Nitinol urethral stents: long-term results in dogs

Implantação de stent autoexpansível para tratamento de estenose uretral em cão: relato de caso

RESUMO O stent endovascular autoexpansível pode ser utilizado como forma de tratamento para estenose uretral em cães. O stent de nitinol é composto por uma liga de níquel e titânio que permite mudanças em suas estruturas sob a influência da temperatura corporal e tem excelente biocompatibilidade na uretra canina. Este estudo relata o caso de um cão, sem raça definida, 14 anos de idade, com histórico de disúria há duas semanas, diagnosticado com grande quantidade de cálculos e microcálculos na bexiga, no colo vesical e na uretra prostática, também cistite e hiperplasia prostática. Depois de tratamento para cistite, orquiectomia e cistotomia para retirada dos cálculos, porém sem melhora clínica após 30 dias de acompanhamento, com o paciente apresentando disúria, foi realizada uretrocistografia retrógrada, que evidenciou estenose da uretra prostática. Esse paciente foi encaminhado para procedimento de implantação de um stent de nitinol na uretra prostática. Imediatamente após o procedimento e até o presente momento, o paciente apresenta micção espontânea, sem qualquer sinal de disúria, sendo este o primeiro relato no Brasil do uso dessa técnica.

Use of an aortic stent graft extension for the treatment of urethral stricture in a dog

A 2-year-old male mixed dog was referred to us for further evaluation and treatment of a 4-week-history of oliguria and abdominal distension after a surgical repair of urethral injury. To relieve the urethral stricture, we placed a self-expanding aortic stent graft extension with a partial coverage with an expanded polytetrafluoroethylene (ePTFE). After the placement of the stent, the dog presented with a normal urinary voiding, despite the presence of urinary incontinence. The current case indicates that the ePTFE-covered, self-expanding ASGE is an effective intervention for the treatment of severe urethral stricture in the dog.

Treatment of traumatic penile urethral stricture in a dog with a self-expanding, covered nitinol stent

CASE DESCRIPTION

An 8-month-old castrated male mixed-breed dog was evaluated because of hematuria, stranguria, and dysuria of approximately 2 weeks' duration that developed immediately following elective castration.

CLINICAL FINDINGS

Results of physical examination, ultrasonography, retrograde double-contrast cystourethrography, and urethroscopy were consistent with a traumatic urethral stricture immediately proximal to the os penis resulting in a partial obstruction of urine outflow. Results of ultrasonographic examination of abdominal organs were considered normal. Digital radiography revealed no evidence of calculi.

TREATMENT AND OUTCOME

Balloon dilation of the urethral stricture was performed and was followed by 2 bougienage procedures during the subsequent 2 weeks when clinical signs returned. The owners declined scrotal urethrostomy, and a self-expanding, covered nitinol stent was placed approximately 3 weeks after the initial evaluation, resulting in amelioration of clinical signs. Results of follow-up urethroscopy and contrast cystourethrography 1 year after stent placement revealed a statically positioned, patent urethral stent, although a small number of polypoid mucosal structures were identified distal to the stent and 1 small structure consistent with tissue ingrowth into the stent was identified.

CLINICAL RELEVANCE

Placement of a covered nitinol stent resulted in long-term resolution of clinical signs associated with traumatic stricture of the penile urethra in this young dog. Because the os penis in dogs limits radial expansion of the urethra, its presence may limit the use of stents in this location.

Fluoroscopic stent placement for the relief of malignant urethral obstruction in a cat

The clinical features and interventional therapy in the case of a female cat with urinary tract obstruction secondary to neoplasia are presented. This form of neoplasia in cats is rare and therapeutic intervention to relieve urinary tract obstruction caused by malignancy has been described only once. This is the first report of a self-expandable metallic stent placed in a feline urethra to relieve obstruction caused by malignancy and the first report of the use of a unique composite metallic stent (Platinol™) in a cat. In conclusion, the palliative stenting of the feline urethra may be a valid therapeutic intervention for malignancies. Further studies are required to determine the optimal size and type of stent that will provide the greatest benefit.

Urethral stenting in a cat with refractory obstructive feline lower urinary tract disease

A 10-year-old male Korean domestic short-haired cat was presented with refractory lower urinary tract obstruction. The cat was treated by urethral stent placement using a self-expanding nitinol intraluminal stent (Zilver 535 biliary stents, COOK, U.S.A.) subsequent with balloon expansion. Although the cat showed 2 days of transient hematuria after the stent placement, no further obstruction was occurred after the stent placement. Follow-up studies performed at monthly intervals have found no re-stenosis or particular complications, to date.

Shape memory alloys: metallurgy, biocompatibility, and biomechanics for neurosurgical applications

SHAPE MEMORY ALLOYS possess distinct dynamic properties with particular applications in neurosurgery. Because of their unique physical characteristics, these materials are finding increasing application where resiliency, conformation, and actuation are needed. Nitinol, the most frequently manufactured shape memory alloy, responds to thermal and mechanical stimuli with remarkable mechanical properties such as shape memory effect, super-elasticity, and high damping capacity. Nitinol has found particular use in the biomedical community because of its excellent fatigue resistance and biocompatibility, with special interest in neurosurgical applications. The properties of nitinol and its diffusionless phase transformations contribute to these unique mechanical capabilities. The features of nitinol, particularly its shape memory effect, super-elasticity, damping capacity, as well as its biocompatibility and biomechanics are discussed herein. Current and future applications of nitinol and other shape memory alloys in endovascular, spinal, and minimally invasive neurosurgery are introduced. An understanding of the metallurgic properties of nitinol provides a foundation for further exploration of its use in neurosurgical implant design.

The Intra-Vascular Stent as a Site-Specific Local Drug Delivery System

The current review focuses on utilization of a tubular structure (coated or uncoated, balloon expandable or self expanding) known as a "stent" for localized intravascular drug delivery. Emphasis of the review is on technologies currently employed for immobilization and coating for drug onto the stent prior to its placement in various lumen of the body. A brief discussion on stent design, comparison of angioplasty and coronary stenting, and market status complements the review for researchers new to this area.

Evaluation of palliative stenting for management of malignant urethral obstructions in dogs

OBJECTIVE

To evaluate use of balloon-expandable and self-expanding metallic stents in management of malignant urethral obstructions in dogs.

DESIGN

Original study.

ANIMALS

12 dogs with malignant urethral obstructions.

PROCEDURES

The extent and location of urethral obstructions and the diameter of adjacent unaffected luminal segments were determined by use of fluoroscopically guided wires and measuring catheters. Stents were chosen to extend approximately 1 cm proximal and distal to the obstruction. Stent diameters were chosen to be approximately 10% greater than the diameter of healthy portions of the urethra to prevent displacement. Stents were placed in the urethra under fluoroscopic guidance to restore luminal patency.

RESULTS

3 dogs received balloon-expandable metallic stents, and 9 dogs received self-expanding metallic stents. The placement procedures were rapid, safe, and effective at restoring luminal patency and were not associated with major complications. Complications included recurrent urethral obstruction secondary to blood clot formation and urethral edema in 1 dog and stent dislodgement into the urinary bladder in 1 dog. All dogs were able to urinate immediately after the procedure. Nine dogs (3/4 females and 6/8 males) were continent or mildly incontinent after stent placement. Of the remaining 3 dogs, 2 developed severe incontinence and 1 had an atonic bladder. Seven dogs were considered to have good to excellent outcome, 3 had fair outcome, and 2 had poor outcome.

CONCLUSIONS AND CLINICAL RELEVANCE

Transurethral placement of metallic stents was a safe and effective palliative treatment option for dogs with malignant urethral obstructions.

BIOCOMPATIBILITY OF NITINOL AND STAINLESS STEEL IN THE BLADDER: AN EXPERIMENTAL STUDY

PURPOSE

We tested the biocompatibility of nitinol, a nickel titanium alloy, and stainless steel (SS) as bladder implant materials.

MATERIALS AND METHODS

Rats received a nitinol implant, an SS implant or were sham controls. Two, 3, 6 and 8 weeks following implantation 24-hour voiding behavior studies were performed to investigate bladder irritation. All animals were sacrificed 8 weeks after implantation and a sample of urine was aspirated for culture. The bladders were examined by light microscopy and scanning electron microscopy (SEM).

RESULTS

No visible encrustations or infections were noted in urine. Voiding frequency in the light period 6 weeks after implantation was significantly decreased in the 2 implant groups compared with sham controls. There were no other significant differences in frequency or mean volume per void in the light or dark periods at any time point. Light microscopy demonstrated similar implant tissue effects in all groups with little or no inflammation or fibrosis. Under SEM all implants showed a brittle, amorphous coating devoid of cells. The transition between the urothelium mucosa and the rod was smoother for SS than for nitinol, suggesting an affinity of SS for mucosa. In all nitinol rods discontinuity was present between the mucosa and rod.

CONCLUSIONS

Nitinol and SS do not cause more irritation than the effects of surgery alone and the 2 materials seem to be biocompatible in the bladder. Nitinol may be more inert than SS based on SEM results.

Porous titanium-nickel for intervertebral fusion in a sheep model: part 2. Surface analysis and nickel release assessment

Porous titanium-nickel (PTN) devices represent an alternative to traditional cage implants. PTN materials possess an interconnecting network of pores with capillarity properties that may promote bone ingrowth, long-term fixation, and intervertebral fusion without the need for bone grafting. However, their considerable surface area and nickel content may elicit concerns over sensitization potential. Therefore, PTN surface corrosion and nickel release resistance must be carefully studied. To evaluate this possibility, a PTN interbody fusion device (IFD) was compared to a conventional nonporous cage made of TiAlV, a well-known biocompatible biomaterial, in a sheep model. PTN and TiAlV IFDs were inserted at two non-contiguous lumbar sites for 3, 6, and 12 months postsurgery. Their surface was then evaluated by scanning electron microscopy (SEM) combined with backscattered electron analysis (BSE). No evidence of surface corrosion was observed either pre- or postimplantation, regardless of device type. Dosage of nickel ions was also performed with the use of inductively coupled plasma-mass spectrometry (ICP-MS). Blood nickel levels were observed to be within acceptable levels at all postinstrumentation times. Nickel content in PTN-adjacent tissue, as well as in detoxification and remote organs, was equivalent both in PTN-treated and control sheep. Therefore, porous titanium-nickel demonstrated resistance to both in vivo surface corrosion and nickel ion release and compared very well with a conventional titanium implant in the course of a 12-month sheep study.

Covered, retrievable, expandable urethral nitinol stent: feasibility study in dogs

PURPOSE

To evaluate the feasibility of using a retrievable urethral stent and to evaluate whether granulation tissue resolves after stent removal in a canine urethra.

MATERIALS AND METHODS

Polyurethane-covered retrievable 0.10-mm-thick (n = 11) or 0.15-mm-thick (n = 10) nitinol wire stents were placed in the urethras of 20 dogs. In one dog, a second stent was placed in the urethra because of complete migration of the first stent into the urinary bladder. The stents were removed with retrieval hook wires 4 weeks (n = 10) and 8 weeks (n = 10) after placement. Fourteen dogs were sacrificed just after stent removal, and the other six dogs were sacrificed 2 weeks after stent removal. Information concerning procedure success, stent migration, and tissue response was obtained.

RESULTS

Stent placement was technically successful in all dogs. Follow-up urethrograms showed partial (n = 4) or complete (n = 1) stent migration. Stent removal failed in two dogs due to partial or complete migration. Granulation tissue was observed at both ends of the stent in 17 dogs. Urethrograms and urethral specimens obtained 2 weeks after stent removal showed diminished granulation tissue and decreased thickness of the papillary projections of the epithelium compared with results obtained immediately after stent removal.

CONCLUSION

Although some design modifications are necessary to reduce current complications, the polyurethane-covered retrievable nitinol stent seems feasible for use in the urethra. Stent-induced granulation tissue formation improved after stent removal.

The cytotoxicity of corrosion products of nitinol stent wire on cultured smooth muscle cells

Although nitinol is one of most popular materials of intravascular stents, there are still few confirmative biocompatibility data available, especially in vascular smooth muscle cells. In this report, the nitinol wires were corroded in Dulbecco's modified Eagle's medium with constant electrochemical breakdown voltage and the supernatant and precipitates of corrosion products were prepared as culture media. The dose and time effects of different concentrations of corrosion products on the growth and morphology of smooth muscle cells were evaluated with [(3)H]-thymidine uptake ratio and cell cycle sorter. Both the supernatant and precipitate of the corrosive products of nitinol wire were toxic to the primary cultured rat aortic smooth muscle cells. The growth inhibition was correlated well with the increased concentrations of the corrosion products. Although small stimulation was found with released nickel concentration of 0.95 +/- 0.23 ppm, the growth inhibition became significant when the nickel concentration was above 9 ppm. The corrosion products also altered cell morphology, induced cell necrosis, and decreased cell numbers. The cell replication was inhibited at the G0-G1 to S transition phase. This was the first study to demonstrate the cytotoxicity of corrosion products of current nitinol stent wire on smooth muscle cells, which might affect the postimplantation neointimal hyperplasia and the patency rate of cardiovascular stents.

Increased corrosion resistance of stent materials by converting current surface film of polycrystalline oxide into amorphous oxide

Current efforts of new stent technology have been aimed largely at the improvement of intravascular stent biocompatibility. Among the chemical characteristics of metallic stents, surface oxide corrosion properties are paramount. Using our unique technique, the currently marketed 316 L stainless steel and nitinol stent wires covered with polycrystalline oxide were chemically etched and then passivated to form amorphous oxide. Excellent metallic-stent corrosion resistance with an amorphous oxide surface was demonstrated in our previous in vitro study. For in vivo validation, we compared the corrosion behavior of different oxide surfaces on various forms of test wires in the abdominal aorta of mongrel dogs using open-circuit potential and cyclic anodic polarization measurements. After conduction, the retrieved test wires were observed under scanning electron microscope. No passivity breakdown was found for wires covered with amorphous oxide, while wires with polycrystalline oxide showed breakdown at potentials between +0.2 to + 0.6 V. It has been proven that severe pitting or crevice corrosion occurred on the surface of polycrystalline oxide, while the surface of amorphous oxide was free of degradations in our experiment. We have demonstrated that this amorphous oxide coating on metallic material provides better corrosion resistance, not only in vitro but also in vivo, and it is superior not only in strength safety but also in medical device biocompatibility.

Materials and biomaterials for interventional radiology

Devices used in interventional radiology have significantly developed in the past few years. In order to understand the trends of this development, we analyzed how new interventional devices are progressively incorporating materials having original physical properties, and how developers are today progressively turning towards biomaterials, with respect to the new regulatory environment, and the requirements of biocompatibility.

In vivo biocompatibility evaluation of nickel-titanium shape memory metal alloy: muscle and perineural tissue responses and encapsule membrane thickness

Nickel-titanium shape memory alloy (Nitinol) has properties that could be very useful in surgical applications. Thermal shape memory, superelasticity, and high damping properties make such alloys behave differently compared to other implant metals. There has previously been a lack of sufficient evidence on the biocompatibility of Nitinol. The purpose of this study was to evaluate general soft tissue response and biocompatibility to Nitinol in vivo, and to clarify neural and perineural responses, previously unreported. Seventy-five rats were randomized into three groups. Test specimens were implanted into paravertebral muscle and near the sciatic nerve. A comparison was made between Nitinol, stainless steel, and Ti-6Al-4V. The animals were euthanized at 2, 4, 8, 12, and 26 weeks after implantation. General morphologic and histologic observations were made under light microscopy. Semiautomatic computerized image analysis was used to measure the encapsule membrane thickness around the implants. The muscular tissue response to Nitinol was clearly nontoxic, regardless of the time period. The overall inflammatory response to Nitinol was very similar to that of stainless steel and Ti-6Al-4V alloy. There were no necroses, granulomas, or signs of dystrophic soft tissue clacification. The immune cell response to Nitinol remained low. Only a few foreign-body giant cells were present. The detected neural and perineural responses were also clearly nontoxic and nonirritating with Nitinol. No qualitative differences in histology between the different test materials could be seen. At 8 weeks, the encapsule membrane of Nitinol was thicker than that of stainless steel (mean 62 +/- 25 microns vs. 41 +/- 8 microns). At the end of the study, the encapsule thickness was equal to all the materials tested. We concluded that Nitinol had good in vivo biocompatibility after intramuscular and perineural implantation in rats in the 26-week follow-up. Based on the results of the present study, Nitinol appears to have good potential for clinical use.

Histological changes associated with long-term urethral stents

OBJECTIVE

To assess the histological changes found in patients with long-term external sphincter, prostatic and urethral stents.

PATIENTS AND METHODS

Eighteen patients with long-term stents (mean time since insertion 3.5 years) were investigated. Three had external sphincter stents for detrusor-sphincter dyssynergia secondary to spinal injury, eight had prostatic stents for obstruction secondary to benign prostatic hyperplasia and seven had urethral stents for recurrent strictures. Nine stents were occluded at investigation, of which seven were entirely removed. The mucosae overlying the remaining two were biopsied, as were mucosae over the nine patent stents, at urethroscopy.

RESULTS

The changes observed included polypoid hyperplasia (11 of 18 patients) between and around the stent mesh wires, nonkeratinizing squamous metaplasia (two) or hyperkeratotic squamous metaplasia (seven), chronic inflammation (15) with prominent plasma cell infiltrates (11), variable foreign-body granuloma (two) and microabscess formation (five), usually associated with clefts formed around the stent wires (three of five).

CONCLUSION

Stents become incorporated into the urethral wall by a process of polypoid hyperplasia through the stent mesh, with at least focal covering of the stent in most cases, and with variable inflammatory infiltrates, most of which are rich in plasma cells. The urothelial and connective tissue proliferation resulted in obstruction of the stent lumen in nine of the patients studied. Further long-term study is necessary to exclude the development of carcinoma in patients with keratinizing squamous metaplasia, although no malignancy was seen in this study.

Cytotoxic, allergic and genotoxic activity of a nickel-titanium alloy

The nearly equiatomic nickel-titanium (NiTi) alloy is known for its shape memory properties. These properties can be put to excellent use in various biomedical applications, such as wires for orthodontic tooth alignment and osteosynthesis staples. The aim of this study was to evaluate the short-term biological safety of the NiTi alloy. We carried out an end-point dilution minimal essential medium (MEM) extract cytotoxicity test, a guinea-pig sensitization test and two genotoxicity tests: the Salmonella reverse mutation test and the chromosomal aberration test. The NiTi alloy showed no cytotoxic, allergic or genotoxic activity, similar to the clinical reference control material AISI 316 LVM stainless steel. This promising biological behaviour was most likely due to a minimal release of ions and in that way a reflection of the good corrosion resistance of the NiTi alloy. Given these very good results, together with the good tissue compatibility as shown in several implantation studies in the literature, the NiTi alloy can be regarded as a biologically safe implant material with many promising clinical applications.