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Basic and Clinical Research

Potential Bone to Implant Contact Area of Short Versus Standard Implants

An In Vitro Micro–Computed Tomography Analysis

Quaranta, Alessandro DDS, PhD; D'Isidoro, Orlando DDS; Bambini, Fabrizio DDS; Putignano, Angelo MD, DDS

Author Information

*Senior Lecturer, Faculty of Dentistry, Division of Periodontology, Deptartment of Oral Sciences, University of Otago, Dunedin, New Zealand.

†Resident and Adjunct Professor, School of Dentistry, University Politecnica delle Marche, School of Dentistry, Silvi, Italy.

‡Associate Professor, Division of Oral Surgery, School of Dentistry, Università Politecnica delle Marche, Ancona, Italy.

§Full Professor and Director of School of Dentale Hygiene, Università Politecnica delle Marche, Ancona, Italy.

Reprint requests and correspondence to: Alessandro Quaranta, DDS, PhD, University of Otago, Faculty of Dentistry, 310 Great King Street, 9054, Dunedin, New Zealand, Phone: 642272918023, Fax: 643 479 0673, E-mail: [email protected]

doi: 10.1097/ID.0000000000000357
  • Free

Abstract

Micro–computed tomography (Micro-CT) analysis is a fast, noninvasive, and accurate x-ray microtomography for in vitro evaluation of specimens1 that has been recently introduced in dentistry.2 In this technique, bidimensional (2D), tridimensional (3D) reconstructions, and volumetric measurements can be obtained. In fact, it is possible to acquire 3D images of specimen with dimensions of 15 mm in extent and a resolution of 5 to 8 μ.3 The availability of this technology has allowed to investigate the trabecular and cortical components of autologous bone specimen and alloplastic grafts without any irreversible sample preparation and consequent destruction.4–7 Micro-CT has been also used to have 3D sample reconstructions, assess material volumes, and study the characteristic of different scaffolds.8–10 Moreover, μCT analysis has been adopted to evaluate the bone-implant interface and assess the osseointegration process.11,12

The atrophy of jaws could jeopardize the use of dental implants with the necessity of bone grafting techniques to allow the placement of standard implants.13,14 Bone augmentative procedures generally require an extensive autogenous donor site from intraoral or extraoral sites.15 These procedures highly depend on soft and hard tissue management and are potentially at risk for a series of early and late complications that could hinder the final result.16,17 In case of reduced crestal bone height, an alternative therapeutic procedure could be the placement of short implants with no need to perform additive surgical techniques provided that strict surgical procedures are adopted.18–20 When placing a short dental implant, bone grafting or sinus floor elevation before implant placement is usually not necessary, the site preparation could be simplified, the potential for overheating is reduced, and the loading forces may be ideally applied on the implant abutment and restoration.21 According to a highly quoted review,22 a short implant is a fixture with an intrabony length of 8 mm or less, whereas wide-diameter implants have a diameter of 4.5 mm or more. On the contrary, a standard implant has a diameter ranging from 3.5 to 4.1 mm, whereas a narrow implant is a fixture of less than 3.5 mm in diameter. A considerable number of articles have documented an increased failure rate with short implants.23–25 Most studies reported implants with machined, nontextured rough surface. More recently, several authors reported that after the placement of a textured-surface short implant with an adapted surgical preparation, survival rates comparable with those obtained with standard implants can be expected.18–21,26–29 Meta-analysis data confirmed favorable survival rates and provided further robust evidence that microrough 6-mm short dental implants are a predictable treatment option. The failures encountered with 6-mm short implants were predominantly early ones, and the survival rate in the mandible was very satisfactory.30,31

The recent increasing adoption of short implants22 to restore severely resorbed posterior areas is also strongly related to the increasing concern of the scientific community regarding the crown to implant (C/I) ratio issue.32–34 When using short implants, the implant supported rehabilitation is usually extremely outsized. As a consequence, a higher (≥1) C/I ratio will be observed.35 To prevent peri-implant bone stress, crestal bone loss, and implant failure, the creation of C/I ratios between 0.5 and 1 was considered essential.36 A very recent review concluded that despite the limited data, high C/I ratio may be related to some prosthetic failures. However, it was observed that is not mandatory to maintain a C/I ratio ≤1 for implant success and survival. In addition, unfavorable C/I ratio did not result in higher biological complications and implant failure.37,38

Implant design is considered a key factor in obtaining and maintaining an optimal osseointegration. Modifications of body design, such as implant threads and surfaces, have been suggested to increase the success in poor bone quality. It has been in fact speculated that these changes provide a better intrabony anchorage, more surface area available for contact with bone, and an ideal distribution of mechanical stress. Moreover, different thread shapes (square, V-shaped, and reverse buttress) are intended to reduce the development of the shear strength at the bone-implant interface so as to improve primary stability, osseointegration, and long-term success.39

The aim of this study was to compare the available potential bone-implant contact (PBIC) area by μCT analysis in 3 short versus 2 standard implants featuring different fixture size, design, and surface topography.

Materials and Methods

The PBIC area of 3 short and 2 regular dental implants was investigated. All measurements were performed at the ISS (Istituto Superiore di Sanita), Rome, Italy by an engineer highly trained in the use of μCT technology applied to implant dentsitry and biomaterials. The measurements on each implant were performed 3 times, and a mean of the measures was calculated for each implant surface. The consistency of the measurements was verified by comparison of repeated measurements on the same sample. Data analysis and interpretation of the results were performed by the first author (A.Q.), Specialist in Periodontics, Oral Surgery, and experienced in dental implantology.

The implants included in this study were arbitrarily chosen among the ones available at the implant center of the Dental School, Universita' Politecnica delle Marche, Ancona, Italy.

The fixtures were chosen so to have a variety of design, microtopography (surface) and macrotopography, wide and regular diameters, and standard and short lengths. The included short implants were 1 wide diameter (Bicon Short implant [4.5 × 6 mm], Bicon) and 2 regular diameter (Endopore [4.1 × 7 mm], Sybrona Implant Solutions; Straumann Standard implant [4.1 × 6 mm] Straumann Switzerland).

The 2 standard-length implants had, respectively, standard (Nobel Biocare Replace, Nobel Biocare, Switzerland [3.5 × 10 mm]) and narrow (Winsix K implant [3.3 × 9 mm], Winsix, Biosafin Italy) diameter and implant length slightly higher than those considered short.22

All the samples were scanned with a μCT device (mod. Skyscan 1072, SKYSCAN, Kartuizersweg 3B 2550 Kontich, Belgium) to calculate implant PBIC values. Samples 1 and 5 were scanned with a straight abutment connected because the scanned images and data were also used as part of a different investigation on the implant-abutment microgap. Therefore, in these 2 samples, at the time of surface analysis, the area corresponding to the abutment area was subtracted. Before μCT analysis, samples were carefully secured in vertical position by embedding the implant collar on a resin base. This step was essential to avoid motion artifacts during specimen acquisition. Afterward, each sample has been placed into μCT device and subjected to 5 x-ray acquisitions. The following parameters were set for implant scanning and 2D slices generation: 0.45-degree rotation step, 180 degree total rotation angle, power source 100KV/98 microA; filter thickness 1 mm; ×16–×35 magnification range, 18.3 to 8.4-μm pixel size, and 20-μm slice thickness. At the end of this phase, approximately 600 2D slices (cross-sectional images) were obtained for each implant. Scan times were approximately 50 minutes for each sample. Next, a specific software (Tview, Kartuizersweg 3B 2550 Kontich, Belgium) was used to dynamically display the 2D images of each sample.

Image acquired data for each scanned implant were then manually processed to find the plane that, in ideal physiological radiographic conditions, would correspond to the most coronal contact point between the crestal bone and implant (ie, the ideal, recommended apico-coronal position of the implant with respect to the alveolar crest). This means that the suggested apico-coronal position of the implant was used as reference point to calculate the surface area of each fixture.

A dedicated image processing software (CtanKartuizersweg 3B 2550 Kontich, Belgium) was used to calculate the available PBIC values expressed as mm2.

Finally, all cross-sectional slices were processed by a dedicated 3D reconstruction software (3D Creator) to view 3D image of each implant and display the microtopography and macrotopography characteristics (Fig. 1).

F1-14
Fig. 1:
Samples 1 to 5: μCT 3D reconstructions. PBIC values were calculated by selecting for each implant the plane that, in ideal conditions, would correspond to the more coronal contact point between the crestal bone and implant. Note that the proportions among the different implants may not correspond to the real ones.

Results

Surface values of each implant are showed in Table 1. The wide-diameter short implant (no. 4) showed the highest PBIC mean value, followed by the 2 standard implants nos. 3 and 5 that had very similar surface areas. Samples nos. 1 and 2 (standard diameter; short implants) resulted the ones with the smallest PBIC. The mean surface values obtained for these 2 implants were, respectively, 47.5% (no. 1) and 38% (no. 2) lower than those of short implant no. 4 and approximately 10% lower than those of the 2 standard implants (Table 1). No artifacts were observed and the 3D reconstructions clearly displayed the differences among the implants in terms of design, threads, and surface features.

T1-14
Table 1:
PIBC Values Obtained for Each Implant

Discussion

The results of our study agree with those of Schicho et al40 that considered μCT analysis a valid method to evaluate implant properties, making them comparable even if geometrical details are not disclosed by manufacturer.

In fact, accurate quantitative information on implant surface area is extremely scarce and the complex implant macrostructure and microstructure make impossible to determine these data with basic arithmetical methods.

For this reason, μCT analysis can be successfully used in implant surface and design analysis. It investigates in a noninvasive and conservative way small radio-opaque objects such as dental implants41 and biomaterials.42

Some authors stated that it is possible to observe artifacts around dental implants because the fixtures are radio-opaque and accurate observations especially at the bone-implant interface could be difficult.43,44,45 The artifacts that can be observed at the μCT analysis are mainly due to the difference of density between alveolar bone and titanium. Since this research was made only on implants without any placement of the fixtures in bone, no artifacts have been experienced and there was not therefore any impact on the data collected and results obtained.

Different macrotopography (implant design and thread type) and microtopography characterize the implants included in this study. According to the manufacturers and the specific surgical manuals, different apico-coronal levels in relation to peri-implant hard and soft tissues are recommended for correct placement of the implants included in this study. Namely, 1 implant should be placed subcrestally (no. 4), 1 at tissue level (no. 1), and the rest at the bone level (nos. 2, 3, 5). Consequently, all the measurements were performed to calculate the entire area potentially available for bone-implant contact depending on the implant design and manufacturer's indications.

Sample 1 was a standard-diameter short implant (4.1 × 6 mm). According to the manufacturer's indications, the implant is placed supracrestally (tissue level) The sand-blasted, large-grit, acid-etched surface is produced by a large grit sand-blasting process with corundum particles, which leads to a macro-roughness on the titanium surface. This is followed by a strong acid-etching bath with a mixture of HCl/H2SO4 at elevated temperature for several minutes.

Sample no. 2 was a standard-diameter short implant (4.1 × 7 mm) with a truncated cone-shaped design and a multilayered porous surface geometry over most of its length. This implant does not feature any thread design.

Sample no. 3 was a root shaped, tapered standard diameter, and length implant (3.5 × 10 mm) that according to the manufacturer's indications has to be placed at the bone (crestal) level. The surface texture is achieved through plasma electrolytic oxidation (anodization) resulting in the transformation of the titanium oxide in to a crystalline state with ceramic like properties.

Sample no. 4 was a wide-diameter short implant (4.5 × 6 mm) with a sloping shoulder and a plateaued tapered root form body designed. According to the producer's indications, the implant should be ideally placed 5 mm subcrestally. This implant is manufactured from surgical grade titanium alloy (Ti 6Al 4V) and is hydroxylapatite treated.

Sample no. 5 was a narrow-diameter standard (3.3 × 9 mm) implant. The fixture features a self-threaded cylindrical design with a polished collar and square-shaped thread. This implant has a microrough surface obtained through a process of subtraction and etching. The fixture is usually placed at crestal bone level.

The potential surface area available for osseointegration was calculated on the implant 3D reconstructions with specific software. The level of initial contact between crestal bone and fixture has been selected in each implant. The choice of the aforesaid level for each fixture was based on the manufacturer's indications about the correct surgical protocol and apico-coronal implant placement level. Therefore, a careful manual selection was performed on each fixture.

In our results, the wide-diameter short implant (no. 4) that is characterized by a square thread design showed the highest PBIC surface mean value, followed by the 2 standard implants nos. 3 and 5. The other 2 short implants (nos. 1 and 2) resulted in lower surface available for bone-implant contact area compared with the rest of the samples.

In a recent review, it has been concluded that short implants could be considered as an alternative to advanced bone augmentation surgeries.46 In fact, despite the reduced length, the implant macrodesign and shape plays an important role on the bone response and bone-implant surface area of short implants.48 These 2 factors highly influence the stress distribution and the initial stability of the implant.37 The surface area of each implant is directly related to the diameter and the length of the fixture with the former (the diameter) probably providing more area of initial bone contact (primary stability) in the cortical area. This could partially explain the excellent results in term of PBIC values observed in this study for the wide-diameter/short implant compared with the standard-diameter/short dental implants and the narrow/standard implants. Moreover, threads could maximize initial stability and enlarge implant surface area.47 However, the present results of our study should be mainly applied to conventional loading where the potential bone to implant contact area is maximum compared with a fresh postextraction socket where there is often a socket atrophy and defect.

Furthermore, it is important to observe that the level of evidence regarding the impact of implant length and diameter on implant survival provided by literature is weak,22–36 and further research with higher level of evidence (randomized controlled studies) is necessary in this field.

Thread depth, thickness, pitch, and face angle determine the functional thread surface and affect the biomechanical load distribution on the implant.49 The square thread type of the wide-diameter/short implant analyzed in this study could contribute to the high PBIC values of this model compared with the other short and standard implants.

The results of this study could suggest the use of short dental implants as a valid alternative to standard dental implants in critical anatomical and clinical conditions. However, other factors such as the implant body design, occlusal force and direction, bone volume, and quality should be considered in the selection of the appropriate dental implant.

Conclusion

Micro-CT analysis has shown to be a very promising technique to evaluate surface area in dental implants with different macrodesign, microdesign, and surface features.

The PBIC values observed in this study suggest that wide-diameter short dental implants show a surface area comparable with regular-diameter standard implants.

Disclosure

The authors declare not have received any funding. The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.

Acknowledgment

The authors would like to thank Dr Rossella Bedini and Dr Raffaella Pecci, ISS, Rome, Italy for their contribution to the making of the manuscript.

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Keywords:

dental implant; short implant; x-ray microtomography

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