You're wondering about the bias in such a study. It's a valid and fair question. And one that researchers must ask themselves when setting up the study. There are ways to avoid this kind of problem.

For example, your study is being done after the operation is over. This is called a cohort or retrospective study. This type of study helps limit patient bias. The study takes place after you've finished your treatment. The results aren't biased by what you expect to happen.

The doctor may choose to compare your results with the final X-rays of the hip joint. In these cases, the X-rays are read by a doctor who didn't do the operation. The radiologist doesn't know by looking at the X-ray which patients had a small incision and which ones had the mini-incision.

Finally, scientists think that independent researchers reduce bias. These are studies done by people who weren't the first ones to try something. They aren't trying to prove something works. They are just looking at the results of using the method developed by someone else.