What Is the TT-TG Distance and Why Does It Matter?

The TT-TG distance is a measurement, taken on a CT scan or MRI, that captures how far the tibial tubercle (the bony bump below your kneecap where the patellar tendon attaches) sits to the outside of the trochlear groove (the channel in the thighbone that the kneecap is supposed to glide through). When that offset is too large, the kneecap gets pulled sideways during movement, raising the risk of dislocation and chronic pain. It has become one of the most commonly ordered measurements in the workup of an unstable or painful kneecap, and it often determines whether surgery is recommended.

The Basic Anatomy Behind the Measurement

Your kneecap rides in a shallow valley on the front of the femur called the trochlear groove. Every time you bend or straighten your knee, the patella slides up and down inside that groove. The patellar tendon, which connects the bottom of the kneecap to the tibial tubercle on your shinbone, acts like a tether pulling the kneecap downward and slightly outward. If the tibial tubercle sits too far to the lateral (outer) side relative to the center of the groove, the tendon’s pull angle becomes more extreme, and the kneecap is effectively being dragged out of its track with every step.

The TT-TG distance puts a number on that mismatch. On imaging, a clinician identifies the deepest point of the trochlear groove on one axial slice and the center of the tibial tubercle on another, then measures the horizontal distance between vertical lines drawn through each. A small number means the tubercle lines up well with the groove. A large number means the kneecap’s anchor point is offset, creating a lateral pull that can destabilize the joint. Reliability studies have found that orthopaedic surgeons and radiologists with varying levels of experience can measure this distance consistently on MRI, with inter-rater agreement generally in the “good” to “almost perfect” range.¹²

Why It Matters for Kneecap Instability

A kneecap that dislocates, meaning it slips completely out of the trochlear groove, usually goes laterally. There are several structural reasons this can happen: the groove itself may be too shallow (trochlear dysplasia), the kneecap may sit too high (patella alta), or the ligament on the inner side of the knee that normally holds the patella in place (the MPFL) may be torn. An increased TT-TG distance is another key piece of that puzzle. A systematic review found that trochlear dysplasia, an elevated TT-TG distance, and patella alta were all statistically significant risk factors for both first-time and recurrent patellar dislocations.³

Research comparing knees in patients with instability on only one side found that the unstable knee had a greater TT-TG distance than the stable opposite knee, along with worse patellar tracking and more lateral tilt. The association was strong enough to suggest that a laterally positioned tibial tubercle is not just a bystander but a direct contributor to instability episodes.⁴

Biomechanical modeling adds detail to the story. One study examined how shifting the tibial tubercle laterally affects contact pressure inside the joint. Lateralizing the tubercle to produce a TT-TG in the 20–25 mm range increased peak contact pressure by roughly 10% compared to a reference range of 10–15 mm, while over-medializing it (0–5 mm) actually increased pressure by about 22%. In other words, the kneecap joint is sensitive to malalignment in either direction, though lateral offset is the direction that sets the stage for dislocation.⁵

What Counts as “Too High”

Thresholds differ depending on who you ask and which imaging modality was used, but in adults, a TT-TG distance above roughly 20 mm on CT has traditionally been treated as abnormal and a strong indication for surgical correction. MRI-based measurements tend to run lower, so the threshold on MRI is typically cited as around 15–16 mm for concern and 20 mm for clear abnormality.

Children present a different challenge. A pediatric study found that a cutoff of about 15 mm on MRI yielded moderate sensitivity (66%) and good specificity (82%) for predicting increased patellar instability risk in children.⁶ Importantly, the TT-TG distance is not fixed in growing patients. A study tracking adolescents found that TT-TG distance could either increase or decrease during growth, making it unreliable as a one-time snapshot for surgical planning in young patients.⁷

Normal values also appear to differ across populations. In a study of Korean adults without knee problems, the mean TT-TG distance on CT was about 10 mm, with no significant difference between men and women.⁸ Meanwhile, other studies in predominantly Western cohorts have reported higher average values. A newer approach uses a TT-TG ratio that adjusts for the width of the knee, which appears to eliminate sex-based differences and may improve diagnostic accuracy compared to a raw millimeter cutoff.⁹

CT Versus MRI Measurements Are Not Interchangeable

This is one of the biggest practical pitfalls with TT-TG distance. CT and MRI consistently give different numbers for the same knee. A meta-analysis pooling multiple studies found that CT-measured TT-TG distances averaged about 1.8 mm higher than MRI-measured ones, and the two modalities showed only fair agreement when compared head-to-head.¹⁰ A study of 59 knees put more specific numbers on it: the mean TT-TG was 16.9 mm by CT but 14.7 mm by MRI. Among patients who were classified as surgical candidates based on a CT reading over 20 mm, the average MRI reading was only 18.7 mm, a difference of nearly 4 mm.¹¹

The reason is partly technical. CT measures bony landmarks, while MRI can visualize cartilage, and the deepest point of the cartilaginous groove does not always line up with the deepest point of the bone underneath. This gap means that applying a 20 mm cutoff blindly to both modalities could lead to different surgical decisions depending on which scan was ordered. The current consensus is that different thresholds should be used for CT and MRI, though exact numbers are still debated.

The Measurement Changes with Knee Position

Standard imaging is done with the knee nearly straight and the patient lying down, but your knee does not live in that position. Studies examining TT-TG distance at varying degrees of flexion have found that the number drops steadily as the knee bends. One study in adolescents reported a decrease of about 0.3 mm for every additional degree of flexion.¹² This makes intuitive sense: as the knee bends, the tibia rotates slightly inward and the patella engages more deeply in the groove, effectively reducing the lateral offset.

Weight bearing makes an even bigger difference. In one study, the mean TT-TG while lying with the knee straight was about 11.6 mm, but dropped to 6.3 mm when the same subjects stood with their knees extended, and fell further to 4.9 mm when standing with knees bent to 30 degrees.¹³ That means a supine MRI performed with the knee near full extension captures the worst-case scenario for lateral offset, which is partly by design, since dislocations most often occur near full extension. But it also means the number on your report does not represent how your knee behaves during most daily activities.

When Other Anatomy Complicates the Picture

An elevated TT-TG distance does not always mean the tibial tubercle itself is in the wrong place. The measurement is a composite: it can be high because the tubercle is too lateral, because the groove is too medial, or because the bones of the leg are rotated in ways that exaggerate the offset. Femoral torsion (the twist of the thighbone) and tibial torsion (the twist of the shinbone) both correlate with TT-TG distance, and patients with severe trochlear dysplasia tend to have higher TT-TG values partly because the groove itself is positioned abnormally.¹⁴

A three-dimensional analysis illustrated this starkly. In patients with high-grade trochlear dysplasia (Dejour types C and D), the “real” three-dimensional TT-TG was substantially greater than what the traditional two-dimensional measurement showed. In type C dysplasia, the 3D value averaged about 24 mm versus 17 mm on the traditional measurement; in type D, it was about 29 mm versus 22 mm.¹⁵ The conventional measurement, in other words, can understate how misaligned things really are when the groove itself is deformed.

Not Just Dislocations — Patellofemoral Pain

You do not have to dislocate your kneecap for a high TT-TG distance to cause problems. Patellofemoral pain syndrome, the vague anterior knee pain that accounts for a large share of orthopaedic knee visits, has also been linked to elevated TT-TG values. A study comparing patients with patellofemoral pain to healthy controls found a mean TT-TG of about 12.3 mm in the pain group versus 9.3 mm in controls.¹⁶ An adolescent female cohort showed a similar pattern, with pain patients averaging about 12.2 mm and controls about 10 mm. A subgroup with the most severe maltracking had TT-TG values around 14.2 mm, well above controls.¹⁷

Even under weight-bearing kinematic MRI, patients with patellofemoral pain demonstrated higher TT-TG distances than healthy volunteers, with the gap widening as the knee approached full extension.¹⁸ These differences are smaller than what you see in dislocation patients, and no one is recommending surgery for pain patients based on TT-TG alone. But the finding does suggest that even modest malalignment can contribute to abnormal loading and chronic symptoms.

The Alternative Measurement — TT-PCL

Because the TT-TG distance is influenced by the shape and position of the trochlear groove, some researchers have proposed measuring the tibial tubercle relative to a more stable bony landmark: the posterior cruciate ligament attachment on the tibia (the TT-PCL distance). The logic is that TT-PCL captures only how lateral the tubercle is on the shin, without being confounded by a malformed or shifted groove on the femur. In theory, comparing TT-TG and TT-PCL together would help surgeons distinguish whether the problem is a lateralized tubercle, an abnormal groove, or both.¹⁹

In practice, however, systematic reviews have found that TT-TG still outperforms TT-PCL for predicting patellar instability. When researchers pooled studies that directly compared the two measurements, TT-TG had better discriminatory power (as measured by the area under the curve) and higher odds ratios for identifying unstable knees. The two measurements had similar reproducibility between raters, so the issue is not reliability but diagnostic usefulness.²⁰ TT-PCL still has a role, though. When TT-TG is elevated and the surgeon needs to figure out whether to move the tibial tubercle, reshape the groove, or both, adding TT-PCL to the assessment helps tease apart the contributions of each side of the joint.

When Surgery Enters the Conversation

If TT-TG distance is high enough and a patient keeps dislocating or has failed rehab, surgeons may recommend a tibial tubercle osteotomy (TTO). The procedure involves cutting the bony prominence where the patellar tendon attaches and shifting it medially, sometimes also moving it slightly downward or forward, then fixing it with screws. The goal is to reduce the TT-TG distance and realign the pull of the extensor mechanism. A retrospective study with ten years of follow-up reported that TTO reduced the median TT-TG from about 23 mm before surgery to 15 mm afterward, with significant improvements in patellar tilt and displacement as well. Functional scores (Kujala and Lysholm) were high at follow-up, with medians around 89–91.²¹

TTO consistently provides symptom relief, with most patients able to return to work within about three months and sport within six.²² But complications are not trivial. One large analysis found an overall complication rate of about 28%, with major complications in nearly a quarter of cases. The most frequent problems were painful hardware requiring removal (about 7%), superficial infection (about 6%), and stiffness requiring a second trip to the operating room (5%). Having had previous surgery on the same knee was a significant predictor of complications, and headed screws were associated with higher rates of hardware-related pain.²³ In that same ten-year follow-up mentioned above, hardware removal was needed in half of patients, and recurrent dislocation occurred in about 14%, mostly in younger females with residual malalignment.²¹

When trochlear dysplasia is the dominant problem rather than an isolated lateral tubercle, some surgeons favor trochleoplasty (reshaping the groove) over TTO. A meta-analysis comparing the two procedures for patients with instability and trochlear dysplasia found that both improved functional scores, but trochleoplasty had far fewer recurrent instability events (3 versus 21 across the studies pooled).²⁴ The choice is not always one or the other; combined procedures are common when multiple anatomical factors are at play.

Pediatric and Adolescent Considerations

Patellar instability tends to peak in teenagers and young adults, yet operating on a growing skeleton introduces extra risk. Tibial tubercle osteotomy is generally avoided in children whose growth plates are still wide open, because cutting into or near the growth plate can cause growth disturbance. Instead, surgeons often reconstruct the MPFL (the ligament on the inner side of the kneecap) and sometimes add a soft-tissue procedure called a Grammont to redirect the patellar tendon’s pull without touching bone. In pediatric patients with elevated TT-TG, adding the Grammont to MPFL reconstruction showed a trend toward lower recurrent instability and fewer revision surgeries compared with MPFL reconstruction alone, without increasing complication rates.²⁵

A separate study in adolescents with high TT-TG values compared MPFL reconstruction alone to MPFL reconstruction plus TTO, and found similar complication rates, recurrent instability, and return-to-sport rates between the two groups.²⁶ That finding has fueled debate about whether adding the bone procedure is worth the extra recovery and complication risk in younger patients, especially when TT-TG values may still change as the skeleton matures.

Reliability Quirks Worth Knowing

Despite being called a reliable measurement, TT-TG distance has a few reproducibility wrinkles that can matter at the margins. The biggest source of variability is slice selection: which axial image the reader chooses to identify the deepest point of the groove and the center of the tubercle. One study found that when raters independently chose their own slices, inter-rater reliability for TT-TG was good (ICC 0.79), and it improved slightly (ICC 0.85) when slices were preselected.²⁷ Interestingly, the improvement with preselection was not statistically significant for TT-TG itself, though it did help with other groove measurements like sulcus angle.

An alternative method using the patellar tendon insertion (the PTTG distance) rather than the bony tubercle has shown slightly better inter-rater reliability on MRI. One four-rater study found an ICC of 0.86 for PTTG versus 0.70 for TT-TG, a statistically significant gap.²⁸ Because the patellar tendon is a soft-tissue structure more easily seen on MRI than a bony prominence, there is less ambiguity about where to place the measurement point. Whether this improved reliability changes clinical outcomes is still an open question, but it may matter for borderline cases where a few millimeters determine whether a procedure is recommended.