While many recognized leaders in the profession struggle to reconcile their faith in various CR theories with the science of neuromuscular occlusion, dentists are often faced with less daunting questions than how to restore a full mouth case. Restoring second molars, by themselves or in combination with quadrant dentistry, poses special problems that are often ignored, leaving patient and dentist with disastrous results. Understanding the special role of the second molar and following a few simple techniques allows the dentist to achieve a high level of clinical success with these restorations.
The second molar is the occlusal stop which prevents the condyle from collapsing superiorly. When this occlusal stop is lost, all manner of TMD symptoms can result as muscles become foreshortened and go into spasm and as the condyle impinges on delicate structures in and around the glenoid fossa. Even if TMD symptoms are not immediately apparent to the clinician or to the patient, there is another more mundane consideration. Without preserving the relationship between the mandible and the maxilla, it is difficult to grind to the pre-existing bite at delivery of a second molar restoration.
The clinician may prepare a second molar with adequate reduction, then check occlusal clearance in habitual closure and notice that there is not enough room for adequate thickness of the restorative material. Why? This is because the condyle has collapsed superiorly. The question is: how does the successful clinician avoid these second molar problems?
The first step is to understand the critical role played by the second molar. The occlusal stop must be preserved. While some CR proponents may argue that loss of this stop allows beneficial superiorization of the condyle, the neuromuscular view is that this can initiate classic TMD sequelae, including muscle spasm, headache, vertigo, facial pain etc. Neuromuscular studies demonstrate that excessive resting muscle activity usually results when the condyle is seated, questioning the long held belief that a seated condyle enables the lateral pterygoid muscle to rest. This begs the question: if CR is not the most comfortable, restful position for the musculature, why should patients be placed in CR? In fact, by measuring EMG readouts, which are largely ignored by CR proponents, neuromuscular researchers have found that the ideal “seated” position of the condyle during maximum intercuspation is often anterior and inferior along the articular eminence. When the condyle is seated in the glenoid fossa, the articular disk is often compressed, leading to perforation and other disk pathology. While the pathologic TMJ is unique in certain respects, it is similar to other pathologic joints in the body in that it often needs to be decompressed. This explains the frequent success of nightguards, splints, and orthotics, which are designed to decompress the TMJ.
What are some techniques to preserve the preoperative occlusion so as to avoid collapsing the TMJ and to make second molar restorations easier clinically?
Preoperatively, the successful clinician begins with full arch impressions and diagnostic casts. Polyvinyl siloxane, such as Take 1 (Kerr), is the state of the art for these impressions. Next, it is critical to take a full arch bite registration. (Figure 1) A firm setting material such as Sapphire (Bosworth) or Affinity (Clinicians Choice) (Figure 2) is recommended. The bite should always be taken–and checked–with the patient in an upright position. When the patient is reclined, the bite changes as the mandible shifts. This bite registration is the blueprint of the occlusion to be preserved. To fabricate a provisional, which will preserve the pre-existing occlusion, a preliminary Triple Tray (Premiere) impression is taken using a firm setting polyvinyl siloxane. When the patient closes into the Triple Tray impression, they should be sitting upright.
During preparation, it is critical to rely on depth reduction guide cuts so that there is no question about adequate reduction. Visually observing or otherwise checking occlusal clearance in habitual closure is of no use once you have prepared the second molar since the second molar prevents the collapse of the condyle. When the quadrant is being restored, the second molar should be prepared last, with a relined bite registration taken when all teeth are prepared except the second molar. Once the second molar is prepared, the bite registration is relined a final time, and then a full arch master impression is made in polyvinyl siloxane. When relining the bite registration, care should be taken to prevent excess material from flowing into areas where the bite has already been recorded. Triple tray impressions are to be avoided after preparation since the bite has been temporarily lost. However, it is critical to fabricate the provisional using the preoperative triple tray impression and a bis-acryl provisional material such as Luxatemp (Zenith), or Integrity (Caulk). These provisionals restore the occlusion to the preoperative condition.
At the laboratory, the technician will mount the models using the bite registration and fabricate the restoration to the preoperative occlusion. Since the provisional is preserving the bite, delivery of the final restoration should take place with only minimal occlusal adjustments. During these adjustments, it is critical to check the bite with the patient in an upright position, taking care to clear out all pathologic interferences, while maintaining anatomy for proprioception.
Understanding the special role of the second molar and following this protocol can prevent numerous problems for the patient and for the dentist.
Dr. Bernstein is a Clinical Instructor at the Las Vegas Institute for Advanced Dental Studies. He lectures throughout the U.S. on “Using Outstanding Services to Provide the Ultimate Dental Experience.” Dr. Bernstein is credited with developing the Bernstein VIP Restoration, a one-piece Empress crown with a Vectris integrated post. In 2002, The Richards Report awarded his office “Practice of the Year.” Dr. Bernstein maintains a private solo practice in Piedmont, California, focusing on restorative cosmetic dentistry, smile design, and five star guest services. He can be reached at www.allnewsmiles.com.