American Board of Oral and Maxillofacial Pathology (ABOMP) Fellowship Examination

Correct: Y-TZP zirconia exhibits a unique property known as transformation toughening. When a crack begins to form, the stress at the crack tip triggers a phase transformation from the tetragonal to the monoclinic state. This transformation is accompanied by a 3-5% volume increase, which creates compressive stresses at the crack tip, effectively pinning the crack and preventing further propagation. This makes it the material of choice for posterior FPDs in high-load areas or in patients with parafunctional habits.

Incorrect: While lithium disilicate offers excellent bonding capabilities due to its glass content, its fracture toughness (approximately 2.5-3.5 MPa·m½) is significantly lower than that of Y-TZP (5-10 MPa·m½), making it less suitable for posterior FPD spans in bruxing patients. Zirconia actually has a high modulus of elasticity, meaning it is very rigid, not flexible. Thermal expansion coefficients are relevant when layering porcelain, but in a monolithic restoration, the primary concern is the bulk mechanical strength and resistance to fracture rather than veneering compatibility.

Takeaway: Transformation toughening is the fundamental mechanical property that allows monolithic zirconia to withstand the high tensile stresses encountered in posterior fixed partial denture connectors.

Correct: In distal extension RPDs (Kennedy Class I and II), the support is derived from both the abutment teeth and the residual ridge. Acrylic resin teeth are preferred over porcelain because they are more resilient, absorb some of the occlusal impact, and do not cause abrasive wear on the opposing natural dentition. Narrowing the buccolingual width of the occlusal table is a standard clinical procedure to reduce the total occlusal load transmitted to the residual ridge, thereby minimizing bone resorption and improving the stability of the denture base.

Incorrect: Porcelain teeth are contraindicated when opposing natural dentition because their high hardness leads to significant wear of the natural enamel. Anatomical cusp heights increase lateral forces, which are destructive to the residual ridge. Placing teeth on the retromolar pad is incorrect because this tissue is displaceable and would lead to instability and soreness. Increasing the width or length of the occlusal table increases the total force applied to the ridge, which accelerates bone resorption rather than distributing it safely.

Takeaway: For distal extension RPDs, using acrylic teeth and a narrowed occlusal table minimizes destructive forces on the residual ridge and protects opposing natural teeth.

Correct: 3Y-TZP (yttria-stabilized tetragonal zirconia polycrystal) undergoes a process known as transformation toughening. When a crack begins to propagate, the localized stress triggers a phase transformation from the tetragonal phase to the monoclinic phase. This transformation is accompanied by a 3% to 5% volume expansion, which creates compressive stresses at the crack tip, effectively halting the crack’s progress and significantly increasing the material’s fracture toughness.

Incorrect: Increasing glass content is characteristic of glass-ceramics like lithium disilicate, which improves esthetics and bondability but reduces fracture toughness compared to polycrystalline ceramics. Leucite crystals are used to reinforce traditional feldspathic porcelains and manage thermal expansion in PFM systems, but they do not provide the high strength of zirconia. A lower modulus of elasticity would indicate a more flexible material; however, zirconia has a high modulus of elasticity (stiffness), which is necessary for structural stability in high-load areas.

Takeaway: Transformation toughening is the unique mechanical property that allows 3Y-TZP zirconia to resist fracture in high-stress posterior clinical scenarios.

Correct: A lingual plate is the indicated mandibular major connector when the floor of the mouth is shallow, specifically when there is less than 7-8 mm of vertical space between the gingival margins and the functional floor of the mouth. Since a lingual bar requires a minimum of 3-4 mm for the bar itself and a 3 mm clearance from the gingival margin to maintain periodontal health, a 5 mm sulcus depth is insufficient for a bar, making the plate the only viable rigid connector that avoids tissue impingement.

Incorrect: A lingual bar is contraindicated because it requires at least 7 mm of total vertical space (3 mm clearance + 4 mm bar height) to avoid periodontal irritation and provide rigidity. A continuous bar (Kennedy bar) still requires a primary major connector (usually a lingual bar) which would face the same space constraints. A labial bar is a secondary choice only used when extreme lingual inclination of the teeth prevents the use of any lingual connector, and it is generally less comfortable for the patient.

Takeaway: The selection of a mandibular major connector is primarily determined by the available vertical space in the lingual sulcus, with a lingual plate being the standard choice when that space is less than 7 mm.

Correct: Zirconia is a polycrystalline ceramic that lacks a glass phase, making it resistant to traditional acid etching with hydrofluoric acid. For non-retentive preparations where a chemical bond is necessary, the ‘APC’ protocol is recommended. This involves Air-particle abrasion (A) to increase surface area and energy, followed by a Primer (P) containing phosphate monomers like MDP which chemically bond to the metal oxides in zirconia, and finally the use of a Composite (C) resin cement. MDP is specifically effective because its phosphate group reacts with the zirconium oxide surface.

Incorrect: Hydrofluoric acid etching and silane are only effective for silica-based ceramics like lithium disilicate or feldspathic porcelain; they do not create a bond with zirconia. Phosphoric acid should never be used to clean zirconia because the phosphate ions in the acid will occupy the bonding sites on the zirconia surface, preventing the MDP primer from reacting and thus significantly weakening the bond. Polishing the internal surface is counterproductive as it reduces the surface area and mechanical interlocking provided by air-abrasion, which is necessary for a durable bond.

Takeaway: Reliable bonding to zirconia requires mechanical surface treatment via air-abrasion and chemical priming with a phosphate monomer (MDP) rather than traditional acid etching and silanation.

Correct: Lithium disilicate is a glass-ceramic that relies on specific thickness (typically 1.0mm to 1.5mm depending on the area) to maintain its structural integrity under occlusal loads. Communicating the preparation design and ensuring the technician has enough space to maintain these minimum dimensions is critical for preventing fractures. This aligns with risk assessment by identifying the primary failure mechanism (insufficient bulk) and mitigating it through clear technical communication.

Incorrect: Option B is incorrect because zirconia is a non-silica-based ceramic and is not etched with hydrofluoric acid; it requires air-particle abrasion for surface treatment. Option C is incorrect because CAD/CAM milling is a subtractive process that does not involve investment materials or a burnout cycle, which are associated with the lost-wax casting or press technique. Option D is incorrect because lithium disilicate crowns are typically used as monolithic or layered ceramic restorations, and a high-noble alloy substructure would describe a metal-ceramic (PFM) restoration, not a lithium disilicate restoration.

Takeaway: Clear communication of material-specific thickness requirements and preparation constraints is the most critical factor in preventing the mechanical failure of glass-ceramic restorations.

Correct: The current scientific consensus, supported by the biopsychosocial model, views TMD as a multifactorial disorder. It involves an interplay of predisposing factors (genetic, systemic, or psychological), initiating factors (macrotrauma or microtrauma), and perpetuating factors (stress, parafunction, or metabolic issues). The condition manifests when the cumulative effect of these factors overcomes the patient’s unique adaptive capacity, rather than being caused by a single mechanical or structural anomaly.

Incorrect: The theory that occlusal disharmony is the primary cause of TMD is largely unsupported by modern longitudinal studies, which show a weak correlation between specific occlusal features and the presence of TMD. Attributing the entire pathogenesis to disc displacement is overly simplistic, as many asymptomatic individuals have displaced discs and many TMD patients have myofascial pain without internal derangement. While skeletal morphology may play a minor role, it is not a primary predictor or the sole etiological driver of the disorder.

Takeaway: TMD is a complex, multifactorial condition best managed through a biopsychosocial framework that accounts for the interaction between physical stressors and the patient’s adaptive capacity.

Correct: In natural dental anatomy, the gingival zenith—the most apical point of the gingival margin—is not centered. For the maxillary central incisor, the zenith is typically located approximately 1 mm distal to the vertical long axis of the tooth. In contrast, the maxillary lateral incisor often exhibits a zenith that is more centrally located relative to its long axis. Adhering to these specific distal displacements is essential for creating a restoration that mimics the natural flow of the periodontium.

Incorrect: Positioning the zenith precisely on the vertical bisector for all teeth fails to account for the natural distal-leaning architecture of the gingiva, leading to an artificial appearance. Placing the zenith mesial to the long axis is anatomically incorrect as it contradicts the natural distal inclination of the tooth roots. Setting the lateral incisor zenith 2mm apical to the central incisor is clinically inappropriate; typically, the lateral incisor gingival margin is level with or slightly incisal (coronal) to the line connecting the zeniths of the central incisor and the canine.

Takeaway: Achieving natural gingival esthetics requires the distal displacement of the gingival zenith on maxillary central incisors while maintaining a more centered zenith on lateral incisors.

Correct: An osseointegrated implant lacks a periodontal ligament (PDL), which in a natural tooth provides a non-linear, biphasic movement pattern. Without the PDL, the implant is in direct contact with the alveolar bone, meaning any displacement is the result of the elastic deformation of the bone itself. This results in a linear load-displacement relationship and a total displacement of approximately 3-5 micrometers, which is significantly less than the 25-100 micrometers seen in natural teeth.

Incorrect: The biphasic displacement pattern is a specific characteristic of the PDL’s viscoelastic response, not the bone-implant interface. The 25-100 micrometer range describes the mobility of a natural tooth, whereas an implant is much more rigid. Cortical bone is significantly stiffer than the PDL and cannot provide an identical range of movement or shock-absorbing capacity.

Takeaway: Osseointegrated implants lack a periodontal ligament, resulting in a rigid, linear load-displacement relationship and significantly lower mobility compared to natural teeth.

Correct: Fracture toughness is the ability of a material containing a crack to resist further fracture, which is critical in implant-supported overdentures because the inclusion of attachment housings creates significant stress concentrations within the acrylic resin. High impact resistance ensures the material can withstand sudden functional forces without brittle failure, especially in areas where the denture base has been thinned to accommodate the restorative components.

Incorrect: Minimal residual monomer content is important for biocompatibility and preventing mucosal irritation, but it does not directly determine the structural resistance to fracture under heavy occlusal loads. A high coefficient of thermal expansion is generally undesirable in denture bases as it can lead to dimensional instability during temperature changes in the oral cavity. Increased surface hardness and wear resistance are beneficial for maintaining the polish and preventing the wear of the denture teeth, but they do not address the bulk structural integrity required to prevent midline or housing-related fractures.

Takeaway: Fracture toughness is the primary material property required to mitigate stress concentrations caused by attachment housings and prevent structural failure in implant-supported overdentures.