magnetic resonance imaging ( mri ) is a highly effective diagnostic modality for the detection of anterior cruciate ligament ( acl ) tears . however , in cases of chronic acl deficiency , mri findings can be misleading , if acl remnants are attached to the posterior cruciate ligament ( pcl ) or some fibrous tissues are well preserved . in such cases , pcl bucking , a secondary sign of chronic acl tear can be a valuable clue to diagnosis1 ) . typical mri findings of acl tear include partial or complete discontinuity of the ligament and abnormal morphology on t1-weighted image2,3 ) . pcl buckling can be observed in cases of acute or chronic acl tears . some studies associated sigmoid or curved appearance of the pcl more with chronic than acute acl tears4 ) . our hypothesis was that pcl buckling would disappear after acl reconstruction if it was associated with anterior tibial translation that results from acl injury . in this study , we evaluated the pre- and postoperative morphological changes of the pcl on mri scans in patients with chronic acl injury . of the patients who underwent acl reconstruction between march 2006 and march 2010 at our institution , 38 patients who were available for mri evaluation within 3 months after surgery were included in this study . there were 34 males and 4 females with a mean age of 32 years ( range , 20 to 45 years ) . the indications for surgery were medial collateral ligament injury in 6 , medial meniscal tear in 14 , and lateral meniscal tear in 6 patients . the causes of injury were football in 15 , basketball in 3 , foot volleyball in 2 , falling while hiking in 3 , and unidentified in 15 patients . acl reconstruction was performed using a hamstring tendon autograft in 19 and a double - bundle semitendinosus tendon autograft in the remaining 19 patients . t1- and t2- , and proton - density - weighted sagittal , coronal , and axial images with a slice thickness of 5 mm were obtained using a 1.5 tesla scanner ( magnetom vision - vb33a , siemens , malvern , pa , usa ) . mri was performed with the patient in the supine position and the knee at 20 flexion and at 15 external rotation in a surface coil . the degree of pcl buckling on the obtained images was assessed using the method of choi et al.1 ) . on the pre- and postoperative sagittal plane mri images that optimally demonstrated the whole pcl , a base line ( bl ) that connects the centers of insertion sites of the pcl in the femur and tibia point a was established at a vertically farthest point from bl , from which line 1 that was extended to the femoral attachment and line 2 to the tibial attachment were drawn . the angle between bl and line 1 was defined as angle a and the angle between bl and line 2 as angle b. a line perpendicular to bl from point a was drawn and the point of intersection was determined as point b. the distance between point a and point b was defined as d. the ratio between the distance from the tibial attachment of the pcl to point b and bl was calculated ( ratio a ) ( fig . 1 ) . we analyzed the pre- and postoperative differences in d , ratio a , angle a , and angle b that were measured on mri images . the relationship between the preoperative manual maximal side - to - side difference ( mmsd ) measured using the kt-1000 arthrometer and the abovementioned parameters were assessed . measurements were performed by two orthopedic surgeons three times each and the average values were used for analysis . a paired t - test was conducted to evaluate the d , ratio a , angle a , and angle b with a significance level set at p<0.05 . the relationship between the preoperative kt-1000 measurements and the angle b was evaluated by pearson correlation analysis with a significance level set at p<0.05 . of the patients who underwent acl reconstruction between march 2006 and march 2010 at our institution , 38 patients who were available for mri evaluation within 3 months after surgery were included in this study . there were 34 males and 4 females with a mean age of 32 years ( range , 20 to 45 years ) . the indications for surgery were medial collateral ligament injury in 6 , medial meniscal tear in 14 , and lateral meniscal tear in 6 patients . the causes of injury were football in 15 , basketball in 3 , foot volleyball in 2 , falling while hiking in 3 , and unidentified in 15 patients . acl reconstruction was performed using a hamstring tendon autograft in 19 and a double - bundle semitendinosus tendon autograft in the remaining 19 patients . t1- and t2- , and proton - density - weighted sagittal , coronal , and axial images with a slice thickness of 5 mm were obtained using a 1.5 tesla scanner ( magnetom vision - vb33a , siemens , malvern , pa , usa ) . mri was performed with the patient in the supine position and the knee at 20 flexion and at 15 external rotation in a surface coil . the degree of pcl buckling on the obtained images was assessed using the method of choi et al.1 ) . on the pre- and postoperative sagittal plane mri images that optimally demonstrated the whole pcl , a base line ( bl ) that connects the centers of insertion sites of the pcl in the femur and tibia point a was established at a vertically farthest point from bl , from which line 1 that was extended to the femoral attachment and line 2 to the tibial attachment were drawn . the angle between bl and line 1 was defined as angle a and the angle between bl and line 2 as angle b. a line perpendicular to bl from point a was drawn and the point of intersection was determined as point b. the distance between point a and point b was defined as d. the ratio between the distance from the tibial attachment of the pcl to point b and bl was calculated ( ratio a ) ( fig . 1 ) . we analyzed the pre- and postoperative differences in d , ratio a , angle a , and angle b that were measured on mri images . the relationship between the preoperative manual maximal side - to - side difference ( mmsd ) measured using the kt-1000 arthrometer and the abovementioned parameters were assessed . measurements were performed by two orthopedic surgeons three times each and the average values were used for analysis . 18 ( ibm , armonk , ny , usa ) . a paired t - test was conducted to evaluate the d , ratio a , angle a , and angle b with a significance level set at p<0.05 . the relationship between the preoperative kt-1000 measurements and the angle b was evaluated by pearson correlation analysis with a significance level set at p<0.05 . the average angle a was 52.37.1 ( range , 39-71 ) preoperatively and 30.13.4 ( range , 24-39 ) postoperatively . the average angle b was 28.94.6 ( range , 20-37 ) preoperatively and 222.9 ( range , 15.9-26 ) postoperatively ( p<0.05 ) . the average d was 6.381.7 mm ( range , 3.4 - 9.9 mm ) preoperatively and 3.771.3 mm ( range , 1.8 - 6.8 mm ) postoperatively ( p<0.05 ) ( fig . 2 ) . the average ratio a was 67.2%5.8% ( range , 55.5 - 79% ) preoperatively and 55.2%7.6% ( range , 43.2 - 66.4% ) postoperatively ( p<0.05 ) . the average preoperative mmsd was 7.202.3 mm ( range , 6 - 13 mm ) , which was not correlated with angle b ( pearson 's correlation coefficient=0.245 , p<0.05 ) . primary mri findings of acl injury include discontinuity of the ligament on t1- or t2-weighted images , an intermediate - signal - intensity mass near the acl on t1-weigthed image , posterior bowing of the acl , poor visualization or nonvisualization of the acl on t1-weighted image in the presence of pcl buckling1,2,4 - 7 ) . secondary signs include bone contusion or fracture of the posterior aspect of the lateral tibial condyle or middle of the lateral femoral condyle2,6,8 ) , anterior translation of the lateral tibial condyle , posterior translation of the posterior horn of the lateral meniscus , deepening of the lateral femoral condylar notch5 ) , and diffuse thickening of the acl , and protrusion or irregularity in the medial border on t1-weighted transverse images1 ) . anterior subluxation of the tibia is a sign of full - thickness acl tears . in the absence of 5 mm subluxation , mri has a sensitivity of 86% and a specificity of 99% for the detection of full - thickness acl tears9,10 ) . anterior subluxation of the tibia relative to the femur induces changes in the appearance of the pcl to a sigmoid shape or hyperbuckling8,9 ) . such morphological changes of the pcl have been well documented in the study by choi et al.1 ) and we also observed increases in the angle a in our patients . choi et al.1 ) suggested that angle a was a significant predictor of the diagnosis of chronic acl tears . however , we found there was little correlation between the angle a and the anterior instability , and thus believe that pcl buckling on mri images is not correlated with anterior instability . in our understanding , anterior subluxation of the tibia may also be influenced by injuries to the medial meniscus , pcl tears , which we could not identify with an mri , and abnormality of the secondary stabilizers of the knee . in the study by choi et al.1 ) , angle a was 56.141.4 in the acl tear group and 39.710.0 in the normal group . in our study , angle a in patients with acl tears decreased from 52.37.1 prepoeratively to 30.13.4 postoperatively . although we could not confirm normalization of the pcl shape , because side - to - side assessemnts were not doable postoperatively , the degree of pcl buckling decreased after acl reconstruction . rak et al.7 ) reported that pcl buckling after acl reconstruction was suggestive of acl graft laxity . horton et al.10 ) reported that pcl buckling was present in 37.5% of full - thickness acl graft tears , in 43% of partial tears , and in 30% of intact grafts . their study showed that pcl buckling had 41% sensitivity , 70% specificity , 76% positive predictive value , and 35% negative predictive value for the diagnosis of full - thickness or partial - thickness tears of the acl reconstruction grafts9,10 ) . the clinical implication of this study is that we could confirm that pcl buckling is caused by anterior subluxation of the tibia in acl tears , considering that hyperbuckling disappears after acl reconstruction . therefore , we think a pcl buckling that is present after acl reconstruction is suggestive of acl laxity . pcl buckling that results from anterior translation of the tibia in chronic acl injury disappeared after acl reconstruction .
purposethe purpose of this study was to compare the morphologic change of the posterior cruciate ligament ( pcl ) on magnetic resonance imaging ( mri ) in chronic anterior cruciate ligament ( acl ) tear before and after reconstruction.materials and methodson the mri image after acl reconstruction , a base line ( bl ) was drawn between the femoral and tibial insertion sites of the pcl . 
 point a was established as the vertically farthest point from bl . 
 line 1 and 2 were drawn respectively between point a and the insertions of pcl . 
 the angles between bl and line1 and 2 were defined as angle a and angle b. correlations between the preoperative anterior knee instability assessed with the kt 1000 arthrometer and the parameters were evaluated.resultsthe average angle a was 52.37.1 preoperatively and 30.13.4 postoperatively , and the average angle b was 28.954.62 preoperatively and 222.9 postoperatively ( p<0.05 ) . 
 the preoperative manual maximal side - to - side difference was 7.22.3 mm , which had no significant correlation with angle b.conclusionsthe preoperative buckling of pcl in the case of chronic acl injury disappeared after acl reconstruction . 
 the degree of pcl buckling had no correlation with the degree of preoperative anterior instability .