Femoral neck strength of mouse in two loading configurations: method evaluation and fracture characteristics
Introduction
Biomechanical testing is an important tool in the evaluation of bone strength, which is associated with susceptibility to fractures. Rat has been employed as a standard rodent model in biomechanical skeletal studies (Ferretti et al., 1993; FDA 1994; Kimmel 1996; Peng et al., 1994; Raab et al., 1990; Tuukkanen et al., 1994), while mouse has been used far less often. According to some earlier studies, genetic variation between different inbred mouse strains results in notable differences in bone density, bone geometry, and mechanical properties (Beamer et al., 1996; Bonadio et al., 1990; Di Masso et al., 1997; Kimmel, 1996; Kuro-o et al., 1997; Mikic et al., 1995; Simske et al., 1994). Transgenic mouse can be considered a suitable animal model for experimental studies of osteoporosis and aging (Kimmel, 1996; Kuro-o et al., 1997).
In the previous biomechanical studies of murine bones, diaphyseal bending strength (Ammann et al., 1997; Anderson et al., 1993; Bonadio et al., 1990; Di Masso et al., 1997; Hiltunen et al., 1993; Jämsä et al., 1998; Simske et al., 1994) and torsion strength (Mikic et al., 1995; Yang et al., 1993) have been measured. However, the femoral neck is clinically a more interesting measuring site than the diaphysis. The studies on rat and rabbit bones have shown that the evaluation of femoral neck strength is a good indicator of bone fragility (Peng et al., 1994; Søgaard et al., 1994; Turner et al., 1996; Tuukkanen et al., 1994). In these studies, the femoral neck has been loaded in a direction parallel to the femoral shaft axis. This simulates one-legged stance in humans. Simulating a fall to the lateral side, on the greater trochanter, is clinically more relevant, because approximately 90% of hip fractures are associated with a fall (Cummings et al., 1990; Grisso et al., 1991). Femoral neck studies on human cadavers have often applied this simulation (Backman, 1957; Cheng et al., 1997; Courtney et al., 1995; Lang et al., 1997; Lotz and Hayes, 1990; Pinilla et al., 1996). The weight-bearing and locomotion of mouse result in different in vivo loads compared to human. However, these clinically relevant loading configurations should be evaluated when developing sensitive experimental skeletal models.
Quantitative computed tomography (QCT) has proven to be an effective tool in evaluating the densitometric and geometric properties of human femoral neck (Augat et al., 1996; Cheng et al., 1997; Esses et al., 1989; Lang et al., 1997; Lotz and Hayes, 1990). Peripheral QCT (pQCT) has been used in experimental studies on the diaphyseal bones of rat (Breen et al., 1996; Ferretti et al., 1995; Ferretti et al., 1996; Gasser, 1995; Österman et al., 1997; Rosen et al., 1995aRosen et al., 1995b; Sato et al., 1995; Vanderschueren et al., 1997). Beamer et al. (1996)applied pQCT to analyses of genetic variability in femoral, vertebral and phalangeal bone density among inbred strains of mice. We have found pQCT fairly precise in evaluating the bending strength of murine femoral and tibial shafts (Jämsä et al., 1998). As far as we know, there have been no earlier attempts to evaluate the properties of the femoral neck of rodents by pQCT.
The purpose of the present study was to evaluate the mechanical testing of the femoral neck strength of mouse in two loading configurations, in the axial direction and in the falling direction, and to assess the usefulness of pQCT in evaluating the mechanical strength of the murine femoral neck in vitro.
Section snippets
Materials and methods
The study was performed on outbred adult male NMRI mice, which were sacrificed by CO2 suffocation. The animals were stored at −20°C and thawed at room temperature. The femora were dissected out and the soft tissues were removed. The femoral necks from 25 animals (weight 39±4 g) were used for a pQCT evaluation and for a comparison of the two loading configurations. The bones were thawed, cut approximately at midshaft, and kept moistened in closed plastic tubes until the experiment was finished.
Results
The method error sM of the mechanical measurements determined from paired samples was 1.6% in the axial configuration and 3.7% in the fall configuration.
All the fractures occurred in the femoral neck area. The fracture pattern and the fracture angle were slightly different in the two loading configurations (Table 2). A load in the fall direction resulted in a basicervical fracture, with a typical fracture angle of 60–80°, while an axial load associated with a mid- or basicervical fracture, with
Discussion
This study shows that the mechanical strength of the femoral neck of mouse, despite its small size, can be measured with high precision using the present testing methods. As far as we know, the strength of murine femoral neck has not been evaluated previously, and the loading configuration simulating a fall on the lateral side has only been studied in the human and some larger mammals. The weight-bearing and locomotion of mouse result in very different in vivo loads in the proximal femur
Acknowledgements
The authors thank Mr Arto Nykänen for technical assistance. This study was in part supported by the Biomedical Engineering Program, University of Oulu.
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