Would negligence be the right call?

It might very well be. NYS requires and expert opinion that 1) actual medical negligence happened and 2) this negligence was the cause of the current harm. A permanent condition could lead to a successful suit. The doctors might defend by claiming your necrosis was the onyl or principal cause of the condition.
A jury would have to decide who was right.

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(Even if you are not filing a lawsuit this information can be useful).

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LEGAL DISCLAIMER
Mr. Sarno is licensed to practice law in NJ and NY. His response here is not legal advice and does not create an attorney/ client relationship. The response is in the form of legal education and is intended to provide general information about the matter in question. Many times the questioner may leave out details which would make the reply unsuitable. Mr. Sarno strongly advises the questioner to confer with an attorney in their own state to acquire more information about this issue.

To begin with there is much, much more information needed in order to determine if you have a valid medical malpractice claim. That being said the proper way for bilateral hip replacement surgery to be done is as you are describing. That is one side first and a lengthening of the leg and approximately six months to a year later the other side done, also with a lengthening of the leg. Usually in between the patient is prescribed an orthodic shoe to compensate for the different length of each leg. It is also not clear why or when you were operated on for the sciatica, but that surgery could cause a further delay in completing the hip replacement. In any case you should consult with an attorney who is experienced in handling claims of this nature, as so much more information is required to determine if you have a valid malpractice claim. You should also be aware the there is a two and one half year statue of limitation in this type of case.