Linear regression analysis of clinical data found that the measured width of the distal humerus directly correlated with the rotation degrees in a 14-year-old male child. As it was shown in Fig. 1, the width of the targeted section was the largest (36.73 mm) when the projection degree was 0 ° (in terms of A-P position). With the increase of the rotation degrees, the width could be decreased gradually, until the minimum value (15.98 mm) was obtained when the projection degree was 90° (in terms of lateral position). Finally, regression analysis was performed and demonstrated that the trend was linearly correlated and had a high degree of fit, and the R square was 0.982.
With the application of 3D printed normal elbow joint, it was found that the measured width at the peak of humeral olecranon fossa was also linearly correlated with the rotation degrees. When the projection degrees was 0°, the width was the maximum (59.8 ± 1.0 mm). With the increase of the projection degrees, the width decreased gradually, until the minimum value (22.6 ± 0.2 mm) was reached in the lateral view (Fig. 2A). Finally, the equation (\(Y=57.39-0.41X\)) was obtained, while Y represented the measured width and X represented projection degrees. The regression analysis demonstrated that the trend was linearly correlated and with the application of 3D printed model, it has a higher fit degree, with its R square was 0.99 (Fig. 2B).
In the lateral 45° oblique position, the width of the distal fracture line could be measured by rotating the distal humerus, while the proximal part was fixed. The diagram in the center of Fig. 3 represented the correlation between the distal fracture line width and rotation degrees, as the distal medial condyle rotated posteriorly defined as internal rotation and the distal medial condyle rotated anteriorly defined as external rotation. Meanwhile, the peripheral X-ray film marked with measured widths corresponded to the representative rotation degrees. It can be found that the width of the distal fracture line decreased from − 45° (external rotation) to 45° (internal rotation) in 5-degree increments, which is basically consistent with the linear regression curve obtained in Fig. 2B. As it was shown in Fig. 3, once the distal fracture line width was larger than that of the proximal fracture line width, it could be determined as external rotation, and vice versa. Therefore, the rotation direction of distal humerus can be intuitively determined based on this. When the distal humeral condyle rotated to 45°, the distal fracture line width reached the maximum or minimum value, which were 59.7 ± 1.8 mm and 22.2 ± 0.4 mm, respectively. As the rotation degree was larger than 45°, the distal fracture line width decreased with the increase of the rotation degrees. Thus, there were two corresponding values of the rotation degrees under the same widths, which would confuse the final judgement. By this time, the rotation degrees can be determined by the size of the medial or lateral condyle of the humerus. Briefly, due to the impediment of the capitellum, the larger the medial condyle rotated anteriorly, the smaller the lateral epicondyle was. Similarly, the larger the medial condyle rotated posteriorly, the more backward the medial epicondyle was. As it rotated less than 45°, the medial condyle was located in front of the longitudinal axis of the humerus, while it rotated beyond 45°, the medial condyle would be behind the midline. Therefore, the corresponding rotation degrees could be determined by the above methods, so as to perform fracture reduction rapidly and precisely.
The width of distal humerus fracture line measured by 5 orthopedic surgeons had excellent reliability, as the overall ICC value can reach to 0.998 (Table 1). Moreover, as it was shown in Table 2, the evaluation accuracy of the rotation direction of distal humeral increased significantly from 76–96% (P = 0.004), especially within 45°, the accuracy can reach to 100% (P = 0.004). When the humeral condyle rotated more than 45°, whether internal or external, the accuracy was 90%, which was higher than that before, but with no statistical difference (P = 0.186). In addition, the overall evaluation accuracy of the rotation degrees was 86.0% by using this established method. Among them, when the rotation degrees were less than 45°, the accuracy could reach to 93.3%, which was higher than 75% (rotation degrees > 45°), but with no statistically significant (P = 0.067).
Table 1
Reliability of Measured Width of Distal Fracture Line
|
ICC
|
95%-L
|
95%-U
|
Overall
|
0.998
|
0.995
|
0.999
|
Rotate degrees < 45°
|
0.998
|
0.993
|
1.000
|
Rotate degrees > 45°
|
0.999
|
0.997
|
1.000
|
Table 2
Accuracy after Application of this Novel Method in Models
|
|
Rotate direction
|
Rotate degrees
|
Lateral view
|
38/50 (76%)
|
-
|
45° Oblique view
|
|
|
Rotate degrees < 45°
|
30/30 (100%)
|
28/30 (93.3%)
|
Rotate degrees > 45°
|
18/20 (90%)
|
15/20 (75%)
|
Overall
|
48/50 (96%)
|
43/50 (86%)
|
In five clinical cases, the rotation direction and degrees of SCHF were estimated by the same surgeons based on the established method. Specifically, the direction of rotation was evaluated accurately (30/30), especially with an evident rotation. The accuracy of rotation degrees is 50% (15/30), as its error was allowed within 5°, due to the large anatomical variation of ages. When the error was allowed within 10°, the accuracy could reach a higher level of 80% (24/30). Meanwhile. the overall ICC value can reach to 0.979 and the 95%CI is 0.933–0.997, which indicated a good reliability. Finally, a typical case of 5-year child suffered from SCHF was shown in Fig. 4.