What it is.
The kneecap runs in a groove at the front of the thigh bone. Both surfaces are covered in articular cartilage, and the forces across that joint are high: several times body weight when descending stairs, and higher again standing up from a low seat. Patellofemoral arthritis is the loss of that cartilage.
It can occur in isolation, which is less common, or as part of arthritis affecting the whole knee, which is more common. Isolated patellofemoral arthritis has a distinct profile: it tends to affect a younger group, more often women, and it is frequently associated with a history of kneecap instability or an underlying shape abnormality of the groove.
Why the symptoms are different.
Arthritis in the main weight-bearing compartments hurts when you load the knee walking. Patellofemoral arthritis hurts when you bend the knee under load, which is a different set of activities:
- Going down stairs more than up.
- Standing up from a low or soft chair.
- Getting out of a car.
- Squatting or kneeling.
- Sitting still for a long time with the knee bent, sometimes called the theatre sign.
Walking on the flat may be comfortable, which is why people with this pattern are sometimes told their knee is fine.
Why it gets missed.
A standard knee X-ray series does not always show the patellofemoral joint well. The joint is best seen on a skyline view, taken looking down the front of the flexed knee. If that view is not requested, significant patellofemoral arthritis can be present on a set of films reported as showing only mild changes.
If you have front-of-knee pain with the pattern above and your imaging did not include a skyline view, that is worth raising.
Treatment.
Non-operative management is the starting point and is often effective. Physiotherapy targeting the quadriceps, particularly control through the range where the kneecap engages the groove, is central. Load modification matters: avoiding deep squatting and using the handrail or a higher seat changes the forces substantially. Weight management has a disproportionate effect here given the force multiples involved. Injections can help symptomatically.
Where symptoms persist and are limiting, and imaging confirms the arthritis, total knee replacement with resurfacing of the patella reliably relieves patellofemoral pain. Isolated patellofemoral replacement exists as an operation but has a higher revision rate in registry data and is not performed in this practice.
Where this practice fits.
Professor Rodda's practice is focused on hip and knee replacement. He does not perform joint-preserving surgery such as arthroscopy or osteotomy. If your condition is at a stage where preservation surgery is the right option, you will be told that and referred to a colleague who does it, within Sunshine Coast Orthopaedic Group or elsewhere. Where the joint has reached the point that replacement is the reasonable option, that is the work done here.
Common questions.
Why does my knee hurt on stairs but not walking?
Because the patellofemoral joint is loaded when the knee bends under weight, not when it is straight. Descending stairs, standing from a low chair and squatting all load it heavily, while walking on the flat loads it comparatively little. That pattern is characteristic of patellofemoral arthritis.
My X-ray was reported as mild but my knee is very painful. Why?
A standard knee series may not include a skyline view, which is the view that shows the patellofemoral joint properly. Significant arthritis there can be present on films otherwise reported as mild. Ask whether a skyline view was taken.
What is the theatre sign?
Pain and stiffness at the front of the knee after sitting for a long time with the knee bent, named for what people notice at the cinema. It is a common feature of patellofemoral problems.
Can I have a partial replacement for just the kneecap?
Isolated patellofemoral replacement exists, but Australian registry data shows a higher revision rate than total knee replacement, and it is not performed in this practice. Where symptoms justify surgery, a total knee replacement with patellar resurfacing is the more reliable operation.
Will strengthening my quadriceps make it worse?
Not if it is done appropriately. The programme is adjusted to work in the ranges that do not compress the worn area, which usually means avoiding deep flexion under load. A physiotherapist will set the range for you.
References.
- Royal Australian College of General Practitioners. Guideline for the management of knee and hip osteoarthritis. 2nd edn. East Melbourne: RACGP.
- Australian Orthopaedic Association National Joint Replacement Registry. Annual Report. Adelaide: AOA.
- Crossley KM, Callaghan MJ, van Linschoten R. Patellofemoral pain. BMJ. 2015;351:h3939.
This page provides general information about a medical condition and a surgical procedure. It is not individual medical advice and does not replace consultation with a qualified practitioner. Outcomes vary between individuals and all surgery carries risk. A referral is needed for a consultation with Professor Daevyd Rodda; without one, you can start with the practice nurse practitioner, who can provide it.