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TitleInterfaces in Medicine and Mechanics—2
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Page 1

INTERFACES IN MEDICINE AND MECHANIC~

Page 2

Proceedings of Interfaces 90, the Second International Conference on
Interfaces in Medicine and Mechanics held at the Instituti Ortopedici
Rizzoli, Bologna, Italy, 9-14 September 1990.

Also published by Elsevier Applied Science Publishers:

Interfaces in Medicine and Mechanics, edited by K. R. Williams and T. H.
J. Lesser, being the proceedings of the First International Conference on
Interfaces in Medicine and Mechanics held in Swansea in April 1988.

Conference Co-ordinators

K. R. Williams, UK
A. Toni, Italy
J. Middleton, UK
G. Pallotti, Italy

Advisory Committee

J. F. Bates
A. W. Blayney
C. A. van Blitterswijk
E. Czerwinski
C. Doyle
K. Fujikawa
P. O. Glantz
M. Green
T. H. J. Lesser
A. Meunier
T.A. Roberts
A. Rohlmann
B. R. Simon
K. Tanne
F. A. Young

UK
Ireland
The Netherlands
Poland
UK
Japan
Sweden
UK
UK
France
UK
Germany
USA
Japan
USA

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IMPROVEMENT OF FRACTURE HEALING BY APPLIED
AXIAL MICROMOVEMENT : A CLINICAL STUDY

S H WHITE, J L CUNNINGHAM, J B RICHARDSON, M EVANS and
J KENWRIGHT (OXFORD)·, and M A ADAMS, A E GOODS HIP,

E SMITH and J H NEWMAN (BRISTOLt
·Oxford Orthopaedic Engineering Centre, University of Oxford, Nuffield

Orthopaedic Centre, Headington, Oxford OX3 7LD, UK.
+Bristol Royal Infinnary, Marlborough Street, Bristol, BS2 8HW

ABSTRACT

A multicentre, randomised, controlled trial of the application of axial micromovement via
external skeletal fixation to open tibial fractures is described. The clinical results showed
a shorter time to bony union, and the mechanical results showed a steeper gradient in the
rise of fracture stiffness measurements than with rigid fixation. This specific regime of
short daily periods of imposed axial cyclical strain is thus shown to be osteogenic to tibial
fractures in man.

INTRODUCTION

Open tibial fractures are still a major challenge and most surgeons opt for external fixation
to stabilise the fracture and yet permit access for management of the soft tissues. There is
however concern that the degree of rigidity imposed by external fixation is detrimental to
healing for one often sees an inhibited callus response. We have designed an external
fixator which allows the surgeon to change the axial rigidity of fixation, by application of
axial micromovement through the pins to the fracture interface, in a precise and
predictable manner. In a series of experiments in ovine osteotomies, we have defined an
axial strain magnitude and rate of application that is osteogenic [1]. This study tests the
hypothesis that the same stimulus of axial micromovement applied early after injury, and
continued throughout healing, will enhance the healing of tibial fractures in patients.

MATERIALS AND METHODS

Patients at two centres (Bristol and Oxford) with open tibial fractures were randomly
selected for treatment using either a rigid external fixator, or the same ftxator but with a
micromovement module attached via sliding clamps. Children were excluded from the

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study as were patients with Grade 3:C fractures. Of the 82 patients entering the trial, one
patient died and another emigrated, leaving 80 patients who were followed through to
radiological consolidation of the fracture. The allocation of patients took account of injury
severity using a stratified randomisation, based upon the soft tissue injury grading of
Gustilo and Anderson [2], combined with a bone injury classification of Johner and Wruhs
[3]:

TABLE I

Classification of Injury Severity

Group Soft Tissue Injury Bone Injury

A Grade I Not comminuted

B Grade 2/3 Not comminuted

C Grade I Comminuted

D Grade 2/3 Comminuted

All patients were treated with a unilateral frame (Dynabrace, Richards Medical (UK) Ltd),
applied to the medial surface of the tibia. The two pins on one side of the fracture were
held by the micromovement module which allowed the option for rigid treatment using
locking nuts, or micromovement treatment using sliding clamps. In the micromovement
group a pneumatic pump was attached to the sliding clamps and small amounts of inter-
fragmentary cyclical axial displacement were applied (Figure I). A daily regime of
applied axial micromovement was started as soon as the leg was comfortable and always
within seven days of frame attachment. This daily mechanical stimulus had the following
characteristics:

1. Maximal initial longitudinal axial displacement of Imm.

2. Constant low force of 400N, so that the fatigue stress limit of the bone
screw interface was not exceeded.

3. A frequency loading of O.5Hz, approximately that of physiological walking.

4. 500 cycles per day (i.e. continuously over 17 minutes)

This externally imposed micromovement was discontinued when the patient was able to
reproduce this axial excursion through his or her own activities. The spring in the
micromovement module was pretensioned to lOkgs to allow movement when the patient
started at least lOkgs weight bearing (Figure 2). In contrast, the patients treated with rigid
fixation had all their clamps locked against the beam.

The external fixator was removed when clinical and radiological fracture union
was diagnosed. Most surgeons applied a light-weight functional cast after frame removal
until it was considered safe to allow unsupported weight bearing. The time to unsupported
weight bearing without fixator or cast was used as one measure of fracture healing. The
stiffness of the fracture was monitored throughout healing, using a strain gauge transducer
which we attached to the fixator column at two weekly intervals (Figure 3). Fracture
stiffness typically increases exponentially with time, and from a logarithmic plot of
stiffness values the time taken to reach a bending stiffness of 15NM/deg was calculated.

Results were analysed using non-orthogonal three-way analysis of variance.

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plethysmogram. The cell and the tip of the optic bundle were placed on
the subcutaneous tissue surface about 5 mm distal from the probe of the
electromagnetic flowmeter.

A catheter for measuring blood pressure was inserted into the femo-
ral artery.

Electrocardiogram. respiration. and rectal temperature too were
recorded simultaneously. All data were taken in through an A~D con-
verter and stored in a computer memory (PC-980!. NEC) for calculations
of each trial. A single trial-time was 99.84 sec. Values were con-
tinuously measured at 5 msec intervals; 19968 samples were taken for
each item in a single trial. Each trial contained between 300 and 600
heart beats. There were from 18 to 45 sampling points in a single
hear t bea t. Each val ue for a. /3. and CR in each hear t bea t was
calcurated according to both the old and the new methods.

PHOTO

ECG

BP

FLOW

RESP

TEMP

--',--_\,---'---L- I 500jJ V

I
---------------------------- I

O.lsec

140mmHg
100

2.5 ml/min

0.5

lmv

Figure 2. Sample polygram. PROTO: photoplethysmogram; ECG: electro-
cardiogram; BP: blood pressure; FLOW: blood flow; RESP: respiration;

TEMP: rectal temperature

Room temperature was maintained at 25' C± 2' C.

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499

As polygraphic estimations show. the anesthetized animals' physi-
ological conditions were kept as stable as possible.

RESULTS

Values for a. /3. and CR in each heart beat were calcurated according
to both the old and the new methods. [n the trial shown in Figure 2.
a sample polygram. with the old method. in CR (sec), mean value was
0.0095 and standered deviation was 0.0057. Wi th the new method.
however. the corresponding values were 0.0087 and 0.0024.

D[SCUSS[ON

Results of calcurations showed that the new method for arriving at a
and /3 is bet ter than the old one. Wi th the new method. standard
deviation was approximately halved. [n the old method. 2 times (t l and
t2) are chosen to caluculate a and /3; but in the new method. all
times in a given single heart beat are used in the calculation. When
physiological conditions as assumed from polygrams remain comparatively
stable. a smaller standard of deviation is thought to represent greater
accuracy of estimation.

Generally. our experimental method combined electrical impedance
plethysmography and electromagnetic frowmetry. although occasionally. to
avoid hemorrhage. we had to make do with photoelectric plethysmography.
We have already demonstrated the possibility of substituting photo-
electric plethysmography for electrical impedance plethysmography with
experimental [2] application of equation (1).

When mean blood flow (f) increased. average values and scattering
in CR tended to decrease. Adequate correlation coefficients between f
and CR did not necessarily result from the measurements. Furthermore.
the source of CR scattering remains unknown. [n the future. we intend
to apply our new method in studying this point.

REFERECES

1. Sasaoka. K. and Ogawa. K .• Analytical study of electrical impedance
plethysmography in peripheral blood flow. Med &: BioI. Eng. &: Comput..
1987. 25. pp.386-90.

2. Sasaoka. K. and Ogawa. [e.. Study of the blood flow in the lingual
artery of rat. Bull. Tokyo dent. ColI.. 1986. 27. Pp. 115-26.

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