although the role of the two functional bundles ( anteromedial ( am ) and posterolateral ( pl ) ) has been defined , few studies exist that attempt to establish the exact pattern or patterns of injury that may occur .
similarly , specific factors that can influence the relative pattern of injury are yet to be defined .
biomechanical studies have shown that variations in acl tunnel placement result in variable postoperative knee kinematics and loads [ 2 , 6 , 16 , 21 , 22 , 25 ] .
employed a straight knee hyperextension model to rupture the native acl and demonstrated a variable pattern of acl tears at high vs. low energy moments .
recently , zantop et al .   studied the rupture pattern of the native acl at the time of arthroscopy .
their results demonstrated that 24% of patients had a partial acl rupture and in 44% of the total cases , the am and pl bundle did not rupture at the same level ( proximal , mid - substance , distal ) . with the increasing number of acl reconstructions
being performed annually , a subsequent increase in the number of revision cases has occurred [ 12 , 18 ] . recently ,
greater attention is being paid to re - rupture cases and what can be learned in this setting [ 4 , 13 ] .
biomechanical studies [ 7 , 16 , 2124 ] have shown that variations in acl reconstruction technique , i.e. difference in tunnel placement and graft type , result in variable postoperative knee kinematics and loads .
theoretically , this exposes the reconstructed acl graft to variable forces and potentially results in different rupture patterns when the graft is stressed to failure . to date , no studies have specifically attempted to determine the most common rupture pattern and causative or related factors in failure of double - bundle acl reconstruction .
double - bundle acl reconstruction may have additional causes of failure than single - bundle reconstruction , associated with 4 tunnels instead of 2 .
if causes of failure can be determined , as well as factors that can identify these causes , they can be prevented in the future .
the primary purpose of this study was to determine the most common rupture pattern in acl revision cases after previous double - bundle reconstruction .
our secondary purpose was to identify variables that can be correlated with rupture pattern of the graft .
the hypothesis of this study was that it is possible to identify factors that influence the rupture pattern of the acl graft , such as the time between the primary surgery and the failure and the mechanism of injury .
between 2005 and 2009 , all patients who underwent revision acl surgery after a previous double - bundle reconstruction were evaluated . after receiving approval from our institutional review board ,
we excluded all patients who had undergone multiple ( > 1 ) revision surgeries on the knee of interest .
three orthopedic surgeons with sports medicine fellowship training independently reviewed the arthroscopic footage of the included subjects to determine the rupture pattern .
the rupture pattern was recorded using the classification system previously described by zantop et al .
specifically , each bundle was classified independently as ruptured proximally , mid - substance or distally , functionally insufficient due to elongation , or intact / uninjured .
if the observers felt they were unable to determine the rupture pattern due to insufficient footage , they could score a bundle as indeterminate .
patient demographic data including age , gender , date of the initial acl reconstruction , date of the re - injury and etiology of failure were collected .
posterior radiographs of the knee , using a commercially available radiographic imaging system ( stentor , philips healthcare , andover , ma ) .
the tunnel angle was defined as the angle between the tunnel and the axis of the femur ( fig . 
1anterior  posterior flexion weight - baring radiograph of the right knee after double - bundle acl reconstruction .
the tunnel angle is measured as the angle between the axis of the femur and the outline of the tunnel anterior  posterior flexion weight - baring radiograph of the right knee after double - bundle acl reconstruction .
the tunnel angle is measured as the angle between the axis of the femur and the outline of the tunnel data were analyzed with pasw statistics ( version 17.0 , spss inc .
inter - observer agreement for the three observers was calculated using the fleiss kappa coefficient for multiple ratings per subject .
statistical analysis of associated variables was performed using the rupture pattern ratings from observer number one .
descriptive statistics were calculated for all the variables , including frequency counts for nominal variables and means , medians , standard deviation and a range for all continuous variables . the kruskal 
wallis one - way analysis of variance was used to determine the influence of patient age , acl reconstruction to re - injury time interval and tunnel angle on the rupture pattern .
when statistical significance was achieved ( p  <  0.05 ) , a post - hoc mann  whitney u test was performed to further specify the difference . for the post - hoc analysis ,
the bonferroni correction was applied to adjust the a priori alpha level to the number of comparisons performed .
this resulted in p  <  0.05/3 for the am bundle and 0.05/6 for the pl bundle as the level of significance .
the chi - square test and cramer v correlation coefficient were used to measure the relationship between the rupture pattern and nominal variables such as gender and etiology of failure .
the a priori alpha level was p  <  0.05 . when statistical significance was achieved , a post - hoc analysis of the standard residuals was performed .
inter - observer agreement for the three observers was calculated using the fleiss kappa coefficient for multiple ratings per subject .
this method generates a single kappa value for all three observers combined . statistical analysis of associated variables was performed using the rupture pattern ratings from observer number one .
descriptive statistics were calculated for all the variables , including frequency counts for nominal variables and means , medians , standard deviation and a range for all continuous variables . the kruskal 
wallis one - way analysis of variance was used to determine the influence of patient age , acl reconstruction to re - injury time interval and tunnel angle on the rupture pattern .
when statistical significance was achieved ( p  <  0.05 ) , a post - hoc mann  whitney u test was performed to further specify the difference . for the post - hoc analysis ,
the bonferroni correction was applied to adjust the a priori alpha level to the number of comparisons performed .
this resulted in p  <  0.05/3 for the am bundle and 0.05/6 for the pl bundle as the level of significance .
the chi - square test and cramer v correlation coefficient were used to measure the relationship between the rupture pattern and nominal variables such as gender and etiology of failure .
the a priori alpha level was p  <  0.05 . when statistical significance was achieved , a post - hoc analysis of the standard residuals was performed .
the kappa for multiple observers was .80 for the am bundle and .76 for the pl bundle .
the most frequent rupture pattern was mid - substance rupture of both the am and pl bundle graft ( 35.1% ) .
after combined mid - substance injuries , the three most common patterns seen were mid - substance rupture of the am graft with either elongation ( 13.5% ) or proximal rupture ( 8.1% ) of the pl graft and proximal rupture of the am bundle with an intact pl bundle ( 8.1% ) ( fig .  2 ) .
since there was an extremely low number of distal ( tibial - sided ) graft ruptures , distal and mid - substance ruptures were collapsed as a single category for further statistical analysis.table  1demographic data ( n  =  40)ratiomeansdrangegender  male : female22:17age19 ( median)1638months between initial surgery and re - injury18.69.5639etiology of failure  traumatic : a - traumatic35:5  tunnel angle am2313141  tunnel angle pl4382762sd standard deviation , am anteromedial bundle graft , pl posterolateral bundle grafttable  2frequencies ( % of total ) of the different rupture patterns for the am and pl bundle graftplproximalmid - substancedistalelongatedintacttotal amam  proximal5005819  mid - substance835014359  distal033038  elongated5300514  intact000000  total pl194131919100pl bundle graft , am anteromedial bundle , pl posterolateral bundlefig . 
2arthroscopic figures of the right knee in 90 of flexion displaying the four most frequent double - bundle graft rupture patterns after anterior cruciate ligament reconstruction .
. a anteromedial portal view , the am and pl bundle have ruptured mid - substance .
b anterolateral portal view , the am bundle ruptured mid - substance and the pl bundle is elongated . c anteromedial portal view , the am bundle ruptured mid - substance and the pl bundle proximal . d anteromedial portal view , the am bundle ruptured proximal and the pl bundle is still intact
. am anteromedial , pl posterolateral , lfc lateral femoral condyle demographic data ( n  =  40 ) sd standard deviation , am anteromedial bundle graft , pl posterolateral bundle graft frequencies ( % of total ) of the different rupture patterns for the am and pl bundle graft pl bundle graft , am anteromedial bundle , pl posterolateral bundle arthroscopic figures of the right knee in 90 of flexion displaying the four most frequent double - bundle graft rupture patterns after anterior cruciate ligament reconstruction .
b anterolateral portal view , the am bundle ruptured mid - substance and the pl bundle is elongated . c anteromedial portal view , the am bundle ruptured mid - substance and the pl bundle proximal . d anteromedial portal view , the am bundle ruptured proximal and the pl bundle is still intact
. am anteromedial , pl posterolateral , lfc lateral femoral condyle isolated analysis of the am bundle graft showed that the time between the initial acl surgery and re - injury significantly influenced the rupture pattern .
the reconstruction to re - rupture interval was significantly greater in proximal ruptures when compared to distal or mid - substance ruptures ( 32 vs. 14  months , p  =  0.001 ) .
rupture pattern of the am graft was also significantly correlated with the etiology of failure ( traumatic vs. atraumatic ) ( cramer v  =  0.436 ; p  =  0.025 ) .
ninety - four percent of proximal , mid - substance or distal tears were traumatic , while only fifty - seven percent of elongated grafts were the result of a traumatic event .
wallis test also showed a main effect of tunnel angle on the am graft rupture pattern , however , post - hoc analysis was unable to further specify this effect .
the am rupture pattern was not significantly different based on the patients age or gender ( table  3).table  3factors that influence the rupture pattern of the anteromedial bundlefactorsproximal ( n  =  7)mid - substance and distal ( n  =  25)elongated ( n  =  7)p valueage ( median , [ 95% ci])20 [ 1827]18.5 [ 1921]23.5 [ 1634]nsgender ( % male)57%56%50%nsmonths out ( median , [ 95% ci])32 [ 2137]14 [ 1218]17 [ 1431]0.002etiology of failure ( % traumatic)86%96%57%0.025tunnel angle ( median , [ 95% ci])14 [ 820]29 [ 2131]20 [ 1130]0.04895% ci  95% confidence interval , ns not significantproximal ruptures had a significantly greater time between the primary surgery and re - injury than the mid - substance and distal ruptures , p  =  0.001cramer v  =  0.436 .
atraumatic injuries were more likely to be elongatedpost - hoc analysis was unable to further specify this difference factors that influence the rupture pattern of the anteromedial bundle 95% ci  95% confidence interval , ns not significant proximal ruptures had a significantly greater time between the primary surgery and re - injury than the mid - substance and distal ruptures , p  =  0.001 cramer v  =  0.436 .
atraumatic injuries were more likely to be elongated post - hoc analysis was unable to further specify this difference with respect to the pl bundle , the time interval between the initial acl surgery and re - injury also influenced the rupture pattern .
distal or mid - substance ruptures occurred at a significantly shorter reconstruction to re - rupture interval than the proximal ruptures ( 13 vs. 24  months , p  =  0.006 ) .
the pl rupture pattern was not significantly different based on the patients age , gender , etiology of failure or tunnel angle ( p  =  0.054 ) ( table  4).table  4factors that influence the rupture pattern of the posterolateral bundlefactorsproximal ( n  =  7)mid - substance and distal ( n  =  16)elongated ( n  =  10)intact ( n  =  5)p valueage ( median , [ 95% ci])21 [ 1826]18 [ 1823]19.5 [ 1827]19 [ 1629]nsgender ( % male)83%56%40%40%nsmonths out ( median , [ 95% ci])24 [ 1532]13 [ 1229]17 [ 1233]22 [ 1521]0.039etiology of failure ( % traumatic)86%100%70%100%nstunnel angle ( median , [ 95% ci])43 [ 3550]45 [
4351]40 [ 3345]37 [ 3345]ns95% ci  95% confidence intervall , ns not significantthe proximal ruptures had a significantly longer time between the primary surgery and re - injury than the mid - substance and distal ruptures , p  =  0.006 factors that influence the rupture pattern of the posterolateral bundle 95% ci  95% confidence intervall , ns not significant the proximal ruptures had a significantly longer time between the primary surgery and re - injury than the mid - substance and distal ruptures , p  =  0.006
the most important finding of the present study was that the most common rupture pattern in acl revision cases after previous double - bundle reconstruction is mid - substance rupture of the am bundle with either mid - substance rupture , proximal rupture , or elongation of the pl bundle , as well as an isolated proximal am bundle rupture with an intact pl graft .
this differs from the most frequent rupture pattern seen in the native acl , as reported by zantop et al . .
their study demonstrated proximal rupture of the am bundle with either proximal or mid - substance rupture of the pl bundle as the most common patterns of injury ( fig .
3 ) . however , despite this difference , we did see similar results for the occurrence of both bundles ruptures at the same level and for the relative incidence of an intact pl bundle ( table  5 ) . in another case
series of acl revision surgeries , kaz et al .   also reported an isolated am bundle rupture with intact pl bundle among their report of three double - bundle re - ruptures .
3arthroscopic figure of the right knee in 90 of flexion showing the most frequent rupture pattern of the native anterior cruciate ligament .
both the am and pl bundle are rupture proximally . when compared to the most common rupture pattern in revision surgery ( fig . 
am anteromedial , pl posterolateral , lfc lateral femoral condyle , mfc medial femoral condyletable  5comparison of the rupture pattern in acl revision surgery to the rupture pattern of the native aclmeasured datazantop et al .  
rupture pattern native aclcurrent study rupture pattern reconstructed acltwo most frequent rupture patternsam proximal pl proximalam mid - substance pl mid - substanceam proximal pl mid - substanceam mid - substance pl elongatedcases where both bundles ruptured at the same location56%43%pl bundle intact12%18.9%acl anterior cruciate ligament , am anteromedial bundle , pl posterolateral bundle arthroscopic figure of the right knee in 90 of flexion showing the most frequent rupture pattern of the native anterior cruciate ligament .
both the am and pl bundle are rupture proximally . when compared to the most common rupture pattern in revision surgery ( fig . 
am anteromedial , pl posterolateral , lfc lateral femoral condyle , mfc medial femoral condyle comparison of the rupture pattern in acl revision surgery to the rupture pattern of the native acl acl anterior cruciate ligament , am anteromedial bundle , pl posterolateral bundle patient - related factors such as the age and gender showed no correlation with the pattern of rupture . the time interval between the initial acl reconstruction and the re - injury did significantly influence the rupture pattern .
a longer time period was associated with more proximal ruptures of the graft , whereas a shorter time resulted in more distal and mid - substance ruptures .
we believe that this finding may be the result of varied degrees of healing or 
stated another way , when the graft was given more time to heal and remodel , the resultant re - injury pattern was most similar to the native acl , i.e. proximal rupture of the graft .
ligamentization may also be influenced by graft type ( allograft vs. autograft ) and procedure ( single- vs. double - bundle ) .
we believe that tunnel positioning is a key determinant of the biomechanical function of the knee .
theoretically , variable graft tunnel positioning should result in differences in forces seen by the graft .
lastly , this study showed a relationship between the rupture pattern and the etiology of failure . when the patient self - reported an actual traumatic event , the graft was more often ruptured . in comparison ,
if the tunnel was placed outside of the native acl insertion site , this could result in a mal - functioning graft that is exposed to unusual forces and as a result elongates over time .
in addition , the rupture pattern was established by review of arthroscopic videos captured at the time of the revision surgery .
therefore , observers were not able to control the arthroscope or to probe the ruptured ligament . still , despite the lack of tactile data , we did find a good to excellent inter - observer reliability .
since a satisfactory correlation was seen between all observers , we deemed it appropriate to utilize a single observer s ratings for further statistical analysis . in only one case , the am bundle was scored as indeterminate , while the pl rupture pattern could be established .
this subject was excluded for the analysis of the am bundle and included for the analysis of the pl bundle .
this dilemma was solved by combining the distal and mid - substance ruptures , so no comparisons could be made between these two classes individually .
finally , the rupture pattern may be determined by multiple of the factors assessed in this study , some of which may be confounding .
the reconstruction techniques for revision surgery after previous double - bundle acl reconstruction are still being developed .
although the outcome of double - bundle reconstruction is generally good [ 8 , 17 ] , with the high incidence of acl injuries , revisions are to be expected . during revision acl surgery ,
the pattern of injury may yield valuable information about the function of the primary acl graft .
isolated rupture of the am bundle has been shown to have a greater effect on the anterior drawer sign than on the lachman test .
the opposite is true for pl bundle ruptures , which can also be tested using the pivot shift .
an isolated pl bundle rupture is more difficult to diagnose because of the oblique course of this bundle taking the ligament out of plane when standard mri sequences are employed .
three - dimensional reconstruction and 3d magnetic resonance imaging ( mri ) can better visualize the rupture pattern preoperatively , but this requires specialized views or use of ultra high - field 3 tesla mri , which are not typically available in routine clinical practice . at present , the rupture pattern can best be observed and classified during arthroscopic examination .
visualizing the rupture pattern during arthroscopy may present a challenge because the am bundle overlies the pl bundle when viewed from the standard anterolateral portal .
using the figure of four position or viewing through the anteromedial portal will help in the visualization of the whole acl and both the tibial and femoral insertion sites .
if there is an isolated rupture of the am or pl graft , augmentation surgery should be considered , similar to primary acl reconstruction [ 13 , 14 ] .
this study shows that after double - bundle reconstruction , the most common rupture pattern is the failure of both grafts mid - substance .
factors that influence the rupture pattern are the time interval between acl reconstruction and re - injury , etiology of failure and graft tunnel angle .
 purposeto determine the most common rupture patterns of previously reconstructed db - acl cases , seen at the time of revision surgery , and to determine the influence of age , gender , time between the initial acl reconstruction and re - injury , tunnel angle and etiology of failure.methodsforty patients who presented for revision surgery after previous double - bundle acl reconstruction were enrolled . 
 three orthopedic surgeons independently reviewed the arthroscopic videos and determined the rupture pattern of both the anteromedial and posterolateral grafts . 
 the graft rupture pattern was then correlated with the previously mentioned factors.resultsthe most common injury pattern seen at the time of revision acl surgery was mid - substance am and pl bundle rupture . 
 factors that influenced the rupture pattern ( proximal vs. mid - substance and distal rupture vs. elongated , but in continuity ) were months between acl reconstruction and re - injury ( p  =  0.002 ) , the etiology of failure ( traumatic vs. atraumatic ) ( p  =  0.025 ) and the measured graft tunnel angle ( p  =  0.048).conclusionsthe most common pattern of graft re - rupture was mid - substance am and mid - substance pl . as the length of time from the initial db - acl reconstruction to revision surgery increased , the pattern of injury more closely resembled that of the native acl . 
 evaluation of patients who have undergone double - bundle acl reconstruction , with a particular focus on graft maturity , mechanism of injury and femoral tunnel angles , and graft rupture pattern assists in preoperative planning for revision surgery . 
Introduction
Materials and methods
Statistical analysis
Results
Discussion
Conclusion