How Are PEEK Screws Used?

PEEK screws are replacing stainless steel and titanium surgical screws in many instances, and they can be found in various orthopedic and trauma fixation applications. Screws can be used to directly interface with bone, or they can be used in conjunction with other trauma fixation and orthopedic devices. For example, PEEK screws can be installed in bone plates to hold a fracture together. They can also be implanted in cortical or cancellous bone, so PEEK’s versatility is a valuable trait in this regard.

Why are PEEK screws emerging as a frontline option?

Stainless steel and titanium have traditionally been the favored materials for surgical screws. In recent years, the introduction of resorbable materials and high performance polymers like PEEK have given surgical teams additional options. Here’s why those surgical teams are increasingly turning to PEEK screws for their patients:

  1. Biocompatibility and bioinertness – PEEK’s biocompatibility has been confirmed through USP Class VI testing and through extensive patient reports. In 20 years of medical use, PEEK hasn’t demonstrated any toxic or allergic properties, so it can be trusted for long-term or permanent implantation.

    PEEK is bioinert in ways that metal implants may not be. For instance, patients can develop sensitivity to certain metals after prolonged exposure, but this hasn’t been seen with PEEK. Further, when PEEK is used as a bearing material, it does not liberate extremely small, potentially toxic particles like some metal implants have done in the past.

    In addition to its biocompatibility, PEEK also resists attack in an organic or aqueous setting, so it will not corrode and it will not absorb minimal moisture. PEEK screws can be implanted anywhere in the body and remain intact for years.

  2. Excellent material properties – PEEK is a high performance polymer, so it possesses an elite array of properties, several of them important for trauma fixation and orthopedic applications. High fatigue strength and an ideal flexural modulus are the standout traits, as they approximate cortical bone well. In short, PEEK ‘micro flexes, compresses and bears weight like bone, so it behaves more like a load-sharing material than a load-bearing one.

    This load-sharing capacity is especially important for trauma fixation and orthopedic procedures. Because it shares the load, PEEK does not cause stress shielding, a common complication among metal, weight-bearing implants. Research published in the European Spine Journal has shown that titanium implants cause bone subsidence (caving in) at rates that far exceed PEEK implants.

    PEEK’s fatigue strength is what makes it an ideal screw, as it can withstand tension and compressive forces without losing shape or integrity. The result is a screw that’s better designed for dynamic areas of the body, like the legs, arms, hands and feet.

    Since PEEK screws can be installed in cortical bone or softer cancellous bone, they can be used in additional hip and knee arthroscopic procedures.

  3. Radiolucency – In its natural state, PEEK is radiolucent, so it will not interfere with any form of medical imaging, whether X-ray, MRI or CT scan. This will allow surgical teams to make more accurate, timely assessments, especially where it concerns osteosynthesis.

    If radiolucency, or imaging transparency is undesirable, additives like barium sulfate can be incorporated into PEEK to provide radiopacity. The addition of barium sulfate does not adversely affect the PEEK, so it can be added without fear of affecting the implant’s function or durability. In either case, PEEK provides an imaging advantage.

  4. Processability – PEEK’s extensive processability may not be an obvious advantage to patients or physicians, but it’s still a critical consideration. PEEK is a thermoplastic, so it is converted at extremely high temperatures and while using special equipment. Experienced converters use a variety of methods, like extrusion and machining, to turn PEEK into an array of useful medical components.

    PEEK is almost endlessly processable when machined, as the polymer retains its strong material properties even when subjected to machining stresses. It is essential, however, that an experienced PEEK converter handle machining, as issues with fiber orientation, for example, may compromise the polymer’s function.

    For surgical teams, PEEK’s processability allows for a variety of screw sizes and designs, so it’s easier to produce and find screws for a particular patient or implant. PEEK can also be machined to extremely tight tolerances, making a custom fit possible.

  5. Easier to adjust – Although infrequent, trauma fixation or orthopedic implants may need to be adjusted after implantation. Some PEEK screw systems have been designed with this potential scenario in mind, and they are easier to remove than stainless steel or titanium screws.

In many areas of medicine, PEEK is so successful that it is pushing other biomaterials into lesser roles. That is also true of fixation hardware like screws, where PEEK’s fatigue strength, pullout strength and all-round durability are valuable properties to have. PEEK’s unmatched processability also ensures component manufacturers can develop a full spectrum of screw and screw system designs.

Advantages of a PEEK Cage vs Titanium Cage

PEEK and titanium are the primary biomaterials used in fusion cages, but PEEK has a few decisive advantages over titanium. Recently published research also suggests that the high-performance polymer is a better fit for fusion procedures.

Though PEEK is one of the newer biomaterials, it has quickly emerged as a top choice in several spinal fusion procedures, including:

  • Anterior cervical discectomy and fusion, or ACDF.
  • Anterior lumbar interbody fusion, or ALIF.
  • Posterior lumbar interbody fusion, or PLIF.

PEEK is also being considered for other spinal devices, including artificial discs. This makes sense, because PEEK is well-suited to handle the physical demands placed on the spine.

PEEK vs. Titanium: Fusion Rates and Subsidence

Studies comparing titanium and PEEK focus on fusion rates and subsidence. Fusion refers to the implant’s capacity to osseointegrate with native bone and subsidence refers to caving in of nearby bone.

Subsidence occurs as a result of stress shielding, and stress shielding occurs when the bone is no longer stimulated by weight bearing forces. Without this stress, affected bone tissue drops in mineral density, compromising its structure to an extent. A loss of bone mineral density could lead to a higher risk of fractures, as has been noted in studies with osteoporosis patients.

PEEK and titanium cages are similar regarding fusion rates, and the research confirms that there isn’t a statistically significant difference between the two. However, there is a significant difference in subsidence rates.

According to a 2013 study published in the European Spine Journal, titanium cages were associated with much higher rates of subsidence (over 30 percent) compared to PEEK (less than 10 percent). Additional studies, including a 2017 study published in the Journal of Clinical Neuroscience, reinforce this research and have found the same thing.

Subsidence remains a significant concern with titanium implants, but PEEK is much more promising in this area due to its bone-like flexural modulus. PEEK, in its unfilled state, possesses a similar flexural modulus to cortical bone, so it bends and bears weight like the body’s own tissues. This is how PEEK is able to avoid stress shielding, and how the polymer encourages more effective healing in native bone.

PEEK vs. Titanium: Radiolucency

PEEK’s flexural modulus is one of its primary advantages, but it’s not the only one. PEEK, again in its unfilled state, offers pure radiolucency. It is completely invisible on MRIs, CT scans and X-rays, so surgical teams can easily monitor post-operative progress and confirm that osseointegration is taking place. If there is a chance of complications emerging, PEEK’s pure radiolucency allows surgical teams to catch them sooner.

PEEK’s pure radiolucency can be modified with the addition of additives like barium sulfate. When mixed in, barium sulfate adds image contrast to the polymer, without affecting PEEK’s properties.

Titanium, unsurprisingly, doesn’t offer the same radiolucency that PEEK does. Titanium, like other metals, is an image-scattering material and creates significant artifacts. This can interfere with attempts to assess the implant’s position and its fusion progress.

PEEK vs. Titanium: Processability

Both PEEK and titanium cages are manufactured using modern CAM processes, and both to excellent tolerances. Titanium, though, poses difficulties during the machining process as it possesses low thermal conductivity. Both expose machining tools to additional wear, which can make the metal more expensive and more challenging to work with.

In the hands of a skilled converter, though, PEEK can be efficiently machined without compromising its properties. The key word here is “skilled,” because PEEK is vulnerable to subtle issues like fiber orientation. A converter experienced with PEEK will be equipped to sidestep these potential obstacles.

In addition to machining, PEEK can be converted using injection molding and extrusion processes. Medical processing facilities can maximize processing economy by opting for large runs of injected PEEK components. Via extrusion, PEEK can also be converted into durable stretches of medical tubing.

PEEK vs. Titanium: The Future

PEEK is a newer biomaterial, so there is more potential to unlock with the polymer. There are already several advanced PEEK cages on the market, mixed with materials that encourage bone growth and osseointegration. Zeolite and hydroxyapatite are two such materials, and initial research confirms that these new PEEK implants are achieving superior bone-in growth.

New PEEK implants are also designed with microporous structures, and these encourage native bone to grow into the implant, with a lock-like fit. Though there are similar initiatives targeted at titanium, PEEK’s superior processability likely means that there is more room to improve upon the polymer.

In the battle of biomaterials, PEEK offers a superior modulus, better radiolucency and a wider range of processing options. These advantages explain why PEEK is the first choice in spinal fusion procedures, and why it is an important biomaterial for several medical fields.

What Medical Devices Can Be Made From PEEK?

PEEK’s superior material properties make it a fit for many areas of medicine, including spinal fusion, dentistry, trauma fixation and cardiovascular medicine, among other fields. This high-performance polymer has been used in medicine for over 20 years, and there are still exciting developments to come for PEEK medical devices.

In the present, there are many medical devices where PEEK serves as a frontline material choice, including:

  1. Interbody fusion cages –

    Interbody fusion cages are used in spinal fusion applications to provide additional stability thereby helping to relieve pain. Spinal fusion is indicated in patients suffering from degenerative disc disease, spondylolisthesis, spinal stenosis, or damage due to fractures or tumors.

    During spinal fusion, the problematic disc is removed from the body and replaced with a bone graft and interbody fusion cage. The cage provides an ideal environment for the bone graft to fuse the two vertebrae together, ensuring predictable, stable growth. The fusion cage becomes a permanent part of the spine, so it must be manufactured from biocompatible materials.

    PEEK provides that total biocompatibility and offers several more advantages. For example, PEEK’s flexural modulus is similar to cortical bone, so it shares weight and handles force like bone. This makes PEEK an ideal bone replacement material, because it will not bear too much weight and cause subsidence. Subsidence is a noted issue with titanium implants for this reason, and PEEK’s subsidence rates are much lower, according to multiple research studies.

    PEEK’s pure radiolucency is also useful in spinal fusion applications, as it allows surgical teams to assess the patient without the implant interfering with medical imaging. This includes CT, MRI and X-ray scans, so PEEK can be used with a variety of imaging technologies.

  2. Dentistry –

    PEEK is a frontline choice for some dental applications, including dental implants and removable partial dentures. PEEK’s biocompatibility means it can be implanted in the jaw or used in the oral cavity without causing issues.

    The rise of CAD/CAM dentistry has been especially beneficial for PEEK, as CAD/CAM dentistry takes precise 3D models of a patient’s mouth and machines a device that fits that patient perfectly. As PEEK tolerates machining extremely well, it is a good fit for a CAD/CAM approach.

    PEEK partial dentures are prized for their excellent aesthetics and comfort. Aesthetically, PEEK can be color matched to surrounding tissues, which is something that can’t be done with other dental materials. This means PEEK devices are inconspicuous, even when the patient’s mouth is open. PEEK partial dentures are also comfortable, as they are lightweight and shaped to fit the patient precisely. Also, PEEK does not alter the patient’s taste and does not irritate tissues, so it doesn’t interfere with the patient’s ability to eat or talk.

  3. Cardiovascular devices –

    PEEK is also a top choice for several cardiovascular devices, including medical tubing. PEEK tubing can be used to deliver cardiovascular devices like replacement valves and stents. It is also incorporated into defibrillators and ablation catheters.

    What makes PEEK an ideal tubing material is its low coefficient of friction and flexibility. Together, these properties make PEEK well-suited for the cardiovascular network, and the polymer offers strong pushability and navigation. Pushability refers to the amount of force required to advance the tubing to the treatment site, while navigation refers to how well the material can move through nonlinear segments of the cardiovascular network. These two traits are usually at odds, because improving one usually adversely affects the other. Fortunately, PEEK tubing provides an optimal mix of both, so it can be used in most locations of the vascular network.

    PEEK’s electrical properties also make it a good fit for ablation catheters, defibrillators and any device that works by delivering precise electrical discharges. PEEK can help control these discharges, reducing risk of electrocution and burn injuries to the patient.

    The polymer is also relied on in some emerging cardiovascular procedures. One of these is the Less Invasive Ventricular Enhancement (LIVE) procedure, which is indicated in patients who have suffered heart damage, usually caused by a heart attack. During the LIVE procedure, a pair of anchors are positioned over the scar tissue and held together by a PEEK tether. The anchors move scar tissue out of the way in order to restore ventricular function, and the tether keeps the anchors properly aligned. The LIVE procedure is a safer alternative to previous forms of ventricular enhancement, which required several incisions to the heart.

  4. Trauma fixation –

    Among biomaterials, PEEK possesses elite mechanical properties, making it a strong choice for bone plates and screw systems. PEEK’s cortical bone-like modulus, resiliency and pull-out strength are particularly important for trauma fixation applications. In short, PEEK can handle the compressive and tension forces applied to trauma fixation devices, resist failure and prevent subsidence with appropriate load-sharing abilities.

Many medical devices are made from PEEK, and some of these devices, like interbody fusion cages, are being improved upon constantly. New fusion cages, for example, are being developed with materials that attract bone growth, like hydroxyapatite and zeolite.

With its existing, impressive capabilities and future potential, PEEK has established itself as a critical biomaterial, and one that will only become more essential with time.

FAQ

Why are PEEK implants replacing titanium implants?

PEEK implants have a flexural modulus that’s similar to cortical bone, so they are less likely to cause stress shielding, and by extension, subsidence. Subsidence refers to settling, or caving in, of bone. Titanium implants are more likely to cause subsidence because titanium bears weight instead of sharing it.

What additives can be mixed with PEEK medical devices?

PEEK can be augmented with chopped carbon fibers (CFR PEEK), and this provides additional strength and stiffness to the polymer. Barium sulfate can also be added to PEEK to make it more radiopaque for applications where it would be helpful to see the implant on medical imaging.

What makes PEEK a good choice for cardiovascular tubing?

PEEK can be converted into extremely small cross sections and is a low-friction material. This makes it an ideal choice for the cardiovascular network, which is nonlinear in design. PEEK tubing offers excellent pushability and navigation, so it can be steered through vascular branches with little difficulty.

The Polymer Engineering and Science Behind PEEK

For PEEK manufacturers, it’s the physics and materials properties behind the polymer that are most relevant. PEEK’s chemistry is also noteworthy because its chemical structure is what makes the polymer so useful. PEEK is a high-performance thermoplastic, so it possesses an elite range of material properties and is processable using heat.

What is a thermoplastic?

A thermoplastic is a polymer that retains its plastic properties at standard temperatures, but can be melted at elevated temperatures. When the polymer is returned to its solid, plastic state, it retains its material properties. This is particularly useful for PEEK manufacturers because it means the polymer can be converted into an array of components without compromising its excellent durability and strength.

Thermoplastics are versatile during the conversion process. For example, PEEK is commonly used in the injection and compression molding processes to manufacture large batches of identical components. PEEK can also be machined using CAM technology to create components that adhere to extremely tight tolerances. PEEK can even be extruded to produce medical tubing or used with film calendaring to produce special, atomically-small films.

In medicine, PEEK is converted using injection molding when manufacturing surgical instruments and laboratory components. This is a cost-effective approach that helps medical facilities control infections with single-use instruments. In vivo and implantable PEEK components, though, are primarily converted via machining, which offers better tolerances, smaller production campaigns, and intricate component designs.

What is a semicrystalline thermoplastic?

There are two primary categories of thermoplastics, which are either amorphous thermoplastics or semicrystalline thermoplastics. In practical terms, all polymers sit between perfectly crystalline and amorphous states, but those with a greater degree of crystallinity are called semicrystalline polymers. Both terms refer to the polymer’s molecular structure, and hint at the kind of properties the polymer is likely to exhibit. Here’s a quick look at what amorphous and crystalline mean in a polymer science context:

  • Amorphous – Amorphous polymers are organized randomly with molecular chains. With little underlying structure, amorphous polymers tend to melt at lower temperatures, though they perform well at lower temperatures. Amorphous polymers are more flexible than crystalline polymers, but they are more likely to crack under stress and possess poor fatigue strength. Amorphous polymers are also usually transparent. Examples of amorphous polymers include polystyrene, polycarbonate, ABS, polyetherimide and polysulfone.
  • Crystalline – Crystalline polymers exhibit structure at the molecular level, most often in small pockets referred to spherulites. In other words, there is significant crystal organization in spots, as well as amorphous areas between the spherulites. This partial crystal structure is what gives PEEK and other semi-crystalline thermoplastics their excellent thermal and fatigue resistance. Because crystalline structures are formed out of more durable bonds, the polymer melts consistently and is tougher once converted into components. Semicrystalline thermoplastics tend to have enhanced bearing and wear properties. Examples of polymers with a high degree of crystallinity include poly-ether-ether-ketone (PEEK), poly-amide (nylon), polyethylene terephthalate (PET), polybutylene terephthalate (PBT) and polytetrafluoroethylene (PTFE).

What are PEEK’s material properties?

PEEK stands out among other polymers with its excellent material properties. It’s these superior properties that have made the polymer a frontline choice in many aerospace, automotive, oil & gas and medical applications. PEEK’s role in medicine has expanded greatly in the last 20 years, and it is now featured in several medical fields, including spinal fusion, orthopedics, cardiovascular medicine, trauma fixation, dentistry and prosthetics, among others.

What has driven this surge in PEEK’s popularity? Several reasons, including:

  1. An ideal flexural modulus – PEEK can be mixed with several additives, but in its unfilled state, PEEK has a flexural modulus similar to cortical bone. Because it bends and handles weight like bone, it can stand in for bone in several applications. This is most important in spinal fusion, orthopedic and trauma fixation procedures, where the implant must not provoke stress shielding in nearby bone. Stress shielding occurs when the implant bears too much weight and robs native bone of the stimulating stresses it needs to heal and grow properly. Stress shielding is a problem with metal implants and can cause nearby bone to drop in mineral density and suffer from subsidence, or caving in.
  2. Pure radiolucency – In its unfilled state, PEEK is completely invisible on medical imaging, including CT scans, MRIs and X-rays. This pure radiolucency is essential in spinal fusion applications and any applications where accurate imaging is critical. Because PEEK does not interfere with medical imaging, surgical teams can better monitor the implant’s positioning and potential for osseointegration.
  3. Total biocompatibility All implantable biomaterials, PEEK included, must undergo the most demanding safety tests available to ensure the material is safe. PEEK has passed through these tests with no concerns, demonstrating zero cytotoxicity, genotoxicity and immunogenic potential. After 20 years of use in patients, PEEK still hasn’t demonstrated any concerns that would threaten its status as a primary biomaterial.
  4. Modifiability – PEEK is plenty capable in its natural state, but it can be mixed with other materials to augment its properties. For example, with the addition of chopped carbon (CFR PEEK), the polymer is stiffer and stronger, so it can be used in applications where additional weight bearing capabilities are needed. PEEK’s radiolucency can also be modified with the addition of barium sulfate. This adds a degree of image contrast without affecting the polymer’s properties.

The polymer engineering and science behind PEEK is astounding, but you don’t have to be a scientist to understand what makes the polymer so useful. With its favorable material and mechanical characteristics, PEEK is at the peak of polymer engineering.

What is PEEK’s Thermal Conductivity?

PEEK is a thermal isolator, so it does not readily conduct heat into nearby materials. It’s also an ideal electrical isolator, and together, these properties make it useful in several medical applications. These applications include cardiovascular devices and surgical tools, where heat and electricity must be controlled for patient comfort and safety.

Some examples include:

  • Ablation catheters
  • Implantable automatic defibrillators
  • Surgical tools designed for electrosurgical procedures

Thermal and electrical isolation may not seem like a primary concern in medicine, but the above devices and tools are used with thousands of patients every year. PEEK’s ability to isolate thermal energy and electrical discharges make it a frontline choice in the above devices and instruments.

PEEK’s Role in Cardiovascular Medicine

The heart runs on electricity, and precise impulses are needed to keep it functioning properly. PEEK’s role as an isolator ensures that these electrical impulses are controlled and that patients are protected from harmful discharges.

For example, ablation catheters are used in patients suffering from heart arrhythmias, specifically to scar cardiac tissue that is contributing to the arrhythmia. Whenever electricity is applied to the heart, it must be done with extreme caution to avoid accidentally burning or scarring healthy tissue. PEEK is a common choice in ablation catheters for this reason, as it can isolate any stray discharges and prevent them from affecting the patient.

For the same reasons, implantable automatic defibrillators must also be sheathed in materials that isolate electrical discharges. PEEK is an ideal choice for this purpose.

PEEK’s Role in Electrosurgery

Electrosurgery is utilized in nearly every field of medicine, as it is an effective means of stopping bleeding and ablating tissues. Electrosurgical instruments work by using an electrical current to generate heat. While in use, these instruments can precisely cauterize and scar tissues, much like ablation catheters.

Like ablation catheters, electrosurgical instruments are frequently built with PEEK, due to its lack of thermal and electrical conductivity. PEEK is especially important in electrosurgery as the surgical environment is conducive to conductivity, with the presence of electrical equipment and body fluids.

A Thermal Isolator and Thermoplastic

PEEK may be an ideal thermal and electrical isolator, but it is still melt processable. As a high performance thermoplastic, PEEK doesn’t melt until it hits about 650 degrees Fahrenheit (343 degrees Celsius), but once it does, it can be processed through one of several methods.

For example, PEEK can be extruded once heated. Extruded PEEK is particularly useful as medical tubing in cardiovascular applications. That’s because PEEK possesses a perfect mix of flexibility and a low-friction surface, so it can be readily steered through the vascular network.

The high performance polymer can be injection molded into an array of components as well. Injection molding is used with implantable devices, but not to the extent that it is used for producing components intended for surgical instruments. These components can be of almost any size, shape or design, as PEEK maintains its excellent material properties through processing.

Why consider PEEK for surgical instruments?

PEEK is used in a large number of surgical instruments, for reasons that go beyond its thermal and electrical conductivity properties. PEEK also offers the following advantages:

  • Versatility – Since PEEK can be converted through various means, it can be adapted for use with a variety of instrument designs. This includes electrosurgical instruments and unpowered instruments.
  • Durability – PEEK is extremely resistant to nearly all forms of damage. It possesses strong impact strength, compression strength, tensile strength and it is impervious to corrosion. PEEK also resists water absorption and can withstand repeated autoclaving without being damaged.
  • Cleanliness – Hospitals have an infection problem on their hands, as there are about 1.7 million instances of hospital-acquired infections (HAIs) every year. These result in close to 100,000 deaths, so the problem is far from trivial. PEEK provides a solution here, too. Since PEEK can be injection molded during large production runs, it is a clean and economical option for single-use instruments. Single-use instruments eliminate the likelihood of infection altogether, and in some cases (like with medical tubing), they may be the only option in preventing infection. PEEK is also highly chemical resistant and will withstand exposure to most hospital cleaning regimes.
  • Comfort – Patient comfort is paramount, but surgeon comfort also needs consideration. Again, PEEK solves a problem because the polymer weighs much less than metal biomaterials. With its lightweight nature, PEEK reduces strain on the surgeon’s fingers, hand and shoulder, and ensures they can control the instrument for longer. PEEK’s versatility also lends itself to any ergonomic-focused design, and it can be converted to fit any hand with maximum comfort.

PEEK’s intriguing thermal conductivity is just one of its many material properties, and one that makes it suitable for an array of medical applications. With its ability to isolate electrical and thermal energy, PEEK has a place in every surgical room, whether in the form of medical tubing, medical equipment or medical instruments.