Biomedical subjects

J S Hodges

Publications and source records attributed to J S Hodges.

At least 19 recordsLinked to original sources

PubMed2003

A clinical pilot study of the time-dependent composition of tooth bleaching systems.

This pilot study was undertaken to determine the compositional changes in tooth bleaching materials as a function of time in vivo. Ten patients were recruited and two bleaching systems were used - one a paste and the other a gel. Each material was placed in a custom bleaching tray and worn by each patient for each of four times - 15, 30, 60 and 120 min. The material was collected and chemically analysed for water by Karl Fischer titration and titrated for carbamide peroxide by the US Pharmacopoeia method. The paste material contained 18.66% water as supplied, and after 2 h this rose to between 28.6 and 64.4%. The gel material contained 2.85% water as supplied, and after 2 h this was diluted to between 28.5 and 73.4%. There was considerable difference in saliva uptake by the custom tray between patients. Most water uptake usually occurred within the first 30 min. Peroxide concentrations decreased in an approximately linear manner with time. There was a significant difference between the materials from baseline to 30 min and thereafter (P < 0.0009). This pilot study is an effective technique for chemical evaluation of bleaching materials. The effect of saliva is an important factor to consider, and is one that has hitherto not always been appropriately emphasized.

Analysis of Variance↗

PubMed2003

Accuracy of a system for creating 3D computer models of dental arches.

Three-dimensional imaging of dental tissues will have a major impact in dentistry if the images are accurate. The purpose of this study was to measure the accuracy and precision of a system for creating three-dimensional images of dental arches. Using vinyl polysiloxane impression materials and improved dental stone, we made 10 stone casts of a "dental" standard with known dimensions. The impressions and casts were scanned by means of a Comet 100 optical scanner. Custom software created three-dimensional images (computer models) from the scanned data. Accuracy was defined as the average of the absolute differences between the computer models and the standard. Precision was the standard deviation of accuracy over 10 repeated measures. Software processing improved the accuracy of the scanner data. Accuracy +/- precision for the casts and impressions was 0.024 +/- 0.002 mm and 0.013 +/- 0.003 mm, respectively. The system produced computer models with sufficient accuracy for clinical application.

Algorithms↗

PubMed2003

Effects of implant healing time on crestal bone loss of a controlled-load dental implant.

The universally accepted concept of delay-loaded dental implants has recently been challenged. This study hypothesizes that early loading (decreased implant healing time) leads to increased bone formation and decreased crestal bone loss. We used 17 minipigs to study implants under a controlled load, with non-loaded implants for comparison. Radiographic and histological assessments were made of the osseointegrated bone changes for 3 healing times (between implant insertion and loading), following 5 months of loading. The effect of loading on crestal bone loss depended on the healing time. Early loading preserved the most crestal bone. Delayed loading had significantly more crestal bone loss compared with the non-loaded controls (2.4 mm vs. 0.64 mm; P < 0.05). The histological assessment and biomechanical analyses of the healing bone suggested that loading and bioactivities of osteoblasts exert a synergistic effect on osseointegration that is likely to support the hypothesis that early loading produces more favorable osseointegration.

Alveolar Bone Loss↗

PubMed2002

New organosilicon maxillofacial prosthetic materials.

OBJECTIVES: The silicone elastomer A-2186 is a widely used maxillofacial prosthetic material. It is a pourable two-component silicone rubber cured by a platinum catalyst. Used as a prosthetic material, A-2186 has short working time and because of its hydrophobic nature, poor adhesion to non-silicone based adhesives. The purpose of this study is to evaluate the physical properties of new prosthetic materials based on methacryloxypropyl-terminated polydimethylsiloxane (MPDS-MF), and to compare the properties with those of A-2186. METHODS: Hardness, tensile strength, ultimate elongation, tear strength and adhesive bonding strength of MPDS-MF and A-2186 with and without additives were determined and compared. The bonding strengths of the extrinsic colorant carrier with the prosthetic materials were also determined. Statistical analyses were done using a two-way analysis of variance (ANOVA). For significant effects, post-hoc tests were done using the Bonferroni correction. RESULTS: The hardness of MPDS-MF is similar to A-2186. However, tensile strength, tear strength, ultimate elongation, and adhesive bonding strength of MPDS-MF are higher than those of A-2186. SIGNIFICANCE: MPDS-MF is cured by free radical thermal polymerization and crosslinking. The working time of MPDS-MF, unlike A-2186, is long. The presence of methacrylate groups in MPDS-MF enhances its adhesion to non-silicone based adhesive. Based on the present study, it appears that MPDS-MF is suitable for use in fabricating of clinical prostheses.

Adhesives↗

PubMed2002

Helical axis errors affect computer-generated occlusal contacts.

A helical axis describing mandibular motion can be calculated from two distinct positions of the mandible; however, as these positions come closer together, calculation errors increase. This study investigated the effects of errors in the calculated helical axis on simulated mandibular motion by the measurement of changes in occlusal contacts. A standard helical axis was calculated from a simulated lateral movement. A series of digital interocclusal records from centric to a 5 degree mandibular rotation about the standard helical axis was created. Digital dental cast models were aligned to the interocclusal records. Helical axis parameters and occlusal contacts calculated with the use of the aligned digital models were compared with those of the standard. Helical axes calculated from mandibular positions separated by 1.5 degrees to 5.0 degrees yielded equivalent occlusal contacts. Qualitatively, contacts for helical axes calculated from jaw rotations of 0.7 degrees or larger were nearly identical to those of the standard.

Computer Simulation↗

PubMed2001

Periodontal disease among indigenous people in the Amazon rain forest.

BACKGROUND: People are not all equally susceptible to periodontitis. To understand the epidemiology and natural history of this disease, it is important to study populations with varying genetic backgrounds and environmental exposures. AIM: Characterize the periodontal condition of a sample of indigenous adults in a remote region of the Amazon rain forest and determine the association of periodontal disease with various demographic, behavioral and environmental factors. METHODS: A cross-sectional evaluation of 244 subjects aged 20-70 years was conducted. Pocket depth (PD), clinical attachment level (CAL), bleeding on probing (BOP), plaque and calculus were assessed for the Ramfjord index teeth. RESULTS: These people had high levels of plaque, calculus and BOP. The mean PD was rather shallow (2.45 mm in 20-29 year-olds to 2.73 mm in 50+ year-olds) and did not increase significantly with age. Mean CAL (0.57 mm in 20-29 year-olds and 2.26 mm in 50+ year-olds) and mean location of the free gingival margin in relation to the cemento-enamel junction changed significantly with age (p<0.0001). Multivariate analysis revealed that increasing age, bleeding on probing and calculus scores were positively associated with mean CAL (p<0.01). Sex, ethnicity, level of modern acculturation, use of coca or tobacco paste, frequency of dental visits and plaque were not associated with mean CAL. CONCLUSIONS: Periodontal disease in these people was mainly associated with gingival recession rather than deep pockets. Most people had clinical attachment loss but despite poor oral hygiene and extensive gingival inflammation, they did not have very severe periodontal destruction.

Acculturation↗

PubMed2001

Factors influencing optical 3D scanning of vinyl polysiloxane impression materials.

PURPOSE: Future growth in dental practice lies in digital imaging enhancing many chairside procedures and functions. This revolution requires the fast, accurate, and 3D digitizing of clinical records. One such clinical record is the chairside impression. This study investigated how surface angle and surface roughness affect the digitizing of vinyl polysiloxane impression materials. MATERIALS AND METHODS: Seventeen vinyl polysiloxane impression materials were digitized with a white light optical digitizing system. Each sample was digitized at 3 different angles: 0 degrees, 22.5 degrees, and 45 degrees, and 2 digitizer camera f-stops. The digitized images were rendered on a computer monitor using custom software developed under NIH/NIDCR grant DE12225. All the 3D images were rotated to the 0 degrees position, cropped using Corel Photo-Paint 8 (Corel Corp, Ottawa, Ontario, Canada), then saved in the TIFF file format. The impression material area that was successfully digitized was calculated as a percentage of the total sample area, using Optimas 5.22 image processing software (Media Cybernetics, LP, Silver Spring, MD). The dependent variable was a Performance Value calculated for each material by averaging the percentage of area that digitized over the 3 angles. New samples with smooth and rough surfaces were made using the 7 impression materials with the largest Performance Values. These samples were tested as before, but with the additional angle of 60 degrees. Silky-Rock die stone (Whip Mix Corp, Louisville, KY) was used as a control. RESULTS: The Performance Values for the 17 impression materials ranged from 0% to 100%. The Performance Values for the 7 best materials were equivalent to the control at f/11 out to a surface angle of 45 degrees; however, only Examix impression material (GC America Inc, Alsip, IL) was equivalent to the control at f/11/\16. At the 60 degrees surface angle with f/11/\16, the Performance Values were 0% for all the impression materials, whereas that for the control was 90%. The difference in the Performance Values for the smooth and rough surface textures was 7%, which was not significant. CONCLUSIONS: The digitizing performance of vinyl polysiloxane impression materials is highly material and surface angle-dependent and is significantly lower than the die stone control when angles to 60 degrees are included. It is affected to a lesser extent by surface texture.

Analysis of Variance↗

PubMed2001

Development of a nondestructive compliance test for resilient denture liners.

PURPOSE: Resilient denture liners are prescribed for patients who cannot adjust to hard-based dentures because of a thin mucosa or severe alveolar ridge resorption. A nondestructive test to evaluate compliance of new soft liner materials will be useful in clinical trials. The purpose of this study was to evaluate a nondestructive compliance testing technique designed to characterize long-term, silicone-based resilient denture liner materials. MATERIALS AND METHODS: Samples of thicknesses of 1.1, 2.2, 3.3, and 4.4 mm of 2 materials (MPDS-SL [Lai Laboratories, Inc, Burnsville, MN] and Molloplast-B [Buffalo Dental, New York, NY]) were assessed for compliance using a closed-loop servohydraulic testing system, applying a 3 lb force following a squarewave pattern; force and position values were recorded using a storage oscilloscope. The oscilloscope values were analyzed using computer software to determine compliance values. The effect of material thickness was examined by testing wedges of the 2 materials. RESULTS: The testing technique used showed that differing thicknesses had significantly different compliance values (p <.0001). In the materials used to evaluate the technique, MPDS-SL behaved more elastically than did Molloplast-B (p <.0001). Material thicknesses beyond 2.2 mm did not increase compliance, although MPDS-SL had a steeper thickness-compliance curve than Molloplast-B. CONCLUSIONS: The method used to test compliance proved to be sufficiently sensitive to distinguish between 2 materials and between varying thicknesses. The sensitivity and nondestructive nature of this test show its suitability for clinical evaluation of resilient denture liners.

Analysis of Variance↗

PubMed2001

Persistent presence of Bacteroides forsythus as a risk factor for attachment loss in a population with low prevalence and severity of adult periodontitis.

BACKGROUND: Previous longitudinal studies investigating the role of microorganisms in periodontitis have focused on subjects with a high prevalence and severity of disease. The complex profile of microbial species in severe cases of periodontitis might not allow us to differentiate which bacterial species initiate disease or which species simply proliferate after disease progression. This prospective longitudinal study followed a group of 205 subjects who showed a low prevalence and severity of adult periodontitis, and thus allowed us to monitor early microbiological changes in the development of periodontitis. METHODS: Subgingival plaque was collected from proximal surfaces of a posterior sextant at 6-month intervals for 2 years. During the monitoring period, 44 subjects had either attachment loss or attachment gain. Using multiplex polymerase chain reaction (PCR), all plaque samples from those 44 subjects were analyzed for the presence of Actinobacillus actinomycetemcomitans, Bacteroides forsythus, and Porphyromonas gingivalis. RESULTS: Both subjects with attachment loss and those with attachment gain had a high prevalence of these 3 periodontal pathogens. The mere presence of any of the 3 species at a site could not predict future attachment loss at that specific site. However, subjects with a persistent presence of B. forsythus at any site across all visits had 5.3 times higher odds of having at least one site in their mouth losing attachment compared to subjects with occasional or no presence of B. forsythus. CONCLUSIONS: The persistence of B. forsythus identified subjects at higher risk, but not which specific sites in those subjects would lose attachment.

Actinobacillus Infections↗

PubMed2000

SYM-2081 a kainate receptor antagonist reduces allodynia and hyperalgesia in a freeze injury model of neuropathic pain.

Cold-freeze injury at -4 degrees C to the rat sciatic nerve produces mechanical allodynia and thermal hyperalgesia [M.A. Kleive, P.S. Jungbluth, J.A. Uhlenkamp, K.C. Kajander, Cold injury to rat sciatic nerve induces thermal hyperalgesia or analgesia, 8th World Congress on Pain, Vancouver, BC, Canada, August 1996 (Abstract).]. The NMDA receptor, an excitatory amino acid (EAA) receptor, appears to be involved in the development of allodynia and hyperalgesia following nerve injury. The role, if any, of the kainate receptor, another EAA receptor, remains unknown. In the current study, we evaluated whether (2S,4R)-4-methylglutamic acid (SYM-2081), a recently developed kainate receptor antagonist, attenuates increased responsiveness following cold injury to the sciatic nerve. During baseline testing, Sprague-Dawley rats were evaluated for frequency of withdrawal from von Frey filaments and latency of withdrawal from a radiant thermal source. Animals were then anesthetized, the left sciatic nerve was exposed, and the nerve was cooled to -4 degrees C for 15 min (n=24). For control rats (n=24), all procedures were identical except that the nerve was maintained at 37 degrees C. Testing resumed on the third day following surgery. On the fifth post-operative day, SYM-2081 (150 or 100 mg/kg), fentanyl citrate (0. 04 mg/kg) or vehicle was injected intraperitoneally. Injury to the rat sciatic nerve induced a significant increase in withdrawal frequency and a significant decrease in withdrawal latency (ANOVA, p<0.05). SYM-2081 and fentanyl significantly reduced these responses (p<0.05). These results suggest that kainate and opioid receptors are involved in the mechanical allodynia and thermal hyperalgesia that develop following cold injury to the sciatic nerve.

Animals↗

PubMed2000

Experimentally induced abutment strains in three types of single-molar implant restorations.

STATEMENT OF PROBLEM: The choice of single-molar implant design is difficult because of a lack of controlled, quantitative biomechanical analyses. PURPOSE: This study determined the effect of 3 single-molar implant designs on implant strains under a variety of homologous loading conditions. MATERIAL AND METHODS: On each implant abutment, 4 strain gauges were placed axially at 90 degrees to each other on the buccal, lingual, mesial, and distal surfaces. Effects of implant design, load location, direction, and magnitude were tested on axial and bending (buccolingual and mesiodistal) strains of 3 single-molar implant designs: (1) single, 3.75-mm (regular) diameter implant, (2) single, 5-mm (wide) diameter implant, and (3) two 3.75-mm diameter (double) implants connected through a single-molar crown. Results were analyzed with ANOVA. RESULTS: Variations in loading conditions induced 3-dimensionally complex abutment strains on the tested implant designs. Peak absolute strains in mesiodistal direction were 6493 microepsilon for design 1 and 3958 microepsilon on design 2, and 3160 microepsilon in buccolingual direction on design 3. For all loading conditions, the single 3.75-mm diameter implant consistently experienced the largest strains compared with wide-diameter and double implant designs. Changes in centric contact location affected implant abutment strains differently among the 3 designs. Angulated force direction resulted in larger bending strains. CONCLUSION: For single-molar implant designs, an increase in implant number and diameter may effectively reduce experimental implant abutment strains.

Analysis of Variance↗

PubMed2000

In vivo evaluation of the surface of posterior resin composite restorations: a pilot study.

STATEMENT OF PROBLEM: Several methods have been used to determine the surface characteristics of resin composites in vivo and compare composite wear rates with enamel wear rates. PURPOSE: This pilot study describes the surface characteristics of resin composites and the wear of resin composites and enamel during 1 year of in vivo service. MATERIAL AND METHODS: A single Class II posterior resin composite restoration (Z100) was placed in 10 patients. Restored teeth and unrestored adjacent control teeth were measured for wear 4 times within the first year. A null point contact stylus profiler and fitting software were used to measure epoxy casts. Maximum depth of wear, average depth of wear, and characteristics of the restoration margin were determined. Paired t tests were used to compare the control and restored teeth, and ANOVA was used to assess the progression of wear over time (P<.05). RESULTS: After 1 year, maximum depth of wear over the entire preparation region was on average 204.8 microm (+/- 129.8), significantly greater than the 36. 8 microm (+/- 10.1) average maximum depth of wear of enamel at occlusal contact areas on control teeth (P=.009). Maximum depth of wear progressed over time (P=.009). Fracture of excess composite, commonly called flash fracture, occurred in 50% of the restored teeth extending over the preparation margin. CONCLUSION: Composite restorations wore significantly faster than enamel contact areas on control teeth. Also of concern were the marginal flash fractures that could facilitate secondary caries.

Adult↗

PubMed2000

Nominal shear or fracture mechanics in the assessment of composite-dentin adhesion?

This study addresses the anticipated problem of discriminating among high-performing dentin adhesives. The simplicity of the nominal shear bond test, despite being heavily criticized, has made it a routine procedure for the determination of bonding efficacy. A fracture mechanics approach has been suggested as a better assessment of bonding efficacy (Versluis et al., 1997). However, experimental complexity is a major limitation. It is hypothesized that a new, simplified interfacial fracture toughness test (Lin, 1994) will evaluate bonding agents differently if compared with the traditional shear bond test. Therefore, the objective of this study was to compare the performances of six dentin bonding agents subjected to the interfacial fracture toughness test (critical plane strain energy release rate) or to the nominal shear bond test (shear bond strength). Their performances were also characterized by scanning electron micrography of the fracture surfaces for evidence of dentin cohesive failure. Statistical analyses showed only marginal differences between these determinants of the two tests. However, when the analysis was applied only to the materials that had 100% frequency of dentin cohesive failure in shear testing, which also had high bonding efficacy, the difference in adhesive strengths between the two tests became significant. The reliability of the nominal shear test is questioned when dentin cohesive failure occurs, which usually is associated with high bonding efficacy. Since it is expected that bonding efficacy will increase further, the interfacial fracture toughness test is the preferred methodology to distinguish among high-performing dentin adhesives.

Adhesiveness↗

PubMed2000

No heritability of temporomandibular joint signs and symptoms.

The causes of temporomandibular joint (TMJ)-related signs and symptoms are largely unknown. We tested the hypotheses that these signs and symptoms, as well as oral parafunctional habits, are substantially heritable. Questionnaire and clinical data were collected from 494 twins, including pairs of reared-apart and reared-together monozygotic (MZ) and dizygotic (DZ) twins. A history of joint-area pain, joint noises, and clenching and grinding habits was scored as present or absent. Twenty-nine percent of the population experienced at least one sign or symptom. Nearly one-quarter of subjects clenched or ground their teeth, and 8.7% reported a history of joint-area pain. Pain was associated with clenching, grinding, and joint noises. MZ twins were no more similar than DZ twins for any outcome, suggesting that genetic factors do not influence these traits in the population. Reared-together MZ twins were no more similar than reared-apart MZ twins, suggesting a negligible effect of the family environment on these outcomes. Environmental factors unique to each twin appeared to be the major determinants of variation in this population.

Adolescent↗

PubMed2000

Clinical performance of novel-design porcelain veneers for the recovery of coronal volume and length.

The present study evaluated the clinical performance of bonded porcelain veneers (PV) restoring substantial coronal volume and length in the anterior dentition. Forty-eight PVs were placed in 16 patients, with systematic coverage and reconstitution of the incisal edge, including well-defined anterior guidance. A standardized protocol comprising diagnostic steps that integrate additive waxups and acrylic mockups was used. PVs were fabricated using feldspathic and low-fusing porcelains in a refractory die technique. Incisal overlaps featured freestanding porcelain spans ranging from 1.5 to 5.5 mm. After a mean clinical service of 4.5 years, 13 clinical parameters for each tooth and 4 parameters that applied to persons were recorded. Permutation tests evaluated the effects of margin location, incisal edge span of porcelain, overbite, opposing contact location, and restoration age on ceramic failure and clinical marginal adaptation and seal. At recall, 100% of the veneers were satisfactory with minor interventions. The effect of slight marginal defects and porcelain cracking was negligible. Biologic, periodontal, and esthetic parameters showed excellent results, which were supported by 100% patient-reported satisfaction. All patients felt comfortable with the newly defined anterior guidance. Aging was negligible, and there were no significant effects of margin location (P > 0.08), incisal edge span of the ceramic, or overbite (P > 0.22) on ceramic failure and marginal performance. Minor alterations of the palatal margin, however, tended to be more frequent compared to facial locations, and were found especially when the opposing tooth contact in centric occlusion was located on the palatal margin (P = 0.028). Bonded ceramic restorations represent a reliable, effective procedure to restore extensive coronal volume and length in the anterior dentition.

Acrylic Resins↗

PubMed1999

Crack propensity of porcelain laminate veneers: A simulated operatory evaluation.

STATEMENT OF PROBLEM: Anterior teeth are especially subject to the thermal variations of ingested food and drinks. Postoperative cracks of porcelain laminates are considered a possible consequence of polymerization shrinkage, function, and thermocycling. PURPOSE: This investigation was conducted to define the parameters associated with the development of cracks in porcelain veneers using cyclic thermal fatigue. MATERIAL AND METHODS: Twenty-seven maxillary incisors were restored with porcelain laminate veneers and subjected to thermocycling (5 degrees C to 50 degrees C) for 1000 cycles. Ceramic cracks were reported for 11 of the 27 specimens. Teeth were sectioned and prepared for SEM analysis. Measurements of the ceramic and the luting composite thicknesses were performed for each specimen at different restoration locations (facial, incisal, and proximal). RESULTS: No significant differences in the ceramic or the luting composite thicknesses were observed between cracked and uncracked specimens. However, significant differences were observed in the ratio of the ceramic and luting composite thicknesses. Most cracked samples exhibited a ratio at the facial location below 3.0 (2.6 +/- 0.35), whereas most noncracked specimens were above this value (3.9 +/- 0.19). Incisal and especially proximal measurements alone were not significantly different between cracked versus uncracked specimens. Ceramic was slightly thinner in the facial aspect than in the proximal aspect, which was also thinner than the incisal aspect. Composite in the facial aspect was thinner in the cervical area than in the incisal third of the tooth. CONCLUSIONS: Significant cyclic temperature changes can induce the development of flaws in porcelain veneers. Control of tooth reduction and the application of die spacers during laboratory procedures undoubtedly represent key elements; a sufficient and even thickness of ceramic combined with a minimal thickness of luting composite will provide the restoration with a favorable configuration with regard to crack propensity, namely, a ceramic and luting composite thickness ratio above 3.

Analysis of Variance↗

PubMed1999

Effects of processing parameters on physical properties of the silicone maxillofacial prosthetic materials.

OBJECTIVES: Maxillofacial prostheses are commonly fabricated using dental stone molds. However, for evaluating physical properties, maxillofacial materials are most often cured in metal molds. A-2186 is a silicone-based maxillofacial prosthetic material. Because its cure may be inhibited by traces of impurities, its physical properties may be different when it contains additives and is cured in dental stone molds, compared to when it is cured in metal molds without additives. This study's purpose is to determine and compare the physical properties of A-2186 cured in stainless steel molds and stone molds. The effects of additives and cure conditions on the physical properties were also studied. METHODS: Hardness, tensile strength, ultimate elongation, and tear strength of A-2186 cured in dental stone molds, stainless steel molds, and with and without additives were determined. The bonding strength of A-2186 to four adhesives was determined by the peel test. All comparisons were made using analysis of variance (ANOVA). RESULTS: Hardness, tensile strength and ultimate elongation of A-2186 cured in stainless steel molds are significantly higher than those cured in stone molds. Adding a small amount of a pigment, a kaolin and a fiber reduces hardness, tensile strength, ultimate elongation and tear strength. Except for Hydrobond, the bond strength of the adhesives to A-2186 was not significantly affected by the cure conditions and additives. SIGNIFICANCE: Physical properties of A-2186 are affected by the additives commonly used in fabricating maxillofacial prostheses, and use of stone molds for curing degrades A-2186's mechanical properties. In fabricating clinical prostheses, special attention should be exercised to avoid contamination of A-2186 with impurities that could inhibit curing and produce inferior prostheses.

Adhesiveness↗

PubMed1999

Cross-study hierarchical modeling of stratified clinical trial data.

Hierarchical random-effects models can be used to estimate treatment or other covariate effects in single-study analyses coordinated over multiple clinical units and can also be extended to a wide variety of cross-study applications. After reviewing the single-study case, we use data from five trial protocols to look for units that tend to have treatment effects consistently above or below the study-specific grand mean across several studies. As a first step, we summarize the patient-level data as study-specific and unit-specific estimated treatment effects and standard errors using independent Cox regression models. We then compare the results of a hierarchical model using these data summaries as input to those produced by a more fully Bayesian method that uses the actual patient-level survival data. We also compare various different models using a deviance information criterion, a recent extension of the Akaike information criterion designed for hierarchical models. Our procedure appears to be effective at answering the question whether certain clinical units of the Terry Beirn Community Programs for Clinical Research on AIDS are better than others at identifying treatment effects where they exist.

AIDS-Related Opportunistic Infections↗