Beneath the periosteum.
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sub·peri·os·te·al -.per-ē-'äs-tē-əl adj situated or occurring beneath the periosteum <\subperiosteal bone deposition> <a \subperiosteal fibroma>
sub·peri·os·te·al·ly -ē adv
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sub·peri·os·te·al (sub″per-e-osґte-əl) beneath the periosteum.Medical dictionary. 2011.