Stent fracture

Factors that make a stent strut vulnerable, which may lead to thrombosis, jailing side branches, or breakage of the struts, include the following polymer/drug coating dissolution, incomplete apposition, stent fracture, and overlap region,... 

Mertens et al. (2011) looked at the long-term results of patients treated with the Zenith Flex stent graft. They analyzed 143 patients for overall survival, intervention-free survival, aneurysm rupture rates, early and late postoperative complications and endoleaks. At 5 years, 72.1% of patients and at 8 years 50.9% had survived. By 8 years 9% of patients (six total) had experienced a late aneurysm rupture, with three fatalities. Forty-seven patients experienced endoleaks, 18 of which were type 1,25 had one or more type II endoleaks (33 cases in total) and three type III one of these required a conversion to open repair and another required a bypass, indicating a defect in the stent graft material rather than an incomplete seal between the aortic limb stubs and the extensions. There was also one case of endotension and six cases of stent fracture. The authors in this study concluded that the Zenith Flex endograft is an excellent device with good long-term results and low aneurysm-related mortality. 

Park and coworkers (2001) also used polyurethane covered Z-stents but with a dog bone configuration and 16 or 18 mm mid diameter. However, the technical success rate was only 75% (18 of 24), mainly due to the device stiffness and the lack of endoscopic assistance. There were no procedural compKcations but a 24% migration rate was observed and stent fracture occurred in three patients (17%). After stent placement symptoms improved in 12 (67%) patients, did not change in five (28%) and progressed in one (6%). 

Stent fracture has only been reported in one series with modified covered Z-stents (Park et al. 2001) but not with any other stent designs implanted in gastroduodenal obstruction. 

Stent fracture in the colon and rectum seems to be a rare event and has only been described in connection with one particular stent design. Odurny et al. (2001) described three cases in which Memotherm nitinol stents were found to be fractured after 3,6 and 7 months. All patients had originally been treated for recurrent anastomotic strictures, one of which was due to local tumour recurrence. This type of nitinol stent does seem unsuitable for long-term management of strictures in the gastrointestinal tract since fracture of Memotherm stents has also been reported in the biliary tract (Peck and Wattam 2000). 

Odurny A (2001) Colonic anastomotic stenoses and memo-therm stent fracture a report of three cases. Cardiovasc Intervent Radiol 24 336 355... 

Fig. 13.20a-c. Rectal carcinoma with rectal stent for palliation, axial, coronal, and sagittal view Stent fracture (arrow) with air leakage (a,b). Beginning tumor invasion (arrow) in the stent graft (c)... 

In an optical micrograph of a commercially available nitinol stent s surface examined prior to implantation, surface craters can readily be discerned. These large surface defects are on the order of 1 to 10 p.m and are probably formed secondary to surface heating during laser cutting. As mentioned above, these defects link the macro and micro scales because crevices promote electrochemical corrosion as well as mechanical instability, each of which is linked to the other. Once implanted, as the nitinol is stressed and bent, the region around the pits experiences tremendous, disproportionate strain. It is here that the titanium oxide layer can fracture and expose the underlying surface to corrosion (9). 

A polymeric implantable device from a PLLA/PDLA blend such as a stent with improved fracture toughness is disclosed. A stereocomplex nucleating agent (a combination of PLLA/PDLA) results in an increase in nucleation density and reduced crystal size, which increases fracture toughness of the formed device. ... 

Peck R, Wattam J (2000) Fracture of Memotherm metallic stents in the biliary tract. Cardiovasc Intervent Radiol 23 55-69... 

Implant design can alter the corrosion performance of alloys in vivo. A case in point of a device whose complex design has spurred much interest in its corrosion behavior is that of the cardiovascular stent [75,76]. To consider another example, many prosthetic devices and fracture fixation implants are by nature multicomponent or modular. This means they have various pieces that mate together, e.g., screws and screw holes in plates. These locations may be foci of localized corrosion processes such as crevice corrosion or (in the case of relative motion) fretting corrosion or both. Careful design of such components can minimize in vivo corrosion problems. 

It is now well identified that bacteria connect to solid supports to shape structured communities called biofilms, also known as biopolymer matrix-enclosed microbial populations adhering to each other and/or surfaces [111]. Biofihns occur on both living and inert supports in all environments [112]. They influence various industrial and domestic areas [113] and are accountable for a broad range of human diseases [111], In view of the ever growing number of implanted patients, biofilm-linked infections of indwelling medical devices are more predominantly a foremost public health issue. Various examples of implants that can be inflated by biofilm formation are mechanical heart valves, catheters, pacemakers/defibriUators, ventricular assist devices, vascular prostheses, coronary stents, neurosurgical ventricular shunts, cerebrospinal fluid shunts, neurological stimulation implants, ocular prostheses, inflatable penile, cochlear, joint prostheses, fracture-fixation devices, breast, and dental implants and contact lenses, intrauterine contraceptive devices [114-116]. 

Balogh Z, Voros E et al. (2003) Stent graft treatment of an external iliac artery injury associated with pelvic fracture. A case report. J Bone Joint Surg Am 85-A(5) 919-22... 

Davis, J. R., ed. 2003. Handbook of Materials for Medical Devices. Materials Park, OH ASM International. This work provides a review of the properties, processing, and selection of materials used in the environment of the human body. Among the application areas described are orthopedics (hips, knees, and spinal and fracture fixation), cardiology (stents, heart valves, pacemakers), surgical instruments, and restorative dentistry. Materials discussed include metals and alloys, ceramics, glasses, and glass-ceramics, polymeric materials, composites, coatings, and adhesives and cements. 

Some metals are used as passive substitutes for hard tissue replacement such as total hip and knee joints, for fracture healing aids as bone plates and screws, spinal fixation devices, and dental implants because of their excellent mechanical properties and corrosion resistance. Some metallic alloys are used for more active roles in devices such as vascular stents, catheter guide wires, orthodontic archwires, and cochlea implants. 

There are various applications for metal implants, including vascular and digestive stents, internal fracture treat-ments, face and dental surgery, and orthopedic joint re-placements. 

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