Comparison of the In Vivo Degradation Progress of Solid Magnesium Alloy Cylinders and Screw-Shaped Magnesium Alloy Cylinders in a Rabbit Model

Resorbable magnesium alloy implants for osteosynthetic surgery would be advantageous to common implants of titanium or surgical steel as a second surgery for implant removal would become unnecessary. To influence the degradation progress, surface modifications are sensible. As plates and screws were used to stabilize fractures, the degradation behavior of threaded cylinders is of particular interest. Therefore each eight solid MgCa0.8 alloy cylinders (3 x 5 mm) with smooth and sandblasted surface, respectively, and eight screw-shaped, threaded MgCa0.8 cylinders (thread pitch 1.25 mm, length 5 mm) were inserted into the medial femoral condyle of adult New Zealand White rabbits. Implantation periods were three and six months, within which the animals were examined daily. To evaluate a possible gas generation radiographs were taken weekly. After euthanasia the bone-implant-compound was scanned in a µ-computed tomograph (µCT80, ScancoMedical). All implants were well tolerated. Smooth implants degraded slowly. The cross sectional area did not reduce obviously after three months implantation duration and only mildly after six months. Sandblasted implants showed the fastest degradation progress after both implantation periods with the most obvious generation of gas. Threaded cylinders revealed pitting corrosion at the thread pitches. They degrade faster than smooth implants but slower than sandblasted cylinders. In summary, surface modification influences the degradation behavior of resorbable magnesium alloy implants. Contrary to common materials, smooth surfaces seem to be favorable. Thread pitches of screw-shaped implants show pitting corrosion. To what extend this result affects future applications of resorbable screws has to be examined in further investigations.