Effective glenoid track: A novel concept for shoulder instability

Abstract

Background

Assessment and quantification of bone loss in cases of shoulder instability is critical for surgical decision making. The glenoid track concept was initially developed to assess Hill Sachs lesions taking into account the native glenoid diameter of the contralateral shoulder and assessing the degree of glenoid bone loss. However, it can not be reverse calculated to determine the effect of an addition of a bone block. We have developed a novel model to help address this problem yielding an “effective glenoid track” (EGT).

Methods

Begin as we always do by using Itoi's concept for assessment of tracking based on the CT scan cuts. Next step is to calucate the Hill Sach's interval (HSI) which will require an MRI scan.

Conclusion

The EGT allows for calculation of residual tracking of Hill Sachs lesions post a bone block addition and will aid in surgical decision making.

1. Introduction

The assessment of glenoid bone loss is critical to the successful outcomes of any anterior shoulder stabilization procedure.¹ We know that significant glenoid bone loss is associated with higher failure rates after arthroscopic Bankart repair, and it is in these patients where the addition of a bone block procedure, such as a Latarjet procedure, helps reduce failure rates.^2,3 The traditional goalpost of glenoid bone loss is undergoing revision, with the prior concept of one-third or one-quarter critical bone loss being revised to a sub-critical bone loss value of 12.5 %.^1,4

Assessment of humeral bone loss constitutes the second pillar of bipolar bone loss. Hill Sach's lesions (HSL) which were previously defined as engaging or non-engaging have also been relooked at through the glenoid track concept.⁵ Itoi's work has been instrumental in helping us understand the concept and their model of glenoid track allows for calculation of risk of engagement both for a native and a deficient glenoid.⁶ One of the biggest fallacies of the model is that it does not allow the user to reverse calculated that when a bone block is added, as it is varied in size, to the deficient glenoid, what is the resultant effect on the glenoid track and whether the HSL becomes an off track lesion with this addition of bone.

Accordingly, we have developed a novel model which we define as “effective glenoid track” (EGT) to serve this purpose. The EGT allows the surgeon to template adding a thickness of bone block to the deficient glenoid and then calculate the resultant effect of the HSL. This model should aid surgeons in decision making for the addition of a remplissage procedure in the setting of a bone block procedure for anterior shoulder stabilization.

2. EGT – effective glenoid track

3D CT scan is crucial. Begin as we always do by using Itoi's concept for assessment of tracking based on the CT scan cuts.⁵ The widest antero-posterior diameter of the glenoid (D) is calculated for both shoulders using any method, we use the circle of best fit method. The resultant bone loss (d) is added to this picture and the final diameter of the deficient glenoid is thus D-d.

We then determine our bone block thickness (d2) based on the coracoid dimensions available (Fig. 1, Fig. 2). Alternatively, if other bone block sources such as iliac crest or allograft are used they may be templated.

Fig. 1.

Method to calculate effective glenoid diameter (EGD) on ‘en face’ view of glenoid with bone loss on 3D CT imaging using circle of best fit method.

Fig. 2.

Schematic representation of calculating Effective Glenoid Track.

Once the bone block (d2) has been added to the glenoid defect, a resultant new effective diameter (EGD) of the glenoid is created. All tracking needs to be assessed based on this new diameter. The new diameter can be calculated using the following equation:

EGD = D + d2 – d

Using Itoi's original work of calculating glenoid track based on the 0.83 concept, we can now calculate the effective glenoid track (EGT) as follows:

EGT = 0.83 × EGD

3. Assessment of tracking based on Hill Sach's interval

The next step is to calucate the Hill Sach's interval (HSI) which will require an MRI scan (Fig. 3). The resultant calculations can now be made of this new EGT which will allow for a truer mathematical assessment of tracking.

Fig. 3.

Method to calculate HSI (Hill-sachs Interval) on MRI axial image. HSI is measured as the width of the Hill-Sachs lesion plus the width of the intact bone bridge between the rotator cuff insertion and the Hill-Sachs lesion.

HSI > EGT – off track lesion needing an additional remplissage.

HSI < EGT – on track lesion which does not require any additional procedure.

4. Reverse calculation of amount of bone block needed

Another utility of this model is that it can also be used to pre-operatively determine the minimum graft diameter (d2) required for converting the off track lesion into an on track lesion.

For example, If the native diameter is 26 mm and the glenoid bone loss is 3 mm with a HIS of 21 mm.

We can calculate the D2 as such, we set the EGT to the HIS which is 21 mm and re model the equation as such:

d2 = (HIS/0.84) + d – D

In the above example

d2 = (21/0.84) + 3–26 = 2

Hence the minimum graft diameter during harvest should be 2 mm to convert the off track lesion into an on track lesion and hence save the need for an additional remplissage procedure.

5. Fallacy of the model

The model oversimplifies the process for bone block principles. There are many variables which must be taken into account when a surgeon decides on a particular model.

Firstly, when we are preparing the native anterior glenoid bone for a bone block procedure we may end up losing 1–2 mm during this preparation which is not factored into the model. Secondly, the compression of fit will be variable, although we all aim for a bone-to-bone perfect fit any change in this fit whether in the sagittal, coronal or axial plane will affect the final model.

6. Conclusion

Surgical decision making for bipolar bone loss in anterior shoulder instability is complex. The flaw of the glenoid track concept is that it does not allow for calculations of tracking to be made after a bone block procedure. Our novel model of effective glenoid track has been developed to address this short coming. Further it can be reverse calculated to determine the minimum bone block thickness needed to convert an off track lesion to an on track lesion. This should aid surgeons when deciding whether to add an additional remplissage procedure while performing a bone block procedure.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflict of interest

None.

Financial disclosures/sponsorship

None.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.