The eruption of mandibular permanent incisors lingual to deciduous dentition, can be perceived by the dentist as:
The primary teeth may have undergone extensive root resorption and may be held only by soft tissues. 
In other instances, the roots may not have undergone normal resorption and the teeth remain solidly in place. 
The tooth buds of succedaneous teeth lie lingual to the primary teeth.
It is common for mandibular permanent incisors to erupt lingually, and this pattern should be considered essentially normal.
It is seen both in patients with an obvious arch length inadequacy and in those with a desirable amount of spacing of the primary incisors.
A pathological finding
An arch length discrepancy
Normal finding
None of the above