What is the clasp structure and the function of each component?

　　ClaspA metal component on the abutment tooth, used for retention, support, and stability of the denture. It can be used as a direct or indirect retainer.

　　I. Clasp arm:

　　1. Retaining arm:

　　1) Clasp shoulder: The part connected to the clasp body, located above the metal wire, is semi-elastic and rigid, playing a role in stabilizing and preventing lateral displacement of the denture.

　　2) Clasp tip: The clasp tip is located below the metal wire and enters the undercut. The undercut is the main part of retention, preventing the denture from dislodging during occlusion.

　　2. Counter arm:

　　It is located above the metal wire on the palatal/lingual side of the abutment tooth and works with the retaining arm for retention. It prevents lateral displacement of the denture during wear.

　　II. Clasp body:

　　It is a rigid component connecting the clasp arms, occlusal rests, and minor connectors, and is non-elastic. It is located in the non-undercut area near the surface of the inter-abutment space. It stabilizes and supports the denture, preventing lateral and gingival displacement.

　　III. Occlusal support:

　　The part of the clasp body that extends towards the occlusal surface of the abutment tooth.

　　1. Occlusal support function:

　　(1) Support to prevent gingival displacement of the denture.

　　(2) Maintain the clasp's position on the abutment tooth.

　　(3) Prevent settling of the base, protecting the periodontal tissues and edentulous area of the abutment tooth.

　　(4) Distribute and transmit occlusal forces to the abutment tooth.

　　(5) Food impaction protector, preventing food impaction.

　　(6) Restore occlusal relationship for teeth with poor occlusal contact.

　　(7) Used alone as an indirect retainer.

　　2. Occlusal support requirements

　　(1) Location: On the marginal ridge near the inter-abutment space. If the occlusion is too tight, it can be placed in the lingual groove of the mandibular molars, the buccal groove of the maxillary molars, and the lingual cusp of the canines.

　　(2) Relationship between the rest and the abutment:

　　View 1: The relationship between the concave base and the long axis of the abutment tooth is 20° (molars) or 100° (premolars).

　　View 2: The concave base is at a 90° angle to the long axis of the abutment tooth.

　　IV. Classification by manufacturing method:

　　1. Cast clasp: Made from a wax pattern, investment, and high-melting-point alloy through a casting process. Cast clasps include circumferential clasps and bar clasps.

　　2. Circumferential clasp: It encircles 3 sides and 4 axial angles of the abutment tooth, i.e., it encircles more than 3/4 of the abutment crown, looking like a circle, so it is called a circumferential clasp. This type of clasp was applied early by Akers (1936), so it is also called the Akers clasp. Circumferential clasps are suitable for abutment teeth with healthy periodontal tissues and well-shaped crowns. It has good support, retention, and stability.

　　3. Bar clasp: The bar clasp was proposed early by Roach (1934), so it is also called the Roach clasp. This clasp has relatively separate buccal and lingual arms. The clasp arm extends from the metal base, the retention mesh in the base, or the minor connector, and passes through the gingiva to the undercut area of the convexity on the buccal surface of the abutment tooth. Its retention function is upward pushing retention, so it is also called a push clasp. It is suitable for abutment teeth of posterior teeth with free-end edentulous spaces, with good retention but poor stability.

　　4. Bent clasp: Bent clasps are made of circular stainless steel wire. Generally, molar clasps use steel wire with a diameter of 0.9~1.0mm (20～19 gauge), and premolar and anterior clasps use steel wire with a diameter of 0.8~0.9mm (20～20 gauge). The rest is made of 1.2mm (18 gauge) stainless steel wire by flattening and bending, using a cast rest. Because this clasp has linear contact with the abutment surface, the contact area is small, smooth, easy to clean, the caries rate of the abutment tooth is low, the manufacturing equipment is simple, the operation is simple, easy to adjust, economical and durable, it is still widely used clinically.