The evolutionary genomics of adaptation to stress in wild rhizobium bacteria

Microbiota comprise the bulk of life's diversity, yet we know little about how populations of microbes accumulate adaptive diversity across natural landscapes. Adaptation to stressful soil conditions in plants provides seminal examples of adaptation in response to natural selection via allelic substitution. For microbes symbiotic with plants however, horizontal gene transfer allows for adaptation via gene gain and loss, which could generate fundamentally different evolutionary dynamics. We use comparative genomics and genetics to elucidate the evolutionary mechanisms of adaptation to physiologically stressful serpentine soils in rhizobial bacteria in western North American grasslands. In vitro experiments demonstrate that the presence of a locus of major effect, the nre operon, is necessary and sufficient to confer adaptation to nickel, a heavy metal enriched to toxic levels in serpentine soil, and a major axis of environmental soil chemistry variation. We find discordance between inferred evolutionary histories of the core genome and nreAXY genes, which often reside in putative genomic islands. This suggests that the evolutionary history of this adaptive variant is marked by frequent losses, and/or gains via horizontal acquisition across divergent rhizobium clades. However, different nre alleles confer distinct levels of nickel resistance, suggesting allelic substitution could also play a role in rhizobium adaptation to serpentine soil. These results illustrate that the interplay between evolution via gene gain and loss and evolution via allelic substitution may underlie adaptation in wild soil microbiota. Both processes are important to consider for understanding adaptive diversity in microbes and improving stress-adapted microbial inocula for human use.

Hosts winnow symbionts with multiple layers of absolute and conditional discrimination mechanisms

In mutualism, hosts select symbionts via partner choice and preferentially direct more resources to symbionts that provide greater benefits via sanctions. At the initiation of symbiosis, prior to resource exchange, it is not known how the presence of multiple symbiont options (i.e. the symbiont social environment) impacts partner choice outcomes. Furthermore, little research addresses whether hosts primarily discriminate among symbionts via sanctions, partner choice or a combination. We inoculated the legume, Acmispon wrangelianus, with 28 pairs of fluorescently labelled Mesorhizobium strains that vary continuously in quality as nitrogen-fixing symbionts. We find that hosts exert robust partner choice, which enhances their fitness. This partner choice is conditional such that a strain's success in initiating nodules is impacted by other strains in the social environment. This social genetic effect is as important as a strain's own genotype in determining nodulation and has both transitive (consistent) and intransitive (idiosyncratic) effects on the probability that a symbiont will form a nodule. Furthermore, both absolute and conditional partner choice act in concert with sanctions, among and within nodules. Thus, multiple forms of host discrimination act as a series of sieves that optimize host benefits and select for costly symbiont cooperation in mixed symbiont populations.

Transgenic Soybeans Expressing Phosphatidylinositol-3-Phosphate-Binding Proteins Show Enhanced Resistance Against the Oomycete Pathogen Phytophthora sojae

Oomycete and fungal pathogens cause billions of dollars of damage to crops worldwide annually. Therefore, there remains a need for broad-spectrum resistance genes, especially ones that target pathogens but do not interfere with colonization by beneficial microbes. Motivated by evidence suggesting that phosphatidylinositol-3-phosphate (PI3P) may be involved in the delivery of some oomycete and fungal virulence effector proteins, we created stable transgenic soybean plants that express and secrete two different PI3P-binding proteins, GmPH1 and VAM7, in an effort to interfere with effector delivery and confer resistance. Soybean plants expressing the two PI3P-binding proteins exhibited reduced infection by the oomycete pathogen Phytophthora sojae compared to control lines. Measurements of nodulation by nitrogen-fixing mutualistic bacterium Bradyrhizobium japonicum, which does not produce PI3P, revealed that the two lines with the highest levels of GmPH1 transcripts exhibited reductions in nodulation and in benefits from nodulation. Transcriptome and plant hormone measurements were made of soybean lines with the highest transcript levels of GmPH1 and VAM7, as well as controls, following P. sojae- or mock-inoculation. The results revealed increased levels of infection-associated transcripts in the transgenic lines, compared to controls, even prior to P. sojae infection, suggesting that the plants were primed for increased defense. The lines with reduced nodulation exhibited elevated levels of jasmonate-isoleucine and of transcripts of a JAR1 ortholog encoding jasmonate-isoleucine synthetase. However, lines expressing VAM7 transgenes exhibited normal nodulation and no increases in jasmonate-isoleucine. Overall, together with previously published data from cacao and from P. sojae transformants, the data suggest that secretion of PI3P-binding proteins may confer disease resistance through a variety of mechanisms.

Negotiating mutualism: A locus for exploitation by rhizobia has a broad effect size distribution and context-dependent effects on legume hosts

In mutualisms, variation at genes determining partner fitness provides the raw material upon which coevolutionary selection acts, setting the dynamics and pace of coevolution. However, we know little about variation in the effects of genes that underlie symbiotic fitness in natural mutualist populations. In some species of legumes that form root nodule symbioses with nitrogen‐fixing rhizobial bacteria, hosts secrete nodule‐specific cysteine‐rich (NCR) peptides that cause rhizobia to differentiate in the nodule environment. However, rhizobia can cleave NCR peptides through the expression of genes like the plasmid‐borne Host range restriction peptidase (hrrP), whose product degrades specific NCR peptides. Although hrrP activity can confer host exploitation by depressing host fitness and enhancing symbiont fitness, the effects of hrrP on symbiosis phenotypes depend strongly on the genotypes of the interacting partners. However, the effects of hrrP have yet to be characterised in a natural population context, so its contribution to variation in wild mutualist populations is unknown. To understand the distribution of effects of hrrP in wild rhizobia, we measured mutualism phenotypes conferred by hrrP in 12 wild Ensifer medicae strains. To evaluate context dependency of hrrP effects, we compared hrrP effects across two Medicago polymorpha host genotypes and across two experimental years for five E. medicae strains. We show for the first time in a natural population context that hrrP has a wide distribution of effect sizes for many mutualism traits, ranging from strongly positive to strongly negative. Furthermore, we show that hrrP effect size varies across host genotypes and experiment years, suggesting that researchers should be cautious about extrapolating the role of genes in natural populations from controlled laboratory studies of single genetic variants.

Evolution of specialization in a plant-microbial mutualism is explained by the oscillation theory of speciation

Specialization in mutualisms is thought to be a major driver of diversification, but few studies have explored how novel specialization evolves, or its relation to the evolution of other niche axes. A fundamental question is whether generalist interactions evolve to become more specialized (i.e., oscillation hypothesis) or if partner switches evolve without any change in niche breadth (i.e., musical chairs hypothesis). We examined alternative models for the evolution of specialization by estimating the mutualistic, climatic, and edaphic niche breadths of sister plant species, combining phylogenetic, environmental, and experimental data on Acmispon strigosus and Acmispon wrangelianus genotypes across their overlapping ranges in California. We found that specialization along all three niche axes was asymmetric across species, such that the species with broader climatic and edaphic niches, Acmispon strigosus, was also able to gain benefit from and invest in associating with a broader set of microbial mutualists. Our data are consistent with the oscillation model of specialization, and a parallel narrowing of the edaphic, climatic, and mutualistic dimensions of the host species niche. Our findings provide novel evidence that the evolution of specialization in mutualism is accompanied by specialization in other niche dimensions.

Decreased coevolutionary potential and increased symbiont fecundity during the biological invasion of a legume-rhizobium mutualism

Although most invasive species engage in mutualism, we know little about how mutualism evolves as partners colonize novel environments. Selection on cooperation and standing genetic variation for mutualism traits may differ between a mutualism's invaded and native ranges, which could alter cooperation and coevolutionary dynamics. To test for such differences, we compare mutualism traits between invaded- and native-range host-symbiont genotype combinations of the weedy legume, Medicago polymorpha, and its nitrogen-fixing rhizobium symbiont, Ensifer medicae, which have coinvaded North America. We find that mutualism benefits for plants are indistinguishable between invaded- and native-range symbioses. However, rhizobia gain greater fitness from invaded-range mutualisms than from native-range mutualisms, and this enhancement of symbiont fecundity could increase the mutualism's spread by increasing symbiont availability during plant colonization. Furthermore, mutualism traits in invaded-range symbioses show lower genetic variance and a simpler partitioning of genetic variance between host and symbiont sources, compared to native-range symbioses. This suggests that biological invasion has reduced mutualists' potential to respond to coevolutionary selection. Additionally, rhizobia bearing a locus (hrrP) that can enhance symbiotic fitness have more exploitative phenotypes in invaded-range than in native-range symbioses. These findings highlight the impacts of biological invasion on the evolution of mutualistic interactions.

Etiology and Clinical Presentation of Concussion: An Age- and Sex-matched Case Series

Purpose

Differential patterns of clinical presentation in concussion have been explored previously based on etiology (e.g., sport vs. non-sport) and may be affected by biomechanical, pathophysiological, and psychological factors. The purpose of the study was to compare neurocognitive performance and symptom presentation across different etiologies in concussed patients.

Methods

36 concussion patients, 12 slip-and-fall (SNF) were age- and sex-matched with motor vehicle accident (MVA) and sport-related concussions (SRC) (20.3±1.5 years; 4 males; 8 females per group). They were evaluated ≤ 2 weeks (7.2±3.7 days) after their injury. Patients completed medical history, an Immediate Post-Concussion Assessment and Cognitive Test (ImPACT), and Post-Concussion Symptom Scale (PCSS) at initial visit. Descriptive statistics, a series of Kruskal-Wallis H Tests, χ² tests and one-way ANOVAs were performed using SPSS 24.

Results

The three groups exhibited no differences in verbal memory (p=0.11), visual memory (p=0.10), visual motor speed (p=0.19), or reaction time (p=0.19) composites. There were no differences in symptom severity (p=0.69) or symptom count (p=0.99) or the cognitive-migraine-fatigue (p=0.69), somatic (p=0.26), affective (p=0.92), or sleep (p=0.73) symptom clusters. Comorbidities (i.e., learning disorder, previous concussion, migraine, seizure, psychological condition, or motion sickness) were not significantly different across groups.

Conclusion

No significant differences were found on ImPACT or PCSS across etiologies. When controlling for comorbidities and demographic variables, these groups appear similar at the initial evaluation. Future research will focus on practice patterns and clinical outcomes.

A high-throughput method of analyzing multiple plant defensive compounds in minimized sample mass

PREMISE OF THE STUDY

Current methods for quantifying herbivore-induced alterations in plant biochemistry are often unusable by researchers due to practical constraints. We present a cost-effective, high-throughput protocol to quantify multiple biochemical responses from small plant tissue samples using spectrophotometric techniques.

METHODS AND RESULTS

Using Solanum lycopersicum and Medicago polymorpha leaves pre- and post-herbivory, we demonstrate that our protocol quantifies common plant defense responses: peroxidase production, polyphenol oxidase production, reactive oxygen species production, total protein production, and trypsin-like protease inhibition activity.

CONCLUSIONS

Current protocols can require 500 mg of tissue, but our assays detect activity in less than 10 mg. Our protocol takes two people 6 h to run any of the assays on 300 samples in triplicate, or all of the assays on 20 samples. Our protocol enables researchers to plan complex experiments that compare local versus systemic plant responses, quantify environmental and genetic variation, and measure population-level variation.

Dynamic genomic architecture of mutualistic cooperation in a wild population of Mesorhizobium

Research on mutualism seeks to explain how cooperation can be maintained when uncooperative mutants co-occur with cooperative kin. Gains and losses of the gene modules required for cooperation punctuate symbiont phylogenies and drive lifestyle transitions between cooperative symbionts and uncooperative free-living lineages over evolutionary time. Yet whether uncooperative symbionts commonly evolve from within cooperative symbiont populations or from within distantly related lineages with antagonistic or free-living lifestyles (i.e., third-party mutualism exploiters or parasites), remains controversial. We use genomic data to show that genotypes that differ in the presence or absence of large islands of symbiosis genes are common within a single wild recombining population of Mesorhizobium symbionts isolated from host tissues and are an important source of standing heritable variation in cooperation in this population. In a focal population of Mesorhizobium, uncooperative variants that lack a symbiosis island segregate at 16% frequency in nodules, and genome size and symbiosis gene number are positively correlated with cooperation. This finding contrasts with the genomic architecture of variation in cooperation in other symbiont populations isolated from host tissues in which the islands of genes underlying cooperation are ubiquitous and variation in cooperation is primarily driven by allelic substitution and individual gene gain and loss events. Our study demonstrates that uncooperative mutants within mutualist populations can comprise a significant component of genetic variation in nature, providing biological rationale for models and experiments that seek to explain the maintenance of mutualism in the face of non-cooperators.

Invasive legumes can associate with many mutualists of native legumes, but usually do not

Mutualistic interactions can strongly influence species invasions, as the inability to form successful mutualisms in an exotic range could hamper a host's invasion success. This barrier to invasion may be overcome if an invader either forms novel mutualistic associations or finds and associates with familiar mutualists in the exotic range. Here, we ask (1) does the community of rhizobial mutualists associated with invasive legumes in their exotic range overlap with that of local native legumes and (2) can any differences be explained by fundamental incompatibilities with particular rhizobial genotypes? To address these questions, we first characterized the rhizobial communities naturally associating with three invasive and six native legumes growing in the San Francisco Bay Area. We then conducted a greenhouse experiment to test whether the invasive legume could nodulate with any of a broad array of rhizobia found in their exotic range. There was little overlap between the Bradyrhizobium communities associated with wild‐grown invasive and native legumes, yet the invasive legumes could nodulate with a broad range of rhizobial strains under greenhouse conditions. These observations suggest that under field conditions in their exotic range, these invasive legumes are not currently associating with the mutualists of local native legumes, despite their potential to form such associations. However, the promiscuity with which these invading legumes can form mutualistic associations could be an important factor early in the invasion process if mutualist scarcity limits range expansion. Overall, the observation that invasive legumes have a community of rhizobia distinct from that of native legumes, despite their ability to associate with many rhizobial strains, challenges existing assumptions about how invading species obtain their mutualists. These results can therefore inform current and future efforts to prevent and remove invasive species.

Association mapping reveals novel serpentine adaptation gene clusters in a population of symbiotic Mesorhizobium

The genetic variants that underlie microbial environmental adaptation are key components of models of microbial diversification. Characterizing adaptive variants and the pangenomic context in which they evolve remains a frontier in understanding how microbial diversity is generated. The genomics of rhizobium adaptation to contrasting soil environments is ecologically and agriculturally important because these bacteria are responsible for half of all current biologically fixed nitrogen, yet they live the majority of their lives in soil. Our study uses whole-genome sequencing to describe the pan-genome of a focal clade of wild mesorhizobia that show contrasting levels of nickel adaptation despite high relatedness (99.8% identity at 16S). We observe ecotypic specialization within an otherwise genomically cohesive population, rather than finding distinct specialized bacterial lineages in contrasting soil types. This finding supports recent reports that heterogeneous environments impose selection that maintains differentiation only at a small fraction of the genome. Our work further uses a genome-wide association study to propose candidate genes for nickel adaptation. Several candidates show homology to genetic systems involved in nickel tolerance and one cluster of candidates correlates perfectly with soil origin, which validates our approach of ascribing genomic variation to adaptive divergence.

Cheaters must prosper: reconciling theoretical and empirical perspectives on cheating in mutualism

Cheating is a focal concept in the study of mutualism, with the majority of researchers considering cheating to be both prevalent and highly damaging. However, current definitions of cheating do not reliably capture the evolutionary threat that has been a central motivation for the study of cheating. We describe the development of the cheating concept and distill a relative-fitness-based definition of cheating that encapsulates the evolutionary threat posed by cheating, i.e. that cheaters will spread and erode the benefits of mutualism. We then describe experiments required to conclude that cheating is occurring and to quantify fitness conflict more generally. Next, we discuss how our definition and methods can generate comparability and integration of theory and experiments, which are currently divided by their respective prioritisations of fitness consequences and traits. To evaluate the current empirical evidence for cheating, we review the literature on several of the best-studied mutualisms. We find that although there are numerous observations of low-quality partners, there is currently very little support from fitness data that any of these meet our criteria to be considered cheaters. Finally, we highlight future directions for research on conflict in mutualisms, including novel research avenues opened by a relative-fitness-based definition of cheating.

The ecological genomic basis of salinity adaptation in Tunisian Medicago truncatula

Background

As our world becomes warmer, agriculture is increasingly impacted by rising soil salinity and understanding plant adaptation to salt stress can help enable effective crop breeding. Salt tolerance is a complex plant phenotype and we know little about the pathways utilized by naturally tolerant plants. Legumes are important species in agricultural and natural ecosystems, since they engage in symbiotic nitrogen-fixation, but are especially vulnerable to salinity stress.

Results

Our studies of the model legume Medicago truncatula in field and greenhouse settings demonstrate that Tunisian populations are locally adapted to saline soils at the metapopulation level and that saline origin genotypes are less impacted by salt than non-saline origin genotypes; these populations thus likely contain adaptively diverged alleles. Whole genome resequencing of 39 wild accessions reveals ongoing migration and candidate genomic regions that assort non-randomly with soil salinity. Consistent with natural selection acting at these sites, saline alleles are typically rare in the range-wide species' gene pool and are also typically derived relative to the sister species M. littoralis. Candidate regions for adaptation contain genes that regulate physiological acclimation to salt stress, such as abscisic acid and jasmonic acid signaling, including a novel salt-tolerance candidate orthologous to the uncharacterized gene AtCIPK21. Unexpectedly, these regions also contain biotic stress genes and flowering time pathway genes. We show that flowering time is differentiated between saline and non-saline populations and may allow salt stress escape.

Conclusions

This work nominates multiple potential pathways of adaptation to naturally stressful environments in a model legume. These candidates point to the importance of both tolerance and avoidance in natural legume populations. We have uncovered several promising targets that could be used to breed for enhanced salt tolerance in crop legumes to enhance food security in an era of increasing soil salinization.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1160) contains supplementary material, which is available to authorized users.

Selection for cheating across disparate environments in the legume-rhizobium mutualism

The primary dilemma in evolutionarily stable mutualisms is that natural selection for cheating could overwhelm selection for cooperation. Cheating need not entail parasitism; selection favours cheating as a quantitative trait whenever less-cooperative partners are more fit than more-cooperative partners. Mutualisms might be stabilised by mechanisms that direct benefits to more-cooperative individuals, which counter selection for cheating; however, empirical evidence that natural selection favours cheating in mutualisms is sparse. We measured selection on cheating in single-partner pairings of wild legume and rhizobium lineages, which prevented legume choice. Across contrasting environments, selection consistently favoured cheating by rhizobia, but did not favour legumes that provided less benefit to rhizobium partners. This is the first simultaneous measurement of selection on cheating across both host and symbiont lineages from a natural population. We empirically confirm selection for cheating as a source of antagonistic coevolutionary pressure in mutualism and a biological dilemma for models of cooperation.

Specialization-generalization trade-off in a Bradyrhizobium symbiosis with wild legume hosts

BACKGROUND

Specialized interactions help structure communities, but persistence of specialized organisms is puzzling because a generalist can occupy more environments and partake in more beneficial interactions. The "Jack-of-all-trades is a master of none" hypothesis asserts that specialists persist because the fitness of a generalist utilizing a particular habitat is lower than that of a specialist adapted to that habitat. Yet, there are many reasons to expect that mutualists will generalize on partners.Plant-soil feedbacks help to structure plant and microbial communities, but how frequently are soil-based symbiotic mutualistic interactions sufficiently specialized to influence species distributions and community composition? To address this question, we quantified realized partner richness and phylogenetic breadth of four wild-grown native legumes (Lupinus bicolor, L. arboreus, Acmispon strigosus and A. heermannii) and performed inoculation trials to test the ability of two hosts (L. bicolor and A. strigosus) to nodulate (fundamental partner richness), benefit from (response specificity), and provide benefit to (effect specificity) 31 Bradyrhizobium genotypes.

RESULTS

In the wild, each Lupinus species hosted a broader genetic range of Bradyrhizobium than did either Acmispon species, suggesting that Acmispon species are more specialized. In the greenhouse, however, L. bicolor and A. strigosus did not differ in fundamental association specificity: all inoculated genotypes nodulated both hosts. Nevertheless, A. strigosus exhibited more specificity, i.e., greater variation in its response to, and effect on, Bradyrhizobium genotypes. Lupinus bicolor benefited from a broader range of genotypes but averaged less benefit from each. Both hosts obtained more fitness benefit from symbionts isolated from conspecific hosts; those symbionts in turn gained greater fitness benefit from hosts of the same species from which they were isolated.

CONCLUSIONS

This study affirmed two important tenets of evolutionary theory. First, as predicted by the Jack-of-all-trades is a master of none hypothesis, specialist A. strigosus obtained greater benefit from its beneficial symbionts than did generalist L. bicolor. Second, as predicted by coevolutionary theory, each test species performed better with partner genotypes isolated from conspecifics. Finally, positive fitness feedback between the tested hosts and symbionts suggests that positive plant-soil feedback could contribute to their patchy distributions in this system.

Adaptive divergence in seed color camouflage in contrasting soil environments

Although adaptive plant population divergence across contrasting soil conditions is often driven by abiotic soil factors, natural enemies may also contribute. Cryptic matching to the native soil color is a form of defensive camouflage that seeds can use to avoid detection by seed predators. The legume Acmispon wrangelianus occurs across a variety of gray-green serpentine soils and brown nonserpentine soils. Quantitative digital image analysis of seed and soil colors was used to test whether genetically based seed color is a closer match to the color of the native soil than to the color of other nearby soils. Lineages bear seeds that more closely match the color of their native serpentine or nonserpentine soil type than the opposing soil type. Further, even within a soil type, lineages bear seeds with a closer color match to the soil at their native site than to other sites. The striking concordance between seed and native soil color suggests that natural selection for locally camouflaged seed color morphs, probably driven by seed predators, may maintain adaptive divergence in pigmentation, despite the opportunity for migration between soil environments.

Mutualism and adaptive divergence: co-invasion of a heterogeneous grassland by an exotic legume-rhizobium symbiosis

Species interactions play a critical role in biological invasions. For example, exotic plant and microbe mutualists can facilitate each other's spread as they co-invade novel ranges. Environmental context may influence the effect of mutualisms on invasions in heterogeneous environments, however these effects are poorly understood. We examined the mutualism between the legume, Medicago polymorpha, and the rhizobium, Ensifer medicae, which have both invaded California grasslands. Many of these invaded grasslands are composed of a patchwork of harsh serpentine and relatively benign non-serpentine soils. We grew legume genotypes collected from serpentine or non-serpentine soil in both types of soil in combination with rhizobium genotypes from serpentine or non-serpentine soils and in the absence of rhizobia. Legumes invested more strongly in the mutualism in the home soil type and trends in fitness suggested that this ecotypic divergence was adaptive. Serpentine legumes had greater allocation to symbiotic root nodules in serpentine soil than did non-serpentine legumes and non-serpentine legumes had greater allocation to nodules in non-serpentine soil than did serpentine legumes. Therefore, this invasive legume has undergone the rapid evolution of divergence for soil-specific investment in the mutualism. Contrary to theoretical expectations, the mutualism was less beneficial for legumes grown on the stressful serpentine soil than on the non-serpentine soil, possibly due to the inhibitory effects of serpentine on the benefits derived from the interaction. The soil-specific ability to allocate to a robust microbial mutualism may be a critical, and previously overlooked, adaptation for plants adapting to heterogeneous environments during invasion.

An evaluation of a case of agenesis of the corpus callosum with Rourke's nonverbal learning disorder model

This article evaluates the case of an 11-year-old male with congenital agenesis of the corpus callosum, against Rourke's NLD models. A comprehensive neuropsychological assessment was completed, and impairments with attention, movement, fine motor and sensory perception, visuo-spatial organization, and verbal memory were identified. Additionally, a volumetric analysis of the magnetic resonance image (MRI) was completed. Although no gross structural abnormalities, beyond agenesis, were present, the possibility of unobservable abnormalities was considered. Many predicted NLD deficits were clearly evidenced in this case, including associated behavioral problems. This study concludes that although Rourke's NLD model is descriptive of many aspects of white matter impairments, it may not account for the full range or severity of deficits that may be observed. In callosal agenesis, children may exhibit verbal learning, auditory attentional, and verbal expressive difficulties.

Long-term performance of Osseotite implants: a 6-year clinical follow-up

In this prospective, multicenter study, 147 Osseotite implants were placed in 75 patients (32 men and 43 women with a mean age of 54 years) using a conventional two-stage surgical protocol with 3 months of healing time in the mandible and 6 months in the maxilla. Of the 147 implants, 69% were inserted in posterior sites and 64% were short implants of 10 mm or less. From the time of implant insertion to second-stage surgery, 5.1 months (+/- 2.4 months) elapsed. Restorative treatments included 25 single-tooth replacements (28.4%), 58 short span fixed bridges (65.9%), 1 full-arch reconstruction (1.1%), and 4 overdentures (4.5%). The mean time from implant placement to final recall was 74.1 months (+/- 8.9 months). At second-stage surgery and at 6-month and annual follow-up examinations, implants were evaluated for mobility, peri-implant radiolucency, gingival health, signs and symptoms of infection, neuropathies, paresthesia, and crestal bone levels. A 3-year interim report identified 5 implant failures, 4 of which occurred as a clustering phenomenon in a single, medically compromised patient. Using the life table analysis method, the cumulative implant success rate was calculated at 96.6%. The 3-year interim report indicates that the implants developed an extended, functional osseous state that remained stable for more than 6 years.

A prospective multicenter clinical study of the Osseotite implant: four-year interim report

This article reports the 4-year interim results of a multicenter study evaluating the clinical performance of the Osseotite dental implant. At 4 study centers, 485 Osseotite implants were consecutively placed in 181 patients (219 were placed in the mandible and 266 in the maxilla). A total of 355 implants were placed in posterior regions. Short implants (10 mm or less) represented 31.5% (n = 153) of all implants placed in this study. Patients were restored with 210 restorations, distributed as 123 short-span prostheses, 58 single-tooth replacements, 28 long-span prostheses, and 1 maxillary overdenture. At this 4-year interim evaluation, the mean time from implant placement to the most recent evaluation was 52.6 +/- 3.0 months, with a mean loading time of 43.3 +/- 3.8 months. Of the 485 implants placed, there have been 6 failures. All implant failures occurred prior to loading and were categorized as early implant failures. Five of the 6 failures occurred in the maxilla. Only one of the 153 short implants failed to integrate. Baseline radiographs were obtained at prosthesis connection. Radiographic analysis 1 year post-restoration showed a mean bone loss of 0.09 +/- 0.7 mm. From baseline to the end of the second year of function, an overall mean bone loss of 0.13 +/- 0.8 mm was recorded, indicating no additional bone was lost after the first year of implant function. At 4 years, the cumulative implant success rate for all implants placed in this study was 98.7%, with a 99.4% success rate in the posterior mandible and 98.4% success rate in the posterior maxilla. Results of this 4-year interim analysis indicate that this implant achieved a high success rate in posterior regions and that all failures with this implant in this patient population occurred prior to implant loading. When the clinical success of implants 10 mm or shorter was compared to that of implants greater than 10 mm in length, the shorter implants in this study performed similarly to longer implants.

A prospective multicenter study evaluating loading of osseotite implants two months after placement: one-year results

Historically, the recommended time between placement and functional loading of machined-surface dental implants has been 3 months for the mandible and 6 months for the maxilla. However, such recommendations are a result of evaluating randomly chosen healing times during the initial phase of implant development and are based on the subsequent clinical outcome of either implant integration or mobility. In recent years, histologic and experimental studies have shown that specifically designed micro-topographic implant surfaces can result in increased bone-to-implant contact at earlier healing times than obtained with machined-surface implants. Histologic and clinical studies investigating early and immediate implant loading support the premise that implants can be placed into function earlier than previously recommended. With the development of specifically designed implant surfaces and the utilization of time-saving surgical (one-stage surgical protocol) and prosthetic (implant position indexing) techniques, patients are now being restored and returning to function sooner than previously thought possible. The purpose of this multicenter clinical investigation is to evaluate the efficacy of loading Osseotite dental implants (3i-Implant Innovations Inc., Palm Beach Gardens, Florida) at 2 months and to determine the effect of early loading on implant performance and survival. A total of 429 Osseotite implants were placed in 155 patients (87 females and 68 males; mean age 54.0 +/- 13.7 yr), at 10 study centers, and subsequently loaded 2.1 +/- 0.7 months following placement. A single-stage surgical protocol was followed, with implants indexed immediately or impressed 4 to 6 weeks following placement. Patient restorative treatments included placement of 83 single-implant provisional restorations and 129 splinted, two-, three-, and four-implant supported maxillary and mandibular provisional restorations. The mean time from prosthetic loading to the most recent follow-up evaluation was 10 +/- 1.3 months. Seven of the 429 implants did not integrate; of these, six were identified prior to loading and one was identified 1 month after loading. The cumulative implant survival rate was 98.5% at 12.6 months. The cumulative post-loading implant survival rate was 99.8% at 10.5 months. The preliminary results of this clinical investigation suggest that successful functional loading of the Osseotite dental implant is possible at 2 months following noncomplicated implant placement.

A human histologic analysis of osseotite and machined surfaces using implants with 2 opposing surfaces

A human histologic study was conducted to compare the percentage of bone-to-implant contact (BIC) at 6 months for Osseotite and machined, commercially pure titanium implant surfaces. To eliminate potential influences caused by differences in bone density at different intraoral locations, 2 mm x 5 mm, threaded, 2-surfaced titanium implants were manufactured; 1 side received the Osseotite surface modification and the opposite side maintained a machined surface. In each of 11 patients, 1 test implant was placed in the posterior maxilla (Types III and IV bone) during conventional dental implant surgery. Following 6 months of unloaded healing, the conventional implants were uncovered, and the test implants and surrounding hard tissue were removed. Histologic analysis indicated that at 6 months of unloaded healing, the mean BIC value for the Osseotite surfaces (72.96% +/- 25.13%) was statistically significantly higher (P < 0.05) than the mean BIC value for the machined surfaces (33.98% +/- 31.04%). When the BIC values for the machined and Osseotite surface pairs were ranked from high to low based on the machined BIC value range of 93% to 0%, the upper 50th percentile (20 surface pairs) mean BIC value was 86.1% +/- 16.7% for the Osseotite surfaces and 60.1% +/- 18.3% for the machined surfaces. The lower 50th percentile (19 surface pairs) mean BIC value was 59.1% +/- 25.3% for the Osseotite surfaces and 6.5% +/- 10.8% for the machined surfaces. Differences between mean BIC values for the 2 surfaces in both the upper and lower 50th percentiles were statistically significant (P < 0.05). The results of this study indicate that in the poorer quality bone typically found in the posterior maxilla, a statistically significantly higher percentage of bone contacts Osseotite surfaces when compared to opposing machined surfaces on the same implant.

Somatosensory-evoked potential monitored during total hip arthroplasty

One hundred consecutive patients were monitored using somatosensory-evoked potential (SEP) monitoring to detect intraoperative sciatic nerve compromise during total hip arthroplasty. The peroneal nerve was stimulated using the contralateral extremity to rule out systemic influences on the SEP tracings. Loss of amplitude or an increase in latency of greater than 10% was considered significant. Of the 18 patients who exhibited changes that met these criteria, 16 were female. Two patients had loss of amplitude of the tracings at the time of closure, and both of these patients exhibited postoperative sciatic nerve palsies. There were no false negatives. Femoral reaming and reduction are the surgical events most commonly associated with nerve reactions. Patients who have had prior hip procedures appear to be at higher risk. There was no correlation with intraoperative SEP changes and age, weight, surgical approach, or leg lengthening. Compared with unmonitored patients, there was no reduction in the incidence of sciatic palsy.

Apparent fracture toughness of all-ceramic crown systems

This study measured apparent fracture toughness (Kc) and elastic modulus-to-hardness ratio (E/H) of dental restorations made with five all-ceramic crown systems. An indentation technique was used whereby Kc was calculated from the length of radial cracks formed on the release of a Vickers Diamond. Twenty-four crowns were made by using five different crown systems: Cerestore, DICOR, Hi-Ceram, Renaissance, and Vitadur-N. After the crowns were fired and glazed, they were embedded, polished, gold-coated, and indented under oil. In addition Knoop indentations were made for E/H calculation. Cerestore had a mean Kc of 1.79 MPa.m1/2 and E/H of 13.9, DICOR 2.10 and 26.8, Hi-Ceram 1.96 and 17.8, Renaissance 2.02 and 24.2, and Vitadur-N 1.96 and 22.3. Statistically significant differences (p less than 0.05) were found between DICOR and Cerestore materials for Kc and between DICOR, Renaissance, and Cerestore materials for E/H. For comparison, previous work found Vita VMK metal ceramic crowns to have a mean Kc of 1.91 MPa.m1/2 and E/H of 20.2.

Colour measurements of all ceramic crown systems

The objectives of this study were: (i) to determine variability among colour parameters of five different ceramic crown systems; and (ii) to measure the effect of using coloured luting agents on restoration colour. The crown systems studied were Cerestore, Dicor, Hi-Ceram, Renaissance, and Vitadur-N. Five crowns for each system were made according to manufacturer's instructions with the same nominal shade (Vita Lumin Vacuum A2) to fit an Ivorine central incisor tooth. Restoration thickness was adjusted to within +/- 0.1 mm (+/- 0.05 mm in the mid-facial area where colour measurements were to be made) with the aid of a dial calliper prior to glazing or, in the case of Dicor, surface staining. Where a core was part of the system this was fabricated to the minimum recommended thickness. The crowns were cemented using luting agents of five different colours in a randomly chosen sequence. The colour of each restoration/cement combination was measured three times using a small-area colorimeter (Minolta CR-121). The variance of each colour parameter (L*, a*, b*) was statistically compared for each crown system using an analysis of variance procedure, as was the effect of the cement. Observed differences were related to visual perception by using the colour difference formula. There were statistically significant differences among the variances of the crown systems and the cements, with significant interactions between crown systems and direction of colour and between cement and direction of colour. Restorations made with different ceramic crown systems had noticeably different colour despite having the same nominal shade. Changing the shade of the luting agent had a perceivable effect on Dicor crowns and, to a lesser extent, on Vitadur-N crowns but not on the other systems due, presumably, to the opacity of their core materials.

Apparent fracture toughness of metal ceramic restorations with different manipulative variables

Apparent fracture toughness (Kc) is a measure of the strain absorption capacity of brittle materials. It relates to the level of tensile stress that must be exceeded at the tip of a crack before failure occurs. Using an indentation technique, this study measured Kc and elastic modulus-to-hardness ratio (E/H) of 65 metal ceramic crowns made with 13 different variables. The variables studied were (1) brand of porcelain, (2) firing temperature, (3) condensation, and (4) modeling liquid. Statistically significant differences in Kc and E/H were found in respect to porcelain brand and between two of the modeling liquids. No significant differences were found in respect to altering firing temperature or condensation.

Intraoperative cortical somatosensory evoked potentials for detection of sciatic neuropathy during total hip arthroplasty

Approximately 3% of patients undergoing hip arthroplasty develop postoperative sciatic neuropathy. The factors associated with changes in somatosensory evoked potentials (SSEP) and sciatic neuropathy were examined in patients undergoing hip arthroplasty, to evaluate whether the use of intraoperative SSEP could help reduce the incidence of postoperative sciatic neuropathy. Eighty-eight patients were assigned to either monitored or unmonitored groups. SSEP were recorded following peroneal nerve stimulation, using contralateral stimulation to detect systemic influences on SSEP. Amplitude reduction of less than 50% of control and/or latency increase of greater than 10% of control was considered significant, and surgical intervention was attempted to restore SSEP. Previous surgery and a lateral incision approach tended to be associated with sciatic neuropathy (p less than 0.053). The incidence of sciatic neuropathy in the monitored group (4.3%) was not different from the unmonitored group (2.4%). Isolated reduction in amplitude or prolongation in latency of the SSEP was not predictive of postoperative neurologic function of the sciatic nerve. Six patients, two of whom developed sciatic neuropathy, demonstrated complete flattening of the SSEP. Both of these patients had flattened SSEP for two or more surgical events (p less than 0.01) and flattened SSEP were present at the end of the surgical procedure. There were no false-negative SSEP changes. Simultaneous amplitude and latency changes appear to be predictive of sciatic nerve function following hip arthroplasty.

Comparison of intravenous nitroprusside, nitroprusside-captopril, and nitroglycerin for deliberate hypotension during posterior spine fusion in adults

Three techniques for deliberate hypotension (mean arterial pressure, 60 to 70 mmHg) were prospectively compared in adults undergoing posterior spine fusion. Patients received either IV sodium nitroprusside, sodium nitroprusside with oral captopril pretreatment, or IV nitroglycerin. Patient groups were comparable in age, sex, weight, baseline hemodynamic and laboratory parameters, duration of surgery, and duration of hypotension. Absolute blood loss was significantly less in the group receiving nitroglycerin; however, there were no differences between groups when corrected for operative exposure (milliliter per spine segment exposed). Nitroprusside was effective in producing target blood pressure in all patients. Nitroglycerin was ineffective in two patients and two other patients required greater than 20 micrograms/kg/min. Both groups receiving nitroprusside developed significant postinfusion increases in arterial pressure. Blood pressure fell significantly after induction of anesthesia in patients receiving captopril. Cardiac index, heart rate, pulmonary capillary wedge pressure, intrapulmonary shunt, and arterial blood gases were comparable and did not change significantly in any group. Systemic vascular resistance fell during infusion in all groups and remained depressed after infusion in patients receiving nitroglycerin. Plasma renin activity was significantly increased in the group receiving captopril due to loss of feedback inhibition of renin release and rose significantly during infusion in those patients receiving nitroprusside alone. There were no complications. Nitroprusside with and without captopril pretreatment was associated with postoperative increases in arterial pressure, although not to hypertensive levels, probably due to loss of captopril activity after single-dose administration. The use of nitroglycerin was limited by lack of potency. There was no demonstrable clinical advantage for any of the three techniques.

Intraoperative allergic reactions to hydroxyethyl starch: a report of two cases

The presentation, evolution, and treatment of two cases of intraoperative allergic reactions are described. In each case, the offending agent was felt to be hydroxyethyl starch (HES), a synthetic polymer colloid solution used for intravascular volume expansion. Symptoms included urticaria, angioedema, and hypotension in the first patient, and urticaria, hypoxia, and haemoconcentration in the second patient. Both patients had earlier received multiple drugs and/or blood products. However, HES administration in both patients immediately produced allergic symptomatology. The first patient had concurrent depression of serum total complement levels (CH50) and no elevation of plasma histamine levels, which suggested a complement-mediated reaction to HES. Levels of CH50 and histamine were not obtained from the second patient. Direct stimulation of mast cell degranulation by the offending agent, complement activation, or conventional antigen-antibody interaction in a previously exposed patient may initiate intraoperative allergic reactions. Anaesthesia personnel should be aware of the risk of intraoperative allergic reactions to HES and be prepared to treat them rapidly and effectively.

Spinal cord and cerebral blood flow responses to subarachnoid injection of local anesthetics with and without epinephrine

Subarachnoid anesthesia with lidocaine, mepivacaine, or tetracaine with and without added epinephrine (1:100 000) produced no demonstrable changes in average cerebral (CBF) or segmental spinal cord blood flow (SCBF) in 38 cats anesthetized with pentobarbital. Blood flow was measured by the injection of radioactive microspheres. Seven groups of cats received either lidocaine 15 mg, lidocaine 15 mg with epinephrine, mepivacaine 10 mg, mepivacaine 10 mg with epinephrine, tetracaine 5 mg, tetracaine 5 mg with epinephrine, or saline with epinephrine 1:100 000. Mean arterial pressure (MAP) decreased significantly (P less than 0.05) in Groups I-VI. Added epinephrine had no effect on the decrease in MAP. Amplitude of the somatosensory cortical evoked response decreased significantly in Groups I-VI, but did not change from control in Group VII. No significant change in CBF or SCBF was demonstrated in any group at any time. Plasma lidocaine and mepivacaine levels were significantly less at 5 min after subarachnoid injection in the groups receiving epinephrine compared to those not receiving epinephrine (P less than 0.05). The data appear to support the hypothesis of a vasoconstrictive reduction in systemic absorption of intrathecal local anesthetics, but suggest that significant segmental spinal cord ischemia does not occur. Maintenance of total flow in the face of a decrease in MAP suggests that autoregulation in brain and spinal cord may be maintained. Changes in regional SCBF or CBF may have been present but were not examined in this study. Further studies of brain and spinal cord blood flow dynamics, regional flow changes, and regulation of flow after intrathecal agents are necessary.

Venous air embolism in a child undergoing posterior fossa craniotomy: a case report

The incidence of venous air embolism in children undergoing neurosurgical procedures in the sitting position may be as high as 33 per cent. It may be more serious in children because smaller total volumes of air can produce serious physiologic disturbances or fatalities, and the risk of systemic air embolism through a patent foramen ovale or patent ductus arteriosus may be greater than in adults. The case of a six-year-old child who sustained four separate episodes of air embolism while undergoing posterior fossa exploration is presented. Early episodes were well tolerated, but later episodes produced clinical instability. A total of 11 cc of air was recovered from an indwelling right atrial catheter, and the patient recovered uneventfully. This case report illustrates the physiologic changes produced by venous air embolism and reviews the importance of adequate monitoring techniques and early treatment.