anterior cruciate ligament surgery is one of the most frequently performed orthopedic operations . with the number of acl reconstructions continuing to increase , revision cases are to be expected .
biomechanical studies [ 6 , 10 , 1316 ] 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 [ 4 , 10 ] .
logically , this should result in variable forces placed upon the reconstructed acl , which could in turn result in differing rupture patterns to the reconstructed acl [ 4 , 11 ] .
recently , a paper was published on acl graft rupture after previous double - bundle acl reconstruction .
factors that influenced graft re - rupture were the months between the initial surgery and re - injury , the etiology of failure , and the tunnel angle . however , the majority of patients undergoing acl revision surgery previously underwent single - bundle reconstruction , not double - bundle reconstruction .
the aims of the current study were as follows : 1 . to determine the most common graft rupture pattern in acl revision cases after previous single - bundle reconstruction and 2 . to determine the relationship between graft rupture and patient age , gender , time between the initial acl reconstruction and re - injury , graft type , tunnel angle , and etiology of failure .
it was hypothesized that a specific graft rupture pattern could be identified that occurs most frequently in single - bundle revision surgery .
in addition , it was hypothesized that there is a relationship between the graft rupture patterns and the etiology of failure and other demographic and surgical factors .
patients were enrolled and data collected following full approval from our institutional review board . between 2005 and 2009 , all patients who underwent revision acl surgery after a previous single - bundle reconstruction were included .
patients who had undergone more than one revision surgery on the same knee were excluded .
three sports - medicine - fellowship - trained orthopedic surgeons independently reviewed the arthroscopic footage of the enrolled subjects to determine the exact pattern of graft rupture .
rupture pattern was recorded using a classification system previously described by zantop et al .   since all included subjects
had undergone previous single - bundle acl surgery , only one bundle was available for evaluation .
the injured single acl bundle was classified as either having ( 1 ) proximal rupture ; ( 2 ) mid - substance rupture ; ( 3 ) distal rupture ; or ( 4 ) functional insufficiency due to elongation .
each observer was able to review patient demographic information and surgical details from the primary acl reconstruction prior to review of the arthroscopic footage from the revision surgery .
if an observer was unable to determine the rupture pattern due to insufficient footage or visualization , a score of  indeterminate  was assigned .
demographic and surgical data that were collected from the patient records included patient age and gender , date of the initial acl reconstruction , date of the re - injury , and etiology of failure .
the etiology of failure was classified as either traumatic or a - traumatic , based on the patient s self - report .
radiographs were reviewed for all patients using the hospital system s commercially available radiographic imaging system ( stentor , philips medical imaging , brisbane , ca , usa ) .
the acl tunnel angle was defined as the angle between the tunnel and the long axis of the femur ( fig . 
1anterior  posterior flexion weight - baring radiograph of the right knee after single - bundle acl reconstruction .
the tunnel angle is measured as the angle between the long axis of the femur and the outline of the tunnel anterior  posterior flexion weight - baring radiograph of the right knee after single - bundle acl reconstruction .
the tunnel angle is measured as the angle between the long axis of the femur and the outline of the tunnel data were entered into and 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 .
this included frequency counts for nominal variables and means , medians , standard deviation , and range for all continuous variables .
wallis one - way analysis of variance was used to determine the influence of patient age , the acl reconstruction to re - injury time interval , and tunnel angle on the resultant graft 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 for the number of comparisons performed .
this resulted in p  <  0.05/3 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 .
data were entered into and 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 .
this included frequency counts for nominal variables and means , medians , standard deviation , and range for all continuous variables .
wallis one - way analysis of variance was used to determine the influence of patient age , the acl reconstruction to re - injury time interval , and tunnel angle on the resultant graft 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 for the number of comparisons performed .
this resulted in p  <  0.05/3 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 .
patient information and demographic data of the included subjects are displayed in table  1 .
the distribution and frequencies of the various graft rupture patterns are displayed in table  2 .
3 ) , then mid - substance rupture ( fig .  4 ) , and lastly distal rupture of the graft ( fig .  5 ) . due to the low incidence of distal graft ruptures , distal and mid - substance ruptures were collapsed as a single group for the purposes of further statistical analysis.table  1demographic data of the 60 included subjectsratiomeansdrangegender  male : female33:27age2591547months between initial surgery and re - injury59595211etiology of injury  traumatic : a - traumatic29:31graft type  allograft : autograft21:33tunnel angle2312452sd standard deviationtable  2rupture patterns following single - bundle acl reconstructionrupture patternfrequency ( % ) proximal21.7mid - substance16.7distal1.7elongated58.3acl anterior cruciate ligamentfig . 
2arthroscopic lateral portal view of the right knee in 90 of flexion displaying the most frequent single - bundle graft rupture pattern after anterior cruciate ligament reconstruction : elongation .
3arthroscopic lateral portal view of the right knee in 90 of flexion displaying proximal rupture of the single - bundle graft . with the probe
, an attempt is made to put the acl stump back toward the lateral femoral condyle .
.  4arthroscopic medial portal view of the right knee in 90 of flexion displaying mid - substance rupture of the single - bundle graft .
5arthroscopic lateral portal view of the right knee in 90 of flexion displaying the only ( partial ) distal rupture of the single - bundle graft found in this study .
lfc lateral femoral condyle demographic data of the 60 included subjects sd standard deviation rupture patterns following single - bundle acl reconstruction acl anterior cruciate ligament arthroscopic lateral portal view of the right knee in 90 of flexion displaying the most frequent single - bundle graft rupture pattern after anterior cruciate ligament reconstruction : elongation .
lfc lateral femoral condyle , acl anterior cruciate ligament arthroscopic lateral portal view of the right knee in 90 of flexion displaying proximal rupture of the single - bundle graft . with the probe
, an attempt is made to put the acl stump back toward the lateral femoral condyle .
lfc lateral femoral condyle , acl anterior cruciate ligament arthroscopic medial portal view of the right knee in 90 of flexion displaying mid - substance rupture of the single - bundle graft .
lfc lateral femoral condyle , pcl posterior cruciate ligament arthroscopic lateral portal view of the right knee in 90 of flexion displaying the only ( partial ) distal rupture of the single - bundle graft found in this study .
lfc lateral femoral condyle the time between the initial acl surgery and the re - injury was of influence on the rupture pattern of the graft . specifically , the reconstruction to re - injury time interval was significantly greater in the elongated graft group compared to distal or mid - substance ruptures ( p  =  0.002 ) .
rupture pattern was also correlated to graft type ( cramer v  =  .543 , p  <  0.001 )
. thirty - three percent of the proximal ruptures and 23% of the elongated grafts were allografts , while 91% of the mid - substance and distal ruptures were allografts .
patient age and gender , tunnel angle , and etiology of failure did not influence the rupture pattern of the graft ( table  3 ) .
the measured kappa value for multiple observers was 0.7.table  3factors that influence the rupture patternfactorsproximal ( n  =  13)mid - substance and distal ( n  =  11)elongated ( n  =  35)p valueage ( median)222025nsgender ( % male)62%72%49%nsmonths out ( median)3011.5450.007*graft type ( % autograft)67%9%77%<0.001etiology ( % traumatic)62%45%46%nstunnel angle ( median)24.517.021.5ns *  the elongated ruptures had a significantly longer time between the primary surgery and re - injury than the mid - substance and distal ruptures , p  =  0.002cramer v  =  0.543 . the distal and mid - substance ruptures were more likely to be allografts reconstructions factors that influence the rupture pattern *  the elongated ruptures had a significantly longer time between the primary surgery and re - injury than the mid - substance and distal ruptures , p  =  0.002 cramer v  =  0.543 .
the most important finding of the present study was that the most common graft re - rupture pattern in acl revision cases following previous single - bundle reconstruction was elongation of the graft , accounting for 58% of all revisions .
this is significantly different from the most frequent rupture pattern seen in the native acl , as reported by zantop et al . , which is proximal rupture of both the anteromedial ( am ) and posterolateral ( pl ) bundle ( fig . 
.  6arthroscopic anteromedial portal view of the right knee in 90 of flexion displaying the most frequent rupture pattern of the native acl .
both the am and pl bundle are ruptured proximally . when compared to the most common rupture pattern in revision surgery ( fig . 
. am anteromedial , pl posterolateral , lfc lateral femoral condyle arthroscopic anteromedial portal view of the right knee in 90 of flexion displaying the most frequent rupture pattern of the native acl . both the am and pl bundle
are ruptured proximally . when compared to the most common rupture pattern in revision surgery ( fig . 
. am anteromedial , pl posterolateral , lfc lateral femoral condyle the time between initial acl reconstruction and the re - injury did show a significant correlation with the rupture pattern . specifically , a longer time frame between surgery and re - injury was consistent with a greater incidence of graft elongation .
in contrast , patients who presented for revision surgery following a shorter postoperative time period seemed to demonstrate a greater incidence of distal and mid - substance graft ruptures . this is similar to the findings presented in a recently published paper on acl revision surgery after previous double - bundle acl reconstruction .
this finding suggests that the graft rupture pattern is an indicator of graft healing and ligamentization .
if a traditional transtibial technique was used , this could have resulted in a graft connecting the tibial pl position to the femoral am position [ 3 , 5 ] .
altered knee kinematics resulting from this vertically oriented graft [ 4 , 10 ] can subsequently expose it to forces that differ from those in a native acl . as a result , these grafts may still  heal  and the patient returns to full activity without difficulty .
however , since the graft is not subjected to the same magnitude and direction of forces seen in the native acl , the graft is exposed to repetitive microtrauma that results in elongation over time . later in the period of recovery ,
the patient may develop a sense of instability without distinct re - injury and in turn present for re - evaluation a long as years after the primary surgery .
autografts were more closely correlated with proximal re - ruptures , more similar to the pattern of injury seen in the native acl . a possible explanation for this finding could be that autografts take a shorter time to incorporate compared to allografts [ 2 , 7 , 8 ] .
furthermore , patients who undergo allograft reconstructions typically experience less postoperative pain and earlier restoration of range of motion .
therefore , these patients will feel ready to return to sports or normal activities at an earlier time .
this would also explain the higher incidence of mid - substance and distal ruptures with the use of allograft , which according to this study is seen more often with a shorter surgery to re - injury interval .
previously published studies have also shown this relationship between graft failure and graft type and activity level
. limitations of the study include that the rupture pattern was established by review of arthroscopic videos taken at the time of the revision surgery .
this was necessary to ensure three independent observers that were blinded to each other assessment of the rupture pattern .
however , this left the observers unable to control the arthroscope or probe the ruptured ligament . despite this limitation ,
because of the satisfactory inter - observer agreement , we determined that the use of one observer s ratings would be appropriate for statistical analysis .
there were some missing data points due to the limitations of the video availability and patient records for the primary acl reconstruction surgery .
the graft type employed at the time of the index surgery was unknown in 6 patients and the observer whose ratings were ultimately used was unable to determine the rupture pattern in one case .
the tunnel angle could not be determined in 16 patients due to the unavailability of plain radiographs and re - injury date was unknown in 2 subjects .
we chose to solve this by combining the distal and mid - substance ruptures , so no comparisons could be made between these two classes .
the present study showed that the variation in graft rupture pattern found during acl revision surgery is associated with certain surgical and demographic variables . during revision acl surgery , close inspection of the knee
the rupture pattern can be determined preoperatively   with the use of magnetic resonance imaging ( mri ) , but also intra - operatively during arthroscopy . by understanding pattern of graft rupture ,
this is clinically relevant as it suggests that the graft rupture pattern is an indicator of the degree of , for example , graft healing and ligamentization and the cause of graft failure .
there are no additional expenses for intra - operative evaluation of graft rupture pattern , making it both a cost and time efficient tool that can provide the surgeon with valuable diagnostic information .
in conclusion , this study showed that after single - bundle acl reconstruction , the most common graft rupture pattern seen at the time of revision surgery is elongation of the graft .
factors that influence the rupture pattern are months between acl reconstruction and re - injury and graft type . a longer time between the primary surgery and re - injury
resulted more often in an elongated graft , than in a true rupture . with the use of autograft ,
the rupture pattern in revision surgery is more similar to that of the native acl .
 purposesthe number of revision anterior cruciate ligament ( acl ) surgeries performed annually continues to rise . 
 the purpose of this study was to determine the most common rupture pattern in acl revision cases after previous single - bundle reconstruction . 
 the second aim was to determine the relationship between rupture pattern and patient - specific factors ( age , gender , time between the initial acl reconstruction and re - injury , and etiology / mechanism of failure ) and surgical factors ( graft type , tunnel angle).methodsthis was a cohort study of 60 patients that underwent revision acl surgery after previous single - bundle acl reconstruction . 
 three sports medicine  
 trained orthopedic surgeons reviewed the arthroscopic videos and determined the rupture pattern of the grafts . 
 the rupture pattern was then correlated to the above - mentioned factors.resultsthe inter - observer agreement had a kappa of 0.7 . 
 the most common rupture pattern after previous single - bundle acl reconstruction is elongation of the graft . 
 this is different from the native acl , which displays more proximal ruptures . with the use of autograft tissue and after a longer period of time , the rupture pattern in revision surgery is more similar to that of the native acl.conclusionthe most common rupture pattern after previous single - bundle reconstruction was elongation of the graft . 
 factors that influenced the rupture pattern were months between acl reconstruction and re - injury and graft type.level of evidencecohort study , level iv . 
Level of evidence
Introduction
Materials and methods
Statistical analysis
Results
Discussion
Conclusion