Fractures caused by shoulder trauma are one of the most common conditions encountered in orthopedic surgery. More than half of extra-articular fractures involve the scapular neck [17]. The proximal part of the fracture line of scapular neck fractures reaches the space delimited by the upper border of the glenoid fossa and the base of the coracoid process (i.e., the coracoglenoid space) [12]. The shape and size of the coracoglenoid space vary greatly among individuals. Therefore, anatomical morphometric and biomechanical studies of these structures may provide information about the etiology of scapular neck fractures.

In this study, the average CGN was 3.74 ± 1.16 mm, which is consistent with the anatomical research results reported by Strnad et al. [4]. Ott et al., a German scholar [18], reported an average CGN size of 6.5 ± 2.1 mm, which is larger than that in the present study. The mean CGD in our study was 12.72 ± 1.46 mm. This is contrary to the mean CGD of 20.0 ± 4.0 mm reported by Alobaidy et al. [2]. This inconsistency may be due to differences in ethnicity. The specimens in our study were from Asians. Previous studies have shown that the dimension of the coracoid process is significantly smaller in Asians than in Europeans [19]. An interesting finding of this study was that there was a significant linear correlation between the CGN and CGD. This finding further shows that anatomical variation of the coracoglenoid space is a common occurrence.

Through this study, we found two coracoglenoid space populations. Although it was very simple to determine the anatomical variation based on the CGD and CGN, this approach served our purpose of assessing morphological differences in the coracoglenoid space. In type I, the average CGD and CGN was 13.81 ± 0.74 mm and 4.74 ± 0.45 mm, respectively, and these values were significantly higher than those in type II (11.50 ± 1.03 mm and 2.61 ± 0.45 mm, respectively). The type I morphology shows a prominent superior pole of the glenoid and a large depression at the base of the coracoid, forming a marked hook-like coracoglenoid space. In contrast, the type II morphology appears similar to a square bracket. The contour of type I reflects a more complex morphology than type II. This result may be explained by the fact that the superior pole of the glenoid serves as the attachment point for the long head biceps tendon. During prolonged shoulder movement, the stress generated by repeated muscle fiber contraction can stimulate bone growth [4]. Consequently, the arch-like structure delimited by the base of the coracoid and the anterosuperior part of the glenoid fossa deepens.

Surgical cases of scapular neck fractures often present challenging fracture line alignment due to the complex anatomical patterns involved and the high-energy injury mechanism. Miller and Ada [8] described a new type of scapular neck fracture, i.e., type IIC, consisting of fracture of the neck inferior to the scapular spine, and controversy regarding the classification of scapular neck fractures remains. After incorporating type IIC into the classification of scapular neck fractures, Goss [12] described this type as a ‘‘fracture of the neck inferior to scapula spine’’. Jaeger et al. [20] designated an anatomic neck fracture (denoted as F0), which was defined as “a fracture of the articular segment, not through the glenoid, but resulting in the fossa being detached from any part of the scapula body’’. The challenge lies in the uncertainty regarding the possible mechanism of scapular neck fractures, which complicates the clinical diagnosis. Therefore, understanding biomechanical patterns of these fractures is of great importance in determining the fracture types.

To the best of our knowledge, no biomechanical compression tests of different coracoglenoid space types have been performed. In the biomechanical failure test, the stiffness was 896.75 ± 281.14 N/mm and 692.91 ± 217.95 N/mm for types II and I, respectively. According to the calculation results of the load versus displacement curve, the failure load and energy were significantly lower for type I than type II. From the biomechanical point of view, stiffness is defined as the resistance of a structure to deformation. The greater the stiffness is, the smaller the deformation [21]. When the applied load gradually increases to the failure load, the deformation and energy of bone reach the maximum, which will eventually lead to fracture [22].

In our study, the proximal fracture line started at the scapular notch area, and the distal fracture line tended to run into the spinoglenoidal notch, which is similar to the typical scapular neck fracture reported by Bartoníček et al. [6]. This is also consistent with the findings of a study by Strnad et al. [4], who used three-dimensional CT reconstruction to observe the relationship between the morphology of the upper rim of glenoid and coracoid base and the fracture line in scapular neck fracture patients. The overall stability of the scapula under axial loading is maintained by the lateral pillar, the area of spine, and the superior border of scapula [23]. Daalder et al. [24] showed that the attachment region of coracoid base has the lowest bone density compared with the lateral border and spine, which might explain the preferred fracture location in our biomechanical compression test. Thus, we speculated type I variation serves as an anatomical factor of predisposition to scapular neck fractures.

There are several limitations to our study. First, the number of specimens (68) was relatively small, but all of them were scapular specimens from human cadavers. As such, we could better capture the natural variation in the shape and mechanical properties of human bone. Second, the biomechanical setup in our study could not be used to calculate the power. A further drawback of this study is that we were unable to assess the effects of muscle forces in vivo because violent voluntary muscle contraction can be a reason for fracture [25]. In future work, we will focus on the influence of muscle forces acting on the shoulder in the context of these fractures.

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