Effect of absorbable membranes on sandwich bone augmentation

Objectives : This study was conducted to evaluate the effect of barrier membranes on sandwich bone augmentation (SBA) for the treatment of implant dehiscence defects. Material and methods : Twenty-six implant-associated buccal dehiscence defects in 22 patients were treated according to the SBA concept – mineralized human cancellous allograft (inner layer), mineralized human cortical allograft (outer layer) and coverage with barrier membrane. The defects were randomly assigned to the bovine collagen membrane (BME) group; acellular dermal matrix (ADM) group; and no membrane group. Measurements at baseline and 6 months re-entry included defect height (DH: from smooth–rough junction to the most apical part of the defect), defect width (DW: at the widest part of the defect), and horizontal defect depth (HDD: at three locations – smooth–rough junction, middle, and most apical portion of the defect). All measurements were taken from a reference stent. Statistical analyses were performed for comparison of intra- and inter-group comparisons. Results : All implants placed were successfully osseointegrated. DH at baseline for three groups were not significantly different ( P =0.858). Mean % DH reductions for ADM, BME, and control groups at 6 months were 73.9±17.6%, 68.1±30.1%, and 63.6±23.9%, respectively, with no significant difference among the groups ( P =0.686). Mean horizontal bone gain, however, was significantly greater for membrane groups (1.7 mm for ADM, 1.6 mm for BME) compared with control group (1 mm) ( P =0.044). Implant exposure resulted in significant reduction in total height gain (79.1±14.3% vs. 57±23.5%, P =0.021). Conclusions : Within the limit of this study, it is concluded that SBA technique achieved predictable clinical outcomes. The addition of absorbable membranes enhanced bone gain in thickness compared with membrane-treated sites. To cite this article: Park S-H, Lee K-w, Oh T-J, Misch CE, Shotwell J, Wang H-L. Effect of absorbable membranes on sandwich bone augmentation. Clin. Oral Impl. Res . 19 , 2008; 32–41. doi: 10.1111/j.1600-0501.2007.01408.xPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74133/1/j.1600-0501.2007.01408.x.pd

The Dynamic Quasiperpendicular Shock: Cluster Discoveries

The physics of collisionless shocks is a very broad topic which has been studied for more than five decades. However, there are a number of important issues which remain unresolved. The energy repartition amongst particle populations in quasiperpendicular shocks is a multi-scale process related to the spatial and temporal structure of the electromagnetic fields within the shock layer. The most important processes take place in the close vicinity of the major magnetic transition or ramp region. The distribution of electromagnetic fields in this region determines the characteristics of ion reflection and thus defines the conditions for ion heating and energy dissipation for supercritical shocks and also the region where an important part of electron heating takes place. All of these processes are crucially dependent upon the characteristic spatial scales of the ramp and foot region provided that the shock is stationary. The earliest studies of collisionless shocks identified nonlinearity, dissipation, and dispersion as the processes that arrest the steepening of the shock transition. Their relative role determines the scales of electric and magnetic fields, and so control the characteristics of processes such as of ion reflection, electron heating and particle acceleration. The purpose of this review is to address a subset of unresolved problems in collisionless shock physics from experimental point of view making use multi-point observations onboard Cluster satellites. The problems we address are determination of scales of fields and of a scale of electron heating, identification of energy source of precursor wave train, an estimate of the role of anomalous resistivity in energy dissipation process by means of measuring short scale wave fields, and direct observation of reformation process during one single shock front crossing