In 1996, Ogawa et al believes that a type 1 coracoid fracture would be more unstable and require an open reduction and internal fixation. They fixed unstable fractures of the coracoid base with a malleolar screw and washer. W. Hill et al described in detail the technique and clinical experience of screw fixation for fracture of coracoid base. The difference with Ogawa is that another screw may be used to supplement the single screw and has the benefit of offering rotational control of the fracture thus enhancing fixation against traction and rotational forces of the upper extremity. We share this view that the common fixation 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. The measurement data on the coracoid base may be useful for safety screw fixation 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 fixation 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 find 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 financial resources, but also can be repeated and verified 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 significant 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 fixation of coracoid process base fractures[2,4-9,11,12]. According to the information in our study, the maximum diameter to avoid cortical breaches is 7.89±0.98 mm(MS), 8.06±0.81 mm(LS) in male and 6.27±0.76 mm(MS) , 6.61±0.87 mm(LS) in female. Anyone possessed a corridor with diameter of at least 4.5mm . Nevertheless, due to individual and sex differences, the use of preoperative measurements and calculations by digital tools is recommended.
W. Hill et al described the screws he used to fix the base fracture of the coracoid process as 30-45cm. In our study We measured the maximum length of the screws just passing through the posterior cortex of the scapula.The length of medial screw is 47.62±4.29mm in male,and 38.60±4.54mm in female. The length of lateral screw is 52.81±5.40mm in male,and 43.52±4.91mm in female. It turns out that we can actually choose a slightly longer screw .
On the basis of mastering the diameter and length of screw, the insertion point and direction are two important factors affecting the safe placement of screws. Previous reports have not given the exact points of entry.Different from previous studies, we found that the optimized insertion points are 12.36±2.70 mm(MS) ,21.08± 2.51mm(LS) away from the closest point in males and 10.89±2.87 mm(MS) , 18.66±2.47mm(LS) in females; simultaneously are 8.21±1.68mm(MS) , 5.06± 0.70mm(LS) away from the posterior line in males and 6.80±1.45 mm(MS) ,4.69±0.96mm(LS) in females. The anatomic landmark of the closest point and the posterior line of coracoid process can be well palpable and identified, so they can be used as effective references intraoperatively.
W. Hill et al used a screw with 15 medial angulation and 30–40 posterior angulation to ensure that the screw remains enclosed in the bone . 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 significant 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 .