A PEEK into carbon fiber: A practical guide for high performance composite polymeric implants for orthopaedic oncology

Abstract

Introduction

The use of carbon fiber implants in orthopaedic oncology has increased within recent years. The most widely used type of polymer is carbon fiber polyether ether ketone (CF-PEEK). Its radiolucency enables targeted radiotherapy and artifact-free tumor surveillance, which provides major advantages over metallic hardware. We aim to summarize the unique benefits within orthopaedic oncology, clinical pitfalls, and recent advancements.

Methods

Four representative patient cases from a single tertiary academic medical center were treated with carbon fiber implants (n = 2 nails, n = 2 plates) from 2021 to 2022.

Results

There were no adverse events noted during intraoperative implantation or postoperative follow up. All patients reported improvements in pain and no difficulties in ambulation. There were no instances of catastrophic failure or implant loosening.

Conclusion

CF implants offer a diverse array of advantages regarding its radiolucency, low scatter density, and bioinert profile. Nonetheless, further research is required to understand the long-term surgical outcomes and robustness of CF implants. Multi institutional trials could address important aspects of durability and stability over extended periods, feasibility and ease-of-use for different anatomical sites and bone quality, as well as cost-effectiveness in post-operative imaging, healthcare resource utilization, and revision rates. Providing orthopaedic surgeons with valuable insight will enable thorough clinically supported, informed decision making regarding optimal use of implants.

1. Introduction

From stainless steel to cobalt chrome to titanium alloys, orthopaedic surgeons have traditionally utilized metallic hardware for reconstruction and fixation purposes. Recent advancements in material science have introduced the use of high-performance, reinforced carbon fiber for implantable medical devices. Originally intended for aeronautics and automotive applications, medical grade carbon fiber is comprised of polymer matrices that have been combined with various resins and reinforcements that offer robust mechanical durability, enhanced transfer of stress, and improved fatigue resistance.1, 2, 3, 4

Orthopaedic surgeons may elect to use carbon fiber implants for a variety of musculoskeletal oncologic indications. Most notably, medical grade CF has been used for prophylactic fixation of radiated bone following soft tissue sarcoma resection, as well as allograft reconstruction.5, 6, 7 Utilization of carbon fiber implants have shown promising applications across orthopaedic subspecialties. Trauma surgeons have incorporated CF implants as an alternative to metallic hardware due to similar union rates and minimal risk of adverse events.^8,9 CF have been successfully employed in spinal surgery for cervical cage reconstructions, and patients have exhibited surgical outcomes similar to titanium mesh cages.^10,11 Despite numerous benefits of carbon fiber compared to conventional metallic hardware, there is a pressing need to investigate long-term outcomes and durability of carbon fiber implants in large, multi-center cohorts.⁷ Our publication serves as a practical guide for orthopaedic surgeons, consolidating existing knowledge and highlighting continued advancements in research.

2. Methods

We present four representative cases of carbon fiber implants (Carbo-Fix Orthopaedic ©, Ocean Isle Beach, NC 28469) with oncologic indications. We reported on 3 males and 1 female that underwent pathologic fracture stabilization from 2021 to 2022 at a single academic tertiary medical center.

3. Case reports

We present four representative clinical indications for carbon fiber implants from within our single institution.

3.1. Case #1 – diaphyseal plate stabilization of proximal humerus

A 79-year-old male presented with an impending pathological fracture of the left humerus secondary to metastatic lung cancer. Two months earlier, he reported increasing humeral pain, and an X-ray demonstrated a growing lytic lesion involving the lateral cortex of the proximal humeral diaphysis (Fig. 1). These findings warranted surgical stabilization over concern of pathological fracture. The patient underwent tumor curettage, cement packing, and left humeral fixation with a carbon fiber nine-hole broad diaphyseal plate and titanium cortex and locking screws (Fig. 1). His postoperative course was uncomplicated, and he was discharged as weight bearing as tolerating. Imaging at two weeks confirmed no evidence of hardware loosening or failure. Three months after surgery, the fracture remains well reduced, and he reports no pain with shoulder or elbow motion.

Fig. 1.

Diaphyseal carbon fiber plate. Prophylactic fixation of an impending pathologic fracture of the proximal left humeral diaphysis using a carbon fiber diaphyseal plate and titanium cortex and locking screws. Preoperative and postoperative X-Rays were captured to confirm implant placement.

3.2. Case #2 – intramedullary nail stabilization of proximal humeral shaft

An 84-year-old male with a history of multiple myeloma and left shoulder pain presented with a pathological fracture of the proximal left humeral shaft. An X-ray five days earlier revealed a 3.6 cm lytic lesion in the proximal humeral shaft with complete erosion of the medial cortex and possible periosteal reaction consistent with an impending pathologic fracture (Fig. 2). The patient returned for additional imaging after hearing a loud crack in his left arm while getting out of bed. A non-displaced fracture through the lesion was present on X-ray. Given the amount of viable bone available, he was indicated for intramedullary nailing for management instead of reverse shoulder arthroplasty. A 260 mm carbon fiber nail was inserted into the intramedullary cavity and secured under fluoroscopy with two proximal and distal titanium screws, respectively (Fig. 2). Upon discharge, he was ambulating independently and was weight bearing as tolerated. His postoperative course was uncomplicated, and imaging at four months demonstrated intact hardware and progressive healing. The patient reports significant improvements in pain and range of motion through 4 months status post fixation.

Fig. 2.

Intramedullary carbon fiber rod. Prophylactic fixation of the left proximal humeral shaft for an impending pathologic fracture via intramedullary carbon fiber rod and titanium screws. Placement of hardware was confirmed using radiographs.

3.3. Case 3 – intramedullary nail stabilization of right proximal femur

A 55-year-old male with a past medical history of plasmacytoma of the jaw treated with definitive radiation presented at a tertiary academic medical center with right hip pain with weightbearing, mechanical characteristics, and increasing serum lambda chains. He was diagnosed with a plasmacytoma of the right proximal femur via MRI, X-ray, and intraoperative curettage. MRI and X-ray showed an impending pathological fracture (Fig. 3). CT staging was negative for further metastases. The patient underwent open biopsy, excision and curettage, bone substitute packing, and surgical stabilization of the right proximal femur using a 380 mm carbon fiber intramedullary nail and 100 mm carbon fiber screw with two distal interlocking screws. He was ambulating with crutches with a weight bearing as tolerated protocol at discharge. His perioperative course was significant for a self-resolved episode of right lower extremity edema negative for DVT. Imaging at 3 months confirms no fracture, implant failure, or complication. At the 3 month follow up, he is ambulating independently with hip flexor weakness and mild discomfort.

Fig. 3.

Intramedullary carbon fiber nail. Stabilization of the right proximal femur for an impeding pathologic fracture of the right proximal femur. Preoperative MRI, X-ray, and intraoperative curettage confirmed diagnosis of a plasmacytoma.

3.4. Case #4 – plate stabilization of proximal tibia

A 67-year-old female with no noteworthy past medical history presented at a tertiary academic medical center with left pretibial pain and enlarging mass over months. She was diagnosed with a grade 2 myxofibrosarcoma of the left pretibial via needle core biopsy, and MRI confirmed an impending pathological fracture (Fig. 4). CT staging showed multiple lung micronodules. The patient underwent staged resection and reconstruction of the left proximal tibia with a fibular strut allograft and large soft tissue reconstruction via ALT free flap with patellar tendon augmentation (Fig. 4). Allograft fixation was performed using a medial carbon fiber plate with proximal locking screws and non-locking cortical screws. She was discharged as non-weight bearing, and she began partial weight bearing at two months post operatively. Her perioperative course was uncomplicated, and imaging at 9 months confirms no fracture, implant failure, or complication. At the 9 month follow up, she is ambulating independently with no pain.

Fig. 4.

Carbon fiber plate. Needle core biopsy and MRI of the left pretibial area confirmed an impending pathological fracture secondary to myxofibrosarcoma. After staged reconstruction and ALT free flap with patellar tendon augmentation (left and center panels), patient underwent prophylactic fixation with a carbon fiber plate.

4. Discussion

Carbon fiber (CF) implants were introduced in the 1980s into orthopaedic surgery as a potential solution to overcome obstacles encountered with traditional metallic alloys. Studies have demonstrated improved osteointegration compared to titanium hardware^1,3 in addition to a relatively lower risk of reactive oxidation and inflammation observed with titanium surfaces.^3,12,13 In this practical guide for orthopaedic oncologists, we aim to provide an up-to-date and comprehensive review on the chemical and physical science of CF implants, clinical applicability, and recent research developments.

Incorporation of composite materials such as carbon fiber enable a more favorable profile of material properties. Medical grade CF devices are composed of various polymer matrices to create a range of implants. These implants, which include reinforced plastic, reinforced thermoplastic, and CF polyether-ether ketone (PEEK), can be customized based on various combinations of resin and reinforcement. Among the types, CF PEEK is one of the most widely used implants in clinical practice due to its advantageous material properties.¹⁰ Apart from the mechanical robustness, CF-PEEK is bioinert with minimal in vitro and in vivo cellular toxicity.^14,15 This biocompatibility is essential for orthopaedic patients, which minimizes the risk of adverse post-operative events and optimizes tissue healing. Petersen et al. reported increased implant osteointegration with CF-reinforced composite rods, measured by percent bone area, compared to their titanium equivalents at two weeks status post fixation in an animal model.² Although current evidence is limited to small scale studies, there is promising data that CF implants serve as a viable alternative to the metallic ones.

4.1. Structural properties of carbon fiber: Young's modulus, fatigue stress, and biocompatibility

Conventional fixation methods in orthopaedic surgery have primarily relied on metallic alloys due to their strength, ease of manufacturing, and relatively low production cost. Nonetheless, these metallic fixation devices are associated with numerous limitations, which have ultimately driven biomaterials research that have investigated cheaper and more biochemically favorable options. The most prominent drawback of alloy implants is the mismatch of modulus of elasticity between bone and metal. Furthermore, heavy metal toxicity has been observed due to wear-and-tear debris. Lastly, radiopacity of metallic hardware on plain radiographs and CT scans can impair perioperative visualization.^16,17

As a means to overcome the material science limitations of metallic alloys, CF were developed to better resemble compressive stiffness of native cortical and cancellous bone, i.e. 12-20 GPa and 1 GPa, respectively.⁶ CF-PEEK implants have a significantly lower Young's modulus (3.5 GPa) compared to titanium (106 GPa).⁴ The decreased rigidity observed in CF-PEEK implants promote better bone healing by improving callus formation and minimal stress shielding.¹⁸ Moreover, carbon fiber allows an improved transfer of stress, which enables an even distribution of axial forces throughout the bone and theoretically prolong the lifespan of a prosthesis when compared to stainless steel. However, additional research is needed to elucidate long term durability and surgical outcomes of CF-PEEK implants, especially in the context of oncologic populations.^19,20

4.2. Global applicability in orthopaedic surgery

Use of CF implants has expanded in recent decades, and utilization currently includes most orthopaedic subspecialties. While initially used within spine surgery, CF implants are now widely employed in trauma surgery, sports medicine, and musculoskeletal oncology, which further supports clinical confidence in their usage.^8,11,21 A study conducted by Pala et al. compared titanium and CF nails in 52 patients with impending or pathological fractures of long bones and found similar outcomes in terms of healing and intraoperative fluoroscopy.²² Although operative length was higher in the CF group, this finding likely identifies a learning curve due to unfamiliar procedural execution rather than an inherent disadvantage of CF implants.

4.3. Management of musculoskeletal oncology patients: pearls and pitfalls

CF implants offer numerous advantages for orthopaedic oncologists during limb salvage and reconstruction procedures, namely optimized imaging and enhanced targeted delivery of radiotherapy. One of the biggest management obstacles of oncologic populations is overcoming the radiodensity generated by metallic hardware.^23,24 Clinicians have traditionally relied on metallic artifact reduction sequences (MARS) to adjust for interference generated by alloys during imaging. Given CF's radiolucency, surgeons have access to improved intraoperative and postoperative visualization of the surgical field. Artifact-free perioperative imaging could lead to improved assessment of radiologic healing of pathological fractures, as well as enhanced surveillance of tumor recurrence.^9,25 In addition, carbon fiber implants do not interfere with delivery of radiotherapy. More specifically, scatter density generated during ionizing radiation is minimized with CF implants due to the lower atomic number of carbon.^21,26 When considering conventional metallic implants, radiation oncologists must adjust for the discrepancy in calculated versus measured dosimetry.^27,28 Depauw et al. demonstrated that CF devices did not significantly impact proton and photon dosimetry compared to titanium alloys.²⁴ The rise in radioresistant tumors necessitates highly accurate perioperative planning, and carbon fiber implants are a promising option to improve dosage precision.^29,30

While CF instrumentation can optimize radiotherapy and tumor surveillance, there are several disadvantages to consider. CF reinforced plastics are relatively expensive, inflexible, and undergo micromovement.³¹ It is important to emphasize that micromotion is a natural and essential phenomenon in fracture healing and rigid fixation.³² Nonetheless, interfragmentary micromotion must be limited to less than 1 mm for optimal secondary bone healing.^33,34 Excessive micromotion across bone can ultimately result in fibrosis on the surface of the implant and impair bone remodeling.^35,36 Adam et al. prospectively evaluated press-fit CF hip prosthesis in 48 patients, and 92% of patients exhibited aseptic loosening and fibrous fixation on carbon stems within 6 years.³⁷ Furthermore, the relative rigidity of carbon fiber limits the ability to contour the implant and makes extraction difficult.³⁸ Monitoring for catastrophic failure is further complicated by implant radiolucency.38, 39, 40 Brittle failure of CF implants occurs typically due to the inability to tolerate stress under physiologic load. A 5-year retrospective, international collaborative study from the Carbon Fiber International Collaboration Initiative Research Group had assessed long term surgical complication rates in 104 oncological patients from February 2015 to May 2021. Three percent of cases were associated with structural failure of carbon fiber plate; two cases were due to non-traumatic origins that were attributed to inappropriate weight bearing and misalignment of the implant.⁷ Moreover, blind screw placement is also more difficult with radiotransparent CF screws due to inability to visualize hardware orientation on fluoroscopy. As an anisotropic material, recent research has focused on the application of high-performance CF composites that have a graded rigidity, which improves the dispersion of stress while optimizing the overall structural integrity.^41,42 Additional research has investigated if surface modifications to carbon fiber polymers such as electro-grafting can further improve the interfacial bonding within the resin.⁴³

4.4. Orthopaedic oncology operative indications

While there are presently no AAOS Clinical Practice Guidelines (CPG) within orthopaedic oncology for CF devices, these implants are commonly employed for prophylactic fixation of radiated bone following soft tissue sarcoma resection,⁷ as well as allograft fixation via excision and curettage.⁶ As reported previously, precise perioperative planning is crucial for radiation mapping and postoperative fracture surveillance.^22,25 No life threatening complications associated with CF implants have been observed in oncological and advanced comorbid populations thus far.^5,7,44, 45, 46 However, further research is necessary to elucidate absolute contraindications for CF implants, if any, in oncologic patients.

4.5. Analysis of perioperative radiographs

A standardized approach for diagnostic evaluation of implantable carbon fiber devices would prove useful for orthopaedic surgeons and diagnostic radiologists alike. At our single tertiary academic center, the authors recommend radiographic imaging of anterior, posterior, and lateral views as the baseline step. Cross sectional imaging using CT or MRI requires adjustment for the metallic contents using metallic suppression techniques as described previously.²⁵

Due to the radiolucent nature, clinicians must have a high index of suspicion of implant failure if there are incorrectly positioned screws with adjacent osteolysis or periosteal reaction within native bone. Radiologists must also be readily familiar with the longitudinal radiopaque markers on plates and nails, which serve as crucial landmarks for implant orientation and future surveillance.²⁵

4.6. Cost effectiveness of carbon fiber implants

Oncology value-based care emphasizes sustainable, outcome-based, patient-centered care. Surgeons and patients alike must consider direct and indirect costs attributed to orthopaedic care, which range from facility fees, devices, acute versus chronic management, as well as restoration of function.⁴⁷ Further robust cost-benefit analyses of CF implantable devices versus metallic alloy hardware are required.

4.7. Current research and next steps

Limited retrospective and prospective studies have reported on the short-term outcomes of carbon fiber fixation methods in oncologic populations.^5,7,44, 45, 46 Surgical outcomes have shown to be promising with minimal osteosynthesis failures, no loss of reduction, and minimal complications. However, postoperative surveillance was limited to mean follow up of 6–12 months.44, 45, 46 Currently, there is a pressing need to prospectively evaluate the long-term surgical outcomes for several years.

Next generation biomaterials research has focused on the application of implantable drug delivery using carbon fiber given its lightweight structure, bioinert profile, and urinary excretion of carbon nanoparticles.⁴⁸ On the small scale, Li et al. had shown carbon fiber can be utilized as a nanocarrier system in vitro and enhance targeted delivery of doxorubicin to malignant cells.⁴⁹ Additional kinetic modeling analysis has reported that carbon fibers can release doxorubicin in a weakly controlled manner, which further supports the call for investigation of chemotherapy-secreting carbon fiber implants.⁵⁰

5. Conclusion

Carbon fiber implants serve as an attractive alternative to alloy-based fixation methods, and they offer a wide range of clinical advantages for the orthopaedic oncologist, namely its radiolucency, low scatter density, and bioinert profile. Future multi-institutional research trials are required to discern the long-term surgical outcomes, feasibility, and cost-effectiveness of carbon fiber implants in orthopaedic surgery.

Funding/sponsorship

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Institutional ethical committee approval

Due to the nature of this paper, an IRB ethical review was not required due to the lack of any effects on patients.

Author contributions

Marilee J. Clunk: conceptualization, methodology, writing – original draft, writing – review and editing.

Marcos R. Gonzalez, Hayley M. Denwood, Joseph O. Werenski, Alisha Sodhi: writing – original draft.

Brett A. Hoffman: data curation.

Nelson Merchan, Santiago A. Lozano-Calderon: supervision, validating, writing – review and editing.

Declaration of competing interest

Author #8 is a paid speaker for CarboFix and Daiichi Sankyo; he is a paid consultant for ONKOS and IlluminOss Medical. He currently serves as a committee member for the Musculoskeletal Tumor Society. The remaining authors have no competing interests to declare.