Reduced mechanical load decreases the density, stiffness, and strength of cancellous bone of the mandibular condyle

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Abstract

Objective. To investigate the influence of decreased mechanical loading on the density and mechanical properties of the cancellous bone of the human mandibular condyle.

Design. Destructive compressive mechanical tests were performed on cancellous bone specimens.

Background. Reduced masticatory function in edentate people leads to a reduction of forces acting on the mandible. As bone reacts to its mechanical environment a change in its material properties can be expected.

Methods. Cylindrical bone specimens were obtained from dentate and edentate embalmed cadavers. Mechanical parameters were determined in the axial and in the transverse directions. Subsequently, density parameters were determined according to a method based on Archimedes’ principle.

Results. The apparent density and volume fraction of the bone were about 18% lower in the edentate group; no age-related effect on density was found. The decrease of bone in the edentate group was associated with a lower stiffness and strength (about 22% and 28%, respectively). The ultimate strain, however, did not differ between the two groups. Both groups had similar mechanical anisotropy; in axial loading the bone was stiffer and stronger than in transverse loading.

Conclusions. Reduced mechanical load had affected the density and herewith the mechanical properties of condylar cancellous bone, but not its anisotropy.

Relevance The change in material properties of the cancellous bone after loss of teeth indicate that the mandibular condyle is sensitive for changes in its mechanical environment. Therefore, changes in mechanical loading of the condyle have to be accounted for in surgical procedures of the mandible.

Introduction

In general, as people grow older and lose their teeth, their masticatory function decreases (Boretti et al., 1995). This decrease in function coincides with an atrophy of the masticatory muscles (Newton et al., 1993) and a reduction in bite force (Helkimo et al., 1977). This leads to a reduction in the forces acting on the mandible, and, as it is known that bone reacts to its mechanical environment (Turner, 1998; Huiskes, 2000), consequently to a change in its material properties.

A number of studies have compared the mandibles of dentate and edentate subjects. All indicate that the density of cancellous bone is lower in edentate than in dentate subjects (mandibular body (Bassi et al., 1999); mandibular condyle (Hongo et al., 1989; Kawashima et al., 1997)). As the mechanical properties of bone are dependent on density (Carter and Hayes, 1977), it can be expected that the cancellous bone in edentate people is less stiff and strong than in dentate people.

The cancellous bone in the mandibular condyle has an anisotropic morphology (Giesen and Van Eijden, 2000). Its principal orientation coincides with the physiologic (superoinferior or axial) loading direction. In accordance with this anisotropic morphology, anisotropic mechanical properties have also been demonstrated (Giesen et al., 2001). In the axial direction, the bone appears to be more than three times stiffer than in the mediolateral direction. Whether this degree of anisotropy changes according to alterations in mechanical loading of the mandible is unknown.

The objective of the present study was to compare the density and mechanical properties of condylar cancellous bone in dentate and edentate subjects. The loss of teeth in the edentate group was used as a model of reduced mechanical loading, and concomitantly reduced density. We hypothesized that the density, stiffness, and strength of the cancellous bone would be lower in the edentate group than in the dentate group. Furthermore, we investigated the presence of differences in mechanical anisotropy between the two groups.

Section snippets

Specimen preparation

Cancellous bone specimens were taken from the mandibular condyles of 49 embalmed human cadavers. Twenty-five of them were edentate (14 female, 11 male, mean age 85.2 (SD 8.5) years). It was not known at what age they had lost their teeth. Twenty-four subjects were dentate (19 female, 5 male, mean age 74.8 (SD 11.7) years); their mean number of teeth was 8.5 in the upper jaw and 10.7 in the lower jaw. The use of human specimens conforms to a written protocol that was reviewed and approved by the

Results

An example of a stress–strain recording (axial loading) in a dentate and an edentate subject is depicted in Fig. 2. The cancellous bone of the dentate subject was stiffer and stronger than that of the edentate subject. In Fig. 3 the density and mechanical parameters are given for both groups. As there was no systematic difference in the density parameters between the axial and transverse specimens, their average values were used to compare the dentate and edentate groups. In the edentate group

Discussion

A number of studies in rats have indicated that the density of the cancellous bone of the mandibular body (Kiliaridis et al., 1996; Bresin et al., 1999) and mandibular condyle (Bouvier, 1988) is affected by a decrease of mechanical loading, i.e., the density of the bone decreased when the animals were fed a soft diet. As a model of reduced mechanical loading in the human masticatory system, we used the edentate mandible. From the literature it is known that with a loss of teeth the

Acknowledgements

This research was institutionally supported by the Inter-University Research School of Dentistry, through the Academic Centre for Dentistry Amsterdam. The authors would like to thank Anette Milton and Jane Pauli (Orthop. Res. Lab., Aarhus) for their laboratory assistance.

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