How knee arthritis progresses

The joint cartilage damage in the arthritic knee progresses and can be monitored in case intervention could be useful to slow its progress.

Page updated July 2023 by Dr Sheila Strover (Clinical Editor)

 

This Primer explains how joint (articular) cartilage damage can lead to osteoarthritis in the knee.

 

Osteoarthritis of the knee progresses through stages.

Joint cartilage is a strange material. It is able to withstand repetitive movement over many years, but at the same time it is relatively fragile.

articular cartilage of knee joint

The cells of joint cartilage are not locked together like most tissues, but are suspended alone or with one or two other cells in little bubbles (lacunae) within a background matrix like a soft plastic, but within which is suspended a mesh of tiny fibres.

The matrix gives the joint cartilage its slipperiness; the fibrils its strength.

What you don't see in joint cartilage is blood vessels - the cartilage is dependent for its nutrition on nutrients filtering in from blood vessels in the bone underneath the joint cartilage or from the fluid within the joint space. So if cartilage is damaged it is in a precarious position - the cells are unable to quickly multiply to cover the defect and nutrition can easily become compromised.

In its advanced stages osteoarthritis IS a destructive disorder. But it doesn't start that way. The joint destruction of arthritis follows a set pattern.

 

Stages of cartilage destruction

Grade I

Illustration of early joint softening

EARLY - softening of the joint cartilage (chondromalacia)

During arthroscopy the surgeon is able to feel the softening of the joint surface when he/she presses it with the tip of an instrument (a 'probe'). At this stage the joint surface has not actually broken down but is under stress and boggy with fluid. At a later stage the cartilage is evidently boggy without being probed.

Grade II

Illustration of cartilage fissuring

LATER - fissuring of the joint cartilage

Often these fissures or cracks cannot be seen - the surgeon will reveal them when probing with the probe. Although this looks minor, the cartilage surface is now on its way to arthritis.

Grade III

BEGINNING TO BE SERIOUS - crabmeat appearance of cartilage("fibrillation")

Illustration of cartilage fibrillation

Note that the damage is still confined to the cartilagenous ('gristle') covering of the bone. The bone is still intact. At this stage surgical intervention may still allow the cartialge to heal (albeit poorly) (eg. abrasion arthroplasty (shaving the surface), microfracture (poking holes in the surface to encourage blood vessels).

Now the damaged joint surface breaks into loose and friable fronds, which release enzymes that further aid the joint surface destruction.

Grade IV

Illustration of deep cartilage damage

SERIOUS - exposure of and damage to underlying bone

Once the destruction has progressed through into the underlying bone the prospect of healing without surgical intervention is nil.

 

Note that osteoarthritis is a progression and a skilled practitioner may be able to intervene at any stage and slow down or arrest the progress. The surgeon will attempt to stage or grade the degree of damage, as this helps to dictate what options are available to improve the situation.

 

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Author

Dr Sheila Strover

Clinical Editor

Dr Sheila Strover is the Founder and Clinical Editor of the KNEEguru website.

Her medical studies were completed at the University of the Witwatersrand in South Africa - BSc(...read more

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