







































Gholizadeh H, Lemaire ED, Salekrostam R. Mechanical evaluation of unity elevated vacuum suspension system. Canadian Prosthetics & Orthotics Journal. 
2019; Volume2, Issue2, No.5. https://doi.org/10.33137/cpoj.v2i2.32941 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 

 
 

 
 

 

 

 

 

 

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OPEN  ACCESS 

MECHANICAL EVALUATION OF ÖSSUR UNITY SUSPENSION SYSTEM  

Volume 2, Issue 2, Article No.5, 2019 

 

 

Journal Homepage: https://jps.library.utoronto.ca/index.php/cpoj/index 

 

RESEARCH ARTICLE 

 

MECHANICAL EVALUATION OF UNITY ELEVATED VACUUM SUSPENSION SYSTEM  

 
Gholizadeh H1*, Lemaire ED1,2, Salekrostam R3 
 

1 Ottawa Hospital Research Institute, Centre for Rehabilitation Research and Development, Ottawa, Canada. 

2 Faculty of Medicine, University of Ottawa, Ottawa, Canada. 

3 Faculty of Engineering, Department of Mechanical Engineering, University of Ottawa, Ottawa, Canada. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

INTRODUCTION   

The method for attaching a prosthesis to the body is 

termed prosthetic suspension. Selecting a suitable 

suspension system is an important step in the prosthetic 

rehabilitation process.1–4 A better understanding of 

prosthetic suspension systems may facilitate selection 

based on the amputee’s needs, leading to better socket 

system performance.2,5,6 Small residual limb-socket 

displacement is a good indicator of prosthetic suspension 

system quality.7,8 

Various prosthetic suspension approaches are used in 

clinical practice. A thigh corset was traditionally used for 

suspension, but introduction of the patellar-tendon bearing 

prosthesis lead to other suspension methods; such as, 

cuff, supracondylar-suprapatellar, and suprapatellar 

strap.2,6 The Icelandic roll-on silicone socket (ICEROSS) 

system was introduced to the rehabilitation market to 

improve suspension via close adhesion of the silicone liner 

to the residual limb skin.2,6 Various methods are used to 

hold the silicone liner inside the prosthetic socket; 

including, single distal pin/lock, lanyard, suction, seal-in, or 

vacuum. A standard lock system for all amputees has not 

been defined.1,2,6 

Vacuum assisted suspension systems (VASS) add an 

externally generated vacuum to a liner-based suspension 

ABSTRACT 

BACKGROUND: Small residual limb-socket displacement is a good indicator of prosthetic suspension 

system quality. Active vacuum suspension systems can decrease vertical movement inside the socket, 

compared to non-active suction systems. This study mechanically evaluated limb-socket displacement 

with the Össur Unity active vacuum system. 

METHOD: Forty-eight conditions were evaluated: four cylindrical and four conical sockets 

(polypropylene, polyethylene terephthalate glycol-modified (PETG), thermoset resin (acrylic), 

Thermolyn soft materials); two Iceross Seal-In V liners (standard, high profile); three vacuum 

conditions (active vacuum, inactive vacuum, no suction with valve open). An Instron 4428 test machine 

applied 0-100N linear ramped tensile loads to each positive mold, with the socket secured in place, 

while displacement between the mold and socket was recorded. Following the displacement tests, the 

load before failure (i.e., 10 mm displacement) was measured. 

RESULTS: Average and standard deviations for movement between the mold and sockets were small. 

The displacement average for all conditions was 0.30±0.16mm for active vacuum, 0.32±0.16mm for 

inactive vacuum, and 0.39±0.22mm for no suction. Across all trials, active vacuum systems tolerated 

significantly (p<0.001) more load before failure (812±221N) compared to inactive vacuum (727±213N), 

and no suction (401±184N). The maximum load before failure (1142±53N) was for the cylindrical 

polypropylene socket and high-profile liner. 

CONCLUSION: The Unity system successfully controlled pistoning inside the socket for regular 

activity loads and also controlled the greatest traction loads. While relative movement was smallest 

for Unity, all conditions (inactive vacuum, no suction) were viable for loads less than 100N. 

Furthermore, similar results can be achieved when using different socket fabrication materials. 

CITATION 

Gholizadeh H, Lemaire ED, 

Salekrostam R. Mechanical 

evaluation of unity elevated vacuum 

suspension system. Canadian 

Prosthetics & Orthotics Journal. 

2019;Volume2, Issue2, No.5. 

https://doi.org/10.33137/cpoj.v2i2.32

941 

 KEYWORDS 

Prosthesis, Amputation, Prosthetic 

limb, Elevated vacuum, Prosthetic 

suspension system, Rehabilitation 

*CORRESPONDING AUTHOR 

Hossein Gholizadeh, 

BSc (P&O), MEng.Sc, PhD, 

Ottawa Hospital Research Institute, Centre for Rehabilitation Research and Development, Ottawa, Canada. 

Email: gholizadeh87@yahoo.com 

ORCID: https://orcid.org/0000-0001-5847-7985 

 

DOI: https://doi.org/10.33137/cpoj.v2i2.32941 

ARTICLE INFO 

Received: July 20, 2019 

Accepted: March 16, 2020 

Published: March 20, 2020 

 

 

https://jps.library.utoronto.ca/index.php/cpoj/index
https://doi.org/10.33137/cpoj.v2i2.32941
https://doi.org/10.33137/cpoj.v2i2.32941
mailto:gholizadeh87@yahoo.com
https://doi.org/10.33137/cpoj.v2i2.32941


 

 

Gholizadeh H, Lemaire ED, Salekrostam R. Mechanical evaluation of unity elevated vacuum suspension system. .Canadian Prosthetics & Orthotics Journal. 2019; Volume2, 
Issue2, No.5. https://doi.org/10.33137/cpoj.v2i2.32941 

 

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Gholizadeh et al. 2019 

 MECHANICAL EVALUATION OF ÖSSUR UNITY SUSPENSION SYSTEM 

system to decrease pistoning within the socket, reduce 

residual limb volume loss over time, and improve 

prosthesis control and proprioception.1,3,9,10 Street11 

mentioned that vacuum could eliminate movement and 

reduce shear, provide a healthier environment for the 

residual limb, and prevent volume loss. Rosenblatt12 also 

reported that elevated vacuum systems can improve 

maximum walking speed, comfort, and gait symmetry 

compared to suction sockets and sleeve suspension 

systems.12 While elevated vacuum systems may have 

some benefits over the other suspension systems and 

improve amputee quality of life, these systems may not be 

appropriate for all amputees since donning the prosthesis 

requires more procedures, and amputees must deal with 

the liners, sleeves, controls, etc.13–16 Moreover, air 

between the liner and skin may create skin blisters.16 

Recently, the Unity elevated vacuum suspension  

system (https://assets.ossur.com/library/31882/IFU) was 

developed by Össur. Unity consists of a mechanical 

vacuum pump in the foot shell, which uses prosthetic foot 

motion to draws air out from the socket in each step. Unity 

includes a hypobaric sealing membrane around a silicon 

liner so that an external sleeve is not required, unlike other 

vacuum systems on the market such as Harmony 

(Ottobock) or LimbLogicVS (Ohio Willow Wood). External 

sleeves can restrict knee range of motion, retain heat and 

therefore create perspiration problems, and may be 

replaced regularly due to sleeve punctures.2 

Seal-In V liners are used and are available as standard 

and high profile options with only cylindrical shape. The 

high profile liner has a more proximal sealing membrane 

and is used when the person has sensitive locations on 

the distal tibial crest or a long residual limb. The high 

profile liner may have better pistoning control than 

standard profile due to larger vacuum area. The 

manufacturer suggests a thin layer of polyethylene 

terephthalate glycol-modified (PETG) materials over the 

positive cast before socket fabrication with thermoset resin 

(acrylic). However, PETG material is rigid and not suitable 

for people who want flexible inner socket. Therefore, 

investigating other materials for socket fabrication could 

be of benefit to amputees. 

Pistoning measurement has been used to evaluate 

suspension system quality for static and dynamic 

conditions.7,8,13,14,18–22 Testing involves applying a tensile 

load to the socket and then measuring displacement 

between the socket and limb. Test loads are based on 

swing phase forces during gait, with typical prosthetic limb 

tensile loads of 44.5N during walking and 88.9N during 

running.1,7 This load is applied to the suspension system in 

less than one second and depends on prosthetic weight 

and walking speed. 

Currently, research is lacking on how the Unity elevated 

vacuum system controls pistoning within the socket, with 

different socket materials. This information is important to 

guide prosthetic prescription and to characterize this 

technology within the current scope of prosthetic 

suspension systems. Therefore, mechanical testing was 

conducted to provide quantitative evidence to guide 

clinical practice. 

 

METHODOLOGY 

The Össur Unity elevated vacuum suspension system was 

used in this project. To provide a repeatable and 

standardized socket and limb surrogate, two reusable 

positive model (cylindrical and conical shapes) were made 

from plaster and covered with Plastazote medium foam 

(2cm) and leather to simulate skin and soft tissue  

(Figure 1). The model dimensions were obtained from 

Campbell, et al.23 Each positive model retained a steel 

mandrel in the proximal end. The reusable positive molds 

were only used during mechanical testing and not for 

socket fabrication. To avoid Plastazote compression under 

vacuum while laminating/thermoforming the sockets 

(Figure 2), molds were casted with Plaster of Paris  

(Figure 2,E) and then reusable positive plaster models were 

fabricated (Figure 2,F). The positive models were used for 

socket fabrication. 

 

Figure 1: Positive mold. Left: cross-sectional view of positive 

mold (conical shape); Right: socket dimensions. 

The cylindrical and conical mold were used to fabricate 

eight sockets, based on the Unity manufacturer guidelines, 

with standard and high profile Iceross Seal-In V liners and 

four materials: polypropylene (PP), polyethylene 

terephthalate glycol-modified (PETG), thermoset resin 

(acrylic), Thermolyn soft (Figure 2). A custom adaptor  

was attached to the socket’s distal end. A valve from the 

Unity transtibial kit was attached to the socket wall below 

the liner seal position. 

Socket displacement was measured for active vacuum  

(-18inHg), inactive vacuum (i.e., acting as a suction 

suspension system), and no suction (i.e., valve in open 

position) conditions. As shown in Figure 2(H), the prosthetic 

foot vacuum pump was attached to the socket and a 

pressure gauge (Mini Dial Air Pressure Gauge Meter) was 

used to check negative pressure inside the socket. The 

gauge was in series with the UNITY tube and pump. We 

created -18inHg for active vacuum by simulating prosthetic 

foot motion to draw air out from the socket. An Instron 

4482 tensile test machine was used to apply ramped 

tensile loads and measure displacement. The positive 

mold’s mandrel was attached to the Instron’s superior grip 

and the socket’s distal adapter was connected to the 

Instron’s inferior grip (i.e., fixed, non-moving attachment). 

The positive molds were pulled with linear ramped loads 

from 0 to 100 N, over 1 second. A 100N maximum load 

encompasses the typical range of lower limb loads in daily 

living.7,21 Following the displacement tests, the load that 

each condition can tolerate before the suspension failed 

was measured. A 10-mm displacement between mold and 

socket was considered as failure since amputees consider 

https://doi.org/10.33137/cpoj.v2i2.32941
https://assets.ossur.com/library/31882/IFU


 

 

Gholizadeh H, Lemaire ED, Salekrostam R. Mechanical evaluation of unity elevated vacuum suspension system. .Canadian Prosthetics & Orthotics Journal. 2019; Volume2, 
Issue2, No.5. https://doi.org/10.33137/cpoj.v2i2.32941 

 

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Gholizadeh et al. 2019 

 MECHANICAL EVALUATION OF ÖSSUR UNITY SUSPENSION SYSTEM 

sockets to fit well and to be secure with displacements of 

less than 10 mm.8 

A total of 48 test conditions were evaluated (Figure 3). 

These conditions included: socket shape (cylindrical, 

conical), socket material (polypropylene, polyethylene 

terephthalate glycol-modified, thermoset resin, Thermolyn 

soft material), liner seal position (high profile, standard 

profile), and vacuum (active vacuum, inactive vacuum, no 

suction). Ten trials were completed for each test condition. 

A review paper by Eshraghi et al.,7 showed that different 

techniques have been used to measure pistoning inside 

the socket and only five studies21,24–27 completed 3 to 5 

trials due to ethical considerations related to the x-ray 

exposure. This study included 10 trials which provide more 

reliable analysis than other reviewed studies. The liners 

and mold were examined after each test to ensure that no 

damage or changes occurred. Moreover, a minimum of 5 

minutes between tests allowed the Plastazote to return to 

its original shape. 

 

Figure 2: Sockets fabrication and mechanical testing. A-G: 

Process of making cylindrical and conical sockets; H: mechanical 

testing. 

Data Analysis 

Positive mold displacements were extracted using the 

Instron Bluehill 2 Software and imported into Excel for 

analysis. Maximum displacement was determined for each 

100N load trial and averages and standard deviations (SD) 

were calculated across the 10 trials for each test condition. 

For the suspension failure tests, the failure load was 

determined for each trial and averages and SD were 

calculated across the 10 trials for each test condition. 

Statistical analyses were performed using SPSS 23 and 

the normality of variables was verified by the Shapiro-Wilk 

Normality test. A one-way Repeated Measures Analysis of 

Variance (ANOVA) with post hoc Bonferroni analysis was 

used to compare active vacuum, inactive vacuum, and no 

suction conditions. Moreover, a paired samples t-test was 

used to compare Seal-In V high profile and standard profile 

liners. The significance level was set at 0.05. 

 

Figure 3: Tensile tests (48 combinations). PP (polypropylene); 

PETG (polyethylene terephthalate glycol-modified); Resin 

(thermoset resin); TS (Thermolyn soft). 

RESULTS 

Average and standard deviations for movement between 

the positive molds and sockets were small (Table 1). 

Across all conditions, the average displacement was 

0.30±0.16mm for active vacuum, 0.32±0.16mm for 

inactive vacuum, and 0.39±0.22mm for no suction. 

Overall, less movement was measured with conical socket 

shapes (0.28±0.16mm) compared to cylindrical sockets 

(0.33±0.17mm) in active vacuum condition when 100N 

traction load applied. 

https://doi.org/10.33137/cpoj.v2i2.32941


 

 

Gholizadeh H, Lemaire ED, Salekrostam R. Mechanical evaluation of unity elevated vacuum suspension system. .Canadian Prosthetics & Orthotics Journal. 2019; Volume2, 
Issue2, No.5. https://doi.org/10.33137/cpoj.v2i2.32941 

 

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Gholizadeh et al. 2019 

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Overall, active vacuum systems tolerated significantly 

(p<0.001) more load before failure (812±221N), compared 

to inactive vacuum (727±213N) and no suction 

(401±184N). With active vacuum, the maximum load 

before failure (1142±53N) was for the cylindrical 

polypropylene socket and high-profile liner. The minimum 

load was recorded with the PETG conical socket and 

standard profile liner in active vacuum (442±42N) and 

open valve conditions (151± 15N). For the passive 

condition, the minimum load was recorded with the 

Polypropylene conical socket and standard profile liner 

(324.8±5N). 

With active vacuum, an average of 958±179N was 

required to displace positive molds with high profile liners, 

compared to 665±155N for standard liners (p<0.001). 

Moreover, the two socket shapes were significantly 

different (p<0.01) since more load was needed for 10 mm 

displacement in cylindrical sockets (905±183N), compared 

to conical sockets (718±227N) when using active vacuum 

(Table 1, Figure 4-Figure 7). 

DISCUSSION  

Mechanical testing was conducted to evaluate the Össur 

Unity elevated vacuum suspension system’s ability to 

minimize socket displacement when external traction 

forces are applied. The results of this study showed that 

the Unity system successfully controlled socket pistoning 

and different materials can be used for socket fabrication.  

From the literature, vacuum and suction suspension 

systems could diminish residual limb displacement inside 

the socket.13,28 Klute et al. measured pistoning with a 

motion analysis system while their participants stood in 

place and shifted their weight from side to side (i.e., 

weighted and un-weighted).13 

Socket Liner  Vacuum 
Displacement at 

100N load  
p-value* 

Load at 10mm  
displacement 

p-value* 

Resin  
conical  

Standard 
profile 

ON 0.45±0.04 (1,2) 1.000 562.4±14.10 (1,2) 0.001 
OFF 0.45±0.04 (1,3) 1.000 521.3±5.22 (1,3) 0.001 

Valve open 0.47±0.05 (2,3) 1.000 214.2±12.78 (2,3) 0.001 

High profile 
ON 0.22±0.03 (1,2) 1.000 701.5±10.85 (1,2) 0.001 
OFF 0.22±0.01 (1,3) 0.001 562.6±15.96 (1,3) 0.001 

Valve open 0.30±0.02 (2,3) 0.001 258.0±5.60 (2,3) 0.001 

Resin cylindrical  

Standard 
profile 

ON 0.37±0.01 (1,2) 0.237 711.9±4.80 (1,2) 0.001 
OFF 0.38±0.02 (1,3) 0.001 671.1±3.57 (1,3) 0.001 

Valve open 0.61±0.03 (2,3) 0.001 252.1±2.35 (2,3) 0.001 

High profile 
ON 0.24±0.04 (1,2) 0.146 905.7±28.02 (1,2) 0.001 
OFF 0.27±0.01 (1,3) 0.206 706.7±31.72 (1,3) 0.001 

Valve open  0.27±0.01 (2,3) 1.000 476.7±8.81 (2,3) 0.001 

PETG  
conical  

Standard 
profile 

ON 0.60±0.11 (1,2) 1.000 442.2±42.37 (1,2) 0.026 
OFF 0.60±0.07 (1,3) 0.022 398.5±45.00 (1,3) 0.001 

Valve open 0.76±0.10 (2,3) 0.010 151.34±14.70 (2,3) 0.001 

High profile 
ON 0.19±0.01 (1,2) 0.222 786.5±55.11 (1,2) 0.986 
OFF 0.20±0.02 (1,3) 0.001 764.5±32.66 (1,3) 0.001 

Valve open 0.21±0.01 (2,3) 0.019 453.7±14.37 (2,3) 0.001 

PETG cylindrical  

Standard 
profile 

ON 0.71±0.23 (1,2) 1.000 741.9±19.32 (1,2) 0.037 
OFF 0.74±0.08 (1,3) 0.314 711.4±22.48 (1,3) 0.001 

Valve open 0.92±0.10 (2,3) 0.002 323.5±10.53 (2,3) 0.001 

High profile 
ON 0.20±0.02 (1,2) 0.153 1060.5±20.48 (1,2) 0.076 
OFF 0.22±0.06 (1,3) 0.001 915.8±140.81 (1,3) 0.001 

Valve open  0.24±0.01 (2,3) 0.813 663.7±116.49 (2,3) 0.013 

Polypropylene 
conical  

Standard 
profile 

ON 0.23±0.10 (1,2) 0.413 446.6±14.98  (1,2) 0.001 
OFF 0.29±0.02 (1,3) 0.439 324.8±5.02 (1,3) 0.001 

Valve open 0.30±0.06 (2,3) 1.000 280.3±31.16 (2,3) 0.001 

High profile 
ON 0.14±0.02 (1,2) 0.020 805.3±18.35 (1,2) 0.001 
OFF 0.19±0.01 (1,3) 0.001 701.4±31.07 (1,3) 0.001 

Valve open 0.23±0.01 (2,3) 0.001 323.1±17.75 (2,3) 0.001 

Polypropylene 
cylindrical  

Standard 
profile 

ON 0.38±0.10 (1,2) 0.657 751.9±54.63 (1,2) 0.001 
OFF 0.38±0.03 (1,3) 0.001 688.5±11.19 (1,3) 0.001 

Valve open 0.54±0.02 (2,3) 0.001 277.1±1.69 (2,3) 0.001 

High profile 
ON 0.17±0.03 (1,2) 0.141 1141.7±52.64 (1,2) 0.003 
OFF 0.19±0.01 (1,3) 0.032 1061.7±40.78 (1,3) 0.001 

Valve open  0.20±0.01 (2,3) 0.158 709.0±99.65 (2,3) 0.001 

Thermolyn soft 
conical  

Standard 
profile 

ON 0.20±0.01 (1,2) 1.000 855.2±23.82 (1,2) 0.002 
OFF 0.20±0.01 (1,3) 0.496 801.3±29.39 (1,3) 0.001 

Valve open 0.21±0.01 (2,3) 0.261 469.6±11.39 (2,3) 0.001 

High profile 
ON 0.18±0.01 (1,2) 0.211 1126.4±36.97 (1,2) 0.001 
OFF 0.19±0.01 (1,3) 0.001 1073.2±11.26 (1,3) 0.001 

Valve open 0.20±0.01 (2,3) 0.004 748.4±28.84 (2,3) 0.001 

Thermolyn soft 
cylindrical  

Standard 
profile 

ON 0.33±0.02 (1,2) 0.732 793.0±40.24 (1,2) 0.001 
OFF 0.34±0.02 (1,3) 0.001 752.2±14.14 (1,3) 0.001 

Valve open 0.47±0.03 (2,3) 0.001 285.9±10.38 (2,3) 0.001 

High profile 
ON 0.21±0.04 (1,2) 1.000 1136.1±58.83 (1,2) 0.001 
OFF 0.21±0.01 (1,3) 0.060 971.0±13.56 (1,3) 0.001 

Valve open  0.23±0.01 (2,3) 0.009 528.6±10.92 (2,3) 0.001 

Table 1: Average and standard deviations for displacement (mm) and load (N) before failure. 

* P values for valve setting comparisons: 1=ON, 2=OFF, 3=valve open. 

https://doi.org/10.33137/cpoj.v2i2.32941


 

 

Gholizadeh H, Lemaire ED, Salekrostam R. Mechanical evaluation of unity elevated vacuum suspension system. .Canadian Prosthetics & Orthotics Journal. 2019; Volume2, 
Issue2, No.5. https://doi.org/10.33137/cpoj.v2i2.32941 

 

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Figure 4: Average displacement (mm) and load (N) in conical socket with Seal-In V standard profile. 

Figure 5: Average displacement (mm) and load (N) in conical socket with Seal-In V high profile. 

https://doi.org/10.33137/cpoj.v2i2.32941


 

 

Gholizadeh H, Lemaire ED, Salekrostam R. Mechanical evaluation of unity elevated vacuum suspension system. .Canadian Prosthetics & Orthotics Journal. 2019; Volume2, 
Issue2, No.5. https://doi.org/10.33137/cpoj.v2i2.32941 

 

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 MECHANICAL EVALUATION OF ÖSSUR UNITY SUSPENSION SYSTEM 

 

 

 

Figure 6: Average displacement (mm) and load (N) in cylindrical socket with Seal-In V high profile. 

Figure 7: Average displacement (mm) and load (N) in cylindrical socket with Seal-In V standard profile. 

https://doi.org/10.33137/cpoj.v2i2.32941


 

 

Gholizadeh H, Lemaire ED, Salekrostam R. Mechanical evaluation of unity elevated vacuum suspension system. .Canadian Prosthetics & Orthotics Journal. 2019; Volume2, 
Issue2, No.5. https://doi.org/10.33137/cpoj.v2i2.32941 

 

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Residual limb pistoning was significantly less (1±3mm) 

with the Harmony suspension system providing active 

vacuum than a pin/lock system (6±4mm).13 The results of 

this study showed that after applying 100N traction load, 

only 0.30±0.16 mm of displacement was found with active 

vacuum across all conditions. 

Another study21 revealed that adding up to 90N loads to 

the prosthesis caused less pistoning in a suction socket 

using Seal-In X5 liner (2±1mm) compared to a pin/lock 

system (5±2 mm). In the current study, 0.32±0.16mm 

pistoning inside the suction socket was recorded overall 

(average for all conditions). This result is lower than the 

previous studies on different suspension system13,21 and 

showed that the Seal-In V could control the movement 

inside the socket successfully. 

Gholizadeh et al., 2014, showed that a Seal-In X5 liner 

with PETG socket tolerated 310N loads before suspension 

failure, pin/lock systems tolerated 580 N, and magnetic 

suspension systems tolerated 351N.29 In this study, the 

maximum load that each condition could tolerate before 

suspension failure was up to 86% higher than other 

studies in the literature.29,30 While the manufacturer 

suggested PETG materials for the Unity socket fabrication, 

the results of this study showed that different materials can 

be used without sacrificing suspension performance. 

Wirta et. al.,31 compared the vertical movement of conical 

and cylindrical residual limb shapes with patellar tendon 

bearing (PTB) sockets and different suspension systems 

(i.e., supracondylar/suprapatellar, supracondylar, cuff, 

waistband and cuff, figure-eight strap, rubber sleeve, 

articulated supracondylar wedge). In both conical and 

cylindrical residual limbs, the rubber sleeve produced the 

least pistoning of the seven evaluated systems (ranged 

from 6 to 31mm for all system), and the cylindrical residual 

limb had more pistoning compared to conical stump.31 

Similarly, less movement was seen in this study with 

conical socket shape compared to cylindrical socket when 

100N traction load applied, but the average difference was 

small (0.05 mm). 

Literature has shown that donning and doffing the 

prosthesis is challenging for elderly amputees using Seal-

In X5 liner.2,22 This study showed that a minimum of 

151±15N was needed with the PETG conical socket and 

standard profile liner in open valve condition to pull off the 

socket, and a maximum of 748±291N was needed for the 

Thermolyne soft socket. Therefore, PETG would be good 

choice for amputees who may have difficulty doffing their 

prosthesis. 

Limitations 

In this study, plaster positive molds were used and 

covered with Plastazote and leather,32 and were pulled 

straight using an Instron test machine. In practice, the 

prosthesis user could wiggle their residual limb to remove 

their prosthesis with less force. Future research is needed 

to evaluate donning/doffing procedures in transtibial 

amputees. Reusable molds, covered with Plastazote foam 

and leather, were used to represent a residual limb; 

however, silicone materials might be a better option to 

simulate the soft tissue than Plastazote.33 

CONCLUSION 

The Unity system successfully controlled pistoning inside 

the socket for regular activity loads and also controlled the 

greatest traction loads. While relative movement was 

smallest for the Unity, the inactive vacuum (suction) 

condition was also viable for loads less than 100N. This 

study showed that the Unity system can hold the residual 

limb inside the socket successfully even if there is a failure 

in the vacuum pump. 

ACKNOWLEDGEMENTS 

The authors would like to thank Knut Lechler, and Kristleifur 

Kristjansson for providing technical advice and support. Natalie 

Baddour and Francois Robitaille are thanked for assistance with 

mechanical testing. 

DECLARATION OF CONFLICTING INTERESTS 

The authors have declared that no competing interests exist. No 

commercial party having a direct financial interest in the results of 

the research supporting this article has or will confer a benefit on 

the authors or on any organization with which the authors are 

associated. 

AUTHOR CONTRIBUTION 

• Hossein Gholizadeh,  

designed the system and the protocol, conducted the 

experiments, collected and analyzed the data, discussed the 

results and drafted the manuscript. 

• Edward D Lemaire,  

supervised the overall project, and helped in writing and 

revising the manuscript. 

• Rasool Salekrostam,  

conducted mechanical testing, collected and analyzed the 

data.  

SOURCES OF SUPPORT 

This study was financially supported by Össur and Mitacs. 

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