What is the risk of partial patellectomy?

Knee weakness. Any procedure performed to remove part or all of the patella runs the risk of causing weakness of the quadricep muscles that stabilize the knee. Be sure to talk to your surgeon about appropriate exercises and physical therapy to help your situation.
Arthritis. Partial patellectomy is usually performed after a severe patella fracture where there are multiple pieces that are unrepairable. The injury itself, and/or removal of part of the patella may place you at risk for knee arthritis.