Mechanical properties of sintered hydroxyapatite and tricalcium phosphate ceramic.

The ultimate values for compressive strength, Young's modulus, and toughness of cylindrical specimens of unitary aspect ratios and uniform grain-size distributions were extrapolated for hydroxyapatite (HAP) to 70 MPa, 9.2 GPa, and 0.36 J cm(-3), and for tricalcium phosphate (TCP), to 315 MPa, 21 GPa, and 2.34 J cm(-3). For total volume porosities of 50%, the corresponding values were determined: for HAP, 9.3 MPa, 1.2 GPa, 0.042 J cm(-3), for TCP, 13 MPa, 1.6 GP, 0.077 J cm(-3). Porosities of HAP specimens ranged from 3%-50%; TCP from 10%-70%. Two pore-size distributions were employed. Exponential dependencies of the mechanical properties were found upon porosity (p < 0.0001). No differences in measured mechanical properties, as determined in compression, could be attributed to pore size. The superiority of TCP increases with density and suggests that a larger or more selective pore-size distribution could be effectively employed in TCP biological implants. This work also suggests the dominant role of secondary calcium phosphates in increasing compressive strengths.

Analysis of nickel-titanium versus stainless steel instrumentation by means of direct digital imaging.

This study compared step-back preparations in curved canals using nickel-titanium (Ni-Ti) K-files and stainless steel K-files. Forty canals in mesial roots of mandibular molars were embedded in casting resin and cross-sectioned at three levels: 1 to 2 mm from the apical foramen, middle of the curve, and coronal. Direct digital computer images were recorded before and after instrumentation. Superimposition of the images combined with digital subtraction computer software allowed direct measurement of area instrumented, distance of transportation, and shape analysis. The computer software calculated absolute center of gravity for each image analyzed to get a full 360-degree interpretation of the canal transport. Time of instrumentation was recorded. Results showed Ni-Ti files to cause significantly less transportation and remain more centered at the apical level (p < 0.05). Area removed by Ni-Ti and stainless steel files was not significantly different (p < 0.05). Time of instrumentation was not significantly different for Ni-Ti and stainless steel instruments (p < 0.05). Cross-sectional shape of the instrumented canal was not significantly different (p < 0.05).

Determination of the existence of hinge movements of the temporomandibular joint during normal opening by Cine-MRI and computer digital addition.

PURPOSE: The purpose of the first phase of this two-part investigation was to determine if the opening motion of the mandible could be illustrated and described using a dynamic imaging method. The purpose of the second phase of the investigation was to determine if a center of rotation would be discovered. MATERIALS AND METHODS: Five volunteer subjects, 2 female and 3 male, whose temporomandibular joints had previously been determined to be asymptomatic, were examined by magnetic resonance imaging (MRI) during opening from a standardized position. The serial static images were reconstructed by the MRI's computer in "cine mode" to simulate dynamic motion, similar to a motion picture. For the second phase, each patient's series of static images were digitally added and manipulated by a computer graphics program to locate the center of hinge motion. RESULTS: After reviewing the animated images recorded on videotape, three independent dentist observers confirmed that the opening movement of the mandible was initially rotational, followed by translation within the glenoid fossa. A center of rotation was calculated to be in the anatomic center of the condylar head of all of the subjects in this study. CONCLUSIONS: This study showed that opening dynamics of the mandibular condyle could be studied by cine-MRI and that an opening hinge axis appears to be located in the anatomic center of the condylar head.

Characterization of 86 bruxing patients with long-term study of their management with occlusal devices and other forms of therapy.

Of 353 patients screened, 86 answered positively to questions designed to identify bruxers. The bruxing patient profile revealed that 100% had working excursive interferences, 78% had balancing excursive interferences, and 95.4% had a premature contact into maximum intercuspation. Most of the bruxing patients had a chief complaint that related to pain, and 89.6% of the patients had a craniomandibular disorder. The patients were initially managed with an anterior deprogrammer and were later managed with other occlusal devices as signs and symptoms dictated. Definitive treatment was determined by the patient's maxillomandibular relationship.

A comparison of intracanal stresses in a post-restored tooth utilizing the finite element method.

The finite element method was used to compare stresses along the inner canal wall in four two-dimensional models of an average maxillary central incisor. The four models evaluated were an intact incisor, an endodontically treated incisor, an endodontically treated crown-restored incisor, and a cylindrical post and crown-restored incisor. A horizontal static force, 1 Newton in magnitude, was applied to the lingual surface of each model and the maximum tensile, compressive, and shear stresses were calculated using the general purpose finite element program PAFEC 75. Results indicate that the stress patterns within the root are altered as a result of post insertion. Specifically, the maximum bending stresses are associated with the apical termination of the post, and post placement does not result in a uniform distribution of stress along the canal wall.

Finite element analysis of six endosseous implants.

Stress magnitudes and contours in bone surrounding six endosteal post-type dental implants were calculated by using the finite element method. Comparisons were made by using Branemark, Core-Vent, Denar, Miter, Stryker, and experimental implant designs. Although certain assumptions were made that could be considered controversial, this study concluded that apical "punching stresses" with all of the implants were probably not clinically significant. Saucerization resulting from biomechanical overloads could be a possibility for three of the implants. Problems related to combinations of overloads and underloads at the same time were suggested for several more popular implants in the United States. Additional research, combining 3-D finite element models and clinical studies, was recommended for all commercially available dental implants.

A finite element survey of eleven endosseous implants.

Eleven different post-type endosseous implants were biomechanically analyzed by use of the finite element method to compile a list of features that could be used to design an optimal post-type endosseous implant. Stress magnitudes and contours within each implant and within the surrounding bone were calculated. Implant features causing high stresses and low stresses, possibly contributing to pathologic bone resorption and bone atrophy were noted. Although this preliminary survey was not complete, tentative suggestions in implant design for improved implant performance were made.

Finite element analysis of bone-adapted and bone-bonded endosseous implants.

The use of bioactive coatings on endosseous implants to induce bone bonding to the implants has become popular in recent years. The actual benefit from these coatings, however, remains controversial. This study compared three endosseous implants by using finite element analysis to determine whether bone-bonding or bone-adaptation (osseo-integration) was biomechanically more beneficial. Results indicated that although a bonded interface between an implant and its host tissues may be biochemically beneficial, bone bonding, by any means, may not be biomechanically beneficial to the implant or the surrounding bone. Neither clinicians nor manufacturers should assume that bioactive coatings or bone-bonding in general improve the biomechanical prognoses of endosteal postdental implants.

Alternative materials for three endosseous implants.

Three endosseous post-type implant geometries were evaluated: a serrated solid with a 2-degree taper and a rectangular cross section, a cylindrical screw-type solid, and a finned solid with a 1-degree 9' taper and a circular cross section. Each implant geometry was analyzed with 10 different moduli of elasticity. Stress contour plots were used to identify which implant material was best suited to each implant geometry. Careful examination of all of the contour plots showed that, for all geometries, increasing the material stiffness transmitted more of the occlusal load to the apical bone. These plots further suggested that an implant material can be too stiff as the punching stresses increase at the apex of the implant. Of the three endosseous implants analyzed, only the finned solid type seemed to be made of the proper material, titanium alloy. The screw-type implant, made of sapphire, should be made of aluminum or possibly titanium. The serrated implant, made of polycrystalline alumina, was too stiff. An implant's elastic behavior is not the only governing factor. An implant's geometry seems to be the determining factor in properly distributing stresses from the implant to the bone.

Submerged alumina dental root implants in humans: five-year evaluation.

This article describes the placement, success rate, and evaluation of an experimental alumina (Al2O3) ceramic dental implant at a mean placement time of 5 years. The study compared the longevity of a submerged implant of the same root design and a refined surgical technique with previously reported studies of the implant exposed to the human oral environment during initial healing. A 54% success rate was achieved for 29 implants placed in the maxillae and mandible, all restored with cast gold dental crowns. A significant difference (p = 0.05) appeared between success and failures at 7.8 months following placement. Crown/root ratio slopes were an earlier and more sensitive indicator of potential success or failure than the bone height slopes. Computer analysis revealed no apparent correlation between implant mobility and/or pocket depths and implant longevity. Delayed implant root fractures and a cast post fracture occurred in 20% of the implants that survived.

Bone stress distribution for three endosseous implants.

Axisymmetric finite element models of three geometries were evaluated: a serrated solid with a 2-degree taper and a rectangular cross section; a cylindrical screw-type solid; and a finned solid with a 1 degree 9' taper and a circular cross section. Ten moduli of elasticity ranging from 0.348 to 74.96 psi x 10(6) were used for each geometry. Contour plots of the resulting Von Mises stresses were used to study the changing stress distribution patterns within the surrounding cortical bone. The results indicated that the serrated geometry led to high-stress concentrations at the tips of the bony ingrowth and near the neck of the implant. Low moduli of elasticity emphasized these concentrations. The nontapered screw-type geometry had high-stress concentrations at the base of the implant when high moduli were studied and at the neck of the implant when low moduli were studied. The conclusion of this study was that a tapered endosseous implant with a high elastic modulus would be most suitable for dental implantology. However, the design must not cause high-stress concentrations at the implant neck that commonly cause bone resorption.

A precision alignment frame for endosseous post dental implants.

The advantages of the precision alignment frame are that it (1) accurately positions and aligns the initial pilot drill at implant surgery, (2) has no adverse effects on the diagnostic cast, (3) eliminates the need for an incision or raising a flap when locating a submerged implant root for transmucosal connection, (4) consistently and accurately records the relation of the implant root to the radiograph, (5) permits long-term evaluation of the implant after restoration, (6) has components that are readily obtainable and inexpensive, and (7) is easily made. With a PAF, accurate placement of an implant can be routinely achieved without drill alignment changes. This is especially important when multiple implants are being placed to support a long framework. In most implant sites, the PAF will record the entire image of the implant on the radiograph. However, patients with advanced alveolar bone resorption, flat or narrow vaults in the premaxilla, or high muscle attachments to the mandible may yield only a coronal image of the implant root on the radiograph. In those patients, the PAF is valuable in evaluating early bone healing and remodeling at the permucosal and neck areas.