Abstract
Background
A resorbable subacromial balloon has been developed to address humeral head migration following posterosuperior rotator cuff tears. The purpose of this experimental assessment was to quantify the effect of balloon augmentation on humeral head position.
Methods
Eight cadaveric shoulders were subjected to 0°, 30°, 60° and 90° of abduction via a shoulder simulator. The deltoid was activated at 40N, then 80N. The subscapularis and infraspinatus with teres minor were then applied independently and together to create four muscle activation states for each deltoid load. The shoulder was tested intact, torn, then with the balloon. The centre of the humeral head was tracked using active optical markers.
Results
When the rotator cuff was torn, the humeral head translated superior by 1.4 ± 1 mm overall (P = 0.009). Following balloon augment, the humeral head translated inferiorly by 2 ± 2 mm relative to the intact state (P = 0.042), and significantly more anterior than the intact (3 ± 2 mm; P = 0.005) state. Rotator cuff variation was only significantly different when the balloon was used, with the subscapularis translating the humeral head posteriorly (P = 0.006).
Discussion
The subacromial balloon inferiorized the humeral head compared to the torn state. Unexpected anterior humeral head translation was attributed to the posterosuperior balloon placement relative to the humeral head.
Keywords: shoulder, rotator cuff tear, InSpace balloon, subacromial balloon, proximal migration, cuff tear arthropathy, reverse shoulder arthropasty
Introduction
Rotator cuff pathology is one of the most common orthopaedics injury, with a prevalence of approximately 50% beyond 60 years of age1 and can arise due to acute trauma or chronic degeneration. The presentation of rotator cuff tendinopathy can range from subacromial impingement, tendonitis, to full thickness tears involving multiple tendons,2 and ultimately results in varying degrees of pain and loss of shoulder function. The massive rotator cuff tear has been defined as a tear greater than 5 cm, which involves two or more tendons.3 These tears have been reported to present with an incidence between 7 and 22%.4 Following a massive tear, the humeral head is free to migrate superiorly reducing the acromiohumeral distance, which can cause pain and range-of-motion alterations that reduce shoulder function.5 The massive irreparable tear can be a challenge to treat due to the associated retraction of tendons, as well as muscle atrophy and fatty infiltration.6 Accordingly, several surgical interventions have been proposed, both arthroscopic (e.g., subacromial debridement with biceps procedure, interpositional graft placement, partial repairs, etc.) and open (e.g., hemiarthroplasty, reverse total shoulder arthroplasty, tendon transfers, etc.),7 each with its own benefits and drawbacks. To-date though, there remains no gold standard for the treatment of massive irreparable rotator cuff tears.
Recently, a resorbable and arthroscopically inserted subacromial balloon spacer has been introduced (InSpace balloon; Orthospace, Caesarea, Israel). This balloon is inserted between the acromion and humeral head, and is proposed to limit superior humeral head translation in massive irreparable rotator cuff tears1 (Figure 1). Following a standard arthroscopic debridement of the subacromial space, the balloon is inserted arthroscopically in a curled form through a lateral subacromial portal and is then inflated with saline to a manufacturer specified volume.1,8
Figure 1.
The InSpace resorbable subacromial balloon spacer is inserted in the folded position through a lateral deltoid split. Once positioned, the balloon is filled with saline to a prescribed volume, and expands.
To date, there have been few clinical prospective trials assessing the performance of the balloon.9–11 One five-year follow-up indicated significant improvements in shoulder functional scores, strength and range-of-motion9; however, a recent two-year follow-up indicated inconsistent results with only 40% of patients clearly benefiting from the procedure.11 Despite this promising clinical evidence, an in-vitro biomechanical assessment of the balloon spacer is still warranted to directly quantify the degree to which superior humeral head migration is prevented. The purpose of this cadaver-based biomechanical investigation was to assess how humeral head position changes following an irreparable rotator cuff tear and after surgical insertion of a subacromial balloon spacer. We hypothesized that superior humeral head translation will be observed following the simulation of the tear, and that the subacromial balloon will significantly inferiorize the humeral head relative to the torn state, thereby mimicking the intact humeral head position.
Methods
Specimen preparation and biomechanical testing
Eight male cadaveric shoulders (mean age: 68 years; range: 60–76 years) were prepared such that the joint capsule, surrounding soft tissues and skin remained intact. None were found to have any rotator cuff or glenohumeral joint pathology via CT review. The distal tendons of the subscapularis, as well as the infraspinatus and teres minor were isolated and tagged using #5 non-absorbable sutures. Similarly, the anterior, middle and posterior heads of the deltoid were tagged through 2 mm holes that were drilled through the lateral humeral cortex at the insertion site. The humerus was resected at the mid-diaphysis just distal to the deltoid insertion, and a custom fixation assembly consisting of an intramedullary rod, load cell (ATI mini45; Apex, NC) and distal restraining rod was rigidly implanted into the humeral intramedullary canal using bone cement. Each shoulder was then secured onto a biomechanical simulator12 using two transosseous bolts through the scapula, such that the centre of rotation of the glenohumeral joint was aligned with the simulator’s abduction arc (Figure 2). The distal humeral rod assembly was then connected to the abduction arc, restraining the shoulder to elevation within the scapular plane. Accordingly, the humerus could be oriented corresponding to 0–90° of shoulder abduction, while also permitting unimpeded proximal translation of the humeral head, as well as rotation and translation about the diaphyseal axis. The relative translation of the humeral head was quantified with respect to the glenoid by affixing two active optical tracking markers (Northern Digital, Ontario, Canada) to the humeral shaft and the scapula (Figure 2).
Figure 2.
The biomechanical shoulder simulator employed in the present investigation permits the humerus to be set at a desired angle of shoulder abduction in the scapular plane, while allowing for proximal translations that are tracked via optical markers positioned on the humeral shaft and the scapula.
Each of the tagged musculotendinous units were connected to pneumatic actuators through cables running along physiologic lines-of-action and were controlled using a custom LabVIEW (National Instruments; Austin, TX) code. The cables connecting the three heads of the deltoid to the pneumatics were wrapped over the remaining deltoid tissue and the acromion to simulate physiological loading. To assess the effect of muscle loading on humeral head translation, the deltoid (Delt) was engaged at 40N, then 80N, while the subscapularis (Sub), and infraspinatus with teres minor (Inf/TM) were independently loaded at 10 N,13–15 resulting in four muscle activation states for each deltoid load (i.e., Delt alone, Delt + Sub, Delt + Inf/Tm and Delt + Sub + Inf/Tm). Independent rotator cuff activation was conducted to quantify the influence of these muscles on humeral head position during biomechanical simulation. This loading protocol was repeated in four quasi-static humeral positions corresponding to 0°, 30°, 60° and 90° of shoulder abduction. The shoulder was tested first in its intact state, followed by an artificially torn state, then finally reconstructed with the subacromial balloon.
Surgical states
Testing was conducted on the intact joint first. Next, the cuff ‘torn’ condition was tested. This was achieved by creating a posteriosuperior tear of the rotator cuff, simulated by severing the suprapinatus and infraspinatus tendons, as well as the underlying superior joint capsule. This was completed through a 5 cm mini-open lateral split in the deltoid that was used to visualize the tendon insertions. The mini-deltoid split was sutured closed prior to testing.
The third testing state was the subacromial balloon (InSpace balloon; OrthoSpace, Caesarea, Israel). The mini-deltoid split was reopened, and a large balloon size was selected, per manufacturer recommendations, as each cadaveric specimen had a subacromial space larger than 5 cm (greater tuberosity to 1 cm medial of the superior glenoid rim). The balloon was inserted into the subacromial space in a curled position and was then inflated with 25 ml of saline. The deltoid split was again closed via sutures.
Outcomes and statistical analyses
Surgical states were compared by tracking the superior–inferior (SI) and anterior–posterior (AP) translation of the centre of the humeral head, which were quantified with respect to the centre of the glenoid articular dish. The humeral head translation was statistically assessed using a four-way repeated measures analysis of variance (RM ANOVA), where the independent variables were abduction position (0°, 30°, 60° or 90°), shoulder state (Intact, Torn or Balloon), rotator cuff activation (Sub, Inf/TM or Sub + Inf/TM) and deltoid load (40N or 80N). The threshold for significance was P < 0.05, and a Bonferroni post-hoc correction was applied to account for the large number of statistical tests performed.
Results
SI humeral head position
Statistically significant differences in humeral head SI position were quantified as a consequence of the degree of shoulder abduction (P < 0.001; Power = 1.000), shoulder state (P < 0.001; Power = 1.000) and deltoid force (P < 0.001; Power = 1.000), but not rotator cuff activation (P = 0.303; Power = 0.294).
Overall, when the rotator cuff was torn, the humeral head translated superiorly by 1.4 ± 1.0 mm from its intact position (P = 0.009; Figure 3). This superior translation was most prominent when the shoulder was in lower angles of abduction (0° = + 2.2 ± 1.3 mm; 30° = + 1.9 ± 1.2 mm, 60° = +0.8 ± 0.8 mm. 90° = +0.5 ± 0.9 mm). Conversely, following the balloon augmentation, the humeral head translated inferiorly by 1.6 ± 1.5 mm overall relative to the intact state (P = 0.042). This relative inferior movement was consistent regardless of shoulder abduction (0° = −1.8 ± 2.5 mm; 30° = −1.5 ± 1.7 mm, 60° = −1.5 ± 1.3 mm. 90° = −1.5 ± 1.9 mm), unlike the torn state.
Figure 3.
Superior–inferior humeral head translations following the simulation of an irreparable rotator cuff tear and subacromial balloon reconstruction for all shoulder abduction configurations and muscle activation states.
Regardless of the injury or repair state, increasing shoulder abduction caused the humeral head to migrate inferiorly; however, this effect was most pronounced when the rotator cuff was torn (0° to 90°: Intact: −3.1 ± 1.1 mm (P < 0.001), Torn: −4.8 ± 2.0 mm (P < 0.001), Balloon: −2.8 ± 3.9 mm). As a consequence, the torn state’s SI divergence from the intact state is less pronounced at higher angles of abduction, as is seen in the relative plot of SI position in Figure 3.
Furthermore, for all shoulder states, increasing the deltoid force from 40N to 80N caused the humeral head to translate superiorly (P < 0.001); though the deltoid’s effect was more prominent on the torn and balloon states (Intact: +0.4 ± 1.8 mm, Torn: +1.5 ± 2.3 mm, Balloon: +1.0 ± 2.7 mm).
AP humeral head position
The AP positioning of the humeral head was found to vary significantly as a result of changes in the shoulder state (P < 0.001; Power = 1.000), deltoid force (P = 0.008; Power = 0.878) and rotator cuff activation (P < 0.001; Power = 0.973), but not the degree of shoulder abduction (P = 0.167; Power = 0.290).
The inflated balloon shifted the humeral head significantly more anterior than the intact (3.1 ± 2.0 mm; P = 0.005). This anterior translation was fairly consistent throughout the tested range of abduction (0° = 2.7 ± 1.9 mm; 30° = 3.1 ± 2.0 mm, 60° = 3.4 ± 2.7 mm. 90° = 3.4 ± 1.9 mm), as is seen in Figure 4. Alternatively, the torn state permitted the humeral head to translate significantly posterior by 0.8 ± 0.8 mm overall from its intact position (P = 0.046). Posterior torn translations were more prominent when the shoulder was in lower angles of abduction (0° = 1.2 ± 1.3 mm; 30° = 1.2 ± 1.4 mm, 60° = 0.6 ± 0.9 mm. 90° = 0.1 ± 0.9 mm).
Figure 4.
Anterior–posterior humeral head translations following the simulation of an irreparable rotator cuff tear and subacromial balloon reconstruction for all shoulder abduction configurations and muscle activation states.
An interaction between the shoulder state and rotator cuff activation indicated that the muscle activation groups were only significantly different from each other when the balloon was used; with solo deltoid activation causing significantly more anterior humeral head positioning compared to either the deltoid with the subscapularis (0.9 ± 0.6 mm; P = 0.007), or the deltoid with both the subscapularis and the infraspinatus and teres minor (0.9 ± 0.6 mm; P = 0.006).
Furthermore, increasing the deltoid force from 40N to 80N caused the humeral head to translate slightly posteriorly in all states, though this was only found to be statistically significant for the intact (0.4 ± 0.2 mm; P = 0.001) and torn (1.1 ± 0.8 mm; P = 0.014) states; not following the balloon augmentation (0.5 ± 0.5 mm; P = 0.062).
Discussion
As hypothesized, the simulation of a massive rotator cuff tear resulted in the superior translation of the humeral head relative to the intact state, and reconstruction with the subacromial balloon resulted in humeral head inferior translation. The subacromial balloon was found to over-correct the superior migration of the humeral head on average, resulting in the humerus sitting slightly below its intact position. Interestingly, the balloon also forced the humeral head to translate anteriorly relative to the intact state. This anteroinferior translation of the humeral head is attributed to the location of the balloon in the subacromial space. As the acromion is located posterosuperior of the glenohumeral articulation, the balloon location causes reaction forces on the humeral head that are directed anteroinferior. It is possible that the magnitude of the translations experienced following balloon augmentation could be adjusted to better match the intact state by tuning the balloon’s fill-volume on a patient-specific basis; however, this should be the subject of further investigation, as the present study only considered the upper-end of the manufacturer’s recommended volume range (i.e., recommended: 23–25 ml; present study: 25 ml).
Superior positioning of the humeral head was not affected by the independent activation of the rotator cuff muscles. Only activation of the deltoid caused SI translations of the humeral head, which makes sense given the muscle lines-of-activation. The activation of the subscapularis did cause the humeral head to shift posteriorly relative to quasi-static loading without the subscapularis; though not enough to correct the anterior humeral head shift present when the balloon augment was used. These findings are also supported by the line-of-action of the subscapularis and suggest that forces through the rotator cuff should be simulated when performing biomechanical simulations of the shoulder. Additionally, during clinical usage of the balloon, it may be reasonable to ensure the subscapularis is intact or repairable to limit excessive anteroinferior humeral head translations. It should also be noted that the rotator cuff loading was limited to a single 10N magnitude in keeping with previous biomechanical simulations. It is possible that physiological loads could exceed this value, and by consequence pull the humeral head further posteriorly, thereby correcting the anterior balloon offset.
Interestingly, for the torn state, the humeral head was found to best match its intact position (both SI and AP) when in 60° and 90° of shoulder abduction. Following balloon augment, the humeral head position was less sensitive to changes in shoulder abduction. In the lower degrees of abduction, the line-of-action of the deltoid pulls the humerus superiorly, and in the absence of a superior joint capsule and the posterosuperior rotator cuff muscles, the unsupported humeral head migrates superiorly; however, as the shoulder abducts to higher degrees, the deltoid’s line-of-action on the humerus shifts more medially resulting in the humeral head lowering closer to its intact position. Alternatively, when the subacromial balloon is in place above the humeral head, it fills the superior void, forcing the humeral head inferiorly. This buttressing effect persists throughout abduction, resulting in a humeral head position that is less sensitive to the orientation of the deltoid’s pull.
The present investigation was not without limitations. The static muscle loads applied do not necessarily represent the dynamic state of muscle loading that presents in-vivo; however, the loads chosen were consistent with Mihata et al.’s biomechanical shoulder simulations.13–15 This quasi-static loading, as well as the estimation of applied muscle loads, is an inherent limitation of an in-vitro cadaveric assessment. The cadaveric model was chosen for the present investigation, as it permitted the direct and invasive quantification of three-dimensional humeral head positioning. As the intact, torn and balloon states were all subject to the same loading configurations, the differences between shoulder states should be valid, though the exact magnitudes of humeral head translations would be expected to vary in living subjects undergoing different activities. As a consequence, the clinical significance of the present findings will require in-vivo investigation. It is also worth noting that the InSpace subacromial balloon is a resorbable construct, and this cadaveric study was limited to testing the immediately post-operative state. As such, the conclusions presented herein remain limited to the function expected directly following surgery. Further, the non-randomized order of procedures (i.e., intact, torn, balloon) may be perceived as a limitation; however, this was done specifically to remain consistent with the progression of injury.
In conclusion, the subacromial balloon effectively prevents superior humeral head migration following a massive rotator cuff tear. Unanticipated anterior humeral head translations were encountered with the subscapularis muscle insufficiency condition, which were partially corrected with subscapularis muscle activation at the presently simulated loads. As such, clinically, these anterior translations can be limited during balloon usage in patients with an intact or repairable subscapularis.
Acknowledgements
We would like to acknowledge the academic support provided by the Collaborative Specialization in Musculoskeletal Health Research (CMHR) program. This work has been communicated in part at the American Orthopaedics Research Society’s 2018 annual meeting (ORS), and the Canadian Orthopaedics Research Society’s 2018 annual meeting (CORS), both times as a poster presentation.
Contributorship
JMR, SS and GDGL participated in data collection, study design and data analysis. GSA and JAJ were involved in study conception and design. JMR wrote the initial draft of the manuscript. All authors reviewed and edited the manuscript and approved the final version of the manuscript.
Declaration of Conflicting Interests
The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: GSA is a consultant for Depuy-Synthes and Wright Medical Inc.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Research support was received from OrthoSpace for research related to the subject of this article. No company had any input in to the study design, protocol, testing, data analysis or manuscript preparation.
Guarantor
JMR.
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