Introduction
The spine is the most prevalent metastasis site, and spinal metastases are associated with quality-of-life reduction [1–3]. The incidence of metastatic spinal tumors has been increasing due to a growing aging population as well as survival rate improvements in patients with cancer [3]. Further, this comes with increased patient mortality secondary to complications, including cord compression, spinal instability, pathological fractures, and pain [4]. Spinal metastasis management has evolved in recent years, with Patchell et al. [5] demonstrating the superiority of decompressive surgery combined with radiotherapy (RT) over RT alone, thereby increasing the surgical treatment of spinal metastases.
Titanium implants have been the established implant material for spinal instrumentation. They exhibit a high strength-to-weight ratio, fatigue strength, and lower density and modulus of elasticity [6,7]. However, they are not ideal for metastatic spine tumor surgery (MSTS) due to their propensity to generate artifacts in postoperative magnetic resonance imaging (MRI) or computed tomography (CT) scans. This complicates tumor recurrence assessment and early postoperative complication detection, such as infections, and interferes with adjuvant RT planning. Improvements have been observed in the metal suppression techniques for CT and MRI; however, varying degrees of artifact generation remain inevitable. Furthermore, titanium implants exhibit scattering effects that impact appropriate dose delivery to the target area during RT [8–10].
Considering these limitations, carbon fiber-reinforced polyetheretherketone (CFR-PEEK) has been considered a promising alternative to titanium in MSTS [11]. Their radiolucency and nonmagnetic properties reduce distortions in postoperative CT and MRI scans, thereby improving the accuracy of dose calculations for postoperative spinal tumor irradiation. Further, these implants minimize radiation scattering and tumor shielding effects, thereby enabling adequate dose delivery to the tumor bed [11,12]. The literature indicated that CFR-PEEK implants demonstrate comparable bending stiffness, loading strength, and yield compared with traditional titanium implants [13–16]. Clinical studies investigating CFR-PEEK instrumentation have also revealed that postoperative outcomes are comparable to those of titanium implants [17]. Local recurrence is a reality with improved survival of patients undergoing MSTS due to enhanced treatment in the form of chemotherapy, RT, immunotherapy, and targeted therapies; thus, early detection of tumor recurrence is crucial. CFR-PEEK implants facilitate this by enabling clearer spine and neural structure visualization for timely intervention [18].
Despite the aforementioned advantages, CFR-PEEK implants have yet to gain widespread popularity, although they were first described in the early 2000s. This can be due to high barriers to entry in using CFR-PEEK as well as unfamiliarity with CFR-PEEK implant application. Most spine surgeons are trained and more familiar with titanium instrumentation, and they may not anticipate the challenges posed by CFR-PEEK implants due to their novelty. This unfamiliarity caused dissatisfaction and reluctance in the regular use of CFR-PEEK implants. However, with an increase in the number of studies highlighting the potential benefits of CFR-PEEK implants in MSTS, the growing role of CFR-PEEK implants in MSTS surgeons’ armamentarium is not to be understated.
CFR-PEEK implant usage is not without challenges, and to the best of our knowledge, independent studies that specifically address the technical considerations and potential pitfalls associated with applying CFR-PEEK instrumentation in MSTS remain currently unavailable. In this review, we aim to determine various surgical nuances and technical differences associated with these implants, enabling MSTS surgeons to anticipate and tackle potential challenges related to CFR-PEEK implant usage. We anticipate that, with reduced barriers to entry and improved evidence for CFR-PEEK implants, the use of these implants will increase among MSTS surgeons globally.
Methods
A narrative review was conducted using PubMed, Medical Literature Analysis and Retrieval System Online, The Cochrane Library, and Scopus through December 31, 2024. The keywords used were (“Carbon fiber PEEK” OR “CFR-PEEK” OR “PEEK” OR “Polyether ether ketone”) AND (“Spine”).
The senior authors of the paper established the inclusion and exclusion criteria of the studies selected. Inclusion criteria were studies that reported on CFR-PEEK usage as a spine implant in metastatic spinal disease (MSD). The exclusion criteria were studies with no full text, not in English, and duplicates throughout the various manuscript resources.
The articles were selected in two stages (Fig. 1). In the first stage, the abstracts determined by the above searches were downloaded and the list was screened using the inclusion and exclusion criteria. In the second stage, the full texts of the articles listed were downloaded and assessed for eligibility. The reference lists of publications were then hand-searched for additional relevant studies. Two of our senior authors independently repeated this process twice.
Additional information in our manuscript comes from the personal experience of the senior authors, who are spine surgeons specializing in MSTS and have applied CFR-PEEK screws and rods in patients with MSD. The CFR-PEEK instrumentation system used by our senior authors included ICOTEC BlackArmor implants (ICOTEC, Altstätten, Switzerland).
Discussion
Surgery with adjuvant RT is reportedly the primary treatment in the majority of patients with MSD [19]. Traditional titanium implants generally create artifacts that critically alter the use of postoperative imaging despite the concomitant application of artifact suppression techniques. In contrast, CFR-PEEK implants significantly reduce artifacts on postoperative imaging due to their “low-Z” characteristics [20]. This enables a clearer visualization of the tumor bed, allowing early detection of recurrence or postoperative complications such as infection. Further, it enables adequate dose calculation for RT by delineating the tumor tissue from the surrounding healthy tissue [21].
Moreover, metal implants caused the scattering of the RT particles, resulting in poorer effective treatment doses regardless of the RT modality used. Artifact generation, causing obscuration of the target tissue, makes it difficult to avoid surrounding normal structures and causes potential toxicity to the normal tissue surrounding the tumor bed [20,21].
CFR-PEEK implants have been available for almost a decade; however, they only recently started to gain popularity for use in MSTS. Most spine surgeons, including the authors, have received their training using titanium implants and are well-versed with it. However, as a non-metallic screw-rod system, the CFR-PEEK implants come with their own set of challenges, which are difficult to anticipate beforehand, unless the surgeon has had ample previous experience with them. The various points when undertaking the CFR-PEEK instrumentation are categorized into preoperative and intraoperative considerations (Table 1).
Preoperative considerations
Extent of the instrumentation
The CFR-PEEK screws currently come in 5.5-, 6.5-, and 7.5-mm diameters. In most Asian spines, the average diameter of the pedicles is approximately 4.5 mm [22]. Hence, proper preplanning is required regarding the extent of instrumentation required to be undertaken in MSTS [23]. Preoperative CT will be useful to plan the diameter of the pedicles for instrumentation levels. Hybrid fixation constructs are important if instrumentation is to be planned to extend to the sacrum (i.e., requiring larger diameter pedicle screws) or the cervical spine (i.e., requiring smaller diameter pedicle screws) (Figs. 2, 3).
Tumor characteristics
A mechanical limitation of CFR-PEEK screws is their lower torsional stress resistance, making them more susceptible to failure under rotational forces. CFR-PEEK is a fiber-reinforced composite that demonstrates anisotropic mechanical properties [24], indicating its various strengths depending on the direction of the applied force. Hence, increased torsional loading places high shear stress on the screw’s fibers and matrix interface, which causes microfracture, delamination, or catastrophic failure. Therefore, tapping the pilot hole for the screw is crucial in reducing the torsional stresses on the CFR-PEEK screws during insertion. Unlike self-tapping titanium screws, which withstand high insertional torque, CFR-PEEK screws exhibit a lower shear strength threshold, making them more vulnerable to damage during excessive torque application.
Thus, accurate pretapping is required to help create a controlled thread path in the bone, thereby reducing the frictional resistance and lowering the torque required for insertion. Without adequate tapping, the CFR-PEEK screws may experience excessive torsional stress during insertion, resulting in localized fiber-matrix failure, which weakens the overall structural integrity of the screw. Moreover, insufficient tapping causes uneven load distribution along the screw threads, thereby increasing the stress concentrations and accelerating the failure under cyclic loading conditions.
Therefore, CFR-PEEK screws are not the best material for sclerotic tumors due to the difficulty in tapping into sclerotic bones. As mentioned above, insufficient tapping may accelerate the torque failure of the screw. Hence, preoperative CT scans will be crucial for MSTS surgeons for thorough planning and anticipating complications such as these [15]. This is especially applicable in osteoblastic spinal metastases, such as prostate cancer, where a significantly increased bone mineral density causes screw breakage.
Investment in a large inventory
Currently, CFR-PEEK rods cannot be cut or contoured according to the patient’s requirement intraoperatively [25]. Therefore, they require a massive inventory for each case, with rods measuring up to 500 mm in length. Further, the rods are precontoured without the ability of the surgeon to contour them to the shape of the spine. However, notably, the contours of these prebent rods are essentially designed to meet the physiological variations of a normal spine. Most patients with pathological fracture(s) secondary to metastasis have an element of increased kyphosis of the spine [26]. This restricts their use in long constructs and complex spinal deformities, where the surgeon ends up having to utilize a hybrid construct with conventional titanium rods. The hybrid construct generates lesser artifacts than an all-titanium construct; however, it still causes significantly more distortion than a purely CFR-PEEK construct (Fig. 3) [27,28]. The requirements for a large inventory significantly increase theater traffic, thereby elevating the risk of surgical field contamination. This challenge is particularly pronounced in smaller theater setups. Care has to be considered to ensure that optimal theater setup is performed when using CFR-PEEK instrumentation.
Higher implant costs
Currently, CFR-PEEK implants still cost more than traditional titanium implants. A shared decision-making process needs to be made between surgeons, medical oncologists, and patients regarding the use of CFR-PEEK instrumentation. Recently, the literature has revealed the decreased barriers to entry for MSTS over the years, with many prognostic scoring methods understating the overall survival of patients undergoing MSTS [29]. Henceforth, the decision to apply CFR-PEEK instrumentation should be shared between patients and the treatment team, weighing the overall benefits against the higher costs as well as considering the prognosis of patients to ensure higher readmission-free survival rates [30].
Intraoperative considerations
Line-to-line tapping
CFR-PEEK pedicle screws, being radiolucent, are challenging to visualize during placement under intraoperative imaging, especially to the untrained eye. This difficulty causes uncertainty in screw positioning and potential misplacement, which may go unnoticed [31]. We urge our readers at this point to familiarize themselves with the structure of these screws. The proximal 15 mm of the screw is coated with titanium, which ideally helps in visualization. Moreover, the tip of the screw harbors a tantalum marker, designed to assist in visualizing the extent of the screw trajectory intraoperatively [32]. Familiarity with this, coupled with a keen eye, helps in detecting potential screw mispositioning intraoperatively. The principles of percutaneous screw placement can be applied with these screws by associating the position of the tantalum-coated screw tip relative to the pedicle shadow on the anteroposterior and lateral views on the image intensifier. Furthermore, centers that have the luxury of advanced technologies, such as intraoperative navigation systems that integrate preoperative CT imaging, can use them for real-time monitoring of the screw trajectory or verify the screw trajectory at the end of the surgery [28]. The placement of metal pedicle screw markers that are inserted after pedicle screw placement can help to better visualize the trajectory of the placed pedicle screw, either with lateral and anteroposterior fluoroscopy or with intraoperative navigation.
The CFR-PEEK pedicle screws are not self-tapping and are weak in torsion, due to which they require line-to-line tapping over the guidewire [33]. This can be complicated by the risk of guidewire advancement or dislodgement, thereby generating anxiousness in the surgeon and increasing the overall operative time. This process is especially challenging in the context of MSD due to the variable bone quality. Line-to-line tapping is recommended in the implant brochures; however, from personal experience, we find that the last 5–6 mm of the screw tract can be left untapped and still cause a good screw purchase. Further, this helps prevent wire dislodgement. Furthermore, we recommend using taps with fluted designs to facilitate bone debris removal and reduce the risk of bone splitting. Guidewires with threaded tips are advisable to ensure that they remain securely anchored within the anterior cortex of the vertebral body, and one should try to avoid tapping beyond the start of the threaded screw tip (Fig. 4). The need to vigilantly monitor the guidewire position to detect any early wire migration, both clinically under direct visualization and during correlation with the image intensifier, cannot be overstated. This obligation falls on both the primary and assisting surgeons. Specialized wire holders are used to secure the guidewire in place instead of relying on the hemostat, which is not designed to secure the sturdy guidewire.
Reduced haptic feedback
The material properties of the CFR-PEEK screws reduced haptic feedback compared with metallic screws. This poses a risk of a higher possibility of missing a pedicle breach during screw insertion, making surgeons more reliant on intraoperative radiography to confirm screw placement, especially in percutaneous minimally invasive spine surgery (MISS) procedures in which surgeons have to forgo visual control of screw placement. This may increase the overall operative time as well as the radiation exposure to every person in the operating theater [30]. We recommend conducting saw-bone workshops and encouraging the participation of surgical team members to become comfortable with the use of CFR-PEEK systems and to become sensitive to haptic feedback. Moreover, we advocate using equipment that is made specifically for CFR-PEEK screw insertion for better insertional control and feedback throughout the procedure. We recognize that significant effort and innovation remain to be performed in this area, and we anticipate greater progress in the future.
Bulky tulips of the CFR-PEEK screws
To mitigate stress at the screw-neck junction, CFR-PEEK screws frequently feature titanium tulips, which are bulkier (Fig. 5). This causes implant prominence, particularly in frail patients, resulting in discomfort and wound-related complications postoperatively. This is especially a challenge if it occurs after RT initiation, as the wound becomes increasingly notorious in healing [33]. Carefully assessing the patient’s anatomy preoperatively and selecting appropriate patient-specific implants are extremely important. For instance, a hybrid fixation strategy can be applied in these patients, with CFR-PEEK screws used closer to the tumor bed and titanium screws utilized as one moves further away from the tumor epicenter. This technique not only retains the advantages of the CFR-PEEK screws with clear visualization of the tumor field on postoperative imaging but also reduces the implant density of bulky tulips. Moreover, it reduces the overall operative time, as most surgeons are well acquainted and comfortable with using titanium screws, and significantly lowers the overall cost of the implants used in surgery. Before using such a hybrid technique combining different systems from different companies, the compatibility of tulips and rods of different diameters needs to be assessed preoperatively. ICOTEC CFR-PEEK rods have a diameter of 5.5 mm, whereas ICOTEC titanium alloy rods come in a diameter of 5.7 mm.
Furthermore, the surgeon needs to be aware that ICOTEC does not provide connectors aside from cross-links, in the form of end-to-end, side-to-side, and so forth, in cases where double rod constructs are planned, especially in cases of primary malignant spine tumors, which require extensive reconstruction of the anterior column. These need to come from other companies that are compatible with rod diameters of 5.5 or 5.7 mm.
The cumulative effect of the visualization difficulties, guidewire management, tapping precision, and reduced feedback increases the rate of pedicle screw misplacements (Figs. 4, 6). Further, overall intraoperative time as well as radiation exposure to the surgical personnel will increase if surgeries are performed in a freehand technique without the support of spinal navigation. We advise our readers to plan the procedure meticulously, considering all potential challenges and preparing contingency plans accordingly to prevent intraoperative confusion. This can also be addressed by streamlining the surgical workflow by conducting and participating in regular hands-on workshops to familiarize the team with the CFR-PEEK implants.
Minimally invasive spine surgery
In recent years, advancements in MSTS have resulted in the development of MISS techniques for MSD treatment. MISS has exhibited benefits in reducing mortality, morbidity, and hospital length of stay as well as enabling the earlier introduction of adjuvant RT and chemotherapy [34]. MISS is a feasible technique with CFR-PEEK instrumentation (Fig. 6), and centers have used minimally invasive CFR-PEEK instrumentation with comparable outcomes to that of titanium instrumentation [35]. However, the haptic feedback from tapping and inserting screws becomes even more crucial with MISS. MSTS surgeons have to be cognizant of possible pedicle breaches during screw insertion, utilizing familiarity with intraoperative fluoroscopic imaging. We present a case of lateral pedicle breach during the CFR-PEEK screw insertion intraoperatively (Fig. 7). The use of intraoperative O-arm checks can be considered with CFR-PEEK instrumentation to ensure proper screw placement intraoperatively and reduce the need for re-operations for patients undergoing MSTS.
Conclusions
In this review, we aim to consolidate our own experiences with CFR-PEEK implants to help our readers comprehend the distinctions between the two systems and to ensure a smooth overall operating experience. This study aimed not to favor one implant material over another but to understand the differences in working with CFR-PEEK systems, especially for those who are new to these implants. Studies have revealed that CFR-PEEK implants are comparable to titanium implants and should be utilized readily in MSTS to take advantage of their radiolucent properties.
With the publication of this manuscript, we aim to equip our readers with ample information for them to be well-prepared before CFR-PEEK implant application by highlighting various preoperative and intraoperative considerations. This will enable MSTS surgeons to fully leverage their benefits and positively affect patient outcomes after MSTS.
Key Points
Carbon-fiber-reinforced polyetheretherketone (CFR-PEEK) instrumentation has a significant role in metastatic spine tumour surgery to help benefit radiotherapy planning and monitoring of locoregional recurrance postoperatively.
Preoperative and intraoperative considerations are crucial in ensuring optimal outcomes when utilising CFR-PEEK instrumentation.
The knowledge of the current limitations of CFR-PEEK instrumentation also helps to minimise potential complications when undertaking surgery with CFR-PEEK instrumentation.
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