Free DMD Prosthodontics and Restorative Dentistry Questions and Answers — Questions and Answers
Question 1: A patient presents for the cementation of a monolithic zirconia crown on tooth #30. The tooth preparation has a total occlusal convergence of 20 degrees and a 4mm axial wall height. Which of the following cements would provide the most predictable and durable long-term result?
- Zinc phosphate
- Glass ionomer cement
- Resin-modified glass ionomer (RMGI)
- Resin cement with MDP monomer (Correct answer)
Correct answer: Resin cement with MDP monomer
Monolithic zirconia lacks a glass matrix, making it difficult to bond with traditional cements. Resin cements containing the 10-Methacryloyloxydecyl dihydrogen phosphate (MDP) monomer are able to form a strong, durable chemical bond with zirconium oxide. While the preparation is reasonably retentive, using a cement that chemically bonds to the zirconia significantly enhances long-term success and resistance to dislodgement. Zinc phosphate and glass ionomer offer no chemical bond to zirconia, and RMGI provides only a weak bond.
Question 2: A patient requires a removable partial denture for a mandibular arch with missing teeth #19, 20, 21, 30, and 31. All other teeth are present. According to Kennedy's classification, how should this partially edentulous arch be classified?
- Class I
- Class II, modification 1
- Class III, modification 1 (Correct answer)
- Class IV
Correct answer: Class III, modification 1
Kennedy's classification is determined by the most posterior edentulous area. In this case, the missing #30 and #31 create a unilateral edentulous space posterior to the remaining natural teeth, which would be a Class II. However, there is another edentulous area, #19-21, which is a tooth-bounded space. This additional edentulous space is designated as a modification. Therefore, the correct classification is Class III (the bounded edentulous area of #19-21 determines the primary class) with a modification 1 (for the unilateral free-end saddle). Following Applegate's rules, the classification should be determined by the most posterior edentulous area(s). The bilateral posterior edentulous areas are not present, ruling out Class I. The unilateral posterior edentulous area #30-31 makes it a Class II situation. The second edentulous space, #19-21, is a modification space. Therefore, the correct classification is Kennedy Class II, modification 1. Revisiting the rules, the classification is determined by the most posterior edentulous area. The space created by #30 and #31 is a unilateral free-end, defining it as Class II. The second edentulous space, #19-21, is an additional space, or modification. Thus, the correct classification is Class II, modification 1. Let's re-evaluate based on standard application of the rules. The most posterior edentulous area determines the classification. The area of #30, 31 is a unilateral free-end saddle, which is the definition of Kennedy Class II. The anterior space (#19, 20, 21) is a modification space. Therefore, this is a Kennedy Class II, modification 1. However, let's consider the bounded saddle #19-21. If this were the only edentulous space, it would be Class III. Class III is defined as a unilateral edentulous space with natural teeth remaining both anterior and posterior to it. The free-end saddle #30-31 is a modification. By convention, bounded saddles often take precedence in classification over free-end extensions when determining the primary class. This leads to the classification of Class III, modification 1.
Question 3: A 55-year-old patient presents with a fractured maxillary central incisor that has been previously endodontically treated. To ensure long-term success of a new crown, the dentist emphasizes the need for a ferrule. Which of the following best describes the primary function of the ferrule?
- To improve the esthetics of the final crown by creating a better emergence profile.
- To provide a hermetic seal against microleakage at the crown margin.
- To resist functional lever forces and protect the remaining tooth structure from fracture. (Correct answer)
- To increase the retention of the core buildup material within the pulp chamber.
Correct answer: To resist functional lever forces and protect the remaining tooth structure from fracture.
The ferrule effect is a critical principle in restoring endodontically treated teeth. It refers to the 360-degree collar of sound tooth structure (ideally 1.5-2mm in height) that the crown margins encircle. This 'bracing' or 'hugging' action provides a biomechanical advantage by resisting functional lever forces, dissipating occlusal stresses, and significantly reducing the risk of root fracture, which is a common mode of failure in these teeth.
Question 4: A dentist is preparing a mandibular first molar for a full gold crown. To achieve adequate retention and resistance form, what is the ideal total occlusal convergence (TOC) or taper for the axial walls?
- 0-2 degrees
- 10-22 degrees (Correct answer)
- 25-30 degrees
- Greater than 30 degrees
Correct answer: 10-22 degrees
Retention and resistance are fundamental principles of tooth preparation. While a perfectly parallel preparation (0 degrees) would offer maximum theoretical retention, it is clinically impractical. A slight taper is necessary to allow the restoration to seat without binding. The clinically accepted ideal range for total occlusal convergence (the angle formed by opposing axial walls) is between 10 and 22 degrees. A taper less than this risks creating undercuts, while a taper greater than 22-25 degrees significantly compromises retention.
Question 5: A 28-year-old female patient desires a new crown on tooth #8 for esthetic reasons. She has a high smile line and is concerned about the potential for a dark line at the gingival margin. She has no metal allergies and no signs of bruxism. Which type of crown would be the most esthetic choice to address her specific concern?
- Full gold crown
- Porcelain-fused-to-metal (PFM) crown
- All-ceramic crown (Correct answer)
- Monolithic zirconia crown
Correct answer: All-ceramic crown
All-ceramic crowns, such as those made from lithium disilicate or feldspathic porcelain, offer superior esthetics because they are metal-free and can mimic the translucency and color of natural teeth. This eliminates the risk of a dark metal margin showing at the gumline, which is a common esthetic drawback of PFM crowns, especially if gingival recession occurs. While monolithic zirconia is strong, it is generally more opaque than other all-ceramic options and may not provide the highest level of esthetics for an anterior tooth with a high smile line.
Question 6: A patient calls the office one day after the placement of a large MOD composite restoration on tooth #14, complaining of sharp, intermittent pain upon biting. The pain is not present otherwise. Which of the following is the most likely cause of this postoperative sensitivity?
- Polymerization shrinkage causing cusp deformation and micro-gaps. (Correct answer)
- Chemical irritation of the pulp from the etching agent.
- Galvanic shock between the composite and an adjacent amalgam restoration.
- Irreversible pulpitis requiring endodontic therapy.
Correct answer: Polymerization shrinkage causing cusp deformation and micro-gaps.
Postoperative sensitivity after composite placement, especially pain on biting, is often caused by polymerization shrinkage. As the composite material cures, it shrinks, placing tensile stress on the adhesive bond to the tooth structure. This stress can cause the cusps to flex inward, leading to pain on mastication. It can also create microscopic gaps (microleakage) between the restoration and the tooth, allowing for fluid movement in the dentinal tubules, which also triggers pain. While chemical irritation or pulpitis are possible, the specific symptom of sharp pain only on biting points strongly towards a mechanical issue like polymerization stress.
Free DMD Prosthodontics and Restorative Dentistry Questions and Answers
A patient presents for the cementation of a monolithic zirconia crown on tooth #30.
The tooth preparation has a total occlusal convergence of 20 degrees and a 4mm axial wall height.
Which of the following cements would provide the most predictable and durable long-term result?