# Effect of Systemic Zoledronic Acid Dosing Regimens on Bone Regeneration in Osteoporotic Rats
## 4 Discussion  
The present study aimed at evaluating bone regeneration in bone defects grafted with a bone substitute material in an ovariectomized rat model treated with intravenous ZA via three different dosing regimens (pre-bone grafting, post-bone grafting, and pre+post-bone grafting). Ovariectomized rats receiving weekly saline injections served as the control group. The study was conducted over a period of 18 weeks, from ovariectomy with beginning of ZA injection until bone graft placement and finally euthanasia. The study time included a six-week healing period for the grafted bone defects. The harvested bone specimens were prepared for descriptive and quantitative histological examination. The  
Figure 4: Bar graph with standard deviation indicating the mean quantitative histomorphometric values of bone area percentage (BA%) for the different study groups. Statistically significant difference between the mean values: \({}^{**}\) indicates \(p=0.031\), \({}^{*}\) indicates \(p=0.043\). Figure 5: Bar graph with standard deviation indicating the mean quantitative histomorphometric values of remaining bone graft percentage (RBG%) for the different study groups. data showed that ZA-Pre and ZA-Post treatments resulted in a significant gain in BA% compared to saline control treatment. No evident effect of the ZA-Pre+Post treatment was found on BA%. Additionally, RBG% showed similar values for all experimental groups. Experiments performed with animal species such as rabbits, cows, dogs, mice, and rats are often used to study bone regeneration with the aid of bone substitutes. For mimicking osteoporoptic bone conditions, predominantly rats are used, in which osteoporosis is induced via gonadectomy (i.e., orchidectomy for male and ovariectomy for female animals). Physiologically, gonadectomized rats possess an increased rate of bone turnover when compared to humans with osteoporosis [46, 47]. Nevertheless, they are excellent preclinical models for studying osteoporoptic changes, as they closely emulate pharmacotherapeutic responses and allow studying the effect of estrogen depletion on the skeleton [42]. However, it should be highlighted that sometimes, there is a disconnection between animal research and clinical research [48]. The relevance of animal models to human health is questioned because of differences between the species. Many studies in animal models are of poor methodological quality. Lack of concordance between animal experiments and clinical trials may be due to bias, random error, or the failure of animal models to adequately represent human diseases. These drawbacks should be kept in mind before clinical translation and while constructing animal model to test a hypothesis. Systematic reviews of animal experiments could promote closer collaboration between research communities and support an iterative approach to improve the relevance of animal models to clinical trial design. When models do not represent the clinical context, they could be adjusted accordingly [49]. For the current study, our published animal systematic review and meta-analysis about anti-osteoporotic drugs in adjunction with bone grafting helped construct the experiment design [38]. Currently, ZA is considered the most potent intravenous BP available for the treatment of osteoporoptic patients and is approved for use by the U.S. Food and Drug Administration (USFDA) [50]. Guidelines for the required dose and administration route of ZA have been devised on the basis of data from pre-clinical models and clinical practice [37]. Consequently, the current protocol for the administration of ZA to osteoporotic patients follows a once-yearly dosing regimen (5 mg IV) for the maintenance of therapeutic drug levels and to facilitate patient compliance [51]. The duration of this therapy is limited to no more than three years. ZA is also administered in a 4 mg dose to patients suffering from hypercalcemia and with advanced malignancies involving bone. ZA treatment by infusion for these patients is every 3-4 weeks.