title: The TT-TG Index: a new knee size adjusted measure method to determine the TT-TG distance.
abstract: OBJECTIVE
  Medial transfer of the tibial tubercle has become a standard procedure in cases of patella instability caused by an increased tuberositas tibae-trochlear groove (TT-TG) distance. However, the TT-TG distance has always been assessed as an absolute value without taking individual joint size into consideration. It was assumed that the pathological influence of the TT-TG distance correlates with individual joint size. Aim of the current study therefore was to develop a method to express TT-TG distance in relation to these joint variables.
  
  
  METHODS
  Two hundred knee MRI scans of healthy individuals (69 females and 131 males) were evaluated retrospectively in a control group. First, the TT-TG distance was measured as described by Schoettle et al. To determine joint size, the proximal-distal distance between the entrance of the chondral trochlear groove (TE) and the onset of the patella tendon at the tibial tubercle (TT) was selected. Subsequently, the TT-TG/TT-TE ratio expresses the relationship between the TT-TG distance and the proximal-distal distance from the entrance of the chondral trochlear groove to the height of the tibial tubercle. The TT-TG Index can also be expressed as an angle (TT-TG angle). Likewise, in another patient group, 54 knee MRTs of patients with patellofemoral instability were evaluated.
  
  
  RESULTS
  The average TT-TG distance of the control group was 7.5±3.5 mm (range 0-17.4 mm) with no significant differences between genders. The mean TT-TE distance was 63.9 mm (range 49-79 mm) with there being significant differences between genders. The resulting mean TT-TG Index was 0.12±0.05 (range 0-0.25). In the patient group, the average TT-TG distance was 13.5±4.1 mm and the average TT-TE distance was 61.3±6.8 mm. The resulting average TT-TG Index of 0.22±0.07 in the patient group (PFI) approximates the threshold determined by the 95% confidence interval in the healthy control group. A direct comparison between the control group and the patient group revealed a significant difference in the TT-TG distance (p=0.0001), in the TT-TE distance (p<0.0042) and in the resulting TT-TG Index (p<0.0001).
  
  
  CONCLUSIONS
  The measurement of the TT-TG Index is a reliable and differentiated approach for determining the lateral displacement of the tibial tubercle in relation to the proximal trochlear groove. The pathological influence of the TT-TG distance in case of patella instability depends on individual joint size, confirming the initial hypothesis. We currently consider a TT-TG Index>0.23 to be pathological based on our findings. Particularly, in case of a marginal TT-TG distance, the additional relative TT-TG Index facilitates a decision concerning an indication for a operative medial transfer of the tibial tubercle.
  
  
  METHODS
  II.
authors.affiliation: Sportklinik Stuttgart, Taubenheimstr. 8, 70372, Stuttgart, Germany.
authors.forename: Swen
authors.initials: S
authors.lastname: Hingelbaum
authors.forename: Raymond
authors.initials: R
authors.lastname: Best
authors.forename: Jochen
authors.initials: J
authors.lastname: Huth
authors.forename: Daniel
authors.initials: D
authors.lastname: Wagner
authors.forename: Gerhard
authors.initials: G
authors.lastname: Bauer
authors.forename: Frieder
authors.initials: F
authors.lastname: Mauch
mesh_terms: D000293:Adolescent; D000328:Adult; D002648:Child; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000473:pathology* / Q000601:surgery; D007719:Knee Joint / Q000473:pathology* / Q000601:surgery; D008279:Magnetic Resonance Imaging; D008297:Male; D017847:Patellar Ligament / Q000473:pathology; D012189:Retrospective Studies; D013977:Tibia / Q000473:pathology* / Q000601:surgery; D055815:Young Adult
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