A partial denture is a removable appliance that replaces one or more missing teeth. It contains artificial teeth that are attached to a pink or gum-colored plastic base. A partial denture may have a metal framework and clasps that connect to your natural teeth. These components help securely fit the partial denture in your mouth.
Rests and direct retainers are important parts of a partial denture. A rest is a component that rests against and provides support from a natural tooth. A direct retainer clamps onto a natural tooth to hold the partial denture in place. Rests and direct retainers help stabilize the partial denture so it doesn’t shift around or become dislodged when eating or speaking.
Functions of a Rest
A rest has several key functions:
Provides Support
A primary function of a rest is to provide support for the partial denture framework. The rest sits against the surface of a natural tooth to help bear some of the forces placed on the partial denture during chewing and other oral functions. This prevents excessive force from being transmitted entirely to the soft tissues and remaining teeth.
By providing support, a rest helps stabilize the partial denture and prevent it from sinking into soft tissues, which could lead to sore spots. It also prevents the partial denture from rotating or rocking during function. Proper support from rests is essential for a partial denture to fit comfortably and securely.
Prevents Tooth Movement
A rest also functions to help prevent natural tooth movement. When teeth are missing, opposing teeth can over-erupt or drift into the empty space. Placing appropriate rests on natural teeth adjacent to the missing teeth can help prevent this unwanted tooth movement.
The rest maintains the vertical dimension and prevents the natural tooth from supraerupting. This helps maintain proper occlusion and the position of remaining teeth.
Indirect Retention
In some cases, a rest can also provide indirect retention to help keep the partial denture in place. Though rests are primarily designed for support, their intimate contact with the natural tooth surface creates friction that can aid retention. This is especially important for partial denture designs where direct retainers cannot be placed on certain abutment teeth.
Indirect retention from a rest is not as secure as a direct retainer. But rests can still contribute to keeping the partial denture from dislodging during function when combined with direct retainers elsewhere on the prosthesis.
Aids Chewing Efficiency
By providing stability, support, and retention, the rests ultimately help keep the partial denture securely seated during chewing and biting. This allows patients to chew more efficiently and comfortably with their partial denture in place. Without proper rests, the partial denture is more likely to move around, impairing chewing ability and potentially damaging soft tissues.
Considerations for Rest Placement
Several factors must be considered when determining where to place rests on abutment teeth for a partial denture:
Tooth Location
Ideally, rests should be placed on teeth located on either side of the areas with missing teeth. These abutment teeth adjacent to the edentulous areas are in the best position to provide support, stability and retention for the partial denture. They bear the forces of chewing while also preventing mesial drift of more distal teeth.
Tooth Health
Rest seats should only be placed on teeth with good bone support and healthy roots without advanced periodontal disease. Teeth with excessive mobility or active decay are not suitable abutments for rests.
Tooth Anatomy
The anatomy and shape of the tooth must be considered. Rests are often placed on cusp tips or marginal ridges of posterior teeth. But if these areas lack sufficient bulk for a rest seat preparation, alternate locations may be utilized.
Patient Occlusion
The patient’s bite and occlusal forces guide appropriate rest position. Centric contacts, excursive movements, and the location of heavy biting forces all impact placement. Rests must be positioned so they do not interfere with dynamic occlusal function.
Partial Denture Design
The number, location, and path of retention claps will influence rest placement. Rests are positioned relative to direct retainers, denture bases, and other components to provide optimal support.
Rest Direction
Rests are prepared perpendicular to the forces they must bear. Proper directional orientation of the rest is key to durability and preventing torque or rotational forces.
Types of Rests
There are two main categories of rests – occlusal and proximal:
Occlusal Rests
Occlusal rests are placed on the biting or chewing surfaces of posterior teeth. Possible locations include the central groove, marginal ridge, cusp tip or triangular ridge of molars and premolars. Occlusal rest seats are prepared parallel to the occlusal plane at approximately a 45 degree angle from vertical. They may be shaped as circular, ovoid, crescent, or compound forms.
Proximal Rests
Proximal rests are placed on the surfaces between adjacent teeth. They may be prepared into the facial or lingual embrasure space or along the cervical third of the tooth crown. Proximal rests are oriented perpendicular to the proximal contour of the tooth surface. They help resist dislodging forces from the side.
In addition to occlusal and proximal rests, indirect retention can also be gained from cingulum rests on the lingual surfaces of anterior teeth. Care must be taken to avoid gingival impingement and poor esthetics with anterior rest seats.
Rest Seat Preparation
Preparing the rest seat is an important step in creating an optimal partial denture:
– The rest seat outline should be shaped with rounded corners to reduce stress concentration.
– Avoid sharp angles which concentrate forces and increase the risk of enamel fracture.
– The rest seat preparation should have smooth, parallel walls for surface area contact with the rest material.
– Ideal rest seat depth is approximately 1.0-1.5mm.
– Rest seats are prepared using diamond burs and fine finishing points.
– A matrix may be used to facilitate uniform, controlled depth and shaped.
– The dentist must ensure proper clearance around the tooth without impinging on gingival or axial surfaces.
Rest Materials
Different materials can be used to fabricate the component of the partial denture framework that contacts the rest seat:
Cast Metal Rests
– Most commonly used material
– Rigid, durable, resistant to wear and deformation
– Requires parallel walls and well-defined preparation
– Can be fabricated chairside or by a dental laboratory
Prefabricated Plastic Rests
– Available in varying anatomies and sizes
– Less expensive option for short-span cases
– More flexibility in preparation requirements
– Prone to wear over time with material degradation
Individual Plastic Rests
– Custom-made acrylic resin rests
– Fabricated on a model from dental stone impression
– Offer a cost-effective alternative to cast metal
– Less durable than metal long-term
Rest Material | Advantages | Disadvantages |
---|---|---|
Cast Metal | Strong, rigid, durable | Higher cost, requires parallel walls |
Prefab Plastic | Inexpensive, readily available | Prone to wear, less adaptable |
Custom Plastic | Low cost alternative to cast metal | Less durable than metal long-term |
Impression for Rest Seats
An accurate impression is critical for fabricating a partial denture framework with well-fitting, contoured rests.
– The rest seat preparation must be clearly recorded in detail.
– Appropriate impression trays and materials for partial denture impressions are utilized.
– The dentist may use retraction cord or other gingival displacement methods to capture unobstructed rest seats.
– Care is taken to avoid voids, pulls, or distortions around the rest seat region.
– The bite registration records the opposing tooth surfaces and relationship to the rest seats.
– Modern digital impression techniques can also effectively capture rest seat details through intraoral scanning.
Adjustment and Maintenance
Once delivered, the partial denture rests may require minor adjustment for optimal fit and function:
– Assess the intaglio surface for full, even contact of each rest with the prepared seat.
– Eliminate any gaps between the rest and seat through selective grinding.
– Smooth any rough areas on the external rest surface.
– Adjust occlusion for comfortable, balanced contacts in chewing and excursions.
– Periodically check rests for wear over time and replace if necessary.
– Have dental professional thoroughly clean rest seats every 6-12 months.
Conclusion
In summary, rests play a critical role in partial denture service and success. When properly designed and fabricated, rests provide necessary support, stability and retention to the prosthesis. Careful attention must be given to rest location, preparation, impressions and material selection. With appropriate rests, patients can function comfortably and efficiently during chewing and speech. Regular professional care and maintenance will maximize the long-term durability and performance of partial denture rests.