Potential of Using an Implant Fixture as a Ridge Expander for Minor Ridge Augmentation: An Ex Vivo Randomized Controlled Study

Significant buccolingual ridge resorption commonly occurs after tooth loss, resulting in 25%–30% of width reduction within the first year and 40% by the third year.^1,2  A ridge deficiency hinders the placement of a prosthetically- and esthetically-ideal implant.^3,4  A number of approaches have been proposed to correct ridge defects and create adequate support for implants.^5,6  One of these techniques, ridge expansion (RE), was first described by Simion et al in 1992⁷ and widened the ridge by splitting the alveolar bone buccolingually with a chisel and placing an implant and graft material between the separated bony plates.⁷  In 1994, Summers et al introduced the use of a series of mallet-driven osteotomes to expand ridge width and densify bone at the surgical site simultaneously.⁸  The literature supports the application of both RE methods to augment bone predictably.^9–11 

Several adaptations of Summers' osteotome expansion technique exist that purport to simply the original protocol and improve upon its results.^12–14  Use of bone expander screws in lieu of malleted osteotomes has been shown to increase operator control, minimize trauma, and result in predictable clinical outcomes.¹²  These instruments consist of a series of self-tapping screws of increasing diameters that are applied sequentially at very low torques to widen the ridge. However, this approach uses mallet that may induce benign paroxysmal vertigo;¹⁵  hence, there is a need to find a better tool without this concern. A human cadaver study found a modest width gain of 0.8 mm using bone screws and noted that the implant itself acted as the final expander.¹⁶  This raises the possibility that the implant fixture alone could be used as a tool for ridge expansion in cases that require minor augmentation (<3 mm), further streamlining the process. The purpose of this study was to quantify the mechanical feasibility of ridge expansion using an implant fixture alone after an abbreviated standard osteotomy preparation in a human cadaver model.

This study was approved by the Institutional Review Board of the University of Michigan (Study ID: HUM00134643). The authors consulted with an independent statistician, who reviewed and approved the study methodology. Twelve fresh cadaver heads donated for educational and research purposes were used. After donor harvesting, the heads were frozen at −20°C and defrosted immediately prior to experimentation.

All heads were imaged presurgically with a CBCT scanner (3D Accuitomo 170, J. Morita, Tokyo Japan) with fixed parameters (120 kV, 18.66 mA, 20 seconds, field of view: and resolution: 200 μm). From the CBCT results, edentulous sites were selected based on the following criteria: (1) ridge width of 4 to 6 mm; (2) ridge height of least 12 mm; (3) type 3 or type 4 bone (with thin a layer of cortical bone), according to the Lekholm and Zarb classification.³  In total, 38 sites were selected and randomly assigned by a computer-generated number sheet to one of 2 study groups: ridge expansion (RE) test group (19 sites) or non-ridge expansion (NE) control group (19 sites).Tapered implants (TSV, Zimmer Biomet, Warsaw, Ind) were used as the sole ridge expanders in the study (Figure 1). Each implant had a length of 11.5 mm, platform diameter of 3.7 mm, and an apex diameter of 3.1 mm.

Implant osteotomy preparation and placement were performed following manufacturer-recommended guidelines (Zimmer Instrument Kit System; Zimmer Biomet) (Figure 2). A full-thickness flap was elevated at each site, and the ridge was prepared to eliminate any sharp protrusions. The prosthetically based ideal osteotomy entry point was marked with a round bur. In line with the osteotomy marker, reference points were created 2 mm with a round bur below the crest on both buccal and lingual plates. The initial ridge width (RW1) was measured as the distance between these plate reference points using a ridge mapping caliper (Pearson Iwanson Crown Gauge, Hu-Friedy, Chicago, Ill) with accuracy to 0.1 mm. The osteotomy was initiated with a pilot drill at 1250 RPM. Further preparation was performed using 2.3- and 2.8-mm diameter drills respectively to a depth of 11.5 mm. Only the control group experienced an additional osteotomy preparation with a 3.4-mm diameter drill to a depth of 11.5 mm. The bone density of each site was further assessed by the resistance at drilling. All implants were placed using hand ratcheting, with 35–45 Ncm insertion torque, in an in-and-out manner. The ridge width after implant insertion (RW2) was measured, and the changes between initial and post-implant widths were calculated (ΔRW = RW2 − RW1). The fixtures were reverse torqued and removed by hand ratchet, and the buccal plate thickness (BPT) was assessed at the buccal reference point with a ridge mapping caliper. Any dehiscence of the buccal plate was also recorded.

All data were analyzed using a statistical software package (SPSS version 23.0, IBM, Armonk, NY). Descriptive statistics were calculated for all parameters. Kolmogorov-Smirnov and Levene's tests were used to evaluate the distribution of data and equality of variance. If the variables were normally distributed and in the homogeneity of variances, then an independent t test was used to determine any significant differences between the two study groups. If not, the Mann-Whitney test was used. Significance for all statistical analyses was set at P < .05.

Thirty-eight implants were successfully placed in posterior segment of the cadaver jaws. The RW1, RW2, BPT, bone type, and the incidence of buccal bone dehiscence are presented in Table 1. Except for ΔRW and BPT in the RE group, all variables showed normal distribution and homogeneity of variances; the Mann-Whitney test was used to analyze ΔRW and BPT. The mean RW1 was 4.69 ± 0.45 mm in the RE group and 4.79 ± 0.40 mm in the NE group; no statistical difference between the groups was noted. The test group RW2 was 5.54 ± 0.35 mm, equivalent to a width increase (ΔRW) of 0.85 ± 0.25 mm (P < .01), while the RW2 of the control group was 4.88 ± 0.42 mm, which correlated to a ΔRW of 0.09 ± 0.13 mm. The BPT was greater in the RE group (1.08 ± 0.28 mm) than in the NE group (0.71 ± 0.37 mm) (P < .001).

Statistical analysis showed significant differences in RW2 (P < .001), ΔRW (P < .001), and BPT (P = 0.001) between the 2 treatment groups (Table 2). When the initial ridge width was <4.8 mm (12 sites), we noted buccal plate dehiscence in 8 of them in the RE group; however, no cases of dehiscence were observed in the NE group.

A certain volume of alveolar bone is critical for the long-term prosthetic and esthetic success of implants; at least 1.8 mm of buccal bone thickness at the crestal level of an implant is recommended¹⁷  to prevent bone resorption and ensuing soft tissue recession. To correct deficient sites, horizontal ridge augmentation is performed using various techniques, including guided bone regeneration,⁶  ridge splitting,^18,19  distraction osteogenesis,²⁰  and expansion instruments.^7,8,12,16  A previous study suggested that the implant fixture itself may act as a ridge expander.¹⁶  In cases requiring very minor (<1 mm) enhancement, the use of an implant-only augmentation protocol would simplify the surgical process. In this technique, rather than using a drill to widen the osteotomy area just shy (<0.5 mm) of the implant diameter, the osteotomy is underprepped by about 1 mm (or more), allowing the implant to perform the final ridge expansion and possibly densify the bone adjacent to its threads.

The present human cadaver study compared ridge dimensional changes after the insertion of tapered 3.7-mm implants between normally prepared (3.4 mm, non-expansion control group) and underprepared (2.8 mm, ridge expansion test group) osteotomy sites and demonstrated that that the ridge width increased in only the implant-driven expansion group (and by 0.85 mm) (Figure 3). The amount of ridge augmentation seen in the RE group was essentially equivalent to the difference between the diameters of the implant and the final osteotomy preparation (3.7 mm − 2.8 mm = 0.9 mm ≈ 0.85 mm), which was logical; the bone expanded to accommodate the implant. This is in agreement with a previous human cadaver study that reported a ridge width gain of 0.8 mm using a screw expander kit (Bone Expander kit, Salvin Dental Specialists, Charlotte, NC) without osteotomy drill preparation; 0.52 mm of that gain in width was attributed to implant insertion itself.¹⁶  Our current study indicates that an implant fixture alone when placed in an underprepped site in type 3 or 4 bone may expand the ridge without the need for accessory bone spreaders in a cadaver model.

In addition to increased ridge width, the RE group had a greater percentage of sites with buccal plate thickness exceeding 1 mm (Table 3), possibly because test osteotomies were underprepped and therefore subjected to less bone removal. Chappuis and colleagues indicated that at least 1 mm of buccal bone thickness was required to maintain blood supply and minimize resorption.²¹  As mentioned previously, 2 mm of buccal bone thickness is preferable to preserve long-term esthetics.¹⁹  Per our observations, use of implant-only ridge expansion requires a minimum initial ridge width (MRW1) of 5.9 mm when a 3.7-mm implant is used. The general calculation as defined here of ideal ridge width (IRW) = 2 mm buccal bone + implant diameter + 1 mm palatal bone. MRW1 = IRW − 0.8 mm, where 0.8 mm is the approximate amount of bone expansion expected from implant-only RE. Thus, the MRW1 for an implant = implant diameter + 2 mm buccal bone + 1 mm palatal bone − 0.8 RE bone expansion = implant diameter + 2.2 mm. For a 3.7 mm implant, the MRW1 is 5.9. If the MRW1 is somewhat less than desired, then further augmentation, such as simultaneous guided bone regeneration, may be warranted. Underpreparing the implant bed helps to preserve the buccal plate thickness to some degree, which may be especially relevant for cases that require the use of grafts; intact native bone is a vascular/nutritive source for regeneration.^22,23 

There were side effects observed in the present study. Eight out of 12 RE sites that had an initial ridge width (RW1) < 4.8 mm demonstrated buccal plate dehiscences; this did not occur in any of the controls. To avoid dehiscence and to obtain the ideal anatomical dimensions, implant-only ridge expansion should be limited to sites that are at the very least 5–7 mm wide, depending on the anticipated implant diameter—as just described, the MRW1 = 2.2 mm + implant diameter. In narrower sites that fail to meet this benchmark, adjunct guided bone regeneration or another treatment modality may be indicated to resolve implant thread exposure and/or provide more predictable augmentation. With regard to the effect of inserting a fixture to an undersized implant bed, the surrounding bone may result in overheating or even pressure necrosis.²⁴  It is suggested to insert the fixture in an in-and-out approach. Moreover, according to the report by Sethi et al, high survival rate (97% at 5 years) was achieved with similar expansion procedure.¹⁰  In addition, it may be fair to assume that tapered implant may have less these concerns when compared to other larger divergent or parallel implants.

Fresh cadaver specimens were used in this experiment, which restricts the clinical relevance of our findings; the data presented here regarding implant-driven bone expansion may be considered stimuli and references for future studies rather than surgical advice. Living alveolar jaw tissue has more elasticity, which may better enable ridge expansion. Implant design may affect results; our investigation used only one fixture type (tapered implant fixture). Human clinical trials are needed to determine the exact effects of this protocol on active bone remodeling, healing, and long-term success for a variety of implant systems and fixture types (parallel vs tapered).

In a fresh cadaver model, it is physically possible to widen a ridge by 0.85 mm solely from the placement of a 3.7-mm diameter implant into an osteotomy site underprepared by 0.9 mm. This procedure leads to buccal dehiscence in 67% of sites with residual ridge widths of less than 4.8 mm. Human trials in regard to such therapy are merited to elucidate its physiological outcomes in and clinical suitability for patients.

Abbreviations

BPT:

buccal plate thickness

ΔRW:

changes of ridge width

IRW:

ideal ridge width

MRW1:

minimum initial ridge width

RE:

ridge expansion

RW1:

initial ridge width

RW2:

ridge width after implant insertion

The authors declare no conflicts of interest.