Tooth loss is very a very common problem; therefore, the use of dental implants is also a common practice. Although research on dental implant designs, materials and techniques has increased in the past few years and is expected to expand in the future, there is still a lot of work involved in the use of better biomaterials, implant design, surface modification and functionalization of surfaces to improve the long-term outcomes of the treatment. This article will discuss the types of implants that have been developed, and the parameters that are presently used in the design of dental implants.
There are have been four main types of dental implant designs that have been developed and used in clinical dentistry, including a subperiosteal form, blade form, ramus frame, and endosseous form. However, the large scope of this review will focus on endosseous implants which are the most used in dentistry today.
Endosseous dental implants are typically screw-shaped, inserted into either the maxilla or mandible, and serve to replace the tooth root. Typically, dental implants are made out of grade 4 commercially pure Ti because it is corrosion resistant and stronger than other grades. However, Ti alloys, mainly Ti6Al4V, are also used since it is stronger and more fatigue resistant that pure Ti18. In bulk form, endosseous implants largely differ by the overall shape of the implant (e.g., tapered versus cylindrical) and macro-topography. Several parameters in the design of endosseous implants affect survival rates of implants, including: body shape, size, chemical surface composition, and topographical features among other factors.
Macro features of endosseous implants
Dental implants are designed to achieve primary mechanical stability and to promote a strong bone-implant interaction over time through osseointegration. For endosseous implants, there are three major macro-aspects: 1) screw threads, 2) solid body press-fit designs (cylindrical, conical), and/or 3) porous-coated designs. Each configuration affects the long-term biomechanical properties at the bone-implant interface and they largely determine success or failure of the implant. Bone adapts to stress concentrations from the implant interface by inducing either hypertrophy or atrophy. Thus, an optimal shape will allow for equal distribution of stress to the surrounding bone matrix and to promote bone growth.
Screw thread type implants are the most popular type of root implant due to their proven success25 and great initial retention strengths. Several parameters in the thread design affect the success of the implant, including thread pitch, thread height, and thread configuration (v-shaped, square-shaped, etc). It should be noted that cortical bone is not significantly affected by the shape of the implant. However, the behavior of trabecular bone is greatly influenced by the shape of the threads.
Thread pitch is the distance from the center of one thread to the center of the next thread. Pitch predominantly plays a role in determining available surface area for bone interaction and is thus an important design parameter. For a fixed length screw, the lower the pitch, the more threads there are available. Chun et al. showed that by decreasing pitch length, maximum effective stresses decrease, thus indicating that less stress exposed to bone is required to hold the implant stable. From the same study, it was shown that increasing the length of the implant decreased the maximum effective stress. In a similar 2-dimensional finite element analysis (FEA) by Motoyoshi et al., a decrease in pitch resulted in a decrease of maximum effective strength. However, the influence of stress distributions as a result of pitch were unclear.
As reviewed by Geng et al. in 2004, there are four common thread configurations: v-thread, thin-thread, reverse buttress, and square thread. Using FEA, Geng et al.29 showed that truncated v-thread (0.1 mm width thread apex) and a large square thread (0.36 mm thread width) designs are beneficial in dissipating stresses evenly and that thin thread type forms (0.1 mm width) should be avoided due to large stress concentrations in bone. However, other FEA studies suggest that thread profiles do not affect von Mises stress distributions in bone30. In an animal study, square thread implants outperformed v-thread and buttress designs in bone-to-implant contact and torque removal after 12 weeks.
Among the various parameters, screws can be self-tapping, thus negating the need to drill pilot holes. However, it was reported that the initial stability of non-self-tapping implants was greater compared to self-tapping implants of the same material using polyurethane blocks to simulate bone with resonance frequency analysis32. This is thought to be attributed to greater surface area available on non-self-tapping screws due the availability of more threads.
There is great debate in the optimal overall shape of the implant (i.e., threaded versus smooth). As previously stated, initial mechanical stability largely determines the success of an implant. As such, mechanical considerations must first be taken into account to minimize micromovement of the implant once loaded. In comparing threaded cylindrical implants versus threaded conical implants, Kim et al. found that the primary stability of conical implants is greater than cylindrical implants. However, the results of this canine study showed that cylindrical implants had higher success rates, though they it was not significantly different. Conical implants were thought to cause over compression on the surrounding bone matrix, thus causing biological damage. In an animal study, comparing threaded Ti implants, threaded hydroxyapatite coated Ti implants, and smooth Ti implants, threaded implants outperformed smooth implants with survival rates of 95.5% to 75.4%, respectively.
The implant-abutment connection can be thought of as the head of the implant; the function of the connection is to provide a means to apply torque to screw the body of the implant into bone and to provide a second-stage connection for the abutment. There are 2 basic forms of the connector consisting of either an internal or external connector which is typically hex shaped. In both cases with respect to coupling, the head must prevent rotation of the abutment and allow for the use of interchangeable parts in the case that a component needs to be replaced. Originally, the external hex connector was developed but was redesigned so that it could withstand higher occlusal forces and minimize micro-movement between the implant and the abutment since this interface determines joint strength. Internal hex implants were then developed to increase stability between the implant and abutment.