The osseointegration and survival of dental implants are linked to primary stability. Good primary stability relies on the mechanical friction between implant surface and surrounding bone with absence of mobility in the osteotomy site immediately after implant placement. Several factors have been found to affect implant primary stability, including bone density, implant design, and surgical technique. Various methods have been used to assess implant primary stability including insertion torque and resonance frequency analysis. This article aims to evaluate the success of osseointegration in the absence of primary stability and to propose recommendations to manage implants that lack primary stability.
Factors influencing primary stability include implant design, bone density, and surgical techniques used.
Implant primary stability is not an absolute prerequisite to osseointegration; however, it has an effect on the implant survival rate.
Resonance Frequency analysis is the most frequent method used by clinician to assess both primary and secondary stability.
The term osseointegration was coined and first defined in 1977 as a direct structural and functional connection between living bone and the surface of a load-carrying implant. Histologically, osseointegration can be identified by the presence of regenerated bone at the implant-bone interface. For dental implant osseointegration to occur, adherence of cells to the surface of the biomaterial is a critical factor. The implant surface characteristics can modulate the adsorption of proteins, lipids, sugar, and ions present in the tissue fluids. Accordingly, several factors have been determined to influence these interactions at the implant-host interface ( Fig. 1 ).
In the Branemark paradigm, implant immobility during the first 6 months of healing is a perquisite for osseointegration to occur. Primary stability is defined as the biometric stability immediately following the insertion of an implant and is a direct result of the mechanical engagement of an implant with the surrounding bone. Primary stability has been widely referenced in the literature as a requirement for the osseointegration of dental implant and the long-term success. This “mechanical stability” gradually decreases during the early stages of healing due to bone remodeling. As new bone is formed along the implant surface, secondary stability is established, which is the direct result of the osseointegration corresponding to both mechanical and biological features ( Fig. 2 ).
Micromotion of dental implants is the minimal displacement of an implant body from the surrounding bone, which is not visible to the naked eye. It has been suggested that micromotion between implant and surrounding bone must not exceed a threshold value of 150 μm for a successful implant healing. Any movement even at the micrometer range can induce stress and strain that may hinder the recruitment of new cells and negatively influence osseointegration and bone remodeling leading to forming fibrous tissues.
Factors influencing primary stability
Primary stability is influenced by multiple factors, including implant design (length/diameter, microscopic/macroscopic morphology of implant), bone condition (quality/quantity), and the surgical protocol specific for each implant system ( Fig. 3 ).
Implant Design (Macro/Microstructure)
The 3-dimensional structural design of an implant plays a vital role in attaining primary stability. The threaded design increases the surface area of the implant in contact with bone, thereby offering a higher percentage of bone-to-implant contact (BIC) in comparison to implants with cylindrical design. Tapered implants were later introduced to provide a degree of compression of the cortical bone in an implant site with inadequate bone.
Several implant surface modifications have been developed to modulate and enhance biological response to improve osseointegration and primary stability ( Fig. 4 ). Studies have shown that surface topography and roughness increases the surface area of the implant and allows a firmer mechanical link to the surrounding tissues, thus enhancing primary stability.
Veis and colleagues showed that implants with acid-etched surfaces can achieve a significantly higher BIC and primary stability in poor bone quality sites in comparison to implants with machined surface. Schätzle and colleagues compared a chemically modified sandblasted/acid-etched titanium surface (modSLA) with a standard SLA surface and found higher implant stability quotient (ISQ) values at 12 weeks in the (modSLA) group versus SLA group.
Bone Density and Quality
Clinical studies have reported that dental implants in the mandible have higher survival rates compared with those in the maxilla, especially the posterior region. The distribution and percentage of both the cancellous and cortical bone in the implant site play a crucial role in determining the insertion resistance of an implant during placement.
A balance between the cancellous and cortical part of the bone is always desired. Type I cortical bone will lead to highly insertion torques with possible negative bone resorption. On the other hand, type IV bone with no cortical bone provides minimal or no primary stability, which in turn may result in no osseointegration due to implant micromotion.
Turkyilmaz and colleagues observed a significance correlation between mean voxel values (gray scale) (751 ± 257) and ISQ values (70.5 ± 7) at implant placement. In a similar study, Fuster-Torres and colleagues found a significance relationship between mean voxel values (623 ± 209) and ISQ values (62.4 ± 8).
Several modalities of osteotomy preparation techniques have been proposed to optimize a high degree of implant stability. Among these techniques are using undersized drilling protocol, osteotomes to laterally condense the bone, and counter-clock wise rotational surgical drills.
It is worth mentioning that drilling into the implant bed not only incurs mechanical damage to the bone but also increases the temperature of the bone directly adjacent to implant surface. Bone cell necrosis occurs when the temperature exceeds 47°C for 1 min. Mechanical and thermal damage to the surrounding tissue around the implant surface can have a destructive effect on the initial state of the osteotomy housing the implant.
Even though primary stability is always desired, it may not be achievable either from patient specific reasons, for an example bone quality, or from operator related factors, such as overpreparing the implant osteotomy. However, it has been reported in the literature that implants that lacked or had low primary stability have comparable survival rates to those with high primary stability.
Measuring primary stability
Although primary stability is widely discussed in the literature, there is no simple and clear method to quantify or measure it rather than unreliable clinicians’ hand tactile sensation. Several investigators presented techniques and methods to quantify implant stability in general and primary stability in particular.
Cutting Torque Resistance Analysis
Cutting torque resistance analysis (CRA) was developed by Friberg and colleagues in 1995. This method measures the required current needed in an electric motor to cut off a unit volume of bone during the low-speed threading of implant osteotomy site. The measured energy is significantly correlated to the bone density because it identifies areas with soft, cancellous bone. The technique consists of incorporating a torque gauge within the drilling unit to measure implant insertion torque in newton centimeter (Ncm) to be converted to J/mm 3 . The major limitation of the CRA technique is that it does not give any information on bone quality until the osteotomy site is prepared. CRA also fails to provide a clear lower cutting torque threshold to indicate the risk of implant failure.
Clinically, insertional torque (IT) is more widely used in assessing primary stability than CRA. However, torque measurement can only be recorded at the time of implant placement and does not assess any changes that happens after implant placement. Ottoni and colleagues investigated the relation between IT and implant survival in single implants. The recommendation of this study to achieve osseointegration was a minimum IT value of 20 Ncm and an optimal torque of 32 Ncm. A high IT value maybe an indication of good primary stability; however, maximum insertion torque can be deceiving; direct pressure of the implant on the dense cortical bone without adequate BTI contact at the rest of implant surface can produce high IT.
IT and resonance frequency analysis (RFA) are the most widely used methods to measure primary stability so their relationship has been extensively analyzed by numerous researchers and yet remains controversial. Lages and colleagues, in a recent systematic review, concluded that insertion torque and RFA are independent and incomparable methods to measure primary stability.
Periotest (Siemens A, Benshein, Germany) has been considered a reliable method to measure primary stability. It was originally developed by Schulte to test natural tooth mobility using a metallic rod that applies an electronically controlled tapping force to the object (tooth or an implant). The response time to this “controlled tapping” is measured by a sensor on the handpiece and converted into a value called periotest value (PTV). This value represents the damping characteristics of the surrounding tissues around the tooth or implant. PTV ranges from −8 (low mobility) to +50 (high mobility). Olive and colleagues found that a normal PTV of an osseointegrated implant falls in a relatively narrow zone (−5 to +5). Other studies found that PTVs of osseointegrated implants falls within a narrower zone (−4 to −2 or −4 to +2).
Many investigators presumed that PTV precisely reflects the condition of BIC. However, the prognostic accuracy of PTV for implant stability has been criticized for poor sensitivity, difficulty of application in posterior region, and susceptibility to operator variables.
Resonance Frequency Analysis
RFA suggested by Meredith in 1998 has recently gained popularity as a noninvasive diagnostic method to measure both primary and secondary implant stability. It uses an L-shaped transducer that is inserted into the implant or abutment containing a vibrating element and a receptor. The vibrating element applies a sinusoidal wave or an impact force wave and the receptor measures the resonance signal from the implant-bone component that has shown to measure implant stability. It represents a measurement of the axial stiffness between implant and bone.
Currently, 2 machines are available in the market that measures implant stability via RFA: Osstell (Integration diagnostics) and Implomates (Bio TechOne). Osstell is a more widely used device and combines the transducer, computerized analysis, and vibration source into one device. Initially, Hertz was used as a measurement unit; then ISQ was developed as a measurement unit by Osstell. The ISQ value ranges from 0 to 100, of which the higher value referring to more implant stability. The Implomates device applies an impact force instead of the sinusoidal effect to trigger the resonance of the implant and a receptor measures in range from 2 to 20 kilohertz. More research has been conducted on the application and efficacy of the Osstell device compared with the Implomates.
Implant bed preparation
The goal of improving primary stability has been a constant objective ever since the development of the early endosseous implants. The concept of placing a longer, wider implant suitable for an implant site is the most commonly cited method to improve primary stability. However, the development of tapered implant with aggressive thread design challenged such a concept.
The quality and dimension of implant site can be assessed through cone beam computed tomography to customize the surgical protocol in sites where softer type III or IV bone is present.
Summers Osteotome Technique
After using the pilot drill, a series of osteotomes can be tapped into the implant site to laterally compact the cancellous bone, which results in better dense bone allowing for better implant primary stability. Markovic and colleagues obtained significantly higher implant stability values using lateral bone condensation versus conventional bone drilling techniques, both immediately after implant placement surgery (74.03 ± 3.53 vs 61.2 ± 1.63 ISQ) and at 6 weeks following the surgery (70.3 ± 1.21 vs 65.23 ± 0.43 ISQ).
Undersized Drilling Technique
Several studies have proved that when dental implants are inserted in underprepared osteotomy sites using smaller diameter drills, maximum bone volume preservation and enhanced bone density are achieved. Furthermore, underpreparation of osteotomy sites will allow translocation of osteogenic bone fragments along the implant surface, which will contribute to bone healing and remodeling.
In the presence of soft trabecular bone, experienced clinicians may choose to underprepare the osteotomy by not using the last drill of the implant system surgical protocol. Degidi and colleagues compared primary stability parameters (insertion torque and RFA) of 3 study groups of implants placed in poor-quality fresh bovine bone where standard osteotomy preparation (control group), 10% undersized preparation, and 25% undersized preparation were performed. They found improved primary stability with 10% undersized preparation group; however, 25% undersized group did not provide an added benefit to stability.
Osseodensification Technique (VERSAH)
Unlike traditional drilling techniques, osseodensification does not excavate and subtract bone tissue. During the drilling process, bone tissue in this technique is simultaneously compacted and autografted in an outward expanding direction from the osteotomy, resembling somewhat a traditional hammered osteotome technique, but without the trauma. When a Densah Bur is rotated at high speed in a reversed, noncutting direction with steady external irrigation, a strong and dense layer of bone tissue is formed along the walls and the base of the osteotomy.
Bicortical fixation has historically been implemented to increase the primary stability of implants. The technique entails the use of long implant to engage 2 layers of cortical bone: the cervical crest and the sinus or nasal floor cortex or lower border of anterior mandibular cortex. In a study by Hsu and colleagues, they used a stopper drill and self-threading implants to firmly engage the sinus floor cortex to improve implant stability in one group, an indirect vertical sinus floor lift in a second group, and unicortical fixation in a third group. They found that primary and secondary implant stabilities of bicortical fixation did not differ significantly from those of unicortical fixation and indirect sinus elevation. However, bicortical fixation technique is simpler and more economical than indirect sinus elevation. In contrast, Ivanoff and colleagues found that bicortically anchored implants failed nearly 4 times more often than the monocortical ones. Bicortical fixation is currently not widely used. Further studies are needed to validate the outcomes of this concept.