Bone healing around a titanium dental implant is a complex biological process not yet fully understood. There is a growing interest especially in investigating the very early events taking place at the interface during the healing process. In particular, research focuses on the posterior region of the maxilla which is characterized by a very thin cortical bone shell, an alveolar bone volume reduced in height, and an overall low bone density. This is consistent with the clinical observation that tooth replacement treatments experience the lowest predictability and long-term success in trabecular bone tissue [4
]. The proximal tibial epiphysis of rabbit was therefore selected as the experimental animal model because of its anatomical heterogeneity regarded as a representative model for the jaw bony architecture [18
]. The aim of the current pilot study was to investigate the early healing events that were triggered by SLA implants inserted in rabbit tibial epiphysis over the period 0–6 days. MS implants were used as control.
Histological and ultrastructural investigations showed a different healing pattern for microrough and machined surfaces since the device insertion step. A few hours after implant placement, a coagulum of blood cells entrapped in a thin network of fibrin was detected at both implant surface types along with primitive marrow cells and inflammatory cells. Many authors have suggested that this immediate interaction with biological microenvironment is governed by surface topography [12
]. In the current study, the blood clot appeared to be less dense on SLA and more abundant on MS surfaces where bundles of fibrin-like fibers were arranged in a tight network firmly adherent to the superficial grooves. The peri-implant gap area also contained several chips of host bone, as consequences of surgical drilling and placing procedures, surrounded by blood and marrow elements.
The following events at the bone-implant interface appeared according to a chronological sequence [22
]. At day 3, the sequence of morphological events which foreshadow newly peri-implant osteoid deposition occurred earlier at SLA surfaces compared to MS. In particular, at the SLA interface, the presence of a more dense and mature connective stroma, more richly vascolarized and organized, was morphologically evident as long as signs of bone resorption by TRAP-positive osteoclasts present on the trabeculae surfaces. Consequently, some ALP-positive osteoblasts starting the deposition of a new osteoid seam were detected. The new bone formation occurred very early in the first days of wound healing. It started as resorption phenomena around preexisting bone at a distance from the implant, and it proceeded directed toward the implant surface, according to a model known as “distance osteogenesis” [10
]. The present authors suggest that the conditioning superficial layer, which appeared less dense on SLA and therefore more easily replaced, could be accountable for this secondary surface cell recruitment pattern. This observation is consistent with data from other researchers [23
By day 6, the morphological differences between the two surfaces were clear. On SLA surface, it was possible to appreciate a cuboidal osteoblast seam depositing intense-stained osteoid matrix characterized by the presence of large osteocyte lacunae. In contrast, on the MS surface, only a few cuboidal cells could be detected and most of the cells appeared flat and elongated. These findings were consistent with literature which showed how attachment was higher and differentiation faster on microroughness surface in the scale dimension of cell adhesion structures [25
]. The new bone formation phenomena, already remarkable at day 3 as distance osteogenesis, was visualized directly in tight contact with some SLA implant areas and raising the possibility of contact osteogenesis. It has been hypothesized that this contact osteogenesis at the implant surface is dependent on a surface-conditioning layer which in turn leads to a specific cell differentiation pattern [18
From day 6, samples osteoclast-like cells (exhibiting the typical “ruffled border”) were detected at the SLA implant surface. According to other authors [28
], osteclast-like cells might be present not only where bone remodeling starts, but in general where osteogenesis occurs as well as, since bone formation process results from osteoblast-osteoclast coupling activity. We speculate that osteoclastic differentiation might be driven by surface topography, according to the geometric tropism of metal peaks and valleys which offer cell anchorage.
In conclusion, the preliminary morphological data hereby described were achieved by means of an experimental in vivo
model system consisting of a titanium plate-like device with different surfaces (microrough versus machined), inserted in rabbit proximal tibial epiphysis. Because of these device shape and placement site, the most portion of the implant surface lacked direct contact with host bone but faced a wide peri-implant space rich in marrow tissue. This wide peri-implant space was intentionally created in order to dismiss a direct contact between host bone and metal implant and to better understand the role of the biological microenvironment at the interface. Therefore, the authors did not consider peri-implant gap area parameter which is known to play an important role in osseointegration process if not too wide [27
]. The study confirmed that the insertion of titanium devices into the proximal tibia elicited a sequence of healing events. Newly formed bone proceeded through an early distance osteogenesis, common to both surfaces, and a delayed contact osteogenesis which seemed to follow different patterns at the two surfaces. In fact, SLA devices showed a more osteoconductive behavior retaining a less dense blood clot, which might be earlier and easier replaced, and leading to a surface-conditioning layer promoting osteogenic cell differentiation and appositional new bone deposition at the titanium surface. This model system is expected to provide a sequence of morphological events useful to clarify the early cellular and biomolecular events occurring at the metal surface.