Trochlear Dysplasia

Trochlear Dysplasia means that the groove (trochlea) for the kneecap (patella) is too flat or even dome-shaped. This is the most important factor for loose kneecap. 
About Trochlear Dysplasia

Below you can find four examples on MRI scans, demonstrating the most upper part of knee with a shallow groove, flat or even dome shaped groove, and this is called Trochlear dysplasia. Trochlear dysplasia can lead to both patella dislocation, anterior knee pain and cartilage breakdown. The thinning of the cartilage is based on too much load on the cartilage and therefore it also predispose to osteoarthritis . Why do you get trochlear dysplasia? Basically this is unknown, however there is some heredity and also breech position during foetal life may also have importance. I have been told by a patient having trochlear dysplasia that the history of loose kneecap could be traced seven generations back in time. Sheila Strover can explain so it is more easy to understand - Click here

Unfortunately is trochlear dysplasia very often overlooked by the radiologist or/and the orthopaedic surgeon on the MRI and X-rays

Lateral Trochlea Inclination angle

The lateral trochlea inclination angle is regarded as the most important measurement for evaluation of Trochlear dysplasia. 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, thereby preventing for coming out of the joint. Further if the angle is below 11 degress it means trochlear dysplasia (shallow groove). Pleae notice the examples below - left is normal - right demonstrates trochlear dysplasia. Read more here

Patella Tilt

When the knee cap tilts, it´s rarely because the lateral ligament is to tight. Importantly the tilt is caused by trochlear dysplasia, meaning there is too much bone in the trochlea. Consequently this causes the knee cap to only articulate on the outer part of the knee cap. 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, since it reduces the pressure.

Normal knee

Patella tilt - Trochlear dysplasia

Patella tilt - Trochlear dysplasia

TT-TG distance

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. 

No Trochlear dysplasi

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

This is 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.

Dejour Classification

Henry Dejour has made a classification that was later updated by Tecklenburg and David Dejour, having four subgroups. The classification is based upon x-ray examination. It contains Dejour type A,B,C and  D. Since we mainly uses MRI or CT scans today and since the classification is with rather low inter correlation (means that is not precise), I use instead the lateral trochlea inclination angle, together with some other measurements, such as Anterior Posterior measurements from Biedert and trochlea asymmetry.

Bieder Anterior Posterior measurement

If you have trochlear dysplasia and you are troubled by patellar instability read more here if you are troubled by anterior knee pain read more here

If you want to read a scientific paper about the evaluation of trochlear dysplasia you can read this paper PDF Version