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Spinal fusion implants enabled by
AM: a design sprint to prove the
process
Philippe Reinders Folmer
Renishaw Benelux BV
Problem
• Design freedom of AM well-known
• Lattices as part of Osseo integrated implants are becoming more desirable
• Knowledge of lattice creation and application is improving
• Design tools in general are evolving
But, these are no good in isolation.
Few resources show an integrated approach
to conception, design and manufacture of
spinal implants
Solution
Tripartite project to
address key factors in
implant design &
development
A collaboration that combines research and development rigor,
powerful design tools and manufacturing expertise to demonstrate
a path for spinal implant design
Idea Product
Spinal fusion devices
• Used to treat pain resulting from spinal cord compression
• Can result from a herniated disc or disc degeneration (spondylosis)
• Fusion device restores the space between the vertebrae and fuses the 2
vertebrae
• C5 – C6 intervertebral space in the cervical spine
C5-C6 = region of interest
for study
Requirements
ASTM
1. Designed for additive manufacture
(DfAM)
2. Allow for direct bone fusion
between vertebral bodies
3. Improve subsidence management
4. Improved fixation between device
and bony endplate
5. Addresses compendial material
requirements
6. Fits within intervertebral space
3. S
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1. D
fAM
Design for additive manufacture (DfAM)3
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1. D
fAM
Leveraging the benefits and avoiding the weaknesses of AM
Minimise
post-
processing
Maximise
parts on
the plate
Reduce
material
use
Design for additive manufacture (DfAM)
Determine:
• Maximum contact patch for manual removal
• Maximum horizontal bridge length
• Minimise angle relative to bed
Manufacturability study
3. S
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1. D
fAM
Test coupon to define key manufacturing geometry
Limiting parameters output from manufacturing geometry study
Objective: remove as much post-processing as
possible whilst retaining manufacturability
Direct bone fusion between vertebral bodies3
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1. D
fAM
Osseointegration
Osteoblast wicking and transmission
Osteoblasts responsible for the synthesis
and mineralisation of new bone tissue
Space for bone growth
Allows enough space for bone ingrowth
whilst retaining interference and
retention
Minimum reliable strut thickness
Size of strut that could be built using a
30 or 60 μm layer thickness
Unit cell restrictions
In addition to the physical
implant size, osteoblast
transmission, space for bone
ingrowth and minimum
manufacturable strut size
need considering.
0.2 – 0.9 mm
0.01 – 0.3 mm
200 µm
Direct bone fusion between vertebral bodies3
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fAM
Unit cell determination
Rhombic Dodecahedron
cell height cell width pore width % density %porous E* (MPa) σy (MPa)
0.6 0.6 0.257797 53.29% 46.71% 33936.31 1616.015
0.92 0.92 0.573591 33.13% 66.87% 13115.49 624.5471
1.2 1.2 0.853591 21.76% 78.24% 5657.744 269.4164
Vertex Centred Cubic
cell height cell width pore width % density %porous E* (MPa) σy (MPa)
0.565685 0.565685 0.282843 0.395385 0.604615 18679.75 889.512
0.707107 0.707107 0.424264 0.353872 0.646128 14963.21 712.5338
0.848528 0.848528 0.565685 0.249 0.751 7408.478 352.7847
0.989949 0.989949 0.707107 0.202241 0.797759 4887.33 232.73
1.131371 1.131371 0.848528 0.164684 0.835316 3240.673 154.3178
Tetrakaidekahedron
cell height cell width pore width % density %porous E* (MPa) σy (MPa)
1.2 1.2 0.6 38.00% 62.00% 17254.36 821.636
1.5 1.5 0.8 11.09% 88.91% 1470.629 70.02997
1.8 1.8 1 8.52% 91.48% 868.2192 41.34377
Tetrakaidecahedron Rhombic
Dodecahedron
Vertex Centred Cubic
3 unit cell types considered
Improve subsidence management3
. S
ub
sid
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ce
2. B
on
e f
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ion
1. D
fAM
What is subsidence?
Stiffness of implant much > stiffness of
adjacent bone
Implant penetrates into structure of
surrounding bone
Improve subsidence management3
. S
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on
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1. D
fAM
What is subsidence?
Cortical -compression
Cancellouscompression
Cortical - tensionCancellous -
bending
Median 17.5 0.225 17 4.6
Max 20 0.35 20 0
Low 15 0.1 14 0
0
2
4
6
8
10
12
14
16
18
20
0
2
4
6
8
10
12
14
16
18
20 Cortical bone
Cancellous bone
Design Iterations
Toothed fixation
Pedicle screw
fixation
Ridged fixation
Design Selection
Concept 2 – ridged porous contact surfaces provide short term fixation and aid
long term fusion
Concept 3a – porous contact surfaces for long term fixation, pedicle screws for
immediate fixation
Design Refinement
Graduated, non-
organised porous
structures
Organised porous
contact surfaces,
with graduated struts
to mitigate inherent
stress at solid/porous
interface
Solid substructure
Design Refinement
Graduated, non-
organised porous
structures
Organised porous
contact surfaces,
with graduated struts
to mitigate inherent
stress at solid/porous
interface
Solid substructure
Final Build Design
• 150 Parts/plate
• Parts hand-removable
• Witness coupons;
• Hardness,
• Metallurgy,
• Chemistry,
• Feedstock
condition
Cost breakdown
Single Laser (RenAM 500M) Quad laser (RenAM 500Q)
€5.38 per part
150 parts per build in 21 hours
€4.12 per part
150 parts per build in 7 hours
A financially viable process
Summary
• A realistic starting point for a product demonstrator was reached from
a standing-start
• More work still required to confirm mechanical and chemical
properties of implants match the specifications
• AM allows design freedom, but user-requirements and DfAM are key
components to achieve success
• 2-month time-frame only possible due to wealth of AM experience
therefore the right partners make a massive difference
• AM of spinal fusion implants is a highly suitable process
Thank you!For more information please search:
Renishaw spinal
Philippe Reinders Folmer
General Manager Benelux
