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Step 2: Optimize instrument geometry to match the specifics of bone structure
In part 1, we highlighted some features of modern industrial drills such as point angle, lip and chisel edge; that drill designers tune to enhance performance. Today, we look at how to tune a rotary instrument design specifically for bone drilling.
Steeper point angles
The point angle is the most visible difference between a bone drill and an industrial drill. 118-135ᵒ is common in metal working drills, but for drilling bone, a steeper point is recommended.
The surface of the cortical layer may often be curving away from the inclination angle of the drill and a shallow point risks sliding across the surface instead of engaging and penetrating.
Bone drills often have points as steep as 70ᵒ, which mean a very sharp tip with good engagement and drive.
A 70ᵒ instrument with conical relief
In many cases a 70ᵒ point will work fine, but there are some drawbacks to such steep pointed instruments. First, steep points are weaker than shallow points and more prone to breakage and blunting. Second, steep points are more prone to grabbing on penetration or layer transition, potentially drawing the instrument deeper than desired into the cancelous layer. And third, it’s difficult (but not impossible) to design a very steep point where the long lip cuts all the way to the outer diameter.
A compromise that’s becoming popular is a 90ᵒ point angle with web-thinning, such as split pointing or S-gashing. The thinned web engages early and reduces the tendency of the instrument to wander over the bone surface. The shallower point angle gives the instrument more strength and makes it easier to control during penetration.
A modern 90ᵒ instrument with facet relief and S-gash web-thinning
Custom flute shapes
As we mentioned in part 1, the shape of the flute and the tip relief surface dictate the shape of the lip and its cutting properties. Due to poor control over flute shape, some lower quality steep pointed instruments can have as much as 0.7mm (.03”) of the lip that is not cutting at all. This non-cutting portion of the lip can cause heat build-up in the outer layer of the wound. At 50ᵒC, bone necrosis sets in and this ultimately impacts patient outcomes by reducing prosthesis fusion to the bone in the heat-effected zone. But this problem can be easily fixed with modern design techniques.
To design a lip that performs safely, you need to pay careful attention to the flute shape. This is one area where industrial drill designers have had the upper hand. Two factors have worked together to advance the art of flute shape design. These days, industrial drills are nearly always made on CNC tool grinders. In the next installment, we’ll cover a number of benefits of grinding over other manufacturing techniques like swissturn. But one that stands out is worth mentioning here.
Modern CNC tool grinders can dress a custom shape onto a grinding wheel to fit a desired flute shape. They can do this quickly and automatically, at micron level accuracy. In contrast, to machine the flute onto a drill, you either need to have a custom mill produced to exactly match the desired flute profile, or else you have to use a standard shaped mill and put up with the resultant errors in flute shape and consequent poor lip performance - not uncommon for instruments manufactured on swissturns.
1) Custom shaped grinding wheel. 2) Dresser mounted directly in the CNC tool grinder
The other factor that’s advanced flute shape design for industrial drills is that because so many people need to design custom flute shapes to match the complex lip profiles of modern industrial drills, the software available on tool grinders and their related CAD systems has advanced to the point where you can specify the flute shape you want and the software will automatically calculate the wheel shape you need to grind it. It’s worth noting that because of the sweeping motion the wheel performs when grinding a helical flute, the wheel shape will be quite different to the flute shape it produces. This is not a simple problem to solve, but modern flute design software such as iFlute, does it with ease and then guides the machine to dress the wheel to the required shape. In tool grinding, it’s not unusual to design a new flute shape and be grinding it minutes later; shortening the prototype phase for new drills considerably.
ANCA’s flute design software (iFlute) being used to
design the grinding wheel shown in the previous diagram
Previously, we showed why helical fluted tools work better than shear fluted tools; better chip evacuation, better stability and less heat build-up. The same principle applies for rotary instruments. A helix of 36-40ᵒ is a good starting point for many bone drill designs, but as we’ve just discussed, adding more helix to a drill complicates the flute design and production process. But this complication is now much less of an issue when using tool design software.
Include a pilot diameter
For instruments that need exceptionally high self centering tendencies, such as drills designed for use on non-flat bone surfaces or for use without a drill guide, consider including a pilot diameter segment. In industry these types of tools are called step drills and are very common in automotive applications such as valve seat drilling, where a multi-diameter hole is needed.
A piloted rotary instrument with a 90ᵒ point angle and split point web-thinning
In orthopedics, you aren’t usually going to drill multi-diameter holes, but the benefits of a piloted instrument can still be significant. Operating a piloted instrument produces a similar outcome to first drilling a very small and accurate pilot hole, then opening the hole out with a larger instrument. This multi-step technique is familiar to orthopedic surgeons; for example, when using a set of graduated femoral reamers (rasps) in preparation for a femoral stem implant during a total hip replacement.
For accurate cylindrical hole drilling, particularly for 3.5mm (0.14”) or larger holes, a piloted instrument can be superior to a single diameter instrument or a multi-step technique because when the larger diameter makes contact with the bone, the thinner segment of the instrument is still seated in the pilot hole ensuring perfect alignment and eliminating flex as the drill enlarges the hole. Also, because there are now 2 pairs of cutting edges operating at different depths in the bone, heat build-up is less of a risk. It can also be helpful to include a very slight back-taper on the pilot segment to reduce friction further reducing heat build-up.
As well as relieving the tip, it’s also important to relieve the step lip. High end software for step drill design will have a number of options for step relief including facet, rolled or eccentric relief and some packages, such as ANCA’s iGrind, can automatically match the surface of the step relief to the adjacent clearance of the pilot segment, without leaving unsightly bumps or gouges. This feature, combined with instant 3D visual feedback makes the programming of piloted instruments relatively straight forward.
Use a double point instead of a pilot when space is limited
A variation on the piloted instrument is the double point angle instrument which has similar characteristics but is tailored to applications, such as cervical drills, where penetration space is limited.
A double point angle bone drill (with different operations highlighted)
Three flute designs
Rotary instruments have high length:diameter ratios and are made from stainless steel so they tend to have more flex than industrial drills. So anything that improves an instrument’s stability is worth considering. Tri-flute instrument designs can be more stable in some circumstances than traditional 2 flute designs. A drill is supported in its hole by the margin and having 3 margins supporting the tool at every cross-section provides more balanced support than 2 margins so a 3 flute instrument will generally resist flex better than a 2 flute design.
There are however, several design and manufacturing challenges in 3 fluted instruments; the biggest one being, to design an effective lip and flute shape. Because the flute is narrower they can be more easily prone to clogging, so the shape of the flute and the gash walk must be carefully designed to break chips up into smaller pieces so they can be properly evacuated up the flute and out of the hole.
Industry is always on a quest to drive down the cost of production. One common method of saving production time is to combine multiple operations into a single, special purpose tool (eg: drilling and deburring, drilling and countersinking). An added benefit is that the quality and consistency of the job improves because there’s no room for error between the 2 operations.
In medical instrumentation, there are several opportunities to combine multiple functions into a single instrument for greater efficiency, reduced risk and better accuracy. The piloted drill described above is one example. An extension on this concept is a drill reamer where the drill portion of the tool drills a slightly undersized hole and the reamer portion gently and accurately scours the hole out to the target size.
A drill-reamer combination tool
Sometimes the instrument can even be combined with the implant. An example of this is the self-tapping bone screw:
A self-tapping bone screw which includes 1) a drilling portion,
2) a thread cutting portion and 3) a screw portion
Tool design software (such as ANCA’s ToolRoom allows you to build up tool designs with dissimilar segments and does the hard work for you of matching flutes and indexes between segments. This same software is just as useful for designing combination rotary instruments. Are you using segments in your rotary instrument designs yet?
Consider a TiN coating
Industrial cutting tools are often coated to improve the tool’s hardness and edge retention. A Titanium Nitride (TiN) coating can improve a cutting tool’s life by a factor of three or more. TiN can also be used as a coating on rotary instruments to improve edge retention.
A TiN coated industrial drill (courtesy Sutton Tools)
It also forms a visual cue for re-use; an instrument where the base metal is starting to show through the gold TiN coating should be inspected for possible replacement.
What to include on your prints
When specifying a rotary instrument, you’ll make the manufacturer’s job a lot easier if you include at least the following dimensions on your print:
- Web (thickness)
- Helix angle (or lead)
- Flute length
- Point angle
- Number of flutes
- Direction of flute spiral (right or left handed)
- Point type (eg: standard, split point, gash walk etc.)
- Relief type (eg: conical, facet or rolled)
- Lip relief angle (primary and possibly secondary)
Next time, we’ll show you why rotary instruments should be ground and how they are being produced, often in a single setup, on the latest generation of CNC tool grinders.
||Step 1: Apply some fresh techniques from industrial cutting tool design
||Step 2: Optimize instrument geometry to match the specifics of bone structure
||Step 3: Leverage the advantages of grinding over Swiss turning
||Step 4: Select a material and grinding wheel to suit your bone drill
||Step 5: Drill your way deep into the medical components market
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