although the glenoid and humeral osseous defects are common injuries associated with anterior dislocation of the shoulder,1011 ) there have been no reports demonstrating when these osseous defects occur . some believe that they are created at the time of dislocation , whereas some state that they occur during reduction or a little later after dislocation . kroner et al.,12 ) in a large - scale epidemiological study of 250,000 subjects , investigated the mechanism of injury in 216 patients with anterior shoulder dislocation . they found that the most common mechanism of injury ( 46% of all injuries ) was a blow against the shoulder . recently , the arm positions have been studied from video analysis , which demonstrates that the dislocation occurred in positions other than abduction and external rotation.1314 ) a hill - sachs lesion ( hsl ) in patients with chronic anterior dislocation of the shoulder is greater than that seen in patients with acute dislocation.15 ) a recent report indicated that the hsl was significantly larger if the time from dislocation until reduction exceeded 5 hours.16 ) considering these reports , we may be able to state that the hsl is created when the humeral head is compressed against the glenoid rim after the dislocation , and it increases as the compression time lengthens . clinicians must understand that when patients who sustain dislocation come to an emergency room , their shoulders are adducted and internally rotated . based on these facts , it can be speculated that the hsl is created during the process to adduction and internal rotation after dislocation . the computed tomography ( ct ) images of 100 shoulders of 100 patients , with recurrent anterior dislocation of the shoulder , were investigated by kawakami et al.,17 ) using an image analyzing software to determine the shoulder position when a hsl is created . they created three - dimensional surface bone models of the scapula and humerus separately ; the humerus was then moved such that the longitudinal axis of the hsl and the anterior glenoid rim fit each other . subsequently , the arm positions were measured three - dimensionally . the average arm position in which a hsl was created was demonstrated to be 74 of abduction , 27 of external rotation , and 3 of horizontal adduction . if the hsl is created at the time of dislocation , the dislocation position should be the same as this position . however , their data showed that the arm position in which the hsl had been created was not the so - called dislocation position ( abduction and maximum external rotation ) , but at a much lower angle of abduction and external rotation . one is that the dislocation occurs in lower angle of abduction and external rotation position . another possibility is that the hsl is created not at the time of dislocation , but a little after the dislocation . it is generally thought that a glenoid osseous defect occurs anteroinferiorly from surgical or radiological findings . however , recent studies18192021 ) have found that the glenoid defect was located anteriorly rather than anteroinferiorly . both griffith et al.18 ) and , more recently , saito et al.19 ) described the glenoid osseous defect as being located anteriorly at approximately the three o'clock position in the right shoulder . however , since the glenoid inclines anteriorly in the sagittal plane relative to the trunk , the true location of the glenoid defect is anteroinferior , relative to the scapula . the location of hsls was quantitatively measured on ct images by saito et al.19 ) their data showed that the hsl existed in the area between 0 and 24 mm from the top of the humeral head ; the inferior portion of the hsl overlaps the bare area when it extends beyond 19 mm from the top of the humeral head . the hsl was oriented toward 6:46 at the top and 8:56 at the bottom of the lesion , with an average orientation of 7:58  0:48 . the depth,22 ) size,23 ) and volume22 ) of the hsl have been thought to be parameters to estimate the risk of engagement . the relative location of the hsl and the glenoid is the most important parameter . in a cadaver study , yamamoto et al.9 ) the location of the glenoid was marked on the humeral head , with the arm elevating along the end - range of glenohumeral motion . as the arm was elevated at 0 , 30 , and 60 with maximum external rotation and horizontal extension , the contact area of the glenoid shifted from inferomedial to superolateral portion of the humeral head , creating a zone of contact . the distance from the medial margin of the footprint of the rotator cuff to the medial margin of this contact area at 60 of abduction was 18.4  2.5 mm , which was equivalent to 84%  14% of the glenoid width . 1 ) . using this new concept , we are able to evaluate whether or not there is a risk of engagement . if the hsl remains within the glenoid track , there is no risk of engagement between the hsl and the glenoid . however , if the hsl extends medially over the glenoid track , there is a risk of engagement . they demonstrated that the width of the glenoid track in live shoulders was 83% of the glenoid width at 90 of abduction , which was very close to the measurement in yamamoto9 ) 's cadaveric study . recently , di giacomo et al.25 ) have developed a method that uses the concept of the glenoid track to determine whether the hsl will engage the glenoid rim with or without an anterior glenoid osseous defect . if the hsl engages , it is called an " off - track " hsl ; if it does not engage , it is an " on - track " lesion . on the basis of this quantitative method , they have developed a treatment paradigm with specific surgical criteria for all patients with anterior instability , both with and without bipolar osseous defect . more recently , cook et al.26 ) validated the on - track vs. off - track concept in anterior shoulder instability , and they concluded that osseous defect defined as on - track or off - track is a better predictor of failed arthroscopic stabilization than solely quantifying glenoid osseous defect . considering the glenoid osseous defect , since the true location of the glenoid is demonstrated to be anterior , we need to re - evaluate the effect of bony defect of the glenoid on anterior shoulder stability . in a biomechanical study , yamamoto et al.7 ) reported that reconstruction of the glenoid concavity might be necessary in shoulders with an anterior glenoid defect of at least 26% of the glenoid width . in their study , all the soft tissues , including the labrum and the capsule , were removed . they performed a similar biomechanical study , taking the soft tissue contribution into consideration.8 ) they reported that the critical size of an anterior glenoid defect was the one with a thickness that was at least 25% of the glenoid width ( fig . 2 ) . thus , in cases with a large glenoid defect ( > 25% of the glenoid width ) , bone grafting is necessary as a surgical treatment . when we consider about the engaging of the hsl , an important point to remember is that engagement always occurs between the hsl and the glenoid rim . in fact , the engagement is known to be observed more frequently if there is a large glenoid defect ( fig . 3).27 ) therefore , when considering the critical size of the hsl , we need to consider the glenoid osseous defect as well . in a new concept proposed by yamamoto et al.,9 ) the glenoid track enables us to take both lesions into consideration at the same time . in case of a glenoid defect , the defect width should be subtracted from 83% of the glenoid width , which is the width of the glenoid track measured in live patients ( fig . kurokawa et al.11 ) reported that in their series of 100 shoulders with recurrent anterior dislocation , 94 shoulders had hsl , and 7 of them ( 7.4% ) were defined as off - track hsl . their study showed that there are two categories of off - track hsl : the large and wide type , and the narrow but medially located type . there is a positive relationship between the sizes of the glenoid defect and hsl : the larger the glenoid defect , the larger the hsl . thus , if we find a large glenoid osseous defect , we need to confirm whether the hsl is an on - track or off - track lesion . if the glenoid defect is less than 25% , which is biomechanically demonstrated to be a critical size,8 ) no treatment is required for this defect . if the latarjet procedure is selected , we can expect to enlarge the glenoid track by bone grafting . in the arthroscopic remplissage procedure , engagement can be avoided by filling the humeral head defect with the infraspinatus tendon . if the glenoid defect is more than 25% , treatment such as bone grafting is the therapy of choice . in these cases , the latarjet procedure is recommended , which will reconstruct the glenoid concavity , at the same time converting the hsl from an off - track lesion to an on - track lesion . if the hsl still remains off - track even after the latarjet , then remplissage or bone graft needs to be performed in addition to the latarjet .
shoulder surgeons need to be aware of the critical size of the glenoid or humeral osseous defects seen in patients with anterior shoulder instability , since the considerable size of osseous defect is reported to cause postoperative instability . 
 biomechanical studies have identified the size of the osseous defect which affects stability . 
 since engagement always occurs between a hill - sachs lesion and the glenoid rim , when considering the critical size of the hill - sachs lesion , we have to simultaneously consider the size of the glenoid osseous defect . with the newly developed concept of the glenoid track , we are able to evaluate whether a large hill - sachs lesion is an " on - track " or " off - track " lesion , and to consider both osseous defects together . in case of an off - track hill - sachs lesion , if the glenoid defect is less than 25% , no treatment is required . in this case 
 , the latarjet procedure or arthroscopic remplissage procedure can be a treatment option . 
 however , if the glenoid defect is more than 25% , treatment such as bone grafting is required . 
 this will convert an off - track lesion to an on - track lesion . 
 after the bone graft or latarjet procedure , if the hill - sachs lesion persists as off - track , then further treatment is necessitated . in case with an on - track hill - sachs lesion and a less than 25% glenoid defect , 
 arthroscopic bankart repair alone is enough .