Is vancomycin MIC "creep" method dependent? Analysis of methicillin-resistant Staphylococcus aureus susceptibility trends in blood isolates from North East Scotland from 2006 to 2010.

This study investigated "creep" in vancomycin and daptomycin MICs among methicillin-resistant Staphylococcus aureus (MRSA) isolates from blood cultures over a 5-year period in a hospital in the United Kingdom, using different susceptibility testing methods. Trends in vancomycin and daptomycin susceptibility were evaluated by using Etest performed prospectively on isolates in routine clinical practice from December 2007 to December 2010 (n = 102). Comparison was made to results from prospective testing of subcultures at the Scottish MRSA Reference Laboratory, using an automated system (Vitek 2) and retrospective testing (Etest and CLSI reference broth microdilution [BMD] method) of stored isolates from 2006 to 2010 (n = 208). Spearman's rank correlations revealed a significant increase in vancomycin MIC (P = 0.012) and a significant decrease in daptomycin MIC (P = 0.03) by year of study for Etest results from the time of isolation. However, neither trend was replicated in MICs from automated or retrospective testing. The Friedman test revealed a significant difference between vancomycin MICs obtained from the same samples by different testing methods (χ(2) [3 degrees of freedom] = 97; P < 0.001). MICs from automated testing and Etest analysis of stored isolates were significantly lower than those from Etest analysis at the time of isolation for both antibiotics (P < 0.001). Effects of storage on the MIC appeared within the first 6 months of storage. Inconsistent evidence on vancomycin MIC creep and the relevance of the MIC to clinical outcome may arise from differences in susceptibility testing methods, including storage of isolates. There is a need to standardize and streamline susceptibility testing of vancomycin against MRSA.

The compatibility of ceramic bone graft substitutes as allograft extenders for use in impaction grafting of the femur.

This study investigates the use of porous biphasic ceramics as graft extenders in impaction grafting of the femur during revision hip surgery. Impaction grafting of the femur was performed in four groups of sheep. Group one received pure allograft, group two 50% allograft and 50% BoneSave, group three 50% allograft and 50% BoneSave type 2 and group four 10% allograft and 90% BoneSave as the graft material. Function was assessed using an index of pre- and post-operative peak vertical ground reaction force ratios. Changes in bone mineral density were measured by dual energy X ray absorptiometry (DEXA) scanning. Loosening and subsidence were assessed radiographically and by histological examination of the explanted specimens. There was no statistically significant difference between the four groups after 18 months of unrestricted functional loading for all outcome measures.

Increased insertion torque delays pin-bone interface loosening in external fixation with tapered bone screws.

OBJECTIVE: To test the null hypothesis that osseomechanical integration is not related to the maximum insertion torque of tapered external fixation pins. DESIGN: Prospective in vivo study in a functionally loading ovine model. In 12 animals, tapered commercial external fixation pins were inserted at predefined locations with measured insertion torques and extraction torque measured at 10 weeks postoperatively. SETTING: Unrestricted stall activity under veterinary supervision. INTERVENTIONS: Under general anesthesia and aseptic conditions, mid-diaphyseal tibial osteotomies were created and a 3-mm gap width stabilized with a custom-made, high-precision, single-sided external fixator, in compliance with United Kingdom government regulations [Animals (Scientific Procedures) Act 1986]. MAIN OUTCOME MEASUREMENTS: Primary pin site stability and interface load were assessed by measuring maximum insertion torque (Nm). At a 10-week postoperative end point, osseomechanical stability was assessed by measuring the extraction torque and a pin performance index determined from the insertion/extraction torque ratio. RESULTS: A positive correlation was found between extraction torque and insertion torque (R2 = 0.322, P < 10(-6)). All pins with an insertion torque equal to or greater than 7 Nm had a measurable extraction torque, as did 98% of the pins with an insertion torque above 5 Nm. Extraction torque decreased both as a function of pin site position by the postoperative end point. High insertion torques were found to enhance end point stability in both diaphyseal and metaphyseal bone. CONCLUSION: The data from this study indicate that tapered external fixation pins should be inserted with a high torque to enhance the long-term integrity of the pin-bone interface.

Effect of nitric oxide donor nitroglycerin on bone mineral density in a rat model of estrogen deficiency-induced osteopenia.

Nitric oxide (NO) may modulate estrogen's anabolic effects on bone homeostasis by restraining osteoclast-mediated bone resorption and stimulation of osteoblast activity. Accordingly, NO donated by organic nitrates, including nitroglycerin, is thought to protect against bone loss associated with estrogen deficiency. In this study, we have explored this phenomenon. Thirty-two 12-week-old female Wistar rats were divided into four groups prior to bilateral ovariectomy or a sham operation. The ovariectomised rats received (1). vehicle control (OVX control), (2). 17-beta-estradiol (OVX+E2), or (3). transdermal nitroglycerin (OVX+NG) for 4 weeks. Femoral and tibial bone mineral density (BMD), serum alkaline phosphatase and urine deoxypyridinoline and NO metabolites were analysed at the end of the study period together with failure torque and torsional rigidity of the tibiae and cellular localisation of the NO-synthase (NOS) isoforms. In OVX+E2 group, proximal and distal femoral and proximal tibial BMD exceeded that of the Sham controls. Nitroglycerin prevented BMD loss at these three sites at levels comparable to that of the Sham controls. Deoxypyridinoline excretion did not change except in the OVX-E2 group that showed an expected reduction when compared to the Sham and OVX controls. There were no treatment-related differences in total alkaline phosphatase or urinary NO metabolites. Tibial failure torque was comparable between the groups but both OVX+E2 and OVX+NG groups showed decreased torsional rigidity compared with the OVX controls. Endothelial and inducible NOS were found in osteoblast-like cells associated with calcifying cartilage spicules in the distal femoral metaphysis. These data confirm previous findings and show that nitroglycerin counteracts the estrogen deficiency-induced osteopenia in the ovariectomised rat model. Organic nitrates may thus be beneficial in conditions where bone turnover is compromised such as in osteoporosis.

Glass ionomer as an expander of allograft in revision arthroplasty of the hip.

The use of glass ionomer as a bone graft expander was investigated in an in vivo model of revision hip arthroplasty. Bone grafts of pure allograft and allograft + glass ionomer particles in a 50:50 by weight mixture were implanted in an ovine hemi-arthroplasty model. Post-operative assessments of locomotor function, radiographic appearance and quantitative changes in mineralisation around the graft were made at 2, 4 and 6 months. Post-mortem assessments of radiographic and histologic appearance of the grafts were made at 6 months. No significant differences were noted in any of the measured or assessed parameters between the two graft types. The glass ionomer particles seemed to be well tolerated within the matrix of new bone, smaller sized particles appearing to be better incorporated than larger ones. The use of particles of glass ionomer as a bone graft expander, in this in vivo model of revision hip arthroplasty, would therefore appear to offer no detriment in performance over pure allograft in the short to medium term.

Asymmetric densitometric and mechanical adaptation of the left fifth metacarpal bone in racing greyhounds.

The densitometric and mechanical structural properties of the left and right fifth metacarpal bones of 10 racing greyhounds with a fracture of the central tarsal bone and 10 without a fracture were investigated by dual energy x-ray absorptiometry and a servo-hydraulic materials testing machine. In all the greyhounds the bone mineral density of the left fifth metacarpal bone was significantly greater than that of the right (P<0.001), but there was no significant difference between the degree of asymmetry in bone mineral density in the two groups. The ultimate torque and energy-to-failure of the left fifth metacarpal bone of the greyhounds with a fracture of the right central tarsal bone were significantly higher than in the right fifth metacarpal bone and higher than in both the left and right fifth metacarpal bones of the greyhounds with no fracture (P<0.05). The ultimate torque and energy-to-failure were significantly related to bone mineral density (P<0.005), and 22 per cent of the variation in these structural properties could be explained by variations in bone mineral density. Fifth metacarpal remodelling in response to asymmetric cyclic loading is bone-specific, and structural properties are enhanced in the left fifth metacarpal of greyhounds with a fracture of the right central tarsal bone.

Dual-energy X-ray absorptiometry measurement of bone-mineral density in the distal aspect of the limbs in racing Greyhounds.

OBJECTIVE: To determine those bones in the distal aspect of the limbs of Greyhounds with fatigue fractures that have the greatest left-to-right differences in bone-mineral density (BMD). SAMPLE POPULATION: Limbs obtained from 20 Greyhounds. PROCEDURE: Dual-energy x-ray absorptiometry (DXA) of the distal aspect of each limb and isolated bones from 10 dogs with a fracture of the central tarsal bone (CTB) of the right pelvic limb was performed. High-resolution scanning was performed on excised bones, and BMD measurements of CTB also were obtained from limbs of dogs without a CTB fracture. RESULTS: The BMD of the accessory carpal bone and calcaneus was not significantly different from the BMD of those bones in the contralateral limb. Although BMD of the CTB of the entire right pelvic limb and isolated bones from dogs with a CTB fracture was lower, compared with values for the entire left pelvic limb, values for isolated CTB from dogs without a CTB fracture were not significantly different. Metacarpal or metatarsal and thoracic or pelvic limb significantly affected BMD for measurements of the entire limb and isolated bones. Left-to-right differences in BMD were greatest for metacarpal 5. CONCLUSIONS AND CLINICAL RELEVANCE: Asymmetric adaptive remodeling of metacarpal 5 can be detected by DXA. The potentially confounding effects of CTB fracture and unknown racing history made it difficult to interpret BMD changes in the CTB of these specimens. Densitometry could be developed as an in vivo assessment for risk of fractures in racing Greyhounds.

Fractographic examination of racing greyhound central (navicular) tarsal bone failure surfaces using scanning electron microscopy.

The greyhound is a fatigue fracture model of a short distance running athlete. Greyhounds have a high incidence of central (navicular) tarsal bone (CTB) fractures, which are not associated with overt trauma. We wished to determine whether these fractures occur because of accumulation of fatigue microdamage. We hypothesized that bone from racing dogs would show site-specific microdamage accumulation, causing predisposition to structural failure. We performed a fractographic examination of failure surfaces from fractured bones using scanning electron microscopy and assessed microcracking observed at the failure surface using a visual analog scale. Branching arrays of microcracks were seen in failure surfaces of CTB and adjacent tarsal bones, suggestive of compressive fatigue failure. Branching arrays of microcracks were particularly prevalent in remodeled trabecular bone that had become compact. CTB fractures showed increased microdamage when compared with other in vivo fractures (adjacent tarsal bone and long bone fractures), and ex vivo tarsal fractures induced by monotonic loading (P < 0.02). It was concluded that greyhound racing and training often results in CTB structural failure, because of accumulation and coalescence of branching arrays of fatigue microcracks, the formation of which appears to be predisposed to adapted bone.