Patellofemoral Cartilage Damage
The patellofemoral joint is the part of the knee where the back of the kneecap (the patella) slides up and down the groove (the trochlear groove) at the front of the femur.
The surfaces of the bone of the back of the kneecap and the front of the femur are covered in a layer of articular cartilage that is a few millimeters thick – the articular cartilage is the smooth, white, glistening layer of tissue in the knee that makes the joint surfaces smooth and low friction. If this articular cartilage becomes overloaded and worn, then it can begin to become soft and rough, and it can then crack and begin to flake off, potentially leaving areas of exposed bare bone in the joint.
Minor damage to the articular cartilage is often referred to as ‘minor wear and tear’. However, ‘wear and tear’ and ‘degeneration’ mean the same thing – and if the damage is severe (particularly if there is any actual bare bone exposed in the joint) then this is normally referred to as arthritis (osteoarthritis).
Cartilage damage in the patellofemoral joint can develop for a variety of different reasons, including:-
- trauma (eg a direct impact onto the front of the knee, such as a fall onto the front of a bent knee onto a hard surface)
- an abnormally shaped patellofemoral joint (patellofemoral dysplasia) can cause increased rates of wear and tear
- a patella that doesn’t sit comfortably in the middle of the trochlear groove, but which instead sits off to the lateral (outer) side can rub and this can also lead to increased rates of wear and tear – this is called patellar maltracking
- if the patella is unstable and dislocates, then each time it dislocates it can cause damage to the articular cartilage
- some people have normal-shaped patellofemoral joints, with normal tracking and no history of any trauma, but they develop patellofemoral arthritis anyway – this can be partly genetic (primary osteoarthritis) and partly just from simple wear and tear.
Damage to the articular cartilage in the patellofemoral joint causes increase stress and pressure on the underlying bone (the cartilage itself does not have any nerve fibres in it, but the underlying bone does), and this causes pain at the front of the knee (‘anterior knee pain’). This pain tends to be aggravated by anything that causes increased loading in the patellofemoral joint, such as things like –
- stairs (especially coming down)
- sitting for long periods with the knee bent
- heavy weights
- impact (running/jumping)
Roughening of the articular cartilage in the patellofemoral often manifests itself as a crunching sound in the knee that is heard particularly on stairs (like the crunching of walking through fresh snow). This is very common, and if it is not actually associated with pain then it is rarely of any real concern.
Pain at the front of the knee that is either severe or getting worse, and which is affecting (restricting) actual function is significant, and warrants further investigation.