The assessment of cortical heat during intramedullary reaming of long bones

Abstract

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

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.