Cover Denture Prosthesis

An overdenture (synonyms: cover denture prosthesis, coverdenture, overdenture, hybrid prosthesis, overlay denture) is used to replace the teeth of a jaw. It is a combination of a removable element and one or more elements that are fixed in the mouth. An overlay denture has the same shape and dimensions as a complete denture (full denture) and, like the latter, is not supported by teeth but by the alveolar ridge or the oral mucosa covering it. However, in contrast to a complete denture, which restores a completely edentulous jaw, there are still a few teeth in the overdenture that are completely covered by the denture. The remaining teeth have hardly any holding function, but they do have a guiding function for the denture and stabilize it against tilting movements and horizontally acting shear forces. The special feature of an overdenture is the so-called resilience telescopes. These are double crowns whose primary crown is firmly cemented to the tooth, while the secondary crown is incorporated into the denture. For an overdenture, double crowns have a built-in resilience margin: although parallel-walled portions of the crowns provide a defined alignment, the teeth are not loaded axially (in the root direction) so that the denture can sink into the jaw and oral mucosa. Sinking into the soft mucosa is called natural resilience (resistance). The prosthesis is mucosa-supported like a total denture.

Indications (areas of application)

An overdenture is planned when there are only a few teeth left in a jaw – usually one to three – and they are periodontally damaged (with regard to the tooth bed), so that their strength is no longer sufficient to allow them to bear a chewing load, but probably to guarantee a stabilizing effect on the position of the denture during chewing. It is often a restoration intended to delay the transition to edentulism.

Contraindications

  • Degree of loosening of the teeth is greater than two
  • Teeth are anchored in the bone less than one third of the root length
  • Intolerance to polymethyl methacrylate (denture acrylic).

Before the procedure

Prior to the provision of an overdenture, the patient’s expectations of the new denture are clarified. The patient will be counseled about alternative treatment options such as a complete denture (full denture) or pre-prosthetic (preceding the denture fabrication) surgery to improve the denture bearing. Placement of implants to anchor a denture is also addressed as a treatment alternative.

The procedure

The procedure is divided into quite a few treatment steps, which alternate between the dental office (hereinafter “ZA”) and the dental laboratory (hereinafter “LAB”). I. Situation impression (ZA)

Impressions of the jaws are taken with standardized impression trays, usually with alginate impression material. II. situation impressions (LAB)

are made by pouring plaster over the alginate impressions and are used for the

  • Orientation about the anatomical conditions of the jaws.
  • Representation of the opposing jaw, if only one jaw is to be provided with an overdenture
  • Production of so-called individual impression trays made of plastic, which meet the individual anatomical features of the jaws.

III. crown preparation (ZA)

  • The teeth to be provided with double crowns are contoured under local anesthesia (local anesthesia) with rotary instruments in such a way that no undercuts interfere with the subsequent placement of the thimble-shaped primary crown. The later crown margin is prepared just below the level of the gingival margin (the gum line).
  • Preparation impression – for example, with addition-curing silicone compound.
  • Facial arch creation – serves to transfer the arbitrary hinge axis of the mandible (connecting line between the temporomandibular joints) into a so-called articulator (devices for simulating the movement of the temporomandibular joint), in which the prosthesis is made
  • Supply of the prepared teeth with temporary crowns.

IV. Primary crown fabrication (LAB)

  • Fabrication of a preparation model from special gypsum based on the preparation impression.
  • Fabrication of the primary crown (cast metal alloy crown).
  • Making the individual impression tray
  • Making bite templates from plastic: wax walls melted onto them simulate the future dental arch and are initially based on average values.
  • Making registration templates to determine the bite position (ZA).

V. Functional impression (ZA)

  • Before the impression is taken with the help of the custom-made tray, its edges are corrected, either by shortening the material with the plastic cutter, or by applying additional thermoplastic material: the initially heated material is applied to the tray in a soft state and slowly hardens in the mouth while the patient performs functional movements (special movements with the mimic muscles and tongue).
  • Functional impression: after positioning the tray coated with impression material in the mouth, the patient performs certain functional movements to shape the margins in a functionally appropriate manner. The aim of functional margin shaping is to ensure that the marginal areas of the new denture fit into the vestibule (space between the alveolar ridge and the lips or cheeks) without interference, but at the same time slightly displacing the soft tissue and thus providing a good seal, and, if a mandible is being restored, into the sublingual area (lower tongue area). The functional marginal design is the decisive step with which a satisfactory denture retention can be achieved via adhesion and negative pressure.
  • Before the functional impression is taken, the primary crowns are placed on the prepared teeth. They remain after the impression in the impression material and are thus transferred to the next working model of the laboratory.

VI. trimming the wax walls (ZA).

The wax walls of the bite templates are individualized and aligned in three dimensions:

  • In the frontal view, the future occlusal plane (masticatory plane; plane where the teeth of the upper and lower jaws meet) must be parallel to the bipupillary line (connecting line between the pupils) and
  • Are located at the level of the lip closure.
  • In lateral view, the masticatory plane must be parallel to Camper’s plane (reference plane on the bony skull: connecting plane between the spina nasalis anterior (bone tip at the anterior end of the crista nasalis of the palatal process of the maxilla) and the porus acusticus externus/opening of the external bony auditory canal (meatus acusticus externus) at the os temporale).
  • The height of the single or both wax walls is to be designed so that the patient has a so-called resting float of 2 to 3 mm: when the chewing muscles are relaxed, the teeth must not touch each other.
  • The centerline is drawn following the centerline of the nose.
  • The canine lines are drawn in line with the width of the nose.
  • The upper wax ridge should still be slightly visible below the upper lip when the mouth is slightly open and the upper lip is relaxed.
  • The smile line is an orientation for the future boundary between teeth and gingiva (gums).

VII. jaw relation determination (ZA).

In the same treatment session, an intraoral support post registration is made: Using a metal pin mounted on bite templates, the patient actively records mandibular movements within the mouth on a color-coated metal plate. In the case of temporomandibular joints without movement restrictions, this results in a so-called arrow angle. The bite templates of the upper and lower jaw are keyed together in a specific position resulting from the arrow angle. In this way, the three-dimensional positional relationship of the jaws to each other can be transferred to the laboratory. VIII. Selection of the anterior teeth (ZA/LAB)

The color and shape of the anterior teeth to be fabricated should always be selected in cooperation with the patient, because otherwise it will be difficult for the patient to accept a prosthesis whose esthetics do not match his or her expectations. The length and width of the teeth must be based on the previously determined parameters such as the midline, smile line and canine line. IX. secondary crown fabrication and wax-up (LAB)

  • Fabrication of the abutments on the primary crowns – first modeling in wax, followed by conversion into a cast secondary crown. This is usually a veneer crown, which is anchored in the denture resin with cast side arms.
  • Placement of the denture teeth in wax, with the dental arch corresponding to the individualized wax wall.
  • Incorporating the secondary crowns into the wax-up, implementing the resilience margin between primary and secondary crowns.

X. Wax try-in (ZA)

On the patient, a try-in of the future prosthesis in wax is now performed. Since the denture teeth are on a wax base, position corrections can still be made. XI. Finalization (LAB)

After the dentist and patient have determined the final position of the anterior and posterior teeth, the denture is finished. Before the denture is pressed in acrylic, the dental technician ensures even better suction adhesion for future maxillary dentures by means of an “artifice”: an approx. 2 mm wide, max. 1 mm deep line is etched (abraded) on the master cast, which lies at the transition of the hard palate to the soft palate: The dorsal dam of the future prosthesis displaces soft tissue and prevents air from penetrating under the prosthesis while the soft palate moves during speech. The prosthesis material is polymethyl methacrylate (PMMA)-based plastic. The denture is manufactured under pressure and heating to achieve the highest possible degree of polymerization or the lowest possible residual monomer content (monomer: individual components from which larger macromolecular compounds, the polymers, are formed by chemical combination). XII. Incorporation of the finished prosthesis (ZA).

  • The finished placed denture is tried in to the patient, and corrections to margins, occlusion (final bite), and articulation (chewing movements) may be required.
  • Attaching the primary crowns – The denture base (underside) and secondary crowns are coated with petroleum jelly to insulate them from the luting cement. The prepared teeth are cleaned and dried, the primary crowns are spread thinly on the inside with e.g. zinc phosphate cement and then placed on the teeth under pressure. Pressed-out excess cement is immediately removed with foam pellets. The denture is placed in the mouth over the primary crowns. During cementing, tiny cotton pellets or a metal foil are required between the primary and secondary crowns to fill the resilience space so that pressure can be applied to the primary crowns during cement hardening.
  • After the cement has set, the prosthesis is removed and checked for cement residue. The first removal can also be done with a few hours interval at an additional appointment.
  • The patient is given care recommendations for the new prosthesis.
  • Insertion and removal of the prosthesis is practiced with the patient.

XIII Follow-up (ZA).

The patient is given a short-term appointment to check for possible pressure points, as well as a recommendation for regular reappearance at the recommended interval, which is based on the state of oral health.

After the procedure

The condition of the denture and the denture bed (hard and soft tissues on which the denture is supported in the mouth), which may be subject to constant change, should be checked at six-month intervals. Timely relining of the denture can prevent damage to the tissue (e.g., pressure points or bone resorption), as well as damage to the denture (e.g., fatigue cracks or denture fracture).

Possible complications

  • Pressure points in the vestibular area of the teeth (vestibulum: space between denture and lips or cheeks).
  • Pressure points in the marginal gingiva area (the gums surrounding the teeth), if not enough space was created in the acrylic base for the gingival bulge or the gingiva is inflamed edematous (swollen) due to lack of dental care.
  • Lack of dental care leads to premature loss of the already periodontally (related to the periodontium) pre-damaged residual tooth stock.
  • Premature denture fracture – The patient is strongly advised to let water into the hand basin before cleaning the denture, so that it lands gently in the water if it falls out of the hand during cleaning.