What is a Dental Implant?

 Introduction.

Modern implantology is based on the principle that dental implants are not a substitute for periodontally diseased dentition; however, they are an excellent treatment for edentulous ridges and hopeless teeth (Greenwell et al. 2019; Pini Prato et al. 2019). The use of dental implants for the rehabilitation of edentulous ridges is today a well-consolidated procedure, with high predictability and patient satisfaction. The present state of the art is the result of 50 years of intense and frenetic research on the biology of osseointegration and clinical use of implants. The market of dental implants is in exponential expansion; it increased by 14% during the last decade, and it was projected that, by 2026, one fourth of the US population will have a least one dental implant (Elani et al. 2018).

What is a dental implant?

The term “Dental Implant” was officially registered in the dictionary of Medical Subject Heading (MeSH) in 1990, and it was defined as

• “Biocompatible materials placed into (endosseous) or onto (subperiosteal) the jawbone to support a crown, bridge, or artificial tooth, or to stabilize a diseased tooth” ^{(https://www.ncbi.nlm.nih.gov/mesh/ )}

From a periodontal perspective, the Online Glossary of Periodontal Terms defined “Oral implant” as ^{(https://members.perio.org/libraries/glossary )}:

• An alloplastic material or device that is surgically placed into the oral tissue beneath the mucosal or periosteal layer or within the bone for functional, therapeutic, or esthetic purposes.
• A graft or alloplastic device inserted into the oral hard or soft tissues for replacement of missing or damaged anatomical parts, or for stabilization of a periodontally compromised tooth or group of teeth.

The proposed definitions of dental implants reconcile the early stages of oral implantology, acknowledged with the sentence “into (endosseous) or onto (subperiosteal) the jawbone”, with the primary use of the implant itself, that is, “to support a crown, bridge, or artificial tooth, or to stabilize a diseased tooth”.

Historical background.

The use of oral implants is much older than the research on osseointegration. The interest in replacing natural dentition has ancient origins, and surprising archaeological specimens challenged the rules of biology and biocompatibility with iron-tissue integration (Becker 1998). More recent and systematic attempts towards implant-retained dentition started with subperiosteal implants, introduced in 1940, and aimed to sit between bone and periosteum. Reported survival rates were usually very poor, with short-term survival of 60% at 3 years (Albrektsson et al. 1986). Later designs were planned for endosseous placement. The blade-vent implants were proposed for both totally and partially edentulous patients, and, even if data were more encouraging than for implants with subperiosteal design, long-term clinical retention did not exceed 50-60% (Smithloff and Fritz 1982).

Osseointegration and the meticulous surgical protocol of the early trials started an unprecedented revolution for the field of implant rehabilitation. The infancy of modern implant dentistry opens with the clinical use of osseointegrated implants for edentulous jaws (Adell et al. 1981; Branemark et al. 1977). In the late 1980s, the use of implants was implemented for partially edentulous patients, and the outstanding results allowed the use of implants for a much wider demographic (van Steenberghe et al. 1990). Implants are now the elective choice for single-tooth replacement, and current challenges have moved from predictability to tissue-bone interactions (Di Gianfilippo et al. 2020), controlled bone remodeling (Di Girolamo et al. 2016), investigation and treatment of peri-implantitis (Wang et al. 2020), and digital technologies.

Osseointegration.

The possibility of an intimate bonding between bone and titanium was noted in Orthopedics before than in Dentistry. When researchers placed titanium screws in rat femurs, it was remarkable how “at the end of 6 weeks, the screws were slightly tighter than when they were originally put in; at 12 weeks, the screws were more difficult to remove; and at the end of 16 weeks, the screws were so tight that in one specimen the femur was fractured when an attempt was made to remove the screw” (Leventhal 1951; Rudy et al. 2008). This reaction was later coined as osseointegration by Per-Ingvar Branemark, who saw the possibilities for human use in dentistry (Rudy et al. 2008). In 1952, Branemark used a titanium implant chamber to study blood flow in the rabbit Tibia and Fibula bones. At the conclusion of the experiment, when it was time to remove the titanium chambers from the bone, he discovered that the bone had integrated so completely with the implant that the chamber could not be removed.

Today, the anatomical and biological differences between the periodontal ligament and osseointegration are well known. While the periodontal ligament interposes collagen fibers to suspend the root of each tooth in its alveolar bone socket (syndesmosis), any appearance of connective tissue between bone and implant fixture would be a negative sign suggesting a failed osseointegration (Branemark et al. 1977). Histology of osseointegrated implants was reported by Schroeder, who described it as a functional ankylosis (Schroeder et al. 1978) with “new bone that laid down on implant surfaces” (Schroeder et al. 1981) and with “direct functional and structural connection between living newly formed bone and the surface of a load implant” (Albrektsson et al. 1981). Osseointegration started a new era of translational research aimed at investigating and improving such bone-implant bonding.

Basics of oral implantology.

While an in-depth evaluation of the implant micro/macro designs, type of abutments, and types of prostheses is left to other seminars, this introductory lecture will be used to overview of the main components of a dental implant.

The implant system includes 3 main components:

1. The implant fixture or body, which is osseointegrated inside the bone it represents an artificial substitute of the root.
2. The abutment, that is an intermediate structure used to connect the fixture with the prosthetic superstructure (crown, bridge, denture…)
3. The superstructure, which could be fixed like a single crown or a bridge, or removable like a partial or complete overdenture.

Primary and secondary stability.

When a fixture is first placed into the recipient site, the implant threads engage the surrounding bone, providing mechanical retention known as primary stability. This mechanical strength is measured in N/cm,² with the suggested range being 25-45 N/cm² to avoid micromovement larger than 100μm (Trisi et al. 2009). Relative motion between the implant body and the surrounding bone during the early healing phase is indeed considered to be a high-risk factor for early implant loss, as failure of osseointegration occurs. As soon as the implant is stabilized in situ, a cascade of cellular and molecular events guides the remodeling of the bone in contact with the threads and appose new bone in direct contact with the implant surface. The direct contact of bone with the titanium surface is called osseointegration or secondary (biological) stability. The transition from primary mechanical stability, provided by the implant design, to secondary biologic stability provided by newly formed bone (osseointegration) takes place during the early wound healing. Raghavendra et al. (2005) hypothesized that there might be a period of time during healing in which osteoclastic activity has decreased the initial mechanical stability of the implant, but the formation of new bone has not yet occurred to the level required to maintain implant stability. During this critical period, a loaded implant would be at greatest risk of micromotion and would be more susceptible to failure. In a landmark narrative review, critical evaluation of available evidence led to the conclusion that 3 weeks is the weakest period for the overall stability due to the drastic reduction of primary stability and a slow increase of secondary stability (Raghavendra et al. 2005).

Paradigm shifts.

Advancements in surface characteristics

Osseointegration started a new era of translational research aimed at investigating and improving such bone-implant bonding. Landmark technological advances relative to surface enhancement were the shift from machined to rough surfaces, from hydrophobic to hydrophilic surfaces, and the recent nano-topography.  The notable difference between smooth (Sa: 0.35 µm) and moderately rough surfaces (Sa: 2.29) relied in the higher bone-to-implant contact for rough surfaces and the different dynamics of bone apposition. Indeed, while bone apposition for rough surfaces was noted from both the implant surface (contact osteogenesis) and the surrounding bone marrow (distance osteogenesis), bone apposition around smooth surfaces occurred solely by distance osteogenesis (Salvi et al. 2015). Hydrophilic rough surfaces represented the following advancement that allowed faster bone deposition (Lang et al. 2011) and upregulation of genes of osteogenesis and angiogenesis (Donos et al. 2011). Latest advances moved to the nanoscale level to prevent biofilm-induced complications and to furtherly improve osteogenesis (Di Gianfilippo et al. 2017). Nano-surface topography preventing bacterial colonization, antimicrobials bonded to the implant surface, and active release of antimicrobials represent the cutting-edge advancement for surface-related research and open a new chapter for prevention of infectious diseases (Hickok et al. 2018).

From external, to internal, to conical connections

Traditionally, implant‐abutment (I-A) connections have been classified according to the presence of a geometric figure extending coronally or apically to the surface of the implant: I-A connections were classified as external, if the geometric figure protrudes coronally to the implant, or internal if it intrudes apically. I-A connections can be further classified based on the geometric features and the relationship between the abutment and the internal aspect of the implant as clearance‐fit (flat-to-flat) connections, conical connections (tube-in-tube), and combined connections. Internal conical connections are comprised of a conical portion of the abutment that is located at the corresponding portion of the internal aspect of the implant, creating a tapered interface between the abutment and the implant. Historically, periodontal literature guided implant companies to shift from external to internal clearance-fit connections to internal conical connections. Data guiding this prosthetic evolution is mainly related to:

• Reduced marginal bone loss occurs internally than around external connections (Berglundh et al. 2005).
• Lower bacterial colonization of the inner implant space for conical connections, followed by greater colonization for internal clearance connections, and greatest for external connections (Canullo et al. 2015; Khorshidi et al. 2016).
• Size of microgap opened during loading stress. During loading, the abutment bends and opens a microgap at the level of the I-A connection. The bending movement with the resulting microgap is lowest with conical connections, higher for internal clearance connections, and greatest for external connections (Canullo et al. 2015).

From free-hand bone-driven placement to prosthetically-driven guided surgery.

The use of dental implants shifted from rehabilitation of edentulous jaws (Branemark et al. 1977) to implant-supported single crowns or bridges (Buser et al. 2017). Interestingly, guidelines regulating implant positioning gradually adapted to accommodate this shift. Position and angulation of implants for a lower overdenture follow, indeed, different rules than those currently applied for single edentulism in the esthetic area. Early trials of dental implants were dominated by oral surgeons who used to place implants where bone allowed; then it was the task of the prosthodontists to connect a denture over the existing, yet rarely parallel, abutments. The increased aesthetic demand, together with the importance of tooth alignment, raised the need for a prosthetically-driven plan. According to the current paradigm, the plan starts with the determination of the ideal crown positioning, and the implants are placed to allow realization of the pre-determined prosthetic plan. To improve the accuracy between the planned and performed implant positioning, a series of stents was developed to guide the drilling protocol. Current terminology stresses the distinction between:

• Free-hand: also called mental navigation, the free-hand surgery consists of drilling and placing the implant without any stent or guiding device.
• Template guide: it is a cast-based guide, opened on the facial side, used to guide the drills. The implants are still placed freehand.
• Partially guided implant placement: a CBCT and a surface scan are used to generate a 3D-printed closed guide to be used during the drilling protocol. The implants are still placed freehand.
• Totally guided implant placement: a CBCT and a surface scan are used to generate a 3D-printed closed guide to be used during the drilling protocol. Another printed closed guide is then used to assist implant placement.
  • Template guides and printed guides are grouped in the category of “static guides”, as opposed to the “dynamic navigation”.
• Dynamic navigation: also called real-time navigation, the dynamic navigation allows the surgeon to see a virtual model of the patient and of the instruments moving in real-time, meanwhile he/she drills and places the implant.

Multiple research models have been tested and agree that the clinical results of guided surgery are more accurate than free-hand surgery when compared with the pre-surgical plan (Chen et al. 2018; Vercruyssen et al. 2015; Vercruyssen et al. 2014). The inaccuracy of fully guided surgery comes from a jiggling movement of the drills into the sleeves, which creates a horizontal deviation of 0.8mm in the coronal entry, 1mm horizontal deviation at the apex, and 3° vertical deviation of the implant shoulder (Chen et al. 2018). On the other hand, the inaccuracy of free-hand placement is 1.4mm in the coronal entry, 2mm horizontal deviation at the apex, and 6° vertical deviation of the implant shoulder (Chen et al. 2018). Dynamic navigation is currently under testing, and recent trials report comparable accuracy between static and dynamic implant placement (coronal horizontal deviation: 1mm, apical deviation: 1.3mm, angular deviation: 3°) (Kaewsiri et al. 2019).

Conclusions.

Dental implants are routinely used in every dental office with the highest rate of satisfaction and predictability. What today is universally accepted as the standard of care resulted from decades of intense translational research. While this seminar provided an overview of history and basic implant features, it aims to the following lectures to investigate more in-depth individual topics. Only a few basic principles were stressed today and, if well understood, gave the bases to successfully proceed with the remaining course of #794 Implant Literature.

What do I need to remember?

• Implants are a valuable treatment for the edentulous ridges and for hopeless teeth. Periodontally affected teeth need periodontal treatment and not extraction.
• What is osteointegration?
• What are the components of a system implant?
• What are primary and secondary stability?
• What surface advances improved the bone-to-implant contact?
• Name the types and the advantages of internal over external connections
• Name the types and the advantages of guides for implant placement.