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VOLUME 4, ISSUE 1 

 2021 

 

RESEARCH ARTICLE 

Walker J, Marable W.R, Smith C, Sigurjónsson B.Þ, Atlason I.F, Johannesson G.A. Clinical outcome of transfemoral direct socket interface (part 2). Canadian 

Prosthetics & Orthotics Journal. 2021;Volume 4, Issue 1, No.6. https://doi.org/10.33137/cpoj.v4i1.36065 

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1 

Walker J, Marable W.R, Smith C, Sigurjónsson B.Þ, Atlason I.F, Johannesson G.A. Clinical outcome of transfemoral direct socket interface (part 2). Canadian 
Prosthetics & Orthotics Journal. 2021;Volume 4, Issue 1, No.6. https://doi.org/10.33137/cpoj.v4i1.36065 

 

 

 
RESEARCH ARTICLE 

 

CLINICAL OUTCOME OF TRANSFEMORAL DIRECT SOCKET INTERFACE (PART 2) 

Walker J1, Marable W.R2,3, Smith C3, Sigurjónsson B.Þ3, Atlason I.F4, Johannesson G.A5 * 
 

1 Virginia Prosthetic & Orthotics, Roanoke, Virgina, USA. 
2 Össur HF, Foothill Ranch, California, USA. 
3 Össur HF, Reykjavik, Iceland.  
4 Quick Lookup, Reykjavik, Iceland. 
5 TeamOlmed, Stockholm, Sweden. 

 

 

 

 

  

 

 

 

 

 

 

 

 
 

 

 

INTRODUCTION   

The primary goal for people that have undergone lower limb 

amputation is to return to the main activities of daily living, 

including recreational and professional activities.1 

Transfemoral (TF) amputation and the subsequent loss of 

knee function is known to negatively impact prosthetic 

fitting,  functional  outcome,  and  quality  of  life  (QoL),  as  

 

 

 

 

compared to transtibial amputation.2-7 Persons with TF 

amputation face significantly more challenges when 

receiving a prosthesis for the first time. With more days 

spent in rehabilitation, the functional outcome and QoL are 

still lower at the time of discharge compared to persons with 

transtibial (TT) amputation.7-11 Additionally, the Certified 

Prosthetist (CP) faces a more significant challenge in fitting 

TF patients versus TT patients.3,12 TF amputations may 

account for approximately 40% of all lower-limb 

amputations in the US alone and result in a twofold higher 

mortality rate than after  TT amputations13 and a lower rate 

 
OPEN  ACCESS Volume 4, Issue 1, Article No.6. 2021 

 

 

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

 

ABSTRACT 

BACKGROUND: Amputation at the transfemoral (TF) level reduces the rate of successful prosthetic 

fitting, functional outcome, and quality of life (QoL) compared with transtibial amputation. The TF socket 

interface is considered the most critical part of the prosthesis, but socket discomfort is still the most 

common user complaint.  Direct Socket for transfemoral prosthesis users is a novel interface fabrication 

process where the socket is shaped and laminated directly on the residual limb and delivered in a single 

visit. 

OBJECTIVE(S): The aim of this study was to investigate if prosthetic users' quality of life (QoL), comfort, 

and mobility with a Direct Socket TF interface were comparable to their experience with their previous 

prostheses. 

METHODOLOGY: The pre/post design prospective cohort study included 47 subjects. From this cohort, 

36 subjects completed the 6-months follow-up (mean age 58 years, 27 males). Outcomes at baseline 

included EQ-5D-5L®, PLUS-M™, CLASS, ABC, AMPPRO, and TUG. At 6-weeks and 6-months, 

subjects repeated all measures. Seven Certified Prosthetist (CP) investigators performed observations 

and data collection at six different sites (from July 2018 to April 2020).   

FINDINGS: Results showed significant improvement in all outcome measures for the 36 subjects that 

completed both 6-weeks and 6-months follow-ups. CLASS sub-scales showed significantly improved 

stability, suspension, comfort, and socket appearance. Improvement in K-Level and less use of assistive 

devices were observed with the AMPPRO instrument, indicating improved user mobility and 

performance. QoL was also increased, as measured in Quality-Adjusted-Life-Years (QALY) from the 

EQ-5D-5L. 

CONCLUSION: Evidence from the findings demonstrate that the Direct Socket TF system and 

procedure can be a good alternative to the traditional method of prosthetic interface delivery. 

ARTICLE INFO 

Received: February 19, 2021 

Accepted: May 26, 2021 

Published: June 8, 2021 

CITATION 

Walker J, Marable W.R, Smith 

C, Sigurjónsson B.Þ, Atlason I.F, 

Johannesson G.A. Clinical 

outcome of transfemoral direct 

socket interface (part 2). 

Canadian Prosthetics & 

Orthotics Journal. 2021;Volume 

4, Issue 1, No.6. 

https://doi.org/10.33137/cpoj.v4i

1.36065 

KEYWORDS 

Transfemoral Amputation, 

Health, Amputee, Prosthesis, 

Socket, Interface, Comfort, 

Outcome Measure, Satisfaction, 

Direct Casting 

 

* CORRESPONDING AUTHOR 

G. Anton Johannesson, PhD 

TeamOlmed, Kistagången 12, 164 40 Kista, Stockholm, Sweden. 

E-mail:  ajohannesson@teamolmed.se 

ORCID: https://orcid.org/0000-0001-8729-458X 

 

https://doi.org/10.33137/cpoj.v4i1.36065
https://jps.library.utoronto.ca/index.php/cpoj/index
https://doi.org/10.33137/cpoj.v4i1.36065
https://doi.org/10.33137/cpoj.v4i1.36065
mailto:ajohannesson@teamolmed.se
https://orcid.org/0000-0001-8729-458X


 

2 

Walker J, Marable W.R, Smith C, Sigurjónsson B.Þ, Atlason I.F, Johannesson G.A. Clinical outcome of transfemoral direct socket interface (part 2). Canadian 
Prosthetics & Orthotics Journal. 2021;Volume 4, Issue 1, No.6. https://doi.org/10.33137/cpoj.v4i1.36065 

ISSN: 2561-987X TRANSFEMORAL DIRECT SOCKET INTERFACE 

Walker et al. 2021 CPOJ 

 
of prosthetic fitting.5 Also, unfortunately, most CPs have 

less experience fitting TF amputees; thus, outcomes and 

proficiency are diminished.14 Moreover, TF interfaces are 

researched less frequently than TT interfaces. Therefore, 

CPs have less published evidence to guide their practice.12  

A meager body of published evidence surrounds TF 

interfaces, and most lack methodological quality. 

Investigations assessing the advantages and 

disadvantages of available socket designs are few and lack 

randomized controls.12 The heterogeneity of intervention, 

study population, and outcome measures make meta-

analysis impossible.3,12,14,15  

The TF socket interface, considered the most important part 

of the prosthesis, allows the user to control the prosthesis 

and provides pelvic stabilization during loading. The ischial 

ramus containment (IRC) socket claims to feature a "bony 

lock" believed to be most effective during full stance phase. 

The "bony lock" is defined as a 3-point support between the 

femoral shaft, the ischium, and a high medial and lateral 

socket trimline.16 While we can observe anecdotally that 

many patients succeed with such socket designs, only 

theoretical models and limited evidence exist to support 3-

point support efficacy.17 Despite limited evidence, the "3-

point support" sockets enjoy wide popularity, and many 

western countries consider them the standard of care, 

especially for active users. However, based on different 

functional philosophies, alternative socket designs are 

available. Performance can be retained, and comfort 

increased while abandoning or altering the 3-point 

support.17-19 

Furthermore, traditional fabrication and delivery of TF 

interfaces often involve multiple client visits to the prosthetic 

clinic for casting or scanning, diagnostic interface (or "check 

socket") fittings and modifications, definitive laminated 

interface fabrication, and final fitting and delivery.2,11,20 The 

user then returns as needed for adjustments and 

modifications mostly related to comfort and functional 

issues. It is understood that the sooner a patient can begin 

gait training with their definitive socket, the better.21,22  

The Direct Socket TF (DS-TF) enables a prosthetist to 

fabricate a custom-made interface directly on the residual 

limb in a single visit, similar to Direct Socket TT.23 The 

proximal portion of a DS-TF interface design differs from the 

proximal portion of sockets, typically included in the above 

described IRC sockets designs,16 as the proximal part of the 

DS-TF incorporates a size-specific silicone brim instead. 

The support provided by the DS-TF brim activates and 

stimulates important hip muscle function24 to enable loading 

and axial/transverse stability during normal walking.23 The 

unique DS-TF interface design has led to greater user 

satisfaction regarding interface function, comfort, and 

overall improvement in the fabrication and delivery 

experience compared to traditional methods.25-27 A deeper 

analysis of the outcome requires more specific tools.12 

Dissonance between the user and the prosthetic interface 

design can negatively impact comfort level and strongly 

correlates with lower functional outcomes.14 It often leads to 

increased residual pain, phantom pain,28 restricted 

movement,29 dermatological problems, or a combination of 

those,30 most often related to the proximal trimline and lack 

of femur stability and positioning. Historically the use of Self-

report instruments has not been the standard in daily 

practice, but the prevalence of their application has been 

increasing.31,32 CPs are improving their implementation of 

such instruments to help screen amputees for prosthetic 

fitting candidacy and monitor mobility and comfort 

outcomes.33 Responses facilitate communication between 

the prosthetic user and clinician, inform training decisions, 

and evaluate care efficacy.34,35 CPs have access to self-

report instruments, specifically applicable to TF prosthetic 

users that measure health outcomes, socket comfort, and 

mobility. Combining self-report and performance-based 

evaluation measures reveals a comprehensive picture of 

overall function. This type of detailed data is increasingly 

used to inform healthcare policies and payments.36  

This paper investigates if DS-TF direct lamination,25 in 

contrast to traditional plaster casting and 3D scanning 

techniques, can result in similar outcomes for the end-user. 

In this way, using plaster or foam model intermediates, that 

are only an approximation of the limb shape, are eliminated. 

We compared the new DS-TF interface versus the existing 

traditional socket using two modules from the Orthotics and 

Prosthetics 'User's Survey (OPUS) in the first article on this 

study.27 This paper, aiming to build on the previous 

publication, quantifies different subjective and objective 

outcomes using instruments applicable to this specific 

population of prosthetic users.  

METHODOLOGY 

Between July 2018 and October 2019, seven CPs in six 

prosthetic clinics across the United States implemented a 

new prosthetic interface for transfemoral prosthesis users. 

In total, 47 subjects were enrolled into this study. The 

previous article was derived from the same cohort27 and 

includes detailed descriptions of principal investigator 

selection criteria, practitioner/clinician training, 

implementation process, and interface design. The 

inclusion criteria (rationale) were as follows:  

• 50Kg<bodyweight<166Kg (the ISO validated 

weight limit of the DS-TF) 

• Cognitive ability to understand all instructions and 

questionnaires in the study 

• Patients who have undergone a TF amputation>1 

year post-amputation (this was to avoid 

https://doi.org/10.33137/cpoj.v4i1.36065


 

3 

Walker J, Marable W.R, Smith C, Sigurjónsson B.Þ, Atlason I.F, Johannesson G.A. Clinical outcome of transfemoral direct socket interface (part 2). Canadian 
Prosthetics & Orthotics Journal. 2021;Volume 4, Issue 1, No.6. https://doi.org/10.33137/cpoj.v4i1.36065 

ISSN: 2561-987X TRANSFEMORAL DIRECT SOCKET INTERFACE 

Walker et al. 2021 CPOJ 

 
postoperative problems and/or adjustments 

related to the initial prosthetic fitting of a new 

amputee) 

• Willing and able to participate in the study and 

follow the protocol 

• Circular dimension of 40-65 cm at the crotch 

(limited to available silicone brim sizes) 

• Residual limb length at least 20 cm from ischium 

to the distal end (fabrication limitation of the DS-

TF) 

• Currently using a prosthetic liner (this was to avoid 

potential confounding influence from transitioning 

an amputee from a skin fitting interface (i.e., 

without a liner) to an interface with a liner) 

Ethical approval was obtained from Advarra® IRB 

(CR00128417), and the investigation was registered at 

Clinical trials.gov NCT04312724. Signed informed consent 

was obtained from all participants. All study subjects 

completed the baseline measurements related to their 

current prosthesis (Figure 1). Each subject was 

consequently fitted with DS-TF by one CP with one 

Technician's assistance. Two subjects received a new knee 

and foot with the new interface.  

Manufacture and features of the DS-TF 

During the fabrication process, a specific casting liner is 

rolled onto the residual limb, followed by application of a 

protective silicone sheath. Next, the CP places a size-

specific silicone brim at the proximal part of the limb. A 

fiberglass or basalt fabric with a pre-attached 4-hole distal 

adapter is then rolled on the length of the limb. An additional 

protective sheath is applied on the outside of the fabric, and 

a two-part resin is injected through the distal adapter. The 

resin saturates the fiber, and then it hardens and cures. 

After 10-15 minutes it has cured enough to be removed. 

Finally, the socket is prepared to be connected to their knee 

and foot.25,26  

The brim is made of flexible silicone and laminated to the 

socket during this process, making the socket flexible 

proximally, while most of the socket is rigid. The flexible 

silicone brim encompasses and compresses the proximal 

thigh muscles when contracted, thereby stabilizing the hip 

at initial contact, loading response, mid-stance, and 

terminal-stance while creating axial and transverse 

stabilization.27 During swing phase, the brim only follows the 

hip movement.  

Outcome measures and data Collection  

Each prosthesis user completed two parts of the Orthotics 

and Prosthetics User's Survey (Client Satisfaction with 

Device (OPUS CSD) and Client Satisfaction with Services 

(OPUS CSS)) for evaluating the new interface and the 

service model. Additional data was collected using multiple 

standardized outcome measures, including: 

• Four subjective self-evaluation measures: health 

status; perceived mobility level; satisfaction 

regarding socket stability, suspension, comfort, 

and appearance; and confidence regarding 

balance and perceived risk of falls. 

• Two objective performance measures. 

 

 

Figure 1: Flow chart of the trial. 

 

Most outcome measures (PLUS-M™,37 CLASS,38 ABC,39  

TUG,40 and AMPPRO41) were collected using the 

Prosthetic Rehabilitation Outcomes Application (PROApp) 

iPad application. Two measures were collected using paper 

forms (EQ-5D-5L® and OPUS). The PROApp systematically 

helps the clinician gather and securely store subjective and 

functional outcomes data.  

DROP OUT BEFORE SIX-WEEKS (N=7) 

• One amputee died  

• Four amputees went back to their previous interface 

       •  One due to severe vascular problems (not device related) 

       •  Three due to poor M-L stability (compared with existing interface) 

•  Two lost to follow-up (one still using the new interface)  

AT SIX-MONTHS FOLLOW-UP (6MFU), N=36 

Subjective and objective evaluations measurement used:  

Subjective: EQ-5D-5L
®

, PLUS-M™, CLASS, (*OPUS-CSD and CSS), ABC  

Objective: TUG, AMPPRO 

* analysed in the previous article
27

  

SUBJECTS ENROLLED INTO THE STUDY (BASELINE), N=47 

Subjects completed self-evaluation measures with their existing socket, including 
subjective and objective evaluations measurement: 

Subjective: EQ-5D-5L®, PLUS-M™, CLASS, ABC, (*OPUS-CSD and CSS), 

Objective:TUG and AMPPRO  

After fitting with Direct Socket TF, subjects completed CLASS and TUG 

 * analysed in the previous article
27 

 

DROP OUT AT SIX-MONTH (N=4) 

•       One amputee went for additional surgery on the contralateral side 

(still uses the new interface) 

•        One lost from follow-up (still using the new interface) 

•        Two missing data 

  

AT SIX WEEKS FOLLOW-UP (6WFU), N=40 

 Subjective and objective evaluations measurement used:  

 Subjective: EQ-5D-5L
®

, PLUS-M™, CLASS, (*OPUS-CSD and CSS), 

ABC  

  Objective: TUG, AMPPRO  

* analysed in the previous article 
27

 

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4 

Walker J, Marable W.R, Smith C, Sigurjónsson B.Þ, Atlason I.F, Johannesson G.A. Clinical outcome of transfemoral direct socket interface (part 2). Canadian 
Prosthetics & Orthotics Journal. 2021;Volume 4, Issue 1, No.6. https://doi.org/10.33137/cpoj.v4i1.36065 

ISSN: 2561-987X TRANSFEMORAL DIRECT SOCKET INTERFACE 

Walker et al. 2021 CPOJ 

 
The PROApp also helps with clinical care decisions, 

communication with referral sources, physical therapy, and 

validating prosthetic care to payor sources.42 Table 1 

contains full description of the outcome measures used. 

Outcome measures were collected on each subject at three 

time periods: baseline, 6-weeks post fitting, and 6-months 

post fitting. Subject completed CLASS, Plus-M, ABC, 

AmpPro, OPUS, EQ-5D, and TUG using their existing 

socket. On the day of fitting, subjects completed TUG (as 

an objective measurement) and CLASS (as a subjective 

measurement)  with their new DS-TF interface (Figure 1). 

Subjects returned to repeat CLASS, Plus-M, ABC, AmpPro, 

OPUS, EQ-5D, and TUG six weeks after fitting and then 

again six months after fitting. The goal was to collect 940 

datasets: meaning every subject (N=47) completed 20 

outcome measurements in total over the six months.  

Sample size and statistical methods 

We conducted a pretrial power analysis for the estimated 

required sample size using GPower version 3.1.9.650 and 

estimated effect size based on published articles7,51,52 for 

the primary endpoint assuming a normally distributed 

amputee population. Therefore, we expected that 38 

subjects were required to complete the protocol with a 

power of 0,95 and α at 0,05. We estimated the drop-out rate 

to be 20%, and therefore, 47 subjects were recruited.  

• EQ-5D-5L®: A valid and reliable questionnaire to describe and value health. The descriptive system comprises five dimensions: mobility, self-care, 
usual activities, pain/discomfort, and anxiety/depression. Each dimension has 5 levels:  

o No problems, Slight problems, Moderate problems, Severe problems and Extreme problems.  
The patient is asked to indicate his/her health state by ticking the box next to the most appropriate statement in each of the five dimensions. This 
decision results in a 1-digit number that expresses the level selected for that dimension. The digits for the five dimensions can be combined into a 5-
digit number that describes the 'patient's health state. This health state is then an indicator of the utility of the patient.  One of the values that can be 

derived from EQ-5D-5L® is quality-adjusted life-year (QALY).43 The mean (standard deviation) utility value for the US population in 2020 was 

estimated to be 0.85 (SD=0.21).43  QALYs can be used to evaluate the efficacy of one healthcare intervention versus another. QALY can be used to 

guide patients and providers to prosthetic solutions that maximize QoL increases while also implementing efficient fitting processes and using 

healthcare funds responsibly.44    

• PLUS-M™: The Prosthetic Limb Users Survey of Mobility (PLUS-M™) measures perceived mobility and was developed for lower limb prosthesis 
users. The survey includes 12 questions that assess mobility with a prosthetic leg and are answered on a five-point scale ranging from "unable to 
do" the activity, to able to do the activity "without any difficulty". A higher PLUS-M™ T-score corresponds to greater mobility. PLUS-M™ T-scores are 
referenced to the PLUS-M™ development sample (n=1091 lower limb prosthesis users).  A T-score has a mean of 50 and a standard deviation of 
10. A PLUS-M™ T-score of 50 represents the mean mobility reported by the development sample.  A respondent that receives a T-score of 60 has 
reported a level of mobility approximately 1 standard deviation above the mean. Conversely, a respondent that receives a T-score of 40 has reported 

their mobility to be about one standard deviation below the mean.45 

• CLASS: The Comprehensive Lower-limb Amputee Socket Survey is a newly developed outcome measurement tool. It reports measures regarding 
the function of the prosthetic interface (e.g., the socket) that is composed of 15 items. The first five context items are scored using a 4-point Likert-
type scale with response options and the corresponding point value of: strongly disagree (1), disagree (2), agree (3), and strongly agree (4). The last 
item of every determinant addressed the overall satisfaction and was scored using a numerical rating scale with values ranging between 0 and 5 
points. The maximum possible score for each of the four determinants was 25 points. 
The CLASS score for each subscale is then represented on a 0%–100% scale (with 100% indicating excellent satisfaction). The average overall 
socket satisfaction including all levels (ankle, transtibial, knee and transfemoral) of the CLASS, administered measured on 124 LLA, was  found to 

be for Stability 70% (SD=18), Suspension 71% (SD=19), Comfort 69% (SD=20) and Appearance 58% (SD=22).15 

• ABC: Activities Based Confidence indicator quantifies 'individuals' confidence in their ability to perform 16 activities of daily living by rating confidence 

from 0% (no confidence) to 100% (complete confidence) for each activity. The scores for each of the activities are averaged to obtain a total score.46 

In a study by Miller et.al., the mean score was 63.8. For subjects amputated due to vascular reasons, it was 54.1 and 74.7 for subjects with amputation 

due to non-vascular reasons.39 

• *OPUS: The OPUS (Orthotics and Prosthetics 'User's Survey) is a set of self-reported outcome measures to be used within O&P clinics for the 

assessment of functional status, quality of life, and client satisfaction and was used in the previous article.
27

 The OPUS instrument consists of five 

independent modules, two of which were used in this study: Client Satisfaction with Device (CSD) and Client Satisfaction with Services (CSS).  The 
CSD and CSS include a total of 21 questions, scored on a 1- 6, discrete scale: Strongly Agree, Agree, Neither agree nor disagree, Disagree, Strongly 

disagree and Don't Know/Not Applicable.47 

Different authors have used scoring for OPUS in a different way, so there is no minimal or maximal score reported. The mean score reported by Jarl 

et.al. for CSD was 36.5 and for CSS was 55.7 on a US sample of 126 prosthetic users (all levels).48 

Performance-based (objective) outcome measurements were: 

• TUG: The Timed Up and Go test is a test where the prosthetic user performs the test by standing up from a chair, walking ten feet, turning and 
returning to the chair, and then sit on the chair. This measure tests a number of tasks that are essential for mobility, such as standing from a seated 
position, walking, turning, and sitting down on the chair, and can be used with or without walking aid. A lower limb prosthetic user who takes ≥19 

seconds to complete the TUG indicates an increased risk of fall.49  

• AMPPRO: This clinical tool is designed for assessing an amputee 'subject's mobility and for assessing existing or potential functional ambulation of 
the lower-limb amputee. It consists of 21 tasks, classified into four categories: sitting balance, simple mobility, standing balance, and gait and 

functional activities. The total score ranges from 0 to 47 points. Higher scores indicate better mobility.41  

The normative data for lower limb amputees has been established according to the K levels classification with the five categories ranging from K 

level 0 (least mobile/not using prosthesis) to K level 4 (most mobile) intended to indicate a 'person's rehabilitation potential,35 using the  AMPPRO 

Score. The normative values of AMPPRO for each of the K Levels classification have been set to:  

• K1 = AMPPRO 15-26, K2 = AMPPRO 27-36, K3 = AMPPRO 37-42,K4 = AMPPRO 43-47  

*Included in previous article
27 

 

Table 1: Full description of the outcome measures used in this study. 

https://doi.org/10.33137/cpoj.v4i1.36065


 

5 

Walker J, Marable W.R, Smith C, Sigurjónsson B.Þ, Atlason I.F, Johannesson G.A. Clinical outcome of transfemoral direct socket interface (part 2). Canadian 
Prosthetics & Orthotics Journal. 2021;Volume 4, Issue 1, No.6. https://doi.org/10.33137/cpoj.v4i1.36065 

ISSN: 2561-987X TRANSFEMORAL DIRECT SOCKET INTERFACE 

Walker et al. 2021 CPOJ 

 
We used R version 4.03 (R-Studio Version 1.2.5033) and 

lme4.53 We performed linear mixed-effects analyses of the 

relationship between outcomes and clinical need (defined 

as users who needed a replacement prosthetic interface, 

according to new referral, due to wear and tear or volume 

changes) and those with no clinical need of replacement. As 

fixed effects, we entered age, gender, and evaluation point 

(tested for interaction with clinical need) into the model. As 

random effects, we included intercepts for subjects and 

investigators and by-subject and by-item random slopes for 

the effect of clinical need. P-values were obtained by 

likelihood ratio tests of the full model with the effect in 

question against the model without the effect.  

RESULTS 

The subjects, with a mean age of 59 years (36-79 years) 

when entering the study, 33 men and 14 women prosthesis 

users, represented a wide range of activity levels (Figure 2).  

Of the initial cohort, 36 participants completed the entire 6-

month follow-up (6MFU; 27 men and 9 women: mean age 

58.2 years (38-81 years). Nine subjects dropped out of the 

study: 7 at or before 6-week follow-up (6WFU; including one 

deceased) and 2 before 6MFU (Figure 1). One subject with 

advanced vascular disease and a small limb withdrew after 

one week. Three subjects withdrew, preferring their 

previous socket. Two subjects in the "clinical need" sub-

group did not complete all measurements at the three time 

periods and therefore were not included in further data 

analysis at 6MFU. One subject became non-ambulatory for 

a portion of the study for reasons unrelated to the 

prosthesis. Though he was considered a drop-out, he 

continued to use the new interface after the study ended. 

Three subjects didn't come in for follow-up measurements. 

At least two of them continued to use the new DS-TF 

interface. All subjects used a liner with their interface. A 

description of their prosthesis can be found in the previous 

study.27 

Subjects represent the full spectrum of K-Levels 1 through 

4: 

• K-level 1, n=4 

• K-level 2, n=11 

• K-level 3, n=21 

• K-level 4, n=11 

Upon inclusion, participants were classified into two sub-

groups: subjects with a "clinical need" of socket 

replacement and subjects with "no clinical need", as 

determined by their CP. The mean age of the "clinical need" 

sub-group was 59.0 year (SD=11.8), and the mean age of 

the "no clinical need" sub-group was 58.8 year (SD=12.3). 

Mean ages in the two sub-groups remained comparable 

throughout the investigation period providing a power of 

95.3% and 94.0% for the follow-ups, respectively (Figure 2). 

 

Figure 2: Age groups at 6MFU divided into "clinical need" (N=26) 

and "no clinical need" (N=10). 

After 6MFU, we had collected 694 datasets, out of a 

maximum potential of 720 datasets, from the 36 subjects 

that completed the follow-up. This gave us a 3.6% rate of 

missing data (Figure 1). Missed appointments and software 

error accounted for the following instances of missing data: 

• At Baseline/delivery: 2 CLASS, 1 AMPPRO, and 2 

TUG data sets 

• At 6WFU: 2 PLUS-M™, 2 CLASS, 2 ABCs, 4 

TUG and 3 AMPPRO 

• At 6MFU: 2 EQ-5D-5L®, 1 PLUS-M™, 1 CLASS, 

2 ABC, 1 TUG, and 2 AMPPRO 

Outcome measure results are presented in three sections 

below. The first section presents data including all 

participants that completed the 6MFU. The following 

sections present findings from the two sub-groups within the 

cohort – "clinical need" and "no clinical need" sub-groups. 

Satisfaction and functional assessment of all 

participants (N=36) (Table 2A): 

The EQ-5D-5L® mean utility score was 0.75 using the 

existing prosthesis at baseline (SD=0.18). Life quality 

increased significantly to 0.82 (SD=0.15) at 6WFU and to 

0.84 (SD=0.12) at 6MFU. 

The PLUS-M™ score at baseline on the existing prosthesis 

was 46 (SD=24). Mean mobility scores rose significantly to 

54 (SD=21) at 6WFU and to 61(SD=16) at 6MFU (e.g., >1 

standard deviation above the mean after 6MFU).   

The CLASS mean overall score improved significantly from 

74% at baseline with the existing interface to 86% on the 

day of fitting with DS-TF. This improvement was maintained 

during the follow-up period (Figure 3).

0

1

2

3

4

5

6

7

8

9

10

(33,39) (39,45) (45,51) (51,57) (57,63) (63,69) (69,75) (75,81)

N

Age in years

Age groups at 6 month follow up

Clinical need No clinical need

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6 

Walker J, Marable W.R, Smith C, Sigurjónsson B.Þ, Atlason I.F, Johannesson G.A. Clinical outcome of transfemoral direct socket interface (part 2). Canadian 
Prosthetics & Orthotics Journal. 2021;Volume 4, Issue 1, No.6. https://doi.org/10.33137/cpoj.v4i1.36065 

ISSN: 2561-987X TRANSFEMORAL DIRECT SOCKET INTERFACE 

Walker et al. 2021 CPOJ 

 

The cohort's ABC mean score at baseline was 63% 

(SD=25) on the existing prosthesis. The scores rose 

significantly to 75% at 6WFU and further to 78% at 6MFU. 

The TUG mean time at baseline on the existing prosthesis 

was 14.7sec. (SD=7.4) compared to 14.0 sec. (SD=6.5) 

with DS-TF. TUG times improved significantly to 13.0 sec. 

(SD=6.5) at 6WFU and 12.8 sec. (SD=6.5) at 6MFU.   

The CP noted whether subjects elected to use an assistive 

device or not when completing the AMPPRO (and TUG) 

(Figure 4). The AMPPRO mobility mean score at baseline 

was 38 (SD=6.0).  Scores increased to 40 (SD=5.4) at 

6WFU and remained at 40 (SD=5.5) at 6MFU (Table 2 and 

Figure 4).  Several subjects exhibited improved K level 

and/or reduced use of assistive devices during the study 

period: 

• One K1 subject increased function up to K2 level 

using 1 cane  

• One K2 progressed from a walking frame to 1 

cane.  

• One K2 who previously used a cane no longer 

required an assistive device 

• One K2 advanced to the K3 level while 

discontinuing the use of a cane  

• One K3 went from currently using a walker to using 

one cane 

• One K3 discontinued the use of two crutches to 

using no assistive device  

• One K2 went from using no aid to using one cane 

• 7 subjects (using no aid) increased from K3 to K4 

• One subject using no aid declined from K4 to K3 at 

the 6MFU (Figure 4) 

All outcome measures (incl. OPUS from the previous article) 

showed significant improvement (Figure 5A) for the cohort. 

Satisfaction and functional assessment of participants 

with the clinical need for new interface (N=26), (Table 

2B): 

EQ-5D-5L®, PLUS-M™, ABC, and AMPPRO results in this 

sub-group are similar to the whole group, showing 

statistically significant (P=<.001) improvement during the 

investigation period (Table 2 and Figure 5B). The mean TUG 

time at baseline on the existing prosthesis was 14.2sec. 

(SD=7.4). Mean TUG time on the day of fitting with the new 

interface was 13.2 sec. (SD=5.5) and the time improved 

further to 13.0 sec. (SD=5.7) at 6WFU and 12.7 sec. 

(SD=5.3) at 6MFU.   

Both TUG and AMPPRO showed statistically significant 

improvement in function (P=0.02) (Table 2B). 

Satisfaction and functional assessment of participants 

without the clinical need for new interface (N = 10), 

(Table 2C): 

The outcome of EQ-5D-5L® and the PLUS-M™ did not 

reach statistical significance in this sub-group. However, the 

Figure 3: CLASS score at baseline (with existing socket) and at delivery of the new socket, including follow-up at 6 weeks and 6 

months. 

Baseline Delivery 6WFU 6MFU Baseline Delivery 6WFU 6MFU 

CLASS SCORE CLASS SCORE 

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7 

Walker J, Marable W.R, Smith C, Sigurjónsson B.Þ, Atlason I.F, Johannesson G.A. Clinical outcome of transfemoral direct socket interface (part 2). Canadian 
Prosthetics & Orthotics Journal. 2021;Volume 4, Issue 1, No.6. https://doi.org/10.33137/cpoj.v4i1.36065 

ISSN: 2561-987X TRANSFEMORAL DIRECT SOCKET INTERFACE 

Walker et al. 2021 CPOJ 

 
outcomes directly related to the new interface function 

(CLASS and AMPPRO) showed significant improvement at 

6MFU. Additionally, the mean TUG time in this sub-group at 

baseline on the existing prosthesis was 15.9sec. (SD=7.6); 

 

 

 

 

 

 

 

 

 

 

 

 

that is higher than the "clinical need" sub-group. Mean TUG 

time on the day of fitting with DSTF was 16.1sec. (SD=5.5) 

and improved to 13.0sec. (SD=5.0) and 12.9sec. (SD=5.1) 

at 6WFU and 6MFU (P=0.04), respectively, (Table 2C). 

 

 

0

2

4

6

8

10

12

14

16

18

20

B 6MFU B 6MFU B 6MFU B 6MFU

K Levels1 K Levels2 K Levels3 K Levels4

Subjects K-Levels at Basline and at 6 Month Follow up by Assistiv device

None On Cane Two crutches Walker

Figure 4: Changes from Baseline to 6MFU in K Levels using the AMPPRO Score, and assistive device use. 

Figure 5: A: Illustrating changes (index value from 0-100%) in outcomes for all participants that completed the study from Baseline (Orange) 

to 6MFU (Blue) for all measures except TUG. B and C: Illustrating changes (measured on a scale from 0-100%) in outcomes for the "clinical 

need" sub-group and the "no clinical need" sub-group (5C), from Baseline (Orange) to 6MFU (Blue) for all measures except TUG.   

ABC

PLUS-M™

CLASS

OPUS CSDAMPPRO

EQ-5D-5L

OPUS CSS

No Clinical Need Six Month Cohort

6MFU Baseline

ABC

PLUS-M™

CLASS

OPUS CSDAMPPRO

EQ-5D-5L

OPUS CSS

Full Six Month Cohort 

6MFU Baseline

ABC

PLUS-M™

CLASS

OPUS CSDAMPPRO

EQ-5D-5L

OPUS CSS

Clinical Need Six Month Cohort

6MFU Baseline
A B C 

0% 

25% 

50% 

75% 

100% 

0% 

25% 

50% 

75% 

100% 

0% 

25% 

50% 

75% 

100% 

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8 

Walker J, Marable W.R, Smith C, Sigurjónsson B.Þ, Atlason I.F, Johannesson G.A. Clinical outcome of transfemoral direct socket interface (part 2). Canadian 
Prosthetics & Orthotics Journal. 2021;Volume 4, Issue 1, No.6. https://doi.org/10.33137/cpoj.v4i1.36065 

ISSN: 2561-987X TRANSFEMORAL DIRECT SOCKET INTERFACE 

Walker et al. 2021 CPOJ 

 
 

Table 2: Outcome measurements from all subjects (A) of the cohort, with clinical need (B) and without clinical need (C). 
  

 A (ALL SUBJECTS) 
Baseline* 

(N=36) 
Delivery** 

(N=36) 
6WFU 
(N=36) 

P 
6MFU 
(N=36) 

P 

EQ-5D-5L® (Index value) 0.75 (0.18) - 0.82 (0.15) <.001 0.84 (0.12) <.001 

Missing  -   2 (5.4%)  

PLUS-M™(Percentile) 46.0 (23.6) - 54.3 (20.6) <.001 61.4 (16.0) <.001 

Missing  - 2 (5.4%)  1 (2.7%)  

CLASS (Percentage) 65.8 (23.1) 84.8 (11.6) 85.1 (13.2) <.001 87.6 (12.2) <.001 

Missing 1 (2.7%) 1 (2.7%) 2 (5.4%)  1 (2.7%)  

ABC (Percentage) 63 (25) - 75 (15) <.001 78 (13) <.001 

Missing  - 2 (5.4%)  2 (5.4%)  

TUG (sec.) 14.7 (7.4) 14.0 (6.5) 13.0 (5.4) <.001 12.8 (5.2) <.001 

Missing  2 (5.4%) 4 (10.8%)  1 (2.7%)  

AMPPRO (Score) 38.0 (6.0) - 39.8 (5.4) <.001 39.6 (5.5) <.001 

Missing 1 (2.7%) - 3 (8.1%)  2 (5.4%)  

 B (WITH CLINICAL NEED) 
Baseline* 

(N=26) 
Delivery** 

(N=26) 
6WFU 
(N=26) 

P 
6MFU 
(N=26) 

P 

EQ-5D-5L® (Index value) 0.71 (0.19) - 0.81 (0.17) <.001 0.84 (0.14) <.001 

Missing  -   2 (7.4)  

PLUS-M™(Percentile) 45.8 (22.2) - 54.5 (21.3) <.001 61.2 (15.7) <.001 

Missing  - 2 (7.4)  1 (3.7)  

CLASS (Percentage) 62.0 (25.3) 85.4 (11.6) 86.0 (14.7) <.001 88.2 (12.6) <.001 

Missing 1 (3.7)  2 (7.4)  2 (7.4)  

ABC (Percentage) 61 (26) - 75 (15) <.001 77 (12) <.001 

Missing  - 2 (7.4)  2 (7.4)  

TUG (sec.) 14.2 (7.4) 13.2 (5.5) 13.0 (5.7) 0.08 12.7 (5.3) 0.02 

Missing  1 (3.7%) 3 (11.1%)  1 (3.7%)  

AMPPRO (Score) 37.8 (6.7) - 39.6 (5.6) 0.01 39.0 (6.1) 0.02 

Missing  - 2 (7.4%)  2 (7.4%)  

 C (WITHOUT CLINICAL NEED) 
Baseline* 

(N=10) 
Delivery** 

(N=10) 
6WFU 
(N=10) 

P Value 
6MFU 
(N=10) 

P 

EQ-5D-5L® (Index value) 0.83 (0.11) - 0.84 (0.09) 0.93 0.86 (0.08) 0.50 

Missing  -     

PLUS-M™(Percentile) 46.5 (28.4) - 53.8 (20.0) 0.50 62.0 (17.9) 0.06 

Missing  -     

CLASS (Percentage) 74.2 (14.9) 83.2 (12.3) 83.2 (9.5) 0.03 86.3 (11.6) <.001 

Missing  1 (10.0%)     

ABC (Percentage) 68 (22) - 73 (17) 0.44 81 (15) 0.01 

Missing  -     

TUG (sec.) 15.9 (7.6) 16.1 (8.8) 13.9 (4.7) 0.02 12.9 (5.1) 0.04 

Missing  1 (10.0%) 1 (10.0%)    

AMPPRO (Score) 38.8 (5.1) - 40.3 (5.1) 0.08 41.1 (3.4) <.001 

Missing 1 (10.0%) - 1 (10.0%)    

All data are presented as mean (SD). Missing data were not included in the statistical tests. 
* Baseline measurement regarding existing interface (as a part of the prosthesis).                                    
 ** Baseline measurement regarding the DS-TF interface (as a part of the prosthesis) 

https://doi.org/10.33137/cpoj.v4i1.36065


 

9 

Walker J, Marable W.R, Smith C, Sigurjónsson B.Þ, Atlason I.F, Johannesson G.A. Clinical outcome of transfemoral direct socket interface (part 2). Canadian 
Prosthetics & Orthotics Journal. 2021;Volume 4, Issue 1, No.6. https://doi.org/10.33137/cpoj.v4i1.36065 

ISSN: 2561-987X TRANSFEMORAL DIRECT SOCKET INTERFACE 

Walker et al. 2021 CPOJ 

 
DISCUSSION 

Aggregate results revealed that the study subjects 

experienced multifaceted improvements during the study. In 

the "clinical need" sub-group, the EQ-5D-5L® outcome 

measure showed significant improvement in the user's 

health state compared with the previous interface.  

The EQ-5D-5L® mean utility score (0.75), using the existing 

prosthesis at baseline, increased significantly to 0.84 at 

6MFU, indicating that average DS-TF prosthesis users in 

this study cohort improved their QoL and reached a level 

similar to the US norm population of 0.85.37 The PLUS-M™ 

score at baseline using existing prosthesis was 46, slightly 

lower than the mean mobility score of 50 reported for the 

PLUS-M™ development sample.45 However, the mean 

mobility scores rose significantly during the study,  to 61 at 

6MFU (>1 standard deviation above the mean (=10)).   

The cohort's ABC mean score was 63% using the existing 

prosthesis at baseline, which is in line with findings by Miller 

-a mean ABC score of 64% among a comparative 

prosthetic-user population.39 However, the score rose 

significantly above the comparative population to 78% at 

6MFU. TUG test times showed significant improvement 

over the study period, indicating the DS-TF interface did not 

increase subjects' risk of falling. OPUS CSS and CSD, also 

discussed in the previous article,27 showed significant 

improvements. The CLASS outcomes after 6-months 

showed improvement in all subscales indicating increased 

user satisfaction with interface stability, suspension, and 

appearance (Figure 5 A&B).  The AMPPRO mean scores 

significantly increase from 38 at baseline to 40 at 6MFU. 

The cohort’s mobility improvements were manifested in K-

Level increases, reduced dependency on assistive devices 

and the combination thereof.  Notice the cohort’s distribution 

on the graphs in Figure 4 shift in a positive direction toward 

increased mobility from baseline to 6MFU. That a socket 

replacement could have a significant impact on K-level was 

an unexpected but enlightening outcome. 

To our knowledge, few studies have investigated QoL 

changes using QALY instruments in the prosthetics field. 

Those that exist were designed to examine the benefits of a 

knee unit rather than an interface.54,55 EQ5D-5L scores in 

our study reveal a significant increase (p<0.001) in QoL in 

a relatively large study cohort (for the prosthetics field) 

attributed to the interface. QoL improvements are the 

culmination of all the other improvements, including 

perceived health status, mobility, satisfaction, stability, 

suspension, comfort, appearance, confidence, balance, 

pain, and fall-risk. No other known studies have investigated 

the effects of the TF interface on QoL.  

Traditional fitting timelines involve multiple fabrication steps, 

patient visits, and adjustments.16 This study's certified 

prosthetists delivered all DS-TF interfaces in a single visit. 

This reduces the number of visits typically required to 

fabricate and deliver a TF socket since DS TF eliminated 

check sockets for all subjects. There are several reasons 

why shortening the fitting timeline is important; however, 

sacrificing quality to gain speed is unacceptable.  

There is a glaring absence of published studies on 

outcomes focusing on fabrication methods and/or different 

interface designs. One study, by Kahle et al., included 15 

subjects comparing 3 socket designs. Results showed no 

significant difference regarding socket position, movement, 

or comfort. Kahle et al. stated that skeletal motion within the 

socket is an important but unquantified factor in outcomes.56 

No other study with more than 5 subjects was found to 

compare interfaces regarding design and function over 6 

months or demonstrate significant outcome improvements 

for an interface type.3 However, Wurdeman et al. enrolled 

multiple subjects (N=509), but they did not compare various 

interfaces. The authors concluded that mobility has a strong 

positive correlation with both QoL and general satisfaction 

in lower limb loss patient care.22 The absence of 

comparative studies could be due to a lack of good testing 

methodology (i.e., lack of sensitivity) and/or a lack of 

standardized subject inclusion criteria complexity, e.g., 

patient needs, cause of amputation, surgical technique, skin 

and soft tissue status/condition, age groups and overall 

health status.3  

This study found improvements across multiple outcome 

measures of both the "clinical need" and "no clinical need" 

sub-groups. It is noteworthy that users with no perceived 

need for a socket replacement still experienced significant 

improvements with DS-TF. As an explanation for this, we 

might speculate that patients are reluctant to reveal their 

true thoughts on their current device because they dread 

the traditionally long fitting procedure and view it as a hurdle 

that is only worth mounting in certain circumstances. This 

study shows that DS-TF can sometimes improve patient 

function and satisfaction regardless of the perceived 

"clinical need of a new socket". While this study presents a 

standardized process for fabrication and delivery of DS-TF, 

an important question remains: would our field (both 

practitioners and users) benefit from a more standardized 

criteria for determining whether or not a patient has a 

"clinical need" for a new interface? CLASS, as a clinical 

instrument, could be a viable choice for standardizing 

socket assessment and replacement. CLASS was 

developed in 2019 with the intention of being more 

descriptive than the Socket Comfort Score.38,57 Clinicians or 

investigators may use it to follow prosthesis users’ socket fit 

and function over time in clinical care or clinical studies, but 

further research is needed. 

Although socket fit is often highlighted in studies as 

important, the descriptions differ and often lack defining 

terms.17 Thus, there is no indication that "good fit" means 

the same things to everyone.14 There is also a void of 

https://doi.org/10.33137/cpoj.v4i1.36065


 

10 

Walker J, Marable W.R, Smith C, Sigurjónsson B.Þ, Atlason I.F, Johannesson G.A. Clinical outcome of transfemoral direct socket interface (part 2). Canadian 
Prosthetics & Orthotics Journal. 2021;Volume 4, Issue 1, No.6. https://doi.org/10.33137/cpoj.v4i1.36065 

ISSN: 2561-987X TRANSFEMORAL DIRECT SOCKET INTERFACE 

Walker et al. 2021 CPOJ 

 
investigations on other socket designs measuring the 

correlation in outcomes regarding mobility and QoL. Of the 

few studies that look at prosthesis user mobility, most focus 

on the TT level. If the study does include transfemoral 

prosthesis users, the focus is on the artificial knee,22 or the 

reference values are still being validated.58 Also, as pointed 

out in the previous article,27 interface designs lack 

standardized descriptions.17 Further limitations exist in the 

disparity of the study subjects. A documented assessment 

of condition and capabilities following a standardized 

approach could improve our ability to compare results 

across different studies.  

The present study possesses four central strengths: A 

comparatively large study cohort (n=47), representative of 

a relatively normal transfemoral prosthesis user 

population,2 with a comparatively long investigational period 

(6 months), and using 7 different validated outcome 

measures. Selecting and combining both subjective and 

objective measures in this way over a 6-month period gives 

a broad and deep picture of DS-TF users' outcomes. We 

are aware of no other published study on TF interfaces of 

this size, duration, and scope of outcome measures. 

CONCLUSION 

DS-TF users experienced improved QoL, satisfaction, 

comfort, and function with their new interface, along with 

increased mobility compared to the previous interface. 

These findings also revealed that interface replacement 

with DS-TF could increase objectively measured user K-

level, even when socket replacement isn't clearly clinically 

indicated. Future studies should include randomized 

controlled comparisons between different prosthetic 

interface designs and their impact on prosthetic user QoL 

and mobility. Additional future studies should contain 

procedures for identifying a standardized approach to 

determine when an interface replacement is appropriate. 

ACKNOWLEDGEMENTS 

We would like to acknowledge the CP´s who participated in 

the study: J. Arnold, B. Sampson, S. Parkinson, C. Smith, 

B. Clark, C.Z. Smith and the technicians involved including 

C. Keeling, R. Camper and E. Thompson. 

DECLARATION OF CONFLICTING 

INTERESTS 

All authors are employees of Össur HF except I. F. Atlason. 

Study principle investigators received no compensation 

from Össur HF.   

AUTHOR CONTRIBUTION 

• Joel Walker: Conceptualization; Study oversight; Data 

collection; Writing original; Review and editing. 

• W. Russ Marable: Conceptualization; Study oversight; 

Writing original; Review and editing. 

• Christian Smith: Conceptualization; Data collection; 

Review and editing. 

• Benedikt Þorri Sigurjonsson: Conceptualization; 

Obtained funding; Study oversight; Data analysis; 

Review and editing. 

• Ingi Freyr Atlason: Data analysis. 

• G. Anton Johannesson: Conceptualization; Study 

oversight; Data analysis; Writing original; Review and 

editing. 

SOURCES OF SUPPORT 

This study was financially supported by Össur HF. 

ETHICAL APPROVAL 

Ethical approval was obtained from Advarra® IRB 

(CR00128417) and the investigation was registered at 

Clinical trials.gov NCT04312724. Signed informed consent 

was obtained from all participants. 

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https://doi.org/10.33137/cpoj.v4i1.36065
https://www.ncbi.nlm.nih.gov/books/NBK531517/table/ch2.tab1
https://www.ncbi.nlm.nih.gov/books/NBK531517/table/ch2.tab1
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https://github.com/lme4/lme4/

