The prosthetics industry has embraced 3D printing as one of the defining technologies of modern device development.
It has enabled faster prototyping, reduced tooling requirements and made complex geometries more accessible to smaller companies and clinical workshops.
However, Fergal Mackie, founder of upper-limb prosthetics company Metacarpal, argues that the sector now needs to move beyond excitement about how a component is manufactured and focus more closely on whether the selected material is appropriate for real-world use.
In a recent article, Mackie describes a shift in the company’s engineering approach from asking, “Can we print this?” to asking, “Should we?”
From prototype technology to everyday product
Mackie explains that 3D printing remains central to Metacarpal’s development process, particularly for rapid iteration, mechanism testing, fixtures and non-structural components.
Yet relatively few printed parts remain inside the company’s finished prosthetic hands, and none are used within the main structural skeleton.
This decision did not result from opposition to additive manufacturing. It emerged through repeated testing and experience with devices being used daily by prosthetic users.
A prosthetic hand is exposed to repeated gripping forces, impacts, twisting, moisture and unpredictable loading. Components must remain compact and lightweight while resisting bending and fatigue over thousands of cycles.
A material that performs adequately in a prototype may not provide the stiffness, hygiene or durability required for a long-term wearable medical device.
Printed polyamides have performance limits
Industrial additive manufacturing in upper-limb prosthetics commonly uses Polyamide 11 or Polyamide 12 with processes such as selective laser sintering and Multi Jet Fusion.
Mackie notes that these materials offer tensile strengths of approximately 48 MPa and stiffness values around 1.6 to 1.7 GPa.
By comparison, extruded Nylon 6 can reach approximately 80 MPa, while some glass-filled nylons may approach 160 MPa and provide significantly higher stiffness.
The difference has practical design consequences.
When a printed finger or structural component is insufficiently stiff, engineers may compensate by making it thicker. This can gradually remove the size and weight benefits that made 3D printing attractive in the first place.
The result may still be printable, but it may not represent the most efficient engineering solution.
Real-world loading is rarely predictable
Another consideration is anisotropy, meaning that a printed part may behave differently depending on the direction of the load.
Layer-by-layer processes have improved substantially, and technologies such as Multi Jet Fusion can produce strong and consistent components. However, printed parts may still lack the uniform mechanical properties of dense machined materials.
A prosthetic hand is not loaded only in the carefully controlled direction used during a laboratory test. It may be dropped, knocked against furniture, twisted during manual work or subjected to unexpected impact from several directions.
For structural components, resistance to varied and repeated loading can be more important than the visual complexity of the geometry.
Porosity and hygiene also matter
Powder-based printed materials can have microscopically porous surfaces that retain sweat, skin oils and contaminants.
Coatings and finishing processes can reduce porosity, but these add production stages and further quality-control requirements.
Dense machined engineering plastics begin with smoother, less porous surfaces that may be easier to clean and more stable during prolonged use.
This does not make 3D-printed materials unsuitable for prosthetics. It means that surface condition and long-term hygiene should be considered alongside strength, cost and ease of manufacture.
Why Metacarpal selected machined aluminium
Metacarpal eventually moved the structural framework of its prosthetic hand to CNC-machined 7075-T6 aluminium.
Although aluminium is denser than polymer, its much greater strength and stiffness allow structural parts to be made considerably thinner.
Mackie cites a tensile strength of approximately 570 MPa and a Young’s modulus of around 71 GPa for 7075-T6 aluminium.
The reduction in material volume can offset its higher density, producing a structure that is lighter, more compact and more durable than a thicker polymer equivalent.
This illustrates an important principle: a heavier material does not always create a heavier finished product.
Why not metal 3D printing?
Metal additive manufacturing could theoretically produce the same components, but Mackie argues that it would not necessarily improve the result.
For geometries that can be manufactured conventionally, CNC machining may offer tighter tolerances, improved surface finish, stronger fatigue performance and lower costs.
Metal printing is most valuable where a component contains internal channels, lattices or forms that cannot be machined effectively.
Using a newer method simply because it attracts more attention can introduce complexity without delivering meaningful clinical or mechanical benefits.
A wider lesson for O&P manufacturers
The article carries an important message for prosthetic and orthotic providers across India, the Middle East and Africa.
3D printing can improve access to rapid prototyping, digital fabrication and customised production. It may be especially valuable where conventional tooling is expensive or difficult to obtain.
However, printed does not automatically mean stronger, lighter, safer or more innovative.
Before selecting additive manufacturing, developers should assess:
- Mechanical strength and stiffness
- Fatigue resistance
- Surface porosity and hygiene
- Heat and moisture exposure
- Repairability
- Availability of replacement materials
- Quality-control requirements
- Cost at the intended production volume
For patient-specific sockets, orthoses, cosmetic covers and prototypes, 3D printing may offer substantial value. For highly loaded structural components, machining, moulding, composites or metal fabrication may remain more appropriate.
The correct choice may also involve combining several methods within one device.
Innovation should be measured by outcomes
Mackie’s argument is not that the prosthetics sector should move away from additive manufacturing.
It is that manufacturing technology should be treated as a tool rather than a marketing claim.
The real measure of innovation is whether the finished device is lighter, stronger, more comfortable, easier to maintain or more functional for the person using it.
As prosthetic manufacturing becomes increasingly digital, engineers and clinicians will need to avoid allowing the production method to become more important than the performance of the device itself.
The most advanced solution may not be the one produced by the newest machine. It may be the one that uses the right material, in the right location, for the right clinical purpose.
- Original LinkedIn article by Fergal Mackie
- Metacarpal
- HP Multi Jet Fusion technology
- International Society for Prosthetics and Orthotics
- World Health Organization assistive technology resources