Fretting corrosion behavior of nitinol spinal rods in conjunction with titanium pedicle screws

Untypical corrosion damage including erosions combined with the build-up of titanium oxide as a corrosion product on the surface of explanted Nitinol spinal rods in the areas where it was in contact with titanium pedicle screw head is reported. It was suggested that Nitinol rods might have inferior fretting corrosion resistance compared with that made of titanium or CoCr. Fretting corrosion of Nitinol spinal rods with titanium (Ti6Al4V) pedicle screws were tested in-vitro by conducting a series of potentiostatic measurements of the peak-to-peak values of fretting corrosion current under bending in a 10% solution of calf serum in PBS. The test included Nitinol rods locked in titanium pedicle screws of different designs. Performance of commercially available titanium (Ti6Al4V) and CoCr spinal rods was also investigated for a comparison. Corrosion damage observed after the in-vitro tests was studied using SEM and EDAX analysis and was compared with patterns on Nitinol rods retrieved 12months after initial surgery. Metal ions level was measured in the test media after in-vitro experiments and in the blood and tissues of the patients who had the rods explanted. The results of this study revealed that Nitinol spinal rods locked in Ti pedicle screws are susceptible to fretting corrosion demonstrating higher fretting corrosion current compared with commercially used Ti6Al4V and CoCr rods. On the surface of Nitinol rods after in-vitro tests and on those retrieved from the patients similar corrosion patterns were observed. Improved resistance to fretting corrosion was observed with Nitinol rods in the in-vitro tests where pedicle screws were used with a stiffer locking mechanism. Since the development of the localized corrosion damage might increase the risk of premature fatigue failure of the rods and result in leaching of Ni ions, it is concluded that Nitinol rods should not be used in conjunction with Ti pedicle screws without special protection especially where the design provides a high degree of mobility to the rods

Metal concentrations in the blood and tissues after implantation of titanium growth guidance sliding instrumentation

BACKGROUND: Growth guidance sliding treatment devices such as Shilla (Medtronic, USA) or LSZ-4D (Conmet, Russia) used for the treatment of scoliosis in children who have high growth potential have unlocked fixtures allowing rods to slide during growth of the spine which avoids periodical extensions. However, the probability of clinical complications associated with metallosis after the implantation of such devices is poorly understood. The content of metal ions in the blood and tissues of pediatric patients treated for scoliosis using fusionless growth guidance sliding instrumentation, have not yet been investigated. PURPOSE: The aim of this study was to measure the content of metal ions in blood and tissues surrounding implanted growth guidance sliding LSZ-4D devices made of titanium alloy (Ti6Al4V) and to identify the incidence of metallosis associated clinical complications in some patients with these devices. STUDY DESIGN: One center case-control retrospective study. PATIENTS SAMPLE: Study group included 25 patients with high growth potential (3 males, 22 females, average age at primary surgery for scoliosis treatment is 11.4±1.2 years old) who had sliding growth guidance instrumentation LSZ-4D (Conmet, Moscow, Russia) implanted on 13 (range 10-16) spine levels for 6±2 years. The LSZ-4D device was made from titanium alloy Ti6Al4V and consisted of two rectangular section rods and fixture elements. Locked fixtures were used on one spinal level, while the others were unlocked (sliding). The control group consisted of 13 patients (12 females and 1 male, 11±1.2 years old) without any implanted devices. OUTCOME MEASURES: The content of Ti, Al and V metal ions in the whole blood and tissues around the implanted device was measured. Incidence of metallosis associated complications in the study group were recorded. METHODS: Metal ion content was measured by ICP-MC on quadrupolar Nexion 300D (Perkin Elmer, USA). RESULTS: 5 of 25 patients in the study group developed metallosis associated complications (two sinuses and three seromas in the lumbar part of the spine). Revisions were carried out in two of these patients. 90% of patients in the study group had increased content of Ti and V ions in the blood (2.8 and 4 times respectively). Median content of Ti ions in soft tissues adjacent to implanted sliding device was more than 1,500 fold higher compared with the control group. These levels are much higher than previously reported for spinal instrumentation. CONCLUSION: Increased content of Ti and V ions in the blood and especially in tissues around the titanium growth guidance sliding device LSZ-4D accompanied by clinical manifestations (seromas and sinuses) indicate the importance of improving of wear resistance of such instrumentation with the coatings and the necessity to exchange sliding instrumentation once the child is fully grown

Analysis of retrieved growth guidance sliding LSZ-4D devices for early onset scoliosis and investigation of the use of nitinol rods for this system

STUDY DESIGN.: Analysis of volumetric wear loss of retrieved growth guidance sliding devices LSZ-4D for treatment of early onset scoliosis and laboratory in vitro wear test for comparison of wear resistance of alloys Nitinol, Ti, and cobalt chromium (CoCr).OBJECTIVE.: To evaluate quantitatively the amount of wear debris from the sliding LSZ-4D device and to investigate the potential of using Nitinol for replacing Ti alloys in spinal instrumentation. To do that, wear resistance of Nitinol, Ti, and CoCr was compared.SUMMARY OF BACKGROUND DATA.: There are little data regarding the amount of wear debris associated with growth guidance sliding devices for patients with early onset scoliosis and the wear resistance of superelastic Nitinol compared with Ti and CoCr.METHODS.: Volumetric wear loss was measured on LSZ-4D devices made from titanium alloy Ti6Al4V and each consisted of 2 rectangular section (6 × 4 mm) rods and 40 ± 8 fixture elements (20 ± 4 hooks and 20 ± 4 clips) retrieved from 3 patients (implantation period, 3.5-5.8 yr). Images of wear scars were taken on Bruker interferometer microscope and incorporated into MATLAB software. Wear resistance of Nitinol, Ti, and CoCr was studied using reciprocation pin-on-disk wear test in bovine serum at 37°C ± 1°C.RESULTS.: The volume wear rate of LSZ-4D device was found to be 12.5 mm per year from which 5 mm per year is the wear debris of the rod and 7.5 mm per year is the contribution of fixtures. Wear resistance of Nitinol is 100 times higher than that of Ti and comparable with that of CoCr.CONCLUSION.: Application of wear-resistant coatings on Ti components in growth guidance sliding devices for the treatment of early onset scoliosis will be useful. High wear resistance of Nitinol combined with its superelastic and shape memory properties could make application of Nitinol rods for spinal instrumentation beneficial. © 2014 Lippincott Williams & Wilkins.