📖 Prosthodontics
Finish lines
ProsthodonticsFinish lines are the marginal configurations at the
interface between a restoration and the tooth structure that are intended to be
refined and polished to a smooth contour. In prosthodontics, they are crucial
for the proper adaptation and seating of restorations, as well as for
maintaining the health of the surrounding soft and hard tissues. Finish lines
can be classified in several ways, such as by their location, purpose, and the
burs used to create them. Here's an overview:
1. Classification by Width:
a. Narrow Finish Lines: These are typically 0.5mm wide or less
and are often used in areas where the restoration margin is tight against the
tooth structure, such as with metal-ceramic restorations or in cases with
minimal tooth preparation.
b. Moderate Finish Lines: These are 0.5-1.5mm wide and are
commonly used for most types of restorations, providing adequate space for a
good margin and seal.
c. Wide Finish Lines: These are 1.5mm wide or more and are
often used in areas with less than ideal tooth preparation or when a wider
margin is necessary for material manipulation or when there is a concern about
the stability of the restoration.
2. Classification by Location and Application:
a. Shoulder Finish Line: This finish line is at a 90-degree
angle to the tooth structure and is often used for metal-ceramic and all-ceramic
restorations. It provides good support and can be easily visualized and
finished.
b. Knife-Edge Finish Line: This is a very thin finish line that
is beveled at an approximately 45-degree angle to the tooth structure. It is
typically used for all-ceramic restorations and is designed to mimic the natural
tooth contour, providing excellent esthetics.
c. Feather Edge Finish Line: Also known as a chamfer, this
finish line is beveled at approximately 90-degrees to the tooth structure. It is
used in situations where the tooth structure is not ideal for a shoulder margin,
and it helps to distribute the forces evenly and reduce the risk of tooth
fracture.
d. Butt-Joint Finish Line: This is when the restoration margin
is placed directly against the tooth structure without any bevel. It is often
used in the lingual areas of anterior teeth and in situations where there is
minimal space for a margin.
3. Classification by Function:
a. Functional Finish Lines: These are placed where the restoration will be
subject to significant occlusal or functional stresses. They are designed to
enhance the durability of the restoration and are usually placed at or slightly
below the height of the free gingival margin.
b. Esthetic Finish Lines: These are placed to achieve a high level of cosmetic
appeal and are often located in the facial or incisal areas of anterior teeth.
They are typically knife-edge margins that are highly polished.
Advantages and Disadvantages:
- Narrow finish lines can be more challenging to clean and may be less visible,
potentially leading to better esthetics and less irritation of the surrounding
tissues. However, they may also increase the risk of recurrent decay and are
more difficult to achieve a good margin seal with.
- Moderate finish lines are easier to clean and provide a better margin seal,
but may be more visible and can potentially lead to increased tooth sensitivity.
- Wide finish lines are more forgiving for marginal adaptation and are easier to
clean, but they can be less esthetic and may require more tooth reduction.
Burs Used:
- The choice of bur for creating finish lines depends on the restoration
material and the desired margin design. For example:
a. Diamond Burs: Typically used for creating finish lines on natural tooth
structures, especially for knife-edge margins on ceramic restorations, due to
their ability to produce a smooth and precise finish.
b. Carbide Burs: Often used for metal-ceramic restorations, as they are less
likely to chip the ceramic material.
c. Zirconia-Specific Burs: Used for zirconia restorations to prevent chipping or
fracture of the zirconia material.
When creating finish lines, the dentist must consider the patient's oral health,
the type of restoration, the location in the mouth, and the desired functional
and esthetic outcomes. The correct selection and preparation of the finish line
are essential for the longevity and success of the restoration.
Anatomy of Maxilary Edentulous Ridge
Prosthodontics
Anatomy of Maxilary Edentulous Ridge
LIMITING STRUCTURES
A) Labial & buccal frenum
- Fibrous band covered by mucous membrane.
- A v-shaped notch (labial notch) should be provided very carefully which should be narrow but deep enough to avoid interference
- Buccal frenum has the attachment of following muscles; levator anguli
- It needs greater clearance on buccal flange of the denture (shallower and wider) than the labial frenum.
B) Labial & buccal vestibule (sulcus)
- Labial sulcus is bounded on one side by the teeth, gingiva and residual alveolar ridge and on the outer side by lips.
- Buccal sulcus extends from buccal frenum anteriorly to the hamular notch posteriorly.
- The size of the vestibule is dependant upon:
i) Contraction of buccinator muscle.
ii) Position of the mandible.
iii) Amount of bone loss in maxilla.
C) Hamular notch
It is depression situated between the maxillary tuberosity and the hamulus of the medial pterygoid plate. It is a soft area of loose connective tissue.
- it houses the disto-lateral termination of the denture.
- Aids in achieving posterior palatal seal.
- Overextension causes soreness.
- Underextension poor retention
D) Posterior palatal seal area (post-dam)
It is a soft tissue area at or beyond the junction of the hard and soft palates on which pressure within physiological limits can be applied by a complete denture to aid in its retention.
Extensions:
1. Anteriorly – Anterior vibrating line
2. Posteriorly – Posterior vibrating line
3. Laterally – 3-4 mm anterolateral to hamular notch
SUPPORTING STRUCTURES
A) Primary stress bearing area / Supporting area
1. Posterior part of the palate
2. Posterolateral part of the residual alveolar ridge
B) Secondary stress bearing area / Supporting area
1. The palatal rugae area
2. Maxillary tuberosity
RELIEF AREAS
A) Incisive papilla
- Midline structure situated behind the central incisors.
- It is an exit point of nasopalatine nerves and vessels.
- It should be relieved if not, the denture will compress the nerve or vessels and lead to necrosis of the distributing areas and paresthesia of anterior palate.
B) Mid-palatine raphe
- Extends from incisive papilla to distal end of hard palate.
- Median suture area covered by thin submucosa
- Relief is to be provided as it is supposed to be the most sensitive part of the palate to pressure
C) Crest of the residual alveolar ridge
D) Fovea palatinae
Few areas like the cuspid eminence , fovea palatinae and torus palatinus may be relieved according to condition required.
Impression Making Techniques
ProsthodonticsImpression making is a critical step in prosthodontics and orthodontics, as it captures the details of the oral cavity for the fabrication of dental prostheses. There are several techniques for making impressions, each with its own principles and applications. Here, we will discuss three primary impression-making techniques: Mucostatic, Mucocompressive, and Selective Pressure Impression Techniques.
1. Mucostatic or Passive Impression Technique
- Proposed by: Richardson and Henry Page
- Materials Used: Plaster of Paris and Alginate
- Key Features:
- Relaxed Condition: Records the oral mucous membrane and jaws in a normal, relaxed condition.
- Tray Design: Utilizes an oversized tray to accommodate the relaxed tissues.
- Tissue Contact: Achieves intimate contact of the tissues with the denture base, which enhances stability.
- Peripheral Seal: This technique has a poor peripheral seal, which can affect retention.
- Outcome: The resulting denture will have good stability but poor retention due to the lack of a proper seal.
2. Mucocompressive Impression Technique
- Proposed by: Carole Jones
- Materials Used: Impression compound and Zinc Oxide Eugenol (ZoE)
- Key Features:
- Functional Recording: Records the oral tissues in a functional and displaced form, capturing the active state of the tissues.
- Retention: Provides good retention due to the compression of the tissues during the impression process.
- Displacement Issues: Dentures made using this technique may tend to get displaced due to tissue rebound when the tissues return to their resting state after the impression is taken.
3. Selective Pressure Impression Technique
- Proposed by: Boucher
- Materials Used: Special tray with Zinc Oxide Eugenol (ZoE) wash impression
- Key Features:
- Stress Distribution: Loads acting on the denture are transmitted to the stress-bearing areas of the oral tissues.
- Tray Design: A special tray is designed such that the tissues contacted by the tray are recorded under pressure, while the tissues not contacted by the tray are recorded in a state of rest.
- Balanced Recording: This technique allows for a more balanced impression, capturing both the functional and relaxed states of the oral tissues.
Kennedy's Classification of Edentulous Arches
ProsthodonticsKennedy's Classification is a system used in dentistry to categorize the
edentulous spaces (areas without teeth) in the mouth of a patient who is fully
or partially edentulous. This classification system helps in planning the
treatment, designing the dentures, and predicting the outcomes of denture
therapy. It was developed by Dr. Edward Kennedy in 1925 and is widely used by
dental professionals.
The classification is based on the relationship between the remaining teeth, the
residual alveolar ridge, and the movable tissues of the oral cavity. It is
particularly useful for patients who are wearing or will be wearing complete or
partial dentures.
There are four main classes of Kennedy's Classification:
1. Class I: In this class, the patient has a bilateral edentulous area with no
remaining teeth on either side of the arch. This means that the patient has a
full denture on the upper and lower jaws with no natural tooth support.
2. Class II: The patient has a unilateral edentulous area with natural teeth
remaining only on one side of the arch. This could be either the upper or lower
jaw. The edentulous side has a complete denture that is supported by the teeth
on the opposite side and the buccal (cheek) and lingual (tongue) tissues.
3. Class III: There is a unilateral edentulous area with natural teeth remaining
on both sides of the arch, but the edentulous area does not include the anterior
(front) teeth. This means the patient has a partial denture on one side of the
arch, with the rest of the teeth acting as support for the denture.
4. Class IV: The patient has a unilateral edentulous area with natural teeth
remaining only on the anterior region of the edentulous side. The posterior
(back) section of the same side is missing, and there may or may not be teeth on
the opposite side. This situation requires careful consideration for the design
of the partial denture to ensure stability and retention.
Each class is further divided into subcategories (A, B, and C) to account for
variations in the amount of remaining bone support and the presence or absence
of undercuts, which are areas where the bone curves inward and can affect the
stability of the denture.
- Class I (A, B, C): Variations in the amount of bone support and presence of
undercuts in the fully edentulous arches.
- Class II (A, B, C): Variations in the amount of bone support and presence of
undercuts in the edentulous area with natural teeth on the opposite side.
- Class III (A, B, C): Variations in the amount of bone support and presence of
undercuts in the edentulous area with natural teeth on the same side, but not in
the anterior region.
- Class IV (A, B, C): Variations in the amount of bone support and presence of
undercuts in the edentulous area with natural teeth remaining only in the
anterior region of the edentulous side.
Understanding a patient's Kennedy's Classification helps dentists and dental
technicians to create well-fitting and functional dentures, which are crucial
for the patient's comfort, speech, chewing ability, and overall oral health.