Appearance
A cannulated screw is a miracle of micro deep hole drilling. The screw is 5 mm in diameter and 80 mm long, with a 1.8 mm hole drilled through its entire length — a hole that must be perfectly straight, perfectly centred, and perfectly smooth. Any deviation causes friction against the guide wire. Any burr generates metallic debris. Any concentricity error weakens the wall on one side, and the screw is implanted in a bone that will be loaded with the patient's full body weight. The hole is drilled before the threads are cut — a gun drill passes through a solid titanium bar at 20,000 rpm with coolant at 2,500 PSI, removing a core of material that is barely wider than a needle. The margin between a successful cannulation and a scrapped blank is measured in microns, and the cost of failure is measured in surgical complications.
Medical Applications of Deep Hole Drilling
| Application | Bore Diameter | Typical Length | Material | Volume |
|---|---|---|---|---|
| Cannulated bone screw | 0.8–2.5 mm | 20–120 mm | Ti-6Al-4V ELI, 316L SS | Millions per year |
| Intramedullary nail | 3–8 mm | 200–500 mm | Ti-6Al-4V ELI | High |
| Biopsy needle cannula | 0.3–1.5 mm | 50–200 mm | 304 SS, 316L SS | Very high |
| Surgical instrument channel | 1–5 mm | 100–400 mm | Stainless steel | Moderate |
| Dental implant abutment | 1–2 mm | 10–20 mm | Ti-6Al-4V, CP Ti | High |
| External fixator pin | 2–5 mm | 100–300 mm | 316L SS or Ti | Moderate |
Cannulated Screw — The Defining Application
The cannulated bone screw is the most demanding high-volume medical deep hole drilling application:
| Parameter | Typical Specification |
|---|---|
| Major screw diameter | 3.5–7.3 mm |
| Cannulation bore diameter | 0.8–2.5 mm |
| Screw length | 20–120 mm |
| L/D ratio (of bore) | 25:1–150:1 |
| Bore straightness | ≤ 0.01 mm over length |
| Concentricity (bore to thread) | ≤ 0.02 mm TIR |
| Bore surface finish | Ra ≤ 0.4 µm (as-drilled) |
| Burr condition | Zero burr — 100% borescope inspection |
| Material | Ti-6Al-4V ELI per ASTM F136 or 316L per ASTM F138 |
Materials for Medical Implants
Implant Material Standards
| Material | Standard | Tensile Strength | Hardness | Machinability |
|---|---|---|---|---|
| Ti-6Al-4V ELI | ASTM F136 | 860–950 MPa | 30–36 HRC | Fair — BUE tendency, low thermal conductivity |
| CP Titanium (Grade 4) | ASTM F67 | 550–700 MPa | 20–25 HRC | Good — softer, less BUE |
| 316L Stainless | ASTM F138 | 490–690 MPa | 20–25 HRC | Good — work-hardens slightly |
| 304 Stainless | ASTM F899 | 500–700 MPa | 20–25 HRC | Fair — work-hardens |
| CoCrMo | ASTM F75 | 650–900 MPa | 35–45 HRC | Poor — high tool wear |
| NiTi (Nitinol) | ASTM F2063 | 600–950 MPa | 30–40 HRC | Very difficult — springy, variable |
Material Selection Guide
| Requirement | Preferred Material | Why |
|---|---|---|
| Highest strength, lowest modulus | Ti-6Al-4V ELI | Best combination for load-bearing implants |
| MRI compatibility | Ti-6Al-4V ELI | Non-magnetic, low susceptibility artefact |
| Low cost, adequate performance | 316L SS | Standard for temporary fixation devices |
| Corrosion resistance in body | Ti-6Al-4V ELI or 316L | Both passivate in physiological fluids |
| Wear resistance (articulating) | CoCrMo | Hardest, most wear-resistant implant alloy |
Micro Gun Drilling Parameters
Machine Requirements
| Parameter | Specification |
|---|---|
| Spindle speed | 5,000–25,000 rpm |
| Feed resolution | 0.001 mm (1 µm) or better |
| Coolant pressure | Up to 2,500 PSI (172 bar) |
| Coolant filtration | 5 µm absolute |
| Workpiece rotation | Counter-rotation for straightness |
| Guide bush | Required for diameters < 3 mm |
| Machine type | Horizontal micro gun drilling machine |
Cutting Parameters by Material
| Parameter | Ti-6Al-4V ELI | 316L Stainless | CP Titanium |
|---|---|---|---|
| Cutting speed | 20–40 m/min | 25–50 m/min | 30–60 m/min |
| Spindle speed (for 2 mm bore) | 3,200–6,400 rpm | 4,000–8,000 rpm | 4,800–9,600 rpm |
| Feed rate | 0.002–0.008 mm/rev | 0.003–0.012 mm/rev | 0.004–0.015 mm/rev |
| Coolant type | EP oil (chlorine-free) | EP oil | EP oil or synthetic |
| Coolant pressure | 150–250 bar (2,200–3,600 PSI) | 120–200 bar | 100–180 bar |
| Tool grade | K10–K15 carbide, uncoated or DLC | K10–K15 carbide, TiAlN | K10–K15 carbide, uncoated |
| Expected bore finish | Ra 0.2–0.4 µm | Ra 0.3–0.6 µm | Ra 0.2–0.5 µm |
Feed Rate and Chip Control
In micro deep hole drilling for medical implants, the chip cross-section is vanishingly small (0.002 mm/rev × 0.05 mm chip width = 0.0001 mm²). The chip is a fine wire that must exit through a narrow flute:
| Challenge | Consequence | Mitigation |
|---|---|---|
| Low feed = thin chip | Chip breaks into fragments, evacuates | Acceptable for cannulated screws |
| High feed = stronger chip | Chip jams in flute, tool breaks | Reduce feed immediately |
| Chip packing | Tool breakage at depth | Use step drilling, peck cycles |
| Built-up edge (Ti) | Oversize bore, rough surface | DLC coating, high coolant pressure |
TIP
For micro gun drilling of titanium cannulated screws, DLC (diamond-like carbon) coating on the gun drill tip provides a significant advantage over uncoated or TiAlN-coated tools. DLC reduces the adhesion of titanium to the cutting edge — the primary cause of BUE in titanium drilling — and the low friction coefficient (0.1) improves chip flow through the narrow flute. Tool life improvements of 3–5× over uncoated carbide have been demonstrated in production.
Manufacturing Process for Cannulated Screws
| Step | Operation | Purpose | Quality Check |
|---|---|---|---|
| 1 | Bar stock inspection | Material certification per ASTM | Chemical analysis, microstructure |
| 2 | Gun drill cannulation | Create central bore | Bore diameter, straightness, borescope |
| 3 | Rough external turning | Form screw blank profile | External dimensions |
| 4 | Thread rolling or cutting | Form threads | Thread form, pitch diameter |
| 5 | Cannulation protection | Insert mandrel to protect bore | — |
| 6 | Head forming | Hex or cruciform drive | Drive geometry |
| 7 | Deburring | Remove all sharp edges | Visual inspection |
| 8 | Cleaning | Remove cutting oil, chips | Cleanliness per ISO 19227 |
| 9 | Passivation | Restore oxide layer | Corrosion resistance test |
| 10 | Final inspection | Dimensional, visual, functional | 100% inspection of all features |
| 11 | Sterilization | Gamma or ethylene oxide | Sterility assurance level (SAL 10⁻⁶) |
Critical Quality Checks During Cannulation
| Check | Method | Frequency | Acceptance |
|---|---|---|---|
| Bore diameter | Air gauge or pin gauge | 100% | ±0.01 mm |
| Bore straightness | Laser or stepped mandrel | Sampling | ≤ 0.01 mm over length |
| Bore concentricity | Optical comparator | Sampling | ≤ 0.02 mm TIR |
| Surface defects | Borescope (magnified) | 100% | No burrs, tears, or pits |
| Exit burr | Microscope inspection | 100% | Zero burr at entry and exit |
Regulatory and Quality Requirements
Applicable Standards
| Standard | Scope | Key Requirements |
|---|---|---|
| ISO 13485 | Medical device quality management | Risk management (ISO 14971), design control, CAPA |
| 21 CFR Part 820 | FDA Quality System Regulation | Device master record, process validation |
| ISO 14644 | Cleanroom classification | Class 7 or better for implant manufacturing |
| ASTM F136 | Ti-6Al-4V ELI for implants | Chemical composition, mechanical properties |
| ASTM F138 | 316L SS for implants | Chemical composition, inclusion content |
| ISO 19227 | Implant cleanliness | Organic contamination limits, particulate limits |
Process Validation (IQ/OQ/PQ)
All deep hole drilling processes for implantable devices must be validated:
| Validation Stage | Activity | Acceptance |
|---|---|---|
| Installation Qualification (IQ) | Verify machine installation per specification | All parameters within manufacturer limits |
| Operational Qualification (OQ) | Verify process runs at parameter extremes | All outputs within specification |
| Performance Qualification (PQ) | Verify process produces conforming product | Cpk ≥ 1.33 for critical dimensions |
Documentation Requirements
| Document | Content | Retention |
|---|---|---|
| Device master record (DMR) | Complete manufacturing specifications | Life of device + 2 years |
| Device history record (DHR) | Production records for each batch | Life of device + 2 years |
| Process validation report | IQ/OQ/PQ results | Life of device |
| Nonconformance report | Any deviation with root cause | Life of device |
| Sterilization record | Cycle parameters, load configuration | Life of device + 1 year |
Surface Finish and Biocompatibility
Surface Requirements
| Surface Condition | Requirement | Why |
|---|---|---|
| As-drilled bore (cannulation) | Ra ≤ 0.4 µm | Minimises friction against guide wire |
| External thread surface | Ra ≤ 0.8 µm | Reduces bone damage during insertion |
| Implant overall (final) | Ra ≤ 0.5 µm | Reduces bacterial adhesion |
| Sharp edges | None permitted | Prevents tissue irritation |
Post-Drilling Surface Treatments
| Treatment | Purpose | Effect on Bore |
|---|---|---|
| Passivation (nitric acid) | Restore protective oxide layer | Minimal — no dimensional change |
| Electropolishing | Smooth surface, remove micro-burrs | Removes 0.005–0.015 mm from bore |
| Cleaning (aqueous + ultrasonic) | Remove all organic residues | No dimensional effect |
| PVD coating (for instruments only) | Wear resistance | Adds 2–5 µm |
Common Defects and Troubleshooting
| Defect | Cause | Fix |
|---|---|---|
| Cannulation off-centre | Drill deflection on entry | Improve guide bush support, reduce entry feed |
| Bore too small | Tool wear, insufficient clearance | Replace drill at scheduled interval |
| Burr at exit | Feed too high at breakthrough | Reduce feed in last 2–3 mm |
| Tool breakage in bore | Chip packing in flute | Increase coolant pressure, check chip form |
| Rough bore surface | Built-up edge on titanium | Switch to DLC coating, increase speed |
| Diameter taper (entry larger) | Drill vibration at start | Use close-tolerance guide bush |
| Concentricity drift | Worn guide bush | Replace bush at scheduled interval |
FAQ
Q: What is a cannulated screw? A cannulated screw is a bone screw with a hollow central bore that allows it to be inserted over a guide wire (K-wire). The surgeon places the guide wire under fluoroscopic guidance, then drills the cannulated screw over it, ensuring precise placement without a large incision.
Q: How is the cannulation hole produced? The cannulation hole is gun-drilled through a solid titanium or stainless steel bar before thread cutting. Micro gun drilling machines operating at 5,000–25,000 rpm with 1 µm feed resolution and high-pressure coolant (up to 2,500 PSI) produce the bore.
Q: What tolerances are required for cannulated screw bores? Bore diameter: ±0.01 mm. Straightness: ≤ 0.01 mm over the screw length. Concentricity to the external thread: ≤ 0.02 mm TIR. Surface finish: Ra ≤ 0.4 µm. Zero burr condition on entry and exit.
Q: What material is most common for cannulated screws? Ti-6Al-4V ELI (extra-low interstitial) per ASTM F136 is the most common material. It offers high strength (860–950 MPa), excellent biocompatibility, low modulus (close to bone), and MRI compatibility. 316L stainless steel per ASTM F138 is used for lower-cost alternatives.
Q: What coolant pressure is needed for micro gun drilling of titanium implants? 150–250 bar (2,200–3,600 PSI) is standard. The high pressure is essential for evacuating fine titanium chips from the narrow flute and for cooling the cutting edge in titanium's low-thermal-conductivity environment.
Q: What are the regulatory requirements for deep hole drilling of implantable devices? Manufacturing must comply with ISO 13485 (quality management system) and 21 CFR Part 820 (FDA Quality System Regulation). The drilling process must be validated (IQ/OQ/PQ), and every device must have a complete device history record with full traceability.
Q: What is the typical L/D ratio for cannulated screw bores? L/D ratios range from 25:1 (short screws) to 150:1 (long femoral neck screws). A typical 7.3 mm × 100 mm cannulated screw with a 1.8 mm bore has an L/D ratio of 56:1 for the bore.
Q: Can cannulated screws be made from materials other than titanium? Yes. 316L stainless steel is common for temporary fixation. CoCrMo is used where wear resistance is critical (articulating implants). Nitinol is used for self-expanding implants. Each material requires specific drilling parameters.
Q: How are cannulated bores inspected in production? Bore diameter is checked 100% with air gauges or pin gauges. Borescope inspection (100%) verifies surface condition. Concentricity and straightness are measured on a sampling basis using optical comparators or laser systems.
Q: What is the most common defect in cannulated screw drilling? Tool breakage from chip packing is the most common defect. The fine, stringy chip produced when drilling titanium can pack in the narrow flute, causing torque to spike and the tool to fracture within the bore, scrapping the blank.