Changes of microstructure and mineralized tissue in the middle and late phase of osteoporotic fracture healing in rats
Introduction
Osteoporosis is a common and preventable disorder of the aging skeleton that predisposes an individual to an increased risk of fracture, i.e. one of the major causes of disability in older people. Although much attention has been focused on fracture prevention and new therapies for conserving bone mass, little emphasis has been given to the study of fracture healing of osteoporotic bone. Fracture healing is a complex cascade of cellular events [1] and the regulation of its healing remains poorly understood. It is still a research topic whether the fracture repair is impaired in postmenopausal osteoporosis. Evidence have been accumulating that osteoporosis does impair the healing of fractures as judged by the formation of callus, mineralization and mechanical properties [2], [3], [4], [5], [6]. However, there are some arguments on the healing of osteoporotic fracture. Namkung-Matthai et al. reported that osteoporosis influenced the early phase of healing of fractures [3], whereas Kubo et al. suggested that osteoporosis influenced the late phase of healing [4]. Cao Y et al. reported a faster fracture healing process in the osteoporotic rats due to osteoporosis related acceleration in its callus remodeling [7]. However, clinical studies are sparse to confirm delayed healing in the elderly fracture patients. In a large study of 436 patients with fractures of the femoral neck, age did not show to influence the rate of bony union [8]. In a recent study on lower extremity fractures, age was not found to be associated with an increased risk of malunion; yet, age was reported to delay full weight bearing and time to union slightly [9]. Thus, it has not yet been sufficiently elucidated whether the fracture healing process is really retarded by advancing age.
Assessing fracture-healing progress is commonly performed clinically using plane X-rays for judging disappearance of fracture line and bony bridging of the fracture gap [10]. Peripheral quantitative computed tomography (pQCT) is a method mostly used for assessing volumetric bone mineral density (BMD) and bone cross-sectional geometry [11], [12]. Compared to plane X-rays, high-resolution and three-dimensional (3D) pQCT has its own unique advantages in assessment of fracture callus and its mineralization [13], [14]. Another imaging modality, MicroCT involves a 3D reconstruction of the bone region of interest, reduces the sample site variation and permits exact measurement of the bone volume [15], [16]. Data from MicroCT present unequivocal evidence that bone geometry reflects bone strength and that MicroCT can provide a non-invasive method to determine or predict bone strength from its structural parameters.
Objectives of the current study are to investigate the temporal changes in microstructure and mineralize tissue in the middle and late phase of fracture healing in ovariectomized (OVX) rats. We hypothesized that significant microstructural changes, in addition to changes in mineralized tissue, would occur in newly formed callus, which could result in the decline of mechanical impairment of fracture healing in ovariectomized rats. The specific aim of this study was to examine the mineralization, microstructure, biomechanical properties and histological events during the middle and late phase of fracture healing in both normal and osteoporotic rats induced by OVX.
Section snippets
Materials and methods
Ninety six Sprague–Dawley (SD) female rats (8 months old) were supplied by an animal experiment center (Sichuan University, Chengdu, China) and 48 rats were used for ovariectomy (OVX) and the other 48 served as controls. All animals received a standard laboratory diet and water ad libitum and housed in cages of a central animal house with 12 h day–night light condition, 60% humidity and room temperature of 21 °C. The experimental protocol was approved by the Animal Experiment Ethics Committee
DXA measurement
After 12 weeks of OVX, aBMD in the OVX rats (0.328 ± 0.009 g/cm2) was on average 19.4% less than that in the controls (0.407 ± 0.012 g/cm2) (P < 0.01), suggesting a successful establishment of osteoporosis model.
pQCT measurement
Volumetric BMD increased healing over time in both the OPF group and the NF group. The BMD in the OPF group was lower than that in the NF group at all post operation time points, which was found statistically significant at 8 weeks and 12 weeks (P < 0.01 for both) (Fig. 1).
The callus area in
Discussion
The present study aimed to establish a fracture model in ovariectomy-induced osteoporotic rat in order to investigate the hypothesized differences in microstructure, mineralized tissue and biomechanical properties of fracture callus between osteoporotic and control rats in the middle and late phases of fracture healing, evaluated using pQCT, MicroCT, biomechanical test and histology. This is the first report that confirmed the decrease in mineralized tissue and the not well connected
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