This study aimed to compare biomechanical characteristics of immediately loaded (IL) and osseointegrated (OS) dental implants inserted into Sika deer antler and lay a foundation for developing an alternative animal model for dental implants studies. Two implants per antler were inserted. One implant was loaded immediately via a self-developed loading device; the other was submerged and unloaded as control. IL implants were harvested after different loading periods. The unloaded implants were collected after OS and the shedding of antler. Specimens were scanned by μCT scanner and finite element models were generated. A vertical force of 10 N was applied on the implant. The mean values of maximum displacements, stresses and strains were compared. The results showed that the density of antler tissue around the implants dramatically increased as the loading time increased. After shedding the antler, 3 pairs of antlers were collected and the density of antler tissue remained in a similar value in all specimens. The maximum values of displacement and stresses in implant and stresses and strains in antler tissue were significantly different among OS models. In one antler, all the biomechanical parameters of IL model were significantly higher than those of OS model of the same animal (P < 0.05) and wider distributions were obtained from IL model. It can be concluded that implants inserted into Sika deer antler might not disturb the growth and calcification process of antler and the use of Sika deer antler model is a promising alternative for implant studies that does not require animal sacrifice.
With the worldwide growing of aging population, there has been considerable increase in the demand for the replacement of lost teeth by means of implant-retained restorations over the last few decades. The clinical success of implant therapy is based on osseointegration, defined as the direct contact between living bone and the implant without the interposition of fibrous tissue [1–3]. Conventionally, loading on implant-retained restoration should be avoided before osseointegration. However, immediately loaded (IL) implants which allow for shorter rehabilitation times have shown similar implant stability and success rate compared with traditional delayed loading implants. Some studies showed that IL is beneficial to delayed loading, since loading is capable of stimulating the healing process [4–6]. IL implants appear to increase patient satisfaction and avoid the difficulty of wearing a conventional temporary restoration during the healing phase as well . Thus, there is a trend in using an immediate loading protocol for implant-retained restoration currently.
While animal models closely represent the mechanical and physiological human clinical situation, they have been widely used in investigating dental implants in loaded or unloaded situations over potentially long time spans and in different tissue qualities (e.g., normal healthy or osteoporotic bone) and ages . Each animal model has unique advantages and disadvantages; therefore for different purposes there are numerous models for testing the properties of implant and its surrounding tissue in vivo. Specifically, for studies investigating bone remodeling process around IL implants, the animal model should have similar bone characteristics to human bone and be appropriate for inserting implants and applying loadings . However, the main disadvantages for the existing animal models are the uncontrolled loads that are exerted on the implants and the sacrificed fate of animals. For these reasons, exploring a novel animal model that is able to apply a controlled force on the inserted IL implant and does not interfere with the animals’ behavior is admirable.
Deer are the only mammals that are capable of fully regenerating a complex organ, called antlers . The ability to fully regenerate stands out as the most impressive feature of antlers. The repeated regeneration each year is even more remarkable, because mammalian appendages are generally considered as being incapable of regeneration. Deer antlers grow annually in defiance of what could be considered nature’s rules . The annual cycle of antler growth starts in spring. After the rapid elongation and the formation of lateral branches in summer, antler gradually becomes calcified in late summer or autumn. The process of calcification is initiated from the base of the antler and proceeds up through the antler and finishes when the distal ends of the tines form sharp tips . After the calcification process has been completed, in conjunction with the loss of blood vessels and nerves, the velvet skin is shed. In winter, the bare bony antlers are firmly attached to the living pedicle and are not “cast” until the following spring. Antler casting triggers another round of antler regeneration . This cycle provides a relative reasonable time span for investigating bone remodeling processes around implants inserted into the antler without sacrificing the animal. Besides, deer antlers are similar to human bones in regard to chemical composition and physiological structure [13,14]. Furthermore, as a muscle- and joint-free bony cranial appendage [15,16], antlers provide a fascinating model to investigate bone remodeling process around IL implant without the influence of external forces (except for gravity).
The primary aim of this study was to compare bone remodeling and biomechanical characteristics of immediately loaded and osseointegrated dental implants inserted into Sika deer antler. Secondly, the aim was to lay a theoretical foundation for developing an alternative animal model for studying bone remodelling around dental implants.
2. Materials and methods
2.1. Animal welfare statement
All animals were handled according to the policies and principles established by the German animal welfare act (TSchG, last amended on 3rd December 2015), approved by the North RhineWestphalia State Agency for Nature, Environment and Consumer Protection as competent authority (Permission No.: LANUV NRW, 84-02.04.2014.A462).
2.2. Surgery procedure
In July of 2015, six 4-year-old male Sika deer (bred at Wildlife Parc Hellenthal, Germany) were anesthetized using 1.2–1.5 ml Hellabrunn’s mixture (100 mg Ketamine and 125 mg Xylazine per ml) according to standard procedures [17,18]. After disinfection and additional local anaesthesia with 3–5 ml lidocaine (Lidocain B. Braun 2%, B. Braun Melsungen, Melsungen, Germany), a longitudinal incision was performed and velvet flap elevated. Implant site was prepared at a position near a branching of the antler by sequential drilling under sterile saline irrigation according to the surgical protocol. Two implants per antler were inserted in a distance of 2.5 cm. The implants were Straumann Roxolid® soft tissue level implants (Institut Straumann AG, Basel, Switzerland) with a length of 10 mm and a diameter of 3.3 mm. After suturing the incision, the most proximal implant was vertically loaded immediately via a self-developed screw retained loading device , while the other one remained unloaded as a control. The motor of the control electronics were fixed on the other antler by colored bandages. Bandage with the different colors, blue, orange, yellow, red and black were used to distinguish the animals (Fig. 1). One deer was dead during the anesthetisation phase. After 2, 3, 4, 5 and 6 weeks, respectively, the loaded implants and surrounding tissue were randomly taken out with a trephine from one animal and the wounds were filled with bone wax (Ethicon®-bone wax, Johnson and Johnson, Hamburg, Germany), sutured and bandage protected. The samples were fixed in buffered formalin (4%). The unloaded implants remained in the antler for osseointegration. In winter, the antlers were collected after their shedding. Finally only three pairs of antlers, blue, orange and black, were able to be collected. The other two pairs of antlers could not be found. Thereafter, the unloaded implant with surrounding antler tissue and the antler tissue adjacent to the former specimen were sectioned for further investigation.
2.3. Numerical analysis
Specimens were prepared and scanned in a μCT scanner (SkyScan 1174, Bruker-micro CT, Kontich, Belgium) using 50 kV and 800 μA, rotation step of 0.25°. Data sets were reconstructed. After scanning the sample, calibration phantom with known density of calcium hydroxyapatite (SkyScan 1174, Bruker-microCT, Kontich, Belgium) was scanned on the same day by using the same parameters. Bone mineral density (BMD) of antler tissue (including cortical and trabecular bone) was calculated by using Bruker-Micro CTAnalyser (Bruker-micro CT, Kontich, Belgium).