Failures of four different 300-series austenitic stainless steel biomedical fixation implants were examined. The device fractures were observed optically, and their surfaces were examined by scanning electron microscopy. Fractography identified fatigue to be the failure mode for all four of the implants. In every instance, the fatigue cracks initiated from the attachment screw holes at the reduced cross sections of the implants. Two fixation implant designs were analyzed using finite-element modeling. This analysis confirmed the presence of severe stress concentrations adjacent to the attachment screw holes, the fatigue crack initiation sites. Conclusions were reached regarding the design of these types of implant fixation devices, particularly the location of the attachment screw holes. The use of austenitic stainless steel for these biomedical implant devices is also addressed. Recommendations to improve the fixation implant design are suggested, and the potential benefits of the substitution of titanium or a titanium alloy for the stainless steel are discussed.

A compression hip screw is a device designed to hold fractures in the area of the femur in alignment and under compression. A side plate, which is an integral part of the device, is attached by screws to the femur, and it holds the compression screw in position. The device analyzed had broken across the eighth hole (of nine holes) from the end of the plate. The detailed metallurgical failure analysis of the device, including metallography and fractography, is reported here. It was found that the device had adequate metallurgical integrity for the application for which it was intended. It is believed that failure was caused by the lack of a screw in the ninth hole. Evidence is also presented which indicates that the device was bent prior to insertion, and the local plastic deformation may have caused structural changes leading to premature crack initiation.

Metallurgical SEM analysis provides many insights into the failure of biomedical materials and devices. The results of several such investigations are reported here, including findings and conclusions from the examination a total hip prosthesis, stainless steel and titanium compression plates, and hollow spinal rods. Some of the failure mechanisms that were identified include corrosive attack, corrosion plus erosion-corrosion, inclusions and stress gaps, production impurities, design flaws, and manufacturing defects. Failure prevention and mitigation strategies are also discussed.

Two type 316L stainless steel orthopedic screws broke approximately 6 weeks after surgical implant. The screws had been used to fasten a seven-hole narrow dynamic compression plate to a patient's spine. The broken screws and screws of the same vintage and source were examined using macrofractography, SEM fractography, and hardness testing. Fractography established that fracture was by fatigue and that the fatigue cracking originated at corrosion pits. Hardness while below specification, still indicated that the screws were in the cold-worked condition and notch sensitive during fatigue loading. Use of a steel with a higher molybdenum content (317L) in the annealed condition was recommended.

Heavy pitting corrosion on type 304 stainless steel bone screw was studied. A screw head that exhibited heavy pitting corrosion attack was observed. Deep tunnels that penetrated the screw head and followed the inclusion lines were revealed. The screw was inserted in a plate made of type 316LR stainless steel and some mechanical fretting and very few corrosion pits were revealed. Type 304 stainless steel was deemed not to be satisfactory as an implant material.

A type 316L stainless steel angled plate failed. The fatigue fracture was found to have occurred at a plate hole. Symmetric cyclic bending forces were revealed by the fatigue damage at the fracture edge at the top surface of the plate. Fatigue striations and slip bands produced on the surface during cyclic loading were observed. The material was showed by the deformation structure to be in the cold-worked condition and was termed to not be the cause of the implant failure.

Threads of a bone screw (Co-Cr-Mo alloy, type ASTM F75) had broken off, and other threads had cracked. 15x sectioning showed porosity, and 155x magnification showed gas holes, segregation, and dissolved oxides. This supports the conclusion that manufacturing defects caused the failure.

Cerclage wire, which was used with two screws and washers for a tension band in a corrective internal fixation, was found broken at several points and corroded after nine months in service. The material was examined using energy-dispersive x-ray analysis and determined not to be in compliance with standards (type 304 stainless steel without molybdenum). The screws and washers were found to be made of remelted implant-quality type 316L stainless steel and were intact. Signs of sensitization, characterized by chromium carbide precipitates at the grain boundaries, were revealed by the microstructure. Intercrystalline corrosion with pitted grains was indicated by SEM fractography. Improper heat treatment of the steel was interpreted to have led to intercrystalline corrosion and implant separation.

TiN coated back surgery wires were made of Ti-6Al-4V. The reported failure was the presence of pits located in the uncoated area of the wires. The uncoated area of the wire is where the wire is fixtured in the coating chamber during coating. Examination and analysis of the pits using SEM/EDX detection unit revealed significant peaks of B, O, Zr and Fe. Moreover, the shape of the pits was similar to an arc crater. The formation of pits in the wire was caused during coating due to microarcing. A contaminated fixture used during the coating most likely caused the microarcing.

Bearing in mind the three-legged stool approach of device design/manufacturing, patient factors, and surgical technique, this article aims to inform the failure analyst of the metallurgical and materials engineering aspects of a medical device failure investigation. It focuses on the device "failures" that include fracture, wear, and corrosion. The article first discusses failure modes of long-term orthopedic and cardiovascular implants. The article then focuses on short-term implants, typically bone screws and plates. Lastly, failure modes of surgical tools are discussed. The conclusion of this article presents several case studies illustrating the various failure modes discussed throughout.

A large portion of the four-hole Lane plate disintegrated and consisted mainly of corrosion products after remaining in the body for 26 years. Transformation structures and carbides were exhibited by the plate which was made from chromium steel. Minimal corrosion was exhibited by the soft austenitic 304 stainless steel used to make the screws. The corrosion products of the plate were revealed by microprobe analysis to impregnate the surrounding tissues. Improper material selection was concluded to be the reason for the general corrosion behavior.

Wear on a titanium screw head with a lip of material that that was transported by fretting at a plate-hole edge was studied. A flat fretting zone was visible on the screw surface over the material lip. A cellular wear structure containing wear debris was found. No morphological signs of corrosion were observed in connection with fretting structures.

Fretting and fretting corrosion at the contact area between the screw hole of a type 316LR stainless steel bone plate and the corresponding screw head was studied. The attack on the 316LR stainless steel was only shallow. Mechanical grinding and polishing structures were exhibited by a large portion of the contact area. Fine corrosion pits in the periphery were observed and intense mechanical material transfer that can take place during fretting was revealed. Smearing of material layers over each other during wear was observed and attack by pitting corrosion was interpreted to be possible.

Type 316LR stainless steel screws that failed by fatigue were studied. It was found that fatigue fracture can occur on different thread levels, depending on the loading situation. The initiation of secondary fatigue cracks was occasionally found parallel to the fracture plane. The screws were used with a relatively rigid plate to treat a fracture complication in the upper end of the femur. The fatigue failures were explained by signs of unstable fixation revealed by radiographs.

The bone cement failed at the distal end of the prosthesis stem of femoral head prosthesis six months after implantation. A small indentation on the lateral contour of the stem was visible where the stem had broken. The degree of loosening (gap between the lateral stem contour and the bone or cement) and implant loading was revealed by the dislocation of fragments of the prosthesis. Secondary cracks that had originated at the lateral aspect of the stem were revealed by metallographic examination of a section parallel to the stem surface and perpendicular to the fracture surface of the distal fragment. Gas pores are apparent in the grain and at the grain boundaries were revealed by a transverse section. Fine parallel line structures that run diagonally through the fractograph may be slip traces were revealed by scanning electron microscopy. One of the cracks was revealed to have propagated through a larger gas pore by a ruptured gas pore. The stresses created through the fatigue process activated glide systems which served the formation of secondary cracks along glide planes.

A narrow bone plate made of type 316 stainless steel and used to stabilize an open midshaft femur fracture failed. A crack at a plate hole next to the fracture site had been revealed by a radiograph taken 13 weeks after the operation. The plate was revealed to be slightly bent in the horizontal plane, and the fracture gap was considerably open. The screws and plates supplied by different manufacturers were revealed to be different with respect to microcleanliness (primary inclusion content) of the materials and only one of them was found to be according to specifications. The local crack formation was influenced by the presence of larger inclusions. The screw failed was revealed to have failed through a fatigue mechanism by the presence of striations in the scanning electron micrograph. The crack in the plate was revealed to have originated at the upper, outer corner of the plate by the beach marks which indicated the action of asymmetric bending and rotational forces.

Stainless steel is frequently used for bone fracture fixation in spite of its sensitivity to pitting and cracking in chloride containing environments (such as organic fluids) and its susceptibility to fatigue and corrosion fatigue. A 316L stainless steel plate implant used for fixation of a femoral fracture failed after only 16 days of service and before bone callus formation had occurred. The steel used for the implant met the requirements of ASTM Standard F138 but did contain a silica-alumina inclusion that served as the initiation point for a fatigue/corrosion fatigue fracture. The fracture originated as a consequence of stress intensification at the edge of a screw hole located just above the bone fracture; several fatigue cracks were also observed on the opposite side of the screw hole edge. The crack propagated in a brittle-like fashion after a limited number of cycles under unilateral bending. The bending loads were presumably a consequence of leg oscillation during assisted perambulation.

Two of four adjustable Moore pins, which had been used to stabilize a proximal femur fracture, were found to be broken and deformed at their threads. The pins were made from a cobalt-chromium alloy and were not in the same condition. Brittle precipitates in the grains and grain boundaries were seen in one of the pins and hence the fracture was revealed to have occurred along the grain boundaries. The other pin made from cold-worked cobalt-chromium alloy was observed to have randomly lines of primary inclusions. Intermingled dimples and fatigue striations were exhibited on the fracture surface of this pin. Thus, the effect of different conditions of cobalt-chromium alloys on failure behavior was demonstrated as a result of this study.

This paper summarizes several cases of metallurgical failure analysis of surgical implants conducted at the Laboratory of Failure Analysis of IPT, in Brazil. Investigation revealed that most of the samples were not in accordance with ISO standards and presented evidence of corrosion assisted fracture. Additionally, some components were found to contain fabrication/processing defects that contributed to premature failure. The implant of nonbiocompatible materials results in immeasurable damage to patients as well as losses for the public investment. It is proposed that local sanitary regulation agencies create mechanisms to avoid commercialization of surgical implants that are not in accordance with standards and adopt the practice of retrieval analysis of failed implants. This would protect the public health by identifying and preventing the main causes of failure in surgical implants.

During the internal fixation, the type 316LR stainless steel cortical bone screw failed. Extensive spiral deformation was revealed by the fracture surface. Dimple structure characteristic of a ductile failure mode was observed with dimples oriented uniformly in the deformation direction. A zone of heavily deformed grains at the fracture edge was revealed by longitudinal metallographic examination. The shearing fractures of a commercially pure titanium screw and a cast cobalt-chromium-molybdenum alloy were discussed for purpose of comparison.