Peri-implant bone organization under immediate loading conditions: collagen fiber orientation and mineral density analyses in the minipig model.

BACKGROUND: Mechanical properties of bones are greatly influenced by percentages of organic and mineral constituents. Nevertheless, information about mineralization level on a microscopic scale and collagen fiber organization in peri-implant bone after immediate loading is scarce. PURPOSE: The aim of this work was to analyze and compare the degree of mineralization and collagen fiber orientation in alveolar bone (AB) and peri-implant bone of immediately loaded (IL) and unloaded (NL) implants. MATERIALS AND METHODS: A total of 25 dental implants of 3.8 mm in diameter and 11 mm in length were used in the present study. In five minipigs, three premolars and the first molar were removed from the left side of the mandible. Three months later, five implants for each animal were inserted. Four implants were loaded immediately with a fixed restoration, while one implant was left unloaded. After a 4-month healing period, all implants were retrieved. Circularly polarized light and scanning electron microscope with backscattered electron imaging were used to analyze both peri-implant and AB retrieved 5 mm from the implant. RESULTS: The bone/implant contact ratio (BIC %) was 77.8 +/- 5.9% for the IL implants and 78.0 +/- 5.8% for the NL implants; the difference was not statistically significant (p = 0.554). In the peri-implant bone, the area related to transverse collagen fibers was 112,453 +/- 4,605 pixels for IL implants and 87,256 +/- 2,428 pixels for NL implants. In the AB, the area related to transverse collagen fibers was 172,340 +/- 3,892 pixels. The difference between groups was statistically significant (p < .001). The degree of mineralization of peri-implant bone was 137 +/- 19 gray level for IL implants and 115 +/- 24 gray level for NL implants, while in the AB, the degree of mineralization was 125 +/- 26 gray level. This difference was statistically significant (p < .001). CONCLUSION: In this study, it was found that IL and NL implants showed the same degree of osseointegration. The bone matrix around IL implants had a higher quantity of transverse collagen fibers and presented a higher level of mineralization.

Bone microvascular pattern around loaded dental implants in a canine model.

The vascularity of the implant bed is a very important parameter in both bone formation and maintenance after dental implants insertion. The relationship between bone and vessels network organization is still unknown. The aim of this study was to investigate the three-dimensional bone vascular canals of the peri-implant bone after loading. A total of ten implants with sandblasted and acid-etched surface were placed in the mandible of a beagle dog. Three months later, the implants were connected and loaded. The dog was killed after 12 months. The specimens were embedded and processed for scanning electron microscopy (SEM) analysis. After a 1-year loading period, a very intricate vessel network could be seen around the implants. The vessels, with neighbouring soft tissues, were round in shape and showed a lot of anastomoses with a mesh-like appearance. They ran circularly around the dental implant. In the bone, the majority of the vessels appeared to ran parallel to the mandibular canal. After a 1-year loading period, the peri-implant bone vasculature looked like a mesh that surrounded the implants. Nevertheless, the presence of many thick vessels inside the peri-implant crestal bone indicates a high metabolic need and also a different bone organization, as no osteons were noted. The crater-like bone loss around the marginal part of the implant could be related to the microvasculature "strain". A high strain level could continuously activate the osteocyte-vessel syncytium, producing a net bone loss.

Screw vs cement-implant-retained restorations: an experimental study in the beagle. Part 2. Immunohistochemical evaluation of the peri-implant tissues.

Crestal bone loss has been reported to occur around dental implants. Even if the causes of this bone loss are not completely understood, the presence of a microgap between implant and abutment with a possible contamination of the internal portion of the implants has been suggested. The aim of this study was to see if there were differences in the vascular endothelial growth factor (VEGF) expression, microvessel density (MVD), proliferative activity (MIB-1), and inflammatory infiltrate in the soft tissues around implants with screwed and cemented abutments. Sandblasted and acid-etched implants were inserted in the mandibles of 6 Beagle dogs. Ten 3.5- x 10-mm root-form implants were inserted in each mandible. A total of 60 implants (30 with screwed abutments and 30 with cemented abutments) were used. After 12 months, all the bridges were removed and all abutments were checked for mobility. A total of 8 loosened screws (27%) were found in the screwed abutments, whereas no loosening was observed in cemented abutments. A gingival biopsy was performed in 8 implants with cemented abutments, in 8 implants with screwed abutments, and in 8 implants with unscrewed abutments. No statistically significant differences were found in the inflammatory infiltrate and in the MIB-1 among the different groups. No statistically significant difference was found in the MVD between screwed and cemented abutments (P = .2111), whereas there was a statistically significant difference in MVD between screwed and unscrewed abutments (P = .0277) and between cemented and unscrewed abutments (P = .0431). A low intensity of VEGF was prevalent in screwed and in cemented abutments, whereas a high intensity of VEGF was prevalent in unscrewed abutments. These facts could be explained by the effects induced, in the abutments that underwent a screw loosening, by the presence of bacteria inside the hollow portion of the implants or by enhanced reparative processes.

Peri-implant bone organization under immediate loading state. Circularly polarized light analyses: a minipig study.

BACKGROUND: Immediate loading of dental implants is currently one of the most examined topics in implant dentistry. Using screw implants with a microstructured surface and bone-quality-adapted insertion procedures, osseointegration is achieved when implants are initially stable and when splinted with the superstructure. Despite reported success, there is a shortage of information relating to remodeling and peri-implant bone formation with immediately loaded implants. METHODS: Four to six immediately loaded and unloaded dental implants with a microstructured surface were placed in the mandible and the maxilla in seven minipigs. A total of 85 implants were placed. After a 4-month healing period, all implants were retrieved. Histomorphometry was performed using a light microscope in transmitted polarized light connected to a high-resolution video camera interfaced to a monitor and personal computer. This optical system was associated with a digitizing pad and a histomorphometry software package with image capturing capabilities. RESULTS: Implants showed osseointegration if the average insertion torque of the implants within one bridge was >35 Ncm. If the primary stability of the bridge was <35 Ncm, all implants in the quadrant were lost after 4 months. The multivariate discriminant analysis showed the highest correlation for implant stability by bridge insertion torque (BIT), localization (mandible or maxilla), and implant insertion torque (IIT) as success parameters. The loaded implants displayed collagen fibers, which were oriented in a more transverse way. In addition, a higher quantity of secondary osteons was present. In comparison, the unloaded implants had collagen fibers with a more parallel orientation, and a higher quantity of marrow spaces was present. CONCLUSIONS: When observed after 4 months, immediately loaded implants showed a higher degree of bone formation and remodeling in comparison to unloaded implants. Immediately loaded implants also demonstrated a prevalence of transversely oriented collagen fibers in the peri-implant bone. In this animal model, an average insertion torque of the implants within one bridge>35 Ncm was associated with the most successful implants.

Screw- vs cement-implant-retained restorations: an experimental study in the Beagle. Part 1. Screw and abutment loosening.

The causes of implant failures can be biological or mechanical. The mechanical causes include fracture of the implant, fracture of the abutment, and loosening of the abutment. Numerous studies show that abutment loosening constitutes one of the marked implant postsurgery complications requiring clinical intervention. The aim of the present study was to evaluate the incidence of the screw loosening in screwed or cemented abutments. Six adult male Beagles were used. In each dog, the first molars and 2 premolars were extracted. The sutures were removed after 7 days. After 3 months, 10 implants were placed in each dog, 5 in the right mandible and 5 in the left mandible. The abutments either were screwed in (n=30) by applying a total strength of 30 N/cm or were cemented (n=30). After 12 months, 8 (27%) loosened screws were present in screwed abutments, whereas no abutment loosening was observed in cemented abutments (P = .0001). Screwed abutments are often submitted to nonaxial loads that determine screw and abutment loosening.

Immediate versus delayed loading of dental implants in the maxillae of minipigs: follow-up of implant stability and implant failures.

PURPOSE: To assess the course of the stability and the failure rate of dental implants placed in the partially edentulous maxillae of minipigs. MATERIALS AND METHODS: Three months after tooth removal, implants were placed in 9 minipigs. Six implants (XiVE; Friadent, Mannheim, Germany) were placed on each side of the posterior maxilla after preparation of the implant sites either by an osteotome technique or with spiral drills. Implant stability was assessed by resonance frequency analysis (RFA) at the time of placement, at second-stage surgery (which took place after a healing periods of 1, 2, 3, 4, or 5 months), and after a loading period of 6 months. RESULTS: Implant stability was significantly influenced by the healing period (P = .007). Implant stability decreased after 1 to 3 months of healing for both of the placement techniques and increased after a healing period of 4 months. After implant site preparation by an osteotome technique, 6 of 12 immediately loaded implants, 18 of 24 implants loaded after healing periods of 1 to 3 months, and 1 of 18 implants loaded after a healing period of 4 or 5 months were lost. After implant site preparation using spiral drills, 7 of 12 immediately loaded implants, 12 of 24 implants loaded after healing periods of 1 to 3 months, and 2 of 18 implants loaded after healing periods of 4 or 5 months were lost. Broad overlapping of confidence intervals for the number of implant failures revealed that there was no relevant difference between immediate and early functional loading for either of the 2 techniques. DISCUSSION AND CONCLUSION: Implant loading after healing periods of 1 to 3 months did not improve implant survival compared to immediate loading in the posterior maxillae of minipigs. Not until a healing period of 4 months was reached did implant stability begin to increase. Only when functional loading was started at this point in time was maximal implant survival achieved.

Interimplant distance and crestal bone resorption: a histologic study in the canine mandible.

BACKGROUND: Crestal bone loss has been shown to occur around dental implants. This crestal bone resorption may determine a more apical position of the gingival margin. A clear trend of increased bone loss with increased interimplant distance has been reported. PURPOSE: The aim of the present study was to evaluate, in the canine mandible, the crestal bone behavior around dental implants inserted with different interimplant distances. MATERIALS AND METHODS: Sandblasted and acid-etched implants (Bone System, Milano, Italy) were placed in the mandibles of six beagle dogs. Each dog received 10 implants in the mandible (five in the right side and five in the left side). A total of 60 implants was used in this study. The implants were divided in four groups: group I, with a 2 mm interimplant distance; group II, with a 3 mm interimplant distance; group III, with a 4 mm interimplant distance; and group IV, with a 5 mm interimplant distance. The dogs were killed after 12 months. RESULTS: No statistically significant differences were found in regard to vertical bone loss whereas on the contrary, statistically significant differences were found in regard to lateral bone loss (p = .0001). Statistically significant differences also were found in regard to vertical crestal bone loss (p = .0001). In fact vertical crestal bone loss decreased, from 1.98 mm in group I to 0.23 mm in group IV. CONCLUSIONS: The clinical significance of these data lies in the fact that the increased crestal bone loss results in an increase in the distance between the base of the contact points of the neighboring implants and the crest of bone, and this fact could determine whether the papilla is present or absent between two implants.

Crestal bone remodeling in loaded and unloaded implants and the microgap: a histologic study.

PURPOSE: The aim of this study was to histologically evaluate the crestal bone response to loaded and unloaded implants in beagle dogs. MATERIALS AND METHODS: Sand-blasted and acid-etched implants (Bone System, Milano, Italy) were placed in the mandible of six beagle dogs. The two premolars and the first molars had been extracted 3 months previously. Each dog received 12 implants in the mandible, and a total of 72 implants were used in this study. Three months after implantation, second-stage surgeries were performed for placement of abutments or healing screws. Three dogs were killed after 6 months, and three dogs were killed after 12 months. All 72 implants were retrieved. RESULTS: No statistically significant differences were found in the amount of bone loss between test and control implants, both at 6 and 12 months. Statistically significant differences were found, in both groups, between the bone loss observed at 6 months and that found at 12 months. CONCLUSION: Loading does not seem to be a relevant factor in the peri-implant bone resorption observed during the first year of function. Our results support previous findings that bone crest level changes could depend on the location of the microgap.

Bone contact, growth, and density around immediately loaded implants in the mandible of mini pigs.

The aim of the study was to compare the bone mineral apposition rate (BMAR) of immediately loaded implants with an unloaded control during the early healing phase in the partially edentulous mandible. In seven mini pigs, three premolars and the first molar were removed in the left mandible. Three months later, five implants were installed. Four implants received a fixed provisional restoration and were loaded immediately. The most anterior implant was used as unloaded control. Polychromatic fluorescence labelling was performed to assess the BMAR. After 4 months, the implants were retrieved together with the adjacent bone. Histological specimens were prepared and subjected to a fluorescence microscopic and histomorphometric analysis. Two provisional restorations were found partially lost at the end of the observation period. One implant that had lost the splinting fixation showed soft connective tissue healing. The BMAR did not differ statistically significantly between loaded and unloaded implants and within the single groups during the observation period (BMARloaded days 14-42=1.8+/-0.2 microm/d, BMARloaded days 42-70=1.8+/-0.1 microm/d, BMARloaded days 70-98=1.6+/-0.1 microm/d, pBMARloaded days 14-42/42-70/70-98 =0.156, BMARunloaded days 14-42=1.7+/-0.1 microm/d, BMARunloaded days 42-70=1.8+/-0.2 microm/d, BMARunloaded days 70-98=1.6+/-0.4 microm/d, pBMARunloaded days 14-42/42-70/70-98=0.368, pBMARloaded/unloaded days 14-42=0.073, pBMARloaded/unloaded days 42-70=0.098, pBMARloaded/unloaded days 70-98=0.262). Four months after implant placement, the bone-to-implant contact was 77.8+/-17.3% for the loaded and 78.0+/-5.8% for the unloaded implants (P=0.753). Immediate loading does not affect the bone mineral apposition rate when compared with unloaded implants. Rigid splinting seems to be the crucial factor for implant success. Uncontrolled masticatory forces can cause failure after partial loss of the provisional restoration.

Osseous healing characteristics of three different implant types.

The macroscopic and especially microscopic properties of implant surfaces play a major role in the osseous healing of dental implants. The aim of this study was to perform a histologic and histomorphometric comparison of the healing characteristics of anodically modified, machined and hydroxyapatite (HA)-coated implant types. A total of 24 machined surface implants (MSI), 24 HA-coated implants (HCI) and 24 anodized titanium surface implants (ASI) were inserted into the mandibles of 12 adult mini-pigs after extracting all mandibular premolars. Four animals each were killed after covered healing for 3, 6 and 12 weeks. Undecalcified ground sections were subjected to histologic and histomorphometric examinations. Primary effects and interactions were statistically evaluated and least square means (Tukey test) were compared. Histologic evaluations showed broad-based bone apposition to HA-coated and anodically roughened surfaces as well as narrow bone contacts to the machined surface. Localized resorption was only observed with the HA-coated implants. Overall, histomorphometric evaluation of bone-to-implant contact percentages for all observation periods showed significant differences between MSI (19.39% +/- 4.53) and HCI (39.05% +/- 4.53; P = 0.0092) and between MSI and ASI (42.72% +/- 4.20; P = 0.0011). In conclusion, the results of this study show that an anodically roughened implant may provide a similar rate of bone-to-implant contact as a HA-coated implant. In the presence of bone quality II to IV, according to Lekholm & Zarb (1985, in: Tissue-Integrated Prostheses: Osseointegration in Clinical Dentistry. Chicago: Quintessence Publishing), this may be of particular benefit, possibly because of higher stability, in maintaining pre-implantation functional strength after implant healing.

Osteoclast activity around loaded and unloaded implants: a histological study in the beagle dog.

The mechanisms of bone loss around dental implants are poorly understood. The osteoclast is the most important bone-resorbing cell. Humoral factors seem able to stimulate the differentiation of osteoclasts from mononuclear phagocytes. Bacterial lipopolysaccharides seem to be directly involved in inflammatory bone loss by stimulation of the survival and fusion of preosteoclasts. Excessive load seems to be able to cause bone loss. The aim of this paper was to evaluate the presence and number of osteoclasts in peri-implant bone in control (unloaded) and test (loaded) implants in order to determine if loading per se could be a contributing factor in peri-implant bone resorption. Forty-eight implants were inserted in the mandibles of 4 beagle dogs. After 3 months, a prosthetic superstructure was inserted on 24 implants, whereas in 24 implants only the healing screws were positioned. Twenty-four implants (12 test and 12 control) were retrieved at 6 months, and 24 implants (12 test and 12 control) were retrieved at 12 months. All implants were osseointegrated. The number of osteoclasts found in the crestal bone in the first 3 mm from the implant surface was evaluated. The mean number of osteoclasts were the following: control implants (6 months), 5.66 +/- 0.81; control implants (12 months), 2.55 +/- 1.05; test implants (6 months), 5.25 +/- 1.55; and test implants (12 months), 2.5 +/- 1.0. No statistically significant differences were observed between the control and test implants. According to our data, loading does not seem to have a relevant importance on the osteoclast activation in peri-implant bone.