Excerpt from Penile Prosthesis Implantation

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Potency always has been something special in human culture. Its effects even have been intertwined within early Common and Church laws in Europe. For instance, laws required that marriages be consummated; unconsummated marriages were grounds for annulment. Considering this happened in an age when divorce was extremely rare shows the magnitude of the "offense" of impotency.

Impotency at that time had far-reaching consequences, which included social stigmatization, societal standing within the community, and legal ramifications on such issues as claims on wills and considerations of the legitimacy of heirs. Although less drastic today, potency still plays a large part in men's self-image and affects the relationship with their partners.

Before 1960, urologic therapy for erectile dysfunction (ED) was rare. ED was branded a psychiatric disorder with little surgical role. More recently, the pathophysiology of male sexual dysfunction has been elucidated, and both medical and surgical treatments of ED are now common.

History of the Procedure

The history of modern ED surgery began with the development of the inflatable penile prosthesis by Scott and Bradley in the early 1970s.¹ This surgical treatment became the treatment of choice in men with significant organic ED. Although the popularity of the inflatable penile prosthesis increased during that time, its design suffered from unacceptable failure rates and the need for surgical revision. Because of this, some urologists began using semirigid intracorporal devices to circumvent the reliability problems of early inflatables.

Today, penile prostheses have proven to be both reliable and durable, with approximately 20,000-30,000 devices implanted annually worldwide. In a 1996 survey, approximately 21,000 prostheses were implanted in North America, with another 5000 worldwide. With new social awareness regarding ED, many experts believe that future device implantations will become more prevalent because of failing medical therapy.

Problem

As defined by Pearman, an early leader in this field, ED is the inability to produce and maintain a functional erection because of pathology of the nervous or vascular system or deformation of the penis.² The surgical treatment of ED largely involves the placement of a penile prosthesis, although penile revascularization procedures have been described.

The most fundamentally basic prosthesis is the semirigid rod prosthesis, which consists of 2 rodlike cylinders that are implanted in the corpora cavernosa. The prosthesis can have a mechanically jointed "backbone" or have a malleable one that allows the phallus to be dressed in the upward or downward position. This prosthesis is generally considered for patients who are significantly obese, who have limited manual dexterity, or in whom abdominal hardware such as reservoir balloons cannot be implanted (ie, patients undergoing extensive abdominal/perineal surgery and those receiving peritoneal dialysis).

Models

• Mechanical rods: Dura II (AMS [formerly produced by Timm Medical Technologies]) prostheses have a series of polyethylene segments that articulate in a ball and socket arrangement and are held in place by a central spring. A silicone membrane covers these devices, and they come in 2 width sizes—10 mm and 12 mm. The standard length of 13 cm is augmented by adding proximal and distal tip extenders.
• Malleable rods (650/600M [AMS]; Acu-Form [Mentor])
• • The AMS models consist of a wire core surrounded by polyester covering and silicone outer jacket. Model 600 is available in 9.5-mm and 11.5-mm width sizes, and the model 650 is available in 11-mm and 13-mm width sizes. Lengths range from 12–20 cm and can vary with tip extenders.
  • The Mentor Acu-Form has a silver wire backbone with a silicone elastomer outer coat and is available in widths of 9.5 mm, 11 mm, and 13 mm. Cylinder lengths range from 14-27 cm. This prosthesis is chosen for its simplicity of usage and durability due to fewer vital moving parts.
• Unitary inflatable penile prosthesis (Dynaflex [AMS])
• • In 1990, the Dynaflex model was introduced as a more robust replacement of the Hydroflex. It is a paired cylinder with all operating components contained within each device, and it consists of the distal tip, central chamber, and proximal reservoir. Rigidity is achieved by pumping 2-3 mL of liquid into the central chamber from the reservoir. Bending the cylinder 55° or more from horizontal operates a pressure switch to deflate the device and to return fluid back to the reservoir. These cylinders are available in 2 widths—11 and 13 mm—and various lengths.
  • This was a good attempt to allow for inflation and deflation in a self-contained unit; however, a patient with a broad penis usually has incomplete filling by the Dynaflex cylinders and, thus, may experience shifting and buckling of the device during intercourse. Some patients complain that it is difficult to inflate because of 2 separate distal pumps. In addition, this device is not suitable for patients with a history of distal urethral erosion because of pressure applied to tissue while the implant is operated.
• Two-piece inflatable devices (Ambicor [AMS] and Mark II [Mentor] [production discontinued in 2000])
• • These devices are marketed to improve the ease of surgical implantation by eliminating the need for reservoir placement in the abdominal region. These prostheses consist of 2 inflatable cylinders that are inserted into the corporal bodies and are connected to a pump-reservoir located in the scrotum. The drawback of these devices is the limited reservoir capacity of 15-20 mL, which is available not only for cycling the 2 cylinders for full rigidity but also for allowing for flaccidity. As much as 5-10 mL of fluid is left in the cylinders during the flaccid state because of limited reservoir space. Patients with larger penises criticize these devices for insufficient volume to fully inflate the cylinders, and those with smaller penises complain of difficulty completely deflating the cylinders because of the limited reservoir capacity and resulting residual cylinder fluid.
  • These prostheses should be considered for patients in whom reservoir implantation is difficult or contraindicated, such as those who have undergone pelvic exenteration or renal transplantation.
• Three-piece inflatable devices (700 series [AMS], Alpha 1, and Titan [Mentor])
• • These devices tend to be more complex and consist of 2 inflatable cylinders placed in the corporal bodies, a small pump that resides in the scrotum, and a large fluid reservoir located in the abdomen. Three-piece prostheses prove to be the most satisfactory devices because they produce the most natural-appearing phallus in both the inflated and deflated states, produce good rigidity (even for larger penises), and offer good flaccidity for social dress. In addition, the flaccid state of the 3-piece allows removal of pressure against the corpora and tunica albuginea. Thus, patients with diabetes, previous implantation extrusion, and previous infections may benefit from this type of inflatable device.
  • The AMS 700 series offers the Ultrex model, which allows for girth and distal expansion, while the CX imparts only girth expansion. The CX line is most applicable to patients with scar tissue or those with a tendency for penile curvature upon tumescence. A new innovation in this line includes the InhibiZone antibiotic surface treatment (rifampin and minocycline hydrochloride), which has been shown to decrease the incidence of infection by roughly half compared with uncoated devices (down to 0.7% in one large study).
  • The Bioflex cylinders used in the Alpha 1 and Titan models from Mentor allow expansion in girth with minimal axial elongation. These have proven to be very durable and resistant to cylinder aneurysm formation. The newer Titan model is identical to the Alpha 1, with the addition of a hydrophilic coating (polyvinylpyrrolidone), which decreases bacterial attachment and enables antibiotic absorption onto the implant when soaked in antibiotic solution prior to implantation. Preliminary data suggest a significant reduction in infection compared with the noncoated model. Both Mentor 3-piece devices now come equipped with a reservoir lockout valve, which has been shown to decrease the incidence of autoinflation.

In summary, selection of the appropriate device for the individual patient is very important. Considerations include patient's preference and underlying medical condition, surgeon's preference, and cost of the device. The advantages of the semirigid devices include easier placement, less dependence on patient manual dexterity, lesser chance of mechanical failure, and lower cost. The disadvantages include higher risk for device erosion, less concealability, and inability to change girth. The semirigid devices may suit patients who have dexterity problems such as those with severe arthritis and others with hand limitations that interfere with working a scrotal pump.

The ideal penile prosthesis would result in a normal-appearing penis when flaccid and erect. Younger patients with good hand dexterity often choose the 3-piece prothesis. This especially is important for those engaged in social settings, such as health club showers, or who wear form-fitting clothes. However, prospective patients should be counseled that penile prostheses do not achieve the full length achieved by natural erections.

Frequency

Studies regarding the incidence of ED have proven difficult. Many experts believe that currently available data are often unreliable, especially when based on patient self-reporting. One study by Massey mentions ED incidence as 17 new cases per 100,000 man-years.³ In other studies, as many as 40% of men interviewed reported some kind of sexual dysfunction. Interestingly, few respondents indicated that the problem was abnormal for their age, and even fewer stated they would seek medical treatment for their sexual problem.

Among men who seek medical help for sexual dysfunction, ED is the single most common presenting problem. It accounts for more than one half of all sexual complaints.

The Massachusetts Male Aging Study by Feldman and colleagues (1994) estimates that men in their early fifth decade of life have a 20% ED rate, and, by their eighth decade of life, close to 45% of men have impotence problems. They further estimate that, between the fifth and seventh decades of life, the probability of having complete impotence triples.⁴

Etiology

The causes of impotence are multifactorial, but diabetes and vascular disease are the most common etiologies. They are usually classified as hormonal, neurologic, psychological, arterial (ie, insufficiency), and venous (ie, venous incompetence).

Injury to nerves and blood vessels can result from trauma, radiation therapy, pelvic surgeries (including prostatectomy), and longstanding medical conditions (eg, diabetes, hypertension). Neurologic causes include multiple sclerosis, tabes dorsalis, and other degenerative disease of the nervous system. Hormonal imbalance or deficiency, such as primary testicular failure, Cushing disease, Addison disease, and diabetes mellitus, can cause ED. Sometimes, adverse effects of drugs such as parasympathetic- and sympathetic-blocking agents, antihistamines, and drugs of abuse may be a cause. Peyronie disease, which is characterized by scarring of the underlying fibrous covering of the erection bodies of the penis, can cause curvature that prevents adequate penetration and requires surgical intervention.

Pathophysiology

Penile tumescence is a complex integration of vascular and neurophysiologic events that culminates in the accumulation of blood under pressure. Fundamentally, the erectile components are surrounded by a thick fibroelastic sheath called the tunica albuginea, which allows a hydraulic effect in which intrapenile blood volume may increase 8-fold. The effect is an increase in intracorporal pressure that approaches systemic blood pressure and produces both penile volume and rigidity.

Blood supply to the penis is derived from the internal pudendal artery, which is a branch of the internal iliac artery. The internal pudendal artery becomes the common penile artery, which then branches into the bulbourethral, cavernosal, and the dorsal penile arteries. Tumescence depends most on the cavernosal arteries.

Penile tumescence is caused by microvascular events in the corpora cavernosa, which allow inflow of blood but restrict outflow. When blood enters the corpora cavernosa, it pressurizes the lacunar spaces within the spongy tissue of the corpora and translates this pressure to the tunica sheath. This pressure impedes venous drainage and results in blood being trapped in the penis and penile erection.

These microvascular events are triggered by neurologic events, which themselves are triggered by sexual stimuli. The parasympathetic nervous system offers the excitatory input to the penis and allows for the vasodilation of the penile vasculature. The parasympathetics originate from sacral spinal cord segments 2-4 and travel via the pelvic plexus into the pelvic nerve. The pelvic nerve joins the sympathetic nerve fibers that originate in the thoracic spine and together become the cavernous nerves as they exit the pelvic plexus. The cavernous nerves travel along the posterolateral aspect of the prostate and are susceptible to damage during pelvic surgery, such as a radical prostatectomy.

The sympathetic nervous system controls ejaculation and detumescence and has a partial role in maintaining erections. Sympathetic preganglionic nerve fibers originate from the intermediolateral cell column to form the thoracolumbar sympathetic pathway (thoracic segment 9 to lumbar segment 2). These preganglionic fibers exit the spinal cord as ventral roots via white rami to become part of the sympathetic chain ganglia. They synapse onto sacral and caudal lumbar ganglion cells. These cells then send postganglionic axons via gray rami to diverge into the pelvic, cavernous, and pudendal nerves that serve the urogenital tract.

Somatic pathways are important for penile sensation and ejaculation. Sensory afferents from the pudendal nerve become the dorsal nerve of the penis, which enters the urogenital diaphragm to innervate the penile skin, prepuce, and glans. The input from these sensory fibers helps maintain erections. These sensory fibers can be damaged by aging and diabetes.

Somatic efferents also originate from the pudendal nerve to innervate motor fibers of the pelvic floor and the bulbocavernous and ischiocavernous muscles. Contraction of these muscles occurs during ejaculation and influences the quality of erections.

Complex synaptic connections of the somatic and visceral neurons facilitate the coordination of erection and ejaculation. The parasympathetic and sympathetic nervous systems were once believed to be in an antagonistic balance to achieve erections; however, parasympathetic and sympathetic pathways are now known to act synergistically. Moreover, well-studied neurochemicals, such as acetylcholine and noradrenaline, are joined by newly discovered ones, such as nitric oxide, vasoactive intestinal peptide, neuropeptide Y, calcitonin gene-related peptide, substance P, and serotonin, which are believed to contribute to normal erectile function.

Erectile function is a very complex and highly tiered system. Disruption along this chain of events can cause varying degrees of ED.

Clinical

See Etiology.

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