33 Practical Failure Analysis Volume 1(4) August 2001 Microstructural Analysis of Failure of a Stainless Steel Bone Plate Implant E. Proverbio and L.M. Bonaccorsi (Submitted 20 April 2001; in revised form 5 May 2001) 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. E. Proverbio and L.M. Bonaccorsi, University of Messina, Salita Sperone 31, S. Agata di Messina, 98166 Messina, Italy. Contact e-mail: proverbi@ingegneria.unime.it. Keywords: bone plate, corrosion, failure analysis, fatigue, implant, stainless steel PFANF8 (2001) 4:33-38 © ASM International Introduction Exposure by orthopaedic implants to the bio- mechanical and biochemical forces and interactions between the implants and the biological environment may lead to failure due to mechanical or biomech- anical reasons. Fatigue or corrosion fatigue damage is one of the major reasons for failure of austenitic stainless steel screws and plates. Local loading con- ditions produce the fatigue stresses. Dynamic loading in the presence of body liquid can also cause surface attack by fretting, fretting corrosion, or wear at im- plant junctions, such as screw heads and plate holes. These types of attacks can be relatively mild; they often occur only on the microscopic level and do not interfere with the functioning of the implant or the healing of the bone. [1,2] A combined attack of fatigue with stress corrosion can also cause implant breakdown, but this is rare. [1,2] The normal pH of the body fluids is almost neutral, with a mean value of about 7.4. [3] At injured sites, the pH shifts to acidic values as low as 4.0, especially in hematoma. [4] Of all the ionic components of blood plasma and interstitial fluids, the chlorine ions are typically the most aggressive to metal implants. Several types of chloride-induced corrosion attacks have been reported to affect stainless steel implants; pitting, intergranular corrosion, and crevice corrosion have all been observed. [5] Various types of forces act on the implants and the bone. In the intact musculoskeletal system, the acting forces are balanced. When a bone is fractured, the balance of forces is destroyed, and the muscle forces pull the bone fragments in various directions. During operative reconstruction of a fractured bone, attaching the fragments to orthopaedic implants stabilizes the fracture. If the bone is perfectly reduced, the entire implant is supported by bone, the acting forces are again in equilibrium, and only relatively small and uncritical loads are exerted on the implant. However, if the bone is not perfectly reconstructed, if fracture gaps are present or fragments of bone are missing, the weight-bearing forces are not completely balanced and the loads may be unevenly distributed. As a result, bending and torsional stresses can con- centrate in areas of the implant where bone support is missing. The implant undergoes cyclic loading in these zones and the risk of fatigue damage may in- crease. It is not necessary for the implant to be loaded in the plastic deformation range for fatigue cracks to develop. Local stress concentrations may be suffi- cient to initiate fatigue cracks in the surface of the implant. The development of fatigue damage de- pends on the number of load cycles and the intensity of loading. An estimate of the number of cycles that an implant may undergo in a given time period varies from 54,000 cycles per month (assuming 1 h motion per day), to 324,000 cycles per month (with 6 h