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Background: We have proposed the concept of glenoid track ("on-track/off-track" lesion) to evaluate the risk of engagement of the Hill-Sachs lesion with the glenoid after arthroscopic Bankart repair. This concept has been widely used and many clinical validation studies have been reported. To measure the glenoid track width, we have recommended to use 3-dimensional computed tomography (CT) images. However, the CT method has the issue of radiation exposure and involves time and effort to make 3-dimensional CT images from 2-dimensional images. For these reasons, there are several reports describing the measurement method using magnetic resonance imaging. Recently, the threshold of the critical glenoid bone loss becomes lower. A zone of bone loss below the critical size is called "subcritical bone loss", which might be related to deterioration of quality of life and bone grafting is recommended. We applied the concept of "subcritical bone loss" to the glenoid track. Patients with "on-track" lesions can be divided into 2 subgroups: those with a "peripheral-track" lesion (most medial 1/4) and those with a "central-track" lesion (the rest 3/4). More recently, similar evaluation methods to evaluate the risk of "off-track" lesions have been reported: ''distance to dislocation'' and "Hill-Sachs interval/glenoid track ratio". Also, similar concept to "peripheral-track" lesion, "near-track" lesion was reported. The concept of "peripheral-track" lesion is a concept of assessing an "on-track" lesion which is very close to the medial margin of the glenoid track (subcritical bone loss). Methods: Similar evaluation methods to evaluate the risk of "off-track" or "peripheral-track" lesions were proposed in the literature. A review was performed by searching PubMed. Journal articles published between January 2014 and January 2023 were taken into account. They were compared and their differences were explained. Results: The "near-track" lesion concept is similar to "peripheral-track" lesion. However, the cutoff value is different: Hill-Sachs occupancy ≥ 75% is the "peripheral-track" lesion, whereas "distance to dislocation" < 8 mm is the "near-track" lesion. Conclusion: We introduced update of the glenoid track concept including the evaluation method, peripheral-track lesion, and its clinical application.

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INTRODUCTION: The purpose of this study was to determine if "off-track" Hill-Sachs lesions in patients with dynamic anteroinferior instability were transformed into "on-track" lesions using iliac bone autografts with screw fixation. The secondary purpose was to observe if postoperative bony remodeling would occur over time, resulting in recurrent "off-track" Hill-Sachs lesions with corresponding instability. MATERIALS AND METHODS: We retrospectively reviewed clinical and CT records of 8 patients with an "off-track" Hill-Sachs lesion who underwent open anatomical glenoid reconstruction with an iliac crest bone autograft. Hill-Sachs lesions, glenoid track widths, and glenoid surface areas were measured on a preoperative and two postoperative (6 weeks, ≥ 2 years) 3D-CT models to determine graft resorption over time. All patients were available for postoperative clinical and CT final follow-up 3 years (2-4 years) postoperatively. RESULTS: In all patients, the Hill-Sachs lesions were "on-track" 6 weeks postoperatively and remained "on-track" at final-follow-up. Compared to preoperative values, the glenoid track width and glenoid surface area both were higher 6 weeks postoperatively (p < 0.001 and p = 0.023, respectively) and at final follow-up (p < 0.001 and p = 0.023, respectively). Whereas the glenoid track width between 6 weeks and final follow-up showed no decrease (p = 0.234), glenoid surface area tended to decrease (p = 0.055). The median SSV was 93 points (85-95 points), the Rowe score 90 points (80-100 points) and the WOSI 1980 points (1783-2067 points) at final follow-up. No recurrent dislocations or subluxations were observed. CONCLUSIONS: An open anatomical glenoid reconstruction with an iliac crest bone autograft technique using screw fixation effectively transformed "off-track" Hill-Sachs lesions to "on-track" lesions, resulting in good short-term clinical outcomes. Whereas glenoid surface area tended to be reduced by bony remodeling processes over time, the glenoid track width did not decrease at final follow-up and consequently no recurrence of "off-track" lesions occurred. LEVEL OF EVIDENCE: Case series; Level of evidence, IV.

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The shoulder is the most frequently dislocated joint in the body due to a larger range of motion and a small area of articulation between the humeral and glenoid surfaces. Traumatic shoulder dislocations, especially those associated with injury to the labroligamentous or bony stabilizers of the joint, lead to further reduction of articular surface contact with resultant glenohumeral instability and recurrent shoulder dislocations. Imaging plays an increasingly important role in the preoperative evaluation of patients with traumatic shoulder instability by evaluating glenohumeral bone loss (uni- or bipolar), assessing soft tissue injuries and identifying patients at risk of postoperative recurrence. Quantification of bone loss is key to differentiate engaging vs. non-engaging Hill-Sachs lesions, while newer concepts of "on-track" vs. "off-track" lesions are being discussed that can determine the required surgical approaches. In this article, we review the preoperative imaging approaches, traditional treatments, outline the bone loss measurement strategies and review these new tracking concepts with relevant case examples.

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BACKGROUND: The glenoid track concept describes the dynamic interaction of bipolar bone loss in anterior glenohumeral instability. Initial studies have successfully demonstrated this concept's application in clinical populations. In clinical practice, the Latarjet procedure is commonly the preferred treatment in addressing "off-track" Hill-Sachs lesions. The effectiveness of this procedure in restoring such lesions to an "on-track" state, however, has not yet been evaluated or described in the literature. HYPOTHESIS: The Latarjet procedure would transform "off-track" Hill-Sachs lesions to "on-track" lesions. Lesions would remain "on-track" during follow-up, despite glenoid remodeling. STUDY DESIGN: Case series; Level of evidence, 4. METHODS: Patients with "off-track" Hill-Sachs lesions treated with the arthroscopic Latarjet procedure between March 2013 and May 2014 were included. Glenoid track and coracoid graft contact surface area measurements using 3-dimensional computed tomography (3D-CT) were performed preoperatively and at 6-week, 6-month, and at least 12-month (final) follow-up. The mean final follow-up was 23 months. The glenoid diameter, as a percentage of the native glenoid, was also calculated from this imaging. RESULTS: Twenty-six patients met the inclusion criteria. 3D-CT scans were available for all patients preoperatively and postoperatively, with 21 patients (81%) undergoing 6-month follow-up CT and 19 patients (73%) undergoing final follow-up CT. Hill-Sachs lesions remained "on-track" at all follow-up time points. The mean glenoid diameter changed significantly from 84.6% preoperatively to 122.8% at 6 weeks ( P < .001) and from 120.5% at 6 months to 113.9% at final follow-up ( P = .005). This was also reflected in significant remodeling seen in the coracoid graft articular contact area (6 weeks to 6 months, P = .024; 6 months to final follow-up, P = .002). This persisting glenoid arc enlargement at final follow-up avoided "off-track" Hill-Sachs lesions in 6 of 19 patients (32%), which would otherwise have occurred had the coracoid graft remodeled to native glenoid dimensions. CONCLUSION: The Latarjet procedure provides an effective treatment for "off-track" engaging Hill-Sachs lesions, despite an evident glenoid remodeling process. At a mean of 23 months postoperatively, a mean persisting enlargement of the glenoid arc of 14% beyond native dimensions remained, avoiding a recurrent "off-track" lesion in 32% of patients, which would otherwise have occurred with complete remodeling.

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Shoulder stability depends on the position of the arm as well as activities of the muscles around the shoulder. The capsulo-ligamentous structures are the main stabilisers with the arm at the end-range of movement, whereas negative intra-articular pressure and concavity-compression effect are the main stabilisers with the arm in the mid-range of movement.There are two types of glenoid bone loss: fragment type and erosion type. A bone loss of the humeral head, known as a Hill-Sachs lesion (HSL), is a compression fracture of the humeral head caused by the anterior rim of the glenoid when the humeral head is dislocated anteriorly in front of the glenoid. Four out of five patients with anterior instability have both Hill-Sachs and glenoid bone lesions, which is called a 'bipolar lesion'.With the arm moving along the posterior end-range of movement, or with the arm in various degrees of abduction, maximum external rotation and maximum horizontal extension, the glenoid moves along the posterior articular margin of the humeral head. This contact zone of the glenoid with the humeral head is called the 'glenoid track'.A HSL, which stays on the glenoid track (on-track lesion), cannot engage with the glenoid and cannot cause dislocation. On the other hand, a HSL, which is out of the glenoid track (off-track lesion), has a risk of engagement and dislocation. Clinical validation studies show that the 'on-track/off-track' concept is able to predict reliably the risk of a HSL being engaged with the glenoid. For off-track lesions, either remplissage or Latarjet procedure is indicated, depending upon the glenoid defect size and the risk of recurrence. Cite this article: EFORT Open Rev 2017;2:343-351.