Cutting Screws Out of Bones with an AWJ
An experimental study by Biskup et al.* showed the feasibility of cutting screws out of bones with an abrasive waterjet(AWJ). This article will highlight their main discoveries.
Bone-patellar tendon-bone (BPTB) implants use metallic interference screws as initial fasteners. These metallic screws present complications for magnetic resonance imaging (MRI), revision surgery, and screw insertion. For these reasons, screws made of bones are used instead. These bony screws are biodegradable. However because of unfavorable reaction of bone to high temperature and its brittleness, machining a screw out of bone presents a great challenge to traditional machining. This study investigated the feasibility of using an AWJ as an alternative method of machining.
An interference screw for this application is a specially-designed screw. Typically it uses a self-tapping design with a tapering end. A deep hex socket or other geometries for screw driving is also needed. The raw material is a 35 mm-long section of buffalo thigh bone. It was preserved in a special solution prior to machining. Machining such a screw with an AWJ used the following five steps.
(1) The first operation was to cut the bones into bar stocks with a 8 mm x 8 mm cross section. (2) Then a hole was pierced through the axle of the bar stock. Because of the brittleness of the bone, a sacrificing metal shield was placed on top of the bony material so that abrasive had a chance to be mixed with water and be accelerated before hitting the bony material. The process parameters were: 150 and 250 Mpa (22 and 36 ksi) pressure, 0.18 mm (.007”) orifice, .4 mm (.016”) focusing tube, and 0.5 g/s (.07 lb/m) 220 mesh abrasive. (3) The third operation was to cut a 2.5 mm hex hole. It used the same orifice and focusing tube as those used in piercing operation. Because the bone is a relatively soft material, the jet tends to create a reversed taper, which was a bad thing for the contour cutting. Therefore, the pressure was reduced to 100 Mpa (15 ksi) and abrasive to 0.2 g/s (.03 lb/min). By testing at varying speeds, the optimum was found to be 100 mm/min for achieving the desired hexagonal shape. However, the hexagonal shape was distorted as the depth of cut increases. It was found that using multiple-runs was able to extend the depth of the hexagonal shape. (4) The fourth operation is turning, to produce the round bar, the tapering end, a chamfer at the other end, and to cut to the desired length of 30 mm. The square bar stock was clamped on a lathe, rotating at 1900 rpm. Traverse speed was 50-100 mm/min. (5) The final operation is tread-cutting. The shape of thread was formed by overlapping 17 traces of the jet. The concentricity of the inside and outside diameters has a direct impact on the shape of the thread when it was held on the inside bore. External clamping eliminated this problem.
As a conclusion, this study proved the feasibility of AWJ cutting of screws out of bones for medical purposes.
* Biskup, C., Louis, H., Pude, F., Kirsch, L., and Schmolke, S., “Machining of bony interference screws by means of an abrasive waterjet,” Proceedings of the 17th International Conference on Water Jetting, Mainz, Germany, September 7-9, 2004, pp 231-243.
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