DETECTION OF PROSTHESIS MICRO-MOBILITY USING FREE VIBRATION ANALYSIS

Evelyn García (1), Rafael Claramunt (1), Francisco Díaz(2), Antonio Ros (1) and Mónica Ledo (1)

1. ETSI Industriales-Universidad Politécnica de Madrid, Spain; 2. EU Informática-

Universidad Politécnica de Madrid, Spain

Introduction

Aseptic loosening is recognized nowadays as the

major reason of re-intervention in total joint

arthroplasties.

Non-cemented prostheses are not so extended like

cemented ones, and so they have been less studied

than the cemented ones. For those prostheses,

aseptic loosening is mainly due to defective bone

ingrowth, in most cases due to micro-mobility

between bone and implant.

The studies carried out trying to explore the micro-

mobility problem, have been conducted trying to

detect torsional mobility after surgery [Lannocca ,

2007] or by means of forced oscillations [Puers,

2000. Li, 1995. Georgiou, 2001. Rowlands, 2008] ,

been the accelerometer the most widely used

sensor.

In this work a new approach to the problem is

presented. The technique used is the analysis of the

free oscillation strain response of the bone after a

sudden release of an applied force, by using

conventional strain gages.

Methods

Six 4 mm conventional strain gages were attached

to both medial and lateral sides of a dried femur.

The gages were aligned with the long dimension of

the femur.

An experienced surgeon then reamed the femur,

until leaving a hole slightly higher than the exact

size needed to assure a good press-fit. An ABG

non-cemented stem was then placed into the hole

and the femur was fixed by the condiles in

horizontal position.

A mass of 300 N was then placed on the morse

cone of the stem and was suddenly released. The

strain register of the gages was recorded at 2 kHz

sampling frequency. The signal was treated in order

to study it in the time domain and in the frequency

domain by means of the Fast Fourier

Transformation (FFT). Three replicas of the

experiment were carried out.

After this experience, called “micro-mobility”, the

gap between bone and implant was filled in with a

soft polymer. The stem was then re-inserted and the

experiment was repeated again, being named as

“fixed”.

Results

In the time domain, the gages readings the

repeatability was much better in the fixed situation.

In the frequency domain, clear changes were

observed between the two situations in the second

harmonic, both in amplitude and in frequency. In

figs. 1 and 2 the recording of the gage placed at the

stem tip position are shown.

Figure 1: Time domain results. Gage on the stem

tip.

Figure 2: FFT results. Gage on the stem tip.

Discussion

The results obtained, at this stage only qualitative

ones, are very encouraging and seem to show that

instrumental defects can be detected with a simple

pre-clinical test.

References

Georgiou et al Clin Biomech, 16:315-323, 2001.

Lannocca et al, Med Eng Phys, 29:886-894, 2007.

Li et al, J Bone Joint Surg, 77:640-644, 1995.

Puers et al, Sensors and Actuators, 85:42-47, 2000.

Rowlands et al, Med Eng Phys, 30: 278–284, 2008.

Presentation 1626 − Topic 25. Implant biomechanics S335

ESB2012: 18th Congress of the European Society of Biomechanics Journal of Biomechanics 45(S1)