Today, modern implant dentistry offers innovative surgical and prosthetic protocols such as the placement of implants in extraction sockets and immediate functional loading [
Although these protocols represent an opportunity to reduce the costs related to implant-prosthetic treatment, they are simultaneously a challenge for the clinician who must ensure the survival/success of implants and related restorations in a context of greater difficulties and risks [
Recently, manufacturers have introduced a series of new micro- and nanorough implant surfaces to the market with the aim of enhancing bone healing, accelerating the time of prosthetic treatment, and reducing the risks arising from the application of modern surgical and prosthetic protocols [
Another innovative procedure for the fabrication of dental implants today is direct metal laser sintering (DMLS) [
To date, the best way to study the interface between the bone and the implant is represented by histological studies on humans [
Several histological studies have shown that, in the short term, the porous surface of DMLS implants can support an excellent bone healing [
However, until now all human histologic/histomorphometric works on the interface between bone and the surface of DMLS implants were based on the evaluation of experimental (small-size) fixtures inserted in a transitional period (for example, to support a complete provisional removable prosthesis) and then removed for histological evaluation [
To effectively evaluate the relationship between the implant surface and bone over time, we should be able to remove the implants after a fairly long period of function, possibly several years. This is rarely possible because most of the implants removed (for infection or progressive loss of bone) may not be used for this purpose [
Therefore, the purpose of the present work was to study the interface between bone and standard size DMLS implants in order to fully understand the dynamics that occur at that level in the long-term. For this purpose, we have histologically evaluated standard size DMLS implants, which were perfectly integrated into the bone but removed for fracture after 5 years of function.
The DMLS fixtures (TixOs®, Leader Implants, Cinisello Balsamo, Italy) were fabricated from Ti-6Al-4V micropowders (diameter: 25–45
The scanning electron microscopy (SEM) evaluation of the DMLS titanium implant showed a porous surface ((a) magnification 47x) with globular protrusions ((b) magnification 842x), rich in cavities interconnected with by pores ((c) magnification 1100x), and irregular crevices ((d) magnification 2270x).
Two DMLS titanium fixtures and the surrounding hard tissues were retrieved after fracture of the implant body occurred after 5 years of functional prosthetic loading. Both of these implants were located in the anterior regions (one in the anterior maxilla and the other in the anterior mandible) of two different patients (a 45-year-old and a 70-year-old man, resp.) where they supported a fixed implant-supported prosthesis and a removable overdenture, respectively. Both of these implants were stable before removal and did not suffer from any infection; the fixtures were removed using a 5 mm trephine bur.
The implants were retrieved after 5 years of prosthetic loading and processed as previously reported [
Bone trabeculae were evidenced around all the implants at low magnification. The first bone-to-implant contact was located at the level of the fracture line. In the apical portion of the interface, small amounts of newly formed bone in close contact to the implant surface could be observed (Figure
Specimen harvested from the anterior maxilla of a 45-year-old patient. Trabecular, mature bone at the interface of the implant. The first bone-to-implant contact was located at the level of the fracture line of the implant. Bone remodeling areas were present. Acid fuchsin-toluidine blue, magnification 12x.
Bone remodeling areas and marrow spaces were present near the implant surface. No gaps were evident at the interface. A rim of osteoblasts making new osteoid matrix on the implant surface was evidenced. Acid fuchsin-toluidine blue, magnification 40x.
Blood vessels of different sizes were present within the marrow spaces. Osteoid matrix was evident inside the marrow spaces and secondary osteons could be seen abutting the implant surface. Acid fuchsin-toluidine blue, magnification 100x.
Bone was in tight contact with the implants surface and adapted to all its microirregularities. Acid fuchsin-toluidine blue, magnification 200x.
In recent years, patients have become increasingly demanding, requiring minimally invasive treatments and a reduction of the number of surgical sessions and time of treatment [
The establishment of new surgical (such as the placement of immediate implants in extraction sockets) [
In order to enhance the integration of the fixture in the bone to reduce healing time and anticipate the functionalization, a series of new implant surfaces with micro- and nanotopographical features have been recently introduced into the market with the aim of stimulating and promoting bone formation [
An alternative solution to these surface treatments is now represented by three-dimensional (3D) printing or additive manufacturing techniques applied to the world of implantology [
Several in vitro studies have demonstrated that titanium DMLS implants possess a highly porous surface structure with an open interconnected porosity where the surface concavities are connected with internal pores through a dense network of tunnels and interconnections [
Several histologic and histomorphometric studies have investigated the osseointegration of DMLS titanium implants in different animal models [
In a biomechanical and histologic/histomorphometric study on the beagle dogs, Witek et al. [
In another interesting histomorphometric and microCT study, Cohen et al. [
These results confirmed the evidence emerging from a previous in vitro study [
In contrast, Bowers et al. [
However, the human histologic/histomorphometric studies are certainly the best way to investigate the bone-implant interfaces [
Previous histologic and histomorphometric researches have investigated the interface between bone and DMLS implants in the first period of healing; however, those were experimental and transitional fixtures of reduced dimension removed 2 months after insertion [
In a histologic/histomorphometric study on the human posterior maxilla (type IV bone), 30 transitional mini-implants (10 DMLS titanium implants, 10 machined implants, and 10 dual acid-etched implants) were inserted in 30 patients (one implant per patient) and left unloaded for a period of 2 months. After that, the fixtures were retrieved for histologic and histomorphometric examination [
Similar results were found in a subsequent human histologic/histomorphometric study where 12 fully edentulous subjects had two DMLS experimental implants in the posterior maxilla installed. One fixture was immediately loaded, whereas the other was left unloaded [
In another study, four patients were installed with experimental, transitional DMLS titanium implants [
In another report, the same authors found a BIC% of 69.5% in experimental, transitional DMLS titanium implants placed in the posterior maxilla. These were left unloaded for a 2-month period and then retrieved for histologic evaluation [
Our present histologic/histomorphometric work is the first that has examined the interface between bone and DMLS implants of standard size that underwent functional loading for a period of 5 years, and it seems to confirm the findings of the previous aforementioned reports. In fact, the histologic sections depicted trabecular, mature bone around the entire implant surface with many remodeling areas. Bone was in tight contact with the implant surface and adapted to all its microirregularities, and rims of osteoblasts depositing osteoid matrix directly on the implant surface could be observed. In accordance with the previous literature, a satisfactory high mean BIC% of 66.1% (±4.5%) was found. The present work confirmed that the 3D environment of cavities, tunnels, and pores of various dimensions obtained with the DMLS technique and the subsequent treatment of the surface with organic acids (oxalic and maleic acids) may provide an optimal substratum for bone tissue ingrowth after functional loading in the long-term.
It could be hypothesized that bone formation within the concavities of the DLMS surface occurs when mesenchymal stem cells migrate into the pores, stop proliferation, and start the differentiation into functional osteoblasts [
In the face of these supposed biological advantages which must be confirmed by further studies, doubts emerge about the mechanical resistance of titanium implants fabricated with DMLS technology [
In this present study, a histologic/histomorphometric evaluation of the peri-implant tissues around two fractured DMLS titanium implants removed from the human mandible after 5 years of functional loading was performed. Bone appeared consistently adherent to the surface, as revealed by the light optical microscopy. The hard tissue grew into the concavities of the titanium surface and completely filled the implant threads. The DMLS implants appeared well integrated over the long-term, with bone tissue around the implant undergoing continuous remodeling. In conclusion, the present study confirms that the DMLS surface may provide an excellent substratum bone tissue ingrowth after functional loading in the long-term. However, controlled histologic/histomorphometric studies are needed to further validate the present results.
The authors have no conflicts of interest related to the present histologic/histomorphometric study, since no materials, grants, or other sources of financial support were provided for the present investigation.
The authors are grateful to Professor Mario Raspanti, from the Department of Human Morphology and Anatomy of the University of Varese, Italy, for helping with preparing this paper.