DEAR DR. ROACH: Isn’t age-related “snap, crackle and pop” likely chondromalacia? How is chondromalacia different from osteoarthritis of the knee? — B.P.

ANSWER: Both chondromalacia and osteoarthritis are causes of knee pain. Both can cause crunching noises and sensations.

Chondromalacia refers to irreversible damage to the cartilage of the patella, most frequently as a complication of patellofemoral syndrome. Patellofemoral syndrome, the most common cause of anterior knee pain in young athletes, occurs when the kneecap doesn’t move straight up and down in the groove at the bottom of the femur, but instead moves toward one side (usually the inside) or the other. This abnormal pressure in the knee, if untreated, leads to degeneration of the cartilage. Patellofemoral syndrome is a clinical diagnosis made by history and physical, but chondromalacia usually is diagnosed by X-ray or MRI. Treatment of patellofemoral syndrome is intended to relieve symptoms and prevent damage to the cartilage. Exercises, supervised by a physical therapist, are the best treatments for patellofemoral syndrome.

Osteoarthritis is more commonly seen in people 50 and older, although I have certainly seen osteoarthritis in much younger people, especially if overweight or with a history of knee injury. Similarly, I have seen patellofemoral syndrome in older athletes.

Osteoarthritis also causes degeneration of the cartilage, but the exact mechanism isn’t known. There are secondary changes to the bone and inflammation. It can be diagnosed by X-ray even at fairly early stages. Treatment includes exercise (again, best if supervised by physical therapy), medication, injection, surgery and many other alternative therapies.

DEAR DR. ROACH: Is ALS related to Lyme disease? — D.T.

ANSWER: Amyotropic lateral sclerosis, commonly referred to as Lou Gehrig’s disease, is a progressive and incurable disease that causes degeneration of the nervous system. The only known risk factors are age and family history, but cigarette smoking may increase risk as well. Because ALS affects both the “upper” motor neurons (in the brain) and the “lower” motor neurons (in the spinal cord and nerve roots), the diagnosis is suspected when people have both kinds of symptoms. Upper motor neuron symptoms include poor coordination and slowness of movement and clonus, a spontaneous contraction of the muscle. Lower motor neuron symptoms are weakness and atrophy, as well as fasciculations, spontaneous twitching of the muscles. Neurologists treat ALS, and treatment is aimed at improving symptoms. There is one medication, riluzole, that slows progression.

Lyme disease, if not treated early, can cause neurologic symptoms, but does not cause ALS. Weeks or months after the tick bite that transmits Lyme disease, symptoms can include damage to the facial nerve (and other nerves) and nerve pain. Lyme disease also can cause a type of meningitis, with fever and headache. People with nervous-system Lyme disease almost always have diagnostic blood tests.

DEAR DR. ROACH: A few years ago, my granddaughter, who was raised in Costa Rica, got Dengue fever. I heard that if you get it a second time, it can be fatal. Is this justified? I am worried about her going back to Costa Rica for holidays.

Is there a vaccination or specific treatment? — A.R.B.

ANSWER: Dengue fever is common, but people who live in areas with Dengue are at much higher risk than travelers. The worst complication of Dengue, called Dengue hemorrhagic fever, happens in people who previously have had an episode of Dengue, so the concern for your granddaughter is justified.

There is no commercially available vaccine for Dengue (yet), and no specific treatment. The most effective prevention for travelers is avoiding mosquitos by staying in well-screened or air-conditioned buildings, wearing protective clothing and using effective mosquito repellant.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Health newsletters may be ordered from www.rbmamall.com.

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