A photograph shows the mandibular left first molar (tooth 19) displaced lingually out of the arch. What does treatment require?

Correct answer: B. Increasing the arch dimension, because the tooth lacks space to be aligned

• A. Extracting a sound first molar to resolve crowding sacrifices a strategically important tooth and is not the appropriate answer.
• B. CORRECT. A tooth displaced out of the arch is displaced because there is INSUFFICIENT SPACE for it. Moving it buccally without first creating room simply pushes it into a new crowded position or relapses. The arch dimension must be increased — by expansion, distalisation, proximal reduction, or extraction elsewhere — before or while the tooth is aligned.
• C. Equilibration adjusts occlusal contacts; it cannot reposition a tooth displaced out of the arch.
• D. A finger spring delivers a light tipping force and is suitable for minor tipping of a single tooth INTO EXISTING SPACE. With no space available it cannot correct the problem and will produce relapse or a new crossbite.

Note: The order of thinking in any crowding problem is always the same: perform a SPACE ANALYSIS first (arch length required versus arch length available), decide how the deficiency will be resolved, and only then choose the appliance. Appliance selection without space analysis is the classic error.