Minimally invasive refers to several factors. A shorter operating time as you suggest is one measure. A shorter operation means less anesthesia. Sometimes there’s less blood loss. Those two things alone can also mean "less invasive" to the pocketbook.

According to a task force of surgeons there are several ways to tell if an operation is minimally invasive. First, the size of the incision is half the length of the standard approach.

Second the location of the cut is often different. The goal is to avoid disrupting the joint capsule or some of the muscles. If the capsule is cut, a smaller incision is used. Third, fewer muscles are cut or detached.

During knee surgery anytime the surgeon can avoid cutting the extensor mechanism, it's considered "less invasive." The extensor mechanism is made up of the quadriceps muscle as it comes down over the front of the thigh and attaches around the patella or kneecap.

Disrupting this muscle can cause weakness in knee extension. The patient may not be able to fully extend the knee, a condition called extensor lag.

There isn’t one single way to define minimally invasive but rather, a group of factors.