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Clasp

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Definition

A part of a removable partial denture which acts as a direct retainer and/or stabilizer for the denture by partially encircling or contacting an abutment tooth.(GPT)
It is that component of the partial denture that rests against the vertical enamel surfaces of an abutment tooth and aids in bracing and retention.

General Consideration

A clasp should be located at the undercut area in relation to the determined path of insertion and removal of the prosthesis.

In 1916 Prothero proposed the Cone theory to explain the basis for clasp retention. He described the shape of the crown of premolars and molars to be equivalent to two cones (upper and lower) sharing a common base. The upper cone resembles the occlusal half of the tooth and the lower cone resembles the cervical half of the tooth. A clasp tip that ends below the junction of the two cones will resist movement in the upward direction, because it has to deform (straighten out) to be released across the junction of the bases of the two cones. The degree of resistance to deformation determines the amount of clasp retention.

Component parts of a clasp

These components may be rigid or flexible. The flexible components are designed below the height of contour so that they provide retention when they engage the undercut at the same time they can flex and pass through the height of contour without requiring much effort during insertion or removal.

In a conventional clasp design, the tip of the retentive arm is the only flexible component. All the other parts are rigid and hence, placed above the height of contour (widest circumference of the tooth).

Retentive arm

“A flexible segment of a removable partial denture which engages an undercut on an abutment and which is designed to retain the denture” – GPT.

It is that part of the clasp comprising of the retentive clasp arm and retentive terminal. The retentive clasp arm is not flexible and is located above the height of contour. The retentive terminal is flexible and lies below the height of contour.

Reciprocal arm

“A clasp arm or other extension used on a removable partial denture to oppose the action of some other part or parts of the prosthesis” - GPT.

It is located on the side of the tooth opposite to the retentive arm. It resists the lateral forces exerted by the retentive arm when it passes through the height of contour during the placement and removal of the RPD.

It is always placed in the supra-bulge area (above the height of contour). It may act as an indirect retainer when placed on an abutment located anterior to the fulcrum line (axis of rotation) of the partial denture. Thus, the rigid reciprocal arm can resist the rocking of the denture base.

General functions of the reciprocal arm can be enlisted as follows:

  • Provides stability and reciprocation against the retentive arm.
  • The denture is stabilized against horizontal movements.
  • Acts as an indirect retainer (prevents rocking) to a minor degree.

Shoulder

It is the part of the clasp that connects the body to the clasp terminals. It lies above the height of contour and provides stabilization against horizontal displacement.

Body

Part of the clasp that connects the rests and shoulders of the clasp to the minor connectors. It is rigid and lies above the height of contour. The body of the clasp is designed such that it contacts the guide plane of the abutment during insertion and removal. The tissue surface of the body of the clasp, which is closely related to the guide planes, is known as a proximal plate of the direct retainer.

Rest

“A rigid (stabilizing) extension of a removable partial denture which contacts a remaining tooth or teeth to dissipate vertical or horizontal forces”- GPT.

It is the part of the clasp that lies on the occlusal or lingual or incisal edge or surfaces of the tooth. It resists tissue ward movement of the clasp by acting like a vertical stop. The rest also helps to maintain the retentive arm of the clasp in position.

Minor connector

“The connecting link between the major connector or base of a removable partial denture and other units of the prosthesis, such as clasps, indirect retainers and occlusal rests” - GPT Here, it joins the clasp with the remaining part of the metal framework. In a gingivally approaching clasp it is also known as the approach arm.

Principles of Clasp Design

The basic principle of clasp design is encirclement i.e. to obtain more than 180° of continous contact for Aker’s clasp and a minimum of 3-point contact for Roach clasps. Other principles of design include:

  1. Occlusal rest should be designed to prevent tissue-ward displacement of the denture.
  2. Each retentive terminal should be opposed by a reciprocal component.
  3. Balanced retention should be present (i.e. if a buccal retentive clasp is present on one side, the same should be present in the opposite side and vice-versa).
  4. Path of escapement should never coincide with the path of removal.
  5. Only the minimum necessary amount of retention should be used.
  6. Primary abutment clasp of a distal extension denture base should never exert tipping forces on the abutment.
  7. It is preferable to place the reciprocal elements at the height of contour and the retentive element below the height of contour.

Functional Requirements of a Clasp

Each of these functional requirements are provided by various components in a clasp. The clasp and its parts should be designed appropriately to achieve the functional requirements.

Retention

“Retention is that quality inherent in the prosthesis which resists the force of gravity, the adhesiveness of foods, and the forces associated with the opening of the jaws” - GPT.

Retention is the most important function of the clasp; hence, it is the most important functional requirement. The purpose of a clasp is lost if the retention is lost. Retention is provided by the retentive arm of the clasp. The tip of the retentive arm (retentive terminal) should lie in an undercut to the selected path of insertion. The undercut used for retention is known as a retentive undercut or preferred undercut. This undercut should be identified during survey. If an undercut is absent it should be created using any one of the four methods described under surveying. The retentive arm should be fabricated according to the following design considerations.

Technical design considerations
  • The retentive arm of the clasp provides retention. The terminal third of the retentive arm is flexible and it engages the undercut. Middle third of the retentive arm has minimum flexibility. Proximal third is rigid and is located above the height of contour.
  • The location and degree of a tooth undercut available for retention is relative to the path of insertion of the partial denture. Path of Insertion is defined as
"The direction in which a prosthesis is placed upon and removed from the abutment teeth"-GPT 
  • A clasp has rigid and flexible components. The rigid components of the clasp should be placed in the non-retentive areas of the tooth for a given path of insertion. This is because they cannot deform to cross the height of contour.
  • The clasp design for each abutment must be separately considered. For a clasp to be retentive, the retentive terminal must be placed in the undercut area of the tooth.

The retentive terminal is forced to deform when a vertical dislodging force is applied. The retentive terminal exhibits a certain amount of resistance to deformation. This resistance is proportional to the flexibility of the clasp arm. It is this resistance to deformation that generates retention. The flexibility of the clasp varies with the type of alloy being used.

  • The retentive undercut will be present only in relation to a given path of insertion. The retentive undercut is absent in conditions where the direction of dislodgement of the clasp arm is similar to the direction along which the clasp arm was inserted. Hence it is important to maintain a single path of insertion that does not coincide with the path of displacement.
  • Retentive undercuts should be located with the help of a surveyor. The cast should be tilted in a surveyor to achieve a unique path of insertion. The following factors should be considered while determining the path of insertion.

• Tissue undercuts • Location of vertical minor connectors • Origin of bar clasp arm • The denture bases.

  • A good path of placement and removal is obtained by the contact of the rigid parts of the framework with the parallel surfaces of the abutment. These parallel tooth surfaces guide the denture during insertion and removal and are called guiding planes. These guiding planes that are prepared on the tooth act as an additional retentive unit.
Guiding planes are defined as “Two or more vertically parallel surfaces of abutment teeth so oriented as to direct the path of placement and removal of partial dentures” -GPT.

Guiding planes should be as parallel as possible to the path of insertion of the denture. If they are not parallel, trauma to the teeth and supporting structures will occur during insertion and removal of the denture. This leads to periodontal breakdown of the abutment teeth and strain to the parts of the denture. In the absence of guiding planes, the retention from the clasp will be meagre or practically non-existent.

  • When the dislodging forces are not acting on the denture, the retentive terminal should be in a passive relationship with the tooth. If the retentive arm is not passive, orthodontic movement of the abutment will occur. This is due to the continuous pressure exerted by the clasps on the abutment teeth.

Stability

It is defined as, “The quality of a denture to be firm, steady, or constant, to resist displacement by functional stresses, and not to be subject to change of position when forces are applied” - GPT.
  • All components of the clasp except the retentive arm provide stability.
  • Cast circumferential clasps offer greater stability because it has a rigid shoulder.
  • Wrought clasps have a flexible shoulder and bar clasps do not have a shoulder; hence, they offer lesser stability.

Support

It is defined as, “To hold up or serve as a foundation or prop for” - GPT.
  • It is the resistance to the movement of the denture in a gingival direction (along the path of insertion). (Whereas retention is the resistance to the movement of the denture against the path of insertion).
  • It is provided by occlusal, lingual and incisal rests.

Reciprocation

It is defined as, “The means by which one part of a prosthesis is made to counter the effect created by another part”- GPT
  • It is provided by a rigid reciprocal arm.
  • It resists the stresses generated by the retentive arm. It also stabilizes the denture against horizontal movement. In other words, it helps to hold the tooth when the retentive arm is active. If the reciprocal arm is absent there will no resistance available for the action of the retentive arm.
  • It should be placed preferably at the junction of the gingival and middle thirds of the abutment tooth (always above or at the height of contour).
  • It should contact the abutment tooth along with or before the retentive arm during insertion and removal.
  • Other parts which offer reciprocation are:

• Lingual plate major connector. • An additional occlusal rest placed on the opposite side of the tooth along with the minor connector.

Encirclement

“It is the property of the clasp assembly to encompass more than 180° of the abutment tooth either by continuous or broken contact to prevent dislodgement during function”

  • Each clasp must encircle more the 180° of the abutment tooth. Encirclement can be either a continuous contact as in circumfrential clasp or a broken contact as in bar clasp with at least 3 different areas of contact. The three

points of contact are:

  1. Retentive terminal
  2. Occlusal rest
  3. Reciprocal arm

This embracement prevents the clasp assembly from moving out of the confines of the tooth during function.

Passivity

It is defined as “The quality or condition of inactivity or rest assumed by the teeth, tissues, and denture when a removable partial denture is in place but not under masticatory pressure” – GPT.

The retentive function should act only when dislodging forces are present. If the clasp is not seated properly, the retentive forces act continuously on the tooth leading to pain and tenderness.

Types of clasps

  1. Simple circlet clasp
  2. Reverse circlet or reverse approach clasp
  3. Multiple circlet clasp
  4. Embrasure clasp or modified crib clasp
  5. Ring clasp
  6. Fishhook or hairpin clasp or reverse action clasp
  7. Onlay clasp
  8. Half and half clasp
  9. Back-action clasp
  10. Grasso’s clasp or VRHR clasp
  1. T clasp
  2. Modified T clasp
  3. Y clasp
  4. I clasp
  5. Infrabulge clasp or mirror view clasp


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