With the advent of dental implants, a revolution has begun in the world of dentistry. The long term clinical success of dental implants have made them the most sought out treatment option for either replacement of a single tooth to full mouth rehabilitations. However, implant stability is critical for achieving and maintaining osseointegration and ensuring long term clinical success. Primary stability of a dental implant is a pre requisite to undisturbed peri-implant bone healing and is defined at the time of implant placement. On the other hand, secondary stability is dynamic and offers biological stability through bone regeneration and remodeling.
It is now well established that primary stability is a critical factor in determining the long-term success of immediately loaded implants. It has been suggested that primary stability largely depends on the three major factors: (1) the implant bed condition such as bone quantity and quality, (2) implant characteristics such as the diameter, length and surface texture and (3) the surgical technique used for implant placement.1 Accurate evaluation of bone structure is essential before implant placement since quality and quantity are important factors in determining the surgical procedure and the type(s) of implant(s) to be used.
Various methods to classify bone quality have been proposed. Lekhom and Zarb index (1985) based on radiographic appearance of bone and its resistance to drilling is popularly used to classify bone density but it remains a subjective method. However, the Hounsfield scale has been used to evaluate bone density before implant surgery since the results are objective, quantitative, and site-specific. Hounsfield unit (HU) is a measure of bone density derived from a CT scan. Hounsfield units, created by and named after Sir Godfrey Hounsfield, are obtained from a linear transformation of the measured attenuation coeffecients.
Misch has classified bone density as follows D1 (>1250 HU), D2 (850-1250 HU), D3 (350-850 HU) and D4 (150-350 HU).2
Various methods of evaluating implant stability such as histology and histomorphometry, percussion test, insertion torque, removal torque, push-through and pull-through, radiographic assessment, Periotest ultrasonic method, and resonance frequency analysis have been reported. Primary stability of dental implants is most typically evaluated in a clinical setting by subjective methods including visual evaluation of implant mobility upon insertion and the percussion (“ping”) test. When assessed visually, inadequate primary stability is defined as “the lack of resistance during final tightening of the cover screw or mobility of the fixture mount when still on the implant”3. In reality, an implant that is visually stable upon insertion into an osteotomy does not always imply a successful clinical outcome. Neither visual evaluation of implant stability nor the percussion test is considered to exhibit an acceptable degree of reliability4. Insertion torque, defined as “the rotational force applied to an object, usually a screw, during placement or tightening”, has been proposed as a more objective measure of primary implant stability. Although it can be measured in a quantitative fashion its reliability to indicate implant stability remains unclear5. So, there is a clear and demonstrable need for a rapid, reproducible, user-friendly, non-invasive technique to clinically assess implant stability and osseointegration.
Meredith in 1996 was the first to demonstrate the use of resonance frequency analysis (RFA) to establish the Implant stability Quotient (ISQ) as a clinical method to measure implant stability and osseointegration6. Through a transducer mounted onto the installed implant, a magnetic element with a frequency spectrum of 3500– 8000Hz is activated. The instrument evaluates the resonance elicited by the bone to implant contact. The frequency with the highest amplitude is used to calculate the so-called implant stability quotient (ISQ) on a scale from 0 to 100. Values above >70 ISQ is considered as high stability, between 60-69 as medium stability and low stability for values lesser than 60 ISQ. If the initial ISQ value is high, a little change in stability is noted over time. A big drop in stability or decrease should be considered a warning sign. Lower values are expected to be higher after the healing period. The opposite could be an indication of an unsuccessful implant and caution should be excercised. Owing to its reproducibility and robustness, this novel technique has recently replaced previously advocated techniques for monitoring implant stability
According to the manufacturer of Ostell apparatus, an increase of the ISQ by 1unit appears to correspond to 50Hz in resonance. It has been postulated that resonance frequency re?ects the bone anchorage of the implant. Since then, it has been suggested as a valuable clinical tool for measuring the implant stability. This non-invasive assessment may be used both for diagnosis and for monitoring changes in implant stability over time.
Originally, clinical protocols were recommended to observe a healing period of 3–6 months before loading implants prosthetically. Schroeder et al. used oral implants with roughened surfaces, other implant systems generally advocated shorter healing periods not exceeding 3 months before functional loading. With the development of more ‘osteogenic’ roughened surfaces during the 1990s reduced healing time of 6–8 weeks was proposed before prosthetic loading. Most recently, such surfaces with biologically active hydrophilic characteristics propagated even shorter healing period of 3–4 weeks.7 However, with the use of RFA the changes in stability can be monitored and it can be objectively determined when to load the implant.
This study was planned with a view to study the primary & secondary stability of implants in areas of varying bone quality & to compare the stability during the phases of healing.
The null hypothesis was that there will be no change in the stability of dental implants placed in different densities of bone as measured by Resonance Frequency Analysis during the healing period.