Measurement of the “Bungee Dip” in Implant Stability Using Resonance Frequency Analysis: Two Case Reports

During the diagnostic, surgical, and healing phases of dental implant care various factors are determined that correlate with successful treatment outcomes. Of these factors, implant stability is highly prognostic for success^1,2 and is, therefore, a critical aspect governing clinical decisions on implant loading.

The stability of an implant at a given point in time is determined by its mechanical and/or biologic stability. At the time of placement, the primary stability of a dental implant is based on intimate contact of the implant with the walls of the prepared osteotomy.³ Because there is no biologic engagement of the implant's surface other than the adherence of a clot, the primary stability is, therefore, solely mechanical. In general, mechanical stability at the time of implant insertion is high and decreases with time as physiologic osseous modeling/remodeling occurs.

Biologic stability, also known as osseointegration, is the formation of new bone onto the dental implant surface within the prepared osteotomy. This process occurs subsequent to the loss of the bone initially in contact with the implant at the time of placement. Secondary stability is, therefore, the sum of mechanical stability, which decreases with time, and biologic stability, which increases with time. Typically, with most implant systems an initial decrease in stability is observed, which is followed by a subsequent increase in stability as the formation of new bone stabilizes the implant. The result of this is an implant that is biologically stable, ie, osseointegrated.^4-6

Resonance frequency analysis (RFA) has been shown to be a noninvasive and reproducible method of assessing both primary and secondary implant stability.^7-12 This testing method provides an objective measurement of an implant's lateral micromotion by analyzing the resonance frequency of a small magnetic peg attached to an implant fixture or abutment. The measurement unit is the implant stability quotient (ISQ). Stiffer bone-implant interfaces and, therefore, greater stability are reflected in high resonance frequencies and high ISQ values.^7-12

After dental implant placement the typical expectation for healing is that implant stability will change over the course of healing/osseointegration. Stability will go from initially higher stability based on ISQ values to slightly lower stability due to physiologic bone modeling/remodeling, then back to ISQ values that are equal to or higher than those originally observed. This physiologic drop in value has been referred to as a "dip" and typically may range between 3 and 9 ISQ units. However, patient healing can vary, and there may be instances where bone deposition may outpace mechanical stability loss in early wound healing, which is known as a "bump."¹³

A third pattern has also been identified in which a small subset of patients, due to variability in healing, exhibits a profound loss of mechanical stability combined with a slower physiologic process of bone deposition. In this wound-healing scenario, after a substantial decrease of 10 or more ISQ units, secondary stability gains momentum and the ISQ measurements inevitably rebound to values equal to or higher than those obtained at initial placement. This process of healing has been termed a "bungee dip." Because bone quality and healing vary from patient to patient, the extent and timing of the so-called bungee dip will also vary. Therefore, accurate measurement of stability is crucial to making a well-informed decision regarding implant loading.

The purpose of this article is to present two case reports that illustrate the measurement of the bungee dip using ISQ values to guide the timing of implant loading.

A 63-year-old female patient in good general health was referred for evaluation and placement of an implant at the site of a failing mandibular left first molar. The tooth previously had undergone endodontic treatment more than a decade prior but had recently become symptomatic. Clinical and radiographic examination of the area revealed no mobility of the teeth, probing depths ranging from 1 mm to 3 mm (Figure 1), and a large radiolucent lesion at the mesial root of the mandibular left first molar (Figure 2). Based on these considerations and consultation with an endodontist, the prognosis for this tooth was questionable to hopeless.

Given this information, the patient chose to have the tooth removed and, if possible, have an implant placed at the time of extraction. In order to achieve primary stabilization above the inferior alveolar nerve, immediate implant placement requires the presence of adequate bone, as was the case in this situation.

The tooth was extracted using a minimally traumatic technique to maintain the integrity of the socket walls to maximize primary implant stability and to contain any graft placed at the time of the procedure. A surgical curette was used to remove soft tissue, which was presumably granulomatous in nature, from the socket. After osteotomy preparation of the site, a dental implant with an electrowetted surface (ProActive^®, Neoss Inc., neoss.com) measuring 6 mm diameter x 9 mm length was placed into the site with a final seating torque of 35 Ncm. Immediately after placement, the Osstell IDx system (Osstell, osstell.com) was used to obtain an RFA assessment of implant primary stability. The appropriate SmartPeg (Osstell) was attached to the implant (Figure 3), and good primary stability was confirmed with ISQ values of 71 in the buccal-lingual and 77 in the mesial-distal directions.

After placement of a polyether ether ketone (PEEK) healing abutment (Neoss Inc.) to enable transgingival healing, the site was grafted with a 70:30 mixture of freeze-dried and demineralized freeze-dried bone allografts (creos^™ allo.gain, Nobel Biocare, nobelbiocare.com), and covered by a barrier of amnion-chorion (BioXclude^®, Snoasis Medical, snoasismedical.com) to facilitate its containment. The soft tissue surrounding the socket was stabilized around the graft-barrier complex with 5-0 polytetrafluoroethylene (PTFE) suture (Omnia S.p.A., omniaspa.eu) using both interrupted and horizontal mattress suturing techniques (Figure 4).

After 3 weeks of uneventful healing, the sutures were removed. At 6 weeks post implant placement, the patient returned for follow-up radiographic imaging and RFA assessment. ISQ values of 35 (buccal-lingual) and 37 (mesial-distal) were recorded (Figure 5), indicating that the implant was not ready for loading. The graver concern, however, was that the reduction in ISQ values from the time of placement reflected not only the loss of mechanical stability due to osseous remodeling, but also that the implant was failing.

A cone-beam computed tomography (CBCT) scan was obtained to ascertain what the problem might be. The CBCT suggested that adequate bone was surrounding the dental implant (Figure 6). Since the patient was in good health and initial insertion torque had been satisfactory, it was decided to maintain the implant while submerging it to allow for further healing and future evaluation. A cover screw was placed instead of the healing abutment, and the tissue was allowed to form over the dental implant in situ for 16 more weeks (Figure 7). The merit of using ISQ values was to provide objective, quantifiable information for clinical decision-making, which in this instance was to submerge the implant, allow for further healing, and delay its loading to avoid failure.

When the patient returned for follow-up 16 weeks later, the implant was exposed, and ISQ values had returned to initial levels of 71 (buccal-lingual) and 77 (mesial-distal) (Figure 8). A PEEK healing abutment was placed again (Figure 9). All clinical signs of a successfully integrated implant were present. Subsequently, an impression was made for the fabrication of a screw-retained metal-ceramic crown (Figure 10).

A 54-year-old female patient in good general health was referred for evaluation and placement of an implant at the site of a missing maxillary left second molar (Figure 11). Pretreatment assessment determined that adequate bone was available in both the buccal-palatal and vertical dimensions for implant placement without the need for any additional procedures.

Following reflection of a full-thickness flap, the implant osteotomy was prepared and a 4.5 mm diameter x 9 mm length tapered dental implant with an electrowetted surface (ProActive^®) was placed into type 2 bone. The final seating torque of 35 Ncm and ISQ values of 71 and 70 (Osstell IDx) in the buccal-palatal and mesial-distal directions, respectively, confirmed adequate primary stability before flap closure (Figure 12). A PEEK healing abutment was placed (Figure 13) prior to flap suturing, and the implant was allowed to heal transgingivally.

At 4 weeks post-placement, the patient returned for RFA assessment of osseointegration (Figure 14). ISQ values had decreased to 56 buccal-palatal and 60 mesial-distal. Studies have shown that higher survival rates are associated with loading of implants with ISQ of at least 60,^4,14-16 and, therefore, loading at 4 weeks was not indicated. At 16 weeks post-insertion, RFA assessment was repeated, and ISQ values of 70 were recorded in both directions, similar to initial stability values (Figure 15). An ISQ of 70 or higher suggests good to high stability,¹⁵ and the implant was considered low risk for proceeding with loading. Treatment progressed to impression and fabrication of a screw-retained metal-ceramic restoration. At 2 years post-treatment, clinical (Figure 16) and radiographic (Figure 17) images showed healthy soft-tissue conditions and stable bone levels.

To determine implant stability an objective measurement such as RFA is valuable for achieving consistently successful treatment outcomes, as monitoring the stability of an implant over time can play a significant role in determining when to load the implant.^14,17,18 To be successful, immediate loading requires high initial or primary stability and an absence of risk factors such as extensive grafting at time of implant placement or substantial parafunctional habits.^17-20 Even for early loading, ie, provisionalization between 1 week and 2 months subsequent to implant placement, or conventional loading 2 or more months after implant placement, the clinician still needs guidance as to determining when the best time might be to load the implant, especially since patients may demonstrate variability in their healing response. Steadily declining stability as measured by diminishing RFA values should prompt the clinician to delay loading until the implant demonstrates a stable ISQ value within a satisfactory range.¹⁸

Thus, monitoring implant stability during the healing period may help to avoid implant failure and the additional treatment time, cost, and inconvenience associated with it that might otherwise occur due to the presumption that all patients heal alike.¹ While most patients may heal in a similar fashion, having to explain to a patient why his or her implant has failed is a discussion clinicians would rather not need to have. From a financial standpoint, implant failure can be catastrophic, especially if recovery from the initial failure is not possible.

The bungee-dip healing presented in these two cases is an example of a specific clinical benefit of measuring implant stability. These cases are evidence of the reliability of objectively measuring an implant's stability for establishing safe and successful loading protocols that are individualized to the patient's course of healing. In essence, this approach is an extension of personalized medicine, which is becoming more prevalent today as practitioners focus on the individual rather than the group.

On the whole, lower ISQ scores, ie, below 60, have been indicative of a higher likelihood of failure when loading the dental implant.¹⁴ On the other hand, RFA values that decrease over time can also serve as a warning against loading, indicating that more healing time is needed before implant stability is achieved. A reasonable clinical approach to minimize risk of implant failure is to consider implants with ISQs of less than 60 to have questionable stability and to delay loading them until ISQs achieve a level of 65 or greater.^4,14,15,21 It is important to note that the relationship between change in ISQ value and decrease in stability is nonlinear; that is, a 10-point ISQ decrease reflects a 50% increase in implant micromotion.^7,8

In the bungee-dip phenomenon the magnitude of the decrease in implant stability and, therefore, the magnitude of the change in ISQ values varies greatly among patients. This is shown in the two cases presented whereby the mandibular implant (Case 1) underwent a 35- to 40-point decrease in ISQ before returning to initial values, while the maxillary implant (Case 2) had a decrease of a much smaller yet potentially consequential 10- to 15-point magnitude.

Understandably, a possible concern with this approach might be the timing of the RFA readings, which were taken at 4 to 6 weeks, as this might be considered an inadequate amount of time for bone maturation to perform an RFA reading, especially in situations where the implant was placed into an extraction socket. However, on the contrary, it has been suggested in the literature that a drop in ISQ for extraction sites, such as that shown in Case 1, does not necessarily occur with the electrowetted-surface dental implant used.¹³ Further, the reading was taken not to proceed to loading, but simply to gauge how healing was progressing. The author considers this a far more desirable option than to subject the patient to radiographic evaluation. Such low RFA values would be disconcerting and this quantitative information would suggest that a midcourse correction be made. Hence, the transgingival abutment was removed and a cover screw was placed. This evidence-guided decision to submerge the implant was intended to create a more favorable situation for healing. The outcome to therapy would appear to support the decision. This same approach might also be applied to immediately provisionalized dental implants. Large dips in RFA might guide the clinician to remove the crown and either switch to a transgingival healing abutment or submerge the implant using a cover screw, depending on the relative change in RFA.

Because bone quality and/or healing response vary from patient to patient, it cannot be assumed that all patients will heal at the same rate. Using RFA assessment as an objective measurement of implant stability enables clinicians to make well-informed decisions about loading protocols on a case-by-case basis. As such, a specified degree of implant stability can serve as an inclusion criterion for progressing to the restorative phase of implant treatment. ISQ values can also support better communication among colleagues and patients, as they provide an objective means of explaining the treatment decisions being made. As a prognosticative diagnostic, RFA measurement of implant stability serves as a form of personalized care, helping to guide clinicians in providing safe and predictable implant procedures for their patients.

Paul S. Rosen, DMD, MS

Clinical Professor of Periodontics, University of Maryland Dental School, Baltimore, Maryland; Adjunct Professor, James Cook University, Cairns, Australia; Private Practices, Yardley, Pennsylvania, and New York, New York