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What is In Steiner's analysis, SNA demonstrates: and explain in detail?
Relating the Maxilla to the Skull: The angle SNA is formed by joining the lines S-N and N-A . The mean reading for this angle is 82°.
Steiners analysis demonstrates the position of maxilla and mandible in relation to the cranial base. 
If the angular reading is more than 82°, it would indicate a relative forward positioning or protrusion of the maxilla. Conversely, should the reading be less than 82°, it would indicate a relative backward or recessive location of the maxilla.