Case Study: Medical Device Testing System To Accommodate Multiple ASTM Standards

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.

Challenge

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.

Solution

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.

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.

ASTM F1717 Spinal Implant Testing

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.

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.

Calculations

• 2% Yield Load
• Compressive Stiffness

ASTM F543 Medical Bone Screw Testing

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:

• Cortical Screws

Used in numerous fracture repair applications

• ACL Screws

Used for knee ligament reconstruction

• Cannulated Screws

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.

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.

Profile 1

Profile 2 included the adjustments of the torque angle.

Profile 2

Calculations

• Torsional Strength

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.

Calculations

• Breaking Angle

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.

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.

Calculations

• 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.

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.

Profile 1

Profile 2

Calculations

• Axial Insertion Torque

ASTM F543 10.1.5. Removal Torque Test Setup

ASTM F543 10.1.5. is an important parameter to avoid failure of the screw during removal and to ensure that the screw may be easily removed by the surgeon.

The setup to run removal torque testing is the same as the one used to run insertion torque tests per ASTM F543 Annex A2.

ASTM F543 Removal Torque Test Method

The MTESTQuattro software was set up with two separate profiles for this test, identical to the insertion torque segments as shown below. This test was programmed for a short pause after specimen insertion, then proceeded with removal torque.

Profile 1

Profile 2

Calculations

• Removal Torque

ASTM F543 10.1.6. Self-Tapping Force Test Setup

ASTM F543 10.1.6. is an important parameter to ensure that the screw may be easily inserted by the surgeon, particularly if the screw is fixed in poor quality or osteoporotic bone.

The self-tapping force test follows ASTM F543 Annex A4.

ASTM F543 Self-Tapping Force Test Method

The MTESTQuattro software was set up with two separate profiles for this test. Profile 1 and Profile 2 segments are shown below. The self-tapping force test takes about 20 seconds per test procedure. On average specimens were tested at three to four different load capacities.

Profile 1

Profile 2

Calculations

• Axial Load
• Maximum Self-Tap Force
• Maximum Torque
• Axial Displacement

Results

The eXpert 8900 axial-torsion testing machine outlined above was built for NexTek, a-US based engineering firm providing testing services to medical device companies. Our customer’s inquiry was to have the testing machine engineered to accommodate different medical devices to enable their business to run a variety of testing for their clients.

Equipped with fixturing and software to run testing for medical ASTM standards, the eXpert 8900 system was installed and calibrated in NexTek’s testing laboratory. An onsite training session was included where an ADMET Applications Engineer explained each of the test fixtures and how they were mounted. NexTek programs their custom test procedures into MTESTQuattro software. ADMET Technical Support was involved in the software training, gains, test setups, and interface commands.

Both US-based companies, NexTek and ADMET were able to clearly communicate and discuss expectations and follow the necessary steps to meet them. Further information on NexTek can be found here. The Company strives to meet customer testing needs and these needs are constantly evolving due to new devices, new techniques, or experimenting with new ideas. In the future, ADMET will continue to work with NexTek to add further capabilities to their eXpert 8900 with accessories, techniques, and setups, as needed.

Contact ADMET

Click here to fill out our Sales Inquiry form and a Sales Engineer will be in touch to discuss how we can help with your testing. To read more about other orthopedic device testing system configurations, click here.

ADMET testing systems are used in a variety of medical testing applications:

• Medical Device Non-Clinical Bench (Mechanical) Testing
• Mechanical Testing of Vascular Stents
• Knee Joint Prosthesis Mechanical Testing & Equipment
• Mechanical Tensile and Torsion Testing of COVID-19 Test Swabs
• Suture Tensile Strength Testing
• Comparative Biomechanics of Hagfish Skins & Planar Biaxial Testing