Review ArticlePolyetheretherketone (PEEK) cages in cervical applications: a systematic review
Introduction
Chronic back pain is a major health problem and results in high socioeconomic costs and loss of quality of life [1], [2]. Degenerative disc disorders are seen as an important source of pain [3], [4]. degenerative disc disorders can initiate secondary changes leading to stenosis, spondylolisthesis, and/or facet joint osteoarthritis. This may also lead to abnormal movement in the affected motion segment, which causes pain [3], [5], [6]. Surgical stabilization of the degenerative segment by a bony fusion is a method to eliminate the pain. Some of the modern surgical techniques involve removal of the degenerated disc and placement of a graft in the intervertebral space to achieve interbody fusion through an anterior approach in the cervical spine or an anterior or posterior approach in the lumbar spine.
To promote interbody fusion of the affected segments, autologous bone grafts were originally used. However, donor site morbidity, together with high failure rates resulting from collapse, subsidence, retropulsion, or resorption of the graft with subsequent pseudoarthrosis or prolonged healing time were frequently seen [7], [8], [9], [10]. Therefore, interbody fusion cages were developed as an alternative for bone grafts [10]. These are designed to contain a bone graft, allowing bony fusion through the cage between the adjacent vertebrae. Cages allow for direct axial load-bearing and restoration of height of the intervertebral and foraminal space.
In 1988 Bagby introduced a stainless steel implant, which he used as a cage to promote spinal fusion and restore the disc height [11]. This Bagby and Kuslich cage showed good fusion rates [12]. In the subsequent years, titanium alloy cages were commercialized. Although high fusion rates and good clinical improvement scores were reported [13], [14], [15], these cages still had some disadvantages. For instance, subsidence is still seen in high percentages, varying from 16% to 60% [16], [17], [18], [19]. Furthermore, titanium is radiopaque, which makes it difficult to visualize bone formation on radiograms after implantation.
To improve visualization of bone formation, radiolucent cages have been developed. Examples of these are resorbable poly(L-lactide-co-D, l-lactide) (PLDLLA) [20] and carbon fiber cages [21]. However, subsidence and pseudoarthrosis are still seen with these types of cages. One-third of patients with PLDLLA cages actually showed a 10% worsening of visual analog scale (VAS) and Oswestry Disability Index (ODI) scores. Also, signs of osteolyses were seen in the PLDLLA treatment group [20].
Polyetheretherketone (PEEK) cages became available during the late 1990s. They reduce stress shielding because of their lower elastic modulus compared with titanium [22], [23]. Radiopaque markers are used to visualize PEEK cages. They cause less artifacts on computed tomography (CT) and magnetic resonance scans as compared with titanium and allow visualization of bony fusion.
PEEK materials were commercialized in the 1980s and belong to the family of polyaryletherketone polymers [24]. PEEK has the ability to withstand high temperatures (up to 300°C) and is resistant to chemicals and radiation. PEEK is compatible with reinforcing agents and strength exceeding many metals [25], [26]. Originally used in industrial applications, PEEK was explored as a biomaterial in prosthetic implants during the 1980s [27], [28]. Not until the late 1990s was PEEK offered commercially as a biomaterial for spinal cages [22]. The first composites consisted of carbon-fiber–reinforced polyetherketoketherketonketon (PEKEKK) and showed positive biomechanical results in a cadaveric cervical and lumbar study [29]. In that study, the PEKEKK implant was compared with allograft human bone blocks of the proximal femur. Compression tests showed the PEKEKK implant had a similar compressive strength as the highest quality of bone implant, and the pullout resistance of the PEKEKK implant exceeded those of the allograft. In addition, an animal study was performed in which Spanish goats received either a PEKEKK cage with autologous iliac crest graft (CFRP cage) or allograft bone blocks [30]. At 6 and 12 months, higher histological and radiographic fusion rates were seen for CFRP cages compared with allograft. Furthermore, the cage was clinically evaluated in a US Food and Drug Administration–approved prospective multicenter study in 221 patients [31]. All patients underwent a lumbar interbody fusion with the carbon-fiber–reinforced PEKEKK cage filled with autologous graft, followed by posterior fixation with pedicle screws and plates. At 24 months, 98.6% of patients who had a 2-year radiographic evaluation (178 patients) achieved fusion. In 13.5% of patients, there were minor device-related complications, the majority of which were broken pedicle screws. There were 10.4% major non–device-related complications, with eight deep wound infections requiring reoperation. Further surgery was performed in 46.1% (102 patients) because of elective removal of screws and plates (35.2%); these addressed new disc levels or repaired dural tears. Five of 221 patients (2.2%) needed revision of the pedicle screws or cages. The lumbar carbon–reinforced PEKEKK cage came to be known as the Brantigan cage [22]. Carbon-fiber–reinforced PEKEKK cages were abandoned however by their industrial supplier for reasons that are not well-documented in literature and thus have ceased to exist [22]. This laid the foundation to the current use of PEEK cages.
PEEK cages have been widely used during the past decade [22], [32], [33], [34]. Their radiolucency and low elastic modulus make them attractive for spinal fusion [35]. Still, drawbacks are seen, such as subsidence and migration of the cages [36], [37], [38]. PEEK has a hydrophobic surface, which allows neither protein absorption nor promotion of cell adhesion [39]. An animal study has reported that PEEK cages are encapsulated by a thin fibrous tissue layer [40]; theoretically, this can interfere with the host–bone integration, which may lead to subsidence and migration. Other than elastic modulus, different design factors are likely to play a large role determining the overall performance of an interbody implant [35]. A radiolucent cage could contribute to difficulties in visualization during surgery, whereas radiopaque cages such as titanium are visible while using fluoroscopy. Despite this, PEEK cages have become more popular over the past few years.
Limited evidence on the clinical outcome of PEEK cages is found in the literature other than in noncomparative cohort studies with only a few randomized controlled trials (RCTs). Therefore, in this study, we systematically reviewed the available literature on the clinical and radiographic outcome of PEEK cages compared with other interbody cages.
Because only a limited number of lumbar interbody fusion studies were found in literature, with large variation in operative techniques and indications for surgery, only cervical interbody fusion studies were included in our review.
Section snippets
Objectives
The objective of this systematic review was to assess the clinical and radiographic outcome of all clinical comparative and long-term noncomparative cohort studies of PEEK cages in the treatment of degenerative disc disorders and/or spondylolisthesis in the cervical spine.
Search strategy
A systematic literature search was performed by the first author on October 5, 2012. The MEDLINE, EMBASE, and Cochrane Library databases were used according to the Preferred Reporting Items of Systematic reviews and
Search and selection
A systematic search of the Medline, Embase, and Cochrane databases identified 223 articles, excluding duplicates. After screening title and abstract by two of the authors (RFMRK, SMvG), 53 articles met the inclusion criteria. After reading the full text, 39 articles were removed based on the exclusion criteria. Of the 14 selected articles, four evaluated PEEK cages after lumbar interbody fusion and were also excluded. Finally, a total of 10 studies were included. A cross-reference check of the
PEEK cage versus human bone graft
A prospective RCT by Celik et al. [58] evaluated the differences in cervical foraminal height changes between standalone PEEK cages in 35 patients on 41 levels in total (29 one-level, 6 two-level) AICGs in 30 patients on 46 levels in total (14 one-level, 16 two-level) after ACDF. Indication for surgery was radiculopathy. Clinical outcome was measured by the VAS score and the JOA myelopathy scoring system. Radiographic outcome was performed by radiogram at the first, third, sixth, twelfth, and
Clinical performance
The primary outcome variable of our study was clinical performance. Five of 10 studies used the VAS score as outcome parameter. No evidence was found in the included studies to indicate that PEEK cages led to significant more reduction of pain compared with other graft materials. MCID of the VAS (scale 1–10), obtained from literature, ranged between 2.5 and 2.6 for neck pain [66], [67], between 2.5 and 4.6 for arm pain [66], [67], and between 1.5 and 2.5 for VAS in general [43], [66]. One of
Conclusion
High fusion rates and good clinical outcome scores are reported for PEEK cages in the cervical spine. Only minimal evidence for better clinical and radiographic outcome is found for PEEK cages compared with bone grafts. No differences were found between PEEK, titanium cages, and carbon fiber cages. Still, limitations are seen with PEEK cages. A lack of osteointegration of the cage and difficulty in radiographic assessment justifies the need for improvement. To improve the quality of research,
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FDA device/drug status: Not applicable.
Author disclosures: RFMRK: Nothing to disclose. SMvG: Nothing to disclose. AdG: Nothing to disclose. FCÖ: Consulting: Medtronic (B, Paid directly to institution/employer), DePuy-Synthes (B, Paid directly to institution/employer); Speaking and/or Teaching Arrangements: AOSpine (B, Paid directly to institution/employer); Research Support (Investigator Salary, Staff/Materials): DePuy-Synthes (E, Paid directly to institution/employer), Medtronic (E, Paid directly to institution/employer); Grants: AOSpine (D, Paid directly to institution/employer); Fellowship Support: DePuy Synthes (D, Paid directly to institution/employer).
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