Primary stability: what does it mean?

Dr. Michael R. Norton asks whether we really know what we’re measuring when assessing the solidity of implant placement

The term of the day is “primary stability.” We have to get good primary stability, don’t we? After all, we don’t want our implant to fail — so primary stability is key, isn’t it? But have you ever stopped to consider what exactly we mean by that? What is primary stability?

The simplest answer is that primary stability is an implant that feels tight in bone. Like a carpenter, you need to know that the screw in the wall is big enough and tight enough to support the heavy picture you are about to hang on it. But bone is not wood, and we are not carpenters.

So perhaps we can be more clinical about things and say that primary stability is an objective measure of tightness for which we use insertion torque. But how does insertion torque measure tightness, and does it correlate to implant success?

Torque or stability?

In order to understand torque, the clinician first needs to appreciate the fact that it is a measure of rotational friction — the friction between the implant and the surrounding bone as the implant is screwed into place. This friction will increase as the difference between the osteotomy diameter and the implant diameter increases or, to put it another way, as the compression of the bone increases. That can occur at the microscopic level by the application of surface roughness, or at the macroscopic level through the adoption of tapered implants and an aggressive thread design.

Of course, we also have a need to appreciate the viscoelasticity of bone (or lack thereof) and the difference between dense cortical lamellar bone and the more yielding spongiosa or cancellous bone. Either way, a high insertion torque equates to a high compression of the bone, and bone is not wood — it is a living vital tissue that responds negatively to high compressive pressure, as seen during orthodontic movement.

The net result is resorption and remodeling to better support the contours of induced stress. High isometric strain, as induced with high insertion torque, is in this sense very destructive, but while this does not impede osseointegration, it has been clearly shown to delay healing and reduce the bone-to-implant contact compared to bone that is subject to lower levels of compression.

Nonetheless, there remains a sense that high insertion torque reduces risk of failure. But this is an illusion. Most experienced implant surgeons will tell you an implant that is a spinner (insertion torque of lower than 10Ncm) still has the capacity to osseointegrate, and many researchers have shown that torque is in fact a poor predictor of success.

Furthermore, the clinician cannot compare torque measurements over time to determine if stability is increasing or decreasing; surely, any valuable objective measure of stability must be able to inform with regard to longitudinal changes in stability so that the clinician can determine if an implant has gained or lost stability during the early osseointegration phase.

At a time when early loading, progressive loading, and immediate loading are all prescribed with increasing regularity, there is a greater need to appreciate subtle changes in stability over time.

Axial versus rotational stability

Perhaps another point to reflect on is that occlusal functional load is rarely, if ever, rotational. Rotational forces do of course occur, as we see with the loosening of abutment screws, for example, but this is not due to direct rotational forces — rather, it is the combination of lateral forces applied from different vectors.

So what is the rational basis for concern with rotational stability, when we all understand from the history of dentistry that the most destructive forces are lateral, horizontal forces, especially those applied through parafunction? Surely, then axial primary stability of an implant counts for greater importance than rotational stability.

Axial stiffness is much to do with the congruence of the osteotomy with regard to the implant diameter and geometry as well as the density of bone, but perhaps contrary to intuition, it has absolutely nothing to do with rotational friction. Consequently, an implant that has low rotational stability can achieve high axial stiffness.

For example, imagine that an implant has been placed in dense cortical bone (Type 1 bone) using a bone tap to cut a thread within the bone exactly to match that of the implant, within an idealized osteotomy. The fit is perfect, but due to the bone tap, there is no rotational compression: The implant is a spinner. Such an implant would yield a high axial stiffness due to its precise fit within the osteotomy and due to the dense quality of the bone.

Conversely, an implant in soft bone under high compression with high initial insertion torque may quickly demonstrate a reduced axial stiffness as the bone undergoes stress release through its viscoelastic properties, and subsequent resorption and the density of bone fail to provide the enhanced axial stiffness desired.

The two examples above lend weight to the question as to whether the implant with lower insertion torque might not actually be preferable to the implant with higher insertion torque. The answer to this question lies in the histology of healing bone in the two scenarios described, which constitutes the different healing pathways adopted by the bone in response to the presence or absence of compression.

It has been very well documented in numerous studies that under the influence of high insertion torque, there is a wider zone of dead or dying cells, a decreased expression of protein biomolecules for bone formation, reduced cell proliferation, the presence of microfractures, reduced vascularization and bone deposition, and ultimately a reduced interfacial stiffness with a lower bone-to-implant contact.

In contrast, the most notable feature of bone healing around implants where there is no bone compression is rapid neo-bone formation and a more rapid osseointegration.

Resonance frequency analysis

Again, no one is saying that high insertion torque prevents osseointegration. It is about quality of osseointegration and thresholds of insertion torque needed to achieve optimal stability.

To this end, this author has worked tirelessly over the past 6 or 7 years to try to identify what represents the threshold for optimal insertion torque with respect to ensuring a highly predictable axial stability at the lowest value of torque as well as from the perspective of the bone, ensuring that the minimum isometric strain builds within the bone-inducing microfractures and delayed healing.

There is much work yet to be done, but we are now homing in on precise values. In my most recent published work (Norton 2017), implants that achieved the very lowest of insertion torques were monitored for axial stiffness, changes in axial stiffness, and successful osseointegration.

All implants in the study osseointegrated, and implant stability quotient (ISQ) values obtained even at the lowest of insertion torques were in the mid to high 60s, which is generally considered clinically stable, increasing to around 80 after a short 12-week period for osseointegration.

Perhaps the most remarkable finding in this study was the variability in axial stiffness for implants below 10Ncm, which was high and differed starkly to those placed with 15Ncm-20Ncm whose variability and standard deviation were extremely small. This is the first insight into a possible threshold for insertion torque at around 10Ncm-15Ncm.

This value is far lower than one could possibly imagine, but it reinforces what this author has always believed and lectured on for many years — that an insertion torque above 25Ncm does not add any clinical value.

This is due to the fact that 25Ncm is well above any level of torque that can be achieved manually, and no one can differentiate between an implant inserted with 25Ncm or one inserted with 50Ncm. Indeed, 25Ncm exceeds the threshold above which it is possible to appreciate differences in tightness.

So the next time you place an implant, in the absence of an ability to measure ISQ, limit your torque to 20Ncm or 25Ncm and feel confident that not only will your implant osseointegrate, but through respecting the bone you will achieve a higher quality of osseointegration, with more rapid healing, and a higher bone-to-implant contact.

We’re not in the game of primary stability; we are in the game of secondary stability — so let’s get there as fast as possible.

Learn more about primary stability!