Mid inter-epicondyle trochlea intersection (MIELTI): Proposal of a new index for identifying the deepest part of the trochlea

Abstract Background Tibial tuberosity trochlear groove distance (TT-TG) is an important radiological measurement in patellofemoral instability (PFI). Where instability is recurrent, a value ≥ 20 mm is considered an indication for tubercle medialisation. Trochlear dysplasia commonly accompanies PFI. It can make identification of the deepest part of the trochlea difficult, which makes the TT-TG difficult or impossible to assess. To address this, we propose a new method of identifying the deepest part of trochlea based on the femoral epicondyles. It is named the tibial tuberosity mid inter-epicondyle trochlea intersection distance (TT-MIELTI). Methods The TT-TG and TT-MIELTI of 30 consecutive non-dysplastic knee MRIs were compared, excluding 96 knees with dysplasia, sulcus angles ≥ 135°, a tibial tuberosity anterior cortex which was not fully demonstrated, artefact, fracture, or Osgood Schlatter's disease. To assess inter-observer reliability three blinded researchers measured the TT-TG and the TT-MIELTI of all 30 knees. To assess intra-observer repeatability one researcher repeated the measurements after six weeks. Results The intraclass correlation coefficient (ICC) test demonstrated good to excellent values for all measurements (TT-TG and TT-MIELTI correlation ICC 0.94–0.97; TT-TG inter-observer reliability ICC = 0.85, intra-observer repeatability ICC = 0.90; TT-MIELTI inter-observer reliability ICC = 0.86, intra-observer repeatability ICC = 0.89. All p values  Conclusions In non-dysplastic knees the mid inter-epicondyle trochlea intersection (MIELTI) accurately identifies the deepest part of the trochlea, and TT-MIELTI is a reliable alternative to the TT-TG. Re-assessment in dysplastic knees would be of benefit to establish its usefulness in the clinical setting.