But indestructible it is not.
"It's too bad men aren't issued an owner's manual for their penis. They don't realize it's possible to injure it during sex," says Dr. Drogo Montague, director of the center for genitourinary reconstruction at the Cleveland Clinic in Ohio.
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Fans of ABC's "Grey's Anatomy" got a peek at this possibility when the character of Dr. Mark Sloan "fractured" his penis during sex in a recent episode. Within hours, "penile fracture" and "broken penis" topped Google's most popular search terms.
Fractures -- in which penis tissue ruptures like a burst tire -- occur more often than one might think. "We usually see a fractured penis every month at our hospitals," says Dr. Tom Lue, professor of urology at UC San Francisco. Even more common, though, are mild, painless injuries to the penis during sex. Such wear and tear can lead to an increasingly prevalent condition known as Peyronie's disease, which leaves the penis with a dramatic bend.
Both conditions are as old as sex itself and documented as far back as the 10th century. But during the last decade a new factor has emerged that may be linked to more complaints of penile trauma: erectile dysfunction medications such as Viagra, Cialis and Levitra.
Experts say these drugs help protect some men from injuries during sex -- but put others at greater risk.
When a man becomes aroused, rapidly pumping blood gets trapped in his penis. Two spongy tubes that run through the shaft swell and strain against the surrounding stretchy sheath -- known as the tunica albuginea, or "white tunic."
As the penis thickens and lengthens, its elastic tunic expands and thins. But if the tunic stretches beyond a certain point -- as the shaft suddenly flexes too far, for instance -- the layers can rip, releasing a small surge of blood. "There will be a sudden blowout," Montague says. "It's usually at the base of the penis, and it's very dramatic."
Doctors call this a fracture (despite the lack of bones in the penis). It is usually accompanied by a popping sound, then swelling and discoloration of the penis to a deep purple hue. Usually, the erection fades and pain sets in (though some couples reportedly try to continue with sex). Men may later find it impossible to urinate.
Young men are most at risk, in part because of the firmness of their erections and athleticism of their activities. Certain sexual positions are more perilous than others. "When a woman is on top, that's a dangerous situation," Lue says. If she bends too far forward or backward, she can create excessive torque on the penis.
Also risky is rear-entry vaginal intercourse because, again, the penis is more likely to bend too far at its base. According to a report published in December in the British Journal of Urology International, this sexual position accounted for half of the penile fractures treated at Brookdale University Hospital in New York between 2003 and 2007.
Careless insertion of the penis can spark a mishap in any position. Men, or their partners, should manually guide the shaft during entry to avoid colliding with a partner's nearby pubic bone, Montague says.
Occasionally, Viagra or related medications are associated with penile fractures. A 2002 case study from India, for instance, described a three-week period in which six men arrived at an emergency room with fractured penises -- all resulting from recreational use of sildenafil, the drug sold as Viagra, to achieve, the authors wrote, "prolonged sexual enjoyment, out of curiosity."
Medical reports have also discussed cases resulting from men attempting to fold an erect penis into tight underwear, or rolling onto their stomachs while asleep with nocturnal erections. And then there are odd habits.
In some Middle East regions, men engage in a practice known as taqaandan ("to click" in Kurdish), explains Dr. Javaad Zargooshi, urology professor at Kermanshah University of Medical Sciences in Iran. It's a painless process, similar to knuckle-cracking, in which the top half of an erect penis is bent forcefully while the rest of the shaft is held stationary.
Usually this produces only a loss of erection and a satisfying popping noise, says Zargooshi, who published a report on the phenomenon in December in the Journal of Sexual Medicine. Other times, it will fracture the penis. Taqaandan is a public health concern in western Iran, where penile fractures are unusually common. "The practice of taqaandan is increasing, and we don't know why," Zargooshi says.
Doctors can easily repair fractures with emergency surgery, with fewer complications than the previously standard treatment of ice packs and sex-free bed rest. A 2004 study found that more than 90% of men with surgically repaired fractures resumed their sex lives without problems.
Usually, penile trauma arrives quietly -- via small tears in just a few layers of the stretchy tunica albuginea. These don't qualify as fractures and often don't even hurt. But experts believe the unnoticed damage can build up and lead to unmistakable problems: the curved penis of Peyronie's, which affects about 1 in 10 middle-aged men. (Exact numbers are tough to know; estimates range from 1% to 24%.)
The body relies on blood flow to repair damaged tissue without leaving a scar, yet very little blood circulates in a penis -- except when it's erect, of course. As a consequence, extensive scars can build up in the organ's elastic layers over time.
Imagine inflating a hot-dog-shaped balloon while pinching one side. As the balloon expands, it bends toward the pinched side. Likewise, a penis with Peyronie's disease looks normal when flaccid but curved when erect. And the greater the scarring, the greater the curve.
Most men who encounter the disease do so in their early 50s. One possible reason is that penile tissue can weaken with passing years. Elastic layers in a middle-aged penis are more likely to give out during vigorous intercourse than in a youthful one.
Also, erections soften with age, which makes even unathletic sex more accident-prone. Just as it's easier to bend and twist a half-blown balloon than a fully inflated one, less-than-rigid penises buckle more easily during intercourse.
The topside, mid-shaft of the penis usually bears the brunt of this buckling pressure, so scars often form there -- giving most affected penises a skyward curve. In severe cases, the penis resembles a hook, curling back to touch the abdomen.
A bend causes dual problems: erection difficulties and logistical issues with intercourse. "Some people will have a rigid penis, but it's so curved that there's nothing they can do," Lue says.
Erectile dysfunction medications such as Viagra might protect some men -- and harm others. In men with mild erection problems, the drugs can bring a medium-soft erection up to full strength, "so there's less chance of injury during normal sexual activity," Montague says.
But for men with severe erectile dysfunction, the drugs might make matters worse, by providing an erection that's just about firm enough for penetration but still soft enough to buckle easily during intercourse.
Cardiovascular problems -- particularly diabetes, high blood pressure and high cholesterol -- boost the risk of Peyronie's as much as six times, according to a 2006 study. Smoking and drinking alcohol each increase chances five-fold. These tend to soften erections and change the structure of the stretchy penile tissue.
Healing from prostate surgery can cause problems, and genetics is probably involved too.
Treatment for Peyronie's disease is still tricky. Dr. Francois de la Peyronie -- who wrote about the condition in 1743 -- suggested that men apply mineral water and mercury to their organs. Electricity, arsenic, cow's milk and a deep heating of the rectal muscles have also been recommended in past times.
Medicines used today directly target the scars on the elastic sheath of the penis. But most have not been tested in large, convincing clinical trials. Two drugs -- interferon and verapamil, both of which help stop new scar tissue from forming -- have shown promise in smaller trials.
Currently underway by Auxilium Pharmaceuticals is a large clinical trial for collagenase, an enzyme that might break down scar tissue in the penis. Also planned is a smaller study by Allergan to investigate the use of botulinum toxin -- Botox -- which might relax penis muscles and reduce scarring.
About 10% of the time, untreated penises with Peyronie's disease will straighten on their own. If not, doctors may recommend surgery after a year or so. Options for reducing the curve include implanting a prosthesis (which adds firmness but requires set-up before sex); permanently pinching a bit of elastic tissue opposite the scar (which also shortens the penis); or replacing scar tissue with a tissue graft (which can sometimes cause other problems, such as numbness or complete loss of erections).
The best approach to Peyronie's disease involves preventing penile trauma in the first place, urologists say.
A bit of biomechanical common sense helps: Stay well-lubricated during sex to reduce friction. Be aware of pubic bones and other hazards, especially during entry and reentry. Keep thrusting motions along the length of the shaft. And avoid maneuvers that bend, twist or buckle the penis.
In short, be careful. It's not a toy.
Read more The Mating Game columns at latimes.com/matinggame.