EXPERIMENTAL EVALUATION OF STRESS-SHIELDING EFFECT IN A NON-CONSTRAINED ELBOW ARTHROPLASTY

Joana Pereira (1), António Completo (1), Fernando Fonseca (2), Carlos Relvas (1), Antonio Ramos (1), Jose Simoes (1)

1. Mechanical Engineering Department, TEMA, University of Aveiro, Portugal;

2. Orthopaedics Department, Coimbra University Hospital, Portugal.

Introduction

The elbow joint has the biggest anatomical

complexity in the human body and it is the most

critical joint in the biomechanical point of view.

Two-thirds of all patients with rheumatoid arthritis

have problems in this joint, and when it is seriously

compromised, total elbow arthroplasty is often

indicated. Numerous models of the elbow

prostheses have been used over the last decade and

these can be divided into two categories: semi-

constrained and non-constrained. Recent studies,

based on records of orthopaedic Norway and

Finland [Fevang, 2009; Skyttä, 2009], revel no

significant differences in success’ rates between

these two types of arthrosplasties, with mean values

of 92% at 5 years and 84% at 10 years. In both

cases the aseptic loosening is pointed as the main

reason for the revision arthroplasty, and this

collapse happens mainly due to the stress-shielding

effect. Biomechanical studies, performed with the

semi-constrained implants, confirmed the presence

of abnormally levels of tension in the bone around

the implant, which can be associated with the risk

of loosening. However, for the non-constrained

implants, biomechanical studies are not available

for corroborating a loosening risk, as identified for

the semi-constrained implants. In order to answer

this question, a comparisons in the structural

behavior of the elbow joint in his native and

arthroplastic states, were carried out.

Methods

For this study were used synthetic models of the

ulna and humerus, in which were set strain gauges

rosettes to obtain cortex strains adjacently to the

elbow joint, before and after replacement. The load

cases applied to the condylar surfaces (intact and

prosthetic) were representative of daily activities.

The implant system used was the IBP

(Instrumented Bone Preserving Total Elbow

System, Biomet, UK).

Results

In the humerus strain levels increased nearly 100%

compared to the situation of intact bone. This

increase can lead to the risk of bone hypertrophy or

bone fatigue failure by overload.

Figure 1: Loading machine used and experimental

setup for humerus and ulna, with representation of

the strain gauges positions in Medial (M),

Posterior (P), Lateral (L) and Anterior (A).

Relatively to the ulna, the placement of the implant

caused a reduction in the strain levels. This

decrease is around 30% and it can lead to the bone

resorption by stress-shielding effect.

Discussion

The experimental results obtained showed that the

introduction of the non-constrained implant in the

humerus and ulna significantly alter the mechanical

behavior when compared to the native state. This

disturbance can be associated with risks of bone

fatigue by overload in the humerus and, at the ulna

with resorption by stress-shielding effect, for elbow

loads in the range of daily activities.

This behavior is similar to that observed in previous

studies with the semi-constrained implant, which is

in accordance with the results observed in the

nordic orthopaedic records.

Acknowledgement

Program COMPETE through the projects

PTDC/EME-PME/103578/2008,PTDC/EME- PME

/111305/2009 and PTDC/EME TME/113039/2009.

References

Fevang et al, J Shoulder Elbow Surg., 18:449-456,

2009.

Skyttä et al, Acta Orthopaedica, 80:472-477, 2009.

S308 Presentation 1029 − Topic 25. Implant biomechanics

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