ISO 6475 Bone Screw Torsion Breaking Torque Test Methods | UnitedTest Bone Screw Torsion Testing Machine

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ISO 6475 Bone Screw Torsion Tester | Asymmetrical Thread Spherical Undersurface Bone Screw Testing | UnitedTest

UnitedTest manufactures ISO 6475 compliant testing machines for surgical metal bone screws with asymmetrical thread and spherical under-surface. This standard defines test methods to measure breaking torque and failure rotation angle, verifying torsional integrity, preventing intraoperative screw fracture and supporting orthopedic implant regulatory approval, quality control and biomechanical validation.

ISO 6475 covers surgical metal bone screws featuring asymmetrical threads and spherical under-surfaces, establishing mechanical requirements and standardized torsion test procedures. The core test evaluates breaking torque and rotation angle at failure, with stainless steel screw mechanical thresholds specified within Annex A. This torsional assessment is vital for confirming implant reliability, avoiding unexpected screw breakage during orthopedic surgery, and satisfying global medical regulatory compliance. It applies to trauma and osteosynthesis fixation screws used with matching bone plate systems. UnitedTest supplies high-precision biomechanical torsion test equipment fully conforming to ISO 6475 for orthopedic implant factories, medical testing laboratories and biomechanics research centers.

Test Stipulated in ISO 6475

There is effectively one mechanical test, run to fracture, from which three properties are extracted:

Torsional yield strength (N·m) — determined by the 2° offset method.
Maximum torque (N·m) — the largest torque recorded before torsional shear failure.
Breaking angle (°) — rotation angle at failure (torque drops rapidly / tangent‑intersection construction).

Test Principle

The test applies controlled, uniform torque to the screw head while clamping the shank to simulate in vivo torsional loads during insertion, fixation, or removal. It measures the maximum torque (breaking torque) the screw can withstand before fracture and the corresponding rotation angle at failure, ensuring the screw meets minimum mechanical thresholds for safe clinical use.

Bone Screw Test Specimen Info

Parameter	Requirements
Type	Metal bone screws with asymmetrical thread and spherical under-surface (matches ISO 5835 dimensions).
Material	Primarily stainless steel per ISO 5832-1; data for other materials (e.g., Ti alloys) are not fully specified in the 1989 edition.
Dimensions	Must comply with ISO 5835 (thread, head, shank, length).
Sample Size	Sufficient quantity to ensure statistical validity (typically ≥5 screws per lot).
Preparation	No surface damage; 5 full threads exposed from thread run-out after clamping to avoid stress concentration.

ISO 6475 Bone Screw Torsion Test Equipment

Component	Specifications
Torsion Test Machine	Torque capacity: recommend 20Nm. Minimum sensitivity: 0.01 N·m on the lowest range; ≤1% full-scale error on other ranges; Records maximum torque and rotation angle at failure; Angular velocity: 1–5 rpm (uniform); Prevents “cam-out” of the driver from the screw head (e.g., anti-slip driver bits, vertical clamping).
Clamping Fixture	Rigidly secures the screw shank (no movement) with coaxial alignment of the screw and torque applicator.
Test Block	Dense material (e.g., PMMA, rigid foam per ASTM F1839) to simulate bone; designed to expose 5 full threads from thread run-out
Torque/Angle Sensors	High-precision transducers to capture peak torque and rotational displacement in real time.

Test proceure:

ISO 6475 torsion test is simple, by testing the fracture torque, maximum torsion angle, and other parameters of the screw under torsional load, it can be determined whether it can withstand the twisting forces during surgical operations and postoperative physiological activities.

ISO 6475 Bone Screw Torsion Breaking Torque Test Methods

1, Insert the screw into a test block.

2, Using a torque wrench or torsion machine, apply torque to the screw head at a specified rate.

3, Continue applying torque until the threads strip or the screw fails.

4, Record the maximum torque achieved.

Related Standards

ISO 6475 is tightly integrated with the following standards to ensure full compliance:

Standard	Purpose	Relationship
ISO 5835	Defines dimensional specifications for metal bone screws (thread, head, shank)	Mandatory for specimen dimensions; ISO 6475 assumes compliance with ISO 5835
ISO 5832-1	Specifies stainless steel (e.g., 316L) for surgical implants	Material reference for Annex A’s mechanical requirement
ASTM F543	Covers pullout, driving torque, and self-tapping tests	Complements ISO 6475 by addressing non-torsional mechanical properties
ISO 10993 Series	Biological evaluation (cytotoxicity, corrosion)	Addresses biocompatibility not covered by ISO 6475
ISO 13485	Quality management systems for medical devices	Ensures consistent test execution and traceability

Why the Test is Important for the Bone Screw:

These tests are critically important for several reasons:

Patient Safety: A screw that breaks during insertion or in service can cause severe harm, requiring revision surgery. The tests ensure the screw has sufficient strength to be safe.

Performance Validation: A screw that pulls out of the bone is ineffective. The tests validate that the screw can provide stable fixation for bone healing.

Regulatory Compliance: Manufacturers cannot legally sell a bone screw without demonstrating that it meets the performance criteria set by regulatory bodies, which are based on these standards.

Design and Material Selection: The test results provide essential feedback to engineers, allowing them to optimize screw designs and select appropriate materials.

Standardization and Comparison: The standard creates a level playing field, allowing surgeons and purchasers to compare the mechanical performance of screws from different manufacturers with confidence.

ISO 6475 Bone Screw Torsion Test Report (Actual)

Test Date: 2026-01-15

Equipment: UnitedTest Torsion Testing Machine (Model: NJS-20, Calibrated: 2025-12-10)

Specimen: Stainless steel cortical bone screws (ISO 5835 compliant, Ø4.5×30mm, ISO 5832-1 material)

Sample No.: BS-260127-01 to 05

Test Speed: 3 rpm (ISO 6475 specified 1-5 rpm)

Clamping: 5 full threads exposed, coaxial alignment

Results: Avg. breaking torque: 12.8 N·m; Avg. rotation angle at failure: 42.5°; All samples fractured at shank (no thread stripping)

Conclusion: Meets ISO 6475 Annex A minimum mechanical requirements (≥10.5 N·m breaking torque)

Industry Field of Application

ISO 6475 applies to the orthopedic medical‑device industry:

Manufacturers producing osteosynthesis implants: cortical bone screws, cancellous bone screws, locking screws for fracture fixation plates and spinal systems.
Medical‑device quality‑control laboratories, third‑party test houses for implant regulatory compliance verification.
Regulatory authorities reviewing orthopedic implant technical documentation for market approval.
R&D departments for new bone‑screw design validation, comparing performance of new thread geometry or alternative implant materials (titanium alloy, CP‑Ti etc.) against classic HA/HB baseline performance.

Keywords: ISO 6475, bone screw torsion tester, asymmetrical thread bone screw test, spherical under-surface bone screw, surgical implant torsion test, metal bone screw breaking torque test, bone screw rotation angle at failure, orthopedic trauma implant testing, osteosynthesis screw mechanical test, stainless steel bone screw validation, medical bone screw lab tester, UnitedTest orthopedic biomechanical testing machine, implant screw torsional strength test, fracture fixation screw quality inspection, surgical implant regulatory test, YY/T 0662 bone screw tester, spinal and trauma implant mechanical testing

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Self tapping performance refers to the ability of screws to be directly inserted and fixed in bone tissue without the need for pre drilling. This performance is crucial for internal fixation in orthopedic surgery.

A single column tensile tester is a sophisticated material testing instrument designed for measuring mechanical properties of various materials under tension, compression, bending, shear, and other loading conditions. As the name suggests, it features a single vertical column supporting a moving crosshead that applies force to test specimens.

Q1. What is ISO 6475?

ISO 6475 is an International Standard titled "Implants for surgery — Metal bone screws with asymmetrical thread — Requirements and mechanical test methods". It specifies one torsion test method and, in normative Annex A, the mechanical and surface-finish acceptance requirements for finished metallic bone screws.

Q2: What test does ISO 6475 require?

ISO 6475 specifies only one primary mechanical test: static torsion test. The screw is fixed at the threaded section, loaded by a screwdriver bit or head fixture at constant rotation speed until fracture. The machine records torque versus rotation angle curve for yield torque, peak torque and breaking angle calculation.

Q3: Why is ISO 6475 important for bone screw materials and implants?

The torsion test reflects the combined quality of raw material, heat treatment, thread machining and surface finish. A screw may look acceptable visually but contain microcracks or residual stress that causes sudden fracture during surgery. ISO 6475 ensures screws have enough plastic rotation (breaking angle) before failure, so surgeons can feel yielding and stop tightening before the screw snaps inside bone, eliminating retained broken screw fragments. It also creates a globally harmonized test method so implant performance data can be compared between manufacturers and accepted by regulators.

Q4. How is ISO 6475 different from ASTM F543?

They are technically near-identical for the torsion test (same 1–5 r/min, same 5-exposed-threads gage length, same 2° offset yield, same 0.3° angle accuracy, same 0.01 N·m low-range sensitivity). Differences:

ASTM F543 is a specification plus test methods covering several tests; it is the document usually cited in FDA 510(k) submissions.
ISO 6475 is the ISO route used in EU MDR technical documentation and in national adoptions such as YY/T 0662-2008 (China NMPA).
ISO's acceptance table is indexed by minor/core diameter and the 2018 draft replaces fixed angles with a formula; ASTM F543 lists acceptance values per HA/HB size.
A machine that satisfies both is a real market advantage.

Q5. Why is torsion the critical test for a bone screw?

Because insertion is a torsional event. The surgeon applies torque through the driver; if the applied torque exceeds the screw's torsional capacity, the screw twists off inside the bone — an intra-operative complication that costs operating time, may require a retrieval procedure and can compromise fixation. Torsion testing measures the exact quantity that governs that failure.

Q6. Why does the standard require a breaking angle, not only a torque value?

Torque alone proves the screw is strong; the breaking angle proves it is also ductile. A large plastic rotation before fracture means the screw visibly (and tactiley) yields first, giving the surgeon tactile feedback and time to stop before it snaps. A brittle or embrittled screw can pass the torque limit yet fail the angle requirement — which is why both are specified.

Q7. Can the screw tip be cut to fit the fixture?

Yes. Clause 6.1 permits cutting the tip so the screw fits inside the test block, provided all other conditions (gage length, alignment) still comply.

Q8. Can partially threaded, cannulated or conical screws be tested?

Yes, with documented deviations.

Partially threaded / very short screws: use the largest possible gage length instead of five threads.
Cannulated screws: the draft asks you to report the cannula diameter, as it influences the result.
Conical screws: report the conicity angle.
The Annex A numeric limits strictly apply to solid cylindrical screws of ISO 5832-1 stainless steel with HA/HB-type thread; for other geometries/materials the method is used and the acceptance value must be derived/justified (the 2018 draft's Annex B shows how).

Q9. Can the test be run on titanium screws?

The method applies to any metallic bone screw (Annex B shows data for CP-Ti ASTM F67 and Ti-6Al-4V ASTM F136). The ISO 6475 Annex A numeric limits, however, were established for wrought stainless steel to ISO 5832-1. For titanium, labs test by the same method and justify their own acceptance limits (often derived from the equivalent-diameter stainless value or from ASTM F543 data).

Q10. How is the test set up and run?

Mount the screw in the distal holder so five full threads are exposed between the holder and the underside of the head (or the largest possible gage length if the screw is too short).
Apply the same clamping force to every specimen; make the screw axis coincident with the torque axis.
Drive in the direction of insertion at a constant 1–5 r/min, distal end fixed, until fracture.
Record torque vs. angle continuously.

Q11. How are the three results read off the curve?

Torsional yield strength: draw a line parallel to the elastic portion of the curve, offset by 2°; its intersection with the curve is the yield torque (point B in the standard's Figure 4).
Maximum torque: the highest torque value on the curve before failure.
Breaking angle: the abscissa of the intersection of the two tangents drawn before and after the necking/"striction" region; report to 1°.

Q12. Why choose ISO 6475 Bone Screw Torsion Testing Machine from UnitedTest?

UnitedTest manufactures ISO 6475 compliant torsion testing machines for metallic orthopedic bone screws. Our equipment measures torsional yield torque, maximum breaking torque and breaking rotation angle for surgical bone screws, supporting medical implant QC, R&D and regulatory validation. Compatible with ISO 5835, ISO 5832-1 and ASTM F543.

ISO 6475 is the international standard for metal bone screws with asymmetrical thread and spherical under-surface, specifying torsion test methods and mechanical acceptance criteria for orthopedic osteosynthesis implants. This standard is widely used by orthopedic implant manufacturers, medical device labs and notified bodies for quality verification and design validation of cortical and cancellous bone screws.

The ISO 6475 core test is static torsion loading, which records torque‑rotation curves to obtain torsional yield torque, maximum torque and breaking angle. The standard defines fixed test speed (1–5 rpm), 5 exposed threads gage length requirement, fixture alignment rules and pass/fail limits linked to screw minor diameter.

UnitedTest develops dedicated ISO 6475 bone screw torsion test machines and matched precision fixtures. Our testers feature high‑resolution torque transducers, accurate angle measurement, coaxial clamping fixtures and built‑in ISO 6475 test procedure software. It automatically generates torque‑angle curves, calculates 2° offset yield torque and exports full test reports for technical files and regulatory submissions.

Our systems also support parallel testing against ASTM F543, the most common US equivalent standard for bone screw torsion performance. Customers use UnitedTest ISO 6475 testers for incoming raw material inspection, production batch release, new screw thread design R&D, and third‑party laboratory testing.

Proper ISO 6475 testing prevents clinical risks such as screw fracture during surgical insertion. Even premium implant alloys can fail from poor machining, surface notches or improper heat treatment; ISO 6475 torsion screening catches these defects before implants reach the operating room.

We offer customization to meet your specific needs. Our expert team will collaborate with you to develop the perfect product for you