Trochlear dysplasia basically means that the groove for the kneecap have not become deep enough. The definiton for trochlear dysplasia is an abnormal shallow, flat or even dome shaped groove. If you have trochlear dysplasia this can lead to both patella dislocation, anterior knee pain and cartilage breakdown. Your tendency to cartilage breakdown is based on too much load in this special part of the knee. Cartilage breakdown predispose to osteoarthritis.
Basically we do not have the answer for this yet. However there is some heredity involved. But breech position during foetal life may also play a role. Once I was told by a patient having trochlear dysplasia and loose kneecaps, that in her family, loose kneecaps could be traced seven generations back. Sheila Strover can explain so it is more easy to understand - Click here.
Since humans are walking upright we have developed from other animals, and the result is that the thigh muscle constantly is pulling the kneecap outwards. To compensate for this high force, humans have developed this groove to provide the kneecap stability and containment. If the groove is missing the kneecap tends to go to the outer site - called a kneecap dislocation.
The lateral trochlea inclination angle is regarded as the most important measurement for evaluation of Trochlear dysplasia. This angle says something about the osseous support for the kneecap. This is the angle of the outer part of the groove (Trochlea), compared to line in the back of the knee between the two femur condyles. In other words the more steep the outer edge of groove, the more stability it provides for the kneecap. A high angle prevents the kneecap for coming out of the joint. Further if the angle is below 11 degress it means trochlear dysplasia (shallow groove). Please notice the examples below - left is normal - right demonstrates trochlear dysplasia. Read more here.
Dejour is a very experienced and skilled orthopedic surgeon. He has made a classification based upon x-rays, having four subgroups. It contains Dejour type A,B,C and D. Today we mainly uses MRI or CT scans instead of x-rays. And also since the classification is not so precise, it is slowly out-faded. Instead the lateral trochlea inclination angle have been more accepted. Also some other measurements can be used.
When your kneecap tilts, it is sometime because the outer kneecap ligament is to tight, but actually quite rare. Importantly your tilt is most often caused by trochlear dysplasia. When you have too much bone in the trochlea, consequently this causes your kneecap only to articulate on the outer part, as you might see below. Moreover this typically gives anterior knee pain or hyperpressure syndrome, based on to much pressure in the patellofemoral joint. In these cases trochleoplasty could be the right solution for you, since it reduces the pressure by unloading your joint.
Patella tilt - Trochlear dysplasia
Another Patella tilt - Trochlear dysplasia
This means Tuberositas Tibia - Trochlear Groove distance. The measure is done by CT or MRI scans. It explain how much offset the groove has in relation to where the patella tendon attaches to the tibia (Tuberositas Tibia). TT-TG distance is normally about 9 mm and if it´s more than 14 mm on MRI scans or 15-20 mm on CT scans, it´s too high.
This is a MRI picture that demonstrates a slice of a normal knee, demonstrating the kneecap and the trochlear groove
Severe increased TT-TG distance on a CT scan in a knee having a degree of trochlear dysplasia
A drawing of how you measure the TT-TG distance. Two axial slices from the MRI or CT scan are overlapping or superimposed and thereby you can measure the distance.
Just a drawing of the same picture at left, demonstrating a TT-TG distance above 40 mm which is extreme. Dejour type D.