Cannulated screw performance relies on accurate deep-hole drilling. In orthopedic implants, accuracy affects both the guidewire alignment and the implant position.
In this post, we’ll look at how drilling drift occurs and how to control it using machining techniques.
What Are Cannulated Screws in Orthopedic Implants?
Cannulated screws are specialized orthopedic implants that have a hollow core called cannulation, which enables them to pass over a guidewire during surgical procedures. The use of such an orthopedic implant helps surgeons place the screw in the exact position when performing surgical operations on the bones and joints.
Why Cannulated Screw Accuracy Depends on Deep-Hole Straightness
They aid in the accurate alignment and optimal location of the implant along the guidewire during preliminary drilling. In surgery, the inner bore serves as the guidewire’s pathway, guiding the implant along its intended course. Because of the need for surgical accuracy, cannulated screws require a longer machined interior bore than solid screws.
In cannulated screw manufacturing, another crucial factor influencing the interaction between the bore hole inside and the thread shape on the outside of the bone screws is the straightness of the deep hole drill. To ensure load distribution and stable fixation, the two parts must remain concentrically aligned.
Why Cannulated Screws are Prone to Deep-Hole Drilling Drift
Long L/D Ratio Geometry
Generally, cannulated screws need to undergo deep hole drilling procedures, with their length-to-diameter (L/D) ratios being between 10:1 and more than 20:1, depending on the design of the implants.
For example, a screw with an L/D ratio of 2.5 mm may have to drill holes that extend up to 30-50 mm in depth or even higher. With increasing L/D ratios, there will be more tendency for drill deflection, vibration, and deviation.
Thin-Wall Structural Constraints
The hollow nature of cannulated screws results in there being very thin walls between the hollow inside and the threads on the outside surface. In most orthopedic screws, there can be thin walls of less than 0.5 to 1.0 mm in some areas. This will make the screw susceptible to machining stress due to vibrations.
Multi-Process Dependency
Cannulated screws are manufactured in a variety of precision processes, including drilling, turning, threading, heat treatment, and finishing. The inner diameter is often measured early in the manufacturing process and used as a reference for the rest of the process.
As a result, any deviation within the bore, even a hundredth of a millimeter, can affect subsequent steps and result in imprecise placement of the implant.
Root Causes of Drift in Cannulated Screw Machining
Tool Deflection
In deep-hole drilling for cannulated screws, drift occurs due to the increased flexibility of the drill as the depth increases. When forces are applied to the drill, they are never entirely aligned with the drill’s center, causing the tool to bend.
As drilling progresses, the length of unsupported material increases, as does the likelihood of bending. In medical CNC machining, bending causes the tool path to drift away from its central axis.
Thermal Instability
The other kind of drift observed while creating deep holes in cannulated screws is due to heat influences. Continuous cutting generates a significant amount of heat. This causes unequal heat expansion along the tool. The drill point expands more than the other parts of the drill. This causes internal stress in the drill, which results in drift.
Chip Evacuation Restriction
As we go deeper, there is less capacity for chip evacuation, and chips begin to accumulate in the flutes. We experience inconsistent friction and chip jamming, resulting in uneven force application on the drill bit and pushing it out of the way. With deep drilling, our drill will gradually deviate.
Material Cutting Resistance
In cannulated screws machining, we are dealing with hard corrosion-resistant biomedical alloys. When we find hard spots while machining, the cutting resistance becomes irregular and causes uneven force distribution around the tool. It makes the drill bit drift naturally towards soft spots in the material.
Spindle and Fixture Runout
The drill may rotate with some eccentricity and will not have perfectly concentric rotation but an orbiting one. This will cause asymmetrical cutting loads each time the drill makes a full rotation. As we continue to make holes in medical CNC machining for cannulated screws, the small difference in concentricity builds up.
Engineering Requirements for Cannulated Screw Manufacturing
Bore Straightness Control
Ensure that the guidewire is put in the same position over the whole length of the cannulated screw. Bore variation leads to difficult insertions, poor screw alignment once implanted, and imprecise surgery.
Bore-to-Thread Concentricity
Maintain the screw’s inner bore canal aligned with its thread axis. This will prevent the screw from improperly threading, progressing over the guidewire, and exhibiting inconsistent mechanical behavior once implanted.
Internal Surface Integrity
In cannulated screw manufacturing, we maintain internal surface integrity because it directly affects how easy the screw is to manipulate during surgery. Increased surface friction, increased resistance to guidewire insertion, and the production of particulates may result from irregular internal surface integrity.
Wall Thickness Uniformity
We maintain wall thickness uniformity because this will have a direct effect on the structural integrity and fatigue strength of the implant. If there are variations in the wall thickness, then the risk of fatigue failure increases due to stress concentrations.
CNC Machining Methods to Control Drift in Cannulated Screws
Gun-Drilling for High-Straightness Bores
Based on our experience with medical CNC machining services, gun drilling remains the preferred method for producing accurate, straight bores in cannulated screws. The drill’s guiding nature, as well as the effective removal of chips from the bore, helps keep the bore route straight.
Multi-Stage Drilling Strategy
Another method that we use is the deep-hole drilling process in multiple stages. Rather than boring the hole to full depth in one go, we continue drilling the hole progressively and consistently.
High-Pressure Coolant Delivery
We have found that the proper application of coolant helps with efficient cooling of the system, lubricating the interface, and removing chips.
Precision Fixturing Systems
It is essential to apply correctly aligned fixtures for securely mounting the workpiece, thereby avoiding vibrations during machining processes. Precise alignment ensures that there are no positional errors since the workpiece will not move during drilling.
Optimized Cutting Parameters
Our machining experience suggests that optimized cutting parameters will provide greater machine stability and reduced tool deflection, resulting in higher accuracy and consistency.
How Cannulated Screw Concentricity is Controlled During Production
Single-setup machining strategy
Whenever possible, important parameters like cannulated bore, outer diameter, thread reliefs, and head shape are manufactured in one setup using a CNC lathe or Swiss-type machine. This prevents misalignment and maintains concentricity within critical process limits.
Datum-based alignment control
The cannulated bore acts as the datum axis for all operations, including cutting, threading, and finishing. All geometric tolerances are measured with respect to the datum axis, which means that the guidewire bore acts as the basis for aligning exterior surfaces.
Minimizing repositioning error between operations
High-precision chucks, guiding fixtures, and precise workholding devices are employed to ensure consistent part positioning. In cannulated screw manufacturing, the layout is set up to minimize the number of part repositionings; hence, fewer errors from poor part repositioning occur.
Threading aligned to drilled reference axis
Cannulated screws are manufactured using CNC threading or CNC cutting operations, with the threading performed on the same reference axis as the drilled portion. Before machining, the tool offset and spindle alignment are performed to ensure uniform pitch diameters and concentricity.
Quality Control for Cannulated Screws
Bore Straightness Inspection
Inspect the straightness of the internal bore throughout the length to ensure proper guidewire alignment and maintain surgical accuracy; usually, deviation should not exceed ≤0.05-0.10 mm based on the diameter of the screws for proper guidewire positioning.
Concentricity Measurement
Measure the concentricity of the inner bore, outer diameter, and the centerline of the threads, which is usually kept within the tolerance limit of ≤0.03-0.08 mm TIR.
CT Scanning
Use a non-destructive testing method to check the internal geometry of the component, capable of detecting any voids, eccentricity, and defects even below ~10-25 μm.
CMM Inspection
We use highly precise measurement techniques for GD&T; the typical measurement tolerance is ±1-3 μm.
Surface Roughness Testing
Inspect the surface roughness to maintain biocompatibility and performance; typical values are ≤0.2-0.8 μm, depending upon the type of surface.
Material Selection for Cannulated Screw Manufacturing
Titanium Alloys
In our experience with orthopedic implant manufacturing, titanium alloys are typically our first choice when titanium cannulated screws are needed. This is due to their high biocompatibility, corrosion resistance, and strength-to-weight ratio, which make them ideal for use in implants.
Stainless Steel Grades
Medical stainless steels remain a cost-effective and manufacturable option for production decisions. Stainless steel is commonly used in orthopedic applications, such as cannulated screws for patella fractures. It provides robust and durable fixation at a lower cost than titanium.
Material Behavior in Deep-Hole Drilling
Material behavior significantly affects how we produce cannulated screws for deep-hole drilling operations. Whether we work on cannulated screws used to treat femoral neck fractures or on another medical application, properties such as hardness, toughness, thermal conductivity, and chip formation play a vital role.
Conclusion
Accurate cannulated screw manufacture necessitates controlling drilling drift in deep, long bores. As we’ve seen, drift is caused by a variety of factors that can be mitigated through precision machining and rigorous process control, thereby maintaining these variations within medical tolerances.
At MedCNCParts, we specialize in high-precision CNC machining. Contact us today to get a free quote.
