A guideline for screw xation of coracoid process base fracture by 3D technology

Fractures of the base of the coracoid process are relatively rare, but an increasing number of studies have reported using screws to x basal coracoid process fractures. This study was performed to simulate the surgical procedure and obtain the ideal diameter, length, insertion point and angle of the screw through the method of 3-D axial perspective in Chinese patients. We randomly collected the right scapula computed tomography (CT) scans of 100 adults. DICOM-formatted CT-scan images were imported into Mimics software. The 3D digital model of the right scapula was established. Two virtual cylinders representing two screws were placed from the top of the coracoid process to the neck of the scapula and across the base of the coracoid process to x the basal coracoid process. The largest secure diameters and lengths of the virtual screws were measured. The positions of the insertion points and the directions of the screws were also researched.

.31 mm and 48.16 ± 6.94 mm, respectively. The screw insertion corridor with a diameter of at least 4.5 mm was found in anyone. We found gender-dependent differences for the mean maximum diameters and the maximum lengths of the two screws. The positions of the two insertion points were statistically signi cant in different genders.

Conclusions
The study provides a valuable guideline for the largest secure corridor of two screws that xed the fracture at the base of the coracoid process. For the ideal screw placement, we suggest an individual preoperative 3D reconstruction simulation. Further biomechanical studies are needed to verify the function of the screws.

Background
Fractures of the coracoid process base are rare, and current treatment guidelines remain unclear [1].
Ogawa et al have proposed a classi cation system for coracoid fractures based on the relationship between the fracture site and the coracoclavicular ligament. Type I fractures are located behind the CC ligaments, whereas type II fractures are anterior to it. They considered that Type-I fractures indeed need operation whenever the scapuloclavicular connection have been destroyed [2]. This is consistent with many reports in the literature [1][2][3][4][5][6][7][8][9][10][11][12]22]. Within these injuries, a fracture of the coracoid process base represents a severe form of the injury, and a variety of classi cations have stressed the importance of recognition of this subtype. The de nitive xation for the fracture of the coracoid process base is done with 1 to 2 bicortical solid screws ± washers [2, 4-9, 11, 12, 19]. W. Hill et al believed that the second screw may be used to supplement the single screw and has the bene t of controling rotation of the fracture thus enhancing xation against traction and rotational forces of the upper extremity. The coracoid process is in close proximity to major neurovascular structures, consisting of the brachial plexus and the axillary artery and vein [13]. A knowledge of the correct location of the insertion point and screw direction is essential to avoid penetrating into joint and injury to neurovascular structures. In addition, the complex anatomy of the coracoid process and the ligaments and muscles attached to it make screw placement more di cult. Bhatia et al described percutaneous coracoid base xation using orthogonal biplanar uoroscopic guidance techniques. Nevertheless, they noted that theoretical complications such as articular perforation, neurovascular injury and damage to coracoclavicular ligaments may emerge even when performed by an experienced shoulder surgeon [12]. Yoshiteru Kawasaki reported a new screw xation technique for coracoid base fracture under uoroscopic guidance and counted anatomic information on the cross-sectional size of the coracoid base obtained in a computed tomography (CT) study [7].
W. Hill et al described that if the fracture is not comminuted and occurs through the base, then a 3.5-mm lag screw is often needed for adequate stability. They used a screw length between 30 and 45 mm with 15 medial angulation and 30-40 posterior angulation to ensure that the screw remains enclosed in the bone [6]. Many reports also show their xation and good postoperative outcomes. Although some methods of open, mini-open, and percutaneous techniques under uoroscopic guidance have been reported previously [2, 4-9, 11, 12, 21], screw insertion into the neck of the scapula across the fracture of coracoid base is di cult due to the complex shape of the scapula [14,15].
At present, there are many studies on the application of CT data into various software for the xation of screws in treatment of different fractures [16,17,18]. In previous studies, only Length of the long and short axes at the thinnest part of the coracoid base in the axial CT plane was measured [7]. The purpose of the study is to specify the ideal insertion points, the largest secure diameters and lengths, and the accurate angles of the two screws through the method of axial perspective.

Materials And Methods
We retrospectively collected the right scapula CT scans of 100 adults who had undergone continuous slice CT scanning at the imaging research center of our hospital between August 2018 and July 2020.
Patients were excluded if they had scapula fractures, tumors or severe deformities. This study was approved by the Institutional Review Board of our hospital, and patients' informed consent was obtained. The mean age of the patients on whom the models were based was 47.96 ± 16.12 years (range 18-85 years).
DICOM-formatted CT-scan images of each patient were imported into Mimics software (21.0; Materialise, Leuven, Belgium). We removed the soft tissue by the function of image segmentation, region growth and multiple slice editing, respectively. A total of 100 right virtual scapula models were created.
We reduced the transparency of the right scapula models and turned it to the axial perspective view, which was parallel to the cross section of the base of the coracoid process from top to bottom (Fig. 1A). We observed and adjusted the position of the model to nd the largest translucent area through the perspective view. Then, a translucent area like an irregular fusiform shape was seen clearly and divided into two basically equal parts to implant two screws (Fig. 1B). The red outline represents the top boundary of the horizontal part of the coracoid process, and the blue outline represents the boundary of the cross section of the base of the coracoid process. The green and orange areas represented the two screw paths,respectively. Two virtual cylinders representing the screws were placed into the translucent area. The diameter was increased progressively and the maximum diameter was de ned when the cylinder did not penetrate the border of the area (Fig. 1C). We observed and adjusted the length of the screw to make sure that the screw just penetrated the posterior cortical bone ( Fig. 2A-C). The diameters and lengths of the virtual screws were measured. In order to con rm the position of screw, the distances from the insertion point to the closest point of coracoid and the posterior border line of the horizontal part of coracoid were measured, respectively. They were recorded as Distance L1, L2 for medial screw (MS) and L3,L4 for the lateral screw(LS) (Fig. 3). The slope of the upper edge of the posterior coracoid process was selected as the reference plane called plane1.The anterior inclination angle between the screw and plane1 was measured and recorded as angle α (Fig. 4A). In addition, we de ned another reference plane perpendicular to plane1 called plane2. The medial inclination angle between the screw and plane2 was also measured and recorded as angle β (Fig. 4B).
The collected data were analyzed by SPSS 25.0 statistical software. The experimental data are represented as the mean ± SD. T tests were used to compare the data. Statistical signi cance was accepted at p < 0.05.

Results
The study subjects included 50 males and 50 females aged between 18 and 85 years old, with a mean age of 47.96 ± 16.12 years. As shown in Fig. 1-B, the screw insertion safe zone can exhibit an irregular fusiform shape from the reconstructed scapula model. Tables 1 and 2, the mean maximum diameters of the medial and lateral screws were 7.08 ± 1.19 mm and 7.34 ± 1.11 mm, respectively. The mean maximum lengths of the medial and lateral screws were 43.11 ± 6.31 mm and 48.16 ± 6.94 mm ,respectively. The mean distance L1 was 11.63 ± 2.87 mm, L2 was 7.50 ± 1.72 mm, L3 was 19.87 ± 2.76 mm, and L4 was 4.88 ± 0.86 mm,respectively. For the data captured above, the intersex difference was signi cant (P < 0.05).  The mean angle α and β of different genders were also recorded in Table 3. The former was 16.40°±6.03°a

As shown in
nd the latter was 10.33°±7.39°. The results of the angle β were statistically signi cant between males and females(P < 0.05). However, the angle α were not statistically signi cant (P > 0.05). The screw insertion corridor with a diameter of at least 4.5 mm was found in everyone in our research.

Discussion
In 1996, Ogawa et al believes that a type 1 coracoid fracture would be more unstable and require an open reduction and internal xation. They xed unstable fractures of the coracoid base with a malleolar screw and washer [2]. W. Hill et al described in detail the technique and clinical experience of screw xation for fracture of coracoid base. The difference with Ogawa is that another screw may be used to supplement the single screw and has the bene t of offering rotational control of the fracture thus enhancing xation against traction and rotational forces of the upper extremity [6]. We share this view that the common xation methods for coracoid base fractures are to pass two parallel screws vertically through the fracture line. He provided detailed surgical approaches and nail placement angles, but the exact point of entry is not given. The Angle and diameter and length of the screw will change with different entry points.
Many anatomic and radiographic measurements of the coracoid process have been reported [14,15,20,23]. Yoshiteru Kawasaki et al reported the cross-sectional size at the level of the coracoid base by the study on CT axial measurement of the coracoid base [7]. The measurement data on the coracoid base may be useful for safety screw xation of coracoid base fracture.
The coracoid process is complex in structure and varies from person to person [14,15]. Previous reports did not give a large number of patients whose coracoid base fracture requires screw xation with their own entry points and angles, as well as screw lengths and diameters statistics. There are few digital anatomical studies on its properties.
Mimics software has been widely used in 3D reconstruction for the development of digital orthopedics technology. In our study, we applied the 3D method of axial perspective as described in previous studies [16,17,18]. We observed and adjusted the position of the model to nd the largest translucent area through the perspective view. Then, the translucent area like an irregular fusiform shape was divided into two basically equal parts to implant two screws. We increased the diameters of two virtual screws progressively and monitored the virtual screws in the views of coronal plane, sagittal plane and horizontal plane, without violating the cortices and articular surface. The method used in our study not only saves manpower, materials and nancial resources, but also can be repeated and veri ed by test results with high reliability.
In our research,we recorded the exact points of entry. And the distances from two screw insertion points to the closest point of coracoid and the vertical distances from two screw insertion points to the posterior border line of the horizontal part of coracoid were all observed in this study. There are signi cant gender differences. For the data captured above, this is due to the obvious anatomic differences in scapula bones between female and male.
Many screws with a diameter of 3.5mm or 4.5mm have been reported for xation of coracoid process base fractures [2,[4][5][6][7][8][9]11,12]. According to the information in our study, the maximum diameter to avoid cortical breaches is 7.  [6]. Because of the difference in the reference plane, the results cannot be compared. We believe that the exact coordinates of the measurement angle are not given in the previous research report, which leads to the imprecision of the measured angle. The angle of measurement will vary depending on the position of the scapula. In our study we measured a signi cant gender difference in Angleβ. The parameters of the two screws may provide the surgeon appropriate information of safe screw placement for the treatment of coracoid base fracture. The large standard deviation of our results indicates great differences among individuals. As a result, preoperative planning should be implemented detailedly for each patient. 3D reconstruction and simulated screw placement technique with digital software before operation are valuable.
There are some limitations to this study. We only analyzed the data according to the gender, not according to different age groups. In addition,we only studied the scapula of Chinese people, who have different skeletal shapes than European and American populations. What is more, more biomechanical studies and related clinical research should be performed .

Conclusion
We indicate a valuable guideline for the safe zone of two screws that xed the coracoid process base fractures. The ideal screw position and the size of the screws can be determined in 3D-models by digital software. Further biomechanical studies are needed to verify the strength and effect of the screw.

Declarations
The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

Ethics approval and consent to participate
This study has obtained ethics approval and consent of the ethics committee in our hospital.

Consent for publication
Not applicable.