As one gets old joints wear out like a tyre of a car. If a joint has been well used (sports and being physically active) the joint will wear out sooner than in a non-active person.
A human knee can be divided into three compartments -
The majority of patients will suffer with medial compartmental arthritis and some with anterior (patellofemoral compartment) arthritis. A few patients will suffer with outer compartment-lateral compartment arthritis. Primary osteoarthritis (wear and tear related) usually affects the medial compartment, whereas inflammatory arthritis (rheumatoid arthritis and related conditions) mainly affect the lateral compartment.
The knee is said to be in varus alignment if the medial compartment is affected. This is when knees get 'bowed'.
Valgus deformity is when the outer compartment of the knee is affected. Here patients stand with a 'knock-kneed' alignment.
If the arthritis process becomes severe and does not respond to conservative management, then surgical treatment is considered. If joint surfaces are damaged beyond repair, knee replacement is a valid option. Surgeons would choose the type of knee replacement (total or partial) based on the amount of arthritis, the type of arthritis, the patient's age, activity level, weight and other associated medical conditions.
Half knee replacement (partial knee replacement) works well in indicated cases. The philosophy is to replace only the damaged part. Patients who have half-knee replacements do rehabilitate well and sooner than total knee replacements.
If arthritis affects more than one compartment, the surgeon would carefully investigate the knee to assess the damage to the rest of the compartments. A detailed discussion would be held after which a treatment option of replacing more than one compartment or total knee replacement can be offered.