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Table 1 Inclusion criteria

From: Three-dimensional assessment of teeth first-, second- and third-order position in Caucasian and African subjects with ideal occlusion

Adult age (not less than 18 years)

No previous orthodontic treatment

Regular arch form with little or no crowding

Complete dentition to second molars

No bridges or implants

Centred midlines

No gingival recession

No joint or muscle pathologies

No ectopic teeth

No supernumerary teeth or tooth agenesis

At least four of Andrews’ six keys with bilateral molar and canine class I occlusion always present

Presence of minimal diastems and/or small premolar rotation or little irregularity (Little’s index less than 3) at incisors in some individuals

Overbite and overjet within normal limits 2 mm ± 1 mm

No anterior or posterior cross-bite

No markedly visible intra-oral or extra-oral symmetry