Abstract
The aim of this animal study was to investigate and compare the osseointegration of zirconia and titanium dental implants. 14 one-piece zirconia implants and 7 titanium implants were inserted into the mandibles of 7 minipigs. The zirconia implants were alternately placed submerged and non-submerged. To enable submerged healing, the supraosseous part was removed, using a diamond saw. The titanium implants were all placed submerged. After a healing period of 4 weeks, a histological analysis of the soft and hard tissue and a histomorphometric analysis of the bone–implant contact (BIC) and relative peri-implant bone-volume density (rBVD; relation to bone-volume density of the host bone) was performed. Two zirconia implants were found to be loose. All other implants were available for evaluation. For submerged zirconia and titanium implants, the implant surface showed an intimate connection to the neighbouring bone, with both types achieving a BIC of 53%. For the non-submerged zirconia implants, some crestal epithelial downgrowth could be detected, with a resultant BIC of 48%. Highest rBVD values were found for submerged zirconia (80%), followed by titanium (74%) and non-submerged zirconia (63%). The results suggest that unloaded zirconia and titanium implants osseointegrate comparably, within the healing period studied.
Titanium is regarded as the ‘gold standard’ for contemporary dental implant materials. Numerous studies have affirmed the high success and survival rates of titanium implants in many different applications . The high biocompatibility of titanium is due to the spontaneous formation of a dense oxide film on its surface. A disadvantage is that it can result in poor aesthetics, especially in anterior sites in the mouth, because of its greyish colour, implant body exposure due to soft tissue recession or thin gingival biotype. Recently, zirconia implants have been introduced into the field of dental implantology and might offer a useful alternative to titanium. Zirconia has an opacity that resembles natural teeth. Whereas titanium implants can give the gingiva an unnatural bluish/grey appearance , the bright white colour of zirconia may provide satisfactory aesthetics.
Ceramics are known for their excellent biocompatibility and high resistance to wear. They are widely used in many clinical applications , for example femoral head and hip replacements in orthopaedics. In comparison to titanium, the molecular covalent-ionic binding structure in ceramics prohibits plastic deformation before failure. This is referred to as a high degree of brittleness. Analyzing material characteristics, ceramics suffer from microstructural flaws resulting in poor resistance to stress concentrations . In dental implantology, the development of yttria-stabilized tetragonal zirconia polycrystal ceramics (PSZ) has rekindled interest in ceramic implant materials. PSZ has a higher fracture resistance and flexural strength than previously available aluminium oxide ceramics, making it less sensitive to stress concentrations. This is mostly due to the effect of transformation toughening, in which, due to stress, a metastable tetragonal grain structure is able to transform into a monoclinic structure at room temperature . This phenomenon inhibits the progression of a crack or flaw in the ceramic due to a simultaneous 3% expansion in volume . PSZ also exhibits a relatively low Young’s modulus (200 GPa) in comparison to aluminium oxide, indicating a higher elastic deformation capability. Various experimental studies reveal the capability of zirconia implants to withstand long-term loading .
Experiments have shown fewer inflammatory infiltrates in soft tissue for zirconia than for titanium . In contrast to titanium, only minimal ion release is detected . As such, the material is considered to be highly biocompatible. Zirconia implants are known to integrate well in the jaw bone . Analyzing the bone–implant interface, A lbrektson et al. showed similar patterns of bone–implant contact (BIC) with either zirconia or titanium implants .
Owing to mechanical characteristics, zirconia implants are produced as one-piece implants that heal non-submerged (i.e. transmucosal) following implantation. In non-submerged healing, a precondition for adequate bone formation at the implant surface is a close adaptation of the soft tissue around the implant neck. In contrast to transmucosal healing, submerged healing allows undisturbed osseous integration. A drawback is the need for a secondary operation, which will penetrate the soft tissue and could cause local inflammation, possibly leading to secondary bone loss.
This animal study was designed to investigate short-term soft and hard tissue formation around unloaded, submerged and non-submerged zirconia implants, and to compare the results with submerged titanium implants.
Materials and methods
The study was performed on 7 one-year-old miniature pigs. Two types of implants were studied: zirconia and titanium. Zirconia implants (diameter 4 mm, length 10 mm) were manufactured from yttria-stabilized tetragonal zirconia polycrystalline (whiteSKY, Bredent, Germany). The surface was sandblasted. According to the producers, the physical surface roughness parameters were: Ra 1.0 μm and Rt 7.2 μm. Titanium implants (Xive, Dentsply, Friadent, Germany) had a sandblasted, acid-etched surface, Ra 2.75 μm and Rt 16.30 μm.
Surgical procedure
Three surgical interventions were performed; all under general anaesthesia (midazolam 1 mg/kg i.m., ketamine 10 mg/kg i.m., atropine 0.05 mg/kg i.m.). Carprofen (2–4 mg/kg s.c.) was administered postsurgery. The animals received a soft diet and water ad libitum .
In the first operation, the primary premolar teeth of the mandible were removed. Two months later, the permanent premolar teeth were extracted. After 9 weeks of healing, 14 one-piece zirconia implants and 7 titanium implants were inserted endosseoussly into the lower jaw of the 7 minipigs. Zirconia implants were alternately placed submerged (7 implants) and non-submerged (7 implants). To allow submerged healing, the supraosseous part of the implant was removed after placement, using a diamond saw under continuous water cooling. Titanium implants received a cover screw after placement, for submerged healing. Each pig received one submerged zirconia, one non-submerged zirconia and one submerged titanium implant, referred to as the three implant groups. For submerged zirconia and titanium implants, a mucoperiosteal flap was elevated from the vestibular region to expose the alveolar crest, under amoxicillin (15 mg/kg i.m.) antibiotic coverage. Any remaining sharp bone edges were removed, using a water-cooled surgical drill. Following this procedure, the alveolar ridge had an approximate width of 6–7 mm. The implants were placed in the posterior area, ensuring a distance of 3 mm from the first molar. The flaps were repositioned and sutured using resorbable material. For non-submerged zirconia implants, a tissue punch was applied. The non-submerged implants extended 6 mm above the gingiva. All implants were inserted with a ratchet, not exceeding an insertion torque of 30 Ncm. No thread cutting tap was applied. The positions of the three implants in each pig within the alveolar crest were randomized prior to placement ( Fig. 1 ).
Histology
4 weeks after implant placement, the animals were killed. Mandibular en bloc resections were retrieved for analysis. The samples were fixed in formaldehyde and dehydrated in a graded series of ethanol. The implants and surrounding bone were embedded in methylmethacrylate (Technovit 9100 neu ® , Heraeus Kulzer, Wehrheim, Germany).
100 μm thick sections along the axis of each implant were cut in an orofacial direction using a diamond saw microsectioning system (Exakt-Apparatebau, Norderstedt, Germany). These sections were reduced to 30 μm thickness using grinding techniques on a roll grinder containing diamond-coated sandpaper. About three middle sections could be obtained per implant. A Masson–Goldner stain was carried out. The sections were imaged and analyzed using light microscopy (Olympus BX 61, Hamburg, Germany) and polarized light analysis. Multiple image alignment was performed using an automated scanning table (Märzhäuser, Wetzlar, Germany).
Histomorphometry
Histomorphometric analysis measured BIC on the implant surface. For each histological section, the length of implant surface in contact with bone tissue was calculated and compared with the total implant surface length.
The percentage of bone within the implant grooves was measured and compared with the percentage of bone within a neighbouring region of reference (RoRef) within the host bone, defining the relative bone-volume density (rBVD). The area within the implant grooves was defined by placing a borderline at the tips of the grooves, parallel to the implant length axis. The neighbouring RoRef had a rectangular shape and was selected from an area distant to the implant, within the host bone. This RoRef had the depth of an implant groove and the length of three grooves ( Fig. 2 ).
Statistics
Data were described by means. Means were supplemented with their 95% confidence intervals. One-way ANOVA was applied to test for differences between means of the three implant groups. Test results were considered significant for P -values below 0.05. All statistical analyses were performed using SPSS version 15.0 software (SPSS, Chicago, IL, USA).
Results
All animals survived the treatment and were available for evaluation. One submerged zirconia implant was lost and was unavailable for histological examination. The clinical intraoral examination performed prior to death showed one non-submerged zirconia implant to be clinically mobile. The remaining 19 implants did not show any signs of inflammation and were clinically stable. The non-submerged implants showed a tight adaptation of the peri-implant mucosa to the implant body. There was no observed dehiscence in the mucosa covering the submerged implants.
Histomorphology
Both submerged and non-submerged healing implants exhibited preservation of the original gingiva biotype and of the original degree of keratinization. In comparison, non-submerged implants showed a downgrowth of sulcular epithelium that corresponded to a triangular resorptive defect in the crestal bone. The thickness of the cell layers decreased from coronal to apical. Crestal bone areas of pre-existing host bone and newly formed bone could be distinguished. Apical to the triangular connective tissue area, direct BIC was present. No inflammatory reactions were found in the transition zone between these different structures.
Polarized light analysis of the fibre structures showed a tight structural connection between the periosteum and connective tissue fibres beneath the mucosa. These fibres allowed direct separation between coronal peri-implant connective tissue, crestal bone and the implant body. The fibres were mostly oriented parallel to the implant surface.
Hard tissue analysis demonstrated differences between submerged and non-submerged zirconia implants. Submerged implants showed BIC around the entire implant surface. Bone lamellae even merged in the most coronal implant areas without any interposition of connective tissue. In comparison, non-submerged implants showed triangular vertical bone resorption defects in the coronal areas, which were filled by connective tissue. In these cases, crestal bone was separated from the implant body by connective tissue. A rounding of the coronal lamella could be observed. The insertion of the lamella into the implant surface was further apical, compared with submerged implants.
The upper third of the implants showed contact osteogenesis for both submerged and non-submerged implants. Further apical, distance osteogenesis was observed. For both healing conditions, no differences could be found for the apical area. Large osteoid formations were found next to woven bone structures. The implant grooves were covered by immature bone without the interposition of connective tissue or any evidence for inflammatory reactions.
The submerged titanium implants developed comparable crestal bone conditions to the submerged zirconia. The middle and lower thirds of the titanium implants showed a pronounced secondary remodelling of the host bone that exceeded the original drill hole. More areas of osteoid formation and immature bone structures were found. The development of trabecular structures that represented adult spongious bone were clearly delayed compared with zirconia. The texture of the hard tissue showed a distinctly tighter BIC ( Figs. 3–7 ).
