***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Purpose The purpose of this study is to assess the reliability of measuring the tibial tubercle to posterior cruciate ( TT-PCL ) distance compared to the tibial tubercle to trochlear groove ( TT-TG ) distance on magnetic resonance imaging ( MRI ) , establish baseline TT-PCL values in patellar instability patients , and determine the predictive value of an excessive TT-PCL distance ( ≥24 mm ) for recurrent patellar instability compared to a TT-TG distance ≥20 mm . Methods  TT-TG and TT-PCL distances  were calculated on MRI in a r and omized and blinded fashion by two review ers on 54 patients ( 59 knees ) with patellar instability .  Interobserver reliability was assessed using interclass correlation coefficients ( ICC ) . TT-PCL distances were also assessed to establish mean values in patellar instability patients . The ability of excessive TT-PCL and TT-TG distances to predict recurrent instability was assessed by comparing odds ratios , sensitivities , and specificities . Results Interobserver reliability was excellent for both TT-TG ( ICC = 0.978 ) and TT-PCL ( ICC = 0.932 ) . The mean TT-PCL in these 59 knees was 21.7 mm ( st and ard deviation 4.1 mm ) . Twelve ( 20 % ) of 59 knees had a single dislocation , and 47 ( 80 % ) exhibited 2 or more dislocations . The odds ratios , sensitivities , and specificities of a TT-TG distance ≥20 mm for identifying patients with recurrent dislocation were 5.38 , 0.213 , and 1.0 , respectively , while those of a TT-PCL distance ≥24 mm were 1.46 , 0.298 , and 0.583 , respectively . Of the 10 knees with a TT-TG distance ≥20 mm , all 10 ( 100 % ) had recurrent instability , while 14 ( 73.7 % ) of the 19 knees with a TT-PCL ≥24 mm experienced multiple dislocations ( n.s . ) . Conclusion Both TT-PCL and TT-TG can be measured on MRI with excellent interobserver reliability . In this series , the mean TT-PCL value in patients with patellar instability was 21.8 mm , but the range was broad . A TT-PCL distance ≥24 mm was found to be less predictive of recurrent instability in this series . For patients experiencing multiple episodes of patellar instability in the setting of a normal TT-TG distance , obtaining the TT-PCL measurement may provide a more focused assessment of the tibial contribution to tubercle lateralization . Level of evidence III Purpose Tibial tubercle – trochlear groove ( TT – TG ) distance is a variable that helps guide surgical decision-making in patients with patellar instability . The purpose of this study was to compare the accuracy and reliability of an MRI TT – TG measuring  technique using a simple external alignment method to a previously vali date d gold st and ard technique that requires advanced software read by radiologists . Methods TT – TG was calculated by  MRI  on 59 knees with a clinical diagnosis of patellar instability in a blinded and r and omized fashion by two musculoskeletal radiologists using advanced software and by two orthopaedists using the study technique which utilizes measurements taken on a simple electronic imaging platform .  Interrater reliability between the two radiologists and the two orthopaedists and inter methods reliability between the two techniques were calculated using interclass correlation coefficients ( ICC ) and concordance correlation coefficients ( CCC ) . ICC and CCC values greater than 0.75 were considered to represent excellent agreement . Results The mean TT – TG distance was 14.7 mm ( St and ard Deviation ( SD ) 4.87 mm ) and 15.4 mm ( SD 5.41 ) as measured by the radiologists and orthopaedists , respectively . Excellent interobserver agreement was noted between the radiologists ( ICC 0.941 ; CCC 0.941 ) , the orthopaedists ( ICC 0.978 ; CCC 0.976 ) , and the two techniques ( ICC 0.941 ; CCC 0.933 ) . Conclusion The simple TT – TG distance measurement technique analysed in this study result ed in excellent agreement and reliability as compared to the gold st and ard technique . This method can predictably be performed by orthopaedic surgeons without advanced radiologic software . Level of evidence II Background : Tibial tubercle – trochlear groove distance ( TT-TG ) is a commonly used measurement for surgical decision making in patients with patellofemoral malalignment and instability . This measurement has historically been performed utilizing axial computed tomography ( CT ) . More recently , magnetic resonance imaging ( MRI ) has been proposed as an equivalent method , but this has not yet been fully vali date d. Purpose : To determine the reliability of TT-TG distance measurements on both MRI and CT and to determine whether the measurements are interchangeable with one another . Study Design : Cohort study ( diagnosis ) ; Level of evidence , 2 . Methods : All patients with patellar instability who underwent both CT and MRI of the knee from 2003 to 2011 were included ( n = 59 knees in 54 patients ) . Two fellowship-trained musculoskeletal radiologists measured the TT-TG distances for each patient by CT and MRI in a r and omized , blinded fashion . Interobserver reliability was calculated between radiologists for both imaging modalities , and inter method reliability was calculated between the 2 imaging modalities . The results are reported using intraclass correlation coefficients ( ICCs ) and Bl and -Altman analysis . Results : The 59 knees had a mean TT-TG distance of 16.9 mm ( range , 8.3 - 25.8 mm ) by CT and 14.7 mm ( range , 1.5 - 25.1 mm ) by MRI . Interobserver reliability between the radiologists was considered excellent for both CT and MRI ( ICC = 0.777 and 0.843 , respectively ) . When comparing CT to MRI , the ICC was considered only fair for each of the raters ( 0.532 and 0.539 ) . Eleven patients ( 19 % ) had a TT-TG distance of ≥20 mm on CT preoperatively and underwent distal realignment by tibial tubercle osteotomy . In this surgical subgroup , the mean TT-TG distance was 22.5 mm ( range , 19.8 - 25.8 mm ) by CT and only 18.7 mm ( range , 14.4 - 22.8 mm ) by MRI for a mean difference of 3.80 mm ( P < .001 ) . Conclusion : The TT-TG distance can be measured with excellent interrater reliability on both MRI and CT ; however , the values derived from these 2 tests may not be interchangeable . This observation should be taken into consideration when MRI is used for surgical planning because MRI may underestimate the TT-TG distance when compared with CT 
    Output:
    
Conclusion A moderate-to-strong positive correlation exists between TT-PCL and TT-TG measurements taken from MRIs of patellar instability patients .
There is excellent interobserver and intraobserver reliability when taking TT-PCL measurements using MRI .