The assessment of cortical heat during intramedullary reaming of long bones
Introduction
Intramedullary nailing of femoral and tibial fractures is the method of choice to achieve fracture stabilisation. Prior to insertion of an intramedullary nail, the medullary canal can be widened with a reamer [1]. Recent studies have demonstrated that the effect of reaming on fracture healing in patients with femoral or tibial fractures favours the use of reamed instead of unreamed nails [2], [3]. For this reason, our research has focussed on the local effects of reaming with renewed interest.
One clinically important, potentially harmful effect of reaming is the generation of heat [4], [5]. This heat is generated by the metal reamer head and conducted to the cortical bone and from there to the soft tissues covering the bone. Case reports show that excessive reaming may cause thermal injury to the bone as well as to the soft tissues surrounding it [6], [7].
Cortical temperature registration when reaming the medullary cavity of long bones in vivo has never been performed. No measuring method has been developed, because the attachment of thermocouples in the cortical wall requires a surgical approach which is not desired in intramedullary nailing techniques. Therefore in vitro methods have been developed of which the direct method of Müller [5] is the most recent one. He described a measuring method which was tested in human cadaveric femora in a 37°C water bath and measured the temperature at the smallest diameter of the medullary cavity with thermal probes inserted at all four quarters radially in the bone. In order to measure the maximum temperature, the final reamer just touched the probe (Fig. 1). They compared blunt and sharp reamers with this method and concluded that no heat-induced osteonecrosis occurs.
Using different methods of determination, the threshold value for irreversible cortical damage may vary and depends on the exposed temperature and the time of exposure. Bone necrosis becomes morphologically visible when using a direct microscopical method at 47°C with an exposure time of 1 min [8]. Histochemical as well as histological methods underestimate the thermal damage [9]. Since reaming is a procedure which can easily be performed in less that one minute under normal conditions, Lundskog [10] has shown that heat necrosis can also occur in a much shorter exposure time. Assuming that the time of inserting and removing a reamer from the medullary canal should not go beyond 40 s under normal conditions, the time of true contact between the reamer and inner cortical wall may be even less than 15 seconds. Therefore it is safe to assume a temperature as high as 77°C to cause irreversible thermal injury to the bone [11].
An undesired discrepancy has now occurred. Thermal complications of reaming in bone has been described [6], but no method of measuring shows that the temperature indeed reaches the threshold level of osteonecrosis. The introduction of new, sharp reamers, designed to minimise the development of cortical heat necrosis, therefore cannot be evaluated. The danger of this phenomenon is that the evaluation of different reamers with the existing measuring method can lead to false conclusions [5].
In this study, we measured the cortical temperature with two positions of the thermocouples in the cortical wall in order to validate an approved mathematical model. This model was used to determine the temperature gradient in the cortical bone in the presence or absence of sensors, of which the latter is the actual situation in vivo.
Section snippets
Measurement
In this study a controlled heat source of 47°C was used instead of a reamer, which was connected to an electrical heat inductor. This artificial reamer fitted exactly in a polycarbonate cylinder with a wall thickness of 6 mm and a canal width of 12 mm, which acted as the long bone. The measurements were carried out on artificial bone to rule out the confounding effect of the heterogenic structure of cortical bone [12]. Polycarbonate (Lexan®, Vink Didam, The Netherlands) is an accepted material
Results
The temperature assessment with the thermocouples in radial position in this experimental model showed a measured temperature of 38.7°C after an exposure time of 15 s with a source temperature of 47°C. The calculated temperature was 41.7°C under the same circumstances (P<0.05) (Fig. 4). With the thermocouples in axial position the measured temperature was 43.1°C after the same exposure time of 15 s with the same source temperature of 47°C. The calculated temperature was 43°C under equal
Discussion
Since the introduction of intramedullary nailing techniques, research has focussed on the measurement of cortical heat development during reaming the medullary cavity in cadaveric and experimental models [4], [5]. The Küntscher principle of achieving rotational stability by hammering a large nail in the medullary canal to obtain interference fit and thus creating a three-point contact between nail and bone demanded a thorough reaming of the medullary cavity prior to insertion of the nail [1].
Conclusion
This study was performed to validate the use of radially placed thermocouples in the cortical wall for measurement of heat through the cortex of long bones during intramedullary reaming. In an artificial bone model we demonstrated that with this method at best only maximally 14% of the generated temperature by the heat source in the medullary cavity is measured. Therefore this method cannot be recommended and it is likely that a much higher temperature is generated and conducted through reaming
Acknowledgements
Gerhard C Reeling Brouwer MSc, Aeronautical Engineer, Delft, The Netherlands. Rob Schuckmann, System Development Engineer, Amsterdam, The Netherlands.
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