The aim to find the perfect biomaterial for spinal implant has been the focus of spinal research since the 1800s. Spinal surgery and the devices used therein have undergone a constant evolution in order to meet the needs of surgeons who have continued to further understand the biomechanical principles of spinal stability and have improved as new technologies and materials are available for production use. The perfect biomaterial would be one that is biologically inert/compatible, has a Young’s modulus similar to that of the bone where it is implanted, high tensile strength, stiffness, fatigue strength, and low artifacts on imaging. Today, the materials that have been most commonly used include stainless steel, titanium, cobalt chrome, nitinol (a nickel titanium alloy), tantalum, and polyetheretherketone in rods, screws, cages, and plates. Current advancements such as 3-dimensional printing, the ProDisc-L and ProDisc-C, the ApiFix, and the Mobi-C which all aim to improve range of motion, reduce pain, and improve patient satisfaction. Spine surgeons should remain vigilant regarding the current literature and technological advancements in spinal materials and procedures. The progression of spinal implant materials for cages, rods, screws, and plates with advantages and disadvantages for each material will be discussed.
Spinal surgery has existed for since Jules Gerin first attempted surgical scoliosis correction in 1839 [
Cages are spinal implants that act as a stabilizer for force distribution between vertebral bodies and to restore the height of the intervertebral and foramina space [
Cages are typically made of metal – ranging from pure titanium (PTi), titanium composite/alloy, ceramic – usually silicon nitride, or plastic – usually PEEK or another bioinert plastic such as acrylic by itself or coated in another material (such as hydroxyapatite [HA] or titanium) [
The most popular materials used today are titanium (titanium-aluminum-vanadium, Ti6Al4V) and PEEK (
PEEK has a similar Young’s modulus compared to bone, without sufficient doping such as titanium plasma spray and vapor deposition, has weak surface interfaces that can fracture upon cage implantation [
Increased research for advancements in disc separation has been made to find more biocompatible materials than titanium and PEEK to improve bone grafting. Experimentation with silicon nitride (Si3N4) showed no significant different compared to PEEK [
Spinal rods are used in conjunction with other spinal implants to add stability to spinal implant structure. Rods are contoured to a specific patient to fit the spine. In 1962, Dr. Harrington introduced the “Harrington Rod” – an SS rod – for surgical treatment of scoliosis [
Rods were initially made of SS to provide sufficient stability and stiffness for proper spine alignment [
The metal rods used today fall into one of 3 alloy families: iron (Fe)-chromium (Cr)-nickel (Ni) alloys and austenitic SS, titanium and its alloys (PTi and Ti6Al4V alloy), and cobalt-chromium alloys (CoCr). These alloys were chosen because they are relatively biocompatible and are protected from corrosion by the presence of a stable oxide layer; however, SS is the least corrosion resistant [
These other metal rods are stiffer than titanium rods. CoCr rods have gained recent popularity with adolescent idiopathic scoliosis with promising frontal correction rate in all-screw constructs due to their increased rigidity compared to titanium, closer reflecting SS [
Nitinol (50% Ni and 50% Ti) rods—also called memory rods —are not as common as Ti since the metal is expensive and notch-sensitive [
Besides the rigid metal rods, there are semirigid rods. One of the most popular is the PEEK rods. It has been shown that PEEK provides comparable stability compared to titanium rods of similar size [
Many biomechanical studies have shown that when rods are bent to fit a patient using a French bender, significant surface defects and weakness are introduced [
New research is being conducted to create more efficient and more failure-resistant materials. Beta-type titanium-molybdenum and oxygen-modified beta-type titanium-chronium alloys have demonstrated promising Young’s Moduli, high bending strength, and high tensile strength [
Pedicle screws are frequently used in spinal surgeries. Pedicle screw and rod posterior spinal fusion has become the clinical standard for the treatment of scoliosis since the pedicle screws can redirect force through the powerful vertebral bodies [
While pedicle screws provide some of the best correctional results in spinal surgery, some of the most common complications of pedicle screw placement include loosening, pullout, and screw breakage that can severely affect the bone healing process [
A study that compared HA, CaP, and PMMA-BC showed that there was no significant difference between the controls versus HA or CaP (pHA = 0.6 and pCaP= 0.234); however, the pullout strength differences between PMMA-BC and control were significantly different (p= 0.026) [
New developments in pedicle screws have not focused much on the Ti6Al4V screw itself, but rather novel coatings and/or new cement on the Ti6Al4V screw to improve fixation and pullout strength. Lotz et al. [
In addition to pedicle screw biomaterial properties, technological advancements in screw design have recently occurred. The first pedicle screws were monoaxial and although provided good vertebral stabilization and correction, had difficulty adequately seating the rod in the screw [
Problems associated with pedicle screws include loss of fixation, improper placement, fatigue and bending failure, dural tears, cerebral spinal fluid leaks, nerve root injury, and infection [
Spinal plates are a pinnacle implant to stabilize and restore normal alignment to the spine. Biomechanical experiments for stress-strain analyses performed from T12–L4 and suggested that a titanium-plate fixation and laminectomy was more stable than just laminectomy or just a plate (p< 0.05) [
Mini-plate systems are extensively applied to cervical laminoplasty for patients with multilevel cervical compressive myelopathy and to secure the posterior elements in the open position after an expansive open-door laminoplasty [
Total disc replacement (TDR) has yielded comparable or superior outcomes compared to lumbar fusion over a 2-year time period, preserving functional movement unlike with spinal fusion [
The ProDisc-L implants are made of a cobalt-chromium-molybdenum with an ultra-high molecular weight polyethylene combined with a rough titanium surface coating to promote bone growth alloy [
The ApiFix system is a new, less-invasive fusionless scoliosis correction system that connects 2 periapical pedicle screws through polyaxial mobile ball-and-socket joints with a rod [
The ProDisc-C implants are made of a cobalt-chromium-molybdenum with an ultra-high molecular weight polyethylene combined with a rough titanium surface coating to promote bone growth alloy [
Another recent development is the Mobi-C cervical disc prothesis. The Mobi-C disc has 3 parts: 2 metal plates (typically made out of cobalt, chromium and molybdenum) covered with a HA coating (to improve bone grafting) and a plastic plate (made from polyethylene) at the center [
Three-dimensional (3D) printing is the most frequently utilized in preoperative planning stages, printing templates out of plastic—including but not limited to acrylonitrile butadiene styrene, acrylate resin, acrylate resin, polyamide photosensitive resin, titanium, and polylactic acid [
Arguably the most exciting application of 3D printing is the ability of surgeons to create specifically-designed implants for each patient. So far, a majority of the implants have been made out of titanium (TiV6Al4) due to its biocompatibility for applications ranging from C1/2 posterior fixation devices to Sacrum replacements [
Spinal implants have come a long way from the original 1890s silver wire fusion methods. Now, the standard material is either PEEK or titanium. New research is being conducted to find materials with increased bone grafting properties (by doping existing materials or developing new materials), improving strength/ Young’s modulus, and developing novel ideas to prevent further postoperative complications by improving range of motion, decreasing pain, distributing anatomic forces to decrease adjacent segment disease, and minimizing the necessity for additional spinal surgery. New developments in biomaterials for spinal implants and the advent of new technologies, like 3D printed patient-specific implants, have made incredible progress in biocompatibility of spinal tools. Spine surgeons should remain vigilant regarding the current literature and technological advancements in spinal materials and procedures.
The authors have nothing to disclose.
Young’s modulus of common biomaterials.
Common biomaterials used in spine surgery
Implant | Implementation procedure | Standard materials | Upcoming materials |
---|---|---|---|
Cage | Titanium | Bioactive glass | |
PEEK | Silicon nitride | ||
Ceramic | Apatite-Wollastonite | ||
Acrylic | poly(ε-caprolactone)+HA (biodegradable) | ||
Screws | Titanium (Ti6Al4V) doped with: | Carbonated apatite | |
HA | |||
CaP | |||
ECM | |||
Tantalum | |||
Rods | Titanium | Ti-Mo | |
CoCr | Oxygen-modified beta-type Ti-Cr | ||
PEEK | Biodegradable materials | ||
Stainless steel | |||
Nitinol | |||
Plates | Spinal stabilization | Titanium | Biodegradable materials |
DDD, degenerative ddisc disease; PEEK, polyetheretherketone; Ti6Al4V, titanium-aluminum-vanadium; HA, hydroxyapatite; CaP, calcium phosphate; ECM, extracellular matrix; CoCr, cobalt-chromium alloys; Ti-Mo, titanium-molybdenum; Ti-Cr, titanium-chronium.
Characteristics of biomaterials
Materials | Advantages | Disadvantages | Application |
---|---|---|---|
Stainless steel | Very strong | Corrosion | Scoliosis correction (rods) |
Very stiff | Relatively poor biocompatibility | Formerly used in screws; now mostly replaced by titanium | |
Easily doped/alloyed to be stronger | High artifacts in imaging | ||
Inexpensive | |||
Titanium | Lightweight | Relatively Expensive | Screws |
Strong | Some artifacts during imaging | Rods | |
Flexible | Plates | ||
Biocompatible | Cages | ||
Easily doped/alloyed to be stronger | |||
PEEK | Lightweight | Low Young’s modulus | Rods |
Flexible | Some grafting issues, but improved with coatings | Cages | |
Relatively Inexpensive | Disc replacement | ||
Biocompatible | |||
Easily doped/coated for improved grafting | |||
Low artifacts on imaging | |||
CoCr | Strong | Relatively expensive | Adolescent scoliosis correction (rods) to provide a more flexible buttress for the spine to curve about. |
Flexible | High artifacts on imaging | ||
Biocompatible | |||
Ceramic | Relatively inexpensive | Brittle | Used in cage biomaterials |
Biocompatible | Grafting issues, but can be improved with coating/doping | Doped with A/W | |
Ware resistant | |||
Easily doped | |||
Nitinol | Strong | Relatively expensive | Not frequently used, but can be implemented for young scoliosis correctional surgery. |
“Memory metal” (shape recovery) | Sometimes not stiff enough for proper correction | ||
Tantalum | High frictional characteristics | Very expensive | Not frequently used due to its price. |
Low Young’s modulus | Not stuff enough for some spinal corrections. | Has primarily been phased out completely by titanium. |
PEEK, polyetheretherketone; CoCr, cobalt-chromium alloys; A/W, Apatite-Wollastonite.