You may be referring to a doctor called an arthroplasty fellow. The doctor is usually in a 12-month period near the end of training to do surgery. The Fellow already has good surgical skills. Now he or she is going to focus on patients having a joint replacement. This gives the surgeon a chance to observe, assist with, and later perform a large number of joint replacements.
A senior surgeon mentors and supervises the Fellow. Any steps in an operation done by the Fellow must be done with the senior surgeon present. The operation may take a few minutes longer than normal. Studies show there's no greater blood loss or increased risk of complications after the operation when a Fellow assists.
The Fellow is also expected to be involved in a research project. At the end of the year an article is written to present the results of the study. You will likely be in good hands with no greater risk of danger than if the senior surgeon was alone.