Requirements
An orthopaedic biomaterial should be selected for the mechanical and biological demands of its role.
Important properties include:
- strength
- stiffness
- fatigue resistance
- corrosion or degradation resistance
- wear behaviour
- biocompatibility
- manufacturability
- imaging characteristics
Stainless steel
Advantages:
- strong
- relatively inexpensive
- familiar handling properties
Limitations:
- higher elastic modulus than bone
- can undergo corrosion in adverse conditions
- greater artefact than some alternatives on certain imaging modalities
Orthopaedic stainless steel is relatively stiff, strong and familiar. It contains chromium, which supports formation of a passive oxide layer, and nickel in common implant alloys. Advantages include cost and established manufacturing properties. Limitations include relatively high modulus and susceptibility to corrosion in adverse circumstances.
Titanium alloys
Advantages:
- lower elastic modulus than stainless steel or cobalt-chromium
- good corrosion resistance
- good biocompatibility
- favourable strength-to-weight ratio
Limitations:
- lower wear resistance than cobalt-chromium
- can be prone to fretting or surface damage in some modular interfaces
Titanium alloys have a lower elastic modulus than stainless steel or cobalt-chromium and excellent corrosion resistance because of a stable oxide layer. They are highly biocompatible and useful in many fixation and arthroplasty applications.
Titanium is relatively notch-sensitive and can suffer galling. It is not chosen simply because it is “stronger”; the whole combination of stiffness, fatigue performance, surface treatment and implant geometry matters.
Cobalt-chromium alloys
Advantages:
- high strength
- high stiffness
- good wear resistance
- commonly used in arthroplasty bearing or structural components
Limitations:
- high elastic modulus
- dense material
- metal debris and ions may be important in specific implant situations
Cobalt-chromium alloys are hard, wear-resistant and corrosion-resistant, with a high elastic modulus. They are used in arthroplasty components where hardness and polished surfaces are valuable. Their high stiffness can contribute to stress shielding depending on component design.
Ceramics
Common orthopaedic ceramics include alumina and zirconia-toughened materials.
Advantages:
- very hard
- excellent scratch resistance
- low wear when used in suitable bearing couples
- chemically inert
Limitations:
- brittle behaviour
- risk of fracture, although modern ceramics are substantially improved
- possible noise phenomena in some hip bearings
Ceramics used in orthopaedics include alumina and zirconia-toughened materials. They are:
- very hard
- scratch-resistant
- highly wear-resistant
- chemically inert
Their key limitation is brittleness. Modern manufacturing has reduced fracture risk, but ceramic components still require careful handling and accurate seating.
Polymers
Important examples include:
- ultra-high-molecular-weight polyethylene in arthroplasty
- PEEK in selected spinal and trauma devices
- bone cement polymers such as PMMA
Polymer properties depend heavily on molecular structure, processing, sterilisation and ageing.
Ultra-high molecular weight polyethylene is widely used as a bearing material. Cross-linking improves wear resistance but can influence other material properties. Oxidation is an important ageing mechanism, particularly in older sterilisation and storage methods.
PEEK and related polymers have niche structural uses because of their favourable imaging characteristics and modulus, but indications depend on implant design.
PMMA bone cement
PMMA acts as a grout rather than an adhesive to bone.
Key points:
- polymerisation is exothermic
- cement interdigitates with cancellous bone
- antibiotic can be incorporated in selected settings
- mechanical properties depend on porosity, mixing and handling
Composite behaviour
No single material has the ideal combination of bone-like stiffness, high fatigue resistance, low wear and biological integration. Implant design therefore combines material choice with geometry and surface engineering.
Clinical principle
Do not select an implant material by a single property. The relevant question is whether the whole implant construct is appropriate for its loading environment, fixation method and biological interface.
Material properties relevant to orthopaedics
Implant choice requires consideration of:
- elastic modulus
- yield strength
- ultimate strength
- fatigue resistance
- fracture toughness
- corrosion behaviour
- wear characteristics
- biocompatibility
- imaging properties
- manufacturability
The mechanical requirement differs between a temporary fracture implant and a long-term bearing or arthroplasty component.
Bone cement
Polymethylmethacrylate is a grout, not a biological adhesive. It interlocks mechanically with cancellous bone and implant surface. Polymerisation is exothermic and monomer-related cardiovascular effects are relevant during cementation.
Good cement technique includes appropriate bone preparation, lavage, drying where possible, retrograde delivery and pressurisation according to the procedure.
Corrosion and tribocorrosion
Metal implants are protected by passive oxide layers, but mechanical disruption and electrochemical conditions can cause:
- crevice corrosion
- fretting corrosion
- galvanic effects
- mechanically assisted crevice corrosion at modular junctions
Corrosion products can contribute to local tissue reactions.
Osseointegration
Porous or roughened surfaces can permit bone ongrowth or ingrowth. Successful biological fixation requires:
- initial mechanical stability
- suitable surface characteristics
- viable host bone
- limited micromotion
Excessive early motion favours fibrous tissue rather than stable bone integration.
FRCS synthesis
Do not answer biomaterial questions as a list of alloys. Compare materials by the property that matters clinically: stiffness for stress shielding, hardness for scratching and wear, toughness for fracture resistance, surface chemistry for corrosion and topography for biological fixation.