Surface Treatment Effects on Disposable Cutters, Bone Drills
Low-friction chromium coating proves to be valuable addition for better overall results
Larry Noble - President
Dave McKay - Process Manager
The Electrolizing Corporation of OHIO
The remarkable results achieved when a unique, low-friction chromium coating is applied to stainless steel substrates are well documented.
Extensive testing has demonstrated the biologic compatibility of this coating; we have seen that the coating greatly improves the durability of surgical instruments due to hardness and corrosion resistance.
But what practical application does this coating have for the majority of the disposable cutters and drill bits that surgeons use?
Arthroscopic surgeons use state-of-the-art tube cutters that allow precise bone removal in knee joints.
These high-speed cutters work by rotating a small-diameter tube with a toothed cutting edge inside a larger tube with a small window at one end.
Stainless steel on stainless steel surfaces produces significant friction, which can quickly lead to seizing and galling of the cutter.
To reduce the friction that can cause these catastrophic failures, prevent heat buildup that can result in bone necrosis and incidentally allow faster RPMs, the inner tube is given a low-friction coating.
Orthopedic surgeons use a wide variety of machined and ground stainless steel bits to drill holes in bone for anchor screws. An almost unlimited selection of diameters, flutes and lengths is available from manufacturers globally.
Generally, these bits are machined, ground and electropolished prior to use.
Titanium nitride (TiNi) coatings are sometimes used to give the bits a “gold” color.
Electropolishing of the bits is required to provide an ultra-passive surface free from contaminants.
These considerations inspired an experiment to compare the heat buildup and wear of surface treatments in a bone-drilling context.
Five surface treatments were tested. The first three surface treatments are well known to manufacturers of surgical instrumentation: low-friction chromium, TiNi and electropolish.
The other two surfaces were selected to compare the effects of electropolish with an unpolished surface: (1) heat-treated and ground, not electropolished and (2) heat-treated and ground, then coated with low-friction chromium with no prior surface prep.
Electropolished drill bit, before drilling. Note the sharp edges.
Electropolished bit after drilling. Note edges, debris on flute.
Low-friction chromium coating drill bit, before drilling. Note edges and point.
Low-friction chromium coating drill bit, after drilling. Edges and point remain unworn.
TiNi drill bit, before drilling.
TiNi drill bit, showing wear on flute after drilling.
The experiment was designed and performed as follows:
• A number of single flute five mm 17-4 SS electropolished bone drills were purchased and inspected for uniformity of the points, cutting edges and flute edges. Nine bits were selected from this lot. Three were coated with TiNi, three with low-friction chromium and the last were left as received. A different lot of bits purchased from the same supplier was machined, heat treated and ground. No electropolish treatment was given to this lot of parts.
Three were coated with low-friction chromium and three were left as received.
• Animal bone material was purchased, milled to a 4 x 1 inch rectangle, at .180 inch thick.
A three-axis (X, Y and Z) CNC mill was selected as the means to perform the actual drilling to ensure consistent, repeatable speed and feed rates of the bone drills.
An infrared thermometer was positioned to measure the temperature of the drill bit as it left the bone.
• A holding fixture was designed to clamp the bone material in place.
The bone material was stacked two high and clamped in place. Five stacks at a time were placed on the fixture.
The mill was programmed to drill 12 holes per bone stack, for a total of 60 holes per drill.
The speed and feed rates were programmed to duplicate as closely as possible the RPM and pressure of a hand-held surgical drill.
• Sixty holes per bit were drilled.
The results of the temperature rise were recorded and charted for each surface treatment.
The low-friction chromium coated bits showed the least heat generation and stabilized rapidly.
In sharp contrast, the electropolished and TiNi coatings showed a quick initial rise and ended in an upward curve.
The uncoated ground bits and low-friction chromium-only bits were significantly lower in temperature than the electropolished/coated bits.
|Top left,TiNi coated bit after 60 holes; top right, electropolished bit after 60 holes; left, low friction chromium bit after 60 holes.|
This was attributed to the fact that electropolish tends to round the sharp cutting edges. From the data it may be concluded that, instead of electropolishing bits after final grind, a low-friction chromium coating offers the best results.
Photographs of the bits were taken prior to and after the test (Figure 1). The electropolished and TiNi coated bits showed definite signs of wear, as did the uncoated ground bit.
However, the low-friction chromium coating showed little if any change.
Moreover, the bone residue left on the bits was considerably less than the electropolished and TiNi bits.
Finally, the bone material was inspected after drilling. Figure 2 shows the results of drilling on the last 12 holes of each type of surface treatment. A nonuniform edge and some bone splintering are evident on the electropolished and TiNi bone. However, both of the low-friction chromium bits showed a smooth bore with uniform edge.
It is evident that low-friction chromium is the best choice to extend the life of surgical instruments and improve the performance of reusable and disposable cutters and bits.
Reduction of excessive heat generation at the bone/drill interface could lead to fewer instances of bone necrosis, a very positive development.