To our knowledge, this is one of the first clinical applications of the glenoid track concept in the evaluation of bone loss with shoulder instability7. This concept attempts to incorporate multiple theories that contribute to the complex nature of shoulder instability and considerations for risk factors for failure of arthroscopic Bankart repairs. Our purpose was to apply this concept of a glenoid track evaluation in a clinical cohort to determine if it predicted postoperative instability after an isolated arthroscopic Bankart procedure. In comparison with the threshold of 20% glenoid bone loss, off-track status proved to be a significantly better predictor for failure of the arthroscopic Bankart repair. This was true for our cohort as a whole. It is worth noting that the negative predictive value was 92%; that is, when the shoulder was on-track, arthroscopic repair rarely failed.
Burkhart and De Beer noted glenoid bone loss to be a factor when they coined the term “inverted pear” glenoid3. Glenoid bone loss is a known risk factor for recurrent instability2,16. Biomechanical models have indicated that >21% could be clinically important4,21. Other authors have reported glenoid bone loss as a risk factor for failure of arthroscopic Bankart repairs in clinical studies with attempts made to quantify the amount of bone loss that increases the risk of failure3,17. Currently, >20% bone loss in the anterior-inferior portion of the glenoid is considered critically high, with subcritical amounts of 13.5% bone loss now being considered a prognostic factor13,22.
The importance of the Hill-Sachs lesion has been noted for some time23. The characteristic of the Hill-Sachs lesion that renders it clinically relevant has been the subject of much investigation. Attempts have been made to quantify the lesion, whether by depth, length, surface area, or volume, all with the hope of being able to predict when a Hill-Sachs lesion requires intervention and when it does not5,6,24,25. The term “engaging Hill-Sachs” became popular as Burkhart et al. evaluated the lesion with a dynamic approach3,26. However, attempting to utilize this to help to determine surgical planning has been difficult, as discussed in depth by Di Giacomo et al.7. Still, the principle that a Hill-Sachs lesion engaging in a functional range of motion results in recurrent instability remained very important.
Yamamoto et al. initiated discussion of a glenoid track1,14,15. They postulated that there was a certain path in which a portion of the humeral head contacted the glenoid in the motion of abduction and external rotation. This track has now been well documented and is able to be calculated with imaging or arthroscopy7,14. Di Giacomo et al. combined the ideas of an engaging Hill-Sachs lesion and the glenoid track to develop a method of calculating the likelihood that a Hill-Sachs lesion will engage in the functional range of motion, accounting for both the size of the Hill-Sachs lesion and the amount of glenoid bone loss7. In theory, this method would allow the surgeon to preoperatively assess the need for addressing a Hill-Sachs lesion or to alter a surgical plan if a high likelihood of failure exists with an arthroscopic Bankart repair. This theory has recently been validated using a biomechanical model18. However, limited clinical applications have utilized this method.
As the glenoid track concept was conceived to evaluate bipolar bone loss, we performed a subgroup analysis including only the patients with bipolar bone loss. We found that 25% of patients who were off-track remained stable, and 92% of patients who were on-track remained stable. The predictive value of the glenoid track was higher than using glenoid bone loss alone, which correctly predicted postoperative stability in 73% of patients. However, with the numbers available, the difference in predictive value between the glenoid track with glenoid bone loss did not achieve significance.
Our cohort was nearly evenly split between those with bipolar bone loss and those with monopolar bone loss (no Hill-Sachs lesion). Although the glenoid track concept is often cited as applicable to bipolar bone loss, we chose to include monopolar lesions in our cohort, which would, by definition, be classified as on-track. We did a subanalysis in patients without a Hill-Sachs lesion, comparing the predictive value of the track concept with that of glenoid bone loss of >20%.
We found the track concept to have a superior positive predictive value compared with glenoid bone loss alone, even in the patients with no Hill-Sachs lesion (monopolar bone loss). In the setting of monopolar bone loss, using the glenoid track concept correctly predicted postoperative stability in 89% of patients. Conversely, if using glenoid bone loss of >20% to predict failure, we would have been incorrect 71% of the time. Thus, we found the glenoid track concept to be a more reliable predictor of postoperative stability than glenoid bone loss alone, even in patients without a Hill-Sachs lesion, and we thus apply it to all patients with shoulder instability.
Increasing glenoid bone loss has recently been correlated with worsening WOSI scores after arthroscopic Bankart repairs22. In our cohort, WOSI scores were significantly lower in the patients who had treatment that failed; additionally, the off-track group had significantly lower WOSI scores than the on-track group. When excluding patients with monopolar bone loss, the off-track group still had worse WOSI scores (p = 0.05). Additionally, when the off-track lesions were combined with those that were nearly off-track (Hill-Sachs interval and glenoid track differing by <2 mm), the WOSI score was significantly worse compared with that for patients who were more on-track. These data add to existing evidence that although bone loss, both glenoid and humeral, is important for recurrent instability, it also weighs heavily on functional outcome.
In 2007, Balg and Boileau introduced the Instability Severity Index Score as a tool to help to predict instability after an arthroscopic Bankart procedure27. This score included points for both glenoid and humeral bone loss. As the dynamic interaction has been better understood with the concepts of an engaging Hill-Sachs lesion and the glenoid track, investigators have sought improvements in the preoperative prediction of those who will likely need concomitant procedures and those who will do well with an isolated arthroscopic Bankart7,14,25,26. Our data validate the glenoid track concept, as evidenced by the high predictive accuracy when applied to our clinical cohort.
There were several weaknesses to this study. First, it was retrospective and had potential biases therein. However, surgeons who measured the imaging were blinded to the clinical result, limiting detection bias. No patient was excluded after data combination, which may have helped in minimizing selection bias. In addition, our sample size was not sufficiently powered to demonstrate the superiority of the track concept to glenoid bone loss when we limited our analysis to strictly bipolar bone loss. Finally, the cohort in this study was a military cohort, and therefore our results may not have been typical of civilian populations. However, it should be noted that this population is characterized by young, active men, who are also common in any instability population. In spite of these weaknesses, we were able to clinically validate the importance of the on-track and off-track paradigm of shoulder instability, as we were able to demonstrate the improved accuracy of the track concept over isolated glenoid bone loss (a positive predictive value of 75% for the track concept compared with 44% for the isolated glenoid bone loss) when evaluating the cohort as a whole, as well as in bipolar and monopolar bone loss individually. Furthermore, to our knowledge, this is the first study to evaluate the glenoid track concept in a clinical population using postoperative functional outcomes. We found it to be highly predictive of outcome and superior to the predictive value of glenoid bone loss of 20%.
In conclusion, the glenoid track concept was recently proposed to evaluate the combined importance of both glenoid and humeral bone loss in shoulder instability. The application of this concept to our clinical cohort demonstrated that the glenoid track was a more accurate predictor of instability compared with isolated glenoid bone loss after primary arthroscopic Bankart repairs. This validation of the glenoid track concept encourages its use in the diagnostic work-up of patients who are undergoing consideration of stabilization for anterior shoulder instability. With this increased understanding of the effect of the track status on postoperative outcomes, we no longer perform an isolated arthroscopic Bankart in a patient with an off-track, unstable shoulder.
Investigation performed at the Tripler Army Medical Center, Honolulu, Hawaii
Disclosure: There was no external source of funding. On the Disclosure of Potential Conflicts of Interest forms, which are provided with the online version of the article, one or more of the authors checked “yes” to indicate that the author had a relevant financial relationship in the biomedical arena outside the submitted work.
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