When used at the beginning of a sentence, this word is a variety of the 'filler' word basically. Another direct replacement for essentially is technically. Other filler words include like, just, and stuff and y'know, but they tend to be in the middle of or at the end of a spoken sentence. Filler words are rarely used in writing and are part of acquired speech.
Essentially, the leg bone is connected to the hip bone.
Replace the sentence with the less-wordy: The leg bone is connected to the hip bone