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)
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.
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.
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.
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.
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Quote from peer-reviewed paper:
" Authors "found a TT-TG distance of 13 mm to be the best cutoff value to use for PFI diagnosis"...and "...accuracy of the MRI-based TT-TG distance as an indicator of PFI risk to be less than that reported for CT scan"
Citation: Vairo GL, Moya-Angeler J, Siorta MA, Anderson AH, Sherbondy PS. Tibial Tubercle-Trochlear Groove Distance Is a Reliable and Accurate Indicator of Patellofemoral Instability. Clin Orthop Relat Res. 2019 Jun;477(6):1450-1458. doi: 10.1097/CORR.0000000000000711. PMID: 31094842; PMCID: PMC6554121.
Forum discussions
- Re: Mpfl reconstruction for chronic pain
Discussion with patient regarding options for a maltracking patella.
Relevant material -
Video and paper -
2017 - The J-Sign - by Dr Lars Blønd (Knee Surgeon)
2017 - TT-TG distance - by Dr Lars Blønd (Knee Surgeon)
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 -
- 2016 - First time patellar dislocation in children – risk factors Authors: Askenberger M et al. - and summarised by Dr Sheila Strover (Clinical Editor)
- 2015 - Current Concepts for Patellar Dislocation Authors: Petri M et al. - and summarised by Dr Sheila Strover (Clinical Editor)
eBook - Factors contributing to an Unstable Patella
Authored by Dr Lars Blønd (Knee Surgeon). Free to download.