Appearance
Cannulated bone screws and spinal implants — those with a hollow central bore — have transformed orthopedic surgery by enabling minimally invasive percutaneous placement over guide wires. The bore that runs through these implants, often no wider than a hypodermic needle yet extending up to 400 mm through titanium alloy, is produced almost exclusively by gun drilling. This is deep hole drilling at its most precise, in materials where any deviation from specification means a rejected implant that can never be reworked.
Cannulated Implant Types
Bone Screws
Cannulated bone screws are used in orthopedic trauma, foot and ankle, hand and wrist, and spinal surgery. The cannulation hole runs the full length of the screw and accommodates a Kirschner wire (K-wire) that guides the screw into precise position.
| Screw Type | Typical Diameter | Cannulation Diameter | Length Range |
|---|---|---|---|
| Small fragment (hand/wrist) | 2.0–4.0 mm | 1.0–1.5 mm | 10–40 mm |
| Large fragment (trauma) | 4.5–7.3 mm | 1.5–2.8 mm | 30–120 mm |
| Pedicle screw (spinal) | 4.5–8.5 mm | 1.6–2.5 mm | 30–80 mm |
| Interference screw (knee) | 7–12 mm | 1.5–2.0 mm | 20–40 mm |
Intramedullary Nails
IM nails are inserted into the medullary canal of long bones (femur, tibia, humerus) to stabilize fractures. They require a cannulation hole through the entire length for guide wire placement during insertion and for later nail removal.
| Nail Type | Typical Length | Cannulation Diameter | Wall Thickness |
|---|---|---|---|
| Tibial nail | 250–400 mm | 2.5–4.0 mm | 3.5–5.0 mm |
| Femoral nail | 300–480 mm | 3.0–5.0 mm | 3.5–6.0 mm |
| Humeral nail | 200–350 mm | 2.5–3.5 mm | 3.0–4.5 mm |
Spinal Implants
Spinal implants requiring deep hole drilling include:
- Pedicle screws: Cannulated screws placed through the pedicle of the vertebra
- Interbody fusion cages: Some designs have drilled passages for bone graft
- Spinal rods: Some rod-based systems use drilled end holes for set screw engagement
- Anchors and hooks: May have drilled through-holes for fixation
Deep Hole Drilling Process
Gun Drilling for Cannulated Implants
Gun drilling is the standard process for producing cannulation holes in medical implants because it uniquely meets the requirements for straightness, surface finish, and repeatability in deep, small-diameter holes.
| Parameter | Typical Range for Medical Implants |
|---|---|
| Hole diameter | 1.0–6.0 mm (micro gun drilling) |
| Length-to-diameter ratio | 20:1–200:1 |
| Concentricity | 0.015 mm TIR over 400 mm |
| Surface finish | Ra 0.4–0.8 μm |
| Diameter tolerance | H7–H8 |
The gun drill operates with a single cutting edge and a high-pressure coolant channel that delivers oil directly to the cutting tip. Chips are evacuated through an external V-flute along the drill body. The single cutting edge produces a natural burnishing effect from the guide pads that results in excellent surface finish.
Titanium Gun Drilling Parameters
For Ti-6Al-4V ELI (Grade 23), the most common medical implant alloy:
| Parameter | Small Diameter (1–3 mm) | Medium Diameter (3–6 mm) | Large Diameter (6–18 mm) |
|---|---|---|---|
| Cutting speed | 15–30 m/min | 25–40 m/min | 30–50 m/min |
| Feed rate | 0.003–0.010 mm/rev | 0.008–0.020 mm/rev | 0.015–0.035 mm/rev |
| Coolant pressure | 150–250 bar | 100–200 bar | 80–150 bar |
| Coolant type | Oil, EP additives | Oil, EP additives | Oil, EP additives |
| Tool material | Micro-grain carbide | Micro-grain carbide | Carbide |
Challenges of Titanium Deep Hole Drilling
Titanium presents specific difficulties that make gun drilling the only viable process for cannulated implants:
| Challenge | Cause | Consequence | Solution |
|---|---|---|---|
| Heat concentration | Low thermal conductivity (7 W/mK vs. 50 for steel) | Tool edge welding, built-up edge | High-pressure coolant, sharp tool geometry |
| Tool deflection | Low modulus of elasticity (114 GPa) | Hole wander, concentricity loss | Counter-rotation, whip guides |
| Chip control | Stringy, segmented chips | Chip packing, tool breakage | Optimized feed, chip breaker geometry |
| Work hardening | Plastic deformation ahead of cutting edge | Increased cutting forces | Consistent feed, no dwell |
| Galling | Chemical affinity between titanium and tool | Poor surface finish | Coated tooling (DLC, TiAlN) |
Counter-Rotation for Straightness
For longer implants (IM nails over 300 mm), counter-rotation — where the workpiece rotates in the opposite direction to the tool — is essential for maintaining concentricity between the cannulation bore and the implant outer diameter.
| Rotation Configuration | Typical Concentricity |
|---|---|
| Tool rotation only | 0.05–0.10 mm |
| Workpiece rotation only | 0.03–0.08 mm |
| Counter-rotation | 0.015–0.05 mm |
| Counter-rotation + steady rests | 0.01–0.03 mm |
Machine Tools for Medical Deep Hole Drilling
Mollart VDMF Micro Drilling Machine
The Mollart VDMF is a vertical micro gun drilling machine designed specifically for medical implants:
| Feature | Specification |
|---|---|
| Hole diameter range | 0.5–6.0 mm |
| Maximum depth | 300 mm |
| Spindle speed | Up to 20,000 RPM |
| Number of spindles | Up to 4 |
| Guide bushing | Tungsten carbide, close-tolerance |
| Coolant system | High-pressure oil, 5 μm filtration |
| Counter-rotation | Available |
Mollart LD Series (Drillsprint)
For larger implants such as IM nails:
| Feature | Specification |
|---|---|
| Hole diameter range | 4–25 mm |
| Maximum depth | 1,000 mm |
| Counter-rotation | Standard |
| Guide bushing | Adjustable, hardened steel |
| Typical application | Femur and tibia nails |
Precihole Micro Gun Drilling Machines
Precihole machines (distributed by Absolute Machine Tools in North America) offer tabletop configurations for micro medical drilling:
| Feature | Specification |
|---|---|
| Hole diameter range | 1.0–6.0 mm (0.04–0.25 in) |
| Maximum depth | 300 mm (12 in) |
| Spindle speed | 1,000–25,000 RPM |
| Spindle power | 1.5 hp per spindle |
| Feed rate | 0–1,000 mm/min |
| Coolant pressure | Up to 2,500 psi (170 bar) |
| Filtration | 5 μm |
| Counter-rotation | Available |
| Monitoring | Vibration and power monitoring standard |
Multi-Spindle Configurations
Modern medical gun drilling machines often use multiple spindles for simultaneous drilling:
- 2-spindle: Common for medium-volume production
- 4-spindle: High-volume screw production
- 8-spindle: Lights-out, high-volume automated production
With multi-spindle setups, one operator can run multiple machines, and automated loading/unloading enables 24/7 production.
Manufacturing Process Flow
Cannulated Bone Screw
| Step | Operation | Process |
|---|---|---|
| 1 | Bar stock preparation | Cut Ti-6Al-4V ELI bar to length |
| 2 | Rough turning | CNC Swiss-type lathe, turn OD and head profile |
| 3 | Cannulation drilling | Gun drill central bore (the critical operation) |
| 4 | Head broaching | Broach hex or multi-lobe drive feature |
| 5 | Thread profile | Thread grinding (preferred) or thread turning/milling |
| 6 | Self-tapping flutes | Cut flute at screw tip |
| 7 | Tip geometry | Machine trocar tip or blunt tip |
| 8 | Deburring | Mechanical or thermal deburr all edges |
| 9 | Cleaning | Ultrasonic wash, DI water rinse |
| 10 | Electropolishing | Surface smoothing, corrosion resistance |
| 11 | Passivation | Nitric acid passivation per ASTM F86 |
| 12 | Final inspection | Dimensional, visual, surface finish |
| 13 | Sterilization | Gamma or EtO sterilization |
| 14 | Packaging | Cleanroom packaging per ISO 13485 |
Intramedullary Nail
| Step | Operation | Process |
|---|---|---|
| 1 | Tube or bar preparation | Pre-cannulated tube or solid bar |
| 2 | Gun drilling (if solid bar) | Drill full-length cannulation (up to 480 mm) |
| 3 | OD turning | Machine external profile, taper, and curvature |
| 4 | Locking screw holes | Gun drill cross-holes for locking screws |
| 5 | Slot cutting | Machine slots for insertion tools |
| 6 | End machining | Machine proximal and distal ends |
| 7 | Deburring and cleaning | Ultrasonic wash, passivation |
| 8 | Final inspection | Dimensions, surface, straightness |
| 9 | Sterilization and packaging | Per ISO 13485 |
The Hollow Bar Alternative
Instead of gun drilling each implant individually, some manufacturers use pre-cannulated bar stock:
- Process: Tubing or hollow bar is drawn to final dimensions with the cannulation already in place
- Advantage: Eliminates the gun drilling step, freeing Swiss-type CNC machines for other work
- Production increase: 50–74% improvement in overall throughput
- Cost trade-off: Raw material cost is higher (tubing vs. solid bar), but drilling cost is eliminated
A cost comparison from Forécreu shows the two approaches are surprisingly close in total cost. For a typical cannulated screw, drilling in-house costs approximately $16.06 per part (including material, drilling time, and tool cost), while using cannulated bar stock costs $15.92 per part — nearly identical, but with higher throughput.
Quality Requirements and Standards
Regulatory Standards
| Standard | Requirement |
|---|---|
| ISO 13485 | Medical device quality management system |
| FDA 21 CFR 820 | Quality system regulation (US) |
| ASTM F136 | Ti-6Al-4V ELI wrought material specification |
| ASTM F67 | Unalloyed titanium material specification |
| ASTM F86 | Surface preparation and passivation |
| ISO 5832 | Implant material standards (multiple parts) |
Critical Quality Characteristics
| Characteristic | Typical Tolerance | Measurement Method |
|---|---|---|
| Cannulation diameter | ±0.025–0.050 mm | Pin gauge, air gauge, optical |
| Concentricity (bore to OD) | 0.015–0.050 mm TIR | CMM with rotary table |
| Straightness | 0.015 mm per 100 mm | Laser gauge or mechanical |
| Surface finish (bore) | Ra 0.4–0.8 μm | Profilometer (entry/exit) |
| Burr condition | No burrs at either end | Visual (10× microscope) |
| Cleanliness | No chips, oil, or debris | Visual, solvent flush |
| Thread fit | Class 2A/2B or tighter | Thread gauge |
Cannulation Bore Inspection
Inspecting the cannulation bore is challenging because it is deep and narrow. Common methods include:
- Go/no-go pin gauges: Quick verification of minimum diameter
- Air gauging: Measures diameter at multiple depths
- X-ray inspection: Verifies straightness and detects voids
- Sectioning: Destructive test for full cross-section verification (first article only)
- Borescope: Visual inspection of bore surface
Cleanliness Requirements
Medical implants have strict cleanliness requirements:
- No cutting oil residue inside the cannulation
- No metal chips or particles
- No burrs that could dislodge in the body
- Surface free of contamination that could affect biocompatibility
Cleaning validation must demonstrate that the cannulation bore is clean, typically through solvent flush analysis or rinse water testing.
Materials
Ti-6Al-4V ELI (Grade 23)
The most commonly used material for cannulated implants:
| Property | Value |
|---|---|
| Tensile strength | 860–965 MPa |
| Yield strength | 795–875 MPa |
| Elongation | 10–15% |
| Elastic modulus | 114 GPa |
| Thermal conductivity | 7 W/mK |
| Biocompatibility | Excellent (ASTM F136) |
Other Implant Materials
| Material | Applications | Deep Hole Drilling Challenge |
|---|---|---|
| Ti-6Al-4V (Grade 5) | General trauma | Moderate (similar to ELI) |
| Ti-6Al-7Nb | Dental, spinal | Similar to Grade 5 |
| Commercially pure Ti (Grade 2/4) | Dental implants | Easier (lower strength) |
| 316L stainless steel | Trauma, spinal | Fair (work hardens) |
| Cobalt chrome (ASTM F75) | Spinal, dental | Very difficult (high strength, abrasion) |
Surface Treatment After Drilling
After deep hole drilling, implant surfaces are typically treated:
| Treatment | Purpose | Effect on Bore |
|---|---|---|
| Electropolishing | Smooth surface, remove micro-burrs | Improves Ra to 0.2–0.4 μm |
| Passivation (nitric acid) | Restore passive oxide layer | No dimensional change |
| Anodic oxidation | Enhanced corrosion resistance | Minimal dimensional change |
| Micro-bead blasting | Surface roughening for osseointegration | Not applied to bore |
Alternative and Emerging Technologies
Modulated Drilling (TriboMAM)
Modulation-assisted machining (MAM) applies high-frequency oscillation to the drilling process:
- How it works: The toolholder oscillates the drill axially at approximately 1,000 cycles/second
- Effect: Creates controlled chip segmentation, improving evacuation in deep holes
- Results (Alphatec Spine pedicle screws):
- Cycle time: 11.9 min → 6.9 min (42% reduction)
- Tool life: doubled
- Eliminated need for expensive cannulated bar stock
- Cost savings exceeding $1,000/day
Laser Drilling
Laser drilling is used for micro features where gun drilling is impractical:
- Hole diameters down to 0.05 mm
- Used for surface porosity in dental implants
- Can create shaped holes (vase, conical) not possible with mechanical drilling
- Typically limited to depths under 5 mm
Hybrid Processing
Some manufacturers combine gun drilling with other processes:
- Gun drill + hone: For ultra-smooth bore surfaces (Ra < 0.2 μm)
- Gun drill + electropolish: Standard process for medical implants
- Gun drill + ultrasonic clean: Ensures bore cleanliness
FAQ
Q: What is a cannulated bone screw? A cannulated bone screw has a hollow central bore (cannulation) that allows it to be placed over a guide wire during surgery. This enables minimally invasive percutaneous procedures where the surgeon can verify screw position with fluoroscopy before insertion.
Q: How are cannulated bone screws manufactured? The cannulation hole is typically produced by gun drilling — a single-lip deep hole drilling process that achieves the required straightness, concentricity, and surface finish in deep, small-diameter holes. After drilling, the screw is turned, threaded, and surface treated.
Q: What diameter cannulation is typical for bone screws? Cannulation diameters range from 1.0 mm for small hand/wrist screws to 2.8 mm for large fragment screws. Intramedullary nails can have cannulation diameters up to 5.0 mm.
Q: Why is titanium difficult to gun drill? Titanium has very low thermal conductivity (7 W/mK), so heat concentrates at the cutting edge. It also has a low elastic modulus (114 GPa), causing tool deflection, and a tendency to gall and smear against carbide tooling.
Q: What machine is used for gun drilling cannulated implants? Specialized micro gun drilling machines such as the Mollart VDMF, Precihole micro gun drill, or similar machines with high spindle speeds (up to 25,000 RPM), high coolant pressure (up to 2,500 psi), and counter-rotation capability.
Q: What is counter-rotation and why is it important? Counter-rotation rotates the workpiece in the opposite direction to the tool. This cancels tool deflection effects and improves concentricity between the bore and the implant outer diameter. It is essential for longer implants like IM nails.
Q: What is the typical straightness requirement for a cannulation bore? Straightness of 0.015 mm total indicator reading (TIR) over the full implant length is achievable with gun drilling. Some implants require this tolerance over 400 mm depth.
Q: Are there alternatives to gun drilling for cannulated implants? Yes. Pre-cannulated bar stock (hollow tubing) eliminates the drilling step entirely, increasing production throughput by 50–74%. Modulation-assisted machining (MAM/TriboMAM) applies high-frequency oscillation to the drill, improving chip evacuation and reducing cycle time.
Q: What quality standards apply to cannulated implant manufacturing? ISO 13485 (QMS), FDA 21 CFR 820 (US quality system regulation), ASTM F136 (Ti-6Al-4V ELI material), and ASTM F86 (surface preparation). ISO 13485 certification is required for CE marking.
Q: What are the cost considerations for cannulated screw production? Gun drilling in-house costs approximately $16 per part including tools and time. Cannulated bar stock costs similar ($15.92 per part) but offers higher throughput by freeing up machine capacity. The breakeven depends on production volume and labor costs.