Arch-length deficiency — fixed appliances
There is less room along the arch than the teeth need, so they overlap, rotate and sit out of line.
Five faults. Each heading below is set in the fault it names and levels itself as you read past it, at the same bounded rate the tooth would move.
Generalised spacing / midline diastema — clear aligners
The opposite problem: more arch than teeth, so gaps open — most often the one between the upper central incisors.
Axial rotation — fixed appliances
A tooth turned about its own long axis. It looks like a small fault and it is one of the more stubborn ones.
Increased overbite / curve of Spee — fixed appliances
The upper incisors cover too much of the lower ones. The occlusal plane is not level, and levelling it is the whole of phase one.
Dental midline deviation — clear aligners
The dental midline does not sit under the facial midline. The whole line is off, not one tooth in it.
Which one is mine?
Most people arrive with two or three of these at once, which is normal and does not make treatment longer in proportion. The consultation exists to sort that out: records, a space analysis in millimetres, and a written plan with a number on it before you commit to anything.