Implant Designs in 2020: Current State of the Art Addresses Primary Stability

In 1993, Tarnow presented the selected surface implant concept with the intention to change the apical micromorphology to enhance integration without sacrificing the periodontal benefits of a machined collar and surface.² Recently, this concept has regained attention due to present-day issues with textured implants and mucositis or peri-implantitis. In 1997, Hahn introduced the tapered implant, which featured a more natural root form while enhancing implant primary stability and was self-limiting in depth due to the wider coronal diameter.³ Dual- or co-axial implants, where the prosthetic angulation changed within the implant body and at implant level rather than abutment level to allow increased incidence of screw retention, have been available since the late 2000s.^4,5 The concept of platform mismatching, or platform switching, was presented by Lazzara and Porter in 2006.⁶ Worhle then proposed the scalloped design at the collar of the implant with the notion that this neck configuration would maintain interdental tissues (papillae).⁷

In the past decade, thread depth and patterns have changed to a more aggressive form to address primary stability in different bone types.⁸ Jacoby and Bichacho created a tri-oval implant with three flat sides at the neck of the implant to decrease crestal bone pressure that could contribute to marginal bone loss.⁹ More recently, Chu and Blackbeard developed a hybrid design featuring an implant "body-shift" in diameter, shape, and thread pattern combined into a single structure to address implant placement in extraction sockets and compromised sites.¹⁰

Implants are offered in an assortment of diameters to meet various clinical situations. Diameter has been shown to have more of an impact on implant success than length especially in soft bone (eg, D3, D4) conditions commonly found in the maxilla.^11-14 Frequently, osteotomy sites are underprepared with regard to diameter to increase primary stability. Also, in extraction sockets, where bone volume can be limited, diameter is important in achieving adequate primary stability. Narrow-diameter implants can be effective when there is limited ridge dimension (buccal-lingual) or there are issues of interproximal space adjacent to the natural dentition. The strength of smaller-diameter implants has been questioned, although recent improvements in metallurgy, whether through alloys or high-strength titanium, have alleviated these concerns.

Implant diameter is categorized as standard or regular (RDI) (3.75 mm to 4 mm), narrow (NDI) (3 mm to <3.75 mm), extra-narrow (<3 mm), wide (4.5 mm to 6 mm), and ultra-wide (7 mm to 9 mm). Several retrospective and meta-analysis studies have shown no difference in survival, prosthesis success rates, and marginal bone levels between NDI and RDI.¹⁵ Wide and ultra-wide implants have been used predominantly in immediate tooth replacement of molar teeth in extraction sockets. While this design has demonstrated stable long-term outcomes, care must be taken to ensure that adequate bone remains around the coronal aspect of wider-diameter implants to maintain crestal bone.

Dental implants also come in a variety of lengths to accommodate clinical needs. Short implants recently have become popular given that vertical ridge augmentation is complex, requires a high level of surgical knowledge and skill, and is associated with an increased incidence of risk.^16,17 Consequently, the use of short implants has filled a void within the industry. Short implants have seen a dramatic progression in decreasing length from 10 mm, to 8 mm, to 6.5 mm, to now even 4 mm in length. Queries arise regarding implant-to-crown ratio because shorter implants imply a resorbed ridge situation with increased interocclusal space. There is no clinically significant evidence to support the presumption that increased implant-to-crown ratio leads to greater marginal bone and implant loss.^18,19 There is, however, an increased risk of prosthetic problems with short implants, such as screw loosening or breakage; therefore, splinting multiple units is suggested to decrease this risk.²⁰

Clinicians must exercise caution when using short implants to avoid innervation anatomy, account for the horizontal (buccal-lingual) component to ridge resorption in addition to the vertical element, and address occlusal relationships of the restoration. If the buccal horizontal defect is not addressed before restoration, there may be potential for buccal-gingival food impaction due to overcontour of the prosthesis in an attempt to reestablish the dentition into a normal occlusal scheme. Food impaction may lead to bone loss that would be significant on shorter implants and could ultimately compromise survival.²¹

Craniofacial implants, such as pterygoid, nasalis, and zygomatic, provide solutions for the severely atrophic maxilla. These longer implants, which require surgical expertise, can also provide an alternative to established grafting procedures, such as sinus augmentation, to decrease treatment time. Immediate loading of zygomatic implants has been shown to be successful, yet clinicians must be aware of the associated potential risks and complications.²²

Thread pattern is categorized by depth and pitch. Depth refers to the distance from the outer edge of the thread to the inner implant diameter. Pitch is characterized as the vertical distance between threads. Studies have demonstrated that an increase in thread depth increases primary stability, especially in soft bone conditions.^23,24 Thread pitch is equally important, especially in compromised extraction sockets or those with limited dimension following tooth removal.

Thread pitch denotes the number of threads per unit length contacting bone. It makes sense that a greater number of threads contacting bone per equal length of bone, eg, ten threads versus five, would afford greater stability. Orsini et al showed not only the importance of thread pitch, but also that the ideal distance between threads should be about 0.5 mm.²⁵ Together, both of these variables-depth and pitch-are critical for primary stability.

The issue of cement-retained versus screw-retained restorations has been a much-debated topic. Studies have shown the negative consequences of retained cement remnants around dental implants and how this contributes to peri-implantitis and, in some cases, eventual implant loss.²⁶ Often, implants require angulation towards the incisal edge position to avoid apical fenestration of the extraction socket or edentulous ridge. Studies have shown a 20% to 80% risk rate of perforation in the anterior maxilla, with central incisors having a 2.5 times greater probability.^27,28 The advent of angulated screw-channel abutments has addressed these needs, although there are limitations to the amount of correction provided with such devices. A threshold of 15 degrees correction at a screw insertion torque or preload value of 25 Ncm has been shown.²⁹

However, the differentiating factor of correcting the prosthetic angle at the abutment (peri-implant soft tissue) level versus implant (subcrestal bone) level is that restorative submergence profile does not become a negative factor in remediation where excess contour in correction can cause soft-tissue recession. The original intent of biaxial implants was to avoid traditional grafting strategies on All-on-4-type cases to treat an edentulous dentition.

Hybrid implants represent a paradigm shift in concept where the best elements from traditional designs can be blended together to meet specific clinical demands. As alluded to earlier, a tri-oval implant, which has been available for about 5 years and focuses on the treatment of edentulous sites, incorporates three flat sides at the coronal 25% of the implant body with the intention of decreasing pressure on the crestal bone in healed ridges. The concept of "pressure necrosis," however, is debatable as some studies have shown no difference in marginal bone levels with increased insertion torque, while other studies have correlated increased insertion torque values, thin bone, and marginal bone loss; the clinical significance re-mains unclear.^30,31

More recently, an inverted-body shift implant design has been developed that combines the properties of a tapered implant with a narrow coronal diameter at the implant platform roughly 40% the length of the implant body.³² The design incorporates changes in di-ameter (wide to narrow), shape (tapered to cylindrical), and thread pattern (deep to shallow) in a singular form. Instead of retaining the existing wider coronal form of the implant body as with tapered designs, the top portion is reduced to a standard (4 mm) or narrow-diameter (3 mm to 3.5 mm) implant body. This implant design specifically addresses the needs associated with placement in maxillary anterior extraction sockets where the apical portion of the implant body is used predominantly to engage the residual socket walls and/or a few millimeters beyond for primary stability, and the reduced coronal portion allows blood clot formation and the placement of graft material to thicken the crestal bone, which has been shown to be important in longitudinal maintenance.³³ A retrospective study compared traditional tapered versus inverted-body shift implant designs and showed better radiographic outcomes for the latter group with a reduced coronal parallel diameter and shape that translated to better esthetic outcomes and pink esthetic scores.³³

Dr. Chu is a consultant for Southern Implants and BioHorizons. He received no financial support for this article.

Stephen J. Chu, DMD,MSD, CDT
Adjunct Clinical Professor, Ashman Department of Periodontology and Implant Dentistry,
Department of Prosthodontics, New York University College of Dentistry, New York, New York