| Abstract|| |
Implant stability is a requisite characteristic of osseointegration. 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 also improves dentist-patient communication. Owing to the invasive nature of histological analysis, various others methods have been proposed like radiographs, cutting torque resistance, reverse torque, modal analysis, resonance frequency analysis and Implatest® . This review focuses on objectives and various methods to evaluate implant stability.
Keywords: Implant stability evaluation, resonance frequency analysis, stability
|How to cite this article:|
Mall N, Dhanasekar B, Aparna I N. Validation of implant stability: A measure of implant permanence. Indian J Dent Res 2011;22:462-7
In today's era, to be successful; quality is of prime importance. With respect to health care too, the concept of quality of life lived by an individual is of paramount importance. To answer the call of such importance, it becomes necessary to maintain a perfect balance between the invention of new technology and its judicious use. The advancements in dental health care have seen the use of implant dentistry as a part of providing a better and stable cosmetic outcome to our demanding patients.
|How to cite this URL:|
Mall N, Dhanasekar B, Aparna I N. Validation of implant stability: A measure of implant permanence. Indian J Dent Res [serial online] 2011 [cited 2013 May 25];22:462-7. Available from: http://www.ijdr.in/text.asp?2011/22/3/462/87071
In 1969, Brånemark et al.  demonstrated that direct contact between bone and titanium implant surface was possible, defining osseointegration as "the direct, structural, and functional contact between living bone and the surface of a functionally loaded implant". Implant stability is a requisite characteristic of osseointegration.  Osseointegration is also a measure of implant stability which can occur at two different stages: Primary and secondary. Primary stability mostly comes from mechanical engagement with cortical bone whereas secondary stability offers biological stability through bone regeneration and remodeling. , Primary stability leads to predictable secondary stability which has shown to increase at 4 weeks after implant placement.  During this period, the lowest implant stability is expected. As a result of osseointegration, initial mechanical stability is supplemented and/or replaced by biological stability and the final stability level for an implant is the sum of the two.
Primary implant stability plays a fundamental role in successful osseointegration. Friberg et al.  reported an implant failure rate of 32% for those implants that showed inadequate initial stability. Ivanoff et al.  in a rabbit study investigated the influence of primary stability on osseointegration by placing titanium implants so that some showed primary stability, some showed rotational mobility and some were totally mobile. It was concluded that high primary stability reduces the risk of micromotion and adverse tissue responses such as fibrous tissue formation at the bone-implant interface during healing and loading. 
Factors influencing primary stability
Factors influencing secondary stability
- Bone quantity and quality
- Surgical technique, including the skill of the surgeon
- Implant (e.g., geometry, length, diameter, surface characteristics) 
- Primary stability
- Bone modeling and remodeling
- Implant surface conditions 
| Objectives for Implant Stability Tests|| |
Objective measurement of implant stability is a valuable tool for achieving consistently good results which are influenced by:
Good decisions about when to load
When a surgeon makes a decision about early loading, objective measurement of implant stability can be valuable. A specified degree of implant stability can serve as an inclusion criterion for immediate loading.
Advantageous protocol choice on a patient-to-patient basis
With objective measurement of implant stability, surgeons can make well-informed decisions about protocol choices on a case-by-case basis. In other words, when low implant stability measurements indicate that immediate loading will jeopardize treatment outcome, a two-step protocol can be applied. In cases where high implant stability measurements are recorded, the implant could be immediately loaded.
Situations in which it is best to unload
Objective measurement of implant stability also supports making the right decisions about unloading. Sennerby and Meredith  point out that when replacing an immediately loaded temporary prosthesis with a permanent prosthesis, "low (secondary) values may be indicative of overload and ongoing failure." To avoid failure, they suggest that surgeons should consider unloading, perhaps placing additional implants and wait until stability values increase before loading the permanent prosthesis.
In a study by Glauser et al.  all implants in a sample group were loaded; those that failed showed significantly lower stability after one month than those that were successful. The authors conclude that, "this information may be used to avoid implant failure in the future by unloading implants with decreasing degree of stability with time.
Supports good communication and increased trust
Implant-stability measurements can also help improve communication between surgeons and patients. When a surgeon refers to measurable values rather than subjective judgments as the basis for decision making, it is easier to explain the treatment choices. The surgeons are also likely to appear more professional to colleagues alike and imbibe patient confidence.
Provides better case documentation
Objective implant stability measurements can be used to document the clinical outcome of implant treatments, which can be useful at a later stage if a problem should arise.
Methods to Measure Implant Stability
Historically the gold standard method used to determine the status of implant stability was microscopic and histological analysis. However, due to the invasiveness of this method and related ethical issues, various other methods have been proposed.
- The surgeon's perception
- Radiographical analysis
- Cutting torque resistance (for primary stability)
- Reverse torque
- Modal analysis
The surgeon's perception
One method of trying to evaluate primary stability is quite simply the perception of the surgeon. This is often based on the cutting resistance and seating torque of the implant during insertion. A perception of "good" stability may be heightened by the sensation of an abrupt stop when the implant is seated. An experienced surgeon's perception is of course invaluable and should under no circumstances be discounted. However, perception is obviously not possible to quantify, to consistently and effectively teach others or to use as a basis for future comparison. Particularly in high-risk cases, relying on perception is often not sufficient to ensure a positive treatment outcome. In addition, one's personal perception is difficult to communicate to others. But most importantly, this type of measurement can only be made when the implant is inserted - it cannot be used later, for example, before loading the implant.
Radiographical evaluation is a noninvasive method that can be performed at any stage of healing. It has been reported that 1.5 mm of radiographical crestal bone loss can be expected in the first year of loading in a stable implant, with 0.1 mm of subsequent annual bone loss. ,, Numerous limitations exist with the use of a conventional radiograph alone in making an accurate, independent assessment of implant stability. First, 1.5 mm is a mean value. Second, due to a low incidence of implant failure, changes in radiographical bone level alone cannot precisely predict implant stability. Third, it is impractical for a clinician to detect changes in radiographical bone loss at 0.1 mm resolution.  Fourthly, conventional periapical or panoramic views do not provide information on a facial bone level, and bone loss at this level precedes mesiodistal bone loss.  Lastly, neither bone quality nor density can be quantified with this method. Even changes in bone mineral cannot be radio graphically detected until 40% of demineralization had occurred.  Computer-assisted measurement of crestal bone level change may prove to be the most accurate radiographical information.  However, this method is not convenient to use in clinical practice.
Cutting torque resistance analysis
It was originally developed by Johansson and Strid  and later improved by Friberg et al. ,, The energy (J/mm 3 ) required for a current-fed electric motor in cutting off a unit volume of bone during implant surgery is measured. This energy was shown to be significantly correlated with bone density, which has been suggested as one of the factors that significantly influences implant stability. To minimize the interoperator variation, hand pressure during drilling was controlled.  Cutting torque resistance analysis (CRA) can be used to identify any area of low-density bone (or poor-quality bone) and to quantify bone hardness during the low-speed threading of implant osteotomy sites. A torque gauge incorporated within the drilling unit can be used to measure implant insertion torque in Ncm to indirectly represent J/mm 3 . 
CRA gives a far more objective assessment of bone density than clinician-dependent evaluation of bone quality based on Lekholm and Zarb classification. Clinical relevance was demonstrated by studies that showed the highest frequency of implant failures in jaws with advanced resorption and poor bone quality, often seen in maxilla. , Therefore, cutting resistance value may provide useful information in determining an optimal healing period in a given arch location with a certain bone quality. The major limitation of CRA is that it does not give any information on bone quality until the osteotomy site is prepared. CRA also cannot identify the lower "critical" limit of cutting torque value (i.e., the value at which the implant would be at risk). 
Reverse torque test
Unlike CRA, which measures the bone density and the resistance to cutting torque, the reverse torque test (RTT), proposed by Roberts et al.  and developed by Johansson and AIbrektsson, , measures the "critical" torque threshold where bone-implant contact (BIC) was destroyed. Reverse torque value (RTV) was reported to range from 45 to 48 Ncm. However, Sullivan et al.  speculated that any RTV greater than 20 Ncm may be acceptable as a criterion for a successful osseointegration, since none of the implants could be removed during abutment connection at 20 Ncm. It was further suggested that RTV is, therefore, a reliable diagnostic aid for verification of osseointegration. However, this method has been criticized as being destructive. Brånemark et al.  cautioned about the risk of irreversible plastic deformation within peri-implant bone and of implant failure if unnecessary load was applied to an implant that was still undergoing osseointegration. The threshold limit varies among patients depending on the implant material and the bone quality and quantity. RTV can only provide information as to "all or none" outcome (osseointegrated or failed) but cannot quantify degree of osseointegration. Also measurement of lateral mobility is more useful than measurement of rotational mobility as an indicator of a successful treatment outcome. A rotationally mobile implant can be laterally stable and reverse torque testing fails to measure, or take into account lateral mobility.
Modal analysis also termed as vibration analysis, measures the natural frequency or displacement signal of a system in resonance, which is initiated by external steady-state waves or a transient impulse force. It can be performed in two models: Theoretical and Experimental.  Two or 3-dimensional finite element modeling (FEM) is an example of computer-simulated theoretical modal analysis. Theoretical modal analysis such as FEM can be used to investigate the vibrational characteristics of objects that may be difficult to excite because of damping effect such as periodontal ligament in an in vivo model.  Experimental or dynamic modal analysis, on the other hand, measures structural changes and dynamic characteristics (e.g., natural characteristic frequency, characteristic mode, and attenuation) of a system that is excited in an in vitro model via vibration testing (e.g., Impactor or Hammer). This in vitro approach provides a more reliable assessment of an object than a theoretical modal. By combining different vibration and response detecting methods, various types of vibration analysis can be performed. 
Experimental modal analysis tests
A percussion test is one of the simplest methods that can be used to estimate the level of osseointegration.  This test is based upon vibrational-acoustic science and impact-response theory. The clinical judgment on osseointegration is based on the sound heard upon percussion with a metallic instrument. A clearly ringing "crystal" sound indicates successful osseointegration, whereas a "dull" sound may indicate no osseointegration. However, this method heavily relies on the clinician's experience level and subjective belief. Therefore, it cannot be used experimentally as a standardized testing method. 
Impact hammer method
Impact hammer method is another example of transient impact as a source of excitement force during experimental modal analysis.  It is an improved version of the percussion test except that, sound generated from a contact between a hammer and an object is processed through Fast Fourier transform (FFT) for analysis of transfer characteristics. By enhancing the response detection using various devices, such as a microphone, an accelerometer, or a strain gauge and by processing the detected response with FFT, it is possible to quantify and qualify the response wave in the form of dislocation, speed, acceleration, stress, distortion, sound and other physical properties. Periotest® and Dental mobility checker® are currently available mobility testers designed according to the impact hammer method.
Dental mobility checker (DMC)® was originally developed by Aoki and Hirakawa.  It has an electromagnetically driven and electronically controlled tapping head that hammers an object at a rate of 4 times per second. Contact time between the tapping head and the object is also measured. DMC® utilizes the same principle of tapping a tooth or implant with a dental hammer. A frequency response function is built-in to detect bone-quality-dependent sound. DMC® may provide quite stable measurements for osseointegrated implants.  There are some problems, however, such as the difficulties of double-tapping and difficulty in attaining constant excitation. Furthermore, the application of a small force to an implant immediately after placement may jeopardize the process of osseointegration.
Periotest® (Seimens, AG, Bensheim, Germany), quantifies the mobility of an implant by measuring the reaction of the peri-implant tissues to a defined impact load. Unlike DMC, ® which applies impact force with a hammer, Periotest® uses an electromagnetically driven and electronically controlled tapping metallic rod in a handpiece. Response to a striking or "barking" is measured by a small accelerometer incorporated into the head. Like DMC,® contact time between the test object and tapping rod is measured on the time axis as a signal for analysis. The signals are then converted to a unique value called the Periotest value (PTV), which depends on the damping characteristics of tissues surrounding teeth or implants. 
Pulsed oscillation waveform
Kaneko et al.  described the use of a pulsed oscillation waveform (POWF) to analyze the mechanical vibrational characteristics of the implant-bone interface using forced excitation of a steady-state wave. POWF is based on estimation of frequency and amplitude of the vibration of the implant induced by a small pulsed force. This system consists of an acoustoelectric driver (AED), acoustoelectric receiver (AER), pulse generator and oscilloscope. Both the AED and AER consist of a piezoelectric element and a puncture needle. A multifrequency pulsed force of about 1 kHz is applied to an implant by lightly touching it with two fine needles connected with piezoelectric elements. Resonance and vibration generated from bone-implant interface of an excited implant are picked up and displayed on an oscilloscope screen. An in vitro study showed that the sensitivity of the POWF test depended on load directions and position.
Resonance frequency analysis
Meredith et al.  developed an electronic method for testing implant stability called resonance frequency analysis (RFA). It is a noninvasive diagnostic method that measures implant stability and bone density at various time points using vibration and a principle of structural analysis. RFA utilizes a small L-shaped transducer that is tightened to the implant or abutment by a screw. The transducer comprises of two ceramic elements, one of which is vibrated by a sinusoidal signal (5-15 kHz) while the other serves as a receptor. The transducer is screwed directly to the implant body and shakes the implant at a constant input and amplitude, starting at a low frequency and increasing in pitch until the implant resonates. Meredith et al.  suggested that this test be performed at implant placement as a baseline reading for future comparison. Resonance peaks from the received signal indicate the first flexural (bending) resonance frequency of the measured object. In vitro and in vivo studies have suggested that this resonance peak may be used to assess implant stability in a quantitative manner. 
Currently, two RFA machines are in clinical use: Osstell® (Integration Diagnostics) and Implomates® (Bio TechOne). Osstell® has combined the transducer, computerized analysis and the excitation source into one machine closely resembling the model used by Meredith.  Osstell® created the implant stability quotient (ISQ) as a measurement unit in place of hertz. Resonance frequency values ranging from 3,500 to 8,500 Hz are translated into an ISQ of O to 100. A high value indicates greater stability, whereas a low value implies instability. A successful implant typically has an ISQ greater than 65. An ISQ<50 may indicate potential failure or an increased risk of failure. 
The most recent version of RFA is a wireless gadget. A metal rod is attached to the implant with a screw connection. The rod has a small magnet attached to its top that is stimulated by magnetic pulses from a handheld electronic device. The rod mounted on the implant has two fundamental resonance frequencies; it vibrates in two directions, perpendicular to each other. One of the vibrations is in the direction where the implant is most stable and the other is in the direction where the implant is least stable. Thus, two ISQs are provided, one higher and one lower. For example, an implant with buccally exposed threads may show one low value, reflecting the lack of bone in the buccal-lingual direction, and one high value, reflecting good bone support in the mesial-distal direction. 
Like Osstell,® Implomates,® which was developed by Huang et al.  uses RFA. However, it utilizes an impact force to excite the resonance of implant instead of a sinusoidal wave. Impact force is provided by a small electrically driven rod inside the transducer. The received response signal is then transferred to a computer for frequency spectrum analysis (range, 2-20 kHz).
Clinical application of RFA
Presently, clinical application of RFA includes establishing
- A relationship between exposed implant length and resonance frequency or lSQ values.
- Differential interarch and intra-arch ISQ values for implants in various locations.
- Prognostic criterion for long-term implants success.
- Diagnostic criterion for implant stability.
- Methods to improve stabilization of implants in low-density bone
The objective of testing implants with electrical impulse methods is to characterize, analyze and monitor their signatures. Conventional impulse testing of an implant requires fastening an accelerometer with associated wires and connectors to the implant, striking it with a calibrated hammer, and then recording and interpreting the data. Implatest® (Q Labs Inc., Providence, R.I.) incorporates all of the features of a conventional impulse test into a compact, portable, self-contained probe. Data can be gathered in seconds and is operator independent (independent of the direction or position of test application on the implant).
Unlike conventional impulse testing that requires attaching an accelerometer to the tested structure, Implatest® floats the accelerometer on a flexible membrane within the body of the instrument itself, and the accelerometer is attached to the instrument's recording tip. During recording, the accelerometer receives an impulse from the instrument's actuator and it floats toward the implant. Acceleration time history (ATH) data are recorded when the tip touches the implant and are fed to a computer. The initial data taken when the tip is in contact with the implant is discarded, and the remaining data are analyzed; these remaining data represent the implant in the free state, unencumbered by the testing apparatus. The stability of the implant then can be monitored by analyzing the changes in the characteristics of the signatures.  However, the Implatest® testing is subjected to some limitations; it uses a single accelerometer that can record acceleration in only one axis. Different recordings are obtained by changing the direction of percussion. Complications may arise when attempting to test an implant with an attached multifixture prosthesis, owing to their splinting effect. The dynamic signature of a multifixture prosthesis is extremely complex owing to the supporting influence of all implants or natural teeth or a combination of these at the particular testing site. 
| Conclusion|| |
The ability to monitor osseointegration and the life expectancy of an implant is a valuable diagnostic and clinical tool that has far-reaching consequences on implant dentistry. RFA has attracted considerable scientific interest in recent years; it can also be used to evaluate the effect of early and delayed loading, assess stability over a period of time and early diagnosis of implant failure. However, information should be established from many different diagnostic aids to assure long-term implant stability.
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I N Aparna
Department of Prosthodontics and Crown and Bridge, Manipal College of Dental Sciences, Manipal University, Manipal