The assessment of cortical heat during intramedullary reaming of long bones.

BACKGROUND: This experimental study was designed to compare different in vitro methods of measuring the cortical temperature when reaming the medullary cavity. This was done to validate an approved mathematical model which can be used to determine the temperature gradient in cortical bone in the presence or absence of sensors. METHODS: Artificial bone was used with an intramedullary heat source instead of a reamer. Temperatures were measured with thermocouples placed radially and axially in the cortical wall. This method with these two measurement positions were compared and used to validate an approved mathematical model. This model was used to determine the temperature gradient in cortical bone in the absence of sensors. RESULTS: The measurement of the cortical temperature with the thermocouples in a radial position only reflects maximally 14% of the temperature of the reamer (calculated 55%). The measurement with the thermocouples in axial position reflects maximally 65% (calculated 70%) of the reamer temperature, which is similar to undisturbed bone. CONCLUSION: The measuring method with the thermocouples in a radial position cannot be recommended. It is likely that a much higher temperature is generated and conducted through reaming than has been assumed until now.

New segmental long bone defect model in sheep: quantitative analysis of healing with dual energy x-ray absorptiometry.

An appropriate animal model is required for the study of treatments that enhance bone healing. A new segmental long bone defect model was developed for this purpose, and dual energy x-ray absorptiometry was used to quantify healing of this bone defect. In 15 sheep, a 3-cm segmental defect was created in the left tibia and fixed with an interlocking intramedullary nail. In seven animals, the defect was left empty for the assessment of the spontaneous healing response. In eight animals serving as a positive control, autologous bone grafting was performed. After 12 weeks, healing was evaluated with radiographs, a torsional test to failure, and dual energy x-ray absorptiometry. The mechanical test results were used for the assessment of unions and nonunions. Radiographic determination of nonunion was not reliably accomplished in this model. By means of dual energy x-ray absorptiometry, bone mineral density and content were measured in the middle of the defect. Bone mineral density was 91+/-7% (mean +/- SEM) and 72+/-6% that of the contralateral intact tibia in, respectively, the autologous bone-grafting and empty defect groups (p = 0.04). For bone mineral content, the values were, respectively, 117+/-18 and 82+/-9% (p = 0.07). Torsional strength and stiffness were also higher, although not significantly, in the group with autologous bone grafting than in that with the empty defect. Bone mineral density and content were closely related to the torsional properties (r2 ranged from 0.76 to 0.85, p < or = 0.0001). Because interlocking intramedullary nailing is a very common fixation method in patients, the newly developed segmental defect model has clinical relevance. The interlocking intramedullary nail provided adequate stability without implant failure. This model may be useful for the study of treatments that affect bone healing, and dual energy x-ray absorptiometry may be somewhat helpful in the analysis of healing of this bone defect.

Type of solution and PCO2 measurement errors during tonometry.

OBJECTIVE: The choice of solution for gastrointestinal tonometry influences the PCO2 measurement bias, precision and the time required for equilibration. We compared saline with buffered solutions during in vitro tonometry, with respect to systematic and accidental measurement errors and equilibration time. DESIGN: A prospective laboratory study. MEASUREMENTS: Saline, phosphate, phosphate bicarbonate and succinylated gelatin solutions were equilibrated in a specialized blood gas tonometer at PCO2s of 2.7, 3.6, 4.5, 6.2 and 9.0 kPa, using calibration gases. Accidental errors were determined: the within-syringe decline of PCO2 and the effects of handling errors (five up and down movements of the plunger). The PCO2 build up in gastrointestinal tonometers was determined in 5000 ml saline baths with fixed PCO2 levels of 2.7 and 9.0 kPa. RESULTS: The build up of PCO2 in phosphate bicarbonate and gelatin was about 4 and 2 times slower than in saline and phosphate, respectively, both for gas and gastrointestinal tonometers. The bias of the measured PCO2 at equilibrium was -15% for saline, and between -1 and 3% for phosphate, phosphate bicarbonate and gelatin. The precision was comparable among the solutions: 2 +/- 1% for saline, 2 +/- 1% for phosphate, 1 +/- 0% for phosphate bicarbonate and 1 +/- 1% for gelatin. The accidental errors were virtually absent with phosphate bicarbonate, intermediate with gelatin and largest with saline and phosphate. CONCLUSION: Phosphate bicarbonate buffer and succinylated gelatin allow accurate PCO2 measurements, but their equilibration is too slow for clinical application. The advantage of phosphate over saline solution is a smaller bias only. Thus, both saline and phosphate are currently the tonometer solutions of choice, provided that strictly anaerobic conditions are applied and the bias by the blood gas analyzer is known.

In vitro evaluation of intragastric PCO2 measurement by air tonometry.

OBJECTIVE: To assess the in vitro performance of a new device, the Tonocap, for semi-continuous air tonometry of regional PCO2 in the gastrointestinal tract. METHODS: The tonometer consists of an air filled balloon-tipped catheter, connected to a prototype Tonocap system. The tonometer was placed in saline baths at steady-state PCO2's ranging from 0 to 105 torr, to evaluate bias, precision and reproducibility to PCO2 measurements. The response time was defined as the time needed to detect 95% of an instantaneous change in bath PCO2. RESULTS: The bias of the PCO2 measurement (mean +/- SD) was -2 +/- 2% and reproducibility (coefficient of variation) was 2 +/- 1%. The response time was 19 +/- 2 min. CONCLUSIONS: Tonocap air tonometry is simple and eliminates most sources of error associated with conventional saline tonometry. The bias, precision, reproducibility and response time in vitro are consistent with a clinically reliable device.