The Safety of FDM-Printed Ankle-Foot Orthoses (AFOs): What O&P Workshops Need to Know

Oktober 22 2025, Von Hugh Sheridan, 9 min Lesezeit

Workshop managers, orthotists, prosthetists and clinicians, the question remains: “Are FDM-printed AFOs safe enough for clinical use?” This article addresses the evidence, key safety considerations, risks and recommendations specific to FDM-printed AFOs.

Additive manufacturing (AM)—particularly fused-deposition modelling (FDM, also called fused-filament fabrication or FFF)—is increasingly adopted for the fabrication of customised ankle-foot orthoses (AFOs). Yet for workshop managers, orthotists, prosthetists and clinicians, the question remains: “Are FDM-printed AFOs safe enough for clinical use?” This article addresses the evidence, key safety considerations, risks and recommendations specific to FDM-printed AFOs.

1. What does the research show so far?

Here’s a summary of what the literature tells us about FDM / 3D-printed AFOs (including but not limited to FDM) and their safety/performance.

• A 2019 feasibility review found that 11 studies met eligibility criteria for 3D-printed AFOs (healthy adults, foot-drop, children) and concluded that designing, manufacturing and delivering 3D-printed AFOs is feasible. 

• A systematic/additive-manufacturing review (2022) examined AM for AFOs and noted that FDM (FFF) is widely used but with variable mechanical testing and material characterisation.

• A more recent review of materials and manufacturing for AFOs (2023) highlights benefits and challenges of using 3D-printed polymers for orthoses, including layered construction, anisotropy and fatigue behaviour. 

• In one experimental study of a 3D printed AFO (using FDM) the device printed in PLA achieved higher stiffness (1.09 Nm/degree) than the traditional anterior AFO (0.14) but required thickness adjustments to avoid fracture at the “neck” region. 

• Another review of custom dynamic AFOs (2022) noted that while additive manufacturing allows custom shapes and improved comfort versus off-the-shelf devices, “no standard testing method to measure AFO stiffness is widespread” and “no clear guidelines yet for customising stiffness relative to impairment severity.” 

Key takeaway: FDM-printed AFOs can be safe and functional — but only when design, materials, manufacturing and testing are carefully controlled. They are not automatically equivalent to conventionally fabricated devices without due diligence.

2. Safety-specific risks for FDM-printed AFOs

When using FDM for AFO fabrication, the workshop must be aware of these unique risk-factors:

• Material anisotropy & layer adhesion
  FDM builds parts layer by layer; mechanical strength is weaker between layers (Z-axis) than within layers. Under cyclic loading (e.g., walking, stance phase loads) delamination or micro-cracks may propagate. For example, studies show stress concentrations at “neck” or hinge-regions of 3D-printed AFOs. 

• Fatigue / cyclic loading durability
  Many gait cycles over months or years can fatigue the orthosis material. While some FDM studies show adequate initial stiffness, long-term fatigue behaviour for some printed polymers is less mature. The literature points out only very few fatigue tests in custom AFOs. 

• Material choice & print parameters
  The mechanical behaviour (stiffness, strength, impact resistance) depends heavily on filament type (PLA, ABS, Nylon, carbon-fibre reinforced), infill density, print orientation, wall thickness and post-processing. A printed AFO in PLA may perform differently than one in Nylon or reinforced composite. 

• Geometry / fit & loading mismatches
  Even if the print is mechanically adequate, if the fit to the patient’s anatomy is poor (pressure points, misalignment) or the stiffness is not matched to the patient’s functional needs, safety issues may arise (skin breakdown, falls, discomfort). Design customization is still evolving. 

• Lack of standard test protocols/standards
  Unlike conventional AFO manufacturing (thermoplastics, laminates) that have more established workflows, standards for 3D-printed AM orthoses are still emerging. This means variation across workshops and devices. 

3. What safeguards and best practices should an O&P workshop adopt?

To ensure safety and performance of FDM-printed AFOs, workshops should implement the following practices:

1. Material validation & print parameter control

   • Choose a filament with known mechanical properties (tensile, flexural, fatigue) for the expected loading scenario.

   • Establish print orientation, wall thickness, infill/infill pattern and layer bonding parameters specific to AFO geometry.

   • Perform sample coupons/parts fatigue tests or adopt published test data.

   • Consider reinforced materials (e.g., Nylon, carbon-fibre composites) for high load zones.

2. Finite element / mechanical testing of design

   • Use simulation (FEA) to identify stress concentrations and modify design accordingly (e.g., thicken neck region). One study with PLA AFO noted that increasing thickness at the neck improved strength. 

   • Subject prototypes to loading profiles that mimic gait: dorsiflexion/plantarflexion moments, heel strike, toe-off, stance phase loads.

3. Fit & anatomical customization

   • Use 3D scanning or structured-light scans to capture the patient’s leg/foot geometry accurately. Custom shells should correspond well to anatomy to avoid misalignment. 

   • Ensure trim-lines, shell geometry, joint alignment and interface surfaces are well designed to avoid skin/bony pressure points.

4. Quality control & inspection

   • Inspect parts for layer delamination, surface defects, warpage post-print.

   • Mark each printed AFO with print orientation, layer height, filament batch, date of manufacture – for traceability.

   • Keep logs of device lifetime; after a certain number of hours or months, inspect or replace.

5. Clinical monitoring & user feedback

   • Monitor patient comfort, donning/doffing ease, skin interface, gait alterations, adverse events (falls, discomfort, breakage).

   • Establish a follow-up schedule (e.g., 1 week, 1 month, 3 months) to assess fit and durability.

6. Patient-specific stiffness tuning

   • Recognise that stiffness of an AFO dramatically affects gait performance and safety (too flexible → insufficient support; too stiff → unnatural gait/compensations). Review literature and adjust print wall thickness, materials or geometry to match impairment level. 

4. Where FDM-printed AFOs are appropriate — and where caution is required

Appropriate when:

• The patient requires a custom shape, one-off device quickly fabricated (for example in low-volume custom workshop).

• The load demands are moderate (e.g., mild dorsiflexor weakness, foot-drop in ambulatory patient) and the material selection/design has been validated.

• Lead time, cost or geometric complexity favour AM over traditional thermoforming/fabrication.

• The workshop has appropriate in-house AM skills, scanning capabilities, design workflows and QC protocols.

Require caution / possibly avoid when:

• High load demands (e.g., heavy patient, high-level activity, sports, uneven terrain) and where long-term fatigue resistance is critical and material data is lacking.

• Patients with significant ankle instability, joint deformity or need for articulated joints where printed shell alone may not suffice.

• Environments where regulatory or reimbursement scrutiny demands fully tested devices under standard norms. If the AM device cannot demonstrate test data, risk may be higher.

• When the workshop lacks standardised print/QC workflows or traceability of devices.

5. Summary & practical takeaway

• FDM-printed AFOs represent a promising innovation: faster turnaround, fully custom fit, design flexibility and potentially lower cost.

• Research shows they can match or exceed stiffness of conventional AFOs in controlled tests, but the evidence base for long-term durability, fatigue behaviour and standardisation is still emerging.

• Safety depends less on “AM vs conventional” and more on the workshop’s process discipline: material selection, print workflow, simulation/testing, quality control, and clinical follow-up.

• For O&P workshops wanting to adopt FDM-printed AFOs: implement rigorous design/validation workflows, track device lifetimes, build in patient monitoring, and treat these devices with the same safety mindset as traditional orthoses.

• Until the industry converges on standardised testing protocols and material certification for AM orthoses, treat FDM-printed AFOs as advanced custom solutions requiring validation rather than “drop-in replacements” for conventional devices.

In short: yes — FDM-printed AFOs can be safe and effective. But only if your workshop treats them with the same rigour as any medical-grade orthosis: right material, right design, right quality process, and right patient matching.

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