HUMAN KNEE PROSTHESIS EQUIPPED WITH FORCE

SENSORS

M. Gazzoli, E. Sardini, M. Serpelloni

Department of Information Engineering, University of Brescia, Brescia, Italy

G. Donzella

Department of Mechanical Engineering, University of Brescia, Brescia, Italy

Keywords: Measurement in human knee, Instrumented implant, Biomechanics, Force transducer, Strain sensor.

Abstract: In-vivo monitoring of human knee implants after total arthroplasty increases the knowledge about articular

motion and loading conditions. The proposed knee prosthesis equipped with force sensors executes force

measurements in polyethylene human knee prosthesis by two sensorized metal bars positioned under the

femoral condyles and fully contained in the polyethylene insert. In this paper, a new force sensor, which

acts in the knee integrated in the prosthesis, is proposed with the aim of reducing the hysteresis of

polyethylene material and increase the rigidity of the insert. The fabricated sensors are characterized and

tested by means of a mechanical press controlled load. The realized conditioning electronics is done by low

power components and it can be integrated in an autonomous system. The forces transmitted across the knee

joint during normal human activities such as walking, running or climbing can be directly measured.

Furthermore, the device can be used to improve design, refine surgical instrumentation, guide post-operative

physical therapy and detect human activities that can overload the system.

1 INTRODUCTION

The measurement of forces acting on knees, while

walking or during normal movements of the leg is a

topic widely discussed in (Gattiker, Umbrecht,

Neuenschwander, Sennhauser, Hierold, 2008),

(Westerhoff, Graichen, Bender, Rohlmann,

Bergmann, 2009), (Heinlein, Graichen, Bender,

Rohlmann, Bergmann, 2007). In Heinlein et al.

(2007), a measurement system detects the forces

using strain gauge sensors: the authors propose the

measurement of six degrees of freedom of the knee

(three forces and three moments) using a titanium

frame inserted in the tibia with six strain gauges. In

(Crescini, Sardini, Serpelloni, 2009), an

instrumented knee prosthesis is proposed for

measure the forces between the femur and tibia by

magnetoresistors. These sensors measure the

deformation of the polyethylene surface of the

prosthesis in contact with the condyles of the tibia.

But, in this configuration and particular design, the

material shows hysteresis after application of forces,

because the polyethylene material has a viscous-

elastic component. The proposed system relies on

the use of a new sensors technology. A typical

prosthesis for human knee is composed by three

principal inserts. In Figure 1, the 3D model of the

adopted prosthesis is represented. The femoral and

tibial insert are made on titanium and the tibial plate

is made of polyethylene. On the top of the implant

two excavations are carried out (Figure 1) centered

under the contact point between the condyles and the

polyethylene insert. These holes are made for

accommodate two sensorized metal bars. Two strain

gauges are placed under the metal bars for

measuring the deformations and correlate the

measured data to the force applied. In this way, the

sensors are closer to the focal point of the condyles

with the prosthesis, and this placing allows

integrating electronics and sensors in the

polyethylene insert. The realized sensors and

conditioning electronics is done by low power

components and it can be integrated in an

autonomous system as proposed in (Crescini et al.,

2009).

349

Gazzoli M., Sardini E., Serpelloni M. and Donzella G..

HUMAN KNEE PROSTHESIS EQUIPPED WITH FORCE SENSORS.

DOI: 10.5220/0003154603490352

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2011), pages 349-352

ISBN: 978-989-8425-37-9

 2011 SCITEPRESS (Science and Technology Publications, Lda.)

Figure 1: Section of upper polyethylene insert with

integrated force sensors.

2 SENSOR

With the aim of reducing the hysteresis of

polyethylene material, in this paper, a new force

sensor that acts in the knee integrated in the

prosthesis is proposed.

The polyethylene insert was

divided in two parts: top and bottom. On the upper

insert two excavations are made, one in the medial

side of the tibial plate and the other in the lateral

side. Two sensorized metal bars are placed on the

hole as shown in Figure 1. The excavations are made

exactly under the contacts point of condyles for

directly transmit the forces on the sensors. In the

bottom insert the conditioning electronic is placed.

The insert was processed with a numeric control

milling machine. The excavations have dimensions

20 mm x 20 mm. The metal bars are made by inox

steel AISI 430 with thickness of 0.8 mm with the

same dimensions of the holes. The two metal bars

are sensorized by strain gauges (with resistance of

120 Ω, Gauge factor 1.88, dimension 5 mm x 5 mm

commercialized by HBM), and placed under the bars

as shown in Figure 1. With the aim of transferring

directly the forces on metal bars, the femoral

condyles are placed directly over the metal bars. To

ensure the biocompatibility of polyethylene insert,

on the superior face of the insert 2 mm thick film of

polyethylene was placed for covering the metal part

of the fabricated sensors. When a force is applied

through the femoral condyles, the metal bars are

bent. Young's modulus of inox steel AISI 430 is

196000 N/mm

and the Young’s modulus of

polyethylene is 2600 N/mm

. Increasing the

Young’s modulus of bent material, the material’s

hysteresis has been reduced.

Another advantage by this configuration is to

increase the rigidity of the insert. The deformation of

metal bars is very small, so that the mechanical

properties of the insert are very similar to the not

sensorized insert.

The resistances of strain gauges are measured by

inserting the strain gauges in two Wheatstone

bridges. The measured differential voltage of each

bridge was amplified and sampled by a digital

multimeter connected by GPIB bus with a PC. The

differential voltage is proportional to the force

applied by the femoral condyles.

Figure 2: Conditioning electronics of integrated sensors.

In Figure 2, the conditioning electronics of

integrated sensors are show. The systems are

composed by two Wheatstone bridges. The R

variable resistors are the strain gauge applied to the

metal bars, while the R resistors are the fixed

resistors of Wheatstone bridge. R

resistors are used

to fix the gain of instrumentation amplifiers. The

differential voltages are measured and amplified by

a micro power dual instrumentation amplifier,

INA2126, produced by Burr-Brown. The INA2126

has two precision instrumentation amplifiers for

accurate, low noise differential signal acquisition.

The two-op-amp design provides excellent

performance with very low quiescent current.

The output voltages are measured by two digital

multimeters (Fluke HP8810).

LateralHole

MetalBars

MedialHole

FemoralInsert

TibialInsert

StrainGauges

LOAD

Polyethylene

LOAD

BIODEVICES 2011 - International Conference on Biomedical Electronics and Devices

350

3 EXPERIMENTAL SETUP

The sensorized polyethylene insert is tested and

characterized by an Instron 8501 force machine. The

polyethylene insert was placed on the fixed part of

the Instron machine and the femoral component was

placed on the mobile component and the forces were

applied. The experimental setup is shown in Figure

Figure 3: Experimental setup with Instron machine and

measurement system.

The force control is made by a load cell integrated

and controlled by the Instron machine. The Instron

machine has an output voltage proportional to the

applied force. This voltage is measured by the digital

multimeter (Fluke HP8810). It lets you know the

profile of force value applied to the polyethylene

insert.

Figure 4: Block diagram of the experimental setup.

In Figure 4, the block diagram of experimental setup

adopted to test the realized sensors was represented.

The two strain gauges are placed in a Wheatstone

bridge, the differential voltages are measured by two

instrumentation amplifiers and the measured

voltages are sampled by two digital multimeters

(Fluke HP8810) connected by GPIB bus to PC. The

Instron machine has an output voltage proportional

to the generated force. The voltage is sampled by a

digital multimeter (Fluke 8810), connected by GPIB

bus to PC. Dedicated software LabVIEW is realized,

it allows to acquire the data and store the differential

voltage (Medial and Lateral Sensors) and the voltage

proportional to the force applied in single file. The

Instron machine is programmed to apply to the

polyethylene insert a very slow variation of load

forces. The applied forces start from 300 N to 1000

N. This force profile is used for the sensor

characterization. Using the experimental setup

showed in Figure 4 the Medial and Lateral Sensor

are characterized. The Instron machine is

programmed to generate the force profile previously

described. The measurement information are stored

in a single file and digitally elaborated.

The data obtained are showed in Figure 5. For

clarity of content only the characterization of Medial

Sensor was reported. The results of Lateral Sensor

are the same.

The output voltage from instrumentation amplifiers

is proportional to the applied force, and the

relationship between force and voltage is linear.

Figure 5: Medial sensor characterization.

The measured voltage was interpolated with a linear

straight and his equation (1) that correlates the

voltage with applied force is:

F = 593392·V-179064 (1)

Where F is the force applied in newtons (N) and V is

the measured voltage in volts (V).

4 CONCLUSIONS

In this paper the authors propose a different sensor

technology for force measurement in a total

prosthesis of human knee and it is fully contained in

the polyethylene insert. The proposed sensors

consist in two metal bars positioned under the

femoral condyle. The metal bars are made on inox

steel AISI 430 because it has a high Young’s



GPIB

e

8810



Fluke8810

LabVIEW

Instron

8510



Medial

Sensor

LabView

Multimeter

Amplifier

Instron

Machine

GPIBBu

Multimeter

Lateral

Sensor

Multimete



Amplifier

HUMAN KNEE PROSTHESIS EQUIPPED WITH FORCE SENSORS

351

modulus. That allows reducing the material’s

hysteresis. The metal bars are sensorized by two

strain gauges. The sensor characterization s was

done and the obtained data show a linear

relationship between the applied forces and the

measured voltages.

These sensors can be integrated in an

autonomous system that does not require internal

power sources, such as batteries. The autonomous

system can integrate an energy harvesting module

that extracts energy from magnetic field.

The forces transmitted across the knee joint

during normal human activities such as walking,

running or climbing can be directly measured.

Furthermore, the device can be used to improve

design, refine surgical instrumentation, guide post-

operative physical therapy and detect human

activities that can overload the system.

The sensorized total knee prosthesis will be

tested by a robotic mannequin for dynamical

characterization. The robotic mannequin can

simulate the human walking. During the test the

polyethylene insert will be subjected to dynamic

forces and it will be possible to make a pseudo in-

vivo dynamic characterization of the sensors.

REFERENCES

Gattiker, F., Umbrecht, F., Neuenschwander, J.,

Sennhauser, U., Hierold, C., 2008. Novel ultrasound

read-out for a wireless implantable passive strain

sensor (WIPSS), Sensors and Actuators A 145–146

291–298.

Westerhoff, P., Graichen, F., Bender, A., Rohlmann, A.,

Bergmann, G., 2009. An instrumented implant for in

vivo measurement of contact forces and contact

moments in the shoulder joint, Medical Engineering &

Physics 31 207–213.

Heinlein, B., Graichen, F., Bender, A., Rohlmann, A.,

Bergmann, G., 2007. Design, calibration and pre-

clinical testing of an instrumented tibial tray, Journal

of Biomechanics 40 S4–S10.

Crescini, D., Sardini, E., Serpelloni, M., 2009. An

Autonomous Sensor for Force Measurements in

Human Knee Implants, Proceedings of the

Eurosensors XXIII conference Procedia Chemistry

1(1) 718-721.

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352