He has severe knee deformity and stiffness. Urging to regain knee movements (pain free!)
Initially addressing gastronemius, soleus and hamstring flexibility followed by AAROM and quad activation strategies. It would be great to obtain a comprehensive examination of the hip, knee, ankle and trunk with an individualized intervention with caution to pain. A multifaceted approach maybe needed including therapeutic modalities, motor control/motor learning strategies, manual therapy, therapeutic exercise including a very good home program based, gait training, biofeedback (visual, sEMG, auditory) based upon the individual's unique presentation. My suggestion is to find the best physiotherapist that has the best success and experience in complex knee rehabilitation.
Apart from above-mentioned strategy , knee replacement can only achieve the objective. However workup for long standing contracted tendon,, capsular structures etc requires planning.
Knee surgery is not my subspecialty, but in my eye this seems like a fixed arthrodesis. This knee will NEVER move. The patella and extensor system is also fused to the arthrodesis, so I do not believe a knee replacement will ever work.
I believe the only way to give better quality of life to this patient is to do an anterior closing wedge osteotomy in the "knee" region to re-fix the knee (re-arthrodesis) in about 10 degrees flexion (instead of 90 degrees as it is now) so that he can walk with it (load-bearing), instead of having a knee in fixed flexion, which does him no good. But watch out for stretching of the posterior structures after surgery.
Maybe a knee sub-specialist can give a better answer.
status of extensor mechanisms , posterior muscles, capsules and neurovascular should be assessed and planned in stages if attempt is done to bring mobility.
According to currently technique, I don't think the case is suitable for arthroplasty. The case is not only involved the problem of bone and articular but also ligament and muscle. The rehabilitation will be so difficult and long, I don't think it's worth to do such a traumatic surgery to get a uncertain treatment effect.
in my opinion arthroplasty won't be an option. There won't be any way to obtain a functional knee, just like Rodolfo Caria Mendes Pompeu Santos already mentioned.
One way to get an improvement in life quality will be to straighten the knee so the patient can use his lower extremity and walk again. The result will be a stiff but straighter knee. Closing wedge osteotomy, shortening and then dynamically gradually @straightening with a hexapod system. The foot has to be included to avoid an equinovarus. After reaching the 10° flexion in the knee one can lengthening the tibia later on with an additional osteotomy.