Looking for advice on exercise that can be done

After 4 arthroscopy ops over a number of years .. no cartilage remains on the lateral side .... developed into severe grade 4 OA
8 years ago as part of trial was fitted with Conformis iForma Interpositional device :
http://www.atmedicals.com/PDFs/publicaciones/2006%20iForma%20ORS%20-%20Dr.%20Koeck.pdf

8 years on - Knee occasionally locking up due to crepitus on patella and a new medial meniscal tear .. resulted in follow up arthrocopy in Apr 2015
Tear removed and rear of patella debrided.

All OK until Nov when without trauma woke up one day with intense pain in knee, very painful Quadriceps and difficulty walking down stairs, getting up from seated position and inability to kneel. The knee feel like it has a steel wire going diagonally across it under the patellar restricting movement. Frequently locking up.

Further visits to surgeon .. have advised grade 3 changes over trochlea and patellar, and very rough patello femoral articulation.
Consideration now is further surgical procedure - a patello fermoral replacement (patello femoral arthroplasty)

It is recommended to build up quadriceps specifically and also the hamstrings before such an operation to aid recovery time.

What exercise would you suggest can be performed to build quadriceps with as little impact on patella as possible.
It is also 'possible' that such exercise 'may' settle knee back down, avoiding need for surgery.