You can always tell a runner....by the toenails.

Toes can be a runner's Achilles heel, in the literary sense, his weak link. For some, it's the knees, for others, the back. And for quite a few, the Achilles is actually the Achilles. And for many, the toes, or at least one of them, can be the problem that dogs an otherwise trouble-free runner. Any runner can get into toe trouble with the right (or wrong) conditions. From blisters, corns, and calluses to stress fractures and gout, those cute little toes can become a big headache.

Toenails alone can be the source of a long list of troubles. Chief among the potential problems is the ingrown toenail--with or without infection. Other, not too pretty problems, are nail thickening due to fungal infections, black toenails due to repeated trauma, I thickened toenails called "runner's nail," and of course other irritations, inflammations and infections. Here are just a few common causes of troubled toenails.

Ingrown Nails

You may be familiar with the warning, "Don't cut the nails too short." Trimming too short can cause the leading edge of the nail to grow forward and cause pressure on the soft tissue at the toe tip. Although this can happen, by far the more common cause of ingrown toenail, and the pain that goes with it, is the curvature of the nail into the tissue on both the medial (inner) and the lateral (next to the other toes) sides of the nail. You can get the picture by looking head-on at your thumbnail. From this vantage point, it looks as if the nail were clipped onto the top of the thumb. Toenails follow a similar curve.

If there is excessive pressure on the nail against the soft tissue from the forefoot of the shoe, problems can develop. Pressure on the side of the nail, where the toe touches the shoe, can push the soft tissue against the incurved edge of the nail. Then the toe gets inflamed, enlarges, and more pressure is felt on the toe. With enough pressure, the nail edge will break the skin surface allowing bacterial or fungal contamination into the toe, causing infection and even more swelling and pain. The warm sweaty environment of a running shoe makes an ideal site for proliferation of infection. Infection can spread around the rest of the nail, further increasing pressure and pain.

Using local antibiotics like bacitracin and antifungal creams may help, but it's hard to reverse an ingrown, infected nail without an office surgical procedure using local anesthesia. Never try to cut the nail "to relieve pressure" yourself. This is a recipe for disaster. It will heal faster and better with a smaller chance of recurrence if you see a doctor for treatment. Ingrown toenails do not lend themselves well to do-it-yourself home treatments.

Runner's Nail

Long distance runners and ballerinas both may be known for their athletic prowess and war-worn, beat up feet. Thickened toenails are so common among long distance runners, the condition has been dubbed, "Runner's Nail." The thickening and changes in color are caused by repetitive pressure of the shoe on the nail. The resulting thickening makes the nail even more vulnerable to additional problems since it is raised closer to the top of the shoe.

Fungal Toenails

A fungus (like the one that causes athlete's foot) can infect the nail and also cause thickening and discoloration of the toenail. This problem is distinguished from the Runner's Nail by infection. The fungus invades the nail when there is an injury or trauma, which may be so subtle you don't even know it is there. Once infected, a fungal nail is very difficult to treat. Prevention is a much better bet. If you notice changes in your toenails, see a podiatrist or a dermatologist for diagnosis and treatment.

Black Toenails

Although you can get a black toenail from a sudden, painful trauma that causes bleeding under the nail, for runners it is more likely a chronic, repetitive trauma to the nail caused by short shoes, running downhill, or wearing loose shoes. This repeated micro-trauma causes only a light amount of bleeding and minimal pressure buildup, so little or no pain is felt. In many cases, you only realize this is happening when you notice your toe is discolored. But don't ignore it. The nail can gradually become thicker, and more problems develop.

Eliminate the cause of the irritation, and a new nail will gradually form. It takes six to nine months for a new nail to form, so be patient. If the nail thickens, you can file it down. If there is pressure and pain under the toenail, see a doctor. Drainage from a blackened nail can be a sign of melanoma and should not be ignored.

RELATED ARTICLE: If the Shoe Fits...

By selecting a shoe that fits your foot, you are on your way to pain-free running. Have both of your feet measured, even if you think you know your size. Feet widen and lengthen as you age or gain weight. Even when you've confirmed your size, remember fit is what matters. Sizing standards can vary from one manufacturer to another--ignore the numbers and get the fit right. This is another good reason to shop in a running specialty store rather than mail order.

Look for a roomy toe box with enough space across the widest part of your foot. Have as much as a thumb's width to spare in front of your longest toe so that toes don't get jammed on downhills. Fit should be snug at the heel, however, to prevent slippage during movement.

No matter how perfectly a shoe fits--it won't stay that way forever. Throw out your old favorites after 300 to 500 miles. Get a few pairs and rotate them. Mark each pair with the date purchased to help you keep track of their age and mileage. Maybe some day they'll come with a built in odometer.

Spend a bit of time finding a good fit, and you will be rewarded with miles of happy, healthy feet.

Why did that toe turn on you?

Trauma, either acute or chronic can contribute to all of these toenail troubles. Trauma--like stubbing a toe--can cause changes in the growth pattern of the nail and eventually it can cause thickening, discoloration, or infection. More often, excess pressure and repetitive trauma is caused by the mechanics of the foot inside the shoe.

Trauma can come from shoes that are too small or shoes that don't grab the midfoot firmly. If the shoe is too small, every step causes the toes to press against the front of the shoe. Tight shoes restrict blood supply to the toes, increasing the risk of infection. If the shoe doesn't grip the midfoot adequately, each time the shoe hits the ground and stops abruptly due to friction, the foot will slide forward unless it is firmly held in place inside the shoe. Momentum forces the foot forward inside the shoe until the toes collide with the front portion of the shoe.

It is important to prevent this sliding or pistoning of the foot inside the shoe. If the shoe comes up high enough on the front of the foot, it can help prevent the foot from sliding forward by holding it at the ankle. Most running shoes, however, don't come up high enough to be effective. In most cases, the lacing across the midfoot has to do the job. You can also glue extra tongue padding in the shoe for a tighter grip on the foot. The extra padding allows the laces to be tighter without pain. Both the padding and tight lacing stop the foot from sliding forward in the shoe.

The best way to prevent most of these problems is to get the right fit. Find a running shoe store where the employees are professionals who understand runners' needs and the differences in the shoes the sophisticated industry is producing. Shop late in the day or after a good run to allow for the natural swelling of the foot. Don't rely on street shoe size or assume that the size is the same from one shoe to the next. Try on both shoes, lace them firmly, and run. A good running shoe store will let you run up and down the block. Get a promise that you can return the shoes after several days of wear inside at home. Don't ever expect running shoes to "break in." They need to fit well from day one.

(Marvin H. Bloom, M.D., is an orthopedic surgeon in Burlingame, CA, and a member of The American Running Association's Editorial Review Board.)