Types of Penile Prosthesis

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Penile implants are currently available in a variety of forms, which can be categorized into two major types: semi-rigid, or inflatable, with a silicon or polyurethane jacket. Details of the available length and diameters can be found on the websites of the two manufacturing companies: American Medical Systems at www.visitams.com; and the Mentor Corporation at www.mentorcorp.com. When implanted into the corpora cavernosa, they can be manipulated by the patient to fit his needs. The Jonas implant, produced in Germany, has a silver core and a silicone covering [12]. A soft silicone implant was introduced by Subrini. Currently these devices are manufactured in France and sold under the names "SSDA" and "Virilis" in many countries [15]. These implants may be indicated in the presence of residual spongy erectile tissue, which permits tumescence and complimentary girth expansion around the central silicone support.

The only "mechanical" prosthesis on the market is the Dura II, which has articulating segments of polyethylene held together by a central spring. These articulating segments are covered by a polytetrafluo-roethylene sleeve surrounded by a silicone jacket. The Dura II prosthesis is a product of the American Medical Systems (AMS). A similar device, called "Genesis", with a hydrophilic coating is planned by the Mentor Corporation. This design provides positional memory, allowing the prosthesis to remain concealed when not in use, yet rigid during intercourse [25]. These semi-rigid implants are cheaper than inflatable prostheses and easier to implant because there is no pump or reservoir to place. Semirigid prostheses may be implanted through a subcoronal, infrapubic or scrotal incision. Local anesthesia may be used if the corpora cavernosa are not too fibrotic and their dilatation is expected to be easy.

Inflatable implants are available in two-piece or three-piece models. Two-piece implants are made by AMS and by the Mentor Corporation. The AMS Ambicor prosthesis consists of a pair of inflatable cylinders, which are implanted into the corpora cavernosa, and a pump, which is implanted into the scrotum. The cylinders and pump are preconnected during the manufacturing process and the system is prefilled with sterile saline. To achieve an erection, the scrotal pump is squeezed and released several times. This inflates the cylinders by transferring saline solution from a small reservoir located at the end of each cylinder into each cylinder shaft. The newly developed Mentor two-piece penile prosthesis is called the Excel Resist® device. It consists of a pair of inflatable cylinders, which are implanted into the corpora cavernosa, and a resi-pump that is implanted into the scrotum. The cylinders and resi-pump are preconnected and filled with sterile saline during the manufacturing process. The volume of fluid in the reservoirs of both two-piece inflatable penile prostheses is less than in the reservoir of the three-piece prostheses. The lesser volume of the two-piece reservoirs diminishes the difference between rigidity and flaccidity of these systems, compared to the three-piece systems.

Three-piece inflatable implants are composed of two cylinders, a scrotal pump and a reservoir, which is typically positioned either in the retrovesical space or within the peritoneal cavity. Three-piece implants manufactured by AMS are the 700 CX, 700 CXR and Ultrex devices. The CXR cylinders have a narrow base and come in shorter lengths than the CX cylinders. CX and CXR cylinders have a three-layer design consisting of an inner layer of elastic silicone, a middle layer of woven Dacron-Lycra and an outer layer of silicone. The inner layer fills with fluid to provide rigidity, the outer layer allows for capsule formation around, but not into, the prosthesis, and the middle layer permits controlled expansion of the cylinders. The recently introduced parylene coating of the cylinders diminishes the risk of aneurysmal dilatation of cylinders. In the CX and CXR cylinders, the middle layer is woven unidirectionally, permitting girth expansion but not length expansion. It is commonly agreed that although very satisfactory results may be obtained in all subsets of patients, the CX implant is particularly useful for patients with Peyronie's disease or any form of severe organic ED. Patients who have significant scarring of the corpora cavernosa, e.g. after priapism or with extensive Peyronie's disease, or having a re-implant after previous removal of an infected prosthesis, may benefit from an inflatable implant with narrow cylinders such as the CXR, which requires dilation of the corpora cavernosa to only 10 mm. The only penile prosthesis that can lengthen with inflation is the AMS Ultrex prosthesis. This prosthesis also has three layers, but the middle layer of Dacron-Lycra has a bi-directional weave that permits both length and girth expansion of the cylinders. However, the Ultrex prosthesis is not appropriate for patients with Peyronie's disease because these patients need greater stiffness than is provided by the Ultrex prosthesis.

Three-piece inflatable implants manufactured by the Mentor Corporation share a similar design to the previously described models. The main difference is that the Mentor implants are made based of an inert aromatic polyurethane polymer called Bioflex™. Compared to silicone, Bioflex™ has increased abrasion resistance and higher tensile strength. An additional feature of the current Mentor "Titan" or three-piece inflatable prostheses, is a lockout valve that reduces the common phenomenon of autoinflation of the prosthesis [16]. The lockout valve is also used in the two-piece Excel Resist® device. The Titan prostheses are also available in a narrow-base version, which is best used in patients with severe fibrosis of the corpora cavernosa, as described above for the AMS CXR prosthesis.

As infection is the most feared complication of penile prosthetic surgery, two- and three-piece implants specifically modified to reduce this risk have been recently introduced into clinical practice. The InhibiZone™ prostheses by AMS are coated with a combination of rifampicin and minocycline to inhibit bacterial growth. In these prostheses, the surfaces of the prostheses, which come into contact with the adjacent tissue, are impregnated with quantifiable doses of these antibiotics that elute into the area surrounding the prosthesis after surgery. Both drugs elute initially at high rates, with a significant decrease in rifampicin after one and minocycline after seven days. The inhibitory effect of the InhibiZone™ treatment is greatest for Staphylococcus epider-midis and Staphylococcus aureus [17,8].

Mentor has also developed a method to reduce penile prosthetic infection. This method uses an intensely hydrophilic coating called ResistTM that is applied to the Titan and Excel prostheses. Its increased lubrication has been shown to decrease bacterial adherence in vitro. Furthermore this coating absorbs antibiotics that can elute into surrounding tissues over 24 to 72 hours to further decrease infection. An antibiotic-soaking solution, which has been tested both in laboratory experi-

Table 12.3 Implant rates of all manufacturers. (Personal data with permission of AMS, Germany [22].)

Year

Overall

USA

Europe

1994

23.280

19.400

3.880

1995

23.760

19.800

3.960

1996

24.360

20.300

4.060

1997

20.760

17.300

3.460

1998

17.640

14.700

2.940

1999

24.960

20.800

4.160

2000

26.600

22.000

4.400

2001

28.800

23.800

5.000

2002

31.100

25.700

5.400

ments and in clinical use, is composed of 2 g of gen-tamycin and 100 000 units of bacitracin in 1000 ml of saline [17,21].

Reduced infection rates have been reported recently for the AMS InhibiZone™ and the Mentor Resist™ products [18,19,20]. The three-piece implants of AMS and Mentor can be seen with their coatings in Figure 12.2. The two-piece device of Mentor is also available with hydrophilic coating, while the AMS two-piece Ambicor device is not.

Notably, to avoid elution of the coating prior to implantation, the various components should not be soaked in saline or other solutions for a prolonged period. However, conventional parenteral prophylactic antibiotic protocols should be maintained.

Penile prosthesis implantation is a frequent operation performed worldwide. The implant statistics over the last years are summarized in Table 12.3.

The introduction of PDE5 inhibitors into the market in 1998 initially caused a significant drop in implant rates worldwide, but in more recent years a constant increase of implants has been noted. Seventy-five percent of penile implant sales are in the United States, where 90% of implants are three-piece and two-piece devices. Outside the United States 40% of all implants are non-hydraulic devices [55].

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