Five pairs of cadaveric shoulders underwent posterior and anterior drawer and inferior sulcus tests in five progressive conditions: intact, vented, following opening of the rotator cuff interval, reconstruction of the interval, and transfer of the coracoacromial ligament. The surgical treatments–vented, open rotator cuff interval, reconstruction, and coracoacromial ligament transfer–had an effect compared to the intact shoulders on the inferior stiffness (P=.00002) and on the anteroposterior stiffness (P=.00031). The difference between the stiffness of the reconstructed rotator cuff interval compared to the coracoacromial ligament transfer was significant for loading in the AP direction (P=.006) and for loading in the inferior direction (P=.005).
The rotator cuff interval lesion was introduced by Nobuhara and Ikeda1 as a painful, unstable shoulder in patients who neither had traumatic instability nor a demonstrable rotator cuff injury. This interval2,3 is completely bridged by the joint capsule and structurally enhanced by both the superior glenohumeral ligament and the coracohumeral ligament. Harryman et al,4 using a cadaveric model, found the rotator interval capsule to serve three functions: to check shoulder motion in flexion, extension, adduction and external rotation; to provide primary restraint to inferior and posterior translation in the adducted shoulder; and to improve stability of the shoulder against posterior dislocation in the position of flexion, abduction and external rotation. Nobuhara and Ikeda1 repaired the rotator cuff interval in patients with inferior instability and had encouraging results, although follow-up was limited. Field et al5 imbricated the rotator cuff interval in patients with atraumatic anterior instability. Unfortunately, many patients had the interval reconstruction in addition to an inferior capsular shift, making it difficult to accurately assess the function of the rotator interval reconstruction in shoulder stability.
From previous studies,6-9 the rotator interval appears to play a role in atraumatic shoulder instability. This article evaluates whether the coracoacromial ligament, when transferred into the imbricated rotator cuff interval, can enhance the reconstruction and improve stability. It is our hypothesis that opening of the rotator cuff interval will offer some instability of the glenohumeral joint in the anterior, posterior, and inferior directions more than venting the joint. Rotator cuff interval reconstruction and/or coracoacromial ligament transfer will then restore stability of the joint.
Materials and Methods
Five pairs of relatively young cadaveric shoulder specimens were obtained from individuals that were aged a mean of 32.3 years at the time of death. The specimens were frozen until the time of preparation. The shoulders were thawed to room temperature for dissection of the skin and overlying subcutaneous tissue. No gross rotator cuff or interval pathology was noted. The medioinferior portion of the scapula was removed, and the remaining bone was cleaned of muscle and periosteum to allow for potting in polyurethane. The cut medial border of the scapula was perpendicular to the base of the potting mold. The midshaft of the humerus was potted in a similar fashion with the longitudinal axis of the humerus parallel to the axis of the cylindrical mold.
Each shoulder was mounted on a custom-built X-Y table that was attached to the MTS biaxial servo-hydraulic materials testing system (Bionix 858 Biaxial Materials Testing System; Bionix, Eden Prairie, Minn). This allowed for 3° of translational motion of the humerus relative to the scapula. In one of two test configurations, the X-Y table allowed for unconstrained mediolateral and superoinferior translation while the MTS controlled anteroposterior translation. In the other configuration, the X-Y table provided unconstrained anteroposterior (AP) and mediolateral translation while the MTS controlled superoinferior translation. A 450 N load cell (Interface Electronics, Scottsdale, Ariz) was placed between the MTS arm and the specimen to improve applied load measurement accuracy. Two linear variable differential transducers (RDP Electrosense Inc, Pottstown, Pa) were attached to the X-Y table to detect obligate translation. A computer data acquisition system was used to obtain data from the MTS unit. For the AP translation experiment the scapula was rigidly fixed to the X-Y table with the plane of the glenoid aligned parallel to the actuator ram of the MTS (vertical). The humerus was attached to the ram of the MTS such that the long axis of the humerus was horizontal and in a physiologically neutral position relative to the scapula (15° abduction, 0° flexion). For the inferior drawer test, the shoulder was rotated 90° so the humeral shaft was parallel to the axis of the MTS actuator and the X-Y table provided unconstrained displacements in the AP and mediolateral directions. For both experiments the specimens were ramp loaded in displacement control at 10 mm per second to a maximum displacement of 20 mm and then ramped down to the starting point. In addition, a maximum force of 100 N was enforced to prevent soft-tissue injury. No delay occurred between the anterior and posterior loading tests.
Each shoulder specimen was numbered before starting the test procedures. Once the glenohumeral joint was rigidly fixed to the testing jig at the desired glenohumeral position, the neutral position was determined. To accomplish this, the glenohumeral joint was manually loaded with a pulley system by pressing the humeral head into the glenoid; it was then translated in the AP and superoinferior planes until the deepest portion of the glenoid was identified. A joint compression load of 20 N was simulated. Each specimen first was preconditioned for 10 cycles with a 15-N anterior-to-posterior and then inferior-to-superior load. A single specimen underwent drawer and sulcus tests, in a random order. These tests were performed with the shoulder in 5 conditions. The shoulders were tested in the following states: intact, vented, following opening of the rotator cuff interval, reconstruction of the interval, and transfer of the caracoacromial ligament. The type of test (drawer or sulcus) and the specimen condition being tested was always randomly chosen. All surgery was performed while the specimen remained on the testing jig. The vented procedure was produced by placing an 18-gauge angiocatheter into the shoulder joint. The rotator interval was identified via a standard deltopectoral approach. The interval was recognized by externally rotating the humerus and a defect was created using scissors. The coracohumeral ligament was then identified by digital palpation at the base of the coracoid and divided. Imbrication of the interval was completed using two #2 cottony dacron sutures (Dekanatel, Fall River, Mass) in a figure eight fashion. The rotator cuff interval imbrication procedure was performed in the external rotation position and every effort was made not to over-tighten the anterior capsule trying to avoid causing posterior translation of the humeral head. The imbrication was symmetrical along the whole length of the incision. The final step involved identifying the coracoacromial ligament and releasing it from its acromion origin. The caracoacromial ligament was then rotated into the interval and imbricated with two mattress stitches of #2 cottony dacron. At each step prior to testing, the humeral head was centered in the glenoid manually using medial pressure.
Friedmans test was selected to evaluate the significance of the differences between mean stiffness values associated with different shoulder conditions. The Friedman test is a nonparametric method (based on ranks) of analysis of variance for repeated measures. Likewise, comparisons between pairs of shoulder conditions were analyzed using the Wilcoxon signed-rank (nonparametric) test. Any value of P<.05 was considered statistically significant (-value=0.05).
Typical AP and inferior load-displacement data were collected through a computer acquisition system (Figures 1 and 2). The surgical treatmentsvented, open rotator cuff interval, reconstructed, and coracoacromial ligament transferhad an effect compared to the intact shoulders on the inferior stiffness (P=.00002) and on the AP stiffness (P=.00031). The difference between the stiffness of the reconstructed rotator cuff interval compared to the coracoacromial ligament transfer was significant for loading in the AP direction (P=.006) and for loading in the inferior direction (P=.005). Differences between the open rotator cuff interval and the reconstructed condition did not reveal any statistical significance (AP/open versus reconstructed: P=.074 and inferior/open versus reconstructed: P=.444), something that attests to the validity of our working hypothesis that coracoacromial ligament transfer substantially contributes to both AP and inferior stability.
Figure 1: Graph demonstrating typical AP load-displacement data. Abbreviation: RCI=rotator cuff interval. Figure 2: Graph demonstrating typical inferior load-displacement data. Abbreviation: RCI=rotator cuff interval.
Anteroposterior Stiffness (N/mm)
Venting of the shoulder joint produced a decrease in AP stiffness. The elimination of the negative intra-articular pressure resulted in a considerable decrease of AP stiffness as expected. Capsule incising (open rotator cuff interval) produced an additional decrease. The differences between the three groups were statistically significant (intact versus vented: P=.016, intact versus open rotator cuff interval: P=.006, vented versus open rotator cuff interval: P=.016). The difference between the last two groups (open rotator cuff interval versus vented capsule) indicates that the needle-vented capsule presents some laxity that is clearly increased after capsule incising. Despite the absence of the negative intra-articular pressure after the needle venting procedure, the rotator cuff interval plays a significant role in maintaining AP stiffness (Figure 3). The symmetrical reconstruction of the rotator cuff interval defect increased the values of the AP stiffness in comparison with the open rotator cuff interval group but without statistical significance (open rotator cuff interval versus repaired: P=.074, repaired versus intact: P=.059). The most important issue after these experiments was that the coracoacromial ligament-transfer group presented a remarkable increase in AP stiffness values, keeping in mind that it was always performed after the rotator cuff interval repair (symmetrical closure) was completed (rotator cuff interval repair versus coracoacromial ligament transfer: P=.0069). This may indicate a possible clinical role for this procedure. The values of AP stiffness obtained after the coracoacromial ligament transfer were even larger than those observed in the intact specimens, but not significantly (intact versus coracoacromial ligament transfer: P=.114) (Figure 3). At this point it must be re-emphasized that every effort was made not to produce a tight anterior capsule. The symmetrical rotator cuff interval closure was performed in the external rotation position. The coracoacromial ligament transfer reinforced the previous construction without compromising the integrity or rigidity of the reconstructed area.
Figure 3: Graph demonstrating the effect of the various operative interventions in the cadaveric shoulder specimens in terms of anteroposterior stiffness (N/mm). It can clearly be seen the pivotal role of venting the capsule in decreasing the mean AP stiffness as well as the final result of the rotator cuff interval repair and the coracoacromial ligament transfer. Abbreviations: CA=coracoacromial ligament and RCI=rotator cuff interval. Figure 4: Graph demonstrating the effect of the various operative interventions in the cadaveric shoulder specimens in terms of inferior stiffness (N/mm). It is well demonstrated the significant role of venting the capsule in decreasing the mean inferior stiffness as well as the final result of rotator cuff interval repair and the coracoacromial ligament transfer. Abbreviations: CA=coracoacromial ligament and RCI=rotator cuff interval.
Inferior Stiffness (N/mm)
Capsule venting produced a decrease in the average inferior stiffness (intact versus vented: P=.006). Rotator cuff interval opening further accentuated this effect (intact vs rotator cuff interval open, P=.005, vented versus open rotator cuff interval: P=.016). The reconstruction of the rotator cuff interval increased the average inferior stiffness (Figures 4 and 5) but did not reach statistical significance (open rotator cuff interval versus reconstructed: P=.444), though it was still significantly different in comparison with the intact specimens (intact versus reconstructed, P=.005). These results reflected the pivotal role of the negative intra-articular pressure in inferior shoulder joint stability (intact versus vented: P=.006, vented versus reconstructed: P=.012). The coracoacromial ligament transfer increased inferior stiffness significantly (reconstructed versus coracoacromial ligament transfer: P=.005), but it was still significantly less than intact (intact versus coracoacromial ligament transfer: P=.005). The important role of capsule venting is again well demonstrated (vented versus coracoacromial ligament A transfer: P=.168).
Figure 5: Graph demonstrating the effect of the various operative interventions in the cadaveric shoulder specimens in terms of anteroposterior and inferior stiffness. The coracoacromial ligament transfer seems to have a more crucial effect in terms of anteroposterior stiffness. Abbreviations: CA=coracoacromial ligament and RCI=rotator cuff interval.
The main components of the rotator cuff interval are the superior glenohumeral ligament, coracohumeral ligament, and parts of the joint capsule. Jost et al3 in an anatomical study reported that the interval was composed of parts of the supraspinatus, subscapularis, superior glenohumeral ligament, coracohumeral ligament, and glenohumeral joint capsule. Specific biomechanical features10,11 have been attributed to each of them according to various studies.12-15 It also has been reported as a direct stabilization function of the rotator cuff interval in the superior direction.2,4,12,16
Although in the past the coracoacromial ligament was thought to be an unnecessary and accessory shoulder structure, it is now known to be a robust structure. Along with the acromion, it constitutes the inferior concave surface that provides resistance to superior translation for the shoulder joint. Lee et al17 reported that at 0° and 30° of abduction, release of the coracoacromial ligament resulted in a significant increase in glenohumeral joint translations in both the anterior and inferior directions. In addition, the differences in translation before and after the release of the coracoacromial ligament decreased in all directions as glenohumeral abduction increased, and they were not significant at 60° of abduction in any of the rotations.17 The results of this study suggest that the coracoacromial ligament has a role as a static restraint of the glenohumeral joint. It provides a suspension function and may restrain anterior and inferior translations through an interaction with the coracohumeral ligament.17 Additionally, one could argue that the procedure of coracoacromial ligament transfer described in our study would trade one problem for another. This does not appear to be true as shown in the previously cited article because all the translations were not significant at 60° of abduction in any of the rotations.17 The importance of the preservation of the subacromial arch has been stressed recently, especially in irreparable lesions of the rotator cuff to prevent anterosuperior migration of the humeral head. More recently, reports have pointed out the importance of regeneration of the coracoacromial ligament after acromioplasty and subacromial decompression.2,18-20 Ovesen and Sojbjerg21 have reported the transposition of the coracoacromial ligament and its bony attachment from the acromion to the lesser tuberosity of the humerus in the treatment of vertical shoulder joint instability. In the current study the protocol included release of the coracoacromial ligament from its acromion origin without any bony resection or fixation. The ligament was transferred into the imbricated rotator cuff interval reconstruction and thus the role of the coracoacromial ligament as a suspension structure of the coracoid was not completely eliminated.
The rotator cuff interval lesion has gained increasing interest as an etiology of instability.2,18 Imbrication of this interval (in the position of external rotation) improved many patients instability symptoms.1 Many open1,5,9,22,23 or arthroscopic2,6,8,24 techniques have been described for this procedure with encouraging results. Harryman et als4 cadaveric study elucidated much about the anatomic function of the rotator interval capsule. Sectioning of the interval increased and imbrication decreased humeral head translation during stress testing, as was also found in our study. Itoi et al12 reported that the rotator interval changes its area with the rotation of the arm. This area change may be necessary for normal motion of the shoulder. In other words, the rotator cuff interval is necessary not only for the stability but also for the motion of the shoulder. This complicated function of the rotator cuff interval appears to be better substituted by grafting a capsule-like tissue that allows the change of the rotator interval area as well as preservation of the intra-articular pressure, than by closing the interval by direct suture of the two tendons.12
This article not only examines the rotator interval function in shoulder stability, but also the effects of transferring the coracoacromial ligament into the reconstruction. The experimental setup offered some advantages. The neutral rotation position was chosen to eliminate any factors relating to the rotation of the humerus as the main focus of interest towards the function of the reconstructed rotator cuff interval. After venting the capsule, the role of the negative joint pressure25-27 was eliminated. In this setting only the static capsuloligamentous components and the presence of the muscles surrounding the joint contribute to joint stability.26-28 The muscles were preserved and all the operative procedures were performed in situ, while the specimens were placed on the testing apparatus to have a more realistic model.
Unique to our study was the transfer of the coracoacromial ligament into the rotator interval reconstruction. Field et al5 reported a suspension of the rotator cuff interval tissue from the coracoacromial ligament near its coracoid origin in three cases when redundant rotator cuff interval capsule tissue was present after closure. Suspension of this rotator cuff interval capsule was performed to maximize use of available tissue in a way that could potentially enhance the stability achieved through rotator cuff interval defect closure. In our study the coracoacromial ligament was transferred in the concept of reinforcing the reconstructed rotator cuff interval. Our results showed that this operative step improved shoulder stability. Stiffness (resistance to deformation) was measured because we felt it represented the most realistic biomechanical value involved with the clinical setting of instability, as the hallmark of this clinical entity is excess capsular redundancy or laxity.
As with all cadaveric studies, this study has limitations. The interaction of muscle forces, intra-articular pressure, and the degree of plastic deformation of the capsuloligamentous structures are clinically important and are difficult to replicate during experiments in cadaveric shoulders. Additionally, the small number of cadaveric specimens may limit its significance. The greatest technical problem was in centering the humeral head in the glenoid prior to each testing cycle.
This novel rotator cuff interval reconstructive procedure appears worthy of further investigation. In patients who present with subtle instability, reconstructive alone may be effective. If residual instability occurs following rotator cuff interval imbrication, coracoacromial ligament transfermay be an effective adjunct.
| What is already known on this topic|
- The rotator cuff interval consists of a thin membranous tissue located within a triangular space that is bounded above by the inferior margin of the supraspinatus tendon, below by the superior margin of the subscapularis tendon, and medially by the base of the coracoid process. The apex of the triangle is formed by the transverse humeral ligament at the biceps intertubercular sulcus. This interval is completely bridged by the joint capsule and structurally enhanced by both the superior glenohumeral ligament and the coracohumeral ligament.
- The rotator cuff interval lesion was originally introduced as painful, unstable shoulder in patients who neither had traumatic instability nor a demonstrable rotator cuff injury and has subsequently gained increasing interest as an etiology of instability. Imbrication of this interval (in the position of external rotation) may improve many of the patients' instability symptoms. Many open or arthroscopic techniques have been described for this procedure with encouraging results.
| What this article adds|
- This article examined the rotator interval function in shoulder stability and also the effects of transferring the coracoacromial ligament into the reconstruction. This novel procedure of rotator interval reconstruction appears worthy of further investigation.
- In patients who present with subtle instability, reconstruction alone may be effective. If there is residual instability following rotator cuff interval imbrication, transfer of the coracoacromial ligament may be an effective adjunct.
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Drs Roidis, Karachalios, and Malizos are from the Department of Orthopedics, University of Thessaly, Larissa, Greece; Drs Stennette and Itamura are from the Department of Orthopedics, University of Southern California, Los Angeles, Calif; and Dr Burkhead is from the Department of Orthopedics, University of Texas, Southwestern Medical School, Dallas, Tex.
Reprint requests: Nikolaos T. Roidis, MD, PhD, 34 Akronos Str, Larissa 41447, Greece.