2015 ISAKOS Biennial Congress ePoster #1801

The Radiological Work Up of Patients with Patellar Femoral Pathology: Should the TT-TG Distance be Measured on MRI and What is its Relationship to the TT-PCL Distance?

Cameron Michael Anley, MBCHB, M Med (Ortho), Paarl, Western Cape SOUTH AFRICA
Guy Morris, MBChB, Birmingham UNITED KINGDOM
Adnan Saithna, MBChB, MSc, BMedSc, DipSEM, FRCS, Ormskirk UNITED KINGDOM
Steve James, MBChB, FRCR, Birmingham UNITED KINGDOM
Martyn Snow, FRCS, Birmingham UNITED KINGDOM

The Royal Orthopaedic Hospital, Birmingham, UNITED KINGDOM

FDA Status Not Applicable

Summary: Accepted values for the TT-TG distance cannot be used interchangeably between CT and MRI and should not be used as an indication for surgery when measure on MRI. The TT-PCL measures true lateralisation of the tibial tubercle while the TT-TG is an amalgamated measure of true lateralisation and KJR. The clinical relevance of these findings requires further investigation.

Rate:

Abstract:

Objective

The Tibia Tubercle- Trochlear Grove (TT-TG) distance is commonly used to assess patients with patellar femoral pathology. A new measurement, the Tibial Tubercle–Posterior Cruciate Ligament (TT-PCL) distance, has shown that not all patients with a pathological TT-TG distance (>20 mm) have lateralization of the tibial tubercle. In addition the interchangeability of the TT-TG distance between CT and MRI has recently been questioned.

The purpose of our study was twofold. Firstly, to determine, with a larger population, if the TT-TG measurement can be used interchangeably between CT and MRI and secondly to confirm the correlation between the TT-PCL and TT-TG distances in determining TT lateralisation.

Methods

This was a retrospective radiographic study. Patients with patellar femoral pathology and both MRI and CT scans of the same knee were identified on the hospital PACS system. The TT-PCL and TT-TG were measured independently by two fellowship trained orthopaedic surgeons. Thirty measurements where repeated by both evaluators on a separate occasion to allow for an assessment of the intrarater reliability.

The intra-class correlation coefficient (ICC) was used to assess reliability of the measurements. Categorical variables were compared using a chi-squared test for association. A comparison of the continuous measurements between two modalities was performed using a Student’s T-Test.

Results

A total of 141 patients were included in this study, 108 females and 33 males (mean age 28.5yrs; range 10-59 yrs, SD ±11.13). The mean TT-TG was 17.72mm on CT (range 6.97-31.33mm; SD ±5.15) and 13.56mm on MRI (range 2-30.04; SD ±6.07). The mean TT-TG measurement was 4.16mm less on MRI. The ICC for each rater comparing the two imaging modalities for TT-TG was only fair (0.54 and 0.48). This would imply that the measurements are not interchangeable between CT and MRI.

The mean TT-PCL measurement was 20.32mm (Range: 10.11-32.01, SD: ± 3.45mm) with an excellent (>0.75) interobserver and intraobserver reliability. Only 14% (20/141) had an abnormal TT-PCL (>24mm). Of this subgroup, 55% (11/20) had an abnormal TT-TG (>20mm) on CT. Based on the TT-TG and TT-PCL measurements, 4 groups of patients can be established. When comparing knee joint rotation (KJR) between groups, we noted that an increased TT-TG may result from either true lateralisation of the TT, an increased knee joint rotation or a combination of both.

Conclusion

Commonly accepted values for the TT-TG distance cannot be used interchangeably between CT and MRI and a TT-TG >20mm measured on MRI should not be used as an indication for surgery. The TT-PCL measurement is a measure of true lateralisation of the tibial tubercle while the TT-TG is an amalgamated measure of true lateralisation and KJR. The clinical relevance of these different groups requires further investigation and may explain the poor results in some patients following tibial tubercle transfers.