Abstract
Background:
Anterior shoulder instability often presents with bony lesions on the glenoid, humeral head, or both. The glenoid track (GT) method was validated to provide surgeons with a method to assess the risk of instability caused by this bone loss. Computed tomography scan is the gold standard of assessment, but subjects patients to a higher dose of radiation. Magnetic resonance imaging can be utilized with the GT to reliably evaluate patients with anterior shoulder instability and bipolar bone loss to determine the existence of an off-track Hill-Sachs lesion (HSL) preoperatively. The study revealed 84.2% accuracy, and statistical analysis established. This as a significant predictor. The purpose of this video is to illustrate how to calculate the GT, Hill-Sachs interval (HSI), and distance to dislocation (DTD) using MRI.
Indications:
Yamamoto et al were the first to present the GT, demonstrating that when assessing HSLs, the most crucial part of evaluation is their location on the humeral head.
Technique Description:
The GT can be defined as the area of contact of the humeral head on the glenoid during shoulder abduction and external rotation. If there is a glenoid bone defect, the GT width is reduced, and the new GT width is calculated. The following equation is used: GT = (0.83 * D) – d, where “D” is the diameter of the intact glenoid, and “d” is the diameter of the glenoid defect. The HSI is the width from the medial edge of the HSL to the insertion of the rotator cuff. The DTD is calculated by subtracting the GT from the HSI. An HSL is “off-track” if the medial margin of the lesion is more medial than the GT, resulting in no bone support. If an HSL is “off-track,” there is a higher risk of recurrence after arthroscopic Bankart repair. Therefore, for “off-track” lesions, adding remplissage is recommended or performing a bone block procedure.
Discussion/Conclusion:
Utilizing the GT technique is crucial when creating a treatment plan for patients with anterior shoulder instability as glenoid bone loss and “off-track” HSL have been shown to be predictors of recurrent instability, even in patients who have undergone prior surgical stabilization procedures.
Patient Consent Disclosure Statement:
The author(s) attest that consent has been obtained from any patient(s) appearing in this publication. If the individual may be identifiable, the author(s) has included a statement of release or other written form of approval from the patient(s) with this submission for publication.
This is a visual representation of the abstract.
Video Transcript
Relevant disclosures are listed.
The purpose of this video is to illustrate how to calculate the glenoid track (GT), Hill-Sachs interval (HSI), and distance to dislocation (DTD) using magnetic resonance imaging (MRI) instead of a computed tomography scan with 3-dimensional reconstruction, the current gold standard when assessing GT. The literature demonstrates that MRI can be used to reliably evaluate shoulder instability using the GT method.
Background
Patients with anterior shoulder instability can present with bony lesions within the glenoid and/or humeral head. Assessment of these lesions is critical in planning treatment. There are many different variations of anterior glenoid bone loss and Hill-Sachs lesions (HSLs). Factors that should be considered include the size, depth, and location of the HSLs and the size as well as the location of the glenoid bone loss. Ultimately, the GT method was introduced and validated to provide surgeons with a method to assess the risk of instability caused by bone loss.
Yamamoto et al, 6 in a cadaver study published in 2007, were the first to present the concept of the GT, demonstrating that when assessing HSLs, the most crucial part of the evaluation is their location on the humeral head rather than depth, volume, or area. This concept has since been refined as we have learned more about subcritical glenoid bone loss and recurrent instability.
Indications
The GT 3 can be defined as the area of contact of the humeral head on the glenoid during shoulder abduction and external rotation. It is positioned along the posterior perimeter of the humeral articular surface. The GT width is 84% of the glenoid width in cadaver shoulders and 83% in live shoulders. If there is a glenoid bone defect, the GT width is reduced, and the new GT width is calculated by subtracting the width of the defect from 83% of the glenoid width.
The GT is measured in millimeters and calculated with the following equation: GT = (0.83 * D) – d. “D” is the measurement of the diameter of the glenoid found by utilizing the perfect circle method, and “d” is the measurement of the glenoid defect, which is the distance from the anterior border of the glenoid to the perfect circle.
Technique Description
To determine which sagittal slice one will use to perform the measurements, we suggest starting by placing the axial and the sagittal slices side by side. We use the reference “toggle” line on the axial view and place that line as parallel as possible to the articular surface of the glenoid. This corresponds to the en face view on the sagittal stack. Once we have located the en face view, we use the circle region of interest tool to place a perfect circle along the posterior and inferior margins of the glenoid on the sagittal T2 plane.
The HSI is measured in millimeters and constitutes the width of the HSL, as well as the intact bone bridge between the attachment of the rotator cuff and the lateral margin of the HSL. The measurement is made on the axial MRI plane at the position with the greatest medial extent of the HSL. It is important to note that the bone bridge must be included in the HSI measurement and not just the size of the HSL.
An HSL is considered on-track if the medial margin of the lesion is within the GT. 2 An HSL is considered off-track if the medial margin of the lesion is more medial than the GT. In other words, if GT > HSI, this is an “on-track lesion,” and if GT< HSI, then it is considered an “off-track” lesion.
The on-track/off-track HSL can be assessed utilizing MRI. 5 In this first video, we will be illustrating how to measure an off-track HSL with bipolar bone loss. You will note we use a T2-weighted axial and sagittal oblique view side by side, using the reference toggle line to make sure we perform our measurements on the best slice. We then scroll over the sagittal stack and identify the cut that includes the entire glenoid face. This can be confirmed using the reference toggle line we mentioned previously. Next, we identify and measure the glenoid diameter using the best circle technique on the sagittal view previously selected. The best-fit circle is placed along the posterior and inferior perimeter of the glenoid. Then, to determine the diameter of the glenoid, a line is placed in the center of the best-fit circle and extended from the posterior to the anterior margins. This line is labeled “D.” In this patient. D = 27.8 mm. To determine the amount of anterior glenoid bone loss, a line is created adjacent to the first line between the anterior margin of the circle and the anterior margin of the glenoid. This line is labeled “d” and represents the anterior glenoid bone loss. In this patient, d = 6.76 mm. The GT is then calculated by utilizing the following equation: (0.83 * D) – d. In this patient, GT is 0.83 times 27.8, which equals 23. We then subtract 6.7, resulting in a GT of 16.3 mm.
Now, we will see how to calculate the HSI. This interval is measured in millimeters and constitutes the width of the HSL, as well as the intact bone bridge between the attachment of the rotator cuff and the lateral margin of the HSL. Scroll over the axial stack and identify the cut where the HSL extends more medially. Then draw a line from the most medial aspect of the HSL to the most medial aspect of the rotator cuff attachment. In this patient, the HSI is 26.5 mm. In this case, the GT is less than the HIS, and therefore the lesion is “off-track.”
Next, we will present a case with an “on-track” lesion. Again, with axial and sagittal slices side by side, we use the reference toggle line to find the en face view. On the sagittal en face view, we place the best-fit circle on the posterior and inferior aspect of the glenoid. Then we draw a line to measure the diameter of this circle, which is labeled “D.” In this patient, “D” is 31.8 mm. There is no anterior glenoid bone loss, and therefore, “d” is 0. So, the GT is 26.4 mm. Now we measure the HSI. Scroll the axial stack and find the slice where the HSL extends most medially. Then, draw a line from the most medial aspect of the HSL to the most medial aspect of the rotator cuff attachment, which in this case measures 22.8 mm, which is less than the GT. Therefore, this is considered an “on-track” lesion.
In this third case, we want to highlight how to correctly measure the HSI because a common mistake is to measure the HSI only to the width of the defect, but one must include the bony bridge, which is between the medial aspect of the rotator cuff insertion to the lateral aspect of the HSL. We measure the diameter of the intact glenoid, or “D,” to be 26.8. There is no anterior glenoid bone loss, and therefore, “d” is 0. So, the GT is 22.2 mm. Now we measure the HSI. Here you can see that if you measure only the width of the HSL, you will get an HSI of 14.4 mm. This will result in the lesion being classified as “on-track” because the GT is greater than the HSI.
Results
However, one must include the bony bridge as shown here, and this will result in an HSI of 24.3 mm, which is classified as an “off-track” lesion because the GT is less than the HSI. This highlights the importance of the correct measurement of the HSI. Moreover, it demonstrates that the location of the HSL is more important than its width, since a medial lesion can be off-track even if it has a small width.
Again, we want to emphasize the correct measurement for the HIS, which is the Hill-Sachs width plus the bone bridge, which is between the medial aspect of the rotator cuff insertion to the lateral aspect of the HSL. The yellow arrow represents the entirety of the HSI.
Discussion/Conclusion
Recently, Li et al 4 suggested a new concept to complement the GT, known as the DTD. The DTD, defined as the distance between the medial edge of the HSL and the anterior edge of the GT, was calculated with the following formula: DTD = GT – HSI. 1
Their study demonstrated that individuals with on-track lesions but low DTD had increased risk of failure after arthroscopic Bankart repair compared to those with a higher DTD. The authors propose a critical threshold DTD value of 8 mm at or below which the risk of failure after arthroscopic Bankart repair rises. They recommend adding a remplissage in patients with DTD less than 8 mm, which has been demonstrated to decrease the failure rates. The critical threshold for DTD decreases and becomes more predictive of failure with increasing age.
Utilizing the GT technique on MRI to predict off- and on-track HSL provides additional information to help in surgical decision-making for patients with anterior shoulder instability. Understanding of risk factors that play a role in recurrent instability is also paramount for the management of this condition. We hope this video helps clarify these concepts.
Thank you to everyone who participated in the production of this video.
Footnotes
Submitted August 12, 2024; accepted June 30, 2025.
One or more of the authors has declared the following potential conflict of interest or source of funding: E.G.M. receives other financial or material support from Arthrex and serves on the editorial board of Arthroscopy. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.
