Drilling efficiency and temperature elevation of three types of Kirschner-wire point

Journal of Bone and Joint Surgery,  Jan 2002  by Piska, M,  Yang, L,  Reed, M,  Saleh, M

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An innovative Kirschner (K-) wire point was developed and compared in fresh pig femora in

terms of drilling efficiency and temperature elevation with the trochar and diamond points currently used in clinical practice. The tips of thermal couples were machined to the defined geometry and the temperature measured during drilling. Using the same drill speed (rev/min) and feed rate, the new K-wire point produced the lowest thrust force and torque as measured by a Kistler dynamometer. Drill point temperatures were highest with the trochar geometry (129 6+/- 6 deg C), followed by the diamond (98 70 deg C). The lowest temperatures were recorded with the Medin K-wire (66 +/- 22 deg C). On repeated drilling it could be used for up to 30 holes before reaching the less satisfactory drill performance of the diamond tip. The new K-wire provides a better alternative as it requires less effort for insertion, generates less heat and may be re-used.

J Bone Joint Surg [Br] 2002;84-B:137-40.

Received 26 October 1999; Accepted after revision 23 April 2001

Kirschner (K-) wires are commonly used in orthopaedics for percutaneous fixation of fractures and circular external fixation. Insertion of these wires involves drilling, a process which may generate heat and cause necrosis of bone. Our aim was to evaluate a new design of K-wire point and to compare it with the trochar and diamond points which are commonly used in terms of drilling efficiency and thermal effects.

Materials and Methods

Three different wire points were tested (Fig. 1). The trochar point had three facets ground to a point with a rake angle of -28 deg and clearance angle of +6 deg. Figure 2 shows the rake and clearance angles. The diamond point had two opposing flat facets ground to a pair of cutting edges with rake angles of 12 deg and clearance angles of +60 deg . The new point design (Medin K-wire; Medin AC, Nove Mesto na Morave, Czech Republic) had two steep flutes for removal of bone fragments during drilling and was ground with a rake angle of +20 deg and a clearance angle of +30 deg. All the wires tested were manufactured by the same manufacturer (Medin AC) using implant steel (ISO 5832-lE).

We obtained the femora of six-month-old pigs from a local butcher within six hours of slaughter. The soft tissues were removed and the bone was cut transversely into blocks. In order to make consistent measurements, wires were inserted in areas where the cortical thickness was 2.5 to 3 mm. All tests were performed at room temperature (22 to 24 deg C).

Drilling efficiency was characterised by the thrust force and torque under the same drilling conditions (rotation velocity n = 280 rev/min and feed rate f = 0.1 mm/rev). We used a Kistler 9272 dynamometer (Kistler Instrumente AG, Winterthur, Switzerland) to measure the thrust force and torque which were sampled at a frequency of 8kHz. The listed drill speed and feed of a round-column drilling machine were calibrated to 5%. Wires of 2.5 mm in diameter were used. For each type of point, three wires were used and each was tested six times. In order to reduce the effect of noise in the digitised data on the results, 100 data points each were selected which included the maximum thrust force and torque moment during drilling and their means calculated for each test giving 18 (3 X 6) mean forces and 18 torque moments for each type of wire. Oneway analysis of variance (ANOVA) and the Scheffe multiple range test (confidence level = 95%) were then performed on the pooled data on the three types of wire using Statgraphics v.7 (STSC Inc, Manugistics Group Inc and Statistical Graphics Corporation, Maryland).

The tool life of the Medin K-wire point was also measured since reinsertion is often necessary in clinical practice. Three new wires of each type were inserted repeatedly through a single cortex while the thrust force and torque were measured. Wear of the drill point was measured at the end according to the ISO 3685 standard.

Temperature elevation during drilling was measured using fast-response coaxial Medtherm thermocouples (Medtherm Corporation, USA, Hundsville, Alabama) with their points being shaped into the defined geometry (Fig. 1). These were calibrated against laboratory mercury thermometers to an accuracy of 1 deg. Each thermocouple was fastened to the tool head of a computer numerical control lathe and inserted into the specimen of bone which was clamped onto a special template. Thermocouple output was sampled at 50Hz and recorded during drilling of the first cortex. Each wire point was tested three times. For each test we selected data for eight seconds which included the highest temperatures for each type of wire. ANOVA and the Scheffe multiple range test (confidence level 95%) were then performed on the pooled files.

Results

The thrust force and torque showed typical 'M' shapes which corresponded to dual-cortex penetration with the higher values occurring during the penetration of the first cortex for nearly all specimens. The box-whisker plots for thrust force and torque moment are shown in Figures 3 and 4. There were significant differences (p