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- Materials and Methods
- Conflict of Interest
The AMS 800 (American Medical Systems, Minnetonka, MN, USA) artificial urinary sphincter (AUS) is currently the gold standard AUS device  used to treat stress urinary incontinence due to sphincter insufficiency when simpler standard techniques fail or are inappropriate. Results from a meta-analysis show that 73% of patients achieve full continence and 88% have improved continence . However, up to 60% of the implanted devices require surgical revisions at 10 years due to technical failures inherent in the hydraulic mechanism or to ischaemic injury of the urethra [3, 4]. Thus, research into alternative solutions seems reasonable.
In this animal study we have tested the hypothesis that an electromechanical AUS (emAUS) might produce continence by applying synchronized sequential compression to the urethra. The purpose of this study was to test the effectiveness of the device, the tissue response after implantation and the animal tolerance.
- Top of page
- Materials and Methods
- Conflict of Interest
Since Foley's first attempt in 1947, various devices and techniques have been developed to replace sphincter activity. Pneumatic, magnetic and hydraulic mechanisms have all been tried [17-20]. Recently, novel slings have been developed and the first results seem promising [7-12]. However, for refractory severe stress urinary incontinence, the AMS 800 is still considered the gold standard . Despite the latest improvements and good clinical outcomes, the revision rate is still high. AMS 800 failures can be divided into two categories, mechanical and non-mechanical, with a variable but roughly equal incidence . Mechanical failures include leakage and malfunction of the device itself. Non-mechanical failures include cuff erosion, infection and urethral ‘atrophy’ affecting the compressed area of the urethra. Since the compressed area of the urethra is always the same the vascularity of this area is potentially compromised by a hydraulic device and urethral ischaemia can occur as a consequence. Although the urethra is commonly considered to be a passive tube through which urine flows, it has been shown that the area of the urethra within the AUS cuff that constitutes the continence mechanism in patients with an AUS is at least partly dependent on the preservation of urethral vascularity .
Attempts have been made to produce a better hydraulic device. In one particular study a self-sealing port was included in the pump to adapt the occlusion pressure to each patient and a stress relief mechanism was added to minimize closure pressure. Although the effectiveness of the device was demonstrated, no conclusion regarding long-term outcomes could be made since only three patients were still in the trial after 12 months and no comparison group with the standard AMS 800 was used in this preliminary clinical investigation .
The ideal AUS should preserve urethral vascularity and be individually adjustable postoperatively. The emAUS was developed with these goals in mind. Furthermore the procedure is simple because dissection is limited and there are no connections to be made. The significant advantage of the emAUS for human application is its adjustability after implantation. The remote control allows the cuff closure pressure to be adjusted exactly to a patient's needs. As a result the lowest closure pressure to give continence will be applied and this will contribute to the long-term preservation of urethral vascularization.
Nitinol was chosen for the contractile unit because of its excellent performance, its low energy consumption and because it can be activated repeatedly if not indefinitely without material damage. Energy consumption of a nitinol device is less than with other materials. Nevertheless the power supply of the device remains a problem. The emAUS used in this animal study was powered by an external energy source. For clinical practice we have developed a remote external transmitter that will operate an implanted receiver to control the contractile unit, rather than the transcutaneous external board used in these experiments. Currently it will work for 1 week without recharging. We plan to supply power by transcutaneous recharging and this is technically feasible but we continue to investigate other potential solutions.
The emAUS is clearly effective in producing continence. Mean BLPP measured during the operations was much higher than BLPP in continent patients in physiological conditions. Indeed the force used to close each cuff could probably be reduced considerably in clinical practice from the 0.7 N used in these experiments.
The major finding in this study was the good tissue response to implantation. The key concept is the use of electronics to produce sequential compression of alternating segments of the urethra, thereby giving continence, with preservation of urethral vascularity by avoiding constant compression of alternating segments since this avoids permanent compression of any single segment of the urethra. That this is actually the case is shown by the absence of structural damage on microscopy.
Clearly this approach to urethral compression requires at least two cuffs. More cuffs are possible but additional cuffs mean that more electromotor and mechanical parts are required. This in turn means that the device will be bigger and heavier and more space will therefore be required to implant it around the urethra. To overcome these issues we settled on a two-cuff emAUS in phase 3.
A comparison with other experimental studies is difficult because we have only been able to identify two studies, both in different animal models. The original AMS device was tested in an animal study in eight mongrel dogs . Explantation was performed between 4 and 26 weeks. There was no evidence of tissue necrosis, urethral stricture formation or cuff migration and the authors found the same foreign body reaction with encapsulation of the prostheses as we found in the present study. The second study, of a magnetically operated sphincter working in an entirely different way, was stopped after 6 weeks because continuous compression on the skin caused significantly reduced blood flow and pressure ulcers in miniature pigs .
One potential criticism of these studies is that the sheep were not actually incontinent but as they were rendered unable to void with the device active and could only void when the device was switched off it was clearly capable of producing continence in incontinent subjects. Another criticism might be the absence of a sham group or a comparative group using the AMS 800. However, as there was no adverse response to the device in the phase 2 study we felt it was unnecessary to include a sham group in the phase 3 study when we would otherwise have considered it; and we felt a comparative study was unnecessary in this pre-clinical phase when we were trying to demonstrate a proof of concept.
Finally, of course, these are only short-term data with a maximum length of 3 months' implantation. However, given that there were no adverse histological consequences whatever at this stage we feel optimistic as we proceed to the next stage of development of this device.
We have developed a new emAUS, based on electronics and implanted contractile units. This electromechanical mechanism provides a sequential alternating compression to close the urethra in successive segments. In this proof of concept, we have demonstrated its effectiveness. Histological findings show that the mechanism can avoid urethral atrophy and ischaemia by the preservation of urethral vascularity. Further technical developments in power supply and energy consumption as well as long-term in vivo tolerance data are required before human use could be considered.