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ePaper created 2015-03-18, 11:28:53 | version 1.33.01
from page 1 Implant Tribune U.S. Edition | March 2015D2 Publisher & Chairman Torsten Oemus t.oemus@dental-tribune.com President/Chief executive Officer Eric Seid e.seid@dental-tribune.com Group Editor Kristine Colker k.colker@dental-tribune.com Managing Editor Implant Tribune Sierra Rendon s.rendon@dental-tribune.com Managing Editor Fred Michmershuizen f.michmershuizen@dental-tribune.com Managing Editor Robert Selleck, r.selleck@dental-tribune.com Product/Account Manager Humberto Estrada h.estrada@dental-tribune.com Product/Account Manager Will Kenyon w.kenyon@dental-tribune.com Product/Account Manager Maria Kaiser m.kaiser@dental-tribune.com Marketing director Anna Kataoka a.kataoka@dental-tribune.com Education Director Christiane Ferret c.ferret@dtstudyclub.com Tribune America, LLC 116 West 23rd Street, Suite 500 New York, NY 10011 Phone (212) 244-7181 Fax (212) 244-7185 Published by Tribune America © 2015 Tribune America, LLC All rights reserved. Tribune America strives to maintain the utmost accuracy in its news and clinical reports. If you find a factual error or content that requires clari- fication, please contact Managing Editor Sierra Rendon at s.rendon@dental-tribune.com. Tribune America cannot assume responsibility for the validity of product claims or for typographical errors. The publisher also does not assume respon- sibility for product names or statements made by advertisers. Opinions expressed by authors are their own and may not reflect those of Tribune America. Editorial Board Dr. Pankaj Singh Dr. Bernard Touati Dr. Jack T. Krauser Dr. Andre Saadoun Dr. Gary Henkel Dr. Doug Deporter Dr. Michael Norton Dr. Ken Serota Dr. Axel Zoellner Dr. Glen Liddelow Dr. Marius Steigmann Tell us what you think! Do you have general comments or critique you would like to share? Is there a particular topic you would like to see featured in Implant Tribune? Let us know by e-mailing feedback@dental-tribune.com. 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IMPLANT TRIBUNE “ STABILITY, Page D1 Mathematically, it can be defined as fol- lows: 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 = Coefficient of friction The important factor in this equation is P, the critical pressure on the bone, as high pressure results in unfavorable bone strain, particularly within the corti- cal compartment. However, the formula indicates that the resisting torque is proportional to the diameter (D) raised to the power of 2. This means that if you double the diameter the resisting torque becomes four times higher. Put another way, if we use the same insertion 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 di- ameter inserted into 1 mm thick cortical bone with a torque of 20 Ncm will trans- mit 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 percent of the torque originates from the pres- sure on the cortical bone, and the contri- bution to torque from bone cutting, etc., is neglected). Yet manufacturers persist in providing a single target value of in- sertion torque across the range of im- plant diameters they offer. It is therefore reasonable to dis- cuss the virtues of insertion torque and ask the pivotal question: Is insertion torque an appropriate mea- sure by which to quantify optimal pri- mary stability? After all, bone is a living tissue, so any measure of primary stabil- ity must also reflect the future viability of the bone. It is clear that higher insertion torques fulfil the desire to achieve a high degree of mechanical stability as interpreted through manual perception. Indeed, it is usual for manufacturers to provide some guidance on optimal insertion torque with some implant designs being specifi- cally tailored to deliver higher insertion 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 sur- rounding bone. Numerous studies have been published that clearly demonstrate that the critical pressure these high torques create leads to micro-fracture of the bone,11,12 with a net resorption in the cortical zone11,12,13 and, indeed, an unfa- vorable delayed healing process with a reduced bone-to-implant contact.14 Such a response might well shift the onset for secondary stability and thereby delay or extend the period of potential vulnerability. This is clearly counter to the goal we are trying to achieve with im- mediate or even early loading protocols, whereby we want to transfer from simple mechanical fixation to full osseointegra- tion in the shortest possible time. The most fascinating aspect of this de- bate is the lack of correlation between in- sertion torque and the implant stability quotient (ISQ) as measured by RFA, which appears to be counterintuitive. How is it possible for an implant that is driven in at 30 Ncm to have the same ISQ as one that required 100 Ncm of torque? None- theless, the weight of literature would seem to suggest this to be the case.15-18 Because ISQ is measuring axial stiff- ness, it must be clear that frictional ro- tational resistance is a completely differ- ent parameter. After all, I don’t doubt we have all have experienced the “spinner” (an implant that exhibits little or no rota- tional stability) that went on to osseoin- tegrate, and there are a number of stud- ies published that report high success rates for immediately loaded implants that were inserted with low insertion torque.19-22 By contrast, implants with an ISQ of less than 50 rarely go on to integrate suc- cessfully, 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 en- suring 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 However, it is not the aim to seek complete elimina- tion of micro-motion, a valuable lesson learned in orthopedics.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 pri- mary stability. Our goal must be the rapid onset of secondary stability, with minimal criti- cal pressure to the poorly vascularised cortical bone so unfavorable resorptive responses and delayed healing are avoid- ed. 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 al.17 I have labeled this objective measure viable constraint (vC), whose central pur- pose is to obtain a clinically relevant de- gree of stability while maintaining a low critical pressure on the vulnerable corti- cal 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 approach to implant dentistry. So I would take this opportunity to ask that we think in terms of viable con- straint. It will, of course, take controlled prospective studies to determine 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 thick- ness that a maximum torque of 20 Ncm and an ISQ of 60 represent the optimal measures we are looking for to ensure safe immediate loading. In the past, we used to think length was important with implants, whereas today there is increasing focus on short im- plants. However, I would point out that a strong correlation has been shown to ex- ist 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 out- come. References available upon request from the publisher. Note This content originally appeared as an editorial in The International Journal of Oral & Maxillofacial Implants, published by Quintessence Publishing. About the author Dr. Michael R. Norton, BDS, FDS, RCS(Ed), gradu- ated from the University of Wales, School of Dental Medicine, in 1988. He runs a world-renowned prac- tice dedicated to implant and reconstructive den- tistry in Harley Street, London. He is a specialist in oral surgery and, in 2007, was awarded a presti- gious fellowship of the Royal College of Surgeons, Edinburgh, without examination, for his contribu- tion to the field of implant dentistry. In 2013, Nor- ton was made adjunct clinical professor to the De- partment 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 United Kingdom, becom- ing one of the world‘s most respected and re- nowned implant surgeons. His considerable port- folio of research has been groundbreaking, and he has become one of the most sought after lecturers in his field. Since 1989, Norton has dedicated all his clinical and postgraduate time to the practice and study of implant reconstructive dentistry. He is sec- retary, board member and fellow of the Academy of Osseointegration (AO) and is past president (1999-2001) and honorary life member of the Asso- ciation of Dental Implantology (ADI), UK. He is past editor of the AO’s Academy News and is currently associate editor of the International Jour- nal of Oral & Maxillofacial Implants (JOMI). He also serves as a referee for a number of other peer-review journals. From 1995 to 2010, he was joint owner and editor of the journal Dental Implant Summaries. ‘We need to employ an objective measure of constraint that reliably ensures the implant can tolerate early or immediate loading.’