Severe tricompartmental osteoarthritis with the symptoms including pain, swelling, stiffness leading to marked restriction of daily life activities does require a total knee replacement.
As far as the Achilles tendonitis is concerned, initially it should be managed conservatively. NSAIDs, ice packs, compression and orthotic devices, elevation of the foot would help. Bed rest should be advised in addition to avoiding exercises that strain the tendon should be avoided. Corticosteroid injections can be an option under ultrasound guidance. If the conservative management fails or the there is significant inflammation on imaging investigations such as an MRI or there is recurrent tendonitis then surgical option should be explored.
The better option in my view would be to treat the Achilles tendonitis conservatively while going for the total knee replacement , first.
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