Partial knee replacement vs total - how does a doctor choose?

Depends: A lot goes into the proper choice: the specific areas of arthritis in the knee, the amount of angulation, the presence or absence of the acl, the age, weight and medical health of the patient, the experience of the surgeon, etc etc etc.

Exam and xray: Patients that have one of their three knee compartments worn down with little to no space on x-ray are candidates by x-ray. Then on exam, the patients should have pain and tenderness in the compartment worn down on the x-ray and should not have pain in the compartments that are not worn down. This is how we choose a partial knee replacement patient. Total knee patients have wear and pain all over.