Evaluation of systemic metal diffusion after spinal pedicular fixation with titanium alloy and stainless steel system: a 36-month experimental study in sheep

It is known that titanium alloys cause more extensive local metallosis due to fretting corrosion than stainless steel implants. The aim of the present study was to investigate possible systemic metal rel eases (Ti, Al, V, Cr, Ni) in sheep where L4-L5 were implanted with titanium alloy (Ti6Al4V; ASTM F 136) and stainless steel (AISI 316 L). 16 sheep wer e used: 8 were implanted with Ti6Al4V and 8 with stainless steel. At 6 12, 24 and 36 months, the following examinations were performed: histology, ato mic absorption spectrophotometry (AAS) and scanning electron microscopy (SE M), on liver, lung, kidney, brain, spleen and lumbo-aortic lymph nodes. Hai r, urine and arteria blood samples were also analysed by AAS before implant ation and at sacrifices. A histologic and ultrastructural study was perform ed on peri-implant tissues, too. Particular attention was paid to avoid con tamination from dissection instruments or use of containers. In basal and i n samples at 6 and 12 months, no metals were found in blood, urine, hair or other target tissues of the animals implanted with either Ti6Al4V or stain less steel. Regarding Al, V; Co and Ni, negative results in all tissues and body fluids were obtained also at 24 and 36 months. On the contrary, Ti tr aces were found in lumbo-aortic lymph nodes and lungs of one sheep only (10 and 30 ng/g, respectively) at 24 months. At 36 months, a systemic diffusio n of Ti was observed in all tissues of both sheep instrumented with Ti6Al4V (2-16.5 ng/g), except for body fluids and hair. Metal research in target tissues by light and SEM micro-probe analysis prov ided negative results. Current data suggest that the amount of Ti found in organs after stable pedicular fixation is extremely low and not biologicall y available. This observation would lead us to exclude the hypothesis of an y toxic reaction and such a release seems to be due to the passive diffusio n through lymphatic fluids. Additional studies are needed to confirm if thi s long-term release of Ti particles might cause tissue damage.