The knee is divided into three sections (compartments). If two or three of these compartments are worn out , a total knee replacement is required. However , if only one compartment is worn out , a much less invasive UKR can be performed to only replace the two surfaces of the single bad compartment. People who are severely obese (BMI greater than 35) or have a severe deformity of their knee , also require a TKR. Approximately 10% of people who need knee replacement are candidates for a UKR.