The Need for Sustained Compression in Ankle Fixation Devices

Douglas Pacaccio, +Christopher M. Yakacki, Ken Gall Private Practice, Yorkville,IL, MedShape Solutions Inc, Atlanta, GA, The Georgia Institute of Technology, Atlanta, GA [email protected] Introduction Tibio-talo-calcaneal (TTC) arthrodesis is a salvage procedure for patients with severe malfunction of the ankle and subtalar joints. Serious ankle disease stems from post-traumatic arthritis, idiopathic osteoarthritis, rheumatoid arthritis, Charcot arthropathy, talar osteonecrosis, or failed arthroplasty, which can lead to severe pain and deformity. Patients with severe ankle disease may face limited salvage options includingfusion and at times, amputation. Diabetic patients who undergo amputation experience a significantly shorter life expectancy. Consequently, ankle arthrodesis is preferred by patients and specialists. Non-union rates reported for TTC fusion have ranged from 11% to 65% in the published literature. While many factors affect bone union, there is a general agreement that secure fixation and compression is desirable and directly relates to bone fusion. The purpose of this study was to characterize the delivery and sustainability of the compressive forces of internal and external fixation devices in response to simulated bone resorption. Methods Two compression IM nails and two external fixators were tested; the PantaNail (Newdeal/Integra) and the VersaNail (DePuy). An AceFischer external fixator (DePuy), denoted as “ExFix”, with a medium foot frame (8180-04-11), medium connecting rods (FA-10000-2), and 2 sets of medium 1/3 and 2/3 rings (FC-10031/32) was tested with 1.8 mm olive wires. A True/Lok (Encore) external fixator was also tested with the same model olive wires. Finally, a prototype dynamic IM nail was manufactured using a nickel-titanium (Ni-Ti) wire to further sustain compression. Testing was performed initially using synthetic bone blocks and then verified using three cadaveric specimens. Figure 1 illustrates the test setup in the cadaveric model. A screw-driven parallel plate mechanism was used to simulate resorption while a donut load cell was used to monitor the forces. Results A summary of the forces during hardware/implant installation and hardware removal can be seen in Figure 2. For the IM nails, the PantaNail instrumentation was able to apply 1898 N of compression in the sawbone model compared to 219 N of compression by the VersaNail. Therefore, the PantaNail was selected for testing in the cadaveric ankle; however, it only generated 1050 N of compression due to relaxation (viscoplasticity) and yielding of the real bone. All IM nail testing showed a decrease in load when the instrumentation was removed. The PantaNail was able to retain 735 and 920 N of compression in synthetic and cadaveric constructs, respectively. The external fixators tested were both capable of loading up to 1200 N with similar loading profiles, and thus the Encore device was selected for cadaveric testing due to author preference. The compressive forces of the devices as a function of resorption can be seen in Figure 3. The commercial IM nails tested showed over a 90% drop in compression force with 1 mm of resorption, whereas the external fixators maintained over 50% of their compression forces with 5 mm of resorption. Lastly, the prototype NiTi-Nail was able to sustain nearly 50% of its compression force with 4 mm of resorption. Discussion Compression is important for the stabilization of a fracture or fusion site and provides a driving force for local bone healing. Compression is also one of two key elements necessary to prevent non-union of a bone in healing according to AO/ASIF principles. Compression, either through external or internal fixation, provides stability between the bone ends, which has been shown to promote "direct" or "primary bone healing".