Orthopaedic Implants — Materials & Corrosion

Overview — Implant Materials in Orthopaedics

Orthopaedic implants must withstand cyclic loading, corrosive biological environments, and mechanical stresses that are among the most demanding in engineering. The choice of material directly determines the mechanical properties (strength, stiffness, fatigue resistance), the biological compatibility (corrosion resistance, absence of toxic ion release, tissue reaction), and the long-term durability of the implant. The principal implant materials in orthopaedics are stainless steel, cobalt-chromium (CoCr) alloys, titanium and titanium alloys, ultra-high molecular weight polyethylene (UHMWPE), and ceramics. Each material has distinct advantages, limitations, and clinical indications.

Key material properties to understand: (1) Young`s modulus (stiffness) — the ratio of stress to strain in the elastic range; the higher the modulus, the stiffer the material; cortical bone has a modulus of ~17–20 GPa; titanium (~110 GPa) is much closer to bone than stainless steel (~200 GPa) or CoCr (~230 GPa); a large mismatch between implant and bone stiffness causes `stress shielding` — the rigid implant bears the load instead of the adjacent bone, leading to bone resorption (stress shielding osteoporosis); (2) Yield strength — the stress beyond which permanent (plastic) deformation occurs; (3) Fatigue strength — the stress below which the material can withstand cyclic loading indefinitely (the endurance limit); implant fatigue failure (rod/nail fracture) occurs at stresses above the fatigue limit over many loading cycles; (4) Corrosion resistance — the ability of the material to resist degradation in the biological environment (ionic solutions, proteins, enzymes); metal corrosion releases metal ions which can cause local tissue necrosis (`metallosis`), systemic ion toxicity, and implant failure

Metallic Implant Materials

Material	Composition	Key Properties	Clinical Uses	Limitations
316L Stainless Steel	Iron + 17–20% chromium + 10–14% nickel + 2–4% molybdenum; `316L` = low carbon (L) to reduce intergranular corrosion	High yield strength; excellent fatigue resistance; widely available; inexpensive; Young`s modulus ~200 GPa (high — significant stress shielding potential); relies on a passive Cr₂O₃ oxide layer for corrosion resistance; susceptible to crevice corrosion (in screw holes, between modular junctions — the oxide layer breaks down in low-oxygen crevices)	Plates, screws, intramedullary nails (temporary implants — designed for removal or in bones not requiring indefinite permanence); K-wires; Steinmann pins; external fixator components; instruments; the most widely used metal for fracture fixation implants	Corrosion susceptibility (crevice corrosion in modular junctions and screw holes); nickel allergy (up to 15% of the population are sensitised to nickel); relatively high stiffness → stress shielding; cannot use in MRI scanners (ferromagnetic — significant artefact); should be removed where possible in young patients to prevent long-term corrosion ion release
Cobalt-Chromium (CoCr) Alloys	Cobalt ~65% + Chromium ~28% + Molybdenum ~6% (CoCrMo — ASTM F75 cast or F799 wrought); also CoCrW (with tungsten) variants	Very high hardness and wear resistance (key advantage); high yield strength; excellent fatigue strength; Young`s modulus ~230 GPa (very high — most stress shielding of common metals); excellent corrosion resistance (superior to stainless steel); the hard, polished CoCr surface provides the lowest friction coefficient of any metal-on-metal bearing surface; however: cobalt and chromium ions released by wear or corrosion are cytotoxic and carcinogenic → ARMD (adverse reaction to metal debris) — soft tissue pseudotumours, osteolysis	Femoral heads in hip arthroplasty (paired with UHMWPE or ceramic liner); femoral stems in some hip arthroplasty systems; tibial tray undersurface in knee arthroplasty; femoral component of knee arthroplasty; wrist and shoulder arthroplasty components; spinal instrumentation (rods, pedicle screws)	Metal ion release (cobalt + chromium ions → ARMD, pseudotumours — particularly with metal-on-metal bearing surfaces; hip resurfacing arthroplasty uses CoCr-on-CoCr → highest metal ion levels); very high Young`s modulus → maximum stress shielding; not appropriate for allergy to cobalt or chromium; MRI artefact (less ferromagnetic than stainless steel but still causes artefact); metal ion monitoring (Co, Cr serum levels) is required for MoM arthroplasty patients
Titanium & Titanium Alloys (Ti-6Al-4V)	Pure titanium (Grade 1–4); Ti-6Al-4V (ASTM F136 — 6% aluminium, 4% vanadium — the most commonly used titanium alloy for implants); Ti-6Al-7Nb (niobium replaces vanadium — reduced cytotoxicity)	Young`s modulus ~110 GPa (closest to cortical bone of all metals — minimises stress shielding); excellent biocompatibility (the TiO₂ passive oxide layer is extremely stable, adherent, and biocompatible — osseointegration occurs directly onto the titanium surface — this is the basis for osseointegration in cementless arthroplasty and dental implants); low density (lightweight); excellent corrosion resistance in biological environments; MRI-compatible (non-ferromagnetic — minimal artefact); low wear resistance (poor tribological properties — NOT suitable for direct bearing surfaces against another material)	Cementless femoral and acetabular components in hip arthroplasty (porous-coated titanium → osseointegration); tibial trays in knee arthroplasty; fracture fixation plates and intramedullary nails (preferred in bones requiring long-term implant retention — tibial nails, spinal fixation); spinal cages (PEEK increasingly preferred); external fixator bodies; dental implants; the preferred metal for cementless arthroplasty and long-term fracture implants	Low wear resistance — titanium femoral heads paired with UHMWPE generate more wear particles than CoCr heads → cannot use Ti femoral heads; NOT suitable for direct bearing surfaces; scratches easily (softer than CoCr); alloy components: aluminium and vanadium in Ti-6Al-4V have been associated with local cytotoxicity (vanadium particularly) → Ti-6Al-7Nb developed as a safer alternative; more expensive than stainless steel; lower fatigue strength than CoCr for the same dimensions → thicker implants needed

Non-Metallic Implant Materials

Material	Properties	Uses	Key Points
UHMWPE (Ultra-High Molecular Weight Polyethylene)	Very high molecular weight (2–6 million g/mol); semi-crystalline polymer; excellent wear resistance (as a bearing surface against CoCr or ceramic); low friction; viscoelastic (absorbs impact); biocompatible; can be sterilised (gamma irradiation or EO gas); NOT MRI-compatible artefact-free (but non-metallic — minimal artefact); conventional UHMWPE susceptible to oxidative degradation (particularly after gamma irradiation in air → free radical formation → embrittlement → delamination and wear); highly cross-linked UHMWPE (HXLPE) — irradiated and then annealed or remelted to quench free radicals → dramatically reduced wear rate (up to 95% reduction vs conventional UHMWPE) but reduced fatigue crack resistance	Acetabular liner in hip arthroplasty (paired with CoCr or ceramic femoral head); tibial insert (bearing surface) in knee arthroplasty; glenoid component in shoulder arthroplasty; patellar component in TKA; ankle arthroplasty bearing surfaces	UHMWPE wear particles: the most important long-term complication of arthroplasty; phagocytosis of submicron UHMWPE particles by macrophages → RANKL release → osteoclast activation → periprosthetic osteolysis (`particle disease`) → implant loosening; HXLPE reduces but does not eliminate wear; second-generation HXLPE with antioxidant additives (vitamin E — tocopherol) provides wear resistance WITHOUT the free radical problem
Alumina ceramic (Al₂O₃)	Extremely hard; excellent scratch resistance; very low friction; biocompatible; produces negligible wear debris; wettable (synovial fluid boundary lubrication); brittle — prone to catastrophic fracture (particularly earlier-generation small femoral heads)	Femoral heads in hip arthroplasty (ceramic-on-ceramic — CoC bearing); ceramic femoral heads paired with UHMWPE or ceramic liners	Ceramic-on-ceramic (CoC) bearings: lowest wear rate of any bearing surface; negligible ion release; ideal for young active patients; disadvantage: squeaking (audible squeaking from dry ceramic contact — in up to 10% of patients; often resolves; rarely requires revision); stripe wear (from impingement); catastrophic ceramic fracture (rare with modern ceramics — fracture rate <0.05%; but catastrophic and extremely difficult to revise — ceramic fragments embed in tissue and must all be retrieved); MRI-compatible
Zirconia-toughened alumina (BIOLOX delta — composite ceramic)	Composite: alumina matrix + zirconia + strontium aluminate; toughened ceramic with much higher fracture resistance than pure alumina while maintaining low wear; currently the most widely used ceramic in arthroplasty	Modern femoral heads and liners in hip arthroplasty; 36 mm and larger heads (reducing dislocation risk); short stems and total hip systems	Superseded pure alumina and zirconia in most arthroplasty systems due to superior toughness + maintained wear properties; the current standard for ceramic-on-ceramic arthroplasty
PEEK (Polyether ether ketone)	High-performance thermoplastic polymer; Young`s modulus ~3.6 GPa (close to cortical bone at 17–20 GPa — closer than any metal); radiolucent (MRI-compatible, allows imaging through the implant); biocompatible; high strength; resistant to sterilisation and chemical degradation; can be combined with carbon fibre reinforcement (CF-PEEK) for increased stiffness	Intervertebral body fusion cages (PEEK cages are standard for PLIF, TLIF, ACDF); trauma bone graft substitutes; custom orthopaedic implants; bone anchors	Spinal cages: PEEK is radiolucent — allows visualisation of the bony fusion mass through the cage on CT/MRI; titanium cages cause scatter artefact on CT making fusion assessment difficult; however: PEEK is not osteoconductive (bone does not bond to it — fibrous tissue interface); titanium-coated PEEK (Ti-PEEK) addresses this by providing an osteoconductive surface while retaining PEEK`s modulus advantages

Corrosion in Orthopaedic Implants

Types of corrosion: (1) Galvanic corrosion — occurs when two dissimilar metals are in electrical contact in the same electrolyte (e.g., titanium screw in a stainless steel plate); the more electronegative metal (anode) corrodes preferentially; the galvanic series ranks metals by their electrochemical potential; titanium and CoCr are close in the galvanic series — can be used together safely; stainless steel and titanium differ significantly — avoid mixing; the clinical rule: NEVER mix stainless steel and titanium implants in the same patient; (2) Crevice corrosion — occurs in confined spaces (screw-plate holes, modular taper junctions — trunnion) where oxygen tension is low; the protective passive oxide layer (Cr₂O₃ in stainless steel; TiO₂ in titanium) breaks down in low-oxygen crevices; accelerated by fluid stagnation and protein concentration; (3) Fretting corrosion — occurs at implant interfaces under micromotion (e.g., modular junctions — head-neck taper, femoral stem-neck junction in dual-modular stems); mechanical fretting damages the oxide layer → accelerated corrosion; fretting produces metal debris + ion release → local tissue necrosis; trunnionosis = corrosion at the head-neck taper junction — a specific and growing problem in modular metal-on-polyethylene THA
Trunnionosis: fretting and crevice corrosion at the femoral head-neck taper (trunnion) junction; metal ions (cobalt + chromium from CoCr heads; titanium + aluminium from titanium trunnions) released locally → adverse local tissue reaction (ALTR); elevated serum cobalt and chromium levels; pseudotumour/ARMD; clinical: unexplained hip pain; effusion; elevated metal ions; MRI: pseudotumour, fluid collection; management: revision if symptomatic with evidence of significant corrosion (CoCr femoral head replacement, trunnion exchange if feasible)
ARMD (Adverse Reaction to Metal Debris): an umbrella term for the local tissue reactions caused by metal particles and ions from arthroplasty implants; includes: ALVAL (aseptic lymphocyte-dominated vasculitis-associated lesion) — a specific histological pattern of T-cell-dominated periimplant tissue destruction seen particularly with MoM bearings; pseudotumours — soft tissue masses (not true neoplasms) composed of necrotic tissue, fibrin, and inflammatory cells caused by metal debris; ARMD is most common with metal-on-metal (MoM) bearing surfaces and trunnionosis; investigation: serum Co and Cr levels; MARS MRI (Metal Artefact Reduction Sequence — specifically designed for MRI around metallic implants); CT for pseudotumour extent; management: cobalt >7–8 µg/L or chromium >7–8 µg/L = requires investigation and surgical review

Bearing Surfaces in Hip Arthroplasty — Summary

Bearing Combination	Wear Rate	Advantages	Disadvantages	Best Indication
CoCr head on conventional UHMWPE (MoP)	~0.1–0.2 mm/year	Long track record; no catastrophic failure; inexpensive; forgiving of impingement	UHMWPE wear particles → osteolysis; limits longevity in active young patients; oxidative degradation of conventional PE	Elderly low-demand patients; THA for fracture (short life expectancy)
CoCr head on HXLPE (MoXLPE)	~0.01–0.03 mm/year	Dramatically reduced wear vs conventional PE; well-studied; lower osteolysis; allows larger femoral head diameters (reduced dislocation risk)	Reduced fatigue crack resistance vs conventional PE; CoCr ion release from head (trunnion corrosion risk)	Most common modern bearing; active patients under 75; standard of care in most UK/US centres
Ceramic head on HXLPE (CoP or CoXLPE)	~0.005–0.02 mm/year	Hardest femoral head — minimal head scratching; lower PE wear than CoCr head; no metal ion release from head; ceramic head tolerates HXLPE well	Ceramic fracture risk (rare); more expensive than CoCr head; squeaking if ceramic liner used	Young and active patients where minimising wear is critical; patients with metal sensitivity
Ceramic on Ceramic (CoC)	Lowest of any bearing — negligible (submicron alumina particles)	Lowest wear rate; no toxic ion release; biocompatible alumina particles; ideal for young active patients; allows large femoral heads (36 mm+ common)	Squeaking (up to 10%); stripe wear (from impingement/edge loading); catastrophic ceramic fracture (rare <0.05% but devastating — ceramic shards must be retrieved); expensive; most unforgiving of component malpositioning	Young active patients (<65 years) with long life expectancy; those requiring maximum bearing longevity
Metal on Metal (MoM — CoCr on CoCr)	Very low volumetric wear BUT generates billions of nanometre-size metal ions	Extremely low volumetric wear; allows large femoral head diameters; hip resurfacing arthroplasty	Metal ion release (cobalt + chromium) → ARMD, ALVAL, pseudotumour; elevated serum metal ions (mandatory monitoring); withdrawn from most markets for THA; hip resurfacing arthroplasty still uses MoM bearing (selected patients — young active males); MHRA alerts on MoM hip monitoring	Hip resurfacing in young active males (selected); large-diameter MoM THA largely abandoned

Exam Pearls

Young`s modulus: cortical bone ~17–20 GPa; titanium ~110 GPa; stainless steel ~200 GPa; CoCr ~230 GPa; PEEK ~3.6 GPa (closest to bone); higher modulus = stiffer = more stress shielding; titanium has least stress shielding of metals; PEEK has least stress shielding of all implant materials
Titanium: best biocompatibility (osseointegration via TiO₂ oxide layer); lowest stress shielding of metals; MRI-compatible; cannot use as a bearing surface (too soft, too much wear); preferred for cementless arthroplasty shells and long-term fracture implants
Stainless steel: ferromagnetic (MRI artefact); nickel sensitisation (15% population); susceptible to crevice corrosion; do NOT mix with titanium (galvanic corrosion); cheapest and most widely used for fracture fixation implants
CoCr: hardest and most wear-resistant metal; best for femoral heads (low friction, scratch-resistant); high Young`s modulus (stress shielding); cobalt + chromium ion release → ARMD, pseudotumour; ARMD monitoring mandatory for MoM patients (serum Co, Cr + MARS MRI)
UHMWPE wear: submicron particles phagocytosed by macrophages → RANKL → osteoclasts → periprosthetic osteolysis → aseptic loosening; HXLPE reduces wear by 95%; ceramic femoral head reduces PE wear further; second-generation HXLPE with vitamin E = wear resistance + antioxidant protection
Ceramic-on-ceramic: lowest wear; no metal ions; but: squeaking (10%), stripe wear, catastrophic fracture (rare but devastating); most demanding of component positioning accuracy
Trunnionosis: fretting + crevice corrosion at head-neck taper; CoCr ions → ALTR; elevated serum Co + Cr; MARS MRI for pseudotumour; revision if symptomatic; avoid large CoCr heads on small titanium trunnions (modulus mismatch + high taper stresses)
PEEK: radiolucent (MRI-compatible); allows fusion mass visualisation through spinal cage; modulus close to bone; NOT osteoconductive (fibrous interface unless Ti-coated); standard for intervertebral cages

Orthopaedic Implants — Materials & Corrosion

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