Overall the colours seem to little too muted - would have more impact in different combinations? #21 could be more easily adapted to the individual brands if it was reversed - the 'D' would be more obviously attached to Dental Space, and the stylised eye could even take the place of an 'i' to form the brand 'iSpace'
One can imagine that it represents eye vision as well as it resembles nervous system of a teeth. Also its inside a sphere the sphere can be represented as the "Space",,,,