Partial/unicompartmental knee replacement is a newer procedure than total knee replacement, and has been added to the armamentarium for those patients where the arthritis is restricted to one compartment
'Uni' = 'one'. 'Compartment' = a functional area of the knee.
So a unicompartmental knee replacement is one which replaces only one of the three compartments of the knee:
Thus one can have -
The two parts of the prosthesis replace the articulating surface of femur and patella.