Space maintainer (fixed unilateral band type) with a hook rest can prevent permanent first molar rotation and tilting efficiently.
For lower first molar, put the rest hook at lingual side to prevent (reduce) occlusal interference.
For upper first molar, put the rest hook at buccal side of the band to reduce occlusal interference.
A curved (contoured) rest to fit the canine can prevent sinking of the spacer also prevent the tilting of the primary second molar.
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