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10 mCME Dental Tribune Middle East & Africa Edition | 5/2017 Primary stability vs. viable constraint: A need to redefi ne mCME articles in Dental Tribune have been approved by: HAAD as having educational content for 1 CME Credit Hour DHA awarded this program for 1 CPD Credit Point CAPP designates this activity for 1 CE Credit By Michael R. Norton, UK Any regular reader of the Journal of Oral & Maxillofacial Implants or indeed of any other publication on dental implants could not fail to have noticed how much attention has been focused on Primary Stabil- ity. The concept of primary stabil- ity is not new; indeed, as early as the 1970s, there were studies emphasiz- ing the need to establish mechanical stability to ensure un-interrupted healing of the bone.1 This was most evident in the orthopedic literature as it pertains to hip prostheses.2 By the 1990s, numerous reports were being published on immedi- ate loading of dental implants3-6 and the ground-breaking work by Neil Meredith on the application of Resonance Frequency Analysis (RFA) This is also apparent in RFA curves which, like a heartbeat, always regis- ter a certain pattern in healthy bone that refl ects this loss of stability at the third or fourth week,10 regard- less of bone density. That said, we still need to defi ne what constitutes primary stability, i.e., that which sets it apart from biological in- tegration. As stated above, mechani- cal stability is one where a friction occurs between the implant and the surrounding bone giving rise to a resisting torque at time of insertion. This resisting torque is proportional to the effort required to seat the im- plant or peak insertion torque; they are in essence one and the same and depend largely on the characteristics of the implant, the density of the bone and the differential size of the osteotomy as it pertains to the diam- bone cutting, etc., is neglected). Yet manufacturers persist in provid- ing a single target value of insertion torque across the range of implant diameters they offer. It is therefore reasonable to discuss the virtues of insertion torque and ask the pivotal question: Is insertion torque an appropriate measure by which to quantify opti- mal primary stability? After all, bone is a living tissue, so any measure of primary stability must also refl ect the future viability of the bone. It is clear that higher insertion tor- ques fulfi l the desire to achieve a high degree of mechanical stability as interpreted through manual per- ception. Indeed, it is usual for manu- facturers to provide some guidance on optimal insertion torque with FIg. 1 FIg. 2 came to the fore7-9 with statements that achievement of implant stabil- ity was a prerequisite for long-term positive outcomes. At the same time, Meredith recog- nized it was possible for clinically fi rm implants with poor axial sta- bility to still be prone to failure.8 Of course, Brånemark recognized this in his early work, proposing as he did a period of submerged healing be- cause of his concerns for any destabi- lization of the bone-to-implant inter- face during the early healing phase. However, today we all recognize that such protective protocols are frequently unnecessary, with wide- spread acceptance of not only trans- mucosal healing but also immediate temporization and/or loading. So how do we defi ne primary stabili- ty? The most simple defi nition is one of mechanical friction between the implant and bone. Certainly, we can all appreciate that this contrasts with secondary implant stability where secondary stability is achieved by biological integration, i.e., osseointe- gration. The gradual shift from pri- mary stability to secondary stability is critically poised at around three weeks. This is seen to be the least stable time point where viscoelastic stress relaxation of the bone along with remodeling results in a loss of primary mechanical stability9 but with an as yet poorly established degree of secondary stability or os- seointegration. eter of the implant. Mathematically, it can be defi ned as follows: Resisting Torque = µ * P * H * π * D2 2 Where: H * ϖ * D2 = Surface Area of implant in contact with bone where H = height of the implant cylinder and D = diameter of implant cylinder P = Critical pressure on the bone µ = Coeffi cient of friction The important factor in this equa- tion is P, the critical pressure on the bone, as high pressure results in un- favorable bone strain, particularly within the cortical compartment. However, the formula indicates that the resisting torque is proportional to the diameter (D) raised to the pow- er of 2. This means that if you double the diameter the resisting torque becomes four times higher. Put an- other way, if we use the same inser- tion torque for a 3 mm wide implant and a 6 mm wide implant, then the critical pressure P will be four times lower for the wider implant! For example, an implant of 3 mm diameter inserted into 1 mm thick cortical bone with a torque of 20 Ncm will transmit the same pressure to the bone as an implant of 6 mm diameter inserted into 2 mm thick cortical bone with a torque of 160 Ncm. (This assumes that 100 per- cent of the torque originates from the pressure on the cortical bone, and the contribution to torque from some implant designs being specifi - cally tailored to deliver higher inser- tion torques, in excess of 75 Ncm. This yields a sense of comfort for the clinician that the implant is initially “stable.” However, such a high torque has not been shown to be propitious to the surrounding bone. Numerous stud- ies have been published that clearly demonstrate the critical pressure these high torques create leads to mi- cro-fracture of the bone11,12, with a net resorption in the cortical zone11,12,13 and, indeed, an unfavorable delayed healing process with a reduced bone- to-implant contact.14 Such a response might well shift the onset for second- ary stability and thereby delay or ex- tend the period of potential vulner- ability. This is clearly counter to the goal we are trying to achieve with immediate or even early loading protocols, whereby we want to trans- fer from simple mechanical fi xation to full osseointegration in the short- est possible time. The most fascinating aspect of this debate is the lack of correlation between insertion torque and the Implant Stability Quotient (ISQ) as measured by RFA, which appears to be counterintuitive. How is it possi- ble for an implant that is driven in at 30 Ncm to have the same ISQ as one that required 100 Ncm of torque? Nonetheless, the weight of literature would seem to suggest this to be the case.15-18 Because ISQ is measuring axial stiff- ness, it is must be clear that frictional rotational resistance is a completely different parameter. After all, I don’t doubt we have all have experienced the “spinner” (an implant that ex- hibits little or no rotational stability) that went on to osseointegrate, and there are a number of studies pub- lished that report high success rates for immediately loaded implants which were inserted with low inser- tion torque.19-22 By contrast, implants with an ISQ of less than 50 rarely go on to inte- grate successfully, and ISQ has been described as a good predictor of success.23, 24 It is this dichotomy that has got me thinking and has led me to write this editorial piece. Could it be that axial stiffness is far more pertinent than rotational friction in ensuring an implant integrates? We already know from the literature that an implant can tolerate a degree of micro-motion, thought to be circa 100-150µm,25, 26 and this is in essence what ISQ measures. Studies have also demonstrated that insertion torque correlates closely to the degree of micro-motion.25 How- ever, it is not the aim to seek com- plete elimination of micro-motion, a valuable lesson learnt in ortho- pedics.27 If it is possible to place an implant with lower insertion torque and still achieve axial stiffness with an ISQ >60, surely this provides us with a more optimal evaluation of primary stability. Our goal must be the rapid onset of secondary stabil- ity, with minimal critical pressure to the poorly vascularised cortical bone so unfavorable resorptive responses and delayed healing are avoided. At the same time, we need to employ an objective measure of constraint that reliably ensures the implant can tolerate early or immediate loading. As much was recently proposed by Barewal et al17. I have labeled this objective measure Viable Constraint (vC), whose central purpose is to obtain a clinically rel- evant degree of stability while main- taining a low critical pressure on the vulnerable cortical tissues through which our implants are inserted. Bone is not wood. It is not inanimate. It would behoove us all to remember this, and avoid the carpenter’s ap- proach to implant dentistry. So I would take this opportunity to ask that we think in terms of Viable Constraint. It will, of course, take con- trolled prospective studies to deter- mine the optimal conditions for vC, but if I were a gambling man (which I most certainly am!) I would guess for a 4.5 mm implant in bone with a cortex of <1.0 mm thickness that a maximum torque of 20 Ncm and an ISQ of 60 represent the optimal measures we are looking for to en- sure safe immediate loading. In the past, we used to think length was important with implants, where- as today there is increasing focus on short implants. However, I would point out that a strong correlation has been shown to exist between ISQ and implant length28,29,30 and, as such, for immediate loading, I also believe a longer implant with a higher ISQ, inserted at a lower insertion torque, will yield a more favorable outcome. Note This content originally appeared as an editorial in The International Journal of Oral & Maxillofacial Im- plants, published by Quintessence Publishing. References 1. Rune B, Jacobsson S, Sarnäs KV, Selvik. A roentgen stereophotogram- metric study of implant stability and movement of segments in the max- illa of infants with cleft lip and palate. Cleft Palate J. 1979;16:267-278. 2. Huiskes R, Weinans H, Dalstra M. Adaptive bone remodelling of bio- mechanical design considerations for noncemented total hip arthro- plasty. Orthopedics 1989;12:1255- 1267. 3. Salama H, Rose LF, Salama M, Betts NJ. Immediate loading of bilater- ally splinted titanium root-form implants in fi xed prosthodontics- a technique re-examined: two case reports. Int J Periodontics Restorative Dent. 1995;15:344-361. Editorial note: A complete list of refer- ences is available from the publisher Dr. Michael R. Norton, UK BDS, FDS, RCS(Ed), graduated from the University of Wales, School of Dental Medicine, in 1988. He runs a world-re- nowned practice dedicated to implant and reconstructive dentistry in Harley Street, London. He is a specialist in oral surgery and, in 2007, was awarded a prestigious fellowship of the Royal Col- lege of Surgeons, Edinburgh, without examination, for his contribution to the fi eld of implant dentistry. In 2013, Norton was made adjunct clinical professor to the Department of Periodontology at the Ivy League Dental School at the University of Pennsylvania. For more than 20 years, Norton has led the way for implant dentistry in the Unit- ed Kingdom, becoming one of the world‘s most respected and renowned implant surgeons. His considerable portfolio of re- search has been ground-breaking, and he has become one of the most sought after lecturers in his fi eld. Since 1989, Norton has dedicated all his clinical and post- graduate time to the practice and study of implant reconstructive dentistry. He is secretary, board member and fellow of the Academy of Osseointegration (AO) and is past president (1999-2001) and honorary life member of the Association of Dental Implantology (ADI), UK. He is past editor of the AO’s Academy News and is currently associate editor of the International Journal of Oral & Maxillo- facial Implants (JOMI). He also serves as a referee for a number of other peer-review journals.