A bone screw drawing is a set of manufacturing decisions. The screw type fixes the thread form, the tip, the head and whether there is a bore through the center, and each of those features decides how the screw is turned, threaded, finished and inspected. For device engineers, sourcing teams and supplier quality engineers at orthopedic, trauma, spine and dental OEMs, the useful question is what each screw type demands from production. Clinical use is a separate subject, outside the scope of a component manufacturer.
Anatomy of a bone screw
Every bone screw, whatever its type, is described by the same five zones. They are also the zones an inspection plan checks.
| Zone | What the drawing defines | Manufacturing point |
|---|---|---|
| Head | Diameter, under-head shape (spherical or conical), locking thread if any | Turned and, for locking heads, threaded in the same setup as the shaft |
| Drive recess | Hex, hexalobular (Torx-type) or cruciform; size and depth | Broached or milled; recess fit with the OEM’s driver is a critical feature |
| Shaft | Shank diameter, threaded and unthreaded lengths | Straightness over length; run-out to the head |
| Thread | Major and minor (core) diameter, pitch, lead, thread form | Bone threads are usually asymmetric (buttress-type), deep and sharp, which favors whirling |
| Tip | Blunt, self-tapping (cutting flutes) or self-drilling | Flute geometry and burr-free cutting edges |
Two terms are often confused. Pitch is the distance between adjacent threads; lead is the distance the screw advances in one turn. They are equal on a single-start thread and differ on multi-start threads, which some designs use to advance faster.
Types of screws in orthopedics
The names overlap: one screw can be cancellous, cannulated and self-tapping at once. What matters on the shop floor is the combination of features. If you are sourcing rather than designing, our page on custom bone screw manufacturing sets out how we machine these screws to OEM drawings.
Cortical screws
Design features: a fine pitch and a relatively shallow thread along the full length, sized for dense bone. Dimensions are often taken from ISO 5835 (hexagonal drive, spherical under-head, asymmetrical thread) or from the OEM’s own system.
Manufacturing implications: a long, fine thread that must keep a constant pitch from tip to head. Thread whirling or single-point threading on a Swiss-type lathe, with the bar supported close to the cut, keeps the thread true over the full length.
Cancellous screws
Design features: a coarser pitch and a deeper thread on the same core, often with a smooth shank and a partial thread.
Manufacturing implications: the deeper the thread, the more material removed and the higher the risk of burrs on the thread crests and at the run-out into the shank. Burr control and a defined thread run-out on the drawing are the two points to agree early.
Locking screws
Design features: a threaded, usually conical head that engages a matching threaded hole in a bone plate, making a fixed-angle connection.
Manufacturing implications: the head thread is as critical as the bone thread. Its pitch, taper and timing relative to the shaft thread must match the plate hole, so both parts are usually inspected against the same gauge or thread definition. Conical under-head dimensions may follow ISO 9268.
Cannulated screws
Design features: a central bore so the screw can be placed over a guide wire. Cannulation is a feature, not a separate class: cortical, cancellous and compression screws can all be cannulated. See cannulated screws machined to drawing.
Manufacturing implications: a long, small bore drilled by gun drilling or deep-hole drilling. The bore must stay straight and concentric to the thread, with clearance for the wire and no burr at either end. Drill drift grows with length; our article on drift in cannulated screw drilling explains how it is controlled.
Self-tapping and self-drilling screws
Design features: a self-tapping tip has one or more cutting flutes that form the thread as the screw advances; a self-drilling tip also starts its own hole with a drill-like point.
Manufacturing implications: flutes are milled after the thread is cut, which leaves burrs on the cutting edges. They are removed under magnification without rounding the edge the design depends on. Flute depth, angle and length are dimensioned on the drawing.
Headless compression screws
Design features: no head, and two threaded zones with different pitch. The leading thread has a coarser pitch than the trailing thread, so the screw draws together the two parts it crosses as it advances.
Manufacturing implications: two threads on one part, with the pitch difference and the phase between them both critical. The threads are cut in one setup to keep them in relation; a cannulated version adds a long bore through both.
Pedicle screws
Design features: a spinal screw with a long threaded shank and a head that connects to a rod. Monoaxial designs have a fixed head; polyaxial designs carry a separate “tulip” head that pivots on the screw and is threaded for a set screw. Some have extended reduction threads.
Manufacturing implications: several parts per assembly (screw, tulip, set screw), each with its own threads and fits. The tulip often needs milling on several faces. See pedicle screws for spinal systems.
Dental and prosthetic screws
Dental abutment and prosthetic screws are much smaller and are defined by the fit with their mating part rather than by a bone thread. For their machining, see our prosthetic screws page and our article on accuracy in dental implant screws.
Materials for bone screws
| Material | Specification | Manufacturing notes |
|---|---|---|
| Ti-6Al-4V ELI | ASTM F136 | The most common metallic screw material; tough to machine, prone to galling, suited to Type II anodizing |
| CP titanium | ASTM F67 | Lower strength; specified where the OEM’s design calls for it |
| 316LVM stainless | ASTM F138 | Work-hardens in cutting; passivated after machining |
| PEEK | ASTM F2026 | Non-metal screws; thread engagement and head design need extra care |
Bioresorbable polymer screws form a separate category, with their own processing routes. Material is always taken from the drawing, and each lot of bar is traced to its mill certificate. We machine implant grades when the drawing calls for them; see implant-grade titanium for the alloy side.
How bone screws are manufactured, step by step
| Step | Purpose | Control or record |
|---|---|---|
| Bar stock receipt | Correct grade and heat lot | Mill certificate checked against the drawing; lot recorded |
| Swiss-type turning | Head, shank and core in one setup | First-article measurement; in-process checks |
| Thread whirling or single-point threading | Full bone thread form | Pitch, major and minor diameter, thread profile |
| Drive recess | Hex or hexalobular recess | Recess size and depth gauged |
| Cannulation (if any) | Guide-wire bore | Bore diameter, straightness, concentricity |
| Flute milling (if any) | Self-tapping or self-drilling tip | Flute geometry; edge condition |
| Deburring | Burr-free threads, flutes and bore ends | Visual check under magnification |
| Cleaning | Remove oils and chips before finishing | Cleaning to the OEM’s specification |
| Anodizing or passivation | AMS 2488 Type II anodizing (titanium); ASTM A967 or F86 passivation (stainless) | Finishing certificate per lot, on request |
| Laser marking | Lot, size or UDI data | Legibility and content checked |
| Final inspection | All critical dimensions | Inspection record; CoC with the lot, on request |
Most of the route runs on one machine. Swiss-type turning holds the bar in a guide bushing just behind the tool, so long screws do not deflect while the thread is cut. Color anodizing for size coding is described on our titanium color anodizing page, and stainless screws go through passivation per ASTM A967.
How screws are tested (by the device OEM)
Mechanical testing is part of the OEM’s design verification, not of machining. ASTM F543 gives the specification and test methods for metallic medical bone screws: torsional properties, driving torque and axial pull-out strength. ISO 6475 sets mechanical requirements and test methods for metal bone screws with an asymmetrical thread and spherical under-surface. The OEM also owns ISO 10993 biological evaluation, sterilization validation and regulatory clearance.
The machining supplier’s part is to deliver test samples made by the production process, then the same geometry on every later lot, with the inspection records that prove it.
Design-for-manufacturing tips
- Define the thread run-out. An undefined transition from thread to shank leaves the machinist to choose it; a dimensioned run-out removes a source of burrs and argument.
- Tolerance the drive recess realistically. Specify what the driver fit needs and no tighter: recess tolerances drive tool wear and inspection time.
- Write burr criteria on the drawing. “Burr-free” needs a magnification and an inspection method to be checkable.
- Check the cannulation length-to-diameter ratio. Long, small bores are possible but cost more and need a straightness requirement you can measure.
- Align color codes with sizes. Set the anodize color for each diameter or length in one table, and keep it the same across the system.
- Leave room for marking. Reserve a flat or a head face for lot or UDI marking before the design freezes.
Screws are one family among the custom medical fasteners a device can need.
Need bone screws machined to your drawing?
Upload your 3D model and 2D drawing, and tell us the material, quantities (samples, pilot build, annual repeat volume) and the documentation you need (FAI, CoC, material certificates, CMM report), all supplied on request. NDA available on request. Lead time and documentation are confirmed with the quote.
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FAQ
What are the main types of screws in orthopedics?
Cortical, cancellous, locking, cannulated, self-tapping or self-drilling, headless compression and pedicle screws. One screw often combines several of these features.
What is the difference between cortical and cancellous screws?
Cortical screws have a finer pitch and a shallower thread for dense bone; cancellous screws have a coarser pitch and a deeper thread. Both geometries come from the OEM’s design.
Why is thread whirling used for bone screws?
It cuts the full thread form in one pass on a Swiss-type lathe, with a good surface finish and minimal burrs. That suits long, fine-pitch and asymmetric bone threads.
Why are titanium screws anodized?
Type II anodizing per AMS 2488 modifies the surface layer of titanium, and color anodizing allows size coding. The OEM specifies which one is required.
MedCNCParts manufactures components to customer specifications; it does not design or market medical devices.
