OTA 1997 Posters - Scientific Basis for Fracture Care

*Mechanical Strength of Used External Skeletal Fixators

Douglas R. Dirschl, MD

UNC School of Medicine, Chapel Hill, North Carolina, USA

Purpose: Although all external skeletal fixators are marketed and sold as single-use items, numerous hospitals and trauma centers have reprocessed and reused the devices. A recent study of a specific program of reuse of external fixator components reported no greater rate of mechanical failures or complications as a result of the reuse program. To date, no published studies have examined the mechanical strength of used external fixation devices. The purpose of this investigation was to investigate the static mechanical strength of used external fixation devices and to compare this with the results of identical testing of new devices.

Conclusion: After one or two clinical uses, the EBI dynafix standard and wrist external fixators showed no decrease in static mechanical strength from values obtained in new fixators. No device tested exhibited catastrophic mechanical failure. One or two clinical uses of these devices does not result in a deterioration of the mechanical properties compared to new devices.

Methods: Thirty-five EBI dynafix external fixation devices (15 standard fixators and 20 wrist fixators) that had been subjected to one or two clinical uses were subjected to static mechanical testing identical to that performed for new devices. The average time the fixators had been applied to patients was 12 weeks for the wrist fixator and 15 weeks for the standard fixator. The fixators were tested using an Instron closed loop, servohydraulic testing apparatus. Fixators were offset 6 degrees from the vertical in order to produce a more realistic biomechanical model and were subjected to a gradually increasing load at an actuator rate of one centimeter per minute until fifteen millimeters of deflection was encountered. Fifteen millimeters of deflection was defined as failure criteria for static loads. In addition, prior to testing each fixator, the serrated joints (standard fixator) or ball joints (wrist fixator) in each of the fixators was examined under 50x magnification for cracks, deep scratches, or wear. All results were compared with identical testing of new devices previously performed by an independent laboratory.

Results: The mean static load to failure for used standard fixators was 721 ± 70.0 N (mean ± SD), while it was 749 ± 81.0 N in new fixators (p>0.5). All fixators failed because of 15 mm of deflection; no fixator underwent catastrophic material failure. There was no difference in load to failure between fixators that had undergone one (N=10) or two (N=5) clinical uses. Examination of the serrated joints revealed no wear, cracks, or deep scratching in any of the fixators, although some surface scratches were observed. The mean static load to failure for used wrist fixators was 252 ± 62 N (mean ± SD), while it was 225 ± 56 N in new fixators (p>0.5). All fixators failed due to slippage of the ball joints, which was also the mode of failure in new fixators. There was no difference in static strength between fixators which had undergone one (N=12) or two (N=8) clinical uses. Examination of the used ball joints revealed surface scratching and shallow pitting, but no deep pitting or cracks were observed.

Discussion: Reuse of external fixators is a controversial subject and this study is the first, to our knowledge, to subject used external fixation devices to the same mechanical testing to which new devices are subjected prior to FDA approval. The EBI dynafix was chosen for this investigation because it is the external fixation device most frequently used (and reused) at our intitiution, making used fixators readily available for mechanical testing. Mechanical testing of used EBI dynafix standard and wrist fixators showed no statistically significant difference in static strength from that of new devices, and no device tested had a static strength below the industry standard for fixators of that type. The stainless steel serrated joint configuration of the EBI standard fixator appears to be quite durable, showing no significant wear after one or two clinical uses. The dynafix wrist fixator, however, did show some signs of wear of the ball joints after one or two clinical uses. This wear, however, did not result in a diminution of the static strength of the device. The results of this study are not surprising, since new devices are generally tested to one million cycles prior to FDA approval, and the used fixators tested in this study were each subjected to far fewer than this number of cycles during clinical. Nonetheless, the results of this study provide documentation that the EBI dynafix retains its mechanical strength after up to two clinical uses. Additional study of the mechanical strength of used external fixators of other designs and other manufacturers is desireable to determine the effects of use of the devices on mechanical properties.