilage tissue engineering is achieving fixation or functional integration into the native tissue [41]. The optimal device must be safely attached to the tissue and must remain in place. Adhesive patches based on bioinspired architectures such as the gecko-/beetle-inspired mushroom-shaped architectures, endoparasite-like microneedles, octopus-inspired suction cups, and slug-like adhesive [32] may be the answer to achieve adequate fixation to the native AF, should this approach be adopted.

32.5.3. Postoperative Imaging

Magnetic resonance imaging (MRI) is a useful imaging technique capable of producing high quality images of the human body and individual tissues. It allows non-invasive assessment of IVD degeneration/herniation and provides excellent spatial resolution and tissue characterization without exposing patients to the potential risks of ionizing radiation and iodinated contrast agents [42]. When monitoring endplate lesions pre- and postoperatively, MRI has proved equally useful [40]. The American Society for Testing and Materials (ASTM) categorizes implants into [43]:
  • “MRI Safe—an item that poses no known hazards resulting from exposure to any MR environment. MR Safe items are composed of materials that are electrically nonconductive, non-metallic, and nonmagnetic.
  • MR Conditional—an item with demonstrated safety in the MR environment within defined conditions.
  • MR Unsafe—an item which poses unacceptable risks to the patient, medical staff, or other persons within the MR environment.”
Safe imaging of the operated area during follow-up appointments is vital in case of a patient experiencing adverse effect or symptom recurrence. Therefore, according to ASTM standards an annular repair strategy must be MRI safe or MRI conditional. For example, even though the Barricaid® device (Intrinsic Therapeutics, Woburn, MA, USA) has metallic components, it can be safely tested with static magnetic fields of 1.5 and 3 Tesla (maximum spatial gradient magnetic field of 3000 Gauss/cm or less) [44].

32.5.4. Same Level Symptomatic Reherniation

Symptomatic IVD reherniation is the most common cause of reoperation after primary IVD surgery [45]. Although there are many theories as to what increases a patient’s chance for reherniation, no one factor has been identified consistently in the literature [46]. Reherniation rates of surgically treated patients with IVD herniation vary greatly across the literature, from 3% to 18% [47] or between 0.5% and 25% [48]. It is therefore difficult to specify an acceptable range of percentage risk; however, recent clinical outcomes of the Barricaid® device vs. controls show that the cumulative incidence of symptomatic reherniation was 8.4% vs. 17.4% at 1 year, 10.7% vs. 23.4% at 2 years, and 14.8% vs. 29.5% at 3 years [49]. A negligible or low reherniation risk would be optimal.

32.5.5. Device Loosening, Failure, and Safe Removal

Clear instructions must be given to the surgical team regarding implant delivery to ensure correct placement and that patients are given the necessary long-term postoperative care guidelines to ensure the AF repair device’s appropriate function. However, in case a patient begins to present adverse effects or pain related to implantation of the device, it is necessary to have a procedure for the safe removal of the device.

32.5.6. Pain Score Improvement

There are currently at least 28 scoring systems available for the evaluation of low back pain [50]. In the lumbar spine, the visual analogue scale (VAS) and the Oswestry disability index (ODI) are used most often. The VAS consists of a straight line with the endpoints defining extreme limits such as ”no pain at all” (score of 0) and ”pain as bad as it could be” (score of 10; 100 mm scale) [51]. In the ODI, the total score ranges from 0% to 100%, with 0% representing no disability and 100% representing maximum disability. For example, a total score between 0% and 20% means minimal disability; between 20% and 40%, moderate disability; between 40% and 60%, severe disability; between 60% and 80%, crippled; and between 80% and 100%, bedbound or symptom magnification [50]. For VAS and ODI scores, a decrease of at least 20mm and 15 points, respectively, is considered a success [26]. In the case of current commercial annular closure devices there was no significant difference in follow-up ODI and VAS scores for both back and leg at 90 days and 2 years when an annular closure device was used compared to microdiscectomy only [50].

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