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Why are bone and soft tissue measurements of the TT-TG distance on MRI different in patients with patellar instability?

  • Knee
  • Published:
Knee Surgery, Sports Traumatology, Arthroscopy Aims and scope

Abstract

Purpose

To determine whether the tibial tuberosity-to-trochlear groove distance (TT-TG) and patellar tendon-to-trochlear groove distance (PT-TG) are equal, whether the bony and cartilaginous points coincide in the trochlea, and whether the insertion of the PT coincides with the most anterior point of the TT in patients with patellar instability.

Methods

Fifty-three MRI scans of patients with patellar instability were examined. TT-TG and PT-TG were measured by three examiners in 31 knees. Additionally, the bone–cartilage distance in the trochlea [trochlear cartilage to trochlear bone (TC-TB)] and the distance between the mid-point of the PT insertion and the most anterior point of the TT (PT-TT) were measured by one examiner. The intraclass correlation coefficient was used to evaluate the reliability of the measurements between the three examiners. The relationships between the measurements were determined, the means of the measurements were calculated, and the correlations between PT-TG and TT-TG, PT-TT, and TC-TB were assessed.

Results

The ICC was above 0.8. PT-TG was 3.7 mm greater than TT-TG. The TC and TB coincided in 73 % of cases, and the mean TC-TB was 0.3 mm. The PT was lateral to the TT in 94 % of the cases, and the mean PT-TT was 3.4 mm. The Pearson’s correlation coefficients between PT-TG and TT-TG, PT-TT, and TC-TB were 0.946, 0.679, and 0.199, respectively.

Conclusion

TT-TG underestimated PT-TG, primarily due to the lateralization of the PT insertion relative to the most anterior point of the TT. Clinical relevance: our study shows that in patients with patellar instability, there are differences in the absolute values of TT-TG and PT-TG, as previously reported for patients without patellar instability. Hence, normal cut-off values based on case–control studies of TT-TG cannot be equivalently used when measuring PT-TG to indicate TT medialization in patients with patellar instability. It is also important to note that the clinical outcomes cannot be directly compared between patients evaluated using TT-TG versus PT-TG measurements.

Level of evidence

III.

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Authors and Affiliations

  1. Department of Orthopaedics and Traumatology, Institute of Orthopaedics and Traumatology, University of São Paulo, Ovídio Pires de Campos St, 333, 2nd Floor, Cerqueira César, São Paulo, SP, 5403-010, Brazil

    Betina B. Hinckel, Riccardo G. Gobbi, Marco K. Demange, José Ricardo Pécora & Gilberto Luis Camanho

  2. Department of Musculoskeletal Radiology, University of São Paulo, São Paulo, SP, 55403-010, Brazil

    Eduardo N. Kihara Filho & Marcelo B. Rodrigues

Authors
  1. Betina B. Hinckel
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  2. Riccardo G. Gobbi
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  3. Eduardo N. Kihara Filho
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  4. Marco K. Demange
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  5. José Ricardo Pécora
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  6. Marcelo B. Rodrigues
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  7. Gilberto Luis Camanho
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Correspondence to Riccardo G. Gobbi.

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Hinckel, B.B., Gobbi, R.G., Kihara Filho, E.N. et al. Why are bone and soft tissue measurements of the TT-TG distance on MRI different in patients with patellar instability?. Knee Surg Sports Traumatol Arthrosc 25, 3053–3060 (2017). https://doi.org/10.1007/s00167-016-4095-0

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