Weight must be limited on the area of the cartilage implant. The reason for limiting weight is that the cells must be allowed to adhere in the area that underwent the ACI procedure.
How to limit weight on the area of the ACI depends on the location. When the ACI is on the tibia (top of the shin bone) or the femur (end of the thigh bone), weight is limited by having a patient use crutches. When the ACI is on the kneecap (patella) or within the groove for the patella (the trochlea), motion must be limited because this will cause compression on the implanted cells.
Range of motion is usually initiated early on after surgery. However, as stated above, if the area of ACI treatment is on the kneecap or within its groove, then motion will be limited for 6-8 weeks.
The reason for starting motion as early as possible is that the movement helps to stimulate healthy cartilage growth. However, this motion must be balanced with the pressure caused by motion. You must strictly adhere to your doctor's recommendations as they will be specific to your injury and treatment.
Weight bearing is usually limited for at least 6-8 weeks, and then gradually progressed over time. After 3-6 months, training can increase in load and intensity, and sport specific activities can begin about 12 months after surgery. Most athletes do not return to full sport until about 16 months after surgery.
Jones DG, Peterson L. "Autologous chondrocyte implantation" Instr Course Lect. 2007;56:429-45.