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ASK DR. KNOWLEDGE

What determines how quickly and cleanly a cut heals?

By John Swain
Globe Correspondent / July 11, 2011

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Sometimes cuts close quickly, while others pull open and require stitches. What determines how quickly and cleanly a cut heals?

Despite its uniform appearance, skin contains loads of elastic fibers, largely made of collagen, laid out parallel to the skin’s surface. They are ordered along lines called “Langer’s lines’’ or “lines of cleavage,’’ which were discovered in cadavers in 1861. Think of a person as covered with latitude and longitude lines but laid out in varying patterns.

The ordering of the skin fibers is similar to the alignment of muscle fibers along a muscle in the direction it is meant to pull. A cut into the muscle parallel to the fibers is less damaging than one that cuts across them, leaving them unable to function.

Unlike muscle fibers, skin fibers do not pull on command, but rather they are under constant tension to keep the skin taught. This explains why loss of collagen results in wrinkles. Also, when skin fibers tear, stretch marks result.

If skin is cut along a line of cleavage, most of the fibers remain intact and the edges of the cut will be held together much as they were before the incision. Surgeons sometime have to use retractors to hold open such a cut, which is likely to heal with little scarring.

On the other hand, if skin is cut across a line of cleavage, the sliced fibers pull apart the skin on the edges of the wound, so it gapes. Such a cut is more likely to need stitches.

“Ask Dr. Knowledge’’ is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.

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