Testing of Spinal Implants

Spinal implants can support and brace the body.
Test fixture_vertibral body

Testing of Spinal Implants 

Pulsating compressive strength tests (R=0.1) are performed on six specimens with a test frequency of up to 5 Hz. Result: Generation of a Wöhler curve with a number of cycles to crack up to 5 million. This corresponds to approximately 2 years in vivo; approximately 7000 cycles per day. The first runout is confirmed.

In addition to purely axial tensile, compression and flexure tests, a pure or combined torsion load can be applied to spinal systems in compliance with standards including ASTM F1717-9, ASTM F2706-8 or ISO 12189-08.

  • ASTM F1717-9 Standard Test Methods for Spinal Implant Constructs in a Vertebrectomy Model
  • ASTM F2706-8 Standard Test Methods for Occipital-Cervical and Occipital-Cervical-Thoracic Spinal Implant Constructs in a Vertebrectomy Model
  • ISO 12189-8 Implants for surgery – Mechanical testing of implantable spinal devices – Fatigue test method for spinal implant assemblies using an anterior support 

These tests can be performed under physiological (in-vivo) conditions (for example, tempered saline solution) using a Zwick thermoregulation bath.

For practical statistical evaluations of fatigue tests in the finite life range (high cycle fatigue) and in the transitional range to fatigue strength (long life fatigue), Zwick offers the technical-scientific program SAFD (Statistical Analysis of Fatigue Data).

Testing of spinal implant constructs in a vertebrectomy model 

For static and dynamic tests on spinal implants in a vertebrectomy model to ASTM F1717-9. A spinal implant, usually consisting of a screw and rod system, is tested under static and dynamic loads. The implants are mounted on a vertebral replacement test block made of ultra-high molecular weight polyethylene (UHMWPE). The use of simulated vertebral bodies improves the reproducibility of the tests compared to those using human material.

Test arrangement with two vertebral replacement test blocks made of highly cross-linked plastic

ASTM F1717 describes a test arrangement with two vertebral replacement test blocks made of highly cross-linked plastic (UHMPWE), which are screwed to fixing elements. The size and design corresponds to the human anatomy. This test facilitates comparisons between various implant systems.

Testing of Vertebral Body Implants to ASTM F2077

ASTM F 2077 describes a number of different quasistatic and oscillating tests to provide a mechanical comparison of intervertebral disc prosthesis. These include shear, compression and torsion tests, which provide a simplified in-vivo simulation of the loads imposed on such components. 
The vertebral body implant is loaded between two plastic (oscillating test) or metal blocks (quasistatic test). These are adapted to the outer contour of the vertebral body. Zwick offers the fixtures required for testing to ASTM F2077 with the corresponding interfaces. Upon request and based on your data, Zwick can design individual blocks for you. All tests can also be performed under physiological (invivo) conditions by using the Zwick thermoregulation bath at 37 °C ±2 °C. Zwick's thermoregulation bath is constructed with DURAN glass and is therefore suitable for almost all biological and chemical test mediums, such as blood, saline solutions, serum, etc.
  • Completely satisfies the requirements of Standard ASTM F 2077 – Characterization and fatigue of intervertebral body fusion device assemblies, and extremely easy to operate
  • The tests can be implemented under “physiological” environmental conditions (for example, temperature conditioned saline solution) with the use of the Zwick temperature conditioning vessel.
  • Head and foot flange assembly for simple and durable installation, taking into consideration the necessary degrees of freedom for the test.
  • The test device is also suitable for subsidence tests to ASTM F2267.
  • Corrosion-resistant container, suitable for disinfection in autoclaves (up to 120 °C)

Torsion Tests on Vertebral Body Implants to ASTM F 2077 and ASTM F1717 (Static & Dynamic)

As a result of comminuted fractures of a vertebral body or tumors in the area of the spinal column it may be necessary to replace a vertebral body with an implant. These vertebral body implants are tested by carrying out quasi-static and oscillating torsion tests in accordance with ASTM F2077 and ASTM F1717. 
With the help of a torsion fixture, a rotational movement and thus a torsion load up to 50 Nm is possible via the linear movement of the single testing actuator. The torsion fixture also applies one more axial load, which is defined by variable dead weights and a lever mechanism. The main advantage of the Zwick torsion fixture is its ability to test multiple axes with a single closed loop controlled linear actuator. The fixture is designed for static and alternating torsion tests. The angle of rotation for static tests is a maximum of 120° and ±20° for oscillating tests. The play-free guide rails can grip specimens of different lengths and test them according to ASTM F2077 and ASTM F1717.
Oscillating torsion testing on an obeliscTM vertebral replacement from ulrich medical®
Torsion fixture_vertibral body implant

The device is also suitable for expulsion tests. Here the expulsion force is determined by positioning the implant between two machined blocks and applying a defined axial preload. A transverse load, which determines the expulsion strength, is then applied to the implant by a testing actuator via a compression die until the implant is expelled. The maximum force determined defines the expulsion force. The Amsler HC Compact servo-hydraulic testing machine features compact design and extremely low noise, providing an optimum solution for testing vertebral body replacements in any laboratory when used in conjunction with the test fixture for torsion tests on vertebral bodies.  

Tests with the Zwick test fixture for torsion tests on vertebral body replacements 

  • Static and cyclic dynamic torsion on corpectomy models to ASTM F1717
  • Static and cyclic dynamic torsion on vertebral body replacement systems to ASTM F2077
  • Static execution of an expulsion test
  • Torsion tests without a torsion testing actuator and with only a linear testing actuator
  • Axial force application via dead weights and a lever mechanism This horizontal system is on a movable carriage which enables guiding in the compression direction of approximately 4 mm.
  • Universal testing options with provided interface: tool fixing on fixed and rotatable side via six M8 threads (pitch circle of 70 mm) 

Medical product safety and reliability enhanced through innovative testing technology

Fatigue tests are an essential element in the development of durable, safe and reliable implants. Prototypes undergo extensive testing before entering the validation process; Komet Medical, the surgical division of Gebr. Brasseler based in Lemgo, Germany, uses a Zwick testing machine with linear drive for these tests.
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