Matrix-plus-Reinforcement and a Bone-like Polymer
- A COMPOSITE material is made of two or more constituents - a continuous MATRIX plus a REINFORCEMENT (fibres or particles) - combined so the result has SUPERIOR properties to either component alone; the body contains natural composites, notably BONE (a composite of collagen, which gives toughness, and hydroxyapatite mineral, which gives stiffness/strength) and tendon.
- PEEK (POLYETHERETHERKETONE) is a semi-crystalline thermoplastic polymer increasingly used in orthopaedic implants because of three favourable properties: an ELASTIC MODULUS (~3-4 GPa) much CLOSER to cortical bone (~18 GPa) than the metals (titanium ~110, stainless steel ~200, cobalt-chrome ~210 GPa); RADIOLUCENCY (it does not obscure radiographs/fusion assessment and produces minimal CT/MRI artefact); and good BIOCOMPATIBILITY, fatigue and chemical/heat resistance.
- Because PEEK's modulus is near that of bone, a PEEK implant transfers more load to the adjacent bone and causes LESS STRESS SHIELDING than a stiff metal implant - stress shielding being the bone resorption (by Wolff's law) that occurs when a stiff implant unloads the bone around it.
- PEEK's main LIMITATION is that it is BIOINERT - it does NOT bond to bone (poor osseointegration) - so it is often SURFACE-MODIFIED (hydroxyapatite or titanium coating, plasma treatment, surface roughening) or made POROUS to encourage bone on-/in-growth; experimental composites add antibacterial or bioactive functionality.
- PEEK can itself be made into a COMPOSITE - CARBON-FIBRE-REINFORCED PEEK (CFR-PEEK) - in which carbon fibres in the PEEK matrix give a tunable, higher strength and a modulus that can be tailored toward bone; CFR-PEEK is used in trauma plates and spinal implants and is also radiolucent.
- Typical orthopaedic USES of PEEK/CFR-PEEK: spinal INTERBODY FUSION CAGES (radiolucent so fusion can be assessed; modulus near bone), TRAUMA PLATES (CFR-PEEK), some arthroplasty components and suture ANCHORS - chosen where radiolucency and reduced stress shielding are advantageous, accepting the need to address its bioinert surface.
- “Composite = matrix + reinforcement (bone = collagen + hydroxyapatite, a natural composite).
- “PEEK: modulus near bone (~4 GPa) → LESS stress shielding; RADIOLUCENT; biocompatible. Limitation = BIOINERT (poor osseointegration) → surface coat (HA/Ti) or roughen.
- “Carbon-fibre-reinforced PEEK (CFR-PEEK) = tunable strength + radiolucent; uses: interbody cages, trauma plates, anchors.
Composite Materials
What a composite is. A composite combines a continuous matrix with a reinforcement, either fibres or particles, so that the combination has better properties than either constituent alone: high strength with a tailored stiffness, for example.
Natural composites. The principle is everywhere in the musculoskeletal system. Bone is a natural composite of collagen, which provides toughness and tensile strength, and hydroxyapatite mineral, which provides compressive stiffness and strength. Tendon is collagen within a ground-substance matrix.
Engineered composites. Orthopaedic examples include carbon-fibre-reinforced polymers and glass-fibre composites, in which strong fibres in a polymer matrix give a high strength-to-weight ratio and a tunable modulus.
Types of Composite: Matrix, Reinforcement and Anisotropy
Composites are classified by the matrix, by the form of the reinforcement and by the directional behaviour that results, and it is these three that determine an implant's strength and stiffness.
The matrix. Composites are grouped first by what the matrix is made of:
- Polymer-matrix composites (PMC), such as CFR-PEEK and glass-fibre composites, the main orthopaedic class
- Metal-matrix composites (MMC)
- Ceramic-matrix composites (CMC)
The reinforcement. Continuous (long, aligned) fibres give the highest directional strength. Discontinuous (short or chopped) fibres and particulate reinforcement give lower but more uniform strength. Bone's hydroxyapatite is essentially a particulate, or platelet, reinforcement in a collagen matrix.
The interface. The reinforcement is the stiffer, stronger phase, and load reaches it from the weaker matrix only across the matrix-fibre interface. A good interface means effective reinforcement. A poor one means debonding and failure: once the interface lets go, the fibre pulls out of the matrix and the material's directional strength collapses. The fracture surface of a failed carbon-fibre composite shows fibre pull-out, voids and matrix-fibre debonding.
Direction. Fibre-reinforced composites are typically anisotropic, their properties depending on direction. A composite of continuous aligned fibres is highly anisotropic, strong along the fibres and weaker across them, and can be laminated in chosen orientations to tune its properties, as carbon-fibre-reinforced PEEK is. Short-fibre and particulate composites are more isotropic.
PEEK: Properties, Limitation and Uses
The material. PEEK (polyetheretherketone) is a semi-crystalline thermoplastic with a favourable orthopaedic profile. It is biocompatible, with good fatigue, wear and chemical and heat resistance.
A modulus close to bone. PEEK's elastic modulus is about 3-4 GPa, against about 18 GPa for cortical bone, and far below the metals: titanium about 110, stainless steel about 200 and cobalt-chrome about 210 GPa. An implant with a modulus closer to bone transfers more load to the adjacent bone and causes less stress shielding, the bone resorption that follows, by Wolff's law, when a stiff implant unloads the bone around it.
Radiolucency. PEEK is radiolucent. It does not obscure radiographs, it allows fusion assessment, and it gives minimal CT and MRI artefact, an advantage over metal. Because the cage is radiolucent on a postoperative CT, what the reader follows is its marker and the bone crossing the interspace, rather than the implant.
The limitation: a bioinert surface. PEEK is bioinert. It does not bond to bone, so osseointegration is poor, and it is commonly surface-modified, by hydroxyapatite or titanium coating, plasma treatment or surface roughening, or made porous to encourage bone on-growth and in-growth. Research composites add antibacterial (for example metal-ion) or bioactive coatings.
Carbon-fibre-reinforced PEEK. PEEK can itself be the matrix of a composite. CFR-PEEK embeds carbon fibres in the PEEK matrix to give higher, tunable strength and a tailorable modulus, the fibres raising stiffness far above that of neat PEEK, while the material remains radiolucent.
Uses. PEEK and CFR-PEEK are chosen where radiolucency and reduced stress shielding help, accepting the need to manage the bioinert surface:
- Spinal interbody fusion cages: radiolucent for fusion assessment, with a modulus near bone
- Trauma plates in CFR-PEEK: radiolucent and fatigue-resistant
- Some arthroplasty components
- Suture anchors
PEEK can be machined or moulded into load-bearing implants, but wear, fixation and long-term biological performance still limit its routine use in arthroplasty.
PEEK Cages in Practice: the Bioinert Interface and the Titanium Response
What happens at the interface. Plain PEEK's smooth, non-bonding surface tends to form a thin fibrous-tissue layer at the implant-bone junction. This can show as a radiolucent halo, or lucency, around a PEEK interbody cage, and is associated with slower or less reliable osseointegration and fusion than an osteoconductive surface.
The titanium response. That drove titanium-coated PEEK and porous-titanium/PEEK hybrid cages, along with roughened and HA surfaces, which add bone on-growth while keeping much of PEEK's modulus and imaging advantage. A solid titanium cage osseointegrates well, but it is stiffer, so shields more stress, and it obscures imaging, with artefact and harder fusion assessment.
PEEK's central trade-off is radiolucency plus a bone-like modulus versus a bioinert, non-bonding surface. Titanium-coated or porous-titanium PEEK tries to get both - the osseointegration of titanium with the modulus and imaging of PEEK.
What the clinical data show. The results do not come out the way the modulus argument predicts. Two meta-analyses of interbody cages agree that titanium fuses more reliably than PEEK (Massaad: OR 0.62 for fusion with PEEK; Tan: OR 2.12 favouring titanium in the lumbar subgroup). On subsidence, the outcome PEEK's bone-like modulus is supposed to prevent, they do not agree: one found no difference, and the other found titanium subsided more, the opposite of the prediction.
The honest position. PEEK's demonstrated clinical advantage is imaging rather than mechanics. Modulus matching remains a sound principle, but at the bone-implant interface the surface appears to decide the outcome, and PEEK's surface is its weakness. That is why the fix has been to put titanium on it rather than to abandon the polymer.
Mnemonics & Memory Aids
PEEK
Hook:PEEK: bone-like modulus, easy imaging, biocompatible, but bioinert (Keeps off bone).
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“What is a composite material, and what are the advantages and limitations of PEEK as an orthopaedic implant material?”
“Why is PEEK used for spinal interbody fusion cages, and what is stress shielding?”
Composites
- Matrix + reinforcement → superior combined properties
- Bone = natural composite (collagen + hydroxyapatite); tendon too
- Fibre-reinforced composites are anisotropic (direction-dependent)
PEEK properties
- Modulus ~3-4 GPa (near bone ~18) → less stress shielding (vs metals 110-210)
- Radiolucent (fusion assessment, minimal CT/MRI artefact)
- Biocompatible, fatigue/chemical resistant
Limitation & solutions
- Bioinert → poor osseointegration (does not bond to bone)
- Surface modify: HA/titanium coating, plasma, roughening, porosity
- Composite functionalisation (e.g. antibacterial coatings) - experimental
Uses
- Spinal interbody fusion cages (radiolucent, modulus near bone)
- CFR-PEEK trauma plates (radiolucent, fatigue-resistant)
- Some arthroplasty components and suture anchors
Evidence & Key Studies
Biofunctionalization of 3D-printed PEEK by integrated plasma coating: antimicrobial and bioactive PEEK
- PEEK is widely used in biomedical engineering for its excellent mechanical properties, biocompatibility and radiolucency.
- However, its BIOINERT nature limits infection prevention and bone integration, so surfaces are modified. Three agents were compared - silver oxide, copper and zinc - and ZINC was the most effective, reducing bacterial adhesion by over 4 log; a TiO2/Zn composite coating combined that antimicrobial activity with cytocompatibility, and the coating was applied by a vacuum arc plasma source built into the 3D-printing process so that printing and biofunctionalisation happen in one step.
- Illustrates both PEEK's advantages and the need for surface modification to overcome its bioinertness.
Enhanced antibacterial activity of copper sulfide/PEEK biocomposites
- PEEK is used for orthopaedic implants, and composites can add functionality - here a CuS/PEEK biocomposite to reduce post-operative infection risk.
- The CuS/PEEK composite achieved over 99.8% antibacterial activity against S. aureus and E. coli (enhanced by light/photothermal effect and copper-ion release).
- Demonstrates the composite strategy of combining PEEK with a reinforcement/additive to gain new properties.
Titanium may be superior to PEEK in lumbar interbody fusion (systematic review and meta-analysis)
- Eleven comparative studies, 743 patients, titanium versus PEEK interbody cages.
- Fusion at final follow-up did not differ overall (OR 1.50, 95% CI 0.57-3.94), but in the LUMBAR subgroup titanium achieved superior fusion (OR 2.12, 95% CI 1.05-4.28, p=0.04).
- COUNTER-INTUITIVELY, in non-infective conditions it was TITANIUM that subsided more, not PEEK (RR 2.17, 95% CI 1.13-4.16, p=0.02) - the opposite of what the modulus argument predicts.
- Operating time, haematoma, neuropathic pain, segmental angle correction and clinical improvement were similar.
PEEK versus titanium cages for posterior lumbar interbody fusion (meta-analysis)
- Eleven studies, 1,094 patients - 421 with titanium or titanium-coated cages and 673 with PEEK.
- PEEK was associated with a significantly LOWER fusion rate than titanium (OR 0.62, 95% CI 0.41-0.93, p=0.02).
- NO difference in subsidence between the two materials (OR 0.91, 95% CI 0.54-1.52, p=0.71).
- Back pain and functional scores were equivalent, though the PEEK group reported less postoperative leg pain (OR 0.61, 95% CI 0.28-0.94).
PEEK's bioinert limitation, and the fact that surface modification rather than bulk change is the route around it, comes from the cited Phruekthayanon study; the composite strategy of dispersing a functional additive in a PEEK matrix from the cited Pan study. Both are benchtop materials papers with no clinical data, so neither supports any statement about how a PEEK implant performs in a patient - that comes from the two cited clinical meta-analyses (Tan 2021; Massaad 2020), which together establish the finding that matters most on this page: titanium fuses more reliably than PEEK, and PEEK's bone-like modulus has not translated into a demonstrated reduction in subsidence.
The definition of a composite, bone as a natural collagen-hydroxyapatite composite, the modulus values themselves and the range of CFR-PEEK applications are standard, well-established biomaterials teaching rather than claims resting on any single paper. (See also our Stress-Strain & Modulus, Stress Shielding, Bone Composition and Ceramics/Polyethylene topics.)