Acromioclavicular (AC) joint separation is a common injury among contact sport athletes and motorcycle accident victims.1 Rockwood et al2 developed a classification system based on the pathomechanisms and roentgenographic characteristics of the various types of AC separation. They described type V AC separation as a disruption of the suspensory mechanism of the shoulder, secondary to complete tear of the AC joint capsule and the coracoclavicular (CC) ligaments, with increased CC distance between 100% and 300% on radiographs.2 The optimal treatment for type V injuries in acute and chronic scenarios is surgical reconstruction.3
Multiple operative techniques have been described for CC ligament reconstruction to restore AC joint stability.4–7 Recently, anatomical CC ligament reconstruction using tendon graft is regaining popularity owing to its numerous biomechanical strength and perceived clinical advantages.8–11 This technique involves drilling the base of the coracoid or looping a tendon graft around it using open or arthroscopic modalities.11,12
Iatrogenic nerve injury is a rare but potentially severe complication of CC ligament reconstruction surgery that can have a devastating impact on clinical outcome.13 The axillary nerve is the closest neurovascular structure to the coracoid base. Several studies evaluated the normal anatomical relation of the nerve to the coracoid base and suggested surgery around the coracoid base is a relatively safe procedure.14 Rockwood type V AC separation injuries are characterized by inferomedial displacement of the scapulohumeral complex, anterior tilt and downward rotation of the scapula, and alteration of the normal anatomical relations of the shoulder girdle structures.15 Consequently, these changes ultimately result in an alteration to the normal anatomical relationship between neurovascular structures near the base of the coracoid.
The purposes of this study were to (1) define the anatomical relationship between the axillary nerve and the base of the coracoid in noninjured AC joints and in type V AC joint separations and (2) evaluate the relative risk for iatrogenic axillary nerve injury during dissection around the base of the coracoid during CC ligament reconstruction. The authors hypothesized that the distance between the axillary nerve and the coracoid base changes during type V AC separation, thereby placing the nerve at greater risk for injury during surgical procedures.
Materials and Methods
Six whole adult cadavers (mean age, 68±7.1 years; range, 45–82 years; 1 female, 5 male) were used. One shoulder with a massive rotator cuff tear and 1 shoulder with previous surgery were excluded from the study. A total of 10 shoulders (5 right and 5 left) were included in the study. The cadavers were stabilized to simulate a typical arthroscopic modified beach chair position in a seated position with 80° back elevation.
Eschler et al16 described a method to simulate Rockwood AC separation types in a cadaveric model. This study used that method to simulate the Rockwood type V AC separation and then measure the alterations in the normal anatomical relationship between the axillary nerve and the coracoid base.
The arm was positioned in 30° of abduction and 10° of forward flexion. All specimens were dissected by the same fellowship-trained shoulder surgeon (N.P.) using a standard deltopectoral approach. The coracoid process was exposed, and the CC ligaments, coracoacromial ligament, pectoralis minor tendon, and con-joint tendon were identified. The most superior aspect of the coracoid base and the undersurface of the clavicle were marked. The CC distance was measured. The conjoint tendon was released to approximately two-thirds of its width at its attachment to the coracoid. This allowed the conjoint tendon to be retracted, exposing the underlying brachial plexus without disturbing it. A metal pin was placed at the anteromedial aspect of the base of the coracoid (this point represents the normal limits of safe visualization of the coracoid during arthroscopic dissection). A second metal pin then was placed at the point of the axillary nerve that was closest to it. The distance between the 2 metal pins was measured.
Next, the AC and CC ligaments were sectioned, and 15 kg of longitudinal tension along the body axis was applied for 30 minutes to simulate orthostatic weight at the upper extremity. The longitudinal tension then was removed from the upper extremity. The CC distance was measured, and an increase of 100% to 300% was verified. The distance between the metal pins was measured again at the anteromedial aspect of the base of the coracoid and the point of the axillary nerve that was closest to it (Figures 1–2). Changes in the distance between the coracoid base and the axillary nerve were measured to estimate the risk of axillary nerve injury during dissection for anatomical reconstruction of high-grade AC joint separations.
Coronal view of normal anatomy (A). Coronal view of anatomy after type V acromioclavicular joint separation with the scapula in downward rotation and anterior tilt (B). Abbreviations: A, acromion; AN, axillary nerve; C, clavicle; CB, coracoid base; CC, coracoclavicular ligaments; CT, coracoid tip; D, minimal distance between coracoid base and axillary nerve; G, glenoid.
Sagittal view of normal anatomy (A). Sagittal view of anatomy after type V acromioclavicular joint separation with the scapula in downward rotation and anterior tilt (B). Abbreviations: A, acromion; AN, axillary nerve; C, clavicle; CB, coracoid base; CC, coracoclavicular ligaments; CT, coracoid tip; D, minimal distance between coracoid base and axillary nerve; G, glenoid.
At this point, the coracoid process was skeletonized from all soft tissue, and the coracoid bony dimensions were measured. The portion of the coracoid distal to the elbow (coracoid tip) was measured. The width was measured in the subscapularis tendon plane, the length was measured from the inner elbow to the tip of the coracoid, and the thickness was measured at the midpoint of the coracoid in the anterior to posterior dimension (Figure 3). To ensure accuracy, all measurements were performed by 2 independent examiners (N.P., M.J.C.) using a digital caliper (Digimatic Series 700; Mitutoyo) (accuracy, 0.2 mm) calibrated to 0.1 mm.
Sagittal view of coracoid tip dimensions (A). Axial view of coracoid tip dimensions (B). Abbreviations: A, acromion; CB, coracoid base; CT, coracoid tip; G, glenoid; L, length; T, thickness; W, width.
Unpaired t tests were used to compare the minimal distance between the coracoid base and the axillary nerve during normal and simulated high-grade AC separation. Results were reported as mean±SEM. P<.05 was considered significant.
Mean width of the coracoid was 15.05±0.93 mm, mean length was 23.1±1.75 mm, and mean thickness was 11.88±1.33 mm. Mean CC distance was 9.72±0.59 mm (range, 9.0–10.5 mm). Mean distance between the coracoid base and the axillary nerve was 26.0±3.9 mm (range, 20.0–31.6 mm). During simulated Rockwood type V AC separation, mean CC distance was 23.34±3.54 mm (range, 19.0–31.3 mm), which was an increase of 140.1%±31.62% (range, 108%–198%). Mean distance between the coracoid base and the axillary nerve was 22.0±3.4 mm (range, 17.3–28.8 mm). The difference of 4 mm was statistically significant (P=.0263; 95% CI, 2.0–5.9 mm).
Iatrogenic axillary nerve injuries during AC reconstruction are uncommon but may have a catastrophic impact on the clinical outcome.13 The normal anatomical relationship of the axillary nerve to the base of the coracoid has been defined by several authors.14,17 In an anatomical cadaveric study of 5 frozen intact shoulders, Lo et al14 found the axillary nerve is the neurovascular structure that is at greatest risk during dissection around the base of the coracoid. They also concluded surgical procedures around the coracoid base are relatively safe as the mean distance measured was 29.3 mm, and the smallest distance measured was never less than 24 mm.14 In their study on 30 shoulders from 15 adult cadavers, Apaydin et al17 found the axillary nerve was a mean distance of 37 mm away from the coracoid tip (minimum, 31 mm), supporting the conclusion of Lo et al14 that surgery around the coracoid is relatively safe.
In the current study, the authors also observed the axillary nerve to be the closest neurovascular structure to the coracoid base in the normal anatomical position and in the simulated type V AC separation. In addition, the current authors found that during a type V AC separation, the axillary nerve moves significantly closer to the coracoid base. The mean distance between the coracoid base and the axillary nerve during simulated Rockwood type V AC separation measured 22.0 mm (minimum, 17.3 mm).
The current authors did not examine the distance from the tip of the coracoid because surgical reconstruction techniques for AC separation do not involve dissection around the tip. The dimensions of the coracoid tip found in the current study were similar to the coracoid tip dimensions reported by Lo et al.14 The axillary nerve to the base of coracoid distance in normal anatomical conditions was smaller in the current study (26.0±3.9 mm) than in the study (29.3±5.6 mm) by Lo et al.14 This difference may be related to variations in body characteristics (eg, height) of the cadavers used in the two studies.
In an in vivo study, Teece et al18 demonstrated the similarity of AC joint kinematics between healthy individuals and cadavers. Urist19 and Fukuda et al20 used biomechanical cadaveric models to study the ligamentous system of the AC joint. Both studies found that rupture of the entire AC capsule contributes to clavicle instability in the anteroposterior plane. However, only concomitant rupture of the CC ligaments and especially the conoid ligament results in major vertical instability of the AC joint.
In a cadaveric study, Oki et al15 found these injuries are characterized by inferomedial displacement of the scapulohumeral complex, anterior tilt, and downward rotation of the scapula and alteration of the normal anatomical relations of the shoulder girdle structures. As such, reconstruction of the stabilizing soft tissues of the AC joint is crucial to restoring normal AC, glenohumeral, and scapulothoracic biomechanics.
The optimal management of acute type V AC separation is surgery. Chronic type V AC separations with continued deformity, pain, and functional impairment are also an indication for surgical reconstruction. Many surgical techniques have been described to reconstruct the AC joint ligaments in acute and chronic constellations. The greatest challenge of AC reconstruction has been maintaining postoperative reduction.11
Recently, open and arthroscopic anatomical techniques for reconstruction of the CC ligaments using a tendon graft have gained popularity due to biomechanical and clinical studies that have demonstrated improved stability, higher load to failure, better reproduction of the native CC ligament function, and most importantly, better clinical outcomes.8–10,12 These techniques, performed with patients in a beach chair position, involve dissecting around the coracoid base for graft transfer or drilling through the coracoid base for graft fixation. In the open approach, suboptimal visibility during this stage of the surgery places the neurovascular structures at risk. An arthroscopic technique may offer greater visibility but is technically challenging.
Regardless of whether the surgical procedure is performed open or arthroscopically, several surgical techniques for type V AC joint reconstruction describe dissection and fixation at the base of the coracoid.8–10,12 The findings of this study emphasize the risk of iatrogenic axillary nerve injury during reconstructive surgery due to a significantly smaller margin of safety than reported previously in other studies. As such, understanding the influence of injury on the anatomical relationship between the neurovascular structures and the base of the coracoid is essential to preventing injury at the time of surgery. The authors recommend performing anatomical reduction of the AC joint prior to dissecting or drilling around the coracoid base.
Strengths and Limitations
This study characterized the position of the axillary nerve in relation to the coracoid base during reconstruction of type V AC separation that requires dissection at the base of the coracoid. This information may be important for preventing iatrogenic axillary nerve injury and therefore deltoid muscle dysfunction and atrophy. Several neurovascular structures lie in close proximity to the coracoid. As such, each of them could be studied with regard to a type V AC joint separation, but the objective of this study was to focus on the structure closest to the base of the coracoid, which is the axillary nerve.
One limitation of this study was that the model of Eschler et al16 was used to simulate type V AC separation. In this model, the AC separation is created by cutting the joint capsule and CC ligament, and applying axial force on the upper extremity rather than by simulating the true mechanism of AC separation, which consists of a forceful impact on the lateral aspect of the acromion. Using serial radiographs, Eschler et al16 demonstrated this model reliably simulates the bony movement and anatomical relations during the various types of AC separation. The effect of this model on the soft tissue and especially the neurovascular structures is less known. To reduce potential bias, full body cadavers were used with minimal dissection around the neurovascular structures.
Another limitation of this study was that due to the small number of shoulders included in this study (10 shoulders in 6 individuals), the correlation could not be calculated between the distance of the nerve from the coracoid and patient characteristics such as height, age, and sex. Moreover, the distances were evaluated only in the typical arthroscopic beach chair position when the elevation of the bed is 80° and the arm is in 30° of abduction and 10° of forward flexion. These specific body and arm positions were used because they are the most common body and arm positions used in the arthroscopic reconstruction technique. Further studies should investigate the anatomical relation in the modified beach chair position that is used for open AC reconstruction where body elevation is 20° to 50°. In addition, the anatomical relations during AC separation should be investigated with the arm in a variety of positions.
The methodology also did not assess the relationship of the axillary nerve to the base of the coracoid in various arm positions. Certainly, that approach could be the focus of future studies. In this study, the objective was to avoid adding that level of complexity and to focus on an analysis with the arm in a position commonly used during surgery for AC joint reconstruction.
During type V AC separation, the axillary nerve is significantly closer to the base of the coracoid than reported in previous studies. This proximity increases iatrogenic risk to the nerve during anatomical reconstruction of the CC ligaments. To reduce this risk, the authors recommend reducing the AC joint prior to surgical dissection around the base of the coracoid.
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