A Bilayer Method for Measuring Toughness and Strength of Dental Ceramics.

The ever-increasing usage of ceramic materials in restorative dentistry necessitates a simple and effective method to evaluate flexural strength σF and fracture toughness KC. We propose a novel method to determine these quantities using a bilayer specimen composed of a brittle plate adhesively bonded onto a transparent polycarbonate substrate. When this bilayer structure is placed under spherical indentation, tunneling radial cracks initiate and propagate in the lower surface of the brittle layer. The failure analysis is based on previous theoretical relationships, which correlate σF with the indentation force P and layer thickness d, and KC with P and mean length of radial cracks. This work examines the accuracy and limitations of this approach using a wide range of contemporary dental ceramic materials. The effect of layer thickness, indenter radius, load level, and length and number of radial cracks are carefully examined. The accuracy of the predicted σF and KC is similar to those obtained with other concurrent test methods, such as biaxial flexure and 3-point bending (σF), and bending specimens with crack-initiation flaws (KC). The benefits of the present approach include treatment for small and thin plates, elimination of the need to introduce a precrack, and avoidance of dealing with local material nonlinearity effects for the KC measurements. Finally, the bilayer configuration resembles occlusal loading of a ceramic restoration (brittle layer) bonded to a posterior tooth (compliant substrate).

A Critical Review of Dental Lithia-Based Glass-Ceramics.

The purpose of this article is to review current understanding of lithia-based glass-ceramics and to identify future research needs for this class of dental materials in relation to novel compositions and fabrication methods. With rapid advances in material development and digital technology, time efficiency of dental workflow and fit accuracy of ceramic restorations are ever improving. Lithia-based glass-ceramics are at the forefront of this advance-new variants with more efficient fabrication routes are continually being introduced into the marketplace. Base glass composition, crystallization heat treatment, nucleant and coloration additives, and property gradation are some pertinent variables. The trend in fabrication is to move from CAD/CAM grinding of partially crystallized glass-ceramics to fully crystallized materials, thereby circumventing the need for postmachining firing altogether. In these endeavors, a better understanding of mechanical properties and evolving shaping technologies, such as ductile grinding, is paramount. Additive manufacturing and 3-dimensional printing methodologies offer a promising alternative to current CAD/CAM subtractive manufacturing routes. Challenges to the implementation of new technologies in efficient development and production of high-quality dental glass-ceramic prostheses are addressed.

Exploring Ductility in Dental Ceramics.

Two damage regimes-"brittle" and "ductile"-have been identified in the literature on ceramic grinding, machining, grit blasting, and wear. In the brittle regime, the damage mechanism is essentially crack formation, while in the ductile region, it is quasiplasticity. Onset of the brittle mode poses the greater threat to strength, so it becomes important to understand the mechanics of ductile-brittle thresholds in these materials. Controlled microcontact tests with a sharp indenter are employed to establish such thresholds for a suite of contemporary computer-aided design/computer-aided manufacturing dental ceramics. Plots of flexural strength S versus indentation load P show a steep decline beyond the threshold, consistent with well-established contact mechanics relations. Threshold dimensions occur on a scale of order 1 µm and contact load of order 1 N, values pertinent to practical grit finishing protocols. The ductile side of ceramic shaping is accessed by reducing grit sizes, applied loads, and depths of cut below critical levels. It is advocated that critical conditions for ductile shaping may be most readily quantified on analogous S(P) plots, but with appropriate machining variable (grit size, depths of cut, infeed rate) replacing load P. Working in the ductile region offers the promise of compelling time and cost economies in prosthesis fabrication and preparation.

Kocher-Debre-Semelaigne syndrome: hypothyroidism with muscle pseudohypertrophy.

Kocher-Debre-Semelaigne syndrome is a rare association of muscular pseudohypertrophy and hypothyroidism in children. We report two cases of this syndrome in this communication. The first case was a seven-years-old female who presented with features of hypothyroidism and muscle pseudohypertrophy. The second child had similar manifestations but was only fifteen months of age at diagnosis. This is one of the youngest patients reported to have the Kocher-Debre-Semelaigne syndrome. A short review of the literature is also presented.

Neurological manifestations of HIV infection.

Case records of HIV infected patients were analyzed for identifying neurological manifestations. Eight patients (7 males) were identified to have probable HIV encephalopathy (in a period of 24 months) as per the CDC revised classification system. Their ages ranged from one year to ten years. The neurological manifestations noted included-developmental delay (2 cases), seizures (6 cases), acute onset alteration of sensorium (4 cases), aphasia (2 cases), loss of vision (2 cases), focal neurological deficits (6 cases), brisk deep tendon reflexes (7 cases), extensor plantar responses (5 cases) and signs of cerebellar dysfunction (2 cases). Other clinical features included growth failure, microcephaly, fever, lymphadenopathy, hepatomegaly, splenomegaly, pneumonia, otorrhea and oral candidiasis. Cerebrospinal fluid studies were normal. The neuroimaging features included cerebral atrophy and ventricular dilatation, cerebral infarction, basal ganglia calcification and cerebellar atrophy. Childhood HIV infection may have a variety of neurological abnormalities. HIV infection should be suspected in children presenting with unexplained neurological manifestations and growth failure.