2 dentists find new body part: a small muscle Something to chew on: Two University of Maryland dentists believe they have discovered a fifth muscle used in chewing.

February 13, 1996|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

By breaking the rules on how to dissect a human body, dental researchers at the University of Maryland at Baltimore believe they have stumbled across a jaw muscle never before described in the anatomy textbooks.

Dr. Gwendolyn F. Dunn and Dr. Gary D. Hack say the 1 1/2 -inch muscle connects a point on the skull directly behind the eye to a spot on the lower jaw behind the teeth.

They suspect it may be responsible for "retro-orbital pain," or pain behind the eye, experienced by some patients suffering disorders of the chewing muscles.

The knowledge that there is a distinct new muscle back there, Dr. Hack said, "might open avenues for new treatments that are not normally accepted."

To be announced today

Dr. Dunn and Dr. Hack were to announce their discovery today in Baltimore at the meeting of the American Association for the Advancement of Science. It will also be published in Cranio: The Journal of Craniomandibular Practice.

It is the second time in a year that Dr. Hack has announced an anatomical discovery.

Last February, Dr. Hack, UMAB neurosurgeon Dr. Walter L. Robinson, and a physical anthropologist at the Smithsonian Institution, Richard T. Koritzer, revealed that their unconventional dissections had uncovered a previously unknown 1/4 -inch connection between a small muscle in the back of the neck, and the dura mater -- the membrane that covers the spinal cord and the brain.

Dr. Hack is a general dentist and assistant professor at the UMAB dental school. Dr. Dunn is an orthodontist and volunteer dissector with the anatomical services division of the UMAB medical school. They have been conducting dissections on cadavers donated to the Maryland State Anatomy Board.

"It is assumed that everything that is known is in the textbooks, and that if you read the text, you know everything," Dr. Hack said. The danger is that anyone dissecting a body will "see only what is described in the text."

Unique frontal approach

The anatomy texts describe a dissection from the side of the head. But approaching from the front opens up entirely new vistas deep inside the face, he said.

That's what Dr. Dunn and Dr. Hack were doing when they found themselves looking at something curious. It was a muscle, 1 1/2 inches long, 3/4 inch wide and 1/2 inch deep. One end attached to a bony "prominence" on the skull, right behind the eye. The other end attached to a bony spot on the mandible, or lower jaw.

The anatomy books say there are only four "muscles of mastication," or chewing muscles. This looked like a fifth.

"We thought it was an anomaly," Dr. Hack said. When the head is dissected from the side, the new muscle looks like a deep part of the temporalis muscle -- one of the four known chewing muscles.

But "if you look in any anatomy text, it describes the temporalis as originating from the side of the head and running to the mandible," Dr. Hack said. "This originates from the undersurface of the skull from an area behind the eye.

'Sat on this for year'

"Dr. Dunn and I sat on this for a year," he said. "We didn't believe it ourselves. We said 'This is too big, too obvious.' " They worried that if they started talking about it too soon, people would say, "What are these crazy dentists up to?"

They began to scour the literature, but found no mention of it. They also continued their dissections and found the same muscle in 25 consecutive cadavers.

Clearly it was no rare malformation. Magnetic resonance imaging revealed the muscle in living patients. Dr. Michael I. Rothman, zTC medical director of the Anna Gudelsky MRI Facility at the University of Maryland Medical Center, plans to present a paper on the finding in April to a meeting of the American Society of Head and Neck Radiology in Los Angeles.

"It was always there, we just never thought to look for it before," said Dr. Rothman. He believes the muscle helps lift the jaw and move it from side to side.

It also seems to be served by a slightly different nerve supply than the other chewing muscles. That, he said, suggests better opportunities for diagnosing nerve damage and better chances for rehabilitating injured patients.

Dr. Hack said he's not sure when a new body part becomes accepted by the medical profession. "It's been so long since a new muscle has been discovered," he said.

The muscle is "clearly new, and undescribed. But is it a fifth muscle of mastication? I don't know." He knows one thing, he said. "Anatomy is not dead."

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