What it is.
The kneecap runs in a groove at the front of the thigh bone, called the trochlea. It is held there by the shape of that groove, by the pull of the quadriceps, and by a ligament on the inner side called the medial patellofemoral ligament. Instability means the kneecap leaves the groove, or comes close enough that the knee feels unreliable.
It almost always goes outwards, because the quadriceps pulls slightly laterally and the inner restraint is the weaker side.
It is usually about shape, not injury.
A first dislocation often follows a twisting movement rather than a blow, and the question worth asking is why that movement was enough. In most cases there is an underlying anatomical predisposition:
- Trochlear dysplasia. A groove that is shallow or flat rather than properly V-shaped. The commonest single factor.
- Patella alta. A kneecap that sits high, so it engages the groove later in flexion and is unprotected through early bend.
- Increased offset between the groove and the attachment of the patellar tendon on the shin, which increases the sideways pull.
- Generalised ligamentous laxity.
This is why first-time dislocation in someone with normal anatomy behaves very differently from the same event in someone with a dysplastic groove: the recurrence rates are not comparable.
Treatment.
A first dislocation without a loose fragment is usually managed without surgery: a short period of protection, then physiotherapy directed at quadriceps control, hip abductor strength and landing mechanics. Many people do not dislocate again.
Recurrent instability is treated surgically, most often by reconstructing the medial patellofemoral ligament, with bony realignment added where the underlying anatomy demands it. That work is done by knee surgeons who specialise in patellofemoral problems.
Why it is on this site.
Repeated dislocation damages the cartilage on the back of the kneecap and in the groove, and the abnormal shape that caused the instability also loads that cartilage abnormally for decades. The result is a recognised route to patellofemoral arthritis, often presenting in the forties and fifties.
Where that arthritis becomes limiting, total knee replacement with resurfacing of the patella is the operation that reliably helps. Previous realignment surgery and retained metalwork are planned for beforehand rather than discovered during the operation.
Common questions.
Will my kneecap dislocate again?
It depends mostly on your anatomy. After a first dislocation in a knee with a normal groove, recurrence is relatively uncommon. Where there is trochlear dysplasia or a high-riding kneecap, recurrence rates are considerably higher, and each episode adds cartilage damage.
Do I need surgery after a first dislocation?
Usually not, unless a fragment of cartilage or bone has broken off, which needs assessment promptly. First-line treatment is protection followed by physiotherapy.
Does patellar instability cause arthritis?
It is a recognised cause of patellofemoral arthritis. Each dislocation damages cartilage, and the underlying shape abnormality loads the joint abnormally over years. Arthritis from this cause often appears earlier than typical.
Can a knee replacement fix an unstable kneecap?
A total knee replacement with patellar resurfacing addresses arthritis and can correct patellar tracking, but it is an operation for established arthritis, not a treatment for instability in an otherwise healthy knee.
Does Professor Rodda perform MPFL reconstruction?
No. This practice is focused on hip and knee replacement. Patellofemoral stabilisation is performed by knee surgeons who specialise in it, and you will be referred if that is the right option.
References.
- Dejour D, Le Coultre B. Osteotomies in patello-femoral instabilities. Sports Med Arthrosc Rev. 2018;26(1):8-15.
- Sanders TL, Pareek A, Hewett TE, et al. Incidence of first-time lateral patellar dislocation: a 21-year population-based study. Sports Health. 2018;10(2):146-151.
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.