When augmenting the back of the head, should the material be put under or above the periosteum? (Photo)

When augmenting the back of the head - must the material be put under the periosteum or can it be put above the periosteum? The reason I ask is because I've had previous head surgery and my scalp tissue is very tight already, so I need tissue expansion before putting the implant material in the head. Apparently the periosteum can not be expanded while the tissue above it can, but I've heard that the implant material "must" be put under the periosteum. Is this true and if so why?

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