After an injury or infection, what part of the nail must be healthy for it to return to normal growth?

The matrix is crucial for the nail's growth and recovery following an injury or infection. It is the tissue located under the base of the nail where new nail cells are generated. If the matrix is healthy and functioning properly, it can produce new nail cells, allowing the nail to grow back to its normal state.

In cases where the matrix is damaged, the growth of the nail can be adversely affected, leading to abnormalities such as ridges, irregular growth patterns, or even permanent changes in the nail's appearance and structure. Therefore, ensuring the health of the matrix is essential for the nail's restoration and normal growth after any trauma or infection has occurred.

Other components of the nail, such as the lateral nail fold, lunula, and nail plate, play significant roles in the structure and protection of the nail but do not directly contribute to the regrowth process in the same critical manner as the matrix.