Practical applications for glass and carbon composites in orthotic & prosthetic appliance manufacture

Glass and carbon composites have moved from specialist labs into everyday orthotic and prosthetic practice. The attraction is simple - high strength and stiffness at very low weight with the ability to tailor behaviour by changing weave, lay-up, and resin. This guide summarises what the materials are, why they matter clinically, and how to use them well from design through to follow up.

Definition

Glass fibre composite - continuous E-glass or S-glass fibres in a polymer matrix such as epoxy, polyester, or thermoplastic. Common in shells, reinforcement layers, and cost sensitive builds.

Carbon fibre composite - carbon fibres in woven or unidirectional forms embedded in a resin matrix. Favoured for ankle-foot orthoses, dynamic foot plates, socket reinforcements, pylons, and energy storing prosthetic feet due to very high stiffness-to-weight and fatigue performance. Peer-reviewed reviews and studies document extensive O&P use and performance tuning via laminate design. Review Medical applications

How it works - Why it matters

Strength-to-weight and stiffness-to-weight are markedly higher than most monolithic plastics and many metals which allows slimmer profiles and lighter devices. Source
Energy storage and return - carefully profiled carbon laminates store elastic energy through stance and return it at push off that can support gait symmetry and propulsion in ESR feet. Evidence for improved step length symmetry exists although effects on metabolic cost can vary by design and user. Study
Design flexibility - fibre orientation, ply count, hybridising glass and carbon, and resin choice tune bending, torsion, and damping which lets you target load paths rather than adding bulk. Hybrid composites
Fatigue and durability - correct lay-up and cure improve cyclic performance. Lamination design directly affects tensile and flexural strength in prosthetic feet. Lamination effects E-glass vs hybrid data
Clinical payoffs - lighter devices can reduce distal mass, improve swing clearance, and enable footwear compatibility without sacrificing control. Carbon AFOs and custom dynamic orthoses show measurable effects on gait mechanics in specific populations. Offloading with carbon AFOs CFO design and gait

Caveats - material cost, process control, and quality assurance are non trivial. Poor wet out, voids, or poorly designed interfaces undermine performance and safety. Detail

What users say

Reports from trials and case series point to lighter feel, crisper rollover, and improved stability when stiffness is well matched to the user. Carbon AFOs can improve balance and walking measures in neurological conditions and vascular disease although results depend on device tuning and rehab context. CMT case report PAD study Pilot offloading study

Step-by-step guide - from brief to delivery

Define functional targets - establish activity level, user mass, terrain, footwear space, desired stiffness, and allowable deflection. For prosthetic feet decide energy return profile and keel geometry. For AFOs define ankle moment envelope and rocker timing. Ground decisions in objective goals such as step length symmetry or walking economy. Example outcome
Choose material system - glass for cost efficiency and ductility, carbon for stiffness and mass reduction, hybrids to blend behaviour. Evidence supports hybrid E-glass/carbon for tuned flexural strength in prosthetic feet. Hybrid data Hybrid O&P work
Design the laminate - specify ply count and orientation (0 - 90 - ±45), local reinforcements, and resin. Cross ply and lattice or sandwich variants can increase energy storage and tune vibration in ESR feet. Cross ply ESR foot Optimising ESR design
Model and verify - use simple beam theory for first pass then FEA for hotspots at interfaces and strut roots. Validate with bench tests for flexural stiffness and cyclic durability where feasible. 3D printed composite foot - numerical and experimental
Manufacture - prepare mould or last, ensure clean edges and radii to reduce stress risers. Use prepreg or controlled resin infusion, vacuum bagging, and appropriate cure with post cure where specified. Keep a laminate traveller for traceability. Lamination quality strongly influences tensile and impact performance. Lamination effects
Integrate and align - manage interfaces with bonded inserts or clamped plates, avoid drilling close to free edges, and finish edges smoothly. Align under load and confirm rollover and shank progression for AFOs and socket-foot alignment for prosthetics.
Fit - educate - review - verify comfort and skin integrity, confirm gait objectives, and set inspection intervals. Teach users to spot red flags such as new noises, visible cracks, softening, or unexpected deflection.
Iterate - if outcomes fall short adjust stiffness by changing ply count or weave, shift reinforcement to high moment regions, or hybridise with glass to modulate feel without a full rebuild. Glass vs carbon in partial foot orthosis

Comparison tables

Glass vs carbon composites

Property	Glass fibre composite	Carbon fibre composite
Material cost	Lower	Higher
Strength-to-weight - stiffness-to-weight	Good	Excellent - superior ref
Energy return potential in feet	Moderate	High - widely used in ESR feet ref
Fatigue behaviour	Acceptable with good process	Often better when laminate and cure are well controlled ref
Manufacturing sensitivity	Lower to moderate	Higher - quality of lay-up and cure strongly affects outcome ref
Best use cases	Reinforcements, shells, cost constrained builds	Dynamic AFO struts, foot keels, high activity users

Typical clinical applications - materials at a glance

Appliance	Common composite choice	Rationale	Notes - evidence
Energy storing prosthetic foot	Carbon - sometimes hybrid with glass	High energy return, low mass, tuned stiffness	Improved step length symmetry in transtibial users has been observed. JNER - laminate design influences strength. MDPI
Custom dynamic AFO - carbon strut	Carbon unidirectional with woven overwrap	Stiffness targeting for plantarflexion restraint and rollover	Offloading and gait effects reported. Pilot study - design affects centre of pressure and perceived smoothness. Clin Biomech
Partial foot orthosis footplates	Glass, carbon, or hybrid	Balance stiffness vs cost and shoe volume	Direct comparison shows material choice changes load response. Case Studies in Thermal Engineering
Socket reinforcement - frame sections	Carbon fabric with local glass	Local strength and crack arrest plus cost control	Mixed-fibre lay-ups used historically in O&P to tune stiffness. Sage archives

FAQ

Are carbon fibre devices always better than glass fibre devices

No. Carbon often gives more stiffness and lower mass which is helpful for energy return and slim profiles. Glass can deliver excellent performance where slightly thicker sections are acceptable and cost matters. Hybrid lay-ups can provide the middle ground. Hybrid evidence

What are the most common failure modes

Delamination at free edges, matrix cracking near stress concentrators, and local fibre breakage at bolt holes. These relate strongly to ply drops, hole placement, and cure quality. Good edge finishing and correct hardware reduce risk. Lamination - strength data

Does composite choice change alignment strategy

Yes. Stiffer struts shift ankle moments and affect rollover timing. Assess under load, capture gait data if possible, and adjust trim lines or ply count for the next build. Clin Biomech

How long will a composite device last

There is no single lifespan. Service life depends on user mass, activity profile, terrain, and process quality. Periodic inspection for cracks or changes in flex is essential. Overview

Can I repair a cracked carbon AFO strut

Local patching is possible with correct fibres and resin although properties may not fully recover. For high load regions replacement is often safer.

Do composites interfere with imaging

Carbon and glass fibres are compatible with MRI and X-ray although metal inserts or ankle joints may create artefacts. Review

References and research sources

Amrithesh R et al. The use of fibre reinforced composite materials for biomedical purposes - orthopaedic medicine and prosthetic devices. Composites Part B
Wong C et al. Energy storing and return prosthetic feet improve step length symmetry in transtibial amputees. Journal of NeuroEngineering and Rehabilitation
Mulvany S. Application of carbon and glass fibre hybrid composites to load bearing orthoses and prostheses. Kingston University
Alzarrad A et al. The impact of laminations on the mechanical strength of carbon fibre composites for prosthetic foot fabrication. Crystals
Rocha I et al. Characterising E-glass and E-glass/carbon hybrid epoxy composites for prosthetic feet. Heliyon
Dziaduszewska M et al. Composites in energy storing prosthetic feet - overview. Medical Technologies
Jiang M, Zhang J. Cross ply effects in carbon lattice sandwich prosthetic foot. Strength of Materials
Thompson J et al. Foot offloading with carbon fibre orthoses - pilot data. Clinical Biomechanics
McGeehan K et al. Carbon-fibre custom dynamic orthosis design - centre of pressure and perceived smoothness. Clinical Biomechanics
Likhanov A et al. The use of carbon fibre reinforced plastics in medicine - review. Fibre Chemistry
Chua Y et al. Carbon vs glass in partial foot orthosis mechanical response. Case Studies in Thermal Engineering
Fayaz N et al. Numerical design and validation of a 3D printed composite prosthetic foot. Composites Part A
Radcliffe R. Carbon fibre and fibre lamination in prosthetics and orthotics. Prosthetics and Orthotics International archive