Dental Tribune United Kingdom Edition

25ClinicalMay 21-27, 2012United Kingdom Edition Telephone 01932 582900 www.indentsystems.com mike@indentsys.co.uk It is only at times of major reconstructive, orthodontic and/or surgical treatments that the option of opening a bite or establishing a new maxillo-mandibular rela- tion may present itself. However, many clinicians still prefer to “play it safe” and retain the existing habitual (CO) maxillo-man- dibular relationship, even during major rehabilitative procedures. By definition, the use of centric occlusion as a treat- ment position excludes re- establishing a proper vertical dimension in an over-closed patient’s. However, if the patients condition is actively deteriorating this may not be a safe option at all, as the continued physiologic break- down may lead to failed den- tistry and/or a flair up of crani- ofacial pain. Centric Relation (CR) The concept of centric relation has a very long history and was originally devised, at least in part, to accommodate the use of articulators during prostho- dontic treatment. Although we now know that the jaw doesn’t function like a hinge, originally it was convenient to make that assumption when using articulators to make prosthe- ses. Today, one clear differ- ence between centric rela- tion procedures and strictly muscle-oriented methodolo- gies is the priority given by CR methods to evaluating the function of the temporoman- dibular joints. Typically, centric relation operators give first priority to establishing stable joint function, while muscle- oriented (neuromuscular) approaches tend to focus al- most exclusively on muscle comfort. Muscle-related Centric (MC) In general, muscle-oriented approaches consider joint po- sition and/or stability second- ary to muscle function. In the extreme, it is simply assumed that creating “happy muscles” will automatically provide good or at least adequate joint function. In a more practical view, both joint function and muscle function are seriously evaluated and, when indicat- ed, a compromise is sought to provide both joint and muscle compatibility. This represents an approach that bridges the gap between strict CR and rig- id MC approaches. Consequently, a variety of methods have evolved to capture and establish a muscle-related centric position, while maintain- ing favorable joint function. Requirements of proper Neuromuscular Occlusion (NMO) The first step in all approaches to NMO requires inducing relaxation in the masticatory musculature, how- ever, there is no rational ex- cuse for not evaluating TM joint function prior to beginning the process. This can be ac- complished quickly and easily with Joint Vibration Analysis (JVA see Fig 1), or with more expensive and invasive imaging such as MRI. Muscle relaxa- tion can be aided by Ultra-Low Frequency TENS (ULF-TENS, see Fig 2), an Aqualizer, soft music or any other technique that reduces the resting hyperactivity of the mastica- tory muscles. Surface elec- tromyography (see Fig 3) is useful for making a quantitative determination whether relaxa- tion has occurred or whether resting muscle hyperactiv- ity still exists. Needles and/or fine wire electrodes not only make relaxation less likely, they record a more localised signal that is less representative of overall muscle activity. However, needle EMG electrodes are required when one is seeking to differentiate a myopathy from a neuropathy. Using the relaxed rest position of the mandible, with respect to the maxilla as a reference, a clinician can select a vertical dimension that allows adequate freeway space, yet avoids over-closing the bite. There are several methods currently used for selecting the treatment vertical. Each has its own rationale and advantages, but all of them benefit from objective diagnostic aids to ensure the best compromise between optimum joint, muscle, and tooth function. DT • The TScan range is dis- tributed in the UK by Indent Systems. For further information please contact Indent Systems on 01932 582900, email mike@ indentsys.co.uk or visit www.in- dentsystems.com * The second part of this article will appear in issue 14 About the author Dr Derek Mahony Specialist Orthodontist BDS(Syd) MScOrth(Lon) DOrth RCS(Edin) MDOrth RCSP(Glas) MOrthRCS(Eng) MOrth RCS(Edin)/ FCDS(HK) FRCD(Can) IBO FICD FIC- CDE 49 Botany Street Randwick NSW 2031 Sydney, Australia www.derekmahony.com ‘There are several different approaches that can be used to determine a “neuro- muscular” maxillo-mandibular relation- ship, even with a fully edentulous case’ Fig 2 Bio-TENS,* a ULF-TENS unit used for muscle relaxation Fig 3 BioEMG II* for monitoring rest position and testing muscle function against the new bite registration