Biomechanical reasoning

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Literature review

Problem definition

Hagberg and Brånemark (2001) have presented findings that show reduced quality of life after having a non-vascular transfemoral amputation. The two main reasons for this is pressure ulcers or shaving from the socket and the increase of heat which leads to sweating (Hagberg & Brånemark, 2001). However, it is well known that many amputees do suffer from vascular diseases, the most common is diabetes (Colgecen, Korkmaz, Ozyurt, Mermerkaya, & Kader, 2016; Vandelanotte et al., 2013). This makes it even more important to be aware of pressure ulcer and wounds in general, since ulcers in the worst case scenario can lead to a reamputation (Vandelanotte et al., 2013). It is therefore interesting to examine what can be done to reduce the risks of ulcers by adjusting the sitting positions.


In general people tend to spend a great amount of time sitting, exactly how much is of course dependent on different aspects of life such as profession, interests and health. Bennie and collueges (2013) report that Europeans on average spend 5-6 hours every weekday sitting down. Another study by Miller and Brown (2004), that examined the level of activity for several professions over a long period of time, found that the average of sitting each day is 9.4 hours. People who worked with management or administration sat even longer (Miller & Brown, 2004), these must be considered low active jobs. A Dutch study by Schoppen and collueages (2001) have shown that Dutch lower limb amputees often switched to a less physically challenging job after the amputation. This could possibly be true in other European countries as well.
Vandelanotte et al. (2013) has shown that the unemployed people sit way more on average during a day compared to employed people. In consonance with this, a large group of amputees with skin problems on the residual limb are unemployed (Colgecen et al., 2016). A return to gainful employment is, from both a social and medical point of view, one of the main challenges of rehabilitation (Fernández, Isusi,Gómez, 2000). However, this is extremely difficult due to both social and physical limitations (Vandelanotte et al., 2013). It is not certain that they can return to their original job even if they start working again.
Larsson, Aaro, Normelli and Oberg (2002) reported that uneven weight bearing can lead to discomfort when sitting, and for people with limited sensation can it lead to pressure sores. Treating these soars also comes with an economical cost (Hobson, 1992), this is an additional reason for preventing uneven load distribution.
It should also be stated that there is a difference between sitting and sitting with a socket. Transfemoral amputees that have used both socket and osseointegration rapport that one of the benefits of not having to use a socket is the increased sitting comfort (Lundberg, Hagberg, & Bullington, 2011). Hagberg, Häggström, Uden and Brånemark (2005) also rapport that it is common for prosthetic socket users to experience discomfort when sitting, moreover they rapport that the discomfort decreases when the hip flexion is more than 90 degrees. The active range of motion in the hip joint is significantly reduced when wearing a transfemoral socket (Hagberg et al., 2005). Which makes it particularly interesting to examine transfemoral amputees.

Pressure and pressure distribution

Pressure is defined as force per area and is measured in pascal (Pa), were 1 Pa equals 1 N/m2 (Nationalencyklopedin, 2016) A high pressure on a small area increases the risk of pressure ulcers on the given area (Ek & Lindgren, 1997). High pressure is therefore sought out to be prevented. Pressure ulcers are defined by the NPUAP as a localized injury as a result of pressure or friction to the skin or underlying tissue (Black et al., 2007). The damage done by the pressure is due to a period of insufficient blood flow, which leads to a deficiency of oxygen in the cell tissue (Ek & Lindgren, 1997).
Forssén and Larsson (2014) found that the pressure distribution relation between the socket- and the healthy side was 30/70. Inconsistently the participants in the study experienced that the pressure was distributed evenly which led to the theory that the pressure inside the socket could be distributed in a different way. One possible explanation considered by Forssén and Larsson is that some load is taken up by the socket and transferred trough the prosthesis to the floor. This led to the recommendation for future studies to examine the pressure inside the socket. Furthermore they found that the angle of the knee may affect pressure distribution as one of the participants had a prosthesis with dissimilar leg length (Forssén & Larsson, 2014).
In this field of knowledge, there is still very little research regarding the amount of pressure inside a transfemoral socket. However Zhang, Zhu, Shen and Zheng (2013) uses a finite element method to analyze the stress in the interface between a transfemoral socket and the residual limb during walking. Zhang and colleges found a calculated normal stress between 50-80 kPa on the distal end of the stump and between 90- 120 KPa on the ischial bearing areas during the gait cycle of an average male amputee.
Research done on wheelchair users have explored the effects on pressure displacement between the buttocks and seat for tilted positions (Giesbrecht, Ethans, & Staley, 2011) and inclinations of backrest and seat (Chen et al., 2014; Hobson, 1992). The theory was that tilting the chair can change the orientation of the gravity in relation to the body, which changes the areas of peak pressure on the tissue (Giesbrecht et al., 2011). This is an interesting theory and indicate that the position is an important aspect to consider when researching the pressure when sitting.

Measurements and methods used by previous studies

According to the literature review made by de Looze, Kuijt-Evers and van Dieën (2013) have some studies reported a significant correlation between pressure distribution and comfort, others only showed an association and some found no clear connection between these, moreover, the associations can currently only be attributed to certain types of seats. A pressure mat has been used for measuring pressure when sitting in a wheelchair in different positions (Chen et al., 2014; Giesbrecht et al., 2011; Lung et al., 2014). Previous studies by students regarding sitting with a prosthesis have also used a pressure mat to investigate either load distribution or maximum pressure of transfemoral amputees when seated (Forssén & Larsson, 2014; Thomasson & Johansson, 2015). Pressure distribution seems to be the objective measurement that best reflects a subjective ranking of seating comfort (de Looze, Kuijt-Evers, & van Dieën, 2003).
Other methods such as electromyography (EMG), measuring the load on the spine or analyzing the subjects posture were not associated as clearly to the subjective rankings, nor were these associations statistically significant (de Looze et al., 2003). This indicate that measuring the pressure distribution is an accurate way to investigate the sitting comfort of transfemoral amputees. Correspondingly, subjective evaluations may to some extent reflect the pressure distribution.

Biomechanical reasoning

Some biomechanical reflections were done concerning various sitting positions, in order to develop a method that could examine the consequences regarding pressure, of altering chair heights for transfemoral amputees.
The authors expect that when a transfemoral amputee sits without foot support the pressure on the residual limb will increase anteriorly/distally. This hypothesis is based on the fact that the weight from the prosthesis not supported by the seat will create a vertical downward force, which affects the distal end of the socket. It is assumed that the distal force will increase the angle between femur and the socket, since expected that the femur bone will remain in an almost horizontal position, while the distal end of the socket will be inclined slightly forward. This should theoretically lead to pressures on the residual limb located anteriorly/distally as well as posteriorly/proximally. It is, however, assumed that the distal downward force on the socket will create a more distal pressure between the socket and chair.
The most equal distribution of distal and proximal pressure inside the transfemoral socket is expected to be found when the amputee sits with the knee flexed 90 .̊ In this scenario it is assumed that the pressure will be distributed almost evenly over the posterior inner side of the socket. The authors are aware that it is not realistic to get an even distribution, since the density and shape of the residual limb is not constant throughout the socket. An even distribution would also require that the posterior wall is parallel to the chair and has no edges. The proximal edge of the socket is often rounded in order to raise comfort for the amputee. This gives an increased posterior/proximal force when siting, which could affect the bone to flex inside the socket and give an anterior and distal pressure. It is assumed, however, that this force rather compresses the soft tissue than affects the bone.
When transfemoral amputees sit on a lower chair with the knee flexed more than 90 ̊the foot support will create a vertical upward force, provided that the prosthetic lower leg is vertical, this affects the distal end of that socket to be pushed upwards. As this happens the gravity will influence the residual limb to move downwards. This theoretically leads to a higher pressure on the residual limb posteriorly and distally as well as anteriorly/proximally.

Justification of the current study

There is, to our knowledge, a shortage of peer reviewed articles on the subject of sitting with a transfemoral prosthetic socket. The majority of studies concerning transfemoral amputees mainly focuses on gait and prosthetic components. It is unclear how the pressure inside the socket is distributed and how this affects the residual limb. The article by Zhang and colleges (2013) who uses the finite element method is one of the few who approaches the pressure in the interface for transfemoral amputees, however it only addresses walking. Furthermore the 3D-modell created in this study were based on several assumptions and compromises, which lowers the validity.
However, no previous studies have examined how the height of a chair influence the pressure distribution inside and underneath a prosthetic socket whilst seated. Nor is it confirmed that high pressure between the socket and seat equals a high pressure inside the socket. Therefore we found it interesting to examine the pressure of the interface between socket and residual limb.
The placement of the sensors is essential when measuring the pressure inside the socket and the procedure is more circumstantial than measuring the pressure between socket and chair. A confirmed relation between these could make it easier to conduct future studies on this subject since they could then use only the pressure mat. Thus is a comparison between the results from the pressure mat and the sensors inside the socket motivated.
Amputees, like all people, spend a large part of their lives sitting as seen in the literature. Therefore is it important that the conditions are as good as possible to avoid discomfort and pressure related problems. Position is according to Hobson (1992) important to research further when discussing pressure-management regimes. By reducing the maximum pressure, the risk of pressure ulcers is lowered (Ek & Lindgren, 1997). To achieve reduced pressure inside the socket more information surrounding the aspects of sitting is required. One way to modify the situation when seated is to adjust the height of the chair. This is easy to implement in everyday life and in the workplace by using an ordinary office chair. A seating device to heighten for instance the toilet seat is a common aid used by amputees according to Olsson and Axtelius (2008).
Arbetsmiljöverkets författningssamling (AFS 2012:2 5 §) states that the employer should arrange the workplace so that the working posture is as beneficial as possible for the employees. Thus knowledge of what is a beneficial position could improve the workplace for the amputees, which, as mentioned, is important for the rehabilitation process.
There is a need to examine the internal and external pressure for the socket, regarding sitting for transfemoral patients in different positions in order to get more information on how to prevent excessive pressure on the tissue and improve comfort.


The primary aims of this study were to examine how the height of a chair influence the pressure distribution in the longitudinal direction of femur, both inside a transfemoral socket as well as between the socket and chair. Furthermore to compare the results of these measurements.
The secondary aims were to examine if the load distribution between the amputated- and the healthy side is affected when the height of the chair is altered, and to investigate how the height of a chair influence the subjective comfort whilst seated.

Research questions

How will the posterior pressure move in the proximal/distal direction underneath and inside a transfemoral socket when the chair height is altered?
For which chair height will the pressure be most evenly distributed in the longitudinal direction of femur?
Is it possible to compare the pressure inside and underneath a transfemoral socket?
How does the load distribution between the left and right side differ when the chair height is altered?
How does the height of the chair influence an amputee’s subjective comfort?
It was expected by the authors that both the pressure inside the transfemoral socket and the pressure between the socket and underlying material would be most evenly distributed when the knee joint was flexed 90 degrees, because then the thigh and socket would be parallel to the surface of the chair. In theory, lifting the distal end of the socket should influence the external proximal peak pressure to increase and the distal pressure to decrease.
Differences in amount of pressure inside the socket is assumed to be lower than the external pressure. This is based on the theory that even if the socket is lifted from one part of the seat the socket still contain the residual limb and some pressure should be registered.
A low chair could be uncomfortable for amputees, as many losses the range of motion in the hip (Hagberg et al., 2005), and it requires considerable flexion. It is also difficult to sit down that low with a prosthetic knee since it does not have any voluntary control. On the other hand a high chair, where the feet does not reach the ground, could be uncomfortable due to the fact that the weight of the prosthesis is not supported through the foot.


This was an experimental study where the subjects were given three interventions. Each subject served as its own control. Repeated measurements were made and results were compared between the studied conditions.


The participants in this study was recruited through contacts of the Department of Rehabilitation at the University of Jönköping. Two subjects participated in this study, both were transfemoral amputated females. The two participants will be referred to as subject X and subject Y throughout the rapport. Subject X and subject Y have respectively been amputated due to cancer and trauma. More information regarding the participants can be found in “Results”.

Inclusion and exclusion criteria

The inclusion criteria were unilateral transfemoral amputees, who was using a prosthetic socket, regardless of design and other components. The subjects should be over 18 years old and should be able to stand up and sit down repeatedly with ease. They should also be able to DON/DOFF the socket themselves and be able to flex the hip- and knee joint 105 degrees bilaterally.
The exclusion criteria were; osseointegration, exuding ulcers on the residual limb or subjects that were dissatisfied with the fit of the prosthetic socket, since poor fit could indicate risk of movement inside the socket, which could affect the trustworthiness of the results. People with cognitive impairment, reduced sensibility in the lower extremities or other pathological conditions that affects the ability to participate in this study were also excluded.


Clinical examination

For the clinical examination a goniometer was used to measure angles, a tape measure was used for all length measurements and a scale was used for measuring the bodyweight and the weight of the prosthesis.

Pressure measurement systems

Two different systems were used for the data collection. Tekscan CONFORMat was used for the measurement between the socket and the seat. The CONFORMat provides information on pressure distribution, by using a 30 x 30 grid of specially designed sensors. The Pliance system from Novel was used for the measurement between the residual limb and the socket. The Pilance sensors used contains 4 x 4 pressure transducers on an area approximately 30 by 30 mm. The default frequencies of each software are used, 10 Hz for Tekscan and 50 Hz for Novel.


The chair used in this study is an ISOMed dynamometer. It is possible to adjust the inclination of the backrest as well as moving it forwards and backwards over the seat. The seat and backrest is flat and relatively firm, which the authors consider ideal when using the pressure mat. The same chair was used for all measurements in order to minimize the amount of independent variables, for example cushioning material, design and inclination. To simulate a chair with different heights a footstool and planks were used to adjust the distance to the footrest. These were secured using tape in order to make it safe for the participants.


A modified questionnaire from a previous study (Donnelly, Callaghan, & Durkin, 2009) was used to acquire quantitative data, to help classify the level of comfort the subject experienced during three different positions. The questionnaire uses a 10 cm scale on which the participants marks the level of experienced discomfort for different parts of the sitting area. The whole questionnaire have been validated. However, this study only include two small section of the original questionnaire. The original questions 13-20, will in this study be referred to as Part 1. It concerns the placement on the body that is in contact with the seat and subjective experienced discomfort. Part 2 consists of the questions 4-6 from the original questionnaire, which concerns discomfort from the chair design, together whit a question where the participants ranked the three positions from most comfortable to least comfortable. The modified questionnaire can be found in Appendix 1.

Data collection

The subjects sat in three different positions whilst the pressure was measured by two different systems simultaneously. Five measurements were carried out for each position, resulting in a total of 15 recordings. Each system measured the general pressure distribution as well as the distal and the proximal peak pressure. For each recording 15 seconds of data were collected.

Clinical Examination

All subjects were examined before participating in the experiment. During the examination it was confirmed that all subjects complied with the physical inclusion and exclusion criteria by investigating the range of motion and looking for ulcers. Additionally measurements were taken. The leg length, length of the remaining limb, bodyweight, the weight of the prosthesis and the distance between knee center and plantar surface were documented. Any apparent differences between the prosthetic and sound leg were also noted.

Table of contents
Literature review
Biomechanical reasoning
Justification of the current study
Research questions
Data collection
Data analysis
Ethical considerations
Proximal-distal pressure between socket and chair
Proximal-distal pressure inside the socket
Load distribution
Comparison between the proximal and distal pressure
Load distribution
Limitations of the study
Recommendations for future studies
How does the height of a chair influence the pressure distribution inside and underneath a transfemoral prosthetic socket whilst seated?

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