The clinical aspects of osteoporosis are of great interest to orthopaedic surgeons, as many problems in the treatment of osteoporosis-associated fractures seem to be related to the reduction in bone mass. There is great need of a suitable animal model for experimental studies on fracture healing and fracture treatment in osteoporotic bone, as well as of a better understanding of the pathogenesis of this affliction[4
Osteoporosis needs to be induced in animals. Ovariectomy simulates the post-menopausal estrogen depletion and a mild bone loss was observed after Ovx in sheep[8
]. Consistent with the literature, in the present study ovariectomy leads to an initial bone loss of 12%. Interestingly, this bone loss seems to be transient and a rebound was observed after 6 months. Further studies are needed to explain this phenomenon, which may be related to the production of estrogen by alternative tissues. However, this mild reduction of BMD does not match the WHO criteria for osteoporosis in humans with a decrease of BMD by -2.5 SD.
Although there is evidence that estrogen deficiency is an important contributory factor, the pathogenesis of post-menopausal osteoporosis is multifactorial. Clinical studies suggest that characteristic changes of bone mass in postmenopausal osteoporosis, resulting from predominance of resorptive process over those leading to bone tissue formation, may be related to melatonin[16
]. A few studies, particularly experimental, suggest that the main neuro-hormone of pineal gland, melatonin, can influence bone tissue metabolism[11
]. Ostrowska et al have been shown that pinealectomy in rat increases collagen degradation marker as biochemical markers of bone resorption. These changes were more pronounced in ovariectomized rats[22
]. Interestingly the surgical pinealectomy in newly-hatched chicks has a significant effect on the development of intervertebral disc degeneration[27
In order to establish a large animal model for osteoporosis, we transferred the findings mentioned above to an ovine model. Bone structural parameters were determined from iliac crest biopsies indicating a significant bone loss after pinealectomy that could be attributed to an increase in trabecular separation and decrease in trabecular thickness. Some of the changes on bone structural parameters after pinealectomy were more pronounced in sheep that also underwent ovariectomy. Similar findings were reported in rat, where pinealectomy had an inducing effect upon the level of investigated markers of bone metabolism in combination with ovariectomy[22
The changes in ovariectomized ewes observed in the present work confirmed those in previous studies. Chavassieux reported a transient increase in biochemical markers 3 months after ovariectomy and changes in cortical bone histomorphometric parameters without alterations in cancellous bone[9
]. In contrast we have shown that pinealectomy in ewe decreases the cancellous bone volume without affecting the cortical structures. A synergistic effect seems to exist when Ovx and Px are combined in the present study. The bone loss affects the cancellous as well as cortical structures. Taken together, these data show that pinealectomy in ewes results in an increase in bone resorption as indicated by the histomorphometric and biochemical changes. These changes result in osteopenia observed at the iliac crest and the distal radius. These data show that Ovx and Px affects bone metabolism resulting in bone loss, supporting the assumption that the pathogenesis of post-menopausal osteoporosis is multifactorial.
At first glance this pinealectomy model including the neurosurgical approach might be a bit extreme for a widely used animal model but it seems to simulate post-menopausal osteoporosis. Leptin is known for the central regulation of bone mass and we could recently demonstrate, that the intra-cerebral injection of leptin causes bone loss in sheep[28
]. However, leptin acts through the inhibition of bone formation and this is not the major cause of bone loss in post-menopausal osteoporosis. Furthermore, and in contrast to recently published sheep models for osteoporosis using leptin or glucocorticoids[28
], there is minimal concern on animal welfare[31
] and animals do not require special care after removal of the pineal gland.
Because this is a study with a limited number of animals, there are still several aspects of pinealectomy on bone remodelling that are not specifically addressed in the present work. These include the potential of melatonin application to reverse the effect of pinealectomy and to inhibit the observed bone loss, as previously shown in small animal models[15
]. Furthermore, a significant bone loss at the iliac crest was seen at 6 months after Px and OvxPx. Although these results go along with the suspected role of melatonin on bone metabolism, the effect of pinealectomy on bone can not be distinguished from the surgical procedure only. The lack of a sham operated group is the major criticism of this study. We omitted the sham group to reduce the number of animals as required by the local ethical committee. However, in order to discriminate any effects due to the neurosurgical procedure from the influence of pinealectomy on bone, we decided to analyze the long term effect of pinealectomy. One year after surgery the effect seen on bone mass must be due to pinealectomy only and additional effects of the procedure can be excluded. Therefore bone mineral density was determined up to 2.5 years after surgery using the pQCT method. Additional bone biopsies were not taken since the healing of the former biopsy holes would have altered the newly formed bone tissue, making the interpretation of the results difficult. This long-term observation clearly shows that the influence of pinealectomy on bone is not transient and, in contrast to ovariectomy, a continuous decrease in BMD was observed. During this study extension an increase of BMD at the distal radius was observed in untreated controls. This effect might be related to an increased access to the paddock which is often associated with higher physical activity.