Application removable Marshall
of the rotational path design concept partial denture with a distal-extension L . Asher
to a base
, DDP
Fort Benning, Ga. Biomechanical considerations for use of the rotational path design concept to construct a remova.ble partial denture for a patient with a tooth-bounded ridge on one side and a distal-extension ridge on the opposite side are presented. The various axes and arcs of rotation that occur during masticatory function are identified and their effects on the prosthesis and supporting structures are analyzed. Sequential steps in the necessary surveys of the master cast are enumerated. Critical details for the most effective and least deleterious placement of the rigid retentive element on the mesial surface of the posterior molar abutment (on the tooth-bounded ridge) are described. (J PROSTHET DENT 1992;68:641-3.)
T
he concepts of “dual path” and “rotational path” removable partial dentures were published in the late 1970s.rS3These approaches to removable partial denture design used precisely fitting minor connectors to engagethe infrabulges of abutment proximal surfaces and thereby provide retention without a conventional retentive clasp arm. These concepts resulted in improved esthetics and stability. In the ensuing years, inadequate understanding of the mechanics of rotational path removable partial denture design sometimes produced removable partial dentures that were actually detrimental to the health of the dentition. In response, a number of articles that provided definitive guidelines to avoid potential errors in design were published.4-8 It is widely recognized that in a rotational path design, there is a potential for unfavorable mechanical leverage when a rigid retentive component is used in a distal-extension base situation. However, there is an undercut area on the molar abutment of a mandibular Kennedy class II (modification 1) removable partial denture that may be used for rotational path retention. The following design is presented for consideration. DESIGN Occlusal rest seats are prepared mesially on the terminal abutment on the distal-extension side, distally on the anterior abutment on the tooth-supported side, and mesially on the molar abutment (Fig. 1). The rest seat on the molar should be as long as possible, and oriented exactly along the planned axis of rotation for the framework (Fig. 1, line B). The floor of the preparation must be rounded to produce
The opinions or assertions contained herein are the private views of the author and are not to be construed as official or as reflecting the views of the Department of the Army. aLieutenant Colonel, U.S. Army, DC; Assistant Director, Advanced Education in General Dentistry.
10/1l39926 THE
JOURNAL
OF PROSTHETIC
DENTISTRY
a rest seat that will allow unimpeded rotation. Care must be taken to ensure that this rest seat preparation is not undercut with respect to the initial path of placement. As with certain rotational path designs, the master cast is surveyed and tripodized at both the initial path of placement and at the final (fully seated) path of placement. The exact area to be engaged by the rotational path retentive element must then be determined and marked by use of the following step-by-step procedure: 1. Place the master cast on the surveyor so that the occlusal plane is perpendicular to the surveying rod. 2. On the master cast, place a straight edge along the axis of rotation and use the surveyor to draw a vertical line from the axis of rotation down the mesiolingual cusp of the molar (Fig. 2, line E). 3. Place a straight edge against the mesiolingual cusp parallel to the plane of occlusion and at right angles to the axis of rotation (Fig. 1, line C). Mark the point at which the straight edge contacts the tooth. Draw a vertical line through this tangent point on the mesiolingual cusp (Fig. 2, line F). The survey line that marks the height of contour of the molar abutment at the final path of placement is the coronal limit of the retentive area (Fig. 2, line D). The area that can be contacted by the rotational path retentive element has now been demarcated (Fig. 2, area C). The framework should be cast to contact this entire area without any relief. The clasp assembly for the molar is completed by adding a bracing arm that extends around the buccal surface, with its gingival edge on the survey line and its tip terminating precisely on the axis or rotation. A clasp assembly that avoids lateral stress during rotational movement is placed on the terminal abutment on the tissue-supported side. The positive retention provided by the rotational path retentive element makes clasp placement unnecessary on the anterior abutment of the tooth-supported segment. This tooth need only provide vertical support and indirect retention through a properly designed rest. 641
ASHER
Fig. 2. View of the molar abutment
Fig. 1. Design features. A, Rotational path retentive element; B, axis of rotation; C, line perpendicular to B and tangent to the tooth; D, bracing arm.
When a distal-extension base is loaded in function, the removable partial denture tends to rotate. The rotational movement of any part of the prothesis anterior to the axis of rotation may have some movement occlusally. It is for this reason that the framework must not contact the infrabulge area anterior to the vertical line drawn through the axis of rotation (Fig. 2, line E). All portions of the prosthesis distal to the axis of rotation will move in a mesiogingival arc. Should the framework contact the tooth distal to the tangent point (Fig. 2, line F), this rotational movement would impinge on the tooth and cause mesiobuccal torque. Between these two lines (Fig. 2, area C), the mesiogingival rotational movement would result in a very slight disengagement of the framework. Therefore, during function there are minimal forces applied to the tooth by a rotational path retentive element that is limited to this area. The buccal bracing arm is not designed to provide reciprocation, but simply to prevent horizontal movement of the framework, thereby ensuring that the rotational path retentive element remains in contact with the tooth when the prosthesis is not in function. When the distal-extension base is depressed in function, the buccal arm will rotate occlusally and disengage. However, the extreme tip will show no perceptible movement because it is placed precisely on the axis of rotation It is apparent that a long channel-type rest seat, guiding plane, SO-degree encirclement, or any feature recommended to prevent drifting of the molar abutment has not been included in this design. Because of the rotation that will occur in function, each of these design items would cause torque on the tooth and therefore should not be used.
642
as seen from the mesiolingual surface. A, Bracing arm; B, occlusal rest; C, area contacted by the rotational path retentive feature; D, height of contour; E, vertical line through the axis of rotation; F, vertical line through the tangent point.
Clinical use of this design over many years has not caused failure due to molar drifting. Perhaps this fortuitous lack of failure is because of the presence of the rotational path rigid retentive element in the mesiolingual undercut. The casting contact actually provides passive resistance to the tendency of the lower molar to drift in a mesiolingual direction in response to occlusal forces. This design is most applicable when the modification space requires the replacement of at least one premolar and one molar. It allows adequate space for the rotational path of placement without excessive blockout on the anterior abutment. Although the framework design allows rotational movement to occur without torque on the abutment teeth, an altered cast impression technique is recommended for the distal-extension base to ensure optimum tissue support and stability of the prosthesis. Its objective is to reduce rotational movement when masticatory forces are applied to the distal extension base.
CONCLUSION By including a rotational path rigid retentive element in a design that accommodates rotational movement in function, exceptional stability is achieved with minimal stress to the abutments. In addition, the total absence of lingual retentive or bracing clasp arms renders the prosthesis more hygienic and comfortable for the patient. The primary disadvantage is the need for direct communication with knowledgeable laboratory personnel. For best results, each prosthesis should be discussed with the technicians who will construct the framework. This may become less necessary once the technicians become familiar with the concept.
OCTOBER
1992
VOLUME
68
NUMBER
4
RPD
WITH
DISTAL-EXTENSION
BASE
REFERENCES 1. King GE. Dual path design for removable partial dentures. J PROSTHET DENT 1978;39:392.
2. King GE, Barco MT, Alson RJ. Inconspicuous retention for removable partial dentures. J PROSTHET DENT 19’78;39:505. 3. Garver DG. A new clasping system for unilateral distal-extention removable partial dentures. J PROSTHET DENT 1978;39:268. 4. Jacobson TE, and Krol AJ. Rotational path removable partial denture design. J PROSTHET DENT 1982;48:370. 5. Jacobsen TE. Satisfying esthetic demands with rotational path partial dentures. J Am Dent Assoc 1982;105:460.
Bound
volumes
available
6. Firtell DN, Jacobsen TE. Removable partial dentures with rotational paths of insertion: problem analysis J PROSTHET DENT 1983;50:8. 7. Daniel RE, Granata JS. The rotational path removable partial denture. Compend Contin Educ Dent 1985;6:716. 8. Bauman R. Rotational path partial dentures: problems and potential. Compend Contin Educ Dent 1986;7:356. Reprint requests to: DR. MARSHALL L. ASHER 508 WICKER~HAM AVE. FORT BENNING, GA 31905
to subscribers
Bound volumes of THE JOURNAL OF PROSTHETIC DENTISTRY are available to subscribers (only) for the 1992 issues from the publisher at a cost of $55.00 ($68.00 international) for Vol. 67 (January-June) and Vol. 68 (July-December). Shipping charges are included. Each bound volume contains a subject and author index, and all advertising is removed. Copies are shipped within 30 days after publication of the last issue in the volume. The binding is durable buckram with the journal name, volume number, and year stamped in gold on the spine. Volumes 65 and 66 are also available. Payment must accompany all orders. Contact Mosby-Year Book, Inc., Subscription Services, 11830 Westline Industrial Drive, St. Louis, MO 63146-3318, USA; phone (800) 3254177, ext. 4351, or (314)453-4351. Subscriptions must be in force to qualify. in place of a regular JOURNAL subscription.
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Bound
volumes
are not available
643
of the rotational path design concept partial denture with a distal-extension L . Asher
to a base
, DDP
Fort Benning, Ga. Biomechanical considerations for use of the rotational path design concept to construct a remova.ble partial denture for a patient with a tooth-bounded ridge on one side and a distal-extension ridge on the opposite side are presented. The various axes and arcs of rotation that occur during masticatory function are identified and their effects on the prosthesis and supporting structures are analyzed. Sequential steps in the necessary surveys of the master cast are enumerated. Critical details for the most effective and least deleterious placement of the rigid retentive element on the mesial surface of the posterior molar abutment (on the tooth-bounded ridge) are described. (J PROSTHET DENT 1992;68:641-3.)
T
he concepts of “dual path” and “rotational path” removable partial dentures were published in the late 1970s.rS3These approaches to removable partial denture design used precisely fitting minor connectors to engagethe infrabulges of abutment proximal surfaces and thereby provide retention without a conventional retentive clasp arm. These concepts resulted in improved esthetics and stability. In the ensuing years, inadequate understanding of the mechanics of rotational path removable partial denture design sometimes produced removable partial dentures that were actually detrimental to the health of the dentition. In response, a number of articles that provided definitive guidelines to avoid potential errors in design were published.4-8 It is widely recognized that in a rotational path design, there is a potential for unfavorable mechanical leverage when a rigid retentive component is used in a distal-extension base situation. However, there is an undercut area on the molar abutment of a mandibular Kennedy class II (modification 1) removable partial denture that may be used for rotational path retention. The following design is presented for consideration. DESIGN Occlusal rest seats are prepared mesially on the terminal abutment on the distal-extension side, distally on the anterior abutment on the tooth-supported side, and mesially on the molar abutment (Fig. 1). The rest seat on the molar should be as long as possible, and oriented exactly along the planned axis of rotation for the framework (Fig. 1, line B). The floor of the preparation must be rounded to produce
The opinions or assertions contained herein are the private views of the author and are not to be construed as official or as reflecting the views of the Department of the Army. aLieutenant Colonel, U.S. Army, DC; Assistant Director, Advanced Education in General Dentistry.
10/1l39926 THE
JOURNAL
OF PROSTHETIC
DENTISTRY
a rest seat that will allow unimpeded rotation. Care must be taken to ensure that this rest seat preparation is not undercut with respect to the initial path of placement. As with certain rotational path designs, the master cast is surveyed and tripodized at both the initial path of placement and at the final (fully seated) path of placement. The exact area to be engaged by the rotational path retentive element must then be determined and marked by use of the following step-by-step procedure: 1. Place the master cast on the surveyor so that the occlusal plane is perpendicular to the surveying rod. 2. On the master cast, place a straight edge along the axis of rotation and use the surveyor to draw a vertical line from the axis of rotation down the mesiolingual cusp of the molar (Fig. 2, line E). 3. Place a straight edge against the mesiolingual cusp parallel to the plane of occlusion and at right angles to the axis of rotation (Fig. 1, line C). Mark the point at which the straight edge contacts the tooth. Draw a vertical line through this tangent point on the mesiolingual cusp (Fig. 2, line F). The survey line that marks the height of contour of the molar abutment at the final path of placement is the coronal limit of the retentive area (Fig. 2, line D). The area that can be contacted by the rotational path retentive element has now been demarcated (Fig. 2, area C). The framework should be cast to contact this entire area without any relief. The clasp assembly for the molar is completed by adding a bracing arm that extends around the buccal surface, with its gingival edge on the survey line and its tip terminating precisely on the axis or rotation. A clasp assembly that avoids lateral stress during rotational movement is placed on the terminal abutment on the tissue-supported side. The positive retention provided by the rotational path retentive element makes clasp placement unnecessary on the anterior abutment of the tooth-supported segment. This tooth need only provide vertical support and indirect retention through a properly designed rest. 641
ASHER
Fig. 2. View of the molar abutment
Fig. 1. Design features. A, Rotational path retentive element; B, axis of rotation; C, line perpendicular to B and tangent to the tooth; D, bracing arm.
When a distal-extension base is loaded in function, the removable partial denture tends to rotate. The rotational movement of any part of the prothesis anterior to the axis of rotation may have some movement occlusally. It is for this reason that the framework must not contact the infrabulge area anterior to the vertical line drawn through the axis of rotation (Fig. 2, line E). All portions of the prosthesis distal to the axis of rotation will move in a mesiogingival arc. Should the framework contact the tooth distal to the tangent point (Fig. 2, line F), this rotational movement would impinge on the tooth and cause mesiobuccal torque. Between these two lines (Fig. 2, area C), the mesiogingival rotational movement would result in a very slight disengagement of the framework. Therefore, during function there are minimal forces applied to the tooth by a rotational path retentive element that is limited to this area. The buccal bracing arm is not designed to provide reciprocation, but simply to prevent horizontal movement of the framework, thereby ensuring that the rotational path retentive element remains in contact with the tooth when the prosthesis is not in function. When the distal-extension base is depressed in function, the buccal arm will rotate occlusally and disengage. However, the extreme tip will show no perceptible movement because it is placed precisely on the axis of rotation It is apparent that a long channel-type rest seat, guiding plane, SO-degree encirclement, or any feature recommended to prevent drifting of the molar abutment has not been included in this design. Because of the rotation that will occur in function, each of these design items would cause torque on the tooth and therefore should not be used.
642
as seen from the mesiolingual surface. A, Bracing arm; B, occlusal rest; C, area contacted by the rotational path retentive feature; D, height of contour; E, vertical line through the axis of rotation; F, vertical line through the tangent point.
Clinical use of this design over many years has not caused failure due to molar drifting. Perhaps this fortuitous lack of failure is because of the presence of the rotational path rigid retentive element in the mesiolingual undercut. The casting contact actually provides passive resistance to the tendency of the lower molar to drift in a mesiolingual direction in response to occlusal forces. This design is most applicable when the modification space requires the replacement of at least one premolar and one molar. It allows adequate space for the rotational path of placement without excessive blockout on the anterior abutment. Although the framework design allows rotational movement to occur without torque on the abutment teeth, an altered cast impression technique is recommended for the distal-extension base to ensure optimum tissue support and stability of the prosthesis. Its objective is to reduce rotational movement when masticatory forces are applied to the distal extension base.
CONCLUSION By including a rotational path rigid retentive element in a design that accommodates rotational movement in function, exceptional stability is achieved with minimal stress to the abutments. In addition, the total absence of lingual retentive or bracing clasp arms renders the prosthesis more hygienic and comfortable for the patient. The primary disadvantage is the need for direct communication with knowledgeable laboratory personnel. For best results, each prosthesis should be discussed with the technicians who will construct the framework. This may become less necessary once the technicians become familiar with the concept.
OCTOBER
1992
VOLUME
68
NUMBER
4
RPD
WITH
DISTAL-EXTENSION
BASE
REFERENCES 1. King GE. Dual path design for removable partial dentures. J PROSTHET DENT 1978;39:392.
2. King GE, Barco MT, Alson RJ. Inconspicuous retention for removable partial dentures. J PROSTHET DENT 19’78;39:505. 3. Garver DG. A new clasping system for unilateral distal-extention removable partial dentures. J PROSTHET DENT 1978;39:268. 4. Jacobson TE, and Krol AJ. Rotational path removable partial denture design. J PROSTHET DENT 1982;48:370. 5. Jacobsen TE. Satisfying esthetic demands with rotational path partial dentures. J Am Dent Assoc 1982;105:460.
Bound
volumes
available
6. Firtell DN, Jacobsen TE. Removable partial dentures with rotational paths of insertion: problem analysis J PROSTHET DENT 1983;50:8. 7. Daniel RE, Granata JS. The rotational path removable partial denture. Compend Contin Educ Dent 1985;6:716. 8. Bauman R. Rotational path partial dentures: problems and potential. Compend Contin Educ Dent 1986;7:356. Reprint requests to: DR. MARSHALL L. ASHER 508 WICKER~HAM AVE. FORT BENNING, GA 31905
to subscribers
Bound volumes of THE JOURNAL OF PROSTHETIC DENTISTRY are available to subscribers (only) for the 1992 issues from the publisher at a cost of $55.00 ($68.00 international) for Vol. 67 (January-June) and Vol. 68 (July-December). Shipping charges are included. Each bound volume contains a subject and author index, and all advertising is removed. Copies are shipped within 30 days after publication of the last issue in the volume. The binding is durable buckram with the journal name, volume number, and year stamped in gold on the spine. Volumes 65 and 66 are also available. Payment must accompany all orders. Contact Mosby-Year Book, Inc., Subscription Services, 11830 Westline Industrial Drive, St. Louis, MO 63146-3318, USA; phone (800) 3254177, ext. 4351, or (314)453-4351. Subscriptions must be in force to qualify. in place of a regular JOURNAL subscription.
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Bound
volumes
are not available
643
