Clinical Validation of the Glenoid Track Concept in Anterior Glenohumeral Instability

Shaha, James S. MD; Cook, Jay B. MD; Rowles, Douglas J. MD; Bottoni, Craig R. MD; Shaha, Steven H. PhD, DBA; Tokish, John M. MD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.15.01099
Scientific Articles
Abstract

Background: Glenoid and humeral bone loss are well-described risk factors for failure of arthroscopic shoulder stabilization. Recently, consideration of the interactions of these types of bone loss (bipolar bone loss) has been used to determine if a lesion is “on-track” or “off-track.” The purpose of this study was to study the relationship of the glenoid track to the outcomes of arthroscopic Bankart reconstructions.

Methods: Over a 2-year period, 57 shoulders that were treated with an isolated, primary arthroscopic Bankart reconstruction performed at a single facility were included in this study. The mean patient age was 25.5 years (range, 20 to 42 years) at the time of the surgical procedure, and the mean follow-up was 48.3 months (range, 23 to 58 months). Preoperative magnetic resonance imaging was used to determine glenoid bone loss and Hill-Sachs lesion size and location and to measure the glenoid track to classify the shoulders as on-track or off-track. Outcomes were assessed according to shoulder stability on examination and subjective outcome.

Results: There were 10 recurrences (18%). Of the 49 on-track patients, 4 (8%) had treatment that failed compared with 6 (75%) of 8 off-track patients (p = 0.0001). Six (60%) of 10 patients with recurrence of instability were off-track compared with 2 (4%) of 47 patients in the stable group (p = 0.0001). The positive predictive value of an off-track measurement was 75% compared with 44% for the predictive value of glenoid bone loss of >20%.

Conclusions: The application of the glenoid track concept to our cohort was superior to using glenoid bone loss alone with regard to predicting postoperative stability. This method of assessment is encouraged as a routine part of the preoperative evaluation of all patients under consideration for arthroscopic anterior stabilization.

Level of Evidence: Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.

Author Information

1Department of Orthopaedic Surgery, Tripler Army Medical Center, Honolulu, Hawaii

2Department of Orthopaedic Surgery, University of Oklahoma, Norman, Oklahoma

3University of Utah, Salt Lake City, Utah

4Department of Orthopaedic Surgery, Steadman Hawkins Clinic of the Carolinas, Greenville, South Carolina

E-mail address for J.S. Shaha: jshaha6@gmail.com

Article Outline

The role of arthroscopy in the treatment of anterior shoulder instability continues to be refined. The surgical technique focuses on anterior capsulolabral reconstruction. The presence of bone loss is a well-known risk factor for failure1. Although both humeral and glenoid-sided defects have been identified as risk factors for failure, the literature has generally treated these contributions independently1-17. The evolution of this evaluation has progressed over the years to include the dynamic interaction of the humeral head with the glenoid. Although this interaction was initially referred to as the “engaging Hill-Sachs lesion,” Yamamoto et al. introduced the concept of the glenoid track, which described the theoretical path of the humerus along the glenoid throughout the range of motion14. In this model, the engaging Hill-Sachs lesion can be predicted, taking into account both glenoid bone loss and the size of the Hill-Sachs lesion.

Recently, these concepts were combined with a description of an on-track and off-track classification to help to predict the risk of recurrent instability. Although the track concept is valid biomechanically18, to our knowledge, there has been limited evidence to support application in the clinical setting7. The purpose of this study was to apply the glenoid track concept to a clinical cohort of isolated, primary, arthroscopic Bankart reconstructions to validate whether the theory correctly predicted postoperative outcome or recurrent instability. A secondary purpose was to compare the predictive value of the on-track and off-track theory with the use of glenoid bone loss alone as a predictor of recurrent instability.

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Materials and Methods

This was an institutional review board-approved, retrospective review of a prospectively gathered collaborative database. The surgical database was queried for all arthroscopic shoulder instability procedures performed over a 2-year period on active-duty U.S. military service members by one of three fellowship-trained orthopaedic sports surgeons. Medical records were reviewed for demographic and medical data. Inclusion criteria were all primary arthroscopic, isolated, anterior Bankart reconstructions with a minimum 2-year follow-up. Any patient who had multidirectional instability, had undergone a concomitant procedure, or had undergone a revision procedure was excluded. All patients had preoperative magnetic resonance imaging (MRI) and postoperative outcome measures including the Single Assessment Numeric Evaluation (SANE) and Western Ontario Shoulder Instability Index (WOSI) scores in addition to return-to-duty status. Patient-reported outcomes were captured using the Standardised Orthopaedic Clinical Research and Treatment Evaluation Software (SOCRATES), and data were merged in the Society of Military Orthopaedic Surgeons (SOMOS) collaborative quality assurance database. Success was defined as the return to unrestricted duty. Patients who were unable to resume full activity, because of redislocation or subjective instability, were classified as having had treatment that failed.

Each patient had undergone a trial of nonoperative treatment prior to surgical intervention including physical therapy and activity modification. Patients who underwent nonoperative treatment that failed were offered surgical reconstruction by the attending physician. A standard arthroscopic Bankart repair was performed in either the lateral or beach-chair position. All surgeons used a similar approach. Following diagnostic arthroscopy confirming an isolated Bankart lesion, anterior labral tissue was mobilized and the glenoid surface was prepared. Suture anchors (SutureTak; Arthrex) were placed on the edge of the glenoid, and labral tissue was repaired and was secured. The postoperative protocol was standardized across all patients.

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Postoperative Rehabilitation

Immediately after the surgical procedure, each patient had the arm placed in a sling and was instructed to range the elbow and perform Codman exercises at least 4 times a day. Therapy protocols were divided into 4 stages. The first stage included range of motion with only forward flexion. The second stage focused on regaining full motion in all directions under the supervision of a licensed physical therapist, and the third stage focused on working on rotator cuff strengthening and periscapular shoulder strengthening. The fourth stage began in the fourth month and focused on dynamic and ballistic exercises. This final stage was typically 2 to 3 months in duration, and patients were typically cleared for return to full, unrestricted activities at 6 months postoperatively.

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Imaging Analysis

Similar to the method used by Di Giacomo et al., bone loss was measured using preoperative MRI by two authors of this study who were blinded to the clinical data7. Although initially described as a technique utilized with either arthroscopy or three-dimensional computed tomography (CT), we utilized MRI as the primary imaging modality, as multiple studies have shown that MRI is reliable for quantifying bone loss19,20. Measurement of glenoid bone loss was performed via the perfect circle technique. This involved placing a circle that best fit the inferior two-thirds of the glenoid, measuring the diameter of the circle, and then measuring the width of the defect. The glenoid track was calculated as 83% of the expected width minus the measured glenoid bone loss. Humeral bone loss was measured on an axial MRI sequence as the distance from the articular insertion of the rotator cuff to the medial margin of the Hill-Sachs lesion. The slice with the largest distance or size was used. Using the calculation steps in Table I, each patient was categorized as either on-track or off-track. If the humeral bone loss (Fig. 1) was larger than the glenoid track (Fig. 2), the patient was classified as off-track7,20. Patients without a Hill-Sachs lesion were placed in the on-track group by definition. Fifteen of 57 patients had an additional three-dimensional CT scan. The same measurements were performed with interobserver and intraobserver reliabilities calculated for all modalities, with all Pearson correlation coefficients found to be ≥0.89. The remainder of the data with regard to outcome measures and recurrent instability were then assessed on the basis of whether the patient was on-track or off-track.

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Statistical Analysis

All statistical analyses and tests were conducted using SPSS version 17 or higher (PASW Statistics; SPSS), and verified with SAS (SAS Institute) when appropriate. F tests (analysis of variance [ANOVA]) and t tests were conducted for contrasts involving continuous variables, and chi-square and t tests for proportions were conducted for contrasting categorical variables. Thresholds for significance were set a minimum of p < 0.05. Statistical analyses were performed by an independent, doctoral-level-trained biostatistician.

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Results

Overall, there were 70 patients with a mean follow-up of 48.3 months (range, 23.5 to 58 months). Of these, 57 patients (81%) had completed all outcome measures and were included in this analysis. The mean age was 25.5 years (range, 20 to 42 years) at the time of the surgical procedure. There were 10 patients (18%) who reported recurrent instability. The mean glenoid bone loss for the cohort was 13.5% (range, 1% to 41.0%).

Eight patients (14%) were noted to be off-track, and 49 patients (86%) were on-track. With respect to treatment failure, 6 (75%) of the 8 off-track patients sustained recurrent dislocations, but only 4 (8%) of 49 on-track patients dislocated postoperatively. With regard to recurrence of instability, 6 (60%) of 10 patients with recurrence of instability were off-track compared with 2 (4%) of 47 patients in the stable group (p = 0.0001). The positive predictive value for an off-track shoulder having recurrent instability was 75%. The negative predictive value was 92%.

There were 30 patients (53%) with bipolar bone loss and 27 patients (47%) without a Hill-Sachs lesion (monopolar bone loss). The monopolar cohort had a mean glenoid bone loss of 11.2% (range, 1% to 22.6%), and the bipolar cohort had a mean glenoid bone loss of 15.6% (range, 2% to 41%). The difference in bone loss was not significant (p = 0.26). Table II demonstrates demographic characteristics of the monopolar and bipolar cohorts.

A subanalysis was performed of the 30 patients with bipolar bone loss. The treatment failure rate in this group was 23% (7 of 30 patients). Of the 7 patients who had treatment that failed and were noted to have a Hill-Sachs lesion, 6 (86%) were off-track and 1 was on-track. Conversely, only 2 (9%) of the other 23 patients with bipolar bone loss who remained stable were off-track. Taken together, classifying bipolar lesions as being on-track or off-track correctly predicted postoperative stability in 90% of patients (27 of 30). Conversely, the presence or absence of 20% glenoid bone loss correctly predicted postoperative stability in 73% of patients (22 of 30) when bipolar bone loss was present.

A subanalysis was then performed of the 27 patients without a measurable Hill-Sachs lesion who, by definition, were on-track. There were 3 patients (11%) who had treatment that failed in this group. The predictive value for stability in this subgroup using the track classification was 89% (24 of 27), or a prediction error in 11% (3 of 27). There were 7 patients in the monopolar group who had glenoid bone loss of >20%. If one were to apply the criterion of glenoid bone loss of >20% to the monopolar subgroup, it would have predicted that 7 of 27 patients had treatment that failed. Of the 7 patients in the monopolar group who had >20% glenoid bone loss, only 2 patients had treatment that failed. Thus, the correct prediction would have been made in 2 (29%) of 7 patients, or a prediction error of 72%, which was significantly inferior to the track classification (p = 0.01).

Functional outcome scores for patients who did not have recurrent instability were significantly better than those who did for both the WOSI score (p = 0.003) and the SANE score (p = 0.002) (Table III). When comparing WOSI scores of the on-track and off-track groups, the on-track group had a significantly better mean score at 668 points compared with the off-track group at 1,134 points (p = 0.011). When looking at only bipolar bone loss, the on-track patients again had a better mean WOSI score at 747 points compared with the off-track group at 1,134 points (p = 0.052) (Table IV). There were no differences in functional outcome or treatment failure when comparing individual surgeons. There was a significant inverse relationship between the size of the glenoid track and the WOSI score (p = 0.01).

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Discussion

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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