Mechanical evaluation of the effect of the rod to rod distance on the stiffness of uniplanar external fixator frames.

PURPOSE: To investigate the effect of the rod-to-rod distance on the mechanical stability of single-rod and double-rod external fixator frames. METHODS:  Four different constructs, one single-rod and three double-rod constructs with different rod-rod distances, were subjected to the axial, bending, and torsional forces. The stiffness of different configurations was calculated. RESULTS:  Single-rod configuration had statistically the lowest stiffness when subjected to the axial, bending, and torsional forces. Maximum stiffness against the axial and anterior-posterior bending forces was achieved when the rod-rod distance was adjusted to 50 mm (halfway between the first rod and the end of the Schanz pins). There was no statistically significant difference in lateral bending stiffness among different double-rod configurations (p value: 0.435). The maximum stiffness against torsional forces was achieved when the rod-rod distance was adjusted to 100 mm (the second rod at the end of the Schanz pins). CONCLUSION:  Double-rod uniplanar external fixator frames are significantly stiffer than the single-rod constructs, and however, the rod-rod distance can significantly affect the construct stiffness. We found that a frame with 50 mm rod-rod distance was the optimum fixator among tested configurations that allowed a balance between axial, bending, and torsional stiffness of the construct.

[Respiratory tract obstruction caused by ballooning of an intubation tube cuff during nitrous oxide inhalation]. / Obstruction des voies aériennes par hernie du ballonnet de la sonde d'intubation au cours d'inhalation de protoxyde d'azote.

A typical case of upper airway obstruction due to deformation of a low pressure tracheal tube cuff is reported. It would seem that this herniation may have been due to nitrous oxide diffusing with in the air-filled cuff, thereby causing it to overdistend. This increase in pressure occurred after 3 hours of inhalation of a mixture of oxygen and nitrous oxide. The deformed cuff may either block the lumen at the end of the tube, or push the tube against the tracheal wall. The result is, in either case, a mechanical obstruction of the airway, with hypoxia, and then anoxia. In the reported case, it was the decrease of SpO2 which alerted the anaesthetist. Pulse oximetry was helpful for an early diagnosis. To avoid such accidents, it is suggested either to inflate the cuff with the gas mixture with which the patient is ventilated, or to deflate it every 30 min.