The day after a knee replacement, most orthopedic surgeons have their patients use a continuous passive-motion machine, which flexes and extends the knee as they lie in bed.

NEWYORKER: Big Med

Some people benefit from a special mouthpiece or a CPAP (continuous positive airway pressure) machine that keeps the airway open and prevents long pauses in breathing during sleep.

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Sleep apnea is commonly treated with a "continuous positive airway pressure, " or CPAP, machine worn at night to keep the airway open.

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