Rotational path removable partial denture: An esthetic alternative Featured in General Dentistry, May/June 2007 Raymond J Byron Jr., DM, D Robert Q Frazer, DDS, Michael C Herren, DMD Missing teeth can be replaced using any of a number of methods Patients may choose to replace missing teeth with a prosthesis that is either removable, fixed, or retained with implants When it is necessary to replace anterior or posterior teeth, a properly designed and fabricated rotational path removable partial denture can be both successful and esthetically pleasing to the patient However, while a patient’s functional and esthetic needs can be met successfully, rotational path removable partial dentures can be more demanding for the laboratory technician to fabricate and for the dentist to seat in the mouth Rotational path removable partial dentures frequently are overlooked as a viable means of treating missing teeth This article reviews the principles of rotational path removable partial dentures, as well as their categories, advantages, and disadvantages, in the hope that more dentists will consider them when the need arises The goal for replacing missing teeth is to restore a patient’s function, stability, and esthetics Modern dentistry offers many options for achieving this goal Sinus lifts and bone augmentation offer more opportunities to replace missing teeth with implant-retained fixed and removable prosthetics More than 700,000 dental implants are placed annually, although they are not always the treatment of choice.1 When patients choose removable prosthodontics to replace missing teeth, a rotational path removable partial denture (RPD) may be the treatment of choice Though rotational path RPDs are more complicated to design than fixed prosthodontics—and can be difficult to fabricate and (at times) to seat—a rotational path RPD can be successful This article discusses the rotational path RPD in terms of its principles, design, advantages, and disadvantages, as well as the difficulty encountered during laboratory fabrication Principles In 1935, Humphreys reported that Hollenback had designed an RPD with a “curved path of placement.”2,3 It wasn’t until 1978 that Garver revived the concept of a rotary path of insertion of RPDs, describing a technique that was limited to a unilateral RPD with a fixed partial denture (FPD) contralaterally.4 In 1978, King proposed an RPD with a rotational path placement that primarily utilized a tooth-supported RPD to replace missing anterior and posterior teeth.5,6 King’s theory was further clarified by Krol and Jacobson, who presented “physiologic and engineering principles” for use in the rotational path of insertion.7,8 According to Krol, a conventional RPD is seated in a single path of insertion, with every occlusal rest seated “more or less simultaneously.”8 By comparison, a rotational path RPD is seated in two segments; the segment that contains the centers of rotation is seated first before the RPD is rotated, positioning the second segment to the RPD’s final seat.7,8 Stated differently, the RPD framework engages desirable proximal undercuts that are essential for retention during the first insertion path At that point, it rotates, utilizing conventional clasps for additional retention The flexible, conventional clasp is the most common form of direct retainer.5 The rotational path design uses a rigid portion of the RPD framework (known as the rigid retainer or rigid retentive element) as the retentive component.9 The rigid portion of the RPD framework provides retention that is comparable to the flexible portion of conventional clasps Once the undercuts are engaged, the framework is rotated to a fully seated position that allows conventional clasps to provide further retention of the RPD framework, completing the second path of insertion.6 The principal advantage of a rotational path design is that the RPD requires a minimal number of conventional clasps to ensure retention.9 In addition, the ro-tational RPD can improve the patient’s esthetic appearance (due to conventional clasps not showing) and decrease plaque retention Categories There are three basic types of rotational path RPDs: anterior-posterior (AP), posterior-anterior (PA), and lateral Jacobson and Krol narrowed the rotational paths to two categories.9 Category I includes all prosthesis designs that first seat the rest that is associated with the rigid retainer Once that first rest is seated, the second segment rotates into place and conventional clasps are engaged.5 The rotational centers are located at the end of unusually long rests.9 The rotational centers on each side of the arch determine the axis of rotation during final placement.10 Category I includes all PA and AP paths of insertion that replace posterior teeth.8 A PA rotational path partial can be used successfully in the mandibular arch when the most distal molars are inclined mesially Figure illustrates a Category I rotational path RPD when a PA path or an AP path of placement is utilized For a PA path of placement, the end of the long occlusal rest contacts the molar first, creating the center of rotation The rigid retainer engages the mesial undercut of the molar The RPD is seated along the arc of rotation until the conventional clasps engage the mesiobuccal undercuts of the premolar The minor connector distal to the premolar must not engage the undercut, which is located between the arc and the distal surface of the premolar For an AP path of placement, the long occlusal rest first contacts the premolar, creating the center of rotation The rigid retainer engages the distal undercut of the premolar The RPD is rotated along an arc until the conventional clasps engage the distobuccal undercuts of the molar The minor connector mesial of the molar must follow the arc of rotation and should not engage any mesial undercut of the molar Category II includes all lateral paths and AP paths that replace anterior teeth The centers of rotation and the rigid retainers are found at the gingival extension of minor connectors.5,9 These prostheses actually use a dual path of insertion, rather than a completely rotational path To seat a Category II rigid retainer, a sliding method must be used to make initial contact with the abutment tooth (Fig 2) The path of insertion is a straight line from an incisal or occlusal direction to the rotational centers At that point, the conventional posterior or contralateral retainer is rotated into position from a conventional path of insertion The initial straight path of insertion along the mesial surfaces of the canines must be parallel with the cingulum rests to permit complete seating (Fig 2) Retainer design requirements The rigid retainer consists of a rest and a minor connector The rest should be prepared to a depth of 1.5–2.0 mm.5,9,10 Preparing the rests to their proper depth prevents movement of the tooth and maintains intimate contact with the rigid retainer Proper rest form also prevents the RPD framework from fracturing at the junction where the rest and minor connector meet.9,10 The occlusal outline form of the rest preparations should be irregular or dovetailed and should extend to more than half of the mesiodistal width of the occlusal surface (Fig 3).5,9 When the RPD is seated, the long occlusal rests become the rotational centers and provide similar bracing to reciprocating clasps The facial and lingual walls of a properly prepared rest should be nearly parallel to each other and both should be perpendicular to the long axis of the tooth It is extremely important for the walls of the rests to remain parallel bilaterally across the arch unless the teeth are both lingually and mesially inclined; in these cases, the facial and lingual walls of the bilateral rest seats are prepared parallel to each other, which actually results in the rests not being parallel to the long axis of the abutment teeth (Fig 4).10 A straight channel preparation (with its floor perpendicular to the long axis of the tooth) is preferred (Fig 5).10 Under occlusal loading, the straight channel preparation exerts the smallest amount of tipping while still permitting slight movement of the tooth during function The rigid retainers are minor connectors that engage proximal undercuts located in the infrabulge area of abutment teeth, eliminating the need for conventional retentive or reciprocating clasps Guiding planes usually are not recommended for rotational path designs.5.9 Preparation of the abutment tooth may be required to enhance proximal undercuts In some instances, natural dentition does not have the necessary shape to provide proper rest seats and/or proximal tooth contours In such cases, the best treatment is full metal or metal ceramic survey crowns fabricated in the dental laboratory The basic requirements of clasp design are satisfied by specially designed long rests in conjunction with the rigid retainer.9 Though rigid retainers are different from conventional direct retainers, they still are capable of satisfying the six biomechanical requirements of retention, support, stability, reciprocation, encirclement, and passivity.5 Laboratory considerations Rotational path RPDs are technique-sensitive and require a careful treatment plan An experienced dental laboratory technician is needed to fabricate the metal RPD framework.8 As with any RPD, diagnostic and master casts must be surveyed by placing the cast on the surveyor at a zero-degree or neutral tilt Undercuts for the rigid retainers and conventional clasps are located when the cast is in a neutral tilt position.5,9-11 Dividers also should be used to design rotational path RPDs One end of the divider tip is placed at the end of the long occlusal rest (the center of rotation) while the second tip is placed in the proximal undercut area and rotated occlusally If the second tip can rotate freely toward the occlusal without being trapped proximally, the undercut and center of rotation are aligned properly.8 When the divider tip is trapped and unable to rotate freely, the undercut is incorrect and the proximal surface of the abutment tooth may need to be recontoured (Fig 6) The second abutment tooth that receives the conventional clasp for retention also must be analyzed to accommodate the rotational path The first tip of the divider is kept at the same position that was used to measure the first segment, the center of rotation The second tip is extended to the marginal ridge of the second abutment tooth and rotated in an occlusogingival direction As the divider rotates, the space that appears between the second abutment and the second tip will require blockout to allow for full seating of the prosthesis as it rotates to its final position (Fig 6) The minor connector should not rest inside this space After the rotation path is analyzed, the second abutment is surveyed (with a zero-degree cast tilt) to determine the proper undercut of the tooth that is to receive the conventional clasp.8 A divider is essential when planning multiple edentulous spaces to replace posterior teeth The divider will determine the amount of blockout that is needed for the multiple minor connectors As edentulous space gets farther from the axis of rotation, the radius becomes straighter and the need for blockout is reduced As the edentulous space gets closer to the axis of rotation, the curvature of the arc increases, more blockout is needed, and the RPD is more difficult to seat (Fig 7).5,12 When fabricating rotational path RPDs, an experienced dental laboratory technician is essential for blocking out undesirable undercuts For Category II designs, the cast first is surveyed at zero degrees to analyze the undercuts of the mesial surfaces of the anterior abutments and the distobuccal undercuts of the posterior abutments The retentive areas of the most anterior teeth should have an undercut of at least 0.010 in.1 A second tilt of the cast is required to identify the initial path of placement This path is determined by adjusting the tilt of the cast upward anteriorly until the undercuts on the mesial surface of the anterior abutment teeth are eliminated The dental surveyor is used to ensure that the mesial undercuts are eliminated and that the cingulum rests are accessible during initial placement During initial contact, the mesial rigid retainers and cingulum rests must contact the anterior abutment teeth almost simultaneously.8,9 For the laboratory’s convenience, the path of insertion for both the anterior/lateral rigid retainers and the posterior conventional retainers should be indicated by two separate tripod marks (Fig 8).9 Using the analyzing rod of the surveyor, the land of the cast should be marked at the upward anterior tilt and the zero-degree tilt positions It is important to mark the cast twice so that the dental laboratory technician will see where the path of insertion of the anterior rigid retainers and the conventional retainer of the posterior teeth are located Proper rest designs are important to ensure success of Category II partial dentures The lingual rest of an anterior tooth (that is, canine) should be U-shaped in appearance In addition, it should extend to more that half the mesiodistal width of the tooth and should be prepared in enamel From a proximal view, the maxillary cingulum rest should be shaped like an inverted V (Fig 9).12 The proximal undercuts and rests must be parallel; otherwise, the rests will not seat completely The surveyor must be used to confirm an absence of interference once the rests and rigid retainers are seated (Fig 10).8 The dental laboratory technician must have a thorough knowledge of rotational path RPDs so that he or she may design the RPD framework correctly and block out undesirable undercuts Seating the RPD framework can be a challenge at times Ideally, the tooth surface of the rigid retainers should not be relieved or finished abrasively Rigid retainers must make close contact with the undercuts of abutment teeth.9 If the RPD framework does not seat fully during its rotational path of insertion, the portion of the framework that is causing the interference should be meticulously reduced with a highspeed handpiece using a dental bur, diamond, or stone If the anterior rigid retainers of a Category II AP RPD are cut back for esthetic reasons, there is a risk that retention will be lost in a worst-case situation Advantage and disadvantages Retention, support, and stability are the principle requirements for successful RPD fabrication A correctly fabricated rotational path RPD will not jeopardize these principles.8 A rotational path RPD is an excellent alternative to an FPD for the anterior part of the mouth Because anterior abutments not require conventional clasps, esthetics will improve and plaque accumulation will decrease Rotational path RPDs are excellent for cases involving mesially inclined mandibular molars In addition, a rotational path RPD with rigid retainers eliminates the possibility of losing RPD retention due to the distortion or breakage that can result from using flexible conventional clasps The major disadvantage of the rotational path design is that it is technique-sensitive.9 Dental laboratory technicians must be skilled at placing the rigid retainers correctly via proximal undercuts and must be careful when finishing and polishing the final framework All adjustments of the rigid retainers must be completed chairside, using pressure indicator paste or other disclosing material Adjustments must be conservative until the framework is seated completely Excessive adjustments mean that the RPD framework must be remade if retention is lost, since rigid retainers cannot be adjusted like conventional clasps can For the rotational path RPD to be successful, the dentist must have a thorough knowledge of rest design and proper blockout and also must communicate well with the dental laboratory Rotational path RPDs are not recommended for distal extensions.13 Functional movement will cause undesirable torque on the anterior abutment teeth due to the rigid retainers The rotational path RPD can be considered for distal extensions, provided the residual ridges are firm and there is little or no displacement A 1987 study by Schwartz and Murchison reported that a spring clasp offered an alternative to a rigid retainer for creating anterior retention in distal extension situations.14 The metal RPD framework has a channel incorporated into its design in which a cast wrought wire spring clasp is soldered Due to the spring clasp’s flexibility, this technique generates less torque to anterior abutment teeth but does not eliminate all torque Summary Rotational path RPDs improve esthetics by allowing dentists to replace missing teeth without placing a conventional clasp on every abutment tooth They are useful when molars are mesially inclined It is important that dentists evaluate each patient carefully and survey casts meticulously to ensure success Working closely with an experienced dental laboratory technician is a must Chairside adjustments to rigid retainers should be made judiciously Though they are technique-sensitive, a correctly designed rotational path RPD can be esthetically pleasing and retentive Acknowledgements Illustrations for this article were produced by Matthew H Hazzard and Thomas J Dole, Teaching and Academic Support Center, University of Kentucky Author information Drs Byron and Frazer are full-time assistant professors, Division of Restorative Dentistry, College of Dentistry, University of Kentucky in Lexington, Kentucky Dr Herren is in private practice in Cynthiana, Kentucky References Garcia LT The use of a rotational-path design for a mandibular removable partial denture Compend Contin Educ Dent 2004;25:552-567 Humphreys K A unilateral removable bridge using a hook attachment J S Calif Dent Assoc 1935;2:332-337 Humphreys K Removable bridge using a hook 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