BIOMECHANICSThe design and function of surgical drills and K-wires
Section snippets
The drilling process
Surgical drills cut holes in bone for the insertion of various implants. The modern drill bit is a complex engineering tool whose various design elements allow penetration of bone in an efficient manner, creating reproducible channels of uniform size. Drilling is the most fundamental surgical skill in orthopaedics. This paper explains the structure and function of surgical drills and K-wires.
To cut a material requires a blade to engage the surface of a material at a critical angle, thus
The engineering structure of a twist drill
A twist drill is a complex engineering tool. Its structure can be broken down to understand the function of its various elements (Fig. 2).
The cutting process
The process of drilling requires a twist drill to penetrate the surface of a material removing through the process of shear material from the cut surface as it advances. The process of drilling is complex, as the various elements of the cutting tip act in different ways during drilling. The chisel edge at the centre does not cut, but instead displaces material ahead of it, like a wedge, as it thrusts forward. The cutting lip engages the material surface, producing plastic deformation along
Rake angle
The geometry of a double edged twist drill is complex. The orthogonal rake angle is measured in a plane perpendicular to the cutting edge and is the angle between the cutting edge velocity vector and tangent to the rake face. Thus the rake angle is not constant along the cutting edge.4
An optimum rake angle facilitates cutting, decreases deformation of material cut by the tool, improves chip flow and reduces specific cutting energy. Increasing the positive rake angle decreases the principal
The influence of bone on the drilling process
The physical properties of bone vary with its state and determine its behaviour when being drilled. Dry bone is described as short chipping, as the debris generated are small particles that are easily cleared by the drill bit. This behaviour is similar to brass, cast iron and hard steel. The optimal helix angle for them is small, that is, a slow helix. At surgery bone debris is mixed with blood and marrow fat and in this wet state it flows differently and tends to clog slow helix flutes; in
The design of K-wires
Kirschner first described the use of his wire in 1909.11 The design of Kirschner wires is far more primitive than modern surgical twist drills. This is partly because their functions are different. K- wires are used in the stabilisation of fractures, they are required to penetrate bone at various angles, generating a smooth drill hole which engages and secures the wire in position. Minimal heat generation ensures a good prolonged grip within bone.
Traditionally there have been two tip designs (
Diamond and trocar tips
Diamond tip wires require the least axial load to penetrate bone and thus generate the least heat during drilling. The flattened portions proximal to the diamond cutting edge improve debris clearance and reduce friction, thus increasing drilling efficiency. The holes generated by diamond tip wires tend to be oval, reducing the immediate holding power of the wire.12, 13
Trocar tip wires require greater load to penetrate bone and generate higher temperatures than diamond tip wires on insertion.
New K-wire designs
Other designs of K-wire have been tested in the literature. Namba et al.13 tested a home-made design produced by cutting a pin obliquely with wire cutters. This generated an eccentrically located tip with a single oblique cutting edge. Insertion load and torque for this design was similar to the trocar tip. Immediate pull out strength was inferior to both trocar and diamond tip designs. With a wide, irregular and burred edge the cut tip produces a hole larger than the diameter of the wire
Conclusion
Surgical drills and K-wires are complex engineering tools finely crafted for precision work. An understanding of their structure will improve ways of using them. Every craftsmen must know his tools.
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