Fractures caused by shoulder trauma are one of the most common diseases of orthopedic surgery, more than half of the extra-articular fractures involve the scapular neck . The proximal part of the fracture line of anatomical neck fractures ran into the space that delimited by the upper border of the glenoid fossa and the base of the coracoid process (i.e., the coracoglenoid space) , the shape and size of this space hinge on its amount of bone mass as well as the fragility. Hence, anatomical morphometric and biomechanical studies of these structures may provide information about the etiology of the scapular neck fractures.
There was no significant difference in mean CGN between left and right side, which is consistent with previous study . In addition, the average CGD at the left side was found to be same as the right side in our study, this confirmed the previous study of Khan et al, who showed that the left and the right side were 28.2 ± 3.5 mm and 27.4 ± 8.3 mm respectively . To the best of our knowledge, many scholars measured the CGD that between the coracoid tip and the nearest point of the glenoid fossa [2, 5], few studies have put forward the distance of the coracoid middle tip to the glenoid. Mohammad et al reported the mean distance of CGD that same as we defined was 20.0 ± 4.0 mm , which is bigger than that of us (12.76 ± 1.59 mm), this difference could be related to the measurement technique or different ethnic of specimens . In our studies, the CGD is the coracoid middle tip to the glenoid, which is located in the same plane as CGN. We classified the coracoglenoid space based on these morphometric proportions, which could better describe the anatomical morphology of the space. We did not include the influence of gender on morphology, but previous studies demonstrated that there was no significant difference between metrological parameters and gender [2, 3].
The findings of this study present that it is a common occurrence for the anatomical variation of coracoglenoid space that was sorted into Type Ⅰ (“hook” shape) and Type Ⅱ (“square bracket” shape) according to the morphometric ratio of 68 scapulae. Among our classification, the incidence of Type Ⅰ was higher than Type Ⅱ, which is similar to previous studies . Type Ⅰ has a prominent superior pole of the glenoid and a large depression at the base of the coracoid that formed a marked hook-like structure with a posterolaterior border of the coracoid. Type Ⅱ resembles a square bracket depression. Therefore, the contour of Type Ⅰ reflects a more complex morphology than another type. This may due to the superior pole of the glenoid serves as the attachment point for the long head tendon of the biceps, the stress that generated by repeated contraction of muscle fibers during prolonged shoulder motion produces stimulates bone growth, consequently, the arch-like structure by the base of the coracoid process and the anterosuperior part of glenoid fossa was deepened . We expected that there would be more Type Ⅰ on the right scapulae owing to most of the Chinese people are right-handed. Contrary to expectation, however, our studies did not find a significant difference between the body sides and the shape of coracoglenoid space in both two types (P > 0.05). Some studies have shown that the coracoid base growth plate was formed by the separate but directly apposed physeal cartilage of the coracoid base and the anterior scapulae, which typically fused by the 14th or 15th years of age; a separate physis forms at the tip of the coracoid and permits longitudinal growth and frequently symmetric in appearance with the contralateral side . This may be the cause of symmetry between the left and the right side.
Surgical cases often present a challenging fracture lines alignment of scapular neck fractures due to the involved complex anatomical patterns, as well as the high-energy injury mechanisms. Miller and Ada had described a new type of scapular neck fractures, i.e., ⅡC type, fracture of the neck inferior to the scapular spine, which is a controversy surrounding the classification of scapular neck fractures . After containing the ⅡC type in the classification of the scapular neck fractures, Goss described this ‘‘fracture of the neck inferior to scapula spine’’ . Jaeger et al designated the anatomic neck fracture(denoted F0), which defined as “a fracture of the articular segment, not through the glenoid, but resulting in the fossa being departed from any part of the scapula body’’ . The challenge is due to unclear about currently the possible patho-mechanical role of scapular neck fractures that complicating the clinical diagnosis. Therefore, understanding biomechanical patterns of bony fracture is of great importance in determining the type of fracture.
The results of our research show, in this study biomechanical setup that simulating an external impact on the glenoid fossa, the average maximum failure load in Type Ⅰ was less than that in Type Ⅱ (P = 0.011). In addition, the average failure stiffness and energy in Type Ⅰ were significantly lower than that of Type Ⅱ (P = 0.001, P = 0.015, respectively). Variation in Type Ⅰ serves as a predisposing factor in the scapular neck fractures, and Type Ⅱ might be substantial enough to improve the stability of scapular neck fractures. To our knowledge, no investigator has compared the biomechanics between different types of coracoglenoid space. The most important clinically relevant finding of this study was that the larger the hook-like structure of the space, the less the external impact force it will be resistant, and it may be more prone to the incidence of fractures. This study yielded similar results as previous reports by Strnad et al., who used the three-dimensional CT reconstruction to observe the morphological characteristics of the coracoid in patients with scapular neck fractures and found that the coracoid was hook shape . Thus, Type Ⅰ may constitute an anatomical predisposition to scapular neck fractures. This variability would be of clinical importance when identified the type of fractures and the surgical procedure for fixation, as it should also guide optional screw fixing positions. In addition, biomechanical compressive tests in our study were performed on human cadaver scapular specimens, which allows for better to capture the natural variation in shape and mechanical properties than a synthetic bone model.
Detailed knowledge of the coracoglenoid space is important to aid in fracture reduction and to improve the mechanics of the bone plate construction. The typical anatomical neck fracture line separates only the glenoid fossa from the scapular body, with a short spike of the lateral border of the body [8, 15, 16]. This typical fracture fragment often results in valgus displacement, which is mainly due to the disruption of both tension and balanced stability of the soft tissues around the scapulae. [16, 18]. Unstable or highly displaced fractures of the glenoid neck have the potential to lead to shoulder instability and chronic pain. A meta-analysis revealed that even after 10–20 years, one-quarter of patients with displaced scapular neck fractures had residual shoulder disabilities and with less favorable results . Several authors have shown good surgical and functional results with operative management of scapular neck fractures [13, 27, 28]. However, there are many different ways of internal fixation for these fractures. The common approach to the fixation of fractures is a single plate that extending along the posterior surface of the lateral scapular border, engaging the glenoid fragment . Sulkar et al. improved an alternative strategy, i.e., the 2-plate neck fixation technique, to repair scapular neck fractures . Anyway, from the standpoint of the photomechanics's mechanism of fracture, morphological variability and biomechanical properties of the coracoglenoid space need to be taken into account for different fixation techniques. Based on an accurate understanding of this information can also help to shorten the surgical cycle, thus reducing tissue irritation and complications in the configuration of the bone-plate construction.
There were several limitations in our study. First, the number of 68 specimens is a relatively small quantity, but all of them are human cadaver scapular specimens. In this way, we can be better capture the natural variation in shape and mechanical properties of human bone. Secondly, we were unable to check the mechanism in vivo of muscle forces around the shoulder. Because besides the fragility associated with the coracoid, it has been believed that the violent voluntary contraction of muscles was one of the reasons for the occurrence of fractures .