ADMET worked with NexTek’s consulting laboratory and supplied a versatile axial-torsion testing system to perform testing on various medical devices for NexTek’s medical device customers. The team at NexTek expressed an interest in a high-capacity biaxial testing machine to run both static and dynamic mechanical tests on medical devices. The versatility of the system was a priority as various medical devices require different testing based on ASTM test methods specific to the characteristics of the device.
The medical device industry is a highly regulated industry where extensive product testing is required for growing startups innovating new implants as well as for large companies that acquire such startups or work on their own innovations. Test procedures often follow standardized test methods to ensure repeatability and reliability and results are used in verification, safety, efficacy, and more, depending on the product and company quality requirements and the FDA guidelines.
NexTek is a US manufacturer specializing in EMC/EMI solutions with a full-fledged R&D facility for product design and development. The Company’s consulting laboratory was developed to provide engineering guidance to medical device companies and run mechanical testing on medical devices including but not limited to orthopedic implants and medical instrumentation such as spinal implants, bone screws, drivers, drill bits, taps, and wire tensioners.
Working with a variety of customers developing different medical devices, NexTek was interested in a flexible and versatile testing system to account for the wide range of static and dynamic testing requirements of various devices. However, the initial inquiries that were discussed with material testing system manufacturers were initially treated as either (1) a very high-cost/long-lead time custom testing system or (2) multiple testing machines to run the various testing requested. NexTek was not satisfied with either one of these solutions and reached out to ADMET.
ADMET proposed the engineered eXpert 8900 axial-torsion testing system with MTESTQuattro to fulfill the specific needs of NexTek Engineers. The applicable fixturing for torsion testing, bend testing, tensile testing, universal joints, as well as those designed per specific test methods such as ASTM F543 and ASTM F1717 were supplied with the system. Further information on these setups are discussed below.
eXpert 8900 Axial-Torsion Testing Machine for Medical Device Testing
In addition to the standard load cells to measure static and torque loads, a compression donut load cell was configured with the system to measure the compression force of lag screw specimen as it was inserted into the substrate. This measurement, in addition to biaxial test results, allowed for data collection at the precise location of the construct while maintaining the ability to run the test profile in load control.
Medical Device ASTM Test Setups on eXpert 8900 with MTESTQuattro Controller & Software
The eXpert 8900 axial-torsion testing system was equipped with ADMET MTESTQuattro, PC-based controller and software. Following test methods are examples of standardized medical device test procedures performed with NexTek’s versatile eXpert 8900 testing system.
Spinal implants are tested according to several ASTM standards. ASTM F1717 is intended to provide a basis for the mechanical comparison among past, present and future spinal implant assemblies. Three static and one fatigue mechanical tests of a spinal construct are governed by ASTM F1717. The three static tests are compression bending, tension bending and torsion. The fatigue test is a compression bending test.
ASTM F1717 8.1.1. Static Compression Bend Test Setup
ASTM F1717 8.1.1 covers spinal implant static compression bend testing. This section of the standard is conducted to determine the displacement at 2% offset yield, compressive bending yield load, and compressive bending stiffness.
To perform this test, ADMET supplied the eXpert 8900 system with the appropriate load cell and the clevis fixture designed per the dimensions specified in the standard.
eXpert 8900 – ASTM F1717 Spinal Implant Test Fixture
Test specimens were pre-assembled spinal implant specimens from NexTek customers and UHMWPE blocks were machined to fit in the ASTM F1717 clevis fixture.
ASTM F1717 Compression Bend (Static) Test Method
The test was set up in the MTESTQuattro software to apply a compressive load at a rate of 25 samples per second. Once the position reached 80mm, the rate was set up to automatically be adjusted to 15 mm/min.
ASTM F543 is used to measure four different mechanical properties of bone screws, including the vertical torsion (Annex A1), the insertion/removal torque (Annex A2), the pullout strength (Annex A3), and the axial compression load force of self-tapping bone screws (Annex A4).
NexTek was looking for a versatile testing system to conduct ASTM F543 on different types of bone screws including:
Used in numerous fracture repair applications
Used for knee ligament reconstruction
Used in numerous fracture repair applications
Headless Compression Screws
Commonly used in fracture repair of the hand/foot
ASTM F543 10.1.1. Torsional Strength Test Setup
ASTM F543 10.1.1. is used to measure the torsional yield strength, maximum torque, and breaking angle of the bone screw under standard conditions. This test method is an important parameter to prevent screw breakage during insertion or removal.
To perform this test, the eXpert 8900 system was supplied with a collet sized appropriately for the test specimen and held in place by a vise. While maintaining contact between the drill bit and the screw, the specimen was torqued at the specified rates found in the standard.
eXpert 8900 – ASTM F543 Torsional Strength Test Setup
ASTM F543 Torsional Strength Test Method
The MTESTQuattro software was set up with two separate profiles for this test. Profile 1 included the following steps where the sample was loaded at 25 samples per second to start testing. The load rate would then be adjusted to 20 N/min until 10N force.
MTESTQuattro ASTM F543 Torsional Strength Test Segments Profile 1
Profile 2 included the adjustments of the torque angle.
MTESTQuattro ASTM F543 Torsional Strength Test Segments Profile 2
ASTM F543 10.1.2. Breaking Angle Test Setup
ASTM F543 10.1.2. provides a measure of the ductility of the screw when undergoing a torsional moment. This test method is an important parameter as it provides information on the breaking angle as well as on when the bone screw will reach its maximum torsional strength.
The test frame and fixturing to measure the torsional strength (10.1.1.) was used to measure the breaking angle following ASTM F543 Annex A1.
ASTM F543 Breaking Angle Test Method
The specimen was prepared and secured in the collet fixture. While maintaining contact between the drill bit and the screw specimen, load was applied at the specified rate found in the standard.
ASTM F543 10.1.3. Axial Pullout Strength Test Setup
ASTM F543 10.1.3. provides a measure of the axial pullout strength following Annex A3 . This test method is an important parameter if the screw is subjected to axial tensile forces, or if the screw is fixed into poor quality or osteoporotic bone.
A custom pullout fixture was designed and mounted on a compression platen on the eXpert 8900. The fixture has a geometry that fits around the test specimen, in this case, the screw head. The test substrate (bone or bone foam) with the previously inserted test specimen is then gripped by a machine vise.
eXpert 8900 – ASTM F543 Pullout Strength Test Setup
ASTM F543 Axial Pullout Strength Test Method
MTESTQuattro was set up to run testing at 25 samples per second until the position adjustment criteria was reached per below.
MTESTQuattro ASTM F543 Axial Pullout Strength Test Segments
Axial Pullout Strength
ASTM F543 10.1.4. Axial Insertion Torque Test Setup
ASTM F543 10.1.4. is an important parameter to avoid failure of the screw during insertion and to ensure that the screw may be easily inserted by the surgeon.
The axial insertion torque is measured following ASTM F543 Annex A2. A universal chuck is mounted to the torsional actuator and the appropriate driver bit is tightened into the chuck. The bone screw test specimen is positioned between the driver and the bone foam sample, which is held by a machine vise.
eXpert 8900 – ASTM F543 Insertion Torque Test Setup
ASTM F543 Insertion Torque Test Method
The MTESTQuattro software was set up with two separate profiles for this test. Profile 1 and Profile 2 segments are shown in the menus below. Under normal testing conditions, this test is run to four revolutions at 1 to 3 rpm and lasts 1.5 to 4 minutes depending on the rpm selected.
MTESTQuattro ASTM F543 Axial Insertion Torque Test Segments Profile 1