Lumbar Spine Cage Material

Lumbar Spine Cage Material

Abstract

Interbody fusion cages are used in spinal fusion to expedite arthrodesis in a vertebral section that is unstable or degenerated. The primary clinical outcome of this implantation is to achieve bony fusion through on-growth and in-growth mechanisms. Currently, the universally selected interbody spacer constructs are titanium (Ti) and polyetheretherketone (PEEK). They are preferred due to their excellent mechanical and biocompatibility features. However, they need further fabrication to sustain osseointegration. The present article aims to analyze the relationship between porosity and mechanical strength of PEEK. In vitro experiments show that the more the porosity of PEEK, the higher the mineralization and proliferation of the cell cultures. Therefore, porous PEEK enhances bioactivity and osseointegration. Reports also indicate that modification of the topography of the surface- chemical and physical treatment- such as creating rougher surfaces improves the cell attachment process. Further, the paper will highlight the advantages of PEEK over the conventional materials and mention the significance of its fabrication process.

            Keywords; PEEK, titanium, spine, spine cage, osseointegration, and mechanical features

Lumbar Spine Cage Material

Introduction

Chronic pain in the lower back vastly disables individuals and reduces the quality of life they lead. The problem is costly to treat, and the available evidence-based treatments are often expensive and challenging. Spinal fusion is among one of the most effective therapies for back pain, with spinal interbody fusion used as the standard method (Dhillon, 2016). Fusion devices are often made of materials that have to accommodate the weight of the patient and the pressure at the point where the invention is to be inserted. A choice of the right material for spinal cages enhances the treatment and management of chronic back pain, thus improving the patient’s quality of life. Various engineering constraints are used to assess different materials to come up with suitable equipment and determine its structure and processing.

Analysis

In carrying out the analysis, peek material was obtained (with the following dimensions; 18 mm depth x 45 mm width x 10 mm height) from Alibaba.  The in vivo vertebrae mostly receives a maximum of 2000N. Therefore, the first part of the analysis involves confirming the axial stiffness of the PEEK. It is important to determine the yielding point as it will show the applicability effectiveness. In obtaining the yielding point, a graph was drawn of stress against strain.

From the formula above, “E” will be obtained from the manufacturer’s information. However, for this case, it – unreinforced PEEK- was obtained from matweb.com, which is 300Mpa. The following formula calculated compression force;

            Compression strength= F/A (where F is applied force, and A is the cross-section area)

The cross-section area is 18 x 45mm. Whereas calculation of strain was through Young Modulus formula, but after obtaining the strength.

E= compression strength/ compression strain (Schaffer et al., 2002)

Table 1

Figure 1: Determination of the yielding point (plotting of stress against strain, data obtained from Table 1)

From table 1, it is clear that the length of the PEEK material when applied a force of 2000N will change by 0.00823mm and that yielding (from Figure 1) does not occur even at as high stresses of 5000N.

By the use of a universal testing machine, a load of 5000N was applied at 5million cycles in a frequency of 5Hz. The PEEK cage was structurally stable because, after the loading cycles, it neither broke or changed. The applied force of 5000N was used as it falls under the range of load threshold value -5000N to 85000N- of human Lumbar static (Kurtz & Edidin, 2007).

A report done by Banse et al. in 2002, revealed that the average stiffness of a 63 adults’ cancellous bone is between 127 to 725Mpa. The PEEK material used, therefore, is within the modulus of the bone, as it has Young’s modulus of 300Mpa.

Selection

It is clear that the above-mentioned material, PEEK can be fabricated through a proprietary process to mimic the three-dimensional structure and elastic modulus of the trabecular bone of a human. Apart from these reasons, pieces of literature narrow down to the two analyzed materials; PEEK and Titanium, because of regulatory concerns over the others. In selecting the materials for analysis, ten were analyzed. Namely, silicon nitride, carbon fibers, bioactive ceramics, resorbable polymers, stainless steel, synthetic bone extenders, demineralized bone matrix (DBM), and bone morphogenetic proteins (BMP).

The last three materials are bone graft substitutes. Although they are rarely used as they inhibit spinal fusion, require to be combined with the bone of the patient and can transfer diseases or cause occasional inflammations. The resorbable polymers like polylactic acid (PLA), and polylactideco-dlactide (PDLLA) have perfect degradation rates. Nonetheless, they are weak in the bearing of loads. On the other hand, titanium, silicon nitride, and bioactive ceramics can meet the demands of load-bearing. Yet, their tensile modulus is exceptionally high.

In addition, Food Drug Act (FDA) demand for interbody cages to meet their regulatory clearance guidelines, ASTM F2077. For the approval of the orthopedic implants, clinical studies with comprehensive follow-ups, usually about 24 months, are a must (Kurtz & Edidin, 2007). Evaluation is based on the achievement of fusion and a patient’s degree of functional improvement. More so, the more extensive the indications for use like devices in class III, the greater the clinical studies. Due to these evaluations, polymethylmethacrylate (PMMA) was disapproved in 1999 as a material for the spine cage, and BMP is only authorized for anterior fusion purposes.

Structure

The physical and mechanical properties enhance the suitability of the material for use in spinal fusion. PEEK (Polyetheretherketone) is a biomaterial that appears in a semi-crystalline form in nature. It is thermoplastic and has a chemical formula (-C6H4-O-C6H4-O-C6H4-CO-)n. and a member of the polyaryletherketones (PAEKs) and is characterized by polythene molecules of ultra-high molecular weight (Muhsin et al., 2019). The material has a crystalline and amorphous phase, with the crystalline content depending on the processing of the material. Its crystallization behavior is complex and is above its glass transition (T_g= -80 to -120^oC) temperature at body temperature. The density of the material determined through x-ray methods is 1.4g/cc in a fully crystalline state and 1.25g/cc in the amorphous state. Except for concentrated sulfuric acid, PEEK cannot be damaged by the action of solvents (Kurtz & Devine, 2007).

PEEK has a bond angle of 125⁰, and its molecular formation allows for a zig-zag chain in structure conformation. The material’s long-axis-c- covers three aryl groups with a 5 Å distance between the centers of the aryl groups (Kurtz & Devine, 2007). The ketone and ether groups finishing the structure of the material are comparable in size to the extent that the crystallization of PEEK is only influenced slightly by its chemical composition.

The material’s crystals are made up of fine string-like lamellae that can organize into larger structures under various conditions. The thickness of these lamellae is based on the processing of the material but is rather small at 50 and 60 Å. A scanning electron microscope with a suitable range can be used for visualization.

Processing

The biocompatibility of PEEK enhances its application in medical fusion as it requires less extensive processing compared to the other materials. Biocompatibility is also a significant requirement for any restorative material used in the body since it has to work well with the primary tissue in the area. Biocompatibility also enhances the permanence of the structure with the probability of an increasing load- such as the patient gaining weight- and long periods of use. Since the mechanical properties of the material, as well as its crystallization, are dependent on the processing, it is vital that the crystalline content is characterized before fabrication (Kurtz & Devine, 2007). Parameters such as temperature, cooling rate, time, and any other post-production additions will need to be calculated to come up with the right thickness for the cage.

The cooling of PEEK is an essential factor in the maintenance of the structure. If the temperature falls rapidly during cooling after melting, the crystallinity of the material differs between the surface skin and the inner core. The skin shows lower crystallinity compared to the rest of the bulk. Composite fillers like carbon fiber are added post-processing to offer more nucleation points in the material (Kurtz & Devine, 2007). Cooling by immersion in cold water, which is a common way of processing thin samples, commonly leads to this thin skin.

Appropriate processing of PEEK guarantees the semi-crystalline nature is maintained, thus improving its osseointegration, mechanical property, chemical resistance, and wear resistance. Furthermore, it has excellent flammability rating, fatigue resistance properties and low level of extractable ionic species.

References

Banse, X., Sims, T. J., & Bailey, A. J. (2002). Mechanical properties of adult vertebral cancellous bone: correlation with collagen intermolecular cross‐links. Journal of bone and mineral research, 17(9), 1621-1628.

Kurtz, S. M., & Devine, J. N. (2007). PEEK biomaterials in trauma, orthopedic, and spinal            implants. Biomaterials, 28(32), 4845–4869.      https://doi.org/10.1016/j.biomaterials.2007.07.013

Kurtz, S. M., & Edidin, A. (2006). Spine technology handbook. Elsevier.

Muhsin, S. A., Hatton, P. V., Johnson, A., Sereno, N., & Wood, D. J. (2019). Determination of             Polyetheretherketone (PEEK) mechanical properties as a denture material. The Saudi         Dental Journal, 31(3), 382–391. https://doi.org/10.1016/j.sdentj.2019.03.005

Schaffer, J. P., Saxena, A., Antolovich, S. D., Sanders, T. H., & Warner, S. B. (2002). The science and design of engineering materials (p. 689). New York: McGraw-Hill.