Hello sir The cavity between two teeth is usually notices when the hole becomes bigger and food starts to Lodge too much. The nerve of the teeth is closer from the side area. If the cavity comes close to the nerve of the tooth, it is advisable to get the root canal treatment done because if simple filling is done, it compresses the nerve and then it starts to pain. So you start feeling after filling pain started. Therefore if your doctor has advised root canal treatment kindly proceed for it. Cap basically depends on your choice. There are a variety of cap available.