The Tibial Tubercle-Trochlear Groove (TT-TG) distance is an orthopedic measurement used to evaluate the alignment of the knee’s extensor mechanism. This metric quantifies the horizontal relationship between the patellar tendon attachment point and the central guiding groove of the thigh bone. It serves as an indicator for assessing patellar malalignment and instability, particularly when patients experience pain or recurrent kneecap dislocation.
The Key Anatomical Structures
The TT-TG measurement connects two specific anatomical landmarks. The Tibial Tubercle (TT) is the bony prominence on the front of the tibia (shin bone), just below the kneecap. This prominence serves as the anchor point where the patellar tendon attaches, transmitting forces from the quadriceps muscle to straighten the knee.
The Trochlear Groove (TG) is the V-shaped channel at the lower end of the femur (thigh bone). This groove acts as a natural guide rail for the patella (kneecap), which is designed to glide smoothly within it as the knee bends and straightens.
The TT-TG distance measures how far the tibial tubercle is positioned laterally (toward the outside of the leg) relative to the deepest point of the trochlear groove. If the tubercle is positioned too far laterally, it applies an excessive lateral pull on the patellar tendon and the kneecap. This lateral force vector is a primary mechanical cause of patellar maltracking and instability.
Imaging and Measurement Techniques
Accurately determining the TT-TG distance requires cross-sectional imaging, as standard two-dimensional X-rays cannot precisely capture the three-dimensional relationship between the two landmarks. Historically, the measurement was taken using Computed Tomography (CT) scans, which provide high-resolution images of the bone structure. The technique involves identifying the deepest point of the trochlear groove on one axial image and the center of the tibial tubercle on another axial image, typically with the knee in full extension.
A common reference line, often drawn across the posterior femoral condyles, is established to ensure measurements are taken along the same perpendicular plane. The horizontal distance between the projected center of the tibial tubercle and the projected deepest point of the trochlear groove is then measured in millimeters. The result reflects the mediolateral offset between the tubercle and the groove.
Today, Magnetic Resonance Imaging (MRI) is often the preferred method because it avoids the use of ionizing radiation and provides superior visualization of soft tissues, such as the articular cartilage and ligaments. However, a systematic bias exists between the two imaging modalities. MRI measurements of the TT-TG distance are consistently lower than those taken on CT scans, sometimes by an average of 2 to 3 millimeters.
This difference is primarily attributed to variations in patient positioning during the scan. The knee may be placed in a slightly different rotation or varus alignment in an MRI machine compared to a CT scanner. Because of this technique-dependent difference, the numerical thresholds used for clinical decision-making must be adjusted based on whether the measurement was derived from a CT or an MRI study.
Clinical Significance and Interpretation
The TT-TG distance is a crucial parameter because an increased measurement signifies a physical malalignment that can lead to significant problems within the patellofemoral joint. When the tibial tubercle is positioned too far laterally, the kneecap is pulled outward as the knee moves, causing it to track abnormally outside the trochlear groove. This phenomenon is known as patellar maltracking, and it can cause chronic pain and damage to the cartilage.
Clinicians typically use established numerical cutoffs to interpret the measurement, though these ranges are based primarily on historical CT data. A TT-TG distance of less than 15 millimeters on a CT scan is generally considered within the normal range for an adult. Measurements falling between 15 and 20 millimeters are often classified as borderline, suggesting a mild to moderate risk of maltracking. A distance greater than 20 millimeters on a CT scan is accepted as an abnormal or pathological finding.
This significant lateralization dramatically increases the risk of recurrent patellar instability, which involves the kneecap partially or completely dislocating from the trochlear groove. For MRI measurements, the threshold values are lower; a distance of 13 millimeters or more may be the optimal cutoff for identifying patients at risk for patellofemoral instability.
While an increased TT-TG distance is a strong risk factor for instability, it is not the sole determinant of a patient’s condition. Recent research emphasizes that the measurement is influenced not only by the position of the tibial tubercle but also by factors like femoral rotation and the depth of the trochlear groove itself, known as trochlear dysplasia. Therefore, a complete diagnosis requires considering the TT-TG distance alongside other anatomical risk factors and the patient’s clinical symptoms.
Treatment Approaches for Abnormal Distance
Treatment for an abnormal TT-TG distance depends on the severity of the malalignment and the patient’s symptoms, especially the frequency of kneecap instability. For patients with borderline measurements and pain without dislocation, conservative management is the first course of action. This involves physical therapy focused on strengthening the hip and thigh muscles, particularly the vastus medialis obliquus, to dynamically stabilize the kneecap.
When conservative measures fail, or in cases of severe recurrent patellar dislocation associated with a significantly increased TT-TG distance, surgical intervention is necessary. The primary procedure to correct this bony malalignment is a tibial tubercle osteotomy (TTO). In a TTO, the surgeon cuts and repositions a segment of bone containing the tibial tubercle.
The bone segment is moved medially to decrease the TT-TG distance, which reduces the lateral pull on the kneecap and improves its tracking. The segment may also be moved anteriorly to reduce pressure within the joint. This procedure is often performed in conjunction with other soft tissue repairs, such as reconstruction of the medial patellofemoral ligament, to provide comprehensive stability to the knee.