TT-TG distance is a measurement, derived from CT or MRI scans, which gives an indication of the sideways alignment of the groove in which the kneecap slides compared to the attachment point of its tendon on the tibia.

Page updated May 2024 by Dr Sheila Strover (Clinical Editor)

TT-TG distance

TG = trochlear groove, and for the measurement it is identified via the scans at its deepest point. TT = tibial tubercle, where the patellar tendon attaches.

 

How to measure TT-TG distance

Two imaging 'slices' from different levels (one at the TG level and the other at the TT level) are overlaid within the computer software to measure this distance.

Since the patellar tendon is attached at its lower end to the tibial tubercle, but runs along the groove as the knee bends and straightens, it is clear that a greater TT-TG distance puts strain on the whole structure.

 

TT-TG distance

The TT-TG distance is the distance measured from side to side, as if you dropped a 'plumb line' down from the deepest part of the groove.

Back to top

 

What is the normal TT-TG Distance?

The TT-TG distance is normally about 9mm, and if it´s more than 15-20mm then that means that the alignment of the groove with the tendon attachment is poor, tending to pull the kneecap over to the outer side.

Back to top

 

Why is the TT-TG measurement important?

Increased TT-TG distance is often noted in patients with patellofemoral pain syndrome.

The finding implies that the poor alignment of the the tendon to the groove is stressing the patella. Calculation of the TT-TG distance is a normal part of the evaluation of a person with a painful or unstable kneecap.

 

Back to top

 

Forum discussions

Back to top

 

Synonyms: 
Tibial tubercle - trochlear groove distance
-

Dr Sheila Strover (Editor)
BSc (Hons), MB BCh, MBA

See biography...


Relevant material -


Video and paper -

Dr Lars Blønd2017 - The J-Sign - by Dr Lars Blønd (Knee Surgeon)

2017 - TT-TG distance - by Dr Lars Blønd (Knee Surgeon)


 

Journal interpretation - patellar instability

Journal interpretation - 2023 - Framing Patellar Instability: From Diagnosis to the Treatment of the First Episode

- Authors: Maggioni DM et al. and summarised for readers by Dr Sheila Strover (Clinical Editor)

Peer-reviewed paper -


 

eBook on patellar instability

eBook - Factors contributing to an Unstable Patella

Authored by Dr Lars Blønd (Knee Surgeon). Free to download.