Early loss of primary dentition needs to be addressed as soon as possible in order to prevent space loss.  In situations where it is determined that the yet-to-erupt permanent tooth may be erupting to the buccal or the lingual a Single Arm Space Maintainer is a good solution. 

The single arm can be soldered to the buccal or lingual of the band as prescribed in order not to interfere with eruption.  Once the erupting tooth has broken the surface a subsequent appliance can be used if deemed necessary to move the tooth into the proper buccal/lingual position.