Secure primary stability is positively associated with successful long-term implant integration to ensure a successful clinical outcome. Initial implant stability is defined as biomechanical stability upon insertion, which is influenced by factors such as bone quantity and quality, geometry of the implant, surgical technique, and insertion torque (IT). New bone develops around the surface of the implant and subsequently undergoes biological fixation (secondary implant stability or osseointegration). Insufficient primary stability is associated with micromotions. After the implant is installed, micromotions > 100 μm may influence osseointegration and bone remodeling by inducing the formation of fibrous tissues and bone resorption at the bone-implant interface.

Optimal implant design is required for sufficient primary stability. For example, thread design is critically important to achieving primary stability. The relevant characteristics of the thread that determine its functional surface and distribute the biochemical load are as follows: depth, thickness, pitch, and face and lead angles. Certain manufacturers have developed double- or triple-threaded implants. Compared with single-threaded implants, multiple-threaded implants can be inserted faster. However, finite element analysis (FEA) revealed that a single-lead thread provides maximum primary stability, followed by the double-lead threaded implant. A triple-threaded implant is the least stable. The implant body design can be modified to improve initial stability to increase the success of immediate loading. The thread improves initial stability by maximizing the initial contact area. Further, the thread depth, thread morphology, pitch, and helix angle affect the biomechanical load distribution of the implant. Therefore, commercially available implant systems require better screw designs.

Differences in implant body pitch include an increase in spiral angle with increasing pitch, as represented by multi-threading, and in the pitch itself. To date, however, no studies have used torque or ISQ values that actually reflect the effect of torsion angle and thread compactness on the stability of implants implanted into low bone density bone.

Unfortunately, the effects of double- or triple-threaded implants on primary stability are known for only a few procedures, such as finite element analysis. An excessive lead angle for these implants may jeopardize their ability to sustain axial load despite faster insertion. Further, when micromotion is compared among implants with different lead angles with the same thread pitch, single-threaded implants demonstrate minimal micromotion, whereas triple-threaded implants show maximum micromotion, with both vertical and horizontal loading. Thus, numerous clinicians believe that double-threaded implants can be inserted faster with greater primary stability compared with single-threaded implants. Thus, double- and triple-threaded implants are used for immediate loading of an implant, and the increase in surface area affords greater primary stability. To date, however, data to confirm these findings have been insufficient. We previously conducted a torque analysis to determine the effects of various thread designs on primary stability. However, we were unable to determine the effects of the double-threaded implants.

Further, we employed an artificial bone model to directly observe the implant/bone interface. This method allowed direct observation of the contact interface between an implant and artificial bone without cutting out the test piece. We were able to match the effect of the torque-time curve by grinding the interface of an implant and artificial bones with the aid of a digital microscope.

This model allows observation of immediate bone damage, characterized by debris and voids during implant placement. This in turn provides critical information about the relation between tissue damage and quantifiable factors associated with primary stability, such as torque and implant stability quotient values (ISQ). Here, we employed this model to measure torque and ISQ, together with direct microscopic observations, to evaluate the primary stabilities of single- and double-threaded implants.