3
Biomaterials for Spinal Implants
3.1 Introduction
Spinal diseases and injuries continue to be common issues in medical treatment, with increasing numbers of elderly patients and incidences of bone disease, traumatic athletic injuries, and traffic incidents [1]. Lower back pain alone itself reportedly affected more than 52 million patients in the United States in 2012, and this issue alone was estimated to cost $253 million in direct medical costs [2]. Implant failure due to a lack of bone healing and anchoring to the device, subsequent expulsion of the material, infection, tensile failure, and loosening continue to be widespread issues. As such it is critical to improve the current biomaterial implants, with the continued development and optimization of biomaterials as one path toward improved implants.
Successful spinal implants will demonstrate resistance to infection, good biocompatibility, bioactivity to encourage bone fusion, and appropriate physical properties (Young’s modulus, stiffness, and fatigue) to prevent stress shielding and degradation. The various types of spinal implants developed include cages, rods, screws, plates, spinal fusions, and replacement spinal disks. Biomaterials with applications in these areas include stainless steels, titanium, polyaryletherketones (PAEKs or PEEKs), cobalt–chromium alloys, ceramics, nitinol, and tantalum. Current research has been focused on finding materials to encourage osseointegration, improving strength, improving range of motion, decreasing pain, distributing forces to decrease adjacent segment disease, and reducing the need for additional/revisional surgeries [3]. The following review discusses typical spinal implants, followed by a review of materials used for said implants, promising advancements highlighted in other reviews, and a summary of notable new works.
3.2 Overview of Implants and Corresponding Material Design Requirements
3.2.1 Spinal Cages
Cages () act as stabilizers to distribute forces between the vertebrae and to encourage spinal fusion through bone growth. They also serve to increase spacing in the intervertebral space to reduce compression caused by various injuries, scoliosis, and degeneration in spinal disks. Bone grafts used to be the top choice for cages but were limited by high failure rates and prolonged healing time [4, 5].
Failure mechanisms include collapse, subsidence, retropulsion, and graft resorption when using bone grafts [5]. Cages must have enough mechanical strength to withstand the compressive forces of the intervertebral space. Shear forces and their negative effect are avoided by staying away from brittle materials, ensuring stable placement, and encouraging good early bone growth. Osteointegration is key for successful spinal fusion as bone growth must happen quickly after implantation to resist the shear forces associated with the spine. Finally, the material should have an elastic modulus close to the surrounding bone to prevent failure from stress shielding [4].
Common materials implemented include titanium and its alloys and composites, silicon nitride ceramics, and PEEK. Historically, stainless steel and cobalt–chromium materials have been used as well but are not as popular. Composites of these materials as well as various surface modifications like coatings have also been explored [5].
3.2.2 Spinal Rods
Spinal rods are typically used with other spinal implants to stabilize their structures and the spine itself. They must be designed to fit the individual’s spine. The process of countering weakens the rod through the creation of notches during deformation (Figure 3.1), reducing the strength and longevity of the device [3].
Figure 3.1 A French bender, employed for shaping spinal rods, exhibited the process of creating notches in the contoured rod.
(From Ref. [6], 2013, Elsevier).
Historically, they are made of stainless steels, but the current trends are dominated by iron chromium, chromium nickel, and austenitic stainless steel, titanium and its alloys, and cobalt–chromium alloys [3].
3.2.3 Pedicle Screws
Pedicle screws are frequently used, with applications in holding spinal vertebrae together when attaching plates and rods. Requirements for screws include high strength and bioinert qualities as they are responsible for redirecting forces through vertebral bodies. Hence, titanium alloys, like Ti6Al4V, are commonly used. Common dopants include hydroxyapatite, calcium phosphate (CaP), extracellular matrix, and tantalum. Several complications arising from materials properties exist for screws. Major issues include loosening of the screws and subsequent pullout because of the loss of fixation, screw breakage, and deformation from fatigue and screw bending [3].
These issues can result in decreased bone healing. Coating research has continued to focus on hydroxyapatite, CaPs, polymethacrylate‐based bone cements, tantalum, and titanium‐plasma sprays [3]. Newer developments include novel coatings and cements like carbonated apatite cancellous bone cement to improve bone fixation and pull‐out strength [3].
3.2.4 Plates
Spinal plates are used for spinal fusion and fixation and serve to stabilize and restore spinal alignment with pure titanium or Ti6Al4V being the traditional material choice. They are installed laterally across two vertebrae and must resist torsion and shear forces. New developments include biodegradable plates made of polymers like polylactic acid (PLA) [3, 7].
3.2.5 Disc Replacements
Low back pain is a common issue that can be traced primarily to the intervertebral disk (IVD). The IVD forms a fibrous, cartilaginous joint between vertebrae and vertebral columns. The disk acts like a ligament to allow movement and is the center of rotation for movement, biomechanically. It also functions as a shock absorber [5]. Implants for total disk replacement, disk repair, and disk regeneration must be flexible enough to allow for range of movement, but strong enough to handle the differential forces.
Lumbar total disk replacement has been as successful post‐surgery as lumbar fusion, with low rates of complications compared to fusion. Lumbar disk replacements were historically steel or titanium balls with polyethylene sliding cores in between them, with ceramics being too fragile alone and being explored instead as coatings to promote osseointegration [3, 5].
Several ceramic coatings and surface functionalization technologies have been explored, including hydroxyapatites, tricalcium phosphate, porous titanium, chromium–cobalt alloys, and various titanium surface coatings aimed at promoting bone growth [3].
Cervical spine disks like ProDisc‐C have been made from cobalt–chromium–molybdenum in conjunction with ultrahigh molecular weight polyethylene and rough titanium surface coatings. Another configuration is the Mobi‐C prosthesis which has two cobalt–chromium–molybdenum plates with HA coatings and polyethylene plates in the center [3].
3.3 Metals
3.3.1 Titanium and its Alloys
Titanium is the most widely used material for spinal implants. It is well known for its reliable strength and biocompatibility. They are lightweight, flexible, and easily doped, but are more expensive than other options and present some artifacts during imaging. Titanium alone is relatively bioinert, suffering from reduced osseointegration. It also has an elastic modulus much higher than bone and therefore is subject to stress shielding, weakening the surrounding bone [3, 4].
Overall, titanium and its alloys have been thought to present better biocompatibility, corrosion resistance, and Young’s modulus when compared to other biocompatible metals like stainless steel and cobalt alloys [3, 8]. In interbody implants, titanium has been used as spacers since it can resist compression and can encourage bone growth given the right surface properties; its weakness lies in the elastic modulus being close to 110 GPa, rather than close to 18 GPa of bone [4].
Titanium has been the focus of a multitude of studies aiming to improve osteointegration through surface roughening, increasing material porosity, and by applying coatings like hydroxyapatite [4]. Other efforts include creating porous titanium that has a reduced elastic modulus and better bone in growth while still preserving load‐bearing capabilities.
Zhang et al. studied the osteogenic and antitumor potential of a 3D‐printed nHA‐loaded porous titanium scaffolds for load bearing bone‐segment implants. The system demonstrated new bone growth and suggested the gradual replacement of apatite with new bone tissue [9].
Various coatings have been explored for titanium substrates. Hydroxyapatite coatings on titanium surfaces are a popular area of study, with HA coatings increasing osseointegration, but suffering from issues of delamination. Johansson et al. coated titanium screws with nano‐HA and found the addition of the coating increased mean bone–implant contact [10].
Li et al. studied strontium‐substituted hydroxyapatite (SrHA) coatings for titanium implants and found that 10% SrHA coatings enhanced osseointegration over plain hydroxyapatite, with increased bone area ratios, bone volume ratios, and bone‐to‐implant contact 12 weeks after implantation in rats. The addition of strontium increased both the max push out force of the implant and its ultimate shear strength (Figure 3.2) [11].
Figure 3.2 (a) Illustration outlining the process of preparing a tumor cell suspension and seeding it into a scaffold. (b) Surgical implantation of tumor cell‐seeded scaffold at a segmental bone defect in the rabbit femur. (c) Assessment of tumor volume in rabbits implanted with either empty scaffolds or nano‐hydroxyapatite (n‐HA)/scaffolds over a five‐week period. Error bars represent standard deviation (SD), with n = 4 per group. **P < 0.01 compared to the empty scaffold, as determined by a two‐way t‐test. (d) Sequential photographs capturing the excised implants with tumors from weeks 2–5. (e) Micro‐computed tomography (Micro‐CT)‐reconstructed images illustrating the implants and adjacent bone tissue. “B” denotes the bone, “S” represents the scaffold, and arrows indicate the cortical bone resorption by the tumor. (f) Histological examination of the implanted scaffolds, with “S” denoting the scaffold, “T” indicating tumor, and red arrows highlighting new bone formation.
(From Ref. [9], 2023, Elsevier).
3.3.2 Stainless Steel
Stainless steels are inexpensive materials that are exceptionally strong and stiff and are easily doped, but suffer from corrosion issues, low biocompatibility, and high artifacts during imaging. Previously commonly used in screws and correction rods, the material has lost popularity in favor of titanium [3].
Stainless steels are protected by a chromium oxide layer compared but have poor corrosion in physiological conditions when compared to titanium and the titanium oxide layer [6]. Nickel and chromium ions are released during stainless steel corrosion and are credited with the suboptimal biocompatibility and bone bonding of the material. Nickel can negatively affect osteoblast formation and is less desirable due to the potential for allergic reactions to nickel in patients. With regard to bone bonding, steels tend to have lower bone contact and removal strength than other materials, with values more comparable to Co–Cr. Overall, they are considered to be less corrosion resistant that titanium materials, but present higher stiffness [6, 12].
3.3.3 Cobalt–Chromium and its Alloys
Like most metals, cobalt chrome is strong, flexible, and biocompatible, but titanium materials are relatively expensive. They also suffer from high artifacts during imaging [3].
One of the benefits of using cobalt–chromium is that it has better antibacterial properties than other materials, including titanium. It was found to suppress Staphylococcus aureus and Propionibacterium acnes bacterium better than titanium both in vitro and in vivo [13].
3.3.4 Nitinol
Nitinols are shape memory alloys of titanium and nickel that are strong and present unique properties as memory metals. They are expensive and lack the stiffness that can be required for certain implant applications. They are not a popular choice, but have potential applications for scoliosis correctional surgery where it is thought they can provide a constant corrective force to the spine as it recovers from deformation when heated [3, 14]. Although nitinol is largely considered biocompatible, there is still concern regarding nickel toxicity [15].
Massey et al. conducted a biomechanical comparison between nitinol memory rods and titanium rods and found that nitinol had comparable compression to titanium rods but had increased torsional toughness and load until torsional failure. They also found that there was no significant difference in fatigue resistance between the two [16].
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