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Knee Pain from Osteoarthritis - What are the Three "Compartments" of the Knee?

Updated on December 4, 2013

Understanding Terminology

Everyone suffering from osteoarthritis of the knee eventually ends up consulting a doctor. It may take years, but such is the nature of this degenerative joint disease that it wears you down.

One common source of frustration is the combination of medical jargon and short consultations with medical professionals. You go in with a long list of questions in your head but before you know it you have said goodbye and are out in the corridor. 

Obviously there are some doctors that use more jargon than others, but the more you understand, the more you can make of those brief minutes discussing your future.

You don't need to study for a University degree, you just need to remember a few terms.


Only 4 to remember:

The Femur: This is your thigh bone

The Tibia: This is your shin bone

The Patella: This is your kneecap

The Fibula: The smaller of the two bones running between the knee and the ankle. It runs down the outside of your leg.


Joints occur where bones move against each other. The two relevant joints in knee arthritis are the tibiofemoral joint (the main, hinge type joint of the knee) and the patellofemoral joint (the articulation between the kneecap and the thigh bone. Healthy joints are covered with smooth articular cartilage that enables pain free movement. Osteoarthritis is the wearing down of this covering.

When people talk of knee osteoarthritis, they usually refer to degenerative changes between the femur and the tibia.

The bottom end of the femur forms two distinct 'knuckle' like projections. One on the inside of the knee, one on the outside. The result is that the joint between the femur and the tibia is divided into these two parts.

They are called the medial compartment (inside compartment) and the lateral compartment (outside compartment).

The most common place to get arthritis is in the medial compartment. 

You can have osteoarthritis in one compartment and this is known as unicompartmental arthritis. This term does not tell you which side it is on. 

Bicompartmental arthritis means there is degeneration on both the inside and outside of the knee.

The third compartment is the patellofemoral joint between the kneecap and the thigh bone.

So you can have it on one side, both sides, or both sides and the kneecap as well.

Knee Compartments

A knee with arthritis in all three compartments. this is a right knee viewed from in front. The kneecap is not shown to enable a better view
A knee with arthritis in all three compartments. this is a right knee viewed from in front. The kneecap is not shown to enable a better view

Why are compartments relevant?

The location and extent of your arthritis will decide which treatment options are likely to work for you. Certain operations are only suitable for arthritis in one compartment - a unicompartmental knee replacement, as the name implies, only resurfaces half of the knee. It is pointless if there is widespread arthritis. When there is arthritis in all three parts of the knee, a total knee replacement is the only suitable procedure.

You will be relying on your surgeons knowledge and experience to choose the most appropriate course of treatment for you. The more you understand the more relaxed you will feel. It also helps your rehabilitation if you understand why you are doing the exercises prescribed to you.

For information on living with arthritis of the knee, and advice on both surgical and non-surgical treatment options, visit The Arthritic Knee

For information on the use of offloading braces for unicompartmental osteoarthritis, click here.


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    • profile image

      Joe Hamilton 2 years ago

      I had a MRI in 2004 of my right knee (my HMO approved of the MRI when I had knee cap pain from weight lifting, and the MRI tech told me the cartilage was thin). Now (started after a 5/2/15 100 mile ride that I was training for and have done 15-17 times in the last 20 years) I'm experiencing a chronic dull ache in my left knee when I'm training hard on my bicycle (I've been a cyclist for 35+ years, and have never had knee joint or knee cap pain from biking).

      Would having a MRI on both knees and comparing the 2004 MRI give my doctor and PT a better idea of the cause of my knee pain?

      No Xrays yet, but I was thinking the MRI comparison would give me a clearer understanding of the cause, and how to restructure my training goals and justifying cutting back on competitive cycling events, hill climbs, spin classes, and step aerobics classes if these activities are going to cause accelerated damage to my knee cartilage.

      I want to avoid Knee replacement when I retire in a few years (I'm 59), and would like to continue my 150-185 miles per week into my retirement years.

    • arthriticknee profile image

      arthriticknee 6 years ago

      Significant metal work such as a rod in your thigh (known as an intramedullary nail) usually has to be removed for a traditional knee replacement to be performed. This is especially the case if the rod and associated locking screws go all the way down close to the knee joint.

      Getting metalwork out can be very difficult especially when it has been in for many years. Unfortunately, it is possible that a knee replacement is not an option for you.

      If you are concerned you should consider a second opinion.

    • profile image

      RICHARD 6 years ago



    • arthriticknee profile image

      arthriticknee 7 years ago

      If you have advanced arthritis in all three compartments (grade IV also known as "bone-on-bone") surgically the only viable option is likely to be joint replacement. This does not mean that you have to have one. If you can manage without it do so.

      Other operations like a high tibial osteotomy and autologous chondrocyte implantation are unlikely to help due to the extensive nature of your arthritis.

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      Charlotte 7 years ago

      I have osteoarthritis in all compartments of both knees, m 47years and had a (r) knee scope, is a knee replacement my only solution.