







































Pousett B, Lizcano A, Raschke S.U. An investigation of the structural strength of transtibial sockets fabricated using conventional methods and rapid 
prototyping techniques. Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.2. https://doi.org/10.33137/cpoj.v2i1.31008 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 

 
 

 
 

 

 

 

 

 

ISSN: 2561-987X 

VOLUME 2, ISSUE 1 

 2019 

 

(ONLINE) 

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Pousett B, Lizcano A, Raschke S.U. An investigation of the structural strength of transtibial sockets fabricated using conventional methods and 

rapid prototyping techniques. Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.2. https://doi.org/10.33137/cpoj.v2i1.31008 
1 

 
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RESEARCH ARTICLE 

 

AN INVESTIGATION OF THE STRUCTURAL STRENGTH OF TRANSTIBIAL SOCKETS 
FABRICATED USING CONVENTIONAL METHODS AND RAPID PROTOTYPING 
TECHNIQUES 
 
Pousett B1, Lizcano A2, Raschke S.U3 
 

1 Barber Prosthetics Clinic, Vancouver, British Colombia, Canada. 
2 Biomedical Engineering Department, Universidad Iberoamericana, Ciudad de Mexico, Mexico. 
3 MAKE + Applied Research, Centre for Applied Research & Innovation (CARI), Burnaby, British Columbia, Canada. 

 

 
ABSTRACT 

BACKGROUND: Rapid Prototyping is becoming an accessible manufacturing 

method but before clinical adoption can occur, the safety of treatments needs to be 

established. Previous studies have evaluated the static strength of traditional 

sockets using ultimate strength testing protocols outlined by the International 

Organization for Standardization (ISO).  

OBJECTIVE: To carry out a pilot test in which 3D printed sockets will be compared 

to traditionally fabricated sockets, by applying a static ultimate strength test.  

METHODOLOGY: 36 sockets were made from a mold of a transtibial socket 

shape,18 for cushion liners with a distal socket attachment block and 18 for locking 

liners with a distal 4-hole pattern. Of the 18 sockets, 6 were thermoplastic, 6 

laminated composites & 6 3D printed Polylactic Acid. Sockets were aligned in 

standard bench alignment and placed in a testing jig that applied forces simulating 

individuals of different weight putting force through the socket both early and late in 

the stance phase. Ultimate strength tests were conducted in these conditions. If a 

setup passed the ultimate strength test, load was applied until failure. 

FINDINGS: All sockets made for cushion liners passed the strength tests, however 

failure levels and methods varied. For early stance, thermoplastic sockets yielded, 

laminated sockets cracked posteriorly, and 3D printed socket broke circumferen-

tially. For late stance, 2/3 of the sockets failed at the pylon. Sockets made for locking 

liners passed the ultimate strength tests early in stance phase, however, none of the 

sockets passed for forces late in stance phase, all broke around the lock mechanism.   

CONCLUSION: Thermoplastic, laminated and 3D printed sockets made for cushion 

liners passed the ultimate strength test protocol outlined by the ISO for forces 

applied statically in gait. This provides initial evidence that 3D printed sockets are 

statically safe to use on patients and quantifies the static strength of laminated and 

thermoplastic sockets. However, all set-ups of sockets made for locking liners failed 

at terminal stance. While further work is needed, this suggests that the distal 

reinforcement for thermoplastic, laminated and 3D printed sockets with distal 

cylindrical locks may need to be reconsidered. 

CITATION 

Pousett B, Lizcano A, Raschke 

S.U. An investigation of the 

structural strength of transtibial 

sockets fabricated using 

conventional methods and 

rapid prototyping techniques. 

Canadian Prosthetics & 

Orthotics Journal. 2019; 

Volume2, Issue1, No.2. DOI: 

https://doi.org/10.33137/cpoj.v

2i1.31008 

 KEYWORDS 

Prostheses, Rapid Prototyping,  

Prosthesis Design,3D Printing, 
Three Dimensional Printing, 

Transtibial, Socket Strength, 
Transtibial Socket,  

Thermoplastic, Lamination, 
Rapid Additive Manufacturing, 

Lower-limb Prostheses,  

*CORRESPONDING AUTHOR 

Brittany Pousett, BSc, MSc, Certified Prosthetist,  

Head of Research at Barber Prosthetics Clinic, 540 SE Marine Dr, Vancouver, British Colombia V5X 2T4, Canada. 

Email: brittany@barberprosthetics.com 

 

DOI: https://doi.org/10.33137/cpoj.v2i1.31008 

ARTICLE INFO 

Received: October 25, 2018 

Accepted: April 4, 2019 

Published: April 18, 2019 

 

https://doi.org/10.33137/cpoj.v2i1.31008
https://jps.library.utoronto.ca/index.php/cpoj/issue/view/2195
https://jps.library.utoronto.ca/index.php/cpoj/index
https://doi.org/10.33137/cpoj.v2i1.31008
https://doi.org/10.33137/cpoj.v2i1.31008
mailto:brittany@barberprosthetics.com
https://doi.org/10.33137/cpoj.v2i1.31008


 

 

Pousett B, Lizcano A, Raschke S.U. An investigation of the structural strength of transtibial sockets fabricated using conventional methods and 

rapid prototyping techniques. Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.2. https://doi.org/10.33137/cpoj.v2i1.31008 
1 

 
OPEN  ACCESS 

Pousett et al., The structural strength of transtibial sockets  

Volume 2, Issue 1, Article No.2, April 2019 

 

 

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INTRODUCTION 

A prosthetic socket is the interface connecting a 

person’s limb to the prosthetic components they use 

to interact with the environment. Typically, sockets 

are manufactured from a plaster mold of a person’s 

limb which is modified to create an optimized shape.1 

The socket is fabricated over the mold using 

materials including thermoplastics and laminated 

composites. 3D scanning systems are an alternate 

method to digitize the patient’s limb and modify the 

shape.1 Often, the optimized shape is milled by a 

computer numerically controlled (CNC) milling 

machine and the socket is fabricated using traditional 

methods.1  

As technology advances, the question emerges: ‘Is 

this hybrid combination of digital scanning and 

design technology with traditional manufacturing 

methods the best approach?’. Rapid Prototyping 

(RP) offers a time efficient way of turning the digital 

design into a physical socket. RP involves sectioning 

the digital 3D socket design into thin slices, and 

sending it to a 3D printer that builds the shape layer 

by layer.1 Over the past three decades, several 

groups have begun to create prosthetic sockets 

using rapid prototyping techniques.2,3,4 

The Prosthetist is responsible for choosing 

fabrication techniques that provide adequate 

strength and safety to their patients while maximizing 

function.5 Currently, their decisions are not grounded 

on an evidence-based foundation as minimal 

evidence is available.  

Furthermore, the evaluation of prosthetic sockets is 

not subject to any specific standard. ISO 10328: 

Prosthetics–Structural testing of lower-limb 

prostheses is the test standard that is most 

commonly used to test prosthetic sockets.5 ISO 

10328 includes both static and cyclic strength tests 

applied in two different loading conditions, Condition 

I: instant of maximum loading occurring early in the 

stance phase of walking, and Condition II: instant of 

maximum loading occurring late in the stance phase 

of walking, for three different weight limits, P3 body 

mass below 60 kg, P4 body mass below 80 kg, and 

P5 body mass above 100 kg.6 

Previously, this standard has been used in to 

evaluate the strengths of different socket attachment 

methods as this is frequently the point of failure in 

transtibial prostheses.5 Current, Kogleg & Barth7 

applied the static portion of the standard and tested 

10 transtibial sockets for P5 at Condition II. They 

compared five reinforcement materials and two 

resins using a 4-hole distal attachment system and 

found that all 10 sockets failed the ISO 10328 

standard, breaking at the attachment plate.7 

Graebner & Current5 investigated the strength of 

different socket attachment methods for composite 

sockets by applying the same portion of the standard 

as above. They found most attachment methods 

tested passed that aspect of the standard, especially 

when carbon reinforcement was used.5 Finally, 

MacKinnon8 used the same jig to perform the same 

test on three different socket attachment methods for 

thermoplastic sockets. He found two of the methods 

passed that portion of the standard when reinforced 

with fiberglass cast.8 

Gerschutz et al.9 took a different approach and 

applied the static part of the ISO Standard 10328 to 

evaluate sockets made in a variety of facilities.  For 

forces applied at Condition II for P6 (a mass being 

further above 100 kg), they found most check 

sockets and definitive laminated sockets and all 

copolymer sockets failed the standard.9 These 

studies show that there is a lot of variability in sockets 

fabrication techniques and attachment methods that 

result in sockets passing or failing this portion of the 

standard. 

The goal of this project was to evaluate how 3D 

printed sockets compare to traditionally fabricated 

sockets made out of thermoplastics and laminated 

composites. This was done by applying the static 

portion of the ISO 10328 standard for a variety of 

weight limits and load both early and late in the 

stance phase following the same testing protocol as 

these previous authors to allow for comparison.5, 7, 8 

METHODOLOGY 

Socket Fabrication & Alignment 

This study chose to evaluate the strength of two 

different types of total surface bearing sockets, those 

made for cushion liners attached to a 5R1 block 

(“cushion sockets”) and those made for a locking 

liner attached distally via a 4-hole pattern lock 

(“locking sockets”) (FIGURE 1). It tested three 

different fabrication materials, thermoplastic, 

laminated composited and 3D printed Polylactic Acid 

https://doi.org/10.33137/cpoj.v2i1.31008
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Pousett B, Lizcano A, Raschke S.U. An investigation of the structural strength of transtibial sockets fabricated using conventional methods and 

rapid prototyping techniques. Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.2. https://doi.org/10.33137/cpoj.v2i1.31008 
2 

 
OPEN  ACCESS 

Pousett et al., The structural strength of transtibial sockets  

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(PLA). A total of 36 sockets were tested. See FIGURE 

2 for more information.  

 

FIGURE 1: The two types of total surface bearing sockets used; 

the left one was made for use with locking liners with a 4-hole 

pattern lock distally and the right one was designed for use with 

cushion liners and was attached to a distal attachment block and 

reinforced with fiberglass wrap. 

The structural test model was manufactured from a 

cast of an 80 kg male with a unilateral transtibial 

amputation that had been modified by an 

experienced Prosthetist using common methods.  

This model was chosen as it was generic total 

surface bearing shape that was slightly conical, 

allowing multiple sockets to be removed without 

damaging the mold.  It was slightly smaller than the 

average transtibial socket fit at the clinic, but it fit 

within the build height of the printer and it was 

feasible to print sockets within a reasonable 

timeframe of 8-9 hours. The socket is 15 cm from the 

patella tendon to the distal end and 32 cm in 

circumference around the patella tendon. One 

physical mold was fabricated identical to the modified 

cast while the other physical mold had the Fillauer 

cylindrical lock dummy (Chattanooga, United States) 

shape incorporated into the bottom of the shape. 

Each mold was digitized using a Spectra Scanner 

(Vorum, Vancouver, Canada) and converted to a 3D 

print file by Additive O&P (Charlotte, United States).   

18 identically shaped sockets were fabricated from 

each of the models; 6 out of each different type of 

material using an identical process for each material 

type. All sockets were fabricated at Barber 

Prosthetics Clinic by a Registered Prosthetic 

Technician. See TABLE 1 for detailed fabrication 

information.  

 

FIGURE 2: An outline of all the sockets that were fabricated for 

this study, which mold and materials they were made from and 

which conditions they were tested for. 

The sockets were identically aligned in a Vertical 

Alignment Jig (Hosmer, Fillauer, Chattanooga, 

United States) using the model patient’s alignment, 

which was 5 degrees of flexion and 2 degrees of 

abduction. This alignment was done similarly to 

previous studies, which do not follow the ISO 10328 

recommendation that the alignment be set in the 

“worst condition”.5,6 This decision was made to 

standardize the process using a realistic alignment 

for the chosen model shape as this bench alignment 

is repeatable whereas the specifics of what makes a 

worse case condition is unspecified and is 

inconsistent with previous studies. The alignment 

chosen will allow future tests to be compared to the 

socket test done in this study. A 5R1 attachment 

block was used for sockets made for cushion liners, 

3
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3 tested for Condition I (early 
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3 tested for Condition II (late stance), 
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3 tested for Condition II (late stance), 
one at each of P3, P4 and P5

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3 tested for Condition II (late stance), 
one at each of P3, P4 and P5

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3 tested for Condition II (late 
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3 tested for Condition II (late 
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3 tested for Condition II (late 
stance), one at each of P3, P4 and P5

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Pousett B, Lizcano A, Raschke S.U. An investigation of the structural strength of transtibial sockets fabricated using conventional methods and 

rapid prototyping techniques. Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.2. https://doi.org/10.33137/cpoj.v2i1.31008 
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as it has been reported to be the most commonly 

used socket attachment methods in Canada.8 The 

sockets for the locking liners were attached using the 

distal 4-hole pattern in the lock mechanism. All 

sockets were then attached to an Ottobock 

(Duderstadt, Germany) titanium pyramid (5R54), a 

23.2 cm aluminum pylon with a titanium connector 

(2R37) and a titanium tube clamp (4R52), all torqued 

to manufacturer’s specifications.           

TABLE 1.  Processes used to produce sockets. Every effort was 

made to ensure an identical process was followed for each socket 

of the same material.  

Method Thermoplastic 
Laminated 
Composite 

3D Printed 

M
a
te

ri
a
l 

12 mm Orfitrans 
Stiff (a transparent, 
rigid and 
thermoformable 
Styrene Co-
Polyester) 

½ oz. 
Dacron Felt, 
Nyglass, 
Carbon 
Cloth, & 
Resin 

PLA 

P
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 f

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Blister-formed 
socket 
Attached 5R1 block 
using OttoBock 
Sealing Resin and 
reinforced it with 3” 
ScotchcastTM 

circumferencial 
wrap. 

Laminated 
1st stage*1, 
attached 
5R1 block 
using 
OttoBock 
Sealing 
Resin, 
laminated 
2nd stage*2  

Print socket using 
fused Deposition 
Modelling 
on Rockstock Max 
V3 Printer.  Attach 
5R1 block using 
OttoBock Sealing 
Resin and reinforced 
it with 3” 
ScotchcastTM 

circumferencial 
wrap. 

P
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s
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 f

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in
g

 

S
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Set lock dummy in 
standard alignment  
Blister-formed 
socket  
Drilled 4 holes 
distally for lock 
installation  

Completed 
1 stage 
lamination*3 
Drilled 4 
holes 
distally for 
lock 
installation 

Print socket using 
fused Deposition 
Modelling 
on Rockstock Max 
V3 Printer with lock 
dummy included 
distally. 

*1  Layup: 1 layer 1/2oz dacron felt, 2 layers nyglass, 1 layer carbon 

cloth from proximal trimline to 1½” distal to posterior brim trim line, 

2 layers nyglass. 

*2 Layup: 1 layer nyglass, 1 layer carbon clock from proximal 

socket trimline to 1½” distal to posterior brim trimline and from 2” 

proximal to the distal end to the distal end of the block, 3 layers of 

nyglass.   

*3 1 layer 1/2oz darcron felt, 2 layers nyglass, 1 layer carbon cloth 

from proximal socket trimline to 1½” distal to posterior brim trimline 

and from 2” proximal to the distal end to the distal end of the 

socket, 2 x nyglass, 1 layer carbon cloth (as described above), 4 

layers nyglass.   

Test Step-Up 

The testing was performed in a Tinius Olsen 

Universal Testing Machine with a 2500kg Revere 

Load Cell (Tinius Olsen Test Machine Co., Horsham, 

United States). ISO 10328 specifies the magnitude 

of load and where the load should be applied at the 

top and bottom of the set up for each condition, also 

called the offsets (TABLE 2).6 A jig was fabricated for 

these conditions, allowing easy and consistent setup 

of the socket fixture for each test done. The vertical 

load was applied using two 19 mm hitch balls 

adapted to the top and bottom lever of the Tinius 

Olsen universal testing machine. To evenly distribute 

the load through the socket a high-density urethane 

resin (Smooth-CastTM 380, Smooth-On, Macungie, 

United States) mold of the limb was made. A steel 

rod was molded into the urethane to generate a 

better grip between the top jig and the limb mold, 

using a 5/8 bolt. The setup can be seen in FIGURE 3. 

TABLE 2. The offset values for the top and bottom load application 

points for all conditions and levels. The forward direction is 

equivalent to anterior/posterior on the socket and the outward 

direction is equivalent to medial/lateral on the socket.  

 

 

 

FIGURE 3.  The experimental set up consisting of the socket and 

pylon held by a custom-made jig in the Tinius Olsen universal 

testing machine.   

 

  P5 P4 P3 

Reference 
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Top 
Forward 82 55 89 51 81 51 

Outward -79 -40 -74 -44 -85 -49 

Bottom 
Forward -48 129 -52 124 -58 124 

Outward  45 -19 39 -22 39 -23 

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Pousett B, Lizcano A, Raschke S.U. An investigation of the structural strength of transtibial sockets fabricated using conventional methods and 

rapid prototyping techniques. Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.2. https://doi.org/10.33137/cpoj.v2i1.31008 
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Test Procedure 

In accordance with the load values (TABLE 3) and 

specifications of the structural testing of lower limb 

prostheses ISO 10328, all sockets were tested for a 

proof test and ultimate static strength test as this 

standard specifies.  

TABLE 3. The static test procedure and load for each condition 

and level. For each condition, the setting force, proof test and 

ultimate static test force is applied following the protocol described 

below. 

 

For the proof test, the settling test force was applied 

for 30 seconds before it was removed and the set up 

rested at zero load for 30 seconds. The test force 

was then smoothly increased at a rate between 100-

250 N/S to the proof test force for 30 seconds before 

it was removed. All load times were recorded with a 

stopwatch.  

The ultimate strength test was conducted for all 

sockets that passed the proof test. Again, the settling 

force was applied for 30 seconds, the set up rested 

at zero load for 30 seconds, and the test fore was 

increased at a rate between 100-250 N/S to the 

ultimate static test force where it was maintained for 

30 seconds. If the set-up had not yet failed, the load 

was increased until failure. Failure was the point at 

where the system could not support any additional 

load. The 10-minute wait time between the setting 

force and the test force specified by ISO 10328 was 

reduced to between 30– 60 seconds.6 This was done 

as no visible deformation or migration occurred 

during this period, and as this was a preliminary 

investigation, it allowed for more expedient testing of 

the samples.  

Statistical Analysis 

Four independent variables were looked at: socket 

type (cushion sockets and locking sockets), 

fabrication method (thermoplastic, laminated 

composite and 3D printed), loading condition 

(Condition I and Condition II), and weight limit (P3, 

P4 & P5). Two dependent variables, “Proof Test 

Performance” and “Ultimate Strength Test 

Performance” each had two possible outcomes: 

“pass” and “fail”. Statistical analysis (SPSS-IBM, 

Armonk, USA) for socket type and loading condition 

were evaluated by Fisher’s Exact test while 

fabrication method and weight limit were evaluated 

by Chi-square test. 

RESULTS  

Cushion Sockets attached distally via a 5R1 

block 

All 9 sockets passed the ultimate strength test for 

both Condition I and II (FIGURE 4A&B), however the 

failure levels and methods varied. For Condition I, 

thermoplastic sockets yielded, laminated sockets 

cracked up the posterior wall and 3D printed socket 

broke circumferentially above the ScotchcastTM 

(FIGURE 5). For condition II, 2/3 set-ups for each of the 

materials failed because the pylon bent and yielded, 

often while the socket was left intact (FIGURE 5). 

 

 

FIGURE 4. A (Top): For cushion sockets at Condition I (early 

stance phase), all set ups failed above the ultimate strength test 

(UST) values specified in ISO 10328; B (Bottom): For cushion 

sockets at Condition II (late stance phase), all set ups failed above 

the ultimate strength test values specified in ISO 10328.  In 2/3 

cases, the modular components were the cause of failure.     

 P5 P4 P3 

Test 
procedure  
and test 
load C

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I 

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II
 

Settling test 
force (N) 1024 920 944 828 736 638 

Proof test 
force (N) 2240 2013 2065 1811 1610 1395 

Ultimate 
static test 
force (N) 

4480 4025 4130 3623 3220 2790 

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Pousett B, Lizcano A, Raschke S.U. An investigation of the structural strength of transtibial sockets fabricated using conventional methods and 

rapid prototyping techniques. Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.2. https://doi.org/10.33137/cpoj.v2i1.31008 
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FIGURE 5. Typical failure methods for sockets made for cushion 

liners for the following conditions: (A) thermoplastic sockets at 

Condition I (B) laminated composite sockets at Condition I (C) 3D 

Printed PLA sockets at Condition I (D) sockets of all material types 

at Condition II. 

The strength to weight ratios were then compared for 

all sockets along with the failure methods (TABLE 4 

A&B). For Condition I, the laminated composites had 

the highest strength to weight ratios, followed by the 

thermoplastic sockets and the 3D printed socket. The 

3D printed sockets were on average approximately 

75% of the weight of thermoplastic sockets and 

withstood approximately 71% of the force. For 

Condition II, the strength to weight ratio is less 

relevant due to the failure methods being in the 

modular components. 

Locking sockets attached distally via a 4-hole 

pattern lock 

All 9 sockets passed the ultimate strength tests at 

Condition I, however, none of the sockets passed the 

ultimate strength test for Condition II, and one socket 

didn’t pass the proof test (FIGURE 6. A&B).  For 

Condition I, thermoplastic sockets yielded around the 

lock, laminated socket broke either along the 

posterior wall or within the lock mechanism, and the 

3D Printed sockets broke circumferentially around 

the distal end and split up the sides (FIGURE 7).  For 

Condition II, the thermoplastic sockets yielded 

around the lock, the laminated sockets’ lock 

mechanisms broke, and the 3D Printed sockets 

broke circumferentially around the distal end (FIGURE 

7).    

TABLE 4. A: Strength to weight ratios of cushion sockets for 

Condition I; B: Strength to weight ration of cushion sockets for 

Condition II.   

 

The strength to weight ratios were then compared for 

all sockets along with the failure methods (TABLE 5 

A&B). For Condition I, the laminated composites had 

the highest strength to weight ratios, followed by the 

3D printed sockets and the thermoplastic sockets. 

The 3D printed sockets weighed on average 

approximately 84% of the weight of thermoplastic 

sockets but withstood approximately 180% of the 

force. For Condition II the thermoplastic sockets 

were slightly stronger than the 3D printed sockets but 

none of them passed the standard.   

A: Condition I 

Material 

F
o

rc
e

 (
N

) 

S
o

c
k
e

t 

W
e

ig
h

t 
(g

) 
 

S
tr

e
n

g
th

 

to
 W

e
ig

h
t 

P
e

rc
e

n
ti
le

 

(%
) 

F
a

ilu
re

 

Laminated 

Composite 
13132 429 30.61 100 Crack - posterior wall 

Laminated 
Composite 

13341 450 29.65 97 
Material yield - 
anterior proximal gap 

Laminated 
Composite 

12113 450 26.92 88 Crack - posterior wall 

Thermoplastic 12566 709 17.72 58 
Material yield - 
anterior proximal gap 

Thermoplastic 11608 738 15.73 51 
Material yield - 

anterior proximal gap 

Thermoplastic 11264 734 15.35 50 
Material yield - 

anterior proximal gap 

3D Printed PLA 7001 541 12.94 42 

Circumferential 

break above 
ScotchcastTM 

3D Printed PLA 6725 542 12.41 41 

Circumferential 

break above 
ScotchcastTM 

3D Printed PLA 5107 544 9.39 31 
Circumferential 
break above 
ScotchcastTM 

B: Condition II 

Laminated 
Composite 

6505 410 15.87 100 
Distal attachment 
screw 

Laminated 

Composite 
4581 420 10.91 69 Pylon  

Laminated 

Composite 
4384 437 10.03 63 Pylon  

3D Printed PLA 4707 544 8.65 55 
Pylon , socket crack- 

posterior  

Thermoplastic 5958 733 8.13 51 Attachment  

3D Printed PLA 4355 547 7.96 50 
Pylon, socket crack - 

posterior 

3D Printed PLA 4143 543 7.63 48 

Circumferential 

break above 
ScotchcastTM 

Thermoplastic 4434 733 6.05 38 Pylon  

Thermoplastic 4340 736 5.90 37 Pylon  

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Pousett B, Lizcano A, Raschke S.U. An investigation of the structural strength of transtibial sockets fabricated using conventional methods and 

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FIGURE 6. A (TOP): For locking sockets at Condition I (early 

stance phase), all of the set ups failed above the ultimate strength 

test values specified in ISO 10328; B (Bottom): For locking 

sockets at Condition II (late stance phase), all the set ups failed 

below the ultimate strength test values specified in ISO 10328.   

 

 

FIGURE 7. Typical failure methods for sockets made for locking 

liners. At Condition I: (A) thermoplastic sockets yielded and 

deformed, (B) all laminated composite sockets failed differently 

with one socket separating from the pyramid when the lock broke, 

and (C) all 3D Printed PLA sockets broke circumferentially at the 

distal end and up the sides. At Condition II: (D) thermoplastic 

sockets yielded and deformed and the locks broke, (E) the 

laminated composite sockets’ lock mechanisms broke and (F) 3D 

Printed PLA sockets broke circumferentially at the distal end.   

 

TABLE 5. A: Strength to weight ration of locking sockets at 

Condition I; B: Strength to weight ration of locking sockets at 

Condition II.     

 

There were no significant differences observed 

between socket types at the proof test however, at 

the ultimate strength test, 100% of all sockets with 

cushion liners passed while only 50% of those with 

locking liners passed the test. Fisher’s Exact test 

found a statistically significant association between 

liner type and ultimate strength test, p=0.001. In 

looking at the strength of the association, results of a 

Phi test showed a strong association between liner 

type and ultimate strength test results, φ=0.577, 

p=0.001. 

Similarly, there were no significant differences 

observed between Condition I and Condition II at the 

proof test however, at the ultimate strength test, 

A: Condition I 

Material 

F
o

rc
e

 (
N

) 

S
o

c
k
e

t 

W
e

ig
h

t 
(g

) 
 

S
tr

e
n

g
th

 

to
 W

e
ig

h
t 

P
e

rc
e

n
ti
le

 

(%
) 

F
a

ilu
re

 

Laminated 
Composite 

11730 256 45.82 100 Lock mechanism 

Laminated 
Composite 

10058 243 41.39 90 Pyramid adaptor 

Laminated 
Composite 

10364 252 41.13 90 Crack - posterior wall 

3D Printed PLA 10925 368 29.69 65 
Circumferential 
break - distal end 

3D Printed PLA 9355 366 25.56 56 
Circumferential 
break - distal end 

3D Printed PLA 9197 364 25.27 55 
Circumferential 
break - distal end 

Thermoplastic 7650 446 17.15 37 
Material yield - 
proximal anterior gap 

Thermoplastic 6091 431 14.13 31 
Material yield - 
proximal anterior gap 

Thermoplastic 5241 423 12.39 27 
Material yield - 
proximal anterior gap 

B: Condition II 

Laminated 
Composite 

3526 249 14.16 100 
Material yield – lock 
broken  

Laminated 
Composite 

3278 253 12.96 91 
Material yield – lock 
broken  

Laminated 
Composite 

2818 252 11.18 79 
Material yield – lock 
broken  

Thermoplastic 3000 426 7.04 50 
Material yield around 
lock 

Thermoplastic 2763 437 6.32 45 
Material yield around 
lock 

Thermoplastic 2853 460 6.20 44 
Material yield around 
lock 

3D Printed PLA 2243 366 6.13 43 

Circumferential 
break around distal 
end 

3D Printed PLA 2189 367 5.96 42 

Circumferential 
break around distal 
end 

3D Printed PLA 2020 365 5.53 39 

Circumferential 
break around distal 
end 

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Pousett B, Lizcano A, Raschke S.U. An investigation of the structural strength of transtibial sockets fabricated using conventional methods and 

rapid prototyping techniques. Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.2. https://doi.org/10.33137/cpoj.v2i1.31008 
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100% of sockets passed at Condition I while 50% of 

passed at Condition II.  Results showed a statistically 

significant strong association between test condition 

and ultimate strength test results (Fisher’s Exact 

p=0.001; φ=0.577, p=0.001) 

When comparing the ultimate strength test results 

based on manufacturing methods or weight 

classification there were no statistically significant 

differences, however, it is worth noting that 25% of 

sockets failed for each manufacturing method, all for 

locking liners at Condition II. Further testing may 

produce more decisive results. 

DISCUSSION  

This study evaluates the static strength of sockets 

made using a variety of fabrication techniques, 

including 3D printing, laminated composites and 

thermoplastics. It employs the methodology used by 

previous studies to test prosthetic sockets, outlined 

in ISO 10328. This study expended beyond this 

methodology as it looked at forces in both early 

stance phase and late stance phase, which previous 

studies do not do.   

Sockets Made for Cushion Liners 

Thermoplastic Sockets: 

Thermoplastic sockets are used as diagnostic 

sockets.  The transparency of this material allows for 

visual inspection of the socket environment to guide 

the prosthetist in adjusting the socket shape. 

Thermoplastic sockets are heavier and have less 

strength than laminated composite sockets and, 

when tested to failure did not break catastrophically. 

A study conducted by Mackinnon8 found that 

thermoplastic sockets attached using resin and 

ScotchcastTM to a 5R1 block failure at Condition II 

occurred at 4792 N. These results are comparable to 

the current study which found thermoplastic sockets 

using the same attachment methods failed, on 

average, at 4910 N, but with the socket tested to P5 

failing at 5958 N. This increased strength could be 

from a variety of factors such as using a different 

socket shape, differences in plastic thickness, or 

differences in the height and thickness of the 

reinforcement material.   

Laminated Composite Sockets: 

Definitive sockets are made from laminated 

composites. In daily clinical practice, laminated 

sockets do not often break over the typical lifetime of 

a prosthesis. In this study, laminated composite 

sockets had the highest strength to weight ratio and 

withstood the highest force. This was especially true 

for Condition I (at early stance) where the sockets 

failed at approximately 3 times the ISO standard.  

This strength is dependent on many factor as 

discussed below.9 Two other studies evaluated the 

strength of laminated composite sockets, for 

Condition II for people weighing over 100 kg, using a 

similar experimental set up. The first study found 

their sockets failed between 1836 – 3160 N with the 

lamination failing at the pyramid attachment point.7   

The second study found that for socket reinforced 

with carbon weighing between 616 – 795g, failure 

occurred between 4247– 5663 N.5 Different material 

lay-ups and socket attachment methods were found 

to increase the strength of laminated composite 

socket.5,7,9 This study also concluded that modular 

components began to fail above 5400 N of force.5  

The sockets tested at Condition II in the current 

study, weighed between 410–450 g and broke 

between 4384 and 6505 N. This is approximately 

double the load reported by the first study and similar 

to results in the second study, despite sockets in this 

study weighing much less. Reasons for this include 

material selection, layer order, laminating protocol 

and socket attachment methods used. This is to be 

expected, as studies have reported a large variation 

in socket strength depending on who manufactures 

it.9 Findings of the second study were supported by 

this study which found that set-ups failed at the 

modular components; either because the distal 

attachment screw sheared or the pylon yielded. The 

current study indicates that for forces applied at 

Condition II, an average force of 4800 N resulted in 

failure of the modular components. Further testing is 

required due to the small sample size. 

3D Printed Sockets: 

3D printing technology has been identified as having 

the potential to benefit the production of prosthetic 

sockets.3,4,10,11 For example, in the current study, the 

3D printed sockets took 9 hours and 9 minutes to 

print but required much less active time from a 

technician than traditional manufacturing methods. 

While 3D printing allows for rapid prototyping of 

custom designs, decreased manufacturing times and 

increased opportunities for collaboration, the main 

limitation continues to be the lack of standardization 

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Pousett B, Lizcano A, Raschke S.U. An investigation of the structural strength of transtibial sockets fabricated using conventional methods and 

rapid prototyping techniques. Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.2. https://doi.org/10.33137/cpoj.v2i1.31008 
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and regulation which may place patients at risk of 

receiving unsafe devices.3,4,12 One way to evaluate 

the safety of 3D printed sockets is to explore how 

they compare strength wise to other fabrication 

methods available.  

For sockets made for use with cushion liners, this 

preliminary testing provides evidence that 3D printed 

sockets are strong enough statically to be used with 

patients as they passed the standards for all weight 

limits. However, when tested to failure, they failed at 

approximately half of the force of traditional 

manufacturing methods. This is hypothesized to be 

as a result of the material properties and manufac-

turing process. While traditional manufacturing 

methods involve either a solid sheet of plastic, which 

yields before it breaks, or layers of sheets of 

reinforcement materials, often braided or weaved for 

strength, 3D printing deposits material in layers, thus 

making it inherently weaker. When force was 

applied, it appeared that the 3D printed material 

sheared between layers. There are many factors in 

the printing process and design that may be able to 

increase the strength of these sockets such as by 

changing print orientation, infill pattern, adding 

corrugations, changing material types or using a 

different type of printer. Subsequent work completed 

by Campbell et al. provides preliminary support that 

for sockets made for cushion liners 10% changes in 

infill percentage does not affect the strength.13  

For Condition I (early in stance phase), the sockets 

failed well above the ISO standard, by cracking 

circumferentially about the ScotchcastTM reinforce-

ment. This indicates that the force is being concen-

trated there, which could be decreased using 

different manufacturing methods described above. 

For Condition II (late in stance phase), the modular 

components failed before the sockets failed. Modular 

components are regularly used in clinical practice 

without negative consequences. It is likely that since 

they are breaking before sockets are breaking, 3D 

sockets will survive the impact put on them statically.  

Another issue raised is that 3D printed sockets break 

catastrophically, while other manufactured materials 

yield or tear more slowly. This catastrophic breaking 

may present dangers to patients who could be 

injured in this process. Additional work is required to 

further investigate this issue and determine if this 

drawback can be avoided, as well as to see how this 

material acts when going through cyclical testing.  

Inherent in rapid prototyping is the adjustability and 

flexibility in the manufacturing methods – there are 

infinite designs, material choices and print settings 

that can be adjusted to influence the final product.  

As in conventional manufacturing methods, this 

variability will largely influence how strong sockets 

are.9 More work is required to evaluate these 

different parameters and give guidance to which 

choices result in better outcomes.  

Sockets Made for Locking Liners 

This study presents some preliminary evidence that 

the use of cylindrical locks in prostheses should be 

re-considered. Regardless of the manufacturing 

methods used, the sockets with locks did not pass 

the ISO standard for forces applied at Condition II, 

and in all cases the material around the lock either 

yielded or cracked. Unless modifications are done to 

relieve the stress concentration from this point or 

include additional reinforcement, these sockets may 

fail when patients are using them. Alternatively, other 

lock mechanisms may be an option as they result in 

different distal socket shapes which may have less 

concentrated stress points and may withstand higher 

forces. This preliminary evidence supports that 3D 

printed sockets should not be used to create sockets 

with distal cylindrical locks.  

Limitations 

Balancing the production of clinically-relevant and 

scientifically sound evidence with the feasibility of 

completing the research leads to several limitations 

which need to be addressed. First of all, sockets are 

not subject to ISO 10328 testing. However, as the 

other components in lower limb prostheses are 

subject to this standard and as several previous 

studies5,7,9 employed this methodology, it is 

reasonable to use 10328 as an evaluation tool for 

socket strength. The 2006 version of ISO 10328 was 

used as the 2016 version was not yet released at the 

beginning of testing.  

The protocol outlined in the standard was followed as 

closely as possible, however several changes were 

made in order to allow the results to be compared to 

other studies and to make it feasible to conduct in a 

timely and cost-efficient manner. At this time point, 

only the static portion of the structural tests were 

conducted due to the length of time required to 

cyclical testing. However, plans are in place to 

continue work on cyclical testing after addressing 

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Pousett B, Lizcano A, Raschke S.U. An investigation of the structural strength of transtibial sockets fabricated using conventional methods and 

rapid prototyping techniques. Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.2. https://doi.org/10.33137/cpoj.v2i1.31008 
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some of the limitations uncovered through this study. 

When selecting a model, the standard does not 

specify a suitable size or shape, so a model was 

chosen that fulfilled technical limitations and resulted 

in a more expedient process. The model chosen was 

slightly smaller than previous studies, but it is a 

realistic mold, copied from a patient’s everyday 

prosthesis, and it was consistent across all samples. 

The alignment for the model was also taken from the 

patient’s every day alignment, which is a fairly neutral 

bench alignment. The standard outlines using a 

worse-case alignment, but as this is not precisely 

defined, and as all previous studies used a standard 

bench alignment, a standard bench alignment was 

used here too. This allows for comparison with other 

studies and consistency between samples.  

Finally, due to financial, time and resource 

constraints, two modifications were made to the 

testing protocol. First, only one sample was tested for 

each weight limit and condition. While the standard 

recommends testing a minimum of two samples of 

each condition, choosing only one sample allowed 

testing at both early stance and at late stance which 

had not been completed in any previous study on 

socket strength. This resulted in new findings and 

directions for future research to be uncovered.  

Second, the wait time between the test force and the 

ultimate strength test force was reduced. This 

significantly reduced the testing time required, 

allowing for more samples to be tested. 

Other limitations arose from the results of the socket 

tests. When completing the testing for Condition II, 

the endoskeleton modular components often failed 

before socket was affected. While pylons are tested 

to ISO 10328, these components broke prematurely 

and prevented the specific testing of the socket.  In 

future studies, solid pylons can be used to isolate the 

force on the socket attachment and evaluate the 

socket strength more directly. Also, while completing 

testing on sockets made for locking liners, the lock 

mechanism frequently broke. In future studies, a lock 

mechanism which does not act as part of the 

structural attachment to the modular components 

may result in stronger sockets.  

Future Work 

There is a need for continued work on this topic of 

3D printing to support its use in prosthetic fabrication 

in an evidence-based and safe manner. For static 

strength testing, future work may include testing 

larger models with worse-case scenario alignment 

and larger sample sizes. There is also an endless 

combination of material choices, design options and 

3D print parameters that can be explored.  More 

specifically, the distal attachment could be 

strengthened, particularly in sockets made for 

locking liners, to extend the use of 3D printing to 

locking liners. Also, if design or material options 

could eliminate the catastrophic nature of the 3D 

printing failure, patient safety would be significantly 

enhanced and the adoption of this technology would 

be more widely accepted. 

Beyond static testing, cyclical testing of 3D printed 

sockets must also be done to complete the testing 

palate. Until information is known on how this 

material performs over time, clinicians cannot be 

confident that this manufacturing method will meet 

the demands of ambulation. Future work should 

focus on expanding the static testing that has been 

done to cyclical tests in order to present a more 

complete picture of how this technology will work for 

patients.  

In addition to strength, there are many other factors 

that can be explored including the personnel and 

material costs of using 3D printing over other 

manufacturing methods, the ease of fabrication, 

quality and consistency of devices fabricated, and 

the methods of introduction of this method into 

clinical practice.  

CONCLUSION 

This study explored the strength of 3D printed 

prosthetic sockets in comparison with two other 

techniques that are currently used in clinical practice.  

It was found that all 3D printed sockets made for use 

with cushion liners withstood the loads specified by 

the ISO standard. In addition, at terminal stance, in 

many cases the pylons yielded before the sockets 

broke. As this is not routinely seen in clinical practice, 

it provides some evidence that the sockets are 

stronger than the modular components and therefore 

statically safe to use on patients. However, one 

notable limitation to the incorporation of 3D printed 

sockets into practice is the catastrophic nature of the 

failure and thus the potential serious risk it can pose 

to the patient. Further evaluation needs to be 

conducted to explore how 3D printing manufacturing 

methods can affect the strength of sockets and the 

nature of the failure.  

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Pousett B, Lizcano A, Raschke S.U. An investigation of the structural strength of transtibial sockets fabricated using conventional methods and 

rapid prototyping techniques. Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.2. https://doi.org/10.33137/cpoj.v2i1.31008 
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ACKNOWLEDGEMENTS 

We would like to thank the following people for their 

contributions to this project: David Moe CP(C), Daryl 

Murphy RTP(C) and Barber Prosthetics Clinic for all 

of the clinical expertise, technical expertise and 

socket manufacturing required for this project; Nigel 

Halsted and Ernie Janzen with BCIT MAKE+ for the 

testing expertise, testing jig design and 

manufacturing;  the BCIT School of Energy for 

access to the testing equipment and Yvette Jones, 

also with BCIT MAKE+, for the statistical analysis.  

Additive Orthotics & Prosthetics for their expertise 

with 3D printing sockets and OrtoPed & Ottobock for 

their generous donations of materials & 

componentry.  

DECLARATION OF CONFLICTING 

INTERESTS 

The authors have no conflicts of interest to declare. 

SOURCES OF SUPPORT 

Materials and components were provided by Barber 

Prosthetics Clinic, OrtoPed & OttoBock.  

AUTHOR CONTRIBUTION 

• Brittany Pousett  

Conception and design of the work.  Supervision 

of fabrication of sockets.  Data analysis and 

interpretation.  Drafting of the manuscript.   

• Aimee Lizcano 

Conception or design of the work.  Data 

collection.  Data analysis and interpretation.  

Drafting of the manuscript.   

• Silvia U Raschke 

Canadian Supervising Academic.  Conception 

and design of the work.  Guided data analysis.  

Critical revision of the manuscript. 

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https://doi.org/10.33137/cpoj.v2i1.31008
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https://jps.library.utoronto.ca/index.php/cpoj/index
http://dx.doi.org/10.1682/JRRD.2011.05.0091
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