Normal Tibial Tubercle-Trochlea Distance Size on Radiography
The tibial tubercle-trochlea (TT-TG) distance quantifies the lateral offset of the tibial tubercle relative to the trochlear groove, reflecting the functional Q-angle of the extensor mechanism. Accurate measurement is essential in evaluating patellar maltracking, instability, and planning corrective osteotomy procedures.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | <2 cm |
Clinical Significance
A TT-TG distance of less than 2 cm is considered within normal limits on lateral radiography. Values exceeding this threshold indicate increased lateral tibial tubercle offset, which predisposes patients to lateral patellar subluxation and recurrent dislocation. Surgical correction — typically tibial tubercle transfer (Fulkerson or Elmslie-Trillat osteotomy) — is often considered when TT-TG exceeds 2 cm in symptomatic patients.
It is important to recognize that TT-TG measurement on plain radiography represents a two-dimensional approximation; CT and MRI are preferred for precise three-dimensional assessment in surgical planning. Rotational limb alignment, trochlear dysplasia, and patella alta are frequently coexisting factors that must be evaluated alongside TT-TG distance.
- Recurrent lateral patellar dislocation
- Chronic patellar instability with maltracking
- Trochlear dysplasia (Dejour classification)
- Patellofemoral pain syndrome with lateral tilt
- Post-traumatic extensor mechanism malalignment
Reference: Robinson P. Essential Radiology for Sports Medicine. Springer Science+Business Media. (2010).
Imaging Notes
On a true lateral radiograph of the knee, the TT-TG distance is estimated by identifying the anterior tibial tubercle and projecting it relative to the deepest point of the trochlear groove. Strict lateral positioning is critical — any rotation introduces significant measurement error by artificially altering the apparent offset. A properly obtained lateral view shows superimposition of the posterior femoral condyles.
While plain radiography provides a rapid screening estimate, CT (axial cuts with knee in extension) or MRI remains the gold standard for definitive TT-TG measurement, allowing simultaneous assessment of cartilage, trochlear morphology, and soft tissue restraints. Radiographic measurement should be interpreted with awareness of its inherent limitations in three-dimensional accuracy.