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The stability of the implant at the time of placement and during the development of the osseointegration process are the two major issues governing the implant survival. Implant stability is a mechanical phenomenon related to local factors such as bone quality, quantity, type of placement technique and type of implant used. The application of a user-friendly, clinically reliable, non-invasive method to assess implant stability and the osseointegration process is considered highly desirable. Resonance frequency analysis (RFA) is one such method which shows almost perfect reproducibility and repeatability after statistical analysis. The aim of this paper is to review the various methods used to assess implant stability and on the currently used RFA method which is being highly accepted in the recent times.
Implant stability plays a critical role in successful osseointegration, which is the direct structural and functional connection between bone and the surface of a load-carrying dental implant.[1,2,3,4] Achieving and maintaining this implant stability is a prerequisite for a successful clinical outcome.[5,6] Therefore being able to measure this implant stability is important for evaluating the success of an implant.
Implant stability is achieved at two different stages: Primary and secondary. Primary stability comes from mechanical engagement of the implant with cortical bone whereas, the secondary stability is the eventual outcome from regeneration and remodeling of the bone and tissue around the implant. Primary stability determines the secondary stability, which dictates the time of functional loading.[6,7]
The degree of implant stability may also depend on the condition of the surrounding tissues. It is, therefore, of utmost importance to be able to quantify implant stability at various points of time and to project a long-term prognosis based upon measured implant stability. Hence, the need to measure implant stability.
Various diagnostic methods, both invasive and non-invasive have been used for this purpose. Histomorphometric analysis, tensional test, push-out/pull-out test and removal torque test involve the disengagement of the implant following the waiting period to determine the extent of osseointegration and hence are limited to animal studies.[8,9] Percussion test, radiography, cutting torque test, periotest and resonance frequency analysis (RFA) being noninvasive are used in the clinical scenario.[10,11,12]
Percussing the implant with a mirror handle is unreliable and no longer used due to subjective variation.
Radiography, the most widely used diagnostic aid for evaluating the quantity and quality of bone in the area for an implant placement, is helpful in predicting implant stability by observing the process of osseointegration or peri-implant lesions.
However, there are many limitations with the conventional periapical and panoramic views. The facial bone loss which precedes the mesiodistal boneloss cannot be viewed, the limitation with image resolution making standardized X-rays difficult to achieve, distortion of images making quantitative measurements challenging and the difficulty in perceiving changes in the bone structure and morphology of the implant-bone interface unless over 30% bone loss has occurred are among the many few.
Computer assisted measurements of crestal bone level may prove to be the most accurate way to use radiographic information as standard deviation between 0.1 mm (0.01 and 0.51 mm) has been reported. However, this method is not practical in clinical practice.
Computed tomography and cone-beam computed tomography are widely used in the implant treatment as diagnostic aids for planning. They are used for determining the bone density, locating vital structures in the vicinity of the proposed implant site, determination of any pathology and for preplanning any augmentation procedures if required. They can also be used during the follow-up periods, following implant placement for studying the osseointegration of the implant.[15,16]
Cutting resistance analysis (CRA) measures the energy required for current fed electric motor in cutting a unit volume of bone during implant surgery. This energy is significantly correlated to the bone density which is one of the important factors in implant stability. A torque gauge incorporated within the drilling unit is used in measuring the implant insertion torque value.
The major limitation of CRA is that it does not give information on bone quality till osteotomy site is prepared, and it also cannot identify the lower critical limit of cutting torque value.
Dr. Schulte developed it to measure tooth mobility, and Teerlick was the first to use it to measure implant stability. Periotest evaluates the damping capacity and the stiffness of the natural tooth or implant by measuring the contact time of an electronically driven and electronically monitored rod upon percussing the test surface. Periotest value (PTV) ranges from −8 (low mobility) to +50 (high mobility) with a PTV of −8 to −6 is considered as good stability.
Periotest can measure all surfaces like the abutment or prosthesis, but the rod must make contact at a correct angle and distance. The perpendicular contact angle should not be more than 20° and the parallel contact angle not more than 4° in which case the measured value becomes invalid. Furthermore, the distance between the rod and the test surface must be maintained between 0.6 and 2.0 mm.[17,18] If the distance is over 5 mm, the measured value is insignificant.
The limitations of periotest are the inability of the instrument to measure the mesiodistal mobility, the possible effect of position and angle of the rods on the measured value and the most failing point is that the percussing force on a poor initial stability implant may further deteriorate the stability.
The need for a non-invasive, clinically applicable method which is user-friendly to measure implants stability lead to the development of RFA.
Meredith et al. in 1996 reported the use of RFA to evaluate implant stability and proved in early in vitro the ability of the device in evaluating the stiffness change of the surface.
Resonance frequency analysis uses the principle of a vibrating fork that is, when a frequency of audibility range is repeatedly vibrated onto an implant, depending on the bone implant interface resonance occurs. The stronger interface, the higher the frequency.
Currently, there are two RFA instruments in clinical use: Ostell (Integration Diagnostics) and Implomates (Biotech One).
The first commercial product of the RFA, the first generation was Osstell™ introduced in 2001(Osstell AB, Goteborg, Sweden) which was, followed by second generation Osstell™ Mentor in 2004 and recently in 2009 Osstell™ implant stability quotient (ISQ) was introduced.
The first generation Ostell uses electronic technology and other devices (Osstell™ Mentor, Osstell™ ISQ) use magnetic technology.
This early model Osstell™ produces alternating sine waves in a specific frequency range by uniform amplitude and makes the transducer connected to the implant or abutment vibrate under 1 mm like an electronic tuning fork. A cantilever small beam connected to the transducer has 2 piezo-ceramic elements attached. One of them receives the signal and vibrates the transducer and the other passes this vibration to the RFA [Figure 1]. Values are displayed on the monitor from 0 to 100. The value of 100 signifies the highest stability.[18,19]
The values are displayed by graphs on the computer monitor or expressed by values between 4500 and 8500 Hz. The obtained output is then calculated by the equation below.
Where, fn is the resonance frequency of the beam, l is the effective vibration length of the beam, E is the young's modulus, I is the moment of inertia, ρ is the mass, α the constant that increases as peri-implant bone density increases. Therefore, when osseointegration is achieved, resonance frequency increases since α value increases. ‘l’ signifies the length of the implant above the bone. Hence, as bone is resorbed, this value increases and thus resonance frequency decreases. In other words, ISQ is affected by the effective implant length, type of bone at implant site and bone density.
Resonance frequency between 3.5 KHz and 8.5 KHz formed from the magnetic field is converted into ISQ values by Osstell Mentor™. Osstell Mentor™ has a magnetic peg which is fixed to the implant fixture or abutment by a screw below. When magnetic resonance frequency is released from the probe, the magnetic peg is activated. The activated peg starts to vibrate, and the magnet induces electric volt into the probe coil and the electric volt is sampled by the magnetic RFA.
After the osteotomy preparation and implant placement, prior to the placement of cover screw [Figure 2] the smart peg (respective for the implant system) is placed onto the implant with the help of the smart peg mount [Figure 3]. The mount is removed after securing the smart peg in the implant [Figure 4]. The RFA instrument is activated and the probe tip is placed maintaining a 1–3 mm distance from the smart peg, at an angle of 90°, and 3 mm above the soft tissue, [Figure 5] otherwise the measured value may be affected. The values are expressed as numbers between 1 and 100 in ISQ. Readings are taken in two directions-mesiodistal, and buccolingual directions since bone is not uniform all over. And the average of the two is recorded as the ISQ. Two readings are taken in each direction [Figures [Figures66 and and77].
It has been reported that ISQ is affected by implant diameter, surface, form, bone contact ratio, implant site, implant system, surgical procedure, bone quality and bone height (Table 1 shows the various studies conducted on factors affecting RFA and its reliability, reproducibility and repeatability). RFA is influenced by the changes in the interface stiffness, and is affected in three aspects.[2,3] First, bone-implant surface stiffness affects RFA, which increases through bone healing and remodeling. Second, the stiffness of bone itself and bone density as well as the ratio of cortical and cancelous bone affects RFA. Finally, the stiffness of implant components can act as a variable, and it is affected by the interlocking structures and the composing elements of the materials.
Bone and implant surface stiffness may be affected using a small-diameter final drill, changes in surgical techniques such as bone compaction technique, self-tapping design implants, and wide tapered implants, but not by implant length.
Histomorphologic studies report that the RFA value has a high correlation with the bone-implant contact. On the contrary, other reports claim that there is no correlation between the bone density and ISQ. Therefore, RFA signifies the bone anchorage of implants but the relation of RFA and bone structure is not yet clear.[23,24] Such diverse results showed RFA value decreases during the first 2 weeks after implant placement, and this change can be related to early bone healing such as biological change and marginal alveolar bone resorption. Bone remodeling reduces primary bone contact and in the early stage after implant placement, the formation of bony callus and increasing lamellar bone in the cortical bone causes major changes in bone density. Thus, in the healing process, primary bone contact decreases and secondary bone contact increases. Furthermore, the three-dimensional implant-bone contact is displayed two-dimensionally in the histological sample and BIC has possibility of inaccuracy to signify bone-implant contact.[26,27] The relationship of bone structure and RFA is not fully understood. Since primary stability is affected by bone volume or bone trabeculae structure, as well as cortical bone thickness and density, the effect of bone quality on implant stability, cannot be explained by bone.
Resonance frequency analysis serves as a user-friendly and reliable, noninvasive method that can be used clinically as a diagnostic tool to measure implant stability during the healing stages, and the subsequent follow-up periods. However, further studies in the form of randomized clinical trials and longitudinal studies are required to establish the efficacy.
Source of Support: Nil
Conflict of Interest: None.