Patellar fractures account for approximately 1% of all fractures. Open reduction and internal fixation is recommended to restore extensor continuity and articular congruity. However, complications such as nonunion and symptomatic hardware, still exist. Furthermore, there is a risk of re-fracturing of the healed bone during the removal of the implants. Magnesium (Mg), a biodegradable metal, has elastic moduli and compressive yield strength that are comparable to those of natural bone. Our previous study showed that released Mg ions enhanced fracture healing. However, Mg-based implants degrade rapidly after implantation and lead to insufficient mechanical strength to support the fracture. Microarc oxidation (MAO) is a metal surface coating that reduces corrosion. We hypothesized that Mg pins, with or without MAO, would enhance fracture healing radiologically, mechanically, and histologically, while MAO would decrease degradation of Mg pins.

Patellar fracture was performed on forty-eight 18-week-old female New Zealand White rabbits according to established protocol. Briefly, the patella is osteotomized transversely and a tunnel (1.1mm) was drilled longitudinally through the two bone fragments. A pin (1 mm, stainless steel, Mg, or MAO-Mg) was inserted into the tunnel. The reduced construct was stabilized with a figure-of-eight band wire (⊘ 0.6 mm stainless steel wire). Cast immobilization was applied for 6 weeks. The rabbits were euthanized at week 8 and 12 post-operation. Microarchitecture and mechanical properties of the repaired patella were analyzed with microCT and tensile testing respectively. Histological sections of the repaired patella were stained. To evaluate the effect of the MAO treatment on degradation rate of Mg pin, the volume of the Mg pins in the patella was measured with microCT.

At week 8, both Mg and Mg-MAO showed higher ratio of bone volume to tissue volume (BV/TV) than the control while there was no significant different between Mg and Mg-MAO. At week 12, Control, Mg, and Mg-MAO groups showed enlarged patella when compared to the normal patella. Tissue volume (TV) and bone volume (BV) of the patella in Mg and Mg-MAO were larger than those in the Control group. However, the Control had higher ratio of bone volume to tissue volume (BV/TV), TV density, and BV density than Mg and Mg-MAO. Tensile testing showed that the mechanical properties of the repaired patella (failure load, stiffness, ultimate strength, and energy-to-failure) of Mg and Mg-MAO were higher than that of the control at both week 8 and week 12. Histological analysis showed that there was significant new bone formation in the Mg and Mg-MAO group compared with the Control group at week 8 and 12. The degradation rate of the MAO-coated Mg pins was significantly slower than those without MAO at week 8 but no significant difference was detected at week 12.

Mechanical, microarchitectural, and histological assessments showed that Mg pins, with or without MAO, enhanced fracture healing of the repaired patella compared to the Control. MAO treatment enhanced the corrosion resistance of the Mg pins at the early time point.