Abstract
Study Design. Combinations of metal implants (stainless steel (SS), titanium (Ti), and cobalt chrome (CC)) were placed in porcine spines. After 12 months, tissue response and implant corrosion were compared between mixed and single metal junctions.Objective.Model development and an attempt to determine any detriment of combining different metals in posterior spinal instrumentation.Methods. Yucatan mini-pigs underwent instrumentation over five unfused lumbar levels. A SS rod and a Ti rod were secured with Ti and SS pedicle screws, SS and Ti crosslinks, SS and CC sublaminar wires, and Ti sublaminar cable. The resulting 4 SS/SS, 3 Ti/Ti, and 11 connections between dissimilar metals per animal were studied after 12 months using radiographs, gross observation, and histology (foreign body reaction (FBR), metal particle count, and inflammation analyzed).Results. Two animals had constructs in place for 12 months with no complications. Histology of tissue over SS/SS connections demonstrated 11.1 ± 7.6 FBR cells, 2.1 ± 1.7 metal particles, and moderate to extensive inflammation. Ti/Ti tissue showed 6.3 ± 3.8 FBR cells, 5.2 ± 6.7 particles, and no to extensive inflammation (83% extensive). Tissue over mixed components had 14.1 ± 12.6 FBR cells and 13.4 ± 27.8 particles. Samples surrounding wires/cables versus other combinations demonstrated FBR (12.4 ± 13.5 versus 12.0 ± 9.6 cells,P= 0.96), particles (19.8 ± 32.6 versus 4.3 ± 12.7,P= 0.24), and inflammation (50% versus 75% extensive,P= 0.12).Conclusions. A nonfusion model was developed to study corrosion and analyze biological responses. Although no statistical differences were found in overlying tissue response to single versus mixed metal combinations, galvanic corrosion between differing metals is not ruled out. This pilot study supports further investigation to answer concerns when mixing metals in spinal constructs.
Highlights
Metal spinal implants are not routinely removed, so corrosion over potentially several decades is a concern [1, 2]
In addition to the concern of fretting corrosion caused by micromotion between components, galvanic corrosion could potentially occur within a spinal implant where stainless steel (SS) or chromium is in physical contact with Ti within an in vivo environment acting as a conducting electrolyte
This study evaluated implants and the surrounding tissue of mixed versus single metal combinations (Ti and SS rods with SS, cobalt chrome (CC) and Ti sublaminar wires, SS and Ti cross links, and SS and Ti screws with SS and Ti connectors) twelve months following posterior spinal instrumentation using a Yucatan minipig model
Summary
Metal spinal implants are not routinely removed, so corrosion over potentially several decades is a concern [1, 2]. Concern of corrosion increases when mixing two different metals in a biological, Advances in Orthopedic Surgery electrolytic system where an oxidation reaction, boosted by temperature, creates the ideal environment for galvanic corrosion. With the increased clinical use of titanium (Ti) and potential use of Ti in combination with existing stainless steel (SS) implants, there is increased concern amongst surgeons of galvanic corrosion related to mixing these metal classes. In addition to the concern of fretting corrosion caused by micromotion between components, galvanic corrosion could potentially occur within a spinal implant where SS or chromium (serving as an anode) is in physical contact with Ti (the cathode) within an in vivo environment acting as a conducting electrolyte
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