Optimization of drill geometry for orthopaedic surgery

Abstract

The fixation of fractures represents the great compromise between biological principles and engineering principles (i). Although a fracture may be very soundly and solidly fixed according to engineering principles, bony union will fail if this violates the biological principles of fracture healing. The aim in all methods of fracture fixation is to produce enough immobilization to allow the process of bony healing to proceed without interruption and yet to do as little as possible which will interfere with the outcome of the important biological processes. The purpose then in almost any type of fixation is to produce a fixation of the bone, either by internal or external means, which is sufficient only to maintain its position while the biological process of healing goes on.
To hold a reduction rigidly, the fracture must be converted into a stable unit. Only if this rigidity is maintained during the whole healing process can patients remain free from pain even while exercising muscles and joints (2). The slightest movement at the fracture site will result in joint stiffness produced by pain. The rigidity of internal fixation therefore determines the success of open fracture treatment. For this form of treatment plates and screws are the devices most commonly used. The successful outcome of this method depends on fundamental mechanical principles and failures
are usually caused by defective materials or improper technique of application. The hole into which the screw is placed must be of correct size, not too large so that the threads of the screw or tap cut a thread in the bone insufficiently deep to hold the screw, but not so small as to start a fracture in the bone or cause the screw to break during insertion. The majority of holes drilled preparatory to the insertion of screws are made in the shaft of long bones. These shafts of long bones are in effect tubes, there being a hard outer cortex and a central medullary cavity. The point of entry of the drill is called the near or proximal cortex and the point of exit the far or distal cortex. Bone as a material, is a specialised type of connective tissue, characterised by the presence of cells with long branching processes (osteocytes) which occupy cavities (lacunae) and
fine canals (canaliculi) in a hard dense matrix consisting of bundles of collagenous fibres in an amorphous ground substance
(cement) impregnated with calcium phosphate complexes (3). The bone structure may therefore be described as a highly
oriented fibre matrix with calcium salts cemented in place to provide rigidity. Bone tissue shows many levels of organisation,
the most prominent of which is the lamellar structure resembling a laminated plywood-type matrix.
As of necessity the holes are drilled freehand which is difficult to do even with a drill guide. With an inefficient cutting action there is considerably more effort required by the surgeon and his control over the drilling process is affected. If the torque on the drill is too great there is a tendency for the drill to jam or even to break in the hole. Should excessive thrust be required the drill may plunge through the far cortex damaging surrounding soft tissue. With the increased use of internal fixation and the associated difficulties of drilling bone an interesting question arose as to whether the drill geometry could be changed in order to improve drill efficiency. Owing to the nature of the material an empirical approach was made to the drilling problem, fresh pig femurs being used for the specimens because of the impracticability of obtaining human bone.
The features of the drill design which affect its performance are the material and its geometry. The materials used 1' or
orthopaedic drills are high speed steel, stainless steel and vitallium. The drill shape includes diameter, length, web and flute proportions and type of point grinding. Some of these features are controlled by the drill manufacturers while others can be changed by the user to suit a specific set of conditions. The importance of selecting particular tool shapes for different materials is generally recognised and in engineering operations it is usual to grind the tools to these recommended angles, these angles being the result of much investigation.
From an analysis of drill geometry it can be seen that helix angle, web thickness and point angle are the three quantities
of greatest significance in determining drill performance (51). In recent years there have been developments in drill point
geometries, these being the spiral point, the four facet point and the radial or rounded lip point (96), each with its own
particular characteristics. Three types of points were used for this investigation, these being the radial relief, four facet and spiral point ground at angles from So - 150 degrees inclusive. The helix angles of the drills used were quick, normal and slow. The effects of both web thickness and point thinning were also considered. This work endeavours to establish what effect the type of drill point, helix angle and web thickness have on drill performance, the knowledge of which will enable the existing orthopaedic drill to be improved. Drill life was not considered important as the number of holes drilled at any
one time would be relatively few and the drill could be resharpened, or replaced at the fjrst indication of dullness or after approximately forty holes (4).