|Year : 2016 | Volume
| Issue : 1 | Page : 84-88
Is primary stability a predictable parameter for loading implant?
Ratnadeep Patil1, Dimple Bharadwaj2
1 Department of Clinical Dentistry, Smile Care, Mumbai, Maharashtra, India; Department of Prosthodontics and Biomaterial, Adjunct Professor, Centre for Dentistry and Oral Hygiene Section, Oral Function, Prosthodontics and Biomaterial, University Medical Centre, The University of Groningen, Groningen, The Netherlands
2 Department of Clinical Dentistry, Smile Care, Mumbai, Maharashtra, India
|Date of Web Publication||12-Feb-2016|
Dr. Dimple Bharadwaj
13 Geetanjali, 234 SV Road, Bandra (West), Mumbai - 400 050, Maharashtra
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Implant stability is important for osseointregration; without it, long-term success cannot be achieved. Continuous monitoring in a quantitative and objective manner is important to determine the status of implant stability. Measurement of implant stability is a valuable tool for making decisions pertaining to treatment protocol and it also improves dentist-patient communication. Owing to the invasive nature of histological analysis, various others methods have been proposed such as radiographs, cutting torque resistance, reverse torque, and resonance frequency analysis (RFA). This review focuses on the objectives and various methods to evaluate implant stability.
Keywords: Insertion torque (IT), primary stability, secondary stability
|How to cite this article:|
Patil R, Bharadwaj D. Is primary stability a predictable parameter for loading implant?. J Int Clin Dent Res Organ 2016;8:84-8
|How to cite this URL:|
Patil R, Bharadwaj D. Is primary stability a predictable parameter for loading implant?. J Int Clin Dent Res Organ [serial online] 2016 [cited 2020 Jan 28];8:84-8. Available from: http://www.jicdro.org/text.asp?2016/8/1/84/176264
| Introduction|| |
The use of dental implants has become widespread and a predictable treatment modality for the restoration of missing teeth. It has become a part and parcel of routine therapy; considering this, success of implant dentistry depends on the stability of the implant, whether biological or mechanical. Initial stability at placement (primary stability) and the development of osseointegration in the following healing process (secondary stability) are two important factors for implant success [Figure 1].
|Figure 1: primary stability comes with old bone. secondary stability comes with new bone|
Click here to view
It is important to verify the status of implant-bone interface to decide the loading time of the implant. Generally, clinicians evaluate primary stability using the percussion test or using their own perception during the implant placement process. In particular, peak insertion torque (IT) and resonance frequency analysis (RFA) are the most used globally.  Peak IT is measured while the implant is inserted. The IT corresponds to a combination of the cutting friction of the tip of the implant in the bone, and the friction between the implant surface and the preparation in the bone. If the osteotome is narrow or the bone quality is high, the torque will be higher. The torque will also depend on how sharp the cutting tip of the implant is, the surface properties of the implant, the lubrication of the preparation (blood), and also the design of the implant itself. The formula of higher IT torque translating into higher primary stability may not always be true because the quantity and quality of bone varies significantly among patients. 
The torque values that we get during the placement of the implant is the first contact of the implant surface with the bone; these values will not be the same all along the osteotome and so we require a smart physiodispenser, which will give us a graphical representation of torque value of each implant thread, which gets embedded inside the bone. The current trends and the changes required to measure the biological and mechanical stability will be explored in this article.
Primary stability is defined as the absence of mobility in the bone bed after the implant has been placed.  It depends on the mechanical engagement of an implant within the fresh bone socket. During the early stages of healing, mechanical stability decreases and biological stability increases. In an osseointegrated implant, the stability depends on the biological component.  Primary stability is important for good secondary stability as it prevents the formation of connective tissue layer between implant and bone, ensuring bone healing.
A key factor for the implant primary stability is the bone-to-implant contact (BIC)  and thus, factors such as implant shape, length, and diameter that cause an increase in the contact area between the implant and bone may increase the implant primary stability. Furthermore, the quality of bone bed plays an important role in shaping the BIC area. 
Factors influencing primary stability: 
Bone density and quality.
Surgical protocol (osteotomy preparation) including the skill of the surgeon.
Mechanical stability occurs where friction occurs between the implant and the surrounding bone, giving rise to resistant torque at time of insertion. This resisting torque is proportional to the effort required to seat the implant; it depends largely on the characteristics of the implant, the density of the bone, and size of the osteotomy, as it pertains to the diameter of the implant. The implants with treated surfaces show greater roughness, a higher friction coefficient, and demand a larger IT than machined implants.  The results of the surface roughness and friction coefficients are in accordance with the results of the IT. The difference, across the IT values, between conical and cylindrical implants can be explained by the different contact surface areas among the thread geometry of these implants. 
It is clear that higher ITs fulfill the desire to achieve a high degree of mechanical stability as interpreted through manual perception. It is typical for manufacturers to provide some guidance on optimal IT, with some implant designs being specifically tailored to deliver higher ITs. This yields a sense of comfort for the clinician that the implant is initially "stable."  However, such a high torque has not been shown to be propitious to the surrounding bone. Numerous studies have been published, which clearly demonstrate that the critical pressure at this high torques leads to microfracture of the bone, with a net resorption in the cortical zone and an unfavorable, indeed delayed healing process with reduced BIC. Such a response might well shift the onset for secondary stability and thereby delay or extend the period of potential vulnerability. This is clearly counter to the goal we are trying to achieve with immediate or even early loading protocols where we want to transfer from simple mechanical fixation to full osseointegration in the shortest possible time. 
Bone density and quality
Bone quality is often referred to as the amount (and their topographic relationship) of cortical and cancellous bone in which the recipient site is drilled. A poor bone quantity and quality have been indicated as the main risk factors for implant failure as it may be associated with excessive bone resorption and impairment in the healing process, compared with higher density bone.  Clinical studies have reported dental implants in the mandible to have higher survival rates compared to those in the maxilla, especially for the posterior maxilla. It has been shown that achieving optimum primary stability in soft bones is difficult and is also related to a higher implant failure rate for the implants placed in such bones. Thus, the density of the surrounding bone seems to play an essential role in high occlusal forces and therefore, the high BIC percentages of a thin, "carpet"-like bone in contact with the implant surface seems to be not clinically significant compared to the lower rate of BIC in a thick bone.
The clinical perception of primary implant stability is frequently based on the cutting resistance of the implant during its insertion. Different surgeons have different preparation protocols, depending on the patient bone densities. Among the surgical factors that influence osseointegration, implant bed preparation is of critical importance. Drilling the implant bed not only causes mechanical damage to the bone but also increases the temperature of the bone directly, adjacent to the implant surface.  Mechanical and thermal damage to the tissue surrounding the implant during drilling can have a destructive effect on the initial state of the cavity housing the implant.  A correlation between cutting resistance at implant placement and primary resonance frequency (RF) values was reported for maxillary implants. 
In clinical work, primary stability can be evaluated by percussion test, Periotest, RFA and placement torque.
The percussion test may involve the tapping of a mirror handle against the implant carrier and is designed to elicit a ringing sound from the implant as an indication of good stability or osseointregration.  However, this method may be subjective according to the examiner and may give inaccurate measurements for implants because of the high rigidity of implants and the lack of periodontal ligaments.  The result is displayed digitally and audibly as Periotest values (PTVs) on a scale of -8 (low mobility) to 50 (high mobility).
Peak insertion torque
The IT used during placement of implants was measured through a surgical handpiece; it can be used to predict implant survival and to estimate healing time.  However, this method is nonsubjective, noninvasive, and extensively used in clinical practice during implant placement to assess primary stability. Peak IT gives us a static measurement, which is taken only once while force that is required for each and every thread of the implant to go through the bone will not be at the same torque. Thus, it allows only a single measurement at implant insertion and cannot be used for evaluating secondary stability. IT only assesses condition at the time of implant placement. 
Periotest (Gulden-Medizintechnik, Bensheiman der Bergstrae, Hesse, Germany) is an electronic instrument originally designed to perform quantitative measurements of the damping characteristics of the periodontal ligament surrounding a tooth, thereby establishing a value for its mobility.  As the outcome of Periotest measurements is influenced by the distance from the striking point to the first bone contact, it is evident that placement of the implant in the vertical dimension, abutment height, the level of marginal bone loss, and the striking position on the abutment/implant are critical factors for accuracy and/or reproducibility.  Single readings of Periotest determinations are of limited clinical value and have not been demonstrated to reflect on the nature of the bone/implant interface. By performing repeated measurements of the same implant over time, implant stability may be confirmed. 
Resonanace frequency analysis
RFA measures the stability by apply a bending load, which mimics the clinical load and direction and provides information about the stiffness of the implant-bone junction. It evaluates the micromobility or displacement of the implant in bone under a lateral load, applying microscopic lateral forces to the implant with a vibrating transducer. , The first commercial version of the RFA technique (Osstellt, Integration Diagnostic AB, Goteborg, Västergötland and Bohuslän, Sweden).
The results are given as implant stability quotients (ISQs),  which are affected by three main factors:
- The stiffness of the implant fixture
- The interface with surrounding tissue
- The design of the transducer and the total effective implant length above bone level.
The stiffer the interface between the bone and implant, the higher the frequency and higher the frequency, higher is the ISQ level. The ISQ unit is based on the underlying RF and ranges from 1 (lowest stability) to 100 (highest stability). We already know from the literature that an implant can tolerate a degree of micromotion, thought to be 100-150 μm, and this is what ISQ measures.
The RFA values are still not definitive as no actual threshold value has been established to differentiate a stable, integrated implant from a failing/failed implant; however, it has been suggested that an ISQ value above 57 at 1 year after loading represents a successful implant outcome  while a value below 50 indicates a risk of implant failure. 
There is a lack of correlation between IT and the ISQ as measured by RFA in an implant that is driven in at 30 ncm and has the same ISQ as the one that required 100 ncm of torque. Since ISQ is measuring axial stiffness, could it be that axial stiffness is far more relevant than rotational friction in ensuring implant integration?
It has been postulated that implants with low ISQ values yield more marked increases in ISQ values with time than implants with high ISQ values, indicating that differences in RFA values between implants may diminish with time.
It may be speculated that similar bone densities will result with time as a consequence of remodeling and adaptation to function.
Although extensively used in clinical research as one parameter to monitor implant stability, it has to be realized that RFA is affected by factors such as bone tissue characteristics and implant sink depth, diameter, and surface characteristics. Research indicates that implants yielding high ISQ values during follow-up appear to maintain stability. Low or decreasing ISQ values may be indicative of developing instability. However, no established normative range of ISQ values is available as yet.
Consequently, a single determination of the ISQ value does not define bone/interface characteristics or provide a quantitative evaluation of bone tissue integration. Similarly, no prognostic value for developing implant instability can be attributed to RFA. Hence, at the present time, the validity and relevance of RFA for clinical use have to be questioned. 
The destructive methodologies, such as removal torque assessment and pullout and pushout techniques are generally used only in preclinical applications. While these methods may be of value as research techniques, they are of limited value in clinical use owing to ethical concerns associated with the invasive nature of such methodology.
The gradual shift from primary stability to biological stability is poised at around 3 weeks; this is seen as to be the least stable time point where viscoelastic stress relaxation of the bone, along with remodeling, results in the loss of primary stability.  While secondary stability is the progressive increase in stability related to biologic events at the bone-to-implant interface such as new bone formation and remodeling,  it is absent at the time of implant placement and increases with time.  Secondary stability is a biological phenomenon, the result of the healing that takes place around the implant, i.e., the osseointregration. This process is dependent on many factors, e.g., the implant surface properties, loading conditions, and the individual host response
The concept of initially placing more bone within the immediate vicinity of the implant surface has been termed initial bone-to-implant contact (IBIC). Maximizing IBIC has two major benefits:
- The greater the IBIC, the greater the mechanical stability, thus enhancing the implant's ability to withstand micromovement while secondary stability develops and
- Reducing the osteogenic migration distance decreases the time for osteoconduction to occur.
| Discussion|| |
The seating torque is the final torque value that is achieved when the implant is inserted. Depending on the implant design and bone properties, the value can be higher or lower. Torque measures the rotational friction between the implant and the bone, together with the force required to cut the bone. However, torque does not necessarily correlate to implant stability. The value which we get during the placement of the implant is the first contact of the implant surface with the bone; this torque value will not be the same all along the osteotome and thus, we require a smart physiodispenser, which would record the torque value at each thread level and a graph can be attained helping us to decide whether we can load an implant immediately or if we need to wait. Although we have fixed IT values, implant with low IT will not be removed and it has to be left to be healed. Due to biology of bone, dynamics process takes place which will lead to ossteointegration and there will be micromechanical fixation.
Osseo integration is the basis of a successful endosseous implant. The process itself is quite complex and there are many factors that influence the formation and maintenance of bone at the implant surface.  The stiffness of the implant is a function of its geometry and material composition (length, diameter, and overall shape). Second, the stiffness of the implant tissue interface depends on the bond between the surface of the implant and the surrounding bone. The stiffness of the surrounding tissue is determined by the ratio of cancellous to cortical bone and the density of the bone with which an implant engages. Stiffness found at the bone-to-implant interface changes over time; thus, primary stability decreases with time and mechanical stability takes over. During this period of transition between primary and secondary stability, the implant faces the greatest risk of micromotion and consequent failure. It is estimated that this period in humans occurs roughly 2-3 weeks after implant placement when osteoclastic activity decreases the initial mechanical stability of the implant but not enough new bone has been produced to provide an equivalent or greater amount of compensatory biological stability.  This is related to the biologic reaction of the bone to surgical trauma during the initial bone remodeling phase; bone and necrotic materials resorbed by osteoclastic activity is reflected by a reduction in implant stability quotient (ISQ) value. This process is followed by new bone apposition initiated by osteoblastic activity, therefore leading to adaptive bone remodeling around the implant. 
Hypothetically, if the level of primary stability can be increased and the rate of osseointegration at the same time can be accelerated, the dip in total stability can be reduced and the implant is made less susceptible to micromovement and potential failure. Our goal must be the rapid onset of secondary stability, with minimal critical pressure to the poorly vascularized cortical bone so that unfavorable resorptive responses and delayed healing are avoided. At the same time, we need to employ an objective measure of constraint that reliably ensures that the implant can tolerate early or immediate loading; the accuracy of primary stability prediction is not good enough to prevent mistakes when using an immediate loading technique. Therefore, a more systematic use of objective measurements is encouraged.  A simple, predictable, noninvasive test to quantify implant stability and osseointegration is highly desirable.
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Conflicts of interest
There are no conflicts of interest.
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