Medical Device Testing: Internal Fixation vs. External Fixation

Posted by Debbi Cohen on Fri, Mar 23, 2012 @ 10:03 AM

External fixation is a surgical treatment used to set bone fractures in which a cast would not allow proper alignment of the fracture. An example of a standard test for external fixation is ASTM F1541.

Internal fixation involves the surgical implementation of implants for the purpose of repairing a bone. Traditionally an internal fixator would be made of stainless steel or titanium, but now bioabsorbable bone screws, acrylic bone screws, and other composite materials are coming into the market. The American Association of Orthopedic Surgeons were asked what they thought the benefits of internal fixation and they responded in favor of several main points. ASTM F543 is an example of a test for internal fixation devices.

A study from an international agency compared internal and external fixation long-term results and found that patients treated with internal fixation had better results anatomically, but did not demonstrate better functionality. In addition, another study from the US National Library of Medicine demonstrates that although it initially looks like internal fixation is stronger than external, after time, the results appear similar for both methods. Also, in a 7 year follow up internal fixation and open reduction of Bennet fractures article explains there is little difference in the techniques. 

How do you determine the similarities and differences between internal and external fixation devices? You can use a testing machine that provides torsional actuation to simulate the movement of the device, the insertion and extraction forces of the bone fixators, and the effect of point and thread variation of the screws.

One way is by simultaneous Torsion and Tension Testing

The eXpert 3600 series and the eXpert 81T series are suited for torsion testing bone, catheters, elastomers, fasteners, joints, metals, plastics, shafts, small components and more. The eXpert 3600 machines are equipped with both a torsion actuator and a second linear actuator for applying tension and compression forces. Pictured below is a static table top version, though the machine is also available in a floor-standing model. Fatigue table-top and floor-standing versions are also available.

eXpert 3600 Biaxial

ADMET's eXpert 3600 Biaxial Testing Machine

For the eXpert 3600 Biaxial Testing System, ADMET's MTESTQuattro, a PC-based data acquisition, analysis, and reporting software controller, will allow the user to set up axial, torsion, and biaxial testing profiles with a wide range of flexible, built-in capabilities.

Another way is by performing torsion only tests with a pulley counter balance weight.

The 81T standalone torsion testing system is well-suited for this application. When the tail stock is left free-floating, the user can apply an axial pre-load to the specimen using a weight and pulley arrangement. Two closed-loop servo controllers are available for performing static or dynamic tests under torque or angle rate control: the MTESTQuattro and the eP Digital Controller — a standalone unit suited for quality-control testing.

ADMET 81T

ADMET 81T Torsion Testing Machine

Ultimately, it all depends on how you want to test your fixation device. A torque test to determine the strength of a bone screw is a straightforward test. Twist at a torque or angle rate to a torque, angle, or time limit. With the ADMET system, you could perform a standard test or create your own profile. Perhaps you are trying to determine what type of tip to use at the end of your screw. You could set up a profile to compress and twist until a designated displacement distance was reached. Our system would report the axial peak and the torsional peak load and additional preprogrammed analyses are available.

Topics: Biomedical, Bone screws and plates