AN IMMERSIVE VIRTUAL ENVIRONMENT FOR PHANTOM LIMB

PAIN REHABILITATION

Steve Pettifer

, Toby Howard

, Ben Blundell

, David Edwards

and Ilan Lieberman

School of Computer Science, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.

University Hospital of South Manchester NHS Foundation Trust, Wythenshawe Hospital,

Southmoor Road, Manchester, M23 9LT, U.K.

Keywords:

Virtual Environments, Phantom Limb Pain.

Abstract:

Phantom Limb Pain is a debilitating condition that affects a signiﬁcant percentage of patients after loss of

an arm or leg. These patients experience chronic pain and other unpleasant sensations in the missing limb,

and the pain resists treatment. Previous research has demonstrated that pain levels can be reduced in some

patients when they are immersed in a virtual environment that presents a 3D computer graphics visualisation

of their missing limb, the movements of which are controlled by sensors attached to the remaining limb. In

this paper we describe a novel approach to the implementation of such a system, using the Kinect game device

for limb motion tracking, in conjunction with wireless motion sensors worn by the patient. We present some

preliminary, but very encouraging, results based on an informal trial with a patient.

1 INTRODUCTION

We present an interactive wireless computer graph-

ics virtual environment (VE) designed to form part of

a treatment regime for amputee patients who suffer

from the debilitating effects of Phantom Limb Pain.

We implement the VE using the Kinect motion track-

ing device, wireless inertial sensors, OpenGL, and the

Cinder C++ library. In this paper we begin with an

introduction to the condition of Phantom Limb Pain,

and then present an overview our our prototype VE,

presenting some preliminary results and observations

from an informal trial with an amputee patient.

2 BACKGROUND

Following the loss of a limb, whether by surgery

or trauma, it is common for amputees to report the

sensation that their missing limb remains attached to

their body (one of the earliest reports being(Mitchell,

1872)). Sometimes under volitional control, while

in other cases frozen in a speciﬁc position, these so-

called ‘phantom limbs’ often present such a vivid il-

lusion that it takes conscious effort for the amputee to

adjust to not using the missing limb as part of their

everyday lives (Jensen and Nikolajsen, 1994). Many

amputees experience some form of Phantom Limb

Pain (PLP), and although studies vary in their meth-

ods of classifying PLP, it is generally accepted that

between 60% and 85% of amputees suffer from some

degree of PLP (Kooijman et al., 2000; Desmond and

MacLachlan, 2006; Ephraim et al., 2005; Ehde et al.,

2000; Black et al., 2009).

The manifestation of PLP varies according to the

individual, including feelings of clenching, burning,

stabbing, severe itching and ‘electric shocks’, which

are often associated with a sense that the phantom

limb is contorted or misshapen in some way (Flor,

2002). Severe PLP (which occurs in around 25%

of sufferers) has been shown to cause societal with-

drawal for extended periods (Sherman et al., 1984),

and amputees with PLP are less likely to use a pros-

thetic limb, which often restricts their normal activi-

ties (Dolezal et al., 1998), and they are prone to de-

pression (Murray, 2005). According to the former Na-

tional Amputee Statistical Database (now ()limbless),

each year in the UK there are 5,500 new referrals to

prosthetic service centres, and 62,000 prosthesis users

in total (NASDAB, 2002). The number of amputees

worldwide is not reliably known, but studies in indi-

vidual countries suggest a ﬁgure well in the millions

(for example, (Zeigler-Graham et al., 2008) report

1.7 million amputees in the USA alone).

It was originally thought that PLP was caused by

nerve and tissue damage at the site of the amputation,

426

Pettifer S., Howard T., Blundell B., Edwards D. and Lieberman I..

AN IMMERSIVE VIRTUAL ENVIRONMENT FOR PHANTOM LIMB PAIN REHABILITATION.

DOI: 10.5220/0003831704260433

In Proceedings of the International Conference on Computer Graphics Theory and Applications (GRAPP-2012), pages 426-433

ISBN: 978-989-8565-02-0

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

and was therefore neuropathic pain, much the same

as that caused by a burn or a cut. However PLP has

proved resistant to the traditional range of pharma-

ceutical e.g.(Bone et al., 2002), surgical, and psycho-

logical pain management techniques (Kooijman et al.,

2000). Coupled with the realisation that children born

with missing limbs (where the problem is congenital

rather than caused by trauma) may also suffer from

PLP, this led to a hypothesis that the brain is in some

sense ‘hard-wired’ to expect signals from four limbs,

and that PLP results from a mismatch between this

mental template and the signals being generated by

the body itself (Flor et al., 1995; Borsook et al., 1998;

McCabe et al., 2005).

2.1 Visual Therapy

In 1996, psychologist V. K. Ramachandran devised an

experiment using a ‘mirror box’ (Ramachandran and

Rogers-Ramachandran, 1996) that allowed amputees

to view a reﬂection of their anatomical limb in the

visual space occupied by their phantom limb. He re-

ported that the mirror box induced vivid sensations of

movements originating from patients’ phantom limbs,

and in some cases relieved their PLP. Since then, oth-

ers have experimented with alternative visual thera-

pies such as the use of video e.g. (Giraux and Sirigu,

2003) or transcranial direct current stimulation e.g.

(Soler et al., 2010) with comparable results.

Hypothesising that interactive visual therapies

may work in similar ways to Ramachandran’s box,

around 2006 a number of research groups indepen-

dently began investigating the use of synthetic com-

puter graphics and virtual environments as a means

of reproducing the positive effects of the mirror box,

but without its physical limitations. MacLachlan’s

group developed an ‘augmented mirror box’ system

in which a data-glove worn on the amputee’s remain-

ing anatomical limb was used to track hand movement

and ﬁnger ﬂexion (Desmond et al., 2006). This was

used to control the movement of a synthetic repre-

sentation of their phantom limb which was displayed

on a ﬂat screen that replaced the mirror box’s mir-

ror. The limb could be customised in detail to reﬂect

any perceived distortions or contortions experienced

by the user. Of the three participants in the study,

two reported an intensiﬁcation of their phantom ex-

perience, one reported gaining increased control over

their phantom ﬁngers, and one reported a reduction

in their phantom pain. (Cole et al., 2009) took a dif-

ferent approach, instrumenting the stump of the af-

fected limb itself, and tracking its movement in order

to control the virtual representation. From a cohort of

seven upper limb and seven lower limb amputees, ﬁve

subjects in each group reported some control over the

movement of their phantom limb, and reported reduc-

tions in their PLP.

Our own previous work (Murray et al., 2006c;

Murray et al., 2010; Murray et al., 2005; Murray

et al., 2006c; Murray et al., 2006b; Murray et al.,

2007b; Murray et al., 2007a) like MacLachlan’s sys-

tem, tracked the movement of the anatomical limb

using a data-glove, transposing this motion to the

computer-generated image of a virtual limb. Unlike

MacLachlan, our system was a fully immersive en-

vironment, presented to the user via a tracked, head-

mounted display. In a small-scale trial, ﬁve partici-

pants whose PLP had resisted all other forms of treat-

ment used our system on a weekly basis; four reported

tangible reduction in their pain levels; two found they

gained some control over their phantom limb’s posi-

tion, being able to manoeuvre it into a more comfort-

able state; and one found that they were even able to

exercise some control over the stump of their ampu-

tated limb, which had previously been paralysed for

over twelve years.

In what, for a clinical application, would be con-

sidered very small-scale trials, all three approaches

demonstrated some success in allowing some (but no-

tably, not all) of the participants to gain a degree of

agency over their phantom limb and a measurable re-

duction in their pain levels. Given such small studies,

it is clearly premature to make any profound claims

about the effectiveness of the therapies. However, at

the same time, it is difﬁcult to ignore the potential

of this approach as a non-invasive mechanism for re-

ducing the considerable suffering of millions of am-

putees.

2.2 The Potential of Virtual

Environments for Treating PLP

Two recent reviews of the use of virtual reality for

pain control cautiously conclude that virtual environ-

ment ‘distraction may be a useful tool for clinicians

who work with a variety of pain problems’ (Mal-

loy and Milling, 2010) and that ‘VR [Virtual Real-

ity] is emerging as both a viable ﬁrst-line interven-

tion and as an adjunctive therapy to pharmacologic

agents’ (Mahrer and Gold, 2009). A further system-

atic review, which includes the use of mirror therapy

for the treatment of Chronic Regional Pain Syndrome

(a phenomenon thought to have similar causes to PLP

(Sato et al., 2010)) ﬁnds that there is value in such an

approach for the treatment of upper limb pain (Ezen-

dam et al., 2009). It must be noted that none of these

reviews speciﬁcally address the use of virtual reality

as a therapy for PLP: Mahrer and Gold and Malloy

AN IMMERSIVE VIRTUAL ENVIRONMENT FOR PHANTOM LIMB PAIN REHABILITATION

427

et al. examine pain caused by physical trauma such

as burns or surgery, whereas Ezendam et al. examine

only the reduction in pain as a result of the use of a

physical mirror box. We hypothesise, however, that

since visual therapies can be shown to improve PLP,

and that synthetic computer graphics can be used to

create illusions that reduce pain, the virtual environ-

ment approach to PLP reduction is a worthwhile av-

enue of research.

All three studies emphasise the need for further

work to better understand these effects. Mahrer et

al. highlight the need for ‘greater scientiﬁc rigor, in-

creased sample sizes, sound methodology, and in-

creased attention to individual user characteristics’

(Mahrer and Gold, 2009) in future studies. Meeting

such criteria using the systems described previously,

however, is likely to present a signiﬁcant practical

challenge: all three systems were built using bespoke

hardware and software, making them difﬁcult to repli-

cate in order to scale up to the kind of large scale trials

necessary to clinically validate their use as effective

therapies. In our own case it rapidly became impossi-

ble to source replacement parts for the specialist com-

ponents as they developed faults (suppliers of virtual

reality equipment are notoriously transient), and the

only existing installation of our system became unus-

able.

Since these initial trials, however, there have

been dramatic advances in computer graphics and re-

lated technologies: wearable displays, unencumbered

tracking devices and powerful 3D graphics engines

are now available as off-the-shelf consumer goods for

PCs, games consoles and home entertainment sys-

tems. Although these devices are in some cases of

lower quality than their specialist counterparts, be-

cause they are designed for general consumer use they

are more robust, cheaper and more easily available.

Driven by the demands of the computer games and

entertainment market, it seems likely that the quality

of these devices is only set to improve, and that their

future availability is essentially guaranteed. For ex-

ample, several groups (Calderwood et al., 2009; Lee,

2008; Scherfgen and Herpers, 2009; Sko and Gardner,

2009) have implemented virtual environments using

the Nintendo Wii (Nintendo Wii, 2011).

With the aim of producing a system that could

easily be replicated in order to support large-scale

clinical trials, we embarked upon a project to re-

produce our previous experimental setup using only

consumer-grade hardware. The remainder of this pa-

per describes this system, which we call ‘PLP2’.

3 SYSTEM DESIGN AND

IMPLEMENTATION

Our previous system used electromagnetic tracking,

with wired sensors attached the patient’s head and

anatomical limb. While providing reasonably accu-

rate absolute six degrees-of-freedom (DOF) tracking

and thus needing no calibration as such, these devices

required cumbersome wiring, and were also sensi-

tive to metal objects and electrical cabling which dis-

torted the tracking space, making ﬁnding a location

to carry out experiments difﬁcult. Equipping the pa-

tient with the sensors required specialist training, and

patients needed careful supervision during their ses-

sions to avoid becoming entangled in the numerous

cables. With the exception of the video feed, our

new system requires no cables and is not sensitive to

metal/electricity in the environment.

The PLP2 system engages the patient in gameplay,

as shown in Figure 1. There are two games, both of

which aim simply to give the patient a purpose to en-

gage with the environment in a way that causes them

to move their virtual limb. In both scenarios, the pa-

tient remains standing but is able to turn around and

has space to move a few steps in any direction. In

the ball-following game, the patient uses their virtual

limb to track the position of a ball, which moves ran-

domly around their position while remaining ‘within

reach’). In the tic-tac-toe game, the ‘game grid’ is

represented by a 3 x 3 array of spheres, where the

patient indicates their move by touching the relevant

sphere. In each game the patient’s task is to use their

intact limb for interaction. The patient wears an ordi-

nary baseball cap, instrumented with lightweight sen-

sors for tracking head movement, sending data wire-

lessly to the control computer. The patient’s anatomi-

cally intact limb is tracked using the Kinect motion

tracking device (Xbox 360 Kinect, 2011), and the

tracked movements are used to control a computer

graphical representation of the missing limb in the vir-

tual environment. The patient views the VE immer-

sively using a Vuzix VR920 headset (Vuzix, 2011).

The Vuzix was chosen for its reasonably high resolu-

tion screen (monoscopic, with two 640x480 panels),

its lightweight build and the addition of a rubber ‘eye-

shield’ that blocks external light, creating a more im-

mersive experience.

Figure 2 shows the software architecture of the

system, which uses only cross-platform components

(though our experiments have been conducted using

Mac OS X 10.6). The Cinder graphics library (Cinder,

2011) provides basic functionality and cross-platform

graphics support. OpenNI (OpenNI, 2011) and the

NITE Middleware (NITE, 2011) allows the Kinect to

GRAPP 2012 - International Conference on Computer Graphics Theory and Applications

428

(a) (b)

Figure 1: Screenshots of the system in use. Panel (a) shows the virtual environment as seen by the participant playing the

‘ball following’ game. Panel (b) shows the same environment as seen by the operator, with the environment’s control display

in the top left of the image, and the Kinect’s skeleton tracking window at bottom left.

be linked to Cinder in order to detect and track par-

ticipants. NITE provides basic information on the po-

sition and orientation of a set number of limbs and

joints for each detected person. A capture pose is re-

quired by the OpenNI middleware in order to estimate

the lengths of the limbs within the system. This must

take place before any skeleton tracking can begin.

This calibration step is performed per-patient, and our

prototype implementation requires an anatomically

complete image, which for an amputee patient may

involve an assistant temporarily holding a ‘dummy’

limb (constructed from cardboard) in place.

To determine the orientation of the patient’s head,

a six-DOF Inertial Measurement Unit from Spark-

fun (Sparkfun, 2011) is linked to an XBee IEEE Stan-

dard 802.15.4 radio (XBee, 2011) run by a lithium

polymer battery. The unit is attached to the patient’s

baseball cap. Orientation is computed using Kalman

ﬁltering (K

alm

an, 1960; Welch, 2009) and an ini-

tial rest point. Raw data from the three gyroscopes

and three accelerometers are sent wirelessly over the

XBee wireless link to the computer running the sim-

ulation.

With the orientation of the head from the gyro-

scope and the orientation of the body and the position

of the joints from the Kinect, a simple 3D skinned

representation of the participant can be created in 3D.

Models are generated from FBX ﬁles (FBX, 2011)

which support the generation of bones, clusters and

associated weights needed to deform a model cor-

rectly.

4 RESULTS

Our goal has been to build a cheap, portable, non-

intrusive system for experiments in use of virtual en-

vironments for PLP rehabilitation, with the aim of

replicating – and then, through larger-scale trials –

bettering the results observed in previous VE/PLP

experiments which used more expensive and fragile

equipment. To this end, we have built a prototype

system that uses mostly ‘off the shelf’ cheap, easily

available and replaceable components, and designed

tasks within the virtual environment that work within

the constraints of the hardware. From a purely ‘virtual

reality technology’ perspective, we believe the results

are very encouraging; for the overall cost of the sys-

tem (an order of magnitude cheaper than our previous

technology), the virtual environment is comparable in

quality, and because of the wireless tracking, using

the system is considerably simpler and more comfort-

able for the patient. The system’s current limitations

(awkward initial calibration, cumulative drift in the

gyroscope data, and lack of ﬁnger ﬂexion or hand ro-

tation tracking), we believe can be addressed in future

developments.

Perhaps more positive than the technological re-

sults, however, have been the system’s effects during

a preliminary, and highly informal, trial with one am-

putee patient. The trial was intended to give us basic

feedback on the behaviour and ‘feel’ of the system

from a patient’s point of view, rather than to invoke

any therapeutic effect. The patient had, ﬁve years pre-

viously, undertaken trials with our ﬁrst VR PLP sys-

tem, and had experienced reduction in pain lasting

AN IMMERSIVE VIRTUAL ENVIRONMENT FOR PHANTOM LIMB PAIN REHABILITATION

429

OpenNI &

NITE

Middleware

FBX SDK

Libraries

Cinder &

Boost

Libraries

Phantom Limb 2

Library

Phantom Limb 2

Environment Engine

Game

Plugins

Kinect(tm) Motion Tracker

Vuzix HMD

XBee Wireless

Transponder

Figure 2: Architecture of the PLP2 system, showing the relationship between the main software components and the hardware

devices.

for several days after each use. In the PLP2 trial,

which lasted three hours, the patient was introduced

to the system and after 15 minutes of free acclimati-

sation to the virtual environment and the limb track-

ing, was asked to play the games. On arrival, the pa-

tient reported their pain level as 8 (on the Numerical

Rating Scale of 10, with 0= no pain and 10=worst).

After 30 minutes using the PLP2 system, the pa-

tient reported his pain level had reduced to 2.5. A

short video illustrating the informal trial is available

at http://aig.cs.man.ac.uk/research/plp2.

5 CONCLUSIONS AND FUTURE

WORK

Our ﬁrst conclusion is simply that it is possible to de-

sign and build a VE system for exploring PLP using

primarily ‘off the shelf’ components. Our second is

that, though taking into account all possible techno-

logical and psychological issues, the number of vari-

ables to control for in future trials of this technology is

daunting, it is nevertheless interesting to observe that

the three approaches to VR PLP therapy described in

Section 2 all had broadly comparable levels of suc-

cess in reducing pain; and this in spite of them taking

radically different approaches to tracking and render-

ing (indeed, they appear to share no common tech-

nology). Even in our informal short trial reported in

Section 4 – which again shares no common technol-

ogy with our previous experimental setup – the patient

reported a non-trivial therapeutic effect. We note that

the various studies cited here of parameters relating to

VR pain reduction all use VR as a source of distrac-

tion from physical/neuropathic pain. It has been sug-

gested that the between-patient variability in response

to visual therapy is related more to the patient’s sus-

ceptibility to the virtual visual feedback than to physi-

cal factors (Mercier and Sirigu, 2009). Because of the

substantial variation in the techniques and parameters

that have been explored in this space with comparable

results, we speculate further that whatever the causes

of PLP, the therapeutic effect of VR is as much an ef-

fect of the process of gaining agency over a phantom

limb using a virtual representation, as it is of any spe-

ciﬁc feature or parameter of the virtual environment.

There remain many interesting questions to be ad-

dressed, which include: how important is the dimen-

sionality of the virtual environment? In our previous

system, the patient saw a true stereo view, from left

GRAPP 2012 - International Conference on Computer Graphics Theory and Applications

430

and right images in the head-mounted display’s re-

spective eye-screens; in PLP2, we currently present

the patient with a 2D view of the VE. This leads to

the general issue of photorealism – what roles, if any,

do the quality of the image, the frame-rate, and the

graphical realism play in any reduction of pain felt by

the patient? Some authors (such as (Hoffman et al.,

2006)) believe that display quality is an important fac-

tor. From our experience we remain unconvinced that

this is necessarily true.

Our research into the use of virtual environments

for PLP rehabilitation is ongoing, and our immedi-

ate plans are to improve PLP2 to enable us to address

the issues of inaccurate tracking and tracking drift, to

engineer a system which is more robust, and to use

the improved system for a set of formal clinic trials

with our project partners, where we will conduct ex-

periments to assess the contributions that factors such

as dimensionality and ‘realism’ make to therapeutic

value.

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