What is Torque?
Torque is sometimes used to describe the stability of an implant. However, torque does not necessarily correlate to implant stability. Torque measures the rotational friction between the implant and the bone combined with the force required to cut the bone if that is the case, and the pressure force from the surrounding bone. Dental implant torque is normally measured in Ncm. 10 Ncm corresponds to having a 10 cm lever attached to the implant, and pushing it at the end, with the force of 1 N (0.1 Kgf).
Insertion torque is torque measured continuously while the implant is inserted. The torque corresponds to a combination of the cutting friction of the tip of the implant in the bone, and the friction between the implant surface and the hole in the bone. If the hole is narrow or the bone quality is high the torque will be higher. The torque will also depend on how sharp the cutting tip of the implant is, on the surface properties of the implant, on the lubrication of the preparation (blood), and also on the design of the implant itself. For instance if the preparation is cylindrical and the implant is tapered, the insertion torque will be higher. The torque will increase as the implant goes down in the bone, due to the increased bone-to implant friction as the implant surface in contact with bone increases. Maybe the most important factor is the diameter of the implant, a wide implant require higher insertion torque than a narrow implant in the same bone for two reasons; the distance from the center of the implant to its contacting surface is longer and the implant surface itself is larger for a wide diameter implant.
The seating torque is the final torque value achieved when the implant is inserted. Depending on the implant design and bone properties, the value can be higher or lower. A tapered implant or an implant with a flange can give a high seating torque even in soft bone. Soft bone with a thin hard cortical plate can also give a high seating torque even if the stability of the implant is poor. For instance in a jaw with poor quality bone and a thin cortical plate, a large portion of the implant will be surrounded by poor bone and the overall stability will then be low. If the cortical plate is thick, the stability will be higher, but the seating torque might be the same since the seating torque mainly consists of torque due to the tapered part or flange is hitting the hard bone.
Reverse torque or abutment tightening torque is sometimes used to test the friction between the implant and the surrounding bone. The friction depends on how much close bone contact there is (osseointegration) and the compression forces between the bone and the implant. The method has been criticized since it can jeopardize or destroy ongoing osseointegration.
ISQs’ Correlation to Torque
In Degidi et al (2010), it is shown that even if ISQ and seating torque correlate in many cases, seating torque can be a poor measurement of implant stability in other cases. This is further investigated in for instance Trisi et al (2010), which show very good correlation between ISQ and implant micro mobility but a less good correlation between seating torque and micro mobility. In Pagliani et al (2012) a very good correlation between ISQ and implant micro mobility is also shown. Torque is not a good baseline for further comparison since testing it could jeopardize ongoing osseointegration, as it is invasive.