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It generally is recognized that the specialty of fixed prosthodontics is based upon sound physical and mechanical principles. Both the associated preparation designs and well as the inserted prosthetic devices have been held to a constant for many decades. In fact, both the materials and the concepts associated with this major backbone of clinical dentistry have changed relatively little for several generations. And as well as most clinicians will attest, the attending success has been most admirable.

Nevertheless, a number of modified techniques as well as materials have challenged the traditional format. The two most important ones that come to mind include the so-called Maryland Bridge and the resin-bonded bridge. Both of these systems have been used by a fairly large number of the profession. Furthermore, a considerable number of publications have been dedicated to these unusual approaches to the permanent replacement of natural teeth.

The Maryland Bridge was introduced by Dr. Gus Lividitis from the University of Maryland in Baltimore. As a member of the faculty at that University, he made a considerable effort to introduce it to the profession as a means for generating a fixed prosthodontics appliance without preparation of the retainer teeth. His efforts were amplified by the scientific contributions of Dr. Van Thompson who also served on the same faculty.

It should be mentioned that through the efforts of both of these individuals, this new method of treatment became the target of numerous investigations and eventual acceptance by a somewhat limited number of practitioners. In spite of an exciting start, this novel approach has all but come to a stop. The number of clinicians who incorporate the Maryland Bridge in their practice today is few and far between.

Reasons for failure are numerous. However, the one major annoying problem is retention failure. Most practitioners will attest to the fact that they require recementing on a continuing basis. The principle etiology for separation from the abutment teeth is due to torque or twisting of the appliance during mastication. And because of this problem the failure rate is commonly greater on posterior teeth as opposed to the anterior segment of the mouth. The only basic solution for this problem lies in the preparation of the abutment teeth. In the case of the anterior teeth, a vertical retentive groove should be generated on the mesial and distal component of the tooth. Furthermore, the normal convex contour that normally occurs on the proximal surfaces should be reduced to a flat surface. In the case of the posterior teeth, mesial and distal occlusal rests of appreciable dimension need to be generated.

Another problem associated with the Maryland Bridge is the over contouring of the lingual surface that occurs after placement of the device. Unless the contour is minimized, health of the periodontal tissue is jeopardized. Finally, the cementation of the appliance is quite critical. In this regard it is suggested that a base-metal alloy be employed that can be electrolytically etched.  This process generates a surface that exhibits a microstructural defects which in turn allow the penetration and retention of a composite resin luting agent. One of the very best luting systems for the Maryland Bridge is Panavia (Kuraray) since it relates to base metal alloys far better than any other system on the market. In spite of all these recommendations however, it is still recommended that the Maryland Bridge be the last choice of systems when replacing a lost natural tooth. In most cases, it generally is not worth the effort.
 
The second prosthetic device that really never achieved the lofty goals promised by numerous, companies, clinicians and dental laboratories was the resin-retained bridge. The two main systems that fall into this category are Targis-Vectris (Ivoclar) and Sculpture-Fibrekor (Zahn/Schein). While these two different systems exhibited a number of different features, they both were based upon the same principle. In essence, the retainer portion of the restoration consisted of an inlay or onlay. The pontic however, consisted of a wide range of systems including a denture tooth or simply fabricated from the same composite resin as the material used for the retainer. Upon completion, the prosthetic device was bonded to the prepared abutment teeth.

There were several novelties associated with the rein-retained bridge. First, the substructure of the appliance was fabricated from a strip of glass fibers all aligned so that there were parallel to one another. It was postulated that this type of configuration would add strength to the bridge as well as preventing torque and bending during mastication. The outer portion of the assembly was fabricated from a supposedly high wear resistant composite resin. In the case of the Sculpture/Fibrekor the wear resistance was quite acceptable whereas in the case of the Targis-Vectris it was not. Furthermore, the glass fibers were actually located on internal surface of the retainer. This then precluded the potential for grinding away any imperfections or interferences to the seating process. However, there were even more serious problems associated with these devices. Specifically the modulus of elasticity or stiffness was not sufficient to prevent significant deformation of the pontic in an apical direction during mastication. In fact the stiffness value was at least 10% of what is needed clinically.

The relatively low stiffness is an inherent property of the material used for construction. Composite resins may be only 10% that of cast metal alloys such as the base alloy systems. This then results in the pontic being unduly deformed in the pulpal direction and the retainer portion of the bridge being force in an occlusal direction. Since the inlay retainer lacks sufficient surfaces for maximum retention, the bonded component begins to leak and subsequently debond from the preparation. Another likely problem is a true fracture at either of the connector sites.

The problems presented above can be resolved to some extent by going to a full crown preparation for the abutment teeth. But in the short term, this is neither conservative (as was the objective of the “inlay-retained” bridge) nor a good long-term replacement for cast alloy such as gold or base metal.  The problems of fracture and debonding could also be addressed through the placement of a base metal alloy bar at the base of the inlay-retained bridge. Unfortunately very little or no clinical information is available regarding the clinical success of such an approach.

It is interesting to note that the resin-retained bridge existed for about four or five years. Targis-Vectris is a matter of history although it is still possible to fabricate the device from Sculpture-Fibrekor. But even this system has been all but abandoned by the profession as a long-term service. All of this points to problems that we unfortunately experience in the dental industry. Too many products are being introduced by various manufacturers to the dental profession without suitable substantiation through clinical testing.

Consider some of the indirect composite resins as well as some of the ceramic materials that have turned into persistent clinical failures. It would appear that we have stepped back into the past where the clinician and the patient performed the clinical testing for the manufacturer and at the clinician’s expense. Profit and reputable products can go hand in hand but that no longer seems to be the rule.