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ISO 6475 Bone Screw Torsion Breaking Torque Test Methods

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ISO 6475 specifies mechanical test methods for determining breaking torque and rotation angle at failure of metal bone screws with asymmetrical threads and spherical under-surfaces, along with minimum mechanical requirements for stainless steel screws in Annex AISO. It is critical for validating torsional integrity to prevent intraoperative/surgical failure and ensure regulatory compliance.


ISO 6475 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
ParameterRequirements
TypeMetal bone screws with asymmetrical thread and spherical under-surface (matches ISO 5835 dimensions).
MaterialPrimarily stainless steel per ISO 5832-1; data for other materials (e.g., Ti alloys) are not fully specified in the 1989 edition.
DimensionsMust comply with ISO 5835 (thread, head, shank, length).
Sample Size

Sufficient quantity to ensure statistical validity (typically ≥5 screws per lot).

ISO 6475 Bone Screw Torsion Breaking Torque Test Methods

PreparationNo surface damage; 5 full threads exposed from thread run-out after clamping to avoid stress concentration.


ISO 6475 Bone Screw Torsion Test Equipment

ComponentSpecifications
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 FixtureRigidly secures the screw shank (no movement) with coaxial alignment of the screw and torque applicator.
Test BlockDense material (e.g., PMMA, rigid foam per ASTM F1839) to simulate bone; designed to expose 5 full threads from thread run-out
Torque/Angle SensorsHigh-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:
StandardPurposeRelationship
ISO 5835Defines dimensional specifications for metal bone screws (thread, head, shank)Mandatory for specimen dimensions; ISO 6475 assumes compliance with ISO 5835
ISO 5832-1Specifies stainless steel (e.g., 316L) for surgical implantsMaterial reference for Annex A’s mechanical requirement
ASTM F543Covers pullout, driving torque, and self-tapping testsComplements ISO 6475 by addressing non-torsional mechanical properties
ISO 10993 SeriesBiological evaluation (cytotoxicity, corrosion)Addresses biocompatibility not covered by ISO 6475
ISO 13485Quality management systems for medical devicesEnsures 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)

Related products and device

ISO 6475 Screws testing machine Driving Torque Insertion and Removal Test

Screw fatigue tester is a multi-functional composite test equipment according customer different test requirements, which can be used for screw rotary torque testing, self-tapping testing and other bone screw-related performance tests.

ISO 6475 Bone Screws Torsion Torque Tester

Torsion tester is mainly used to execute torsion test for various kinds materials, manual loading, high accuracy torsion transducer to measure torque, photoelectric encoder to measure torsion angle, digital display test result. Easy to operate and with budgetary price, most suitable for research institute and colleague, factory laboratory etc. to check the torsion character of different materials.

ISO 6475 Metallic Medical Bone Screws Self Tapping test

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.

ISO 6475 Bone nails Pull-out tester, tensile tester

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.

Related Standard

ASTM F543 Metallic Medical Bone Screws Test Driving Torque, Pullout Load Self-Tapping

ASTM F543 Standard Specification and Test Methods for Metallic Medical Bone Screws


ASTM F543 is a testing standard used in the biomedical industry that defines specifications for metallic medical bone screws. Bone screws are used in surgical procedures for securing implants, osteosynthesis devices, and fracture fixation plates to the skeletal system. In normal clinical use, a surgeon applies combined axial and torsional forces to implant a bone screw into the body. Bone screws are designated Class 2 FDA devices, requiring significant testing and analysis  of mechanical properties prior to approval and release. It provides requirements for materials, finish and marking, care and handling, and the acceptable dimensions and tolerances for metallic bone screws that are implanted into bone. The dimensions and tolerances in this specification are applicable only to metallic bone screws described in this specification.

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