The common choice of rats as experimental subjects is due to the fact that they are easy to breed and relatively inexpensive compared to larger animal models. The emergence of new procedures, such as the reliable reproduction of a disorder, e.g., osteoporosis or diabetes, makes this model especially useful for analyzing the potential use of treatments and/or the influence of the disease in the biological response to drugs and surgical devices or procedures1,2.
The rat's bone mass gain occurs mostly during the first 6 months of life, although some researchers believe that the long bone grows constantly for at least a year with a progressive increase in length1. With aging, there is a transition from modeling to remodeling, which does not occur in all cases equally throughout the bones2. Female Sprague Dawley rats grow more slowly than male rats and achieve a lower peak in weight than male rats1. Continuous bone elongation and varied bone remodeling dynamics in rats are factors that have to be taken into account when addressing human health issues; however, it has not yet been possible to find any experimental research that shows either lifelong rat bone development or the species' inability to remodel bone1. If the experimentation starts around 10 months of age, a margin of at least 1 mm from the growth plate of the tibia should be left intact due to this longitudinal bone growth, an issue to be considered in dental implant studies2. Hormones are also a key parameter in bone research since at 8 months of age, male rats were found to have 22% greater bone width and 33% greater breaking strength than females in the tibia3.
The reliable reproduction of a disorder is thus very important in orthopedics and implant dentistry since osseointegration of an orthopedic screw or a dental implant is a complex process that depends on numerous factors influencing the systemic response to the device implantation into the bone. Systemic disorders like osteoporosis and diabetes are known to affect the success rate in orthopedics and implant dentistry, so the reliable reproduction of those disorders in rat models can be applied to explore ways to overcome these limitations.
The rat tibia, due to the easy surgical access, moderate bone volume, and the flat shape on the medial plate, makes it suitable for surgical bone implantation experiments4,5, and it has been used in numerous research studies exploring the effects of implant surface on osseointegration4,5,6. A growing number of studies assess the effects on osseointegration of coatings and substances added to the implant surface in both healthy animals7 and in compromised animals affected by diabetes or osteoporosis8,9,10,11,12,13,14.
The number of implant devices placed in one rat's tibia is limited and can differ depending on the type of study. Depending on the number of implants or study conditions, the dimensions of the devices must be adapted to minimize surgical trauma. In studies with one implant, a nearly human-size implant can be placed (2.0 mm in diameter and 4 to 5 mm in length), and bi-cortical anchorage can be achieved6,7,15,16. The dimensions of the implants in multi-implant protocols should adopt an appropriate implant size (1.5 mm in diameter and 2.5 mm in length)4,17.
The present study aims to describe a standardized surgical protocol for titanium implant placement on the tibia of two rat models: the osteoporosis and the diabetes rat model. Moreover, this study permits testing the surgical protocol to assess different types of implant surface biofunctionalization and its effect on osseointegration.
A sample of 80 rats was divided into two groups. In group 1, 40 ovariectomized Sprague Dawley females and 5 sham animals were selected, with a mean weight of 484 g and a mean age of 12 weeks. Based on vendor recommendations (see Table of Materials), three months after neutering, the experiment started. This waiting period ensured the disappearance of sex hormones. Osteoporosis was confirmed at the time of surgery based on micro-computed tomography (micro-CT) bone analysis, which reflected an average of 20% bone loss compared to the sham group. Group 2 consisted of 40 BBDR (Bio Breeding Diabetes Resistant) genetically modified Sprague Dawley rats with type II diabetes. The mean weight was 730 g, and the average age was 12 weeks. Prior to surgery, the diabetic status was confirmed with three consecutive days of glucose measurements with results higher than 200 mg/dL. Glucose was measured with a glucometer in 6 h of fasting, and a blood drop was collected by tail puncture.
Grade 3 titanium implants measuring 2 mm in length and 1.8 mm in diameter were used. All implants were sterilized in clean room conditions, by being ultrasonically cleaned in cyclohexane (3 times for 2 min), acetone (once for 1 min), deionized water (3 times for 2 min), ethanol (3 times for 2 min), and acetone (3 times for 2 min) using an ultrasound bath (230 VAC, 50/60 Hz, 360 W). Then, the samples were dried with nitrogen gas, and a nitrogen beam at 0.5 bar was applied directly onto the samples. Prior to implantation, the implants were first soaked in deionized water and then immersed in 70% ethanol (v/v) for 10 min. After this, the implants were transferred to sterile microcentrifuge tubes, and kept under sterile conditions until the surgery.