In vivo assessment of changes in bone due to osteoporosis and its possible treatments

Osteoporosis is a skeletal disease characterized by a decrease in bone mass and deterioration of bone microarchitecture, resulting from an unbalance in the amount of bone formed and resorbed during bone remodeling. It often takes place after menopause in women due to estrogen deficiency and results in decreased bone strength and, subsequently, a greater risk of fracture. Pharmaceutical treatments for osteoporosis can roughly be divided into bone resorption inhibitors and bone formation enhancers. To evaluate possible treatments, postmenopausal osteoporosis can be simulated in animals by performing an ovariectomy, which leads to estrogen deficiency and subsequent bone loss. This loss of bone mass and the subsequent microarchitectural deterioration is often analyzed by micro-CT, which until recently was only possible to do ex vivo, after sacrifice. Recently, however, in vivo micro-CT scanners have become available with which bone in living rats can be scanned. In vivo micro-CT, combined with image registration software, offers a potentially more powerful method to identify effects of osteoporosis and treatments over time. Additionally, local changes in bone within the same animal can be monitored over time, which taken together can provide novel and unique information In this dissertation, we focused on the development of osteoporosis and several treatments in rats. We first concentrated on bone resorption inhibitors and then on bone formation enhancers. Changes over time in bone microstructure were determined as well as mechanical properties after sacrifice using mechanical tests or finite element models. We first ruled out that radiation damage due to scanning affected our studies. Then two different animal models that simulate bone loss due estrogen-deficiency (i.e. after menopause) and immobilization (e.g. after long bed resting) were compared, as their effects on bone structure and strength may differ. In the metaphysis, the loss of bone volume fraction was found to be similar for both models, while structure and strength were more affected after immobilization. In the epiphysis, changes in bone volume fraction and structure were different. The difference in response between the meta- and epiphysis may be related to different mechanisms underlying the bone loss after estrogen-deficiency and immobilization. These findings offer insight into the aetiology and possible treatment of different types of osteoporosis. Zoledronic acid (ZOL) is a novel, potent bone resorption inhibitor. In the rat tibia, we found that preventive treatment with ZOL prevented all bone microstructural changes seen after ovariectomy. Recovering treatment significantly improved bone microstructure, though not back to original levels. These results indicate that the timepoint of initiation of treatment is important for the final bone microstructure and strength. Both preventive and recovering treatments also led to inhibition of loss of bone mass and static compressive strength in the lumbar vertebra, a clinically relevant site. However, no significant influence of time-point of treatment was found here. Vertebral fractures mostly result from cyclic loading. ZOL may influence mineralization and lead to accumulation of microdamage, possibly affecting fatigue behavior. A method was developed to assess compressive fatigue properties in rat vertebrae. ZOL treated rats were found to have similar fatigue properties as normal rats, indicating that any altered mineralization and accumulated microdamage due to ZOL treatment did not affect fatigue properties. After exploring the effects of bone resorption inhibitors, we continued with studying the effects of bone formation enhancers. It was found that PTH leads to a linear, constant increase in trabecular and cortical bone mass over time and that mechanical properties improved. Micro-analysis showed that bone was formed on trabeculae, there where most beneficial for structure and strength. This indicates that bone formation resulting from PTH may be mechanically driven. In another study, the effects of a daily period on a vibration platform, which has been described in the literature to increase bone formation, were studied in osteoporotic rats over time. Within six weeks, no significant effects were found to take place. The potential of vibration as treatment for osteoporosis thus could not be established. Summarizing, for the first time, the comparison between two types of osteoporosis and the effects of several treatments for osteoporosis on bone microstructure were analyzed over time in vivo, offering insight into the temporal and spatial effects of bone resorption inhibitors and bone formation enhancers in osteoporotic rats.