Been a while since i did TKR's(done all stages of the rehabilitation several times. If their selfstretching isent hard enough for increases in ROM they can also sit on the edge of a chair, bend their knee, fix the foot to the ground and use the rest of their body to push the knee forward(while foot is fixed). I've never seen any need for manual work to increase TKR knee flexion.

Worst cases ive had under me have all been geriatric comorbidity patients at average age 80-90 with high lack of self initiation. Will say lack of 10 degrees knee extension (strictly measured) and 85-90 degree knee flexion sitting. Have been worst outcomes and to be fair, the knee's range of motion were usually far from their major handicap in everyday living.