Cervical Stent Failure Analysis
- PDF / 1,097,070 Bytes
- 6 Pages / 595.276 x 790.866 pts Page_size
- 64 Downloads / 207 Views
CASE HISTORY—PEER-REVIEWED
Cervical Stent Failure Analysis Wayne Reitz
Submitted: 9 July 2013 / in revised form: 11 September 2013 / Published online: 28 September 2013 Ó ASM International 2013
Abstract Harrington rods failed after a short period in service. Metallurgical analysis showed (1) notches were present on the rods, (2) small cracks present in the bent regions of the rod, and (3) the fractures occurred at clamped locations. All of these conditions can shorten the fatigue life by eliminating the crack initiation stage of fatigue and allowing corrosion fatigue to occur. Keywords Titanium
Annealing Biomaterials Failure analysis
Investigation The investigation included visual inspection at 91, macroscopic inspection, Knoop microhardness, chemical analysis, scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS), and metallography. Metallographic etch consisted of immersion in a solution of 10 ml KOH, 5 ml H2O2, and 20 ml H2O for 10 h to highlight grain size.
Discussion Introduction The Harrington rod, developed in 1953 by Paul Harrington, a professor of orthopedic surgery at Baylor College of Medicine in Houston, Texas, was implanted along the spinal column to treat lateral curvature of the spine. Harrington rods were intended to provide a means to reduce the curvature and to provide more stability to a spinal fusion. The device was implanted and secured onto the vertebral laminae [1]. A Harrington rod cervical stent fractured while in-service. The device was implanted in 2005 and retrieved in 2006 and then submitted for metallurgical examination to determine cause of failure.
W. Reitz (&) Talbott Associates, Inc., 7 SE 97th Ave., Portland, OR 97216, USA e-mail: [email protected]
123
The components for this cervical stent were shown in Fig. 1. Indentations were present at the clamp positions; the Harrington rod fractured adjacent to a retaining clamp. The fracture occurred on both rods in essentially the same location based on clamp marks. Semi-quantitative chemical analysis was performed using SEM/EDS and the results were listed in Table 1. The data showed that rod ‘‘B’’ had low aluminum and that iron was present, which might be a surface contaminant. Rod ‘‘A’’ chemical analysis met the chemistry specification for ASTM F136. Microhardness measurements were conducted on the longitudinal and transverse cross-sections with all samples exhibiting the same hardness of 30 Rockwell C. The mechanical properties of the rods and the ASTM specification and typical annealed values were presented in Table 2. All the hardness values were in reasonable agreement. The low ductility in ASTM F136 was for cold worked material, while annealed material was typically 30%. Figures 2 and 3 showed the fractured rods and their mating surfaces. The general location of the crack initiation
J Fail. Anal. and Preven. (2013) 13:678–683
679
Fig. 1 Partial assembly of components; note fractures at black arrows
Fig. 2 Fractured Rod ‘‘B’’; crack initiated at black arrow
Table 1 Chemical analysis
Data Loading...
