Anatomical parameters of the coracoclavicular ligament: A study of 480 cases

Background: Although the optimal surgical technique for treatment of acromioclavicular (AC) joint dislocation still remains controversial, reconstruction of the coracoclavicular (CC) ligament has become the current tendency, particularly the transclavicular-transcoracoid drilling technique. The study investigated the anatomical characteristic of CC ligament by obtained anatomical parameters based on magnetic resonance imaging (MRI), for guiding surgeons to create transosseous tunnels precisely in reconstructing CC ligament. Material/Methods : Four hundred and eighty MRI scans of left shoulders from patients (ages from 11 years to 80 years, mean age of 51 years) were analyzed for the study. Some anatomical parameters were defined and measured in images of the coronal and sagittal plane, including four angles and seven distances. All measurement results were analyzed by SPSS 20.0. Results : In the coronal plane, the distance form the midpoint of AC joint to the insertion at the clavicle of male patients was greater than the average value of female patients. In the sagittal plane, in addition to the diameter of insertion at the coracoid process, other average distance values of male patients were greater than the average values of female patients, which were significantly different (P<0.05). Conclusion : These anatomical parameters obtained from MRI could completely describe the anatomical characteristic of CC ligament, which are beneficial for the reconstruction of CC ligament to create transosseous tunnels accurately, contributing to reduce the incidence of postoperative complications and promote the operative treatment of AC joint dislocation.

injury is common in certain populations, such as teenagers and athletes, and its incidence is raising with the increase of traffic accidents and sports injuries (25). Depending on the rupture of ligaments and varying degrees of injury to the deltoid and trapezius muscles and fascia, Rockwood proposed a classification of AC joint dislocation to include type I to VI in 1984 (2). So that in accordance with different types, the treatment for AC joint dislocation have not been uniform and the results also vary. Most AC joint dislocations can be treated nonoperatively, symptomatic and high-grade dislocations like type III to VI may be managed by a myriad of surgical techniques (2,9). Overall, more than 60 surgical techniques have been described so far (19), but the optimal one still remains controversial (4,22). Due to a slice of biomechanical and anatomical studies confirmed that the coracoclavicular (CC) ligament and AC capsule had played an important role maintaining the stability of AC joint (2,6,17,20,26), reconstruction of CC ligament gains more popularity to treat AC joint dislocation operatively. Particularly, using grafts like tendon graft (4), or synthetic materials, such as anchors (23) and endobutton with suture (20,25), to drill transosseous tunnels in the coracoid process and the clavicle for restoring the primitive anatomy of CC ligament has become the current tendency over the years (19). However, despite multiple clinical studies have indicated effective clinical and radiographic outcomes of the transclavicular-transcoracoid drilling technique (14,20), there is no gold standard for precisely how to create transosseous tunnels (14,19), which means a surgeon might reconstruct CC ligament from experience or in a routine manner of his own, so as to it is quite a few differences between the reconstructed ligament and the native CC ligament. Once CC ligament was reconstructed improperly, it may lead to poor reduction of AC joint, cause fixed anterior subluxation or contribute to fractures (5,11,14,19). Hence, it is no wonder that recent clinical studies have reported complication rates with CC reconstruction techniques as high as 23% to 80% (3).
In order to determine the ideal transosseous tunnels in the clavicle and the coracoid process, many studies researched the anatomical characteristic of CC ligament. Most of them came to their values and conclusions by means of cadaveric specimens (18,26,27), which could visually represented the anatomic morphology of CC ligament. Such as Mazzocca et al. (18) and Zhu et al (13), who have described the origin of CC ligament by measuring the distance from the lateral edge of the clavicle to the center of the conoid and trapezoid ligaments. Otherwise, Zhu et al. also used two K-wires to measure the valgus angle and retroversion angle of the two ligaments and described the orientation of CC ligament. In addition, a limited studies have brought forth more accurate date about CC ligament with computed tomography (CT). Sella et al. (19) analyzed 74 CT scans and obtained a convergence point (cP) to determine the exact point on which the drilling of the bone, but actually what they emphasized was the anatomical relationship between the clavicle and the coracoid process, which only shows anatomical information about the attachments of CC ligament; Xue et al. (24) reported values about the trapezoid and conoid footprint at the clavicle and the coracoid process in terms of CT scan data, though their main goal was to verify the feasibility of drilling technique and recommended the non-collinear drilling technique provided the capability to prepare bony tunnels without any risk of cortical breach. With respect to magnetic resonance imaging (MRI), some studies have focused on the advantage of MRI for the diagnosis (1,8,12) and prognosis (20) of AC joint dislocation, rather than the detailed anatomy of CC ligament. Only several studies dynamically described the length of CC ligament when analyzing the CC ligament As a result of MRI has an excellent soft tissue resolution and could provide multiple plane imaging, the present study analyzed MRI scans to obtain the anatomical parameters of CC ligament for investigating its anatomical characteristic, in order that guiding surgeons to create transosseous tunnels accurately in reconstructing CC ligament. then CC ligament had no significant changes were included.

Materials And Methods
We have used PACS for viewing the images and taking measurements as well. The measurements were made by 3 researchers engaged in the work of radiology more than five years, henceforth referred to as observers. These observers would take measurements all alone and each measurement was repeated three times, next averaging the three values obtained. The measurements of lines recorded in millimetre.

Methods
To demonstrate the anatomical characteristic of CC ligament based on MRI scans, some parameters were defined and measured separately in the coronal and sagittal plane. We had access to collected scans at PACS and images in the two planes were performed initially. In each plane, what we observed carefully was the hypointensity region attached to the clavicle and the coracoid process, in which was the location of CC ligament ( Figure   1). Afterwards, the following measurements were taken in this region.

Measurements in the coronal plane
The following points of this plane were marked: A: the insertion at the clavicle (the center of the attachment on the clavicle), B: the insertion at the coracoid process (the center of the attachment on the coracoid process), C: the midpoint of AC joint ( Figure 2).
Anatomical structures in linear and angular orientation parameters of CC ligament were quantified, linear dimension including: the length of CC ligament was assessed with the distance from the insertion at the clavicle to the insertion at the coracoid process (AB).
Another distance form the midpoint of AC joint to the insertion at the clavicle (AC) was measured for explaining the position of CC ligament at the clavicle. Furthermore, angular orientation including two angles: one was the angle between CC ligament and the clavicle (∠α), the other was the angle between CC ligament and the coracoid process (∠β).

Measurements in the sagittal plane
The measurements in this plane were a little different from the coronal plane. The following points of this plane were marked: a: the insertion at the clavicle (the center of the attachment on the clavicle), b: the insertion at the coracoid process (the center of the attachment on the coracoid process), c: the tip of the coracoid process, x: the midpoint of the supraclavicular plane of the clavicle (represents the supraclavicular plane of the clavicle), y: the point closest to the tip of the coracoid process in the subcoracoid plane (represents the subcoracoid plane of the coracoid process) (Figure 3).
In addition to the distance from the insertion at the clavicle to the insertion at the coracoid process assessed the length of CC ligament (ab), we measured the distance form the tip of the coracoid process to the insertion at the coracoid process (bc) simultaneously for explaining the position of CC ligament at the coracoid process. The third distance was from the supraclavicular plane of the clavicle to the subcoracoid plane of the coracoid process (xy). What's more, the other quantified anatomical parameters including: the diameters of insertions at the clavicle (mn) and the coracoid process (m n ) for evaluating the footprint of CC ligament. Two angles between CC ligament and the clavicle (∠γ) and the coracoid process (∠δ) were obtained similarly.

Statistical Analysis
Statistical analysis was performed using SPSS 20.0 software (IBM Corp, Armonk, NY, USA).
All data were presented as the mean ± standard deviation (SD). Independent samples ttest was applied to identify differences between measurements in male patients and female patients. All hypothesis tests were implemented by adopting a 5% significance level and P values equal to or smaller than 0.05 were considered statistically significant.

Discussion
Aiming to investigate the anatomical characteristic of CC ligament based on MRI and raise a reference for surgeons to create transosseous tunnels precisely in reconstructing CC ligament, we designed to review 480 cases of patients and analyze their MRI scans of left shoulders. By taking different measurements, we obtained some anatomical parameters related to the length of ligament, the location and footprint of the insertions at the clavicle and the coracoid process as well as the orientation of ligament in the coronal and sagittal plane, respectively. With these abundant parameters, the present study permits a better understanding of the anatomy of CC ligament and conduces to find the applicable points where transosseous tunnels should be drilled in the clavicle and the coracoid process. More importantly, MRI offers more possibilities for anatomical research of CC ligament. This is an improvement on previous studies in which similar measurements have been made using cadaveric specimens. In contrast, imaging techniques are able to acquire numerous samples in a short time and avoid potential errors caused by manual measuring due to the automatic operation. This study reveals that MRI not only has advantages in clinical diagnosis and prognosis of AC joint dislocations, also contributing to restore the primitive anatomy of CC ligament with high accuracy to reduce the incidence of postoperative complications and promote the operative treatment of AC joint dislocation.
The AC joint is a diarthrodial joint formed by the distal clavicle and the medial facet of the acromion (2), and its stability is mostly maintained by CC and AC ligaments and capsule (15). The AC ligament and capsule are essential in providing its anterior-posterior stability. The CC ligament is the laterally located trapezoid ligament and the more medial conoid ligament, which prevents superior-inferior displacement of the clavicle (2, 17, 27).
The mechanism of AC joint dislocation usually involves a direct blow to the lateral aspect of the adducted shoulder, leading to downward displacement of the scapula opposed by impaction of the clavicle onto the first rib (7). The force initially damages AC ligament, then further injuries CC ligament as the force perpetuates. Thus AC joint dislocation range from a simple sprain of AC ligament to a complete dislocation of the joint. Variable severity of injuries results in the great diversity of treatment. However, no matter how many treatment exist, the controversy of almost all the literature about the therapies of AC joint dislocation eventually focus on the options of operative or nonoperative techniques. At present, most scholars generally accept that Rockwood type Ⅰ and II choose conservative treatment while Rockwood type IV to VI use operative management. The ideal treatment of type-III injuries is debated (9,21), each individual treatment should be carried out according to the specific conditions of patient, namely the type of injury, age, amount of exercise and aesthetic requirements.
In spite of there is ongoing debate as to which technique should be the gold standard of the surgical management of high-grade AC joint dislocations, operative techniques continue to advance with technology and an improved characterization of the anatomy (15). Early surgical procedures using K-wires, Steinman pins and cerclage wires to fixate AC joint. Because of there are a lot of shortcomings and the use of hardware across AC joint may worsen the intra-articular injury and might hasten the onset of joint arthrosis (2), these techniques have been largely eliminated. Later, AC joint reconstruction has gradually become the mainstream as clinicians gained a deeper understanding of the anatomical structure of AC joint. Hardware such as hook plate and screw was the main surgical material for reconstructing the anatomy of AC joint at first, but researches reported that it had been associated with numerous complications (2) and required a secondary operation for implant removal (25). Then clinicians began to shift their attention to ligament reconstruction. In 1972, the Weaver-Dunn procedure was first described, which utilized the native AC ligament in AC joint reconstruction (15).

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Data availability
The related data used to support the findings of this study are restricted by the medical ethics committee of School of Basic Medical Sciences, Southwest Medical University. Data are available from Lei Zhang (email: zhanglei870722@126.com) for researchers who meet the criteria for access to confidential data.

Competing Interests
The authors declare that they have no competing interests.