Hello, Thanks for your query. But first let me address what that hard lump is..... since it is not draining, it is most likely a cyst that has formed in the bone and grown into the hard tissue. This is seen frequently with root canaled teeth because it is caused by the bacteria that has been residing in your root canaled tooth. Not just from the missed canal , but from the microscopic canals that cannot be cleaned out. So the re treatment may have removed some dead nerve material from the missed canal, but it did nothing to access the microscopic ones. Therefore, the tooth will continue to be infected. An apicoectomy is a surgical root canal, it accomplishes the same thing that a re treatment does with the exception that the surgeon physically goes in and removes the cyst. I do hope that you have found something helpful and I will be glad to answer any further query. Take care