A retractor assembly for percutaneous surgery in a patient that includes first and second retractor portions positionable opposite one another in an incision of the patient. A working channel extends between the first and second retractor portions and provides access to a location within the patient adjacent to distal ends of the first and second retractor portions. The first and second retractor portions are movable relative to one another to adjust the size of the working channel. An add-on retractor element is provided that is mounted to the first and second retractor portions and spans the gap between the first and second retractor portions to prevent creep of soft tissue into the working channel when the retractor portions are separated.