Biomechanical studies via computational finite element simulation can provide deeper insight into the stability and functionality of bone constructs, and therefore can be used to make comparisons among various fixation techniques that are difficult to be accomplished through other methods[20]. This study is the first to establish a pediatric bone model for three-dimensional finite element analysis through a novel combination of commonly available commercial software. In this model, we compared the stiffness of prevalent Kirschner wire configurations and the variations in fixation patterns of supracondylar fracture of the humerus. We observed that 2-crossed pins are not stiffer than 2-divergent lateral pins in some circumstances, as it used to be believed by many surgeons. A third pin going through the olecranon fossa, inserted through the ossific nucleus of the capitellum substantially enhanced the stiffness of 2-divergent lateral pins configuration.
Previous finite element analytic studies on pediatric fractures exclusively utilized synthetic bone models[14]. The dramatic difference in the anatomy of the distal humerus between adults and children would inevitably lead to divergence from clinical reality. In this study, through a combination of of innovative software packages, we established a pediatric humerus model with the computed tomography images of a 6-year-old boy, in which the child-characteristic cartilaginous component and the ossific nucleus of the capitellum were reconstructed for the finite element analytic tests.
Among all the pinning configurations, divergent lateral pins were more resistant than crossed pins to translational forces. Crossed pins were however more resistant to rotational forces. The stiffness values of the pinning configurations were mainly influenced by the entry and exit points of pins, pin pathway and number of pins. These findings may partly explain the different conclusions of previous studies, since most of those studies differed in the entry or exit points of pins.
It is mostly believed that, the more proximal pins exit, the more stable the fixation would be. In our crossed pinning configurations however, the best stiffness was achieved when both pins exited around the upper border of the metaphyseal-diaphyseal junction (MDJ). This superiority was most obvious in the anti-rotation ability. It had been reported that proper pin spread at the fracture site could influence the stability of pins[5]. We observed from our study that, by exiting around the upper border of the MDJ in typical SHFs, the ideal pin spread could be achieved and therefore optimum stability may be obtained. Although the higher the crossed pins exited the more pin spread achieved along the fracture line, stiffness however reduced as they exited higher. This was probably because the center of rotation was brought closer to the crossing point as the pins exited higher, thereby making them less stable. Pins that crossed within the MDJ region would have their centre of rotation brought closer to the fracture line, also making them less stable. Similar observation was made with crossed pins in a previous biomechanical study conducted with composite bones using varying fracture heights of metaphyseal-diaphyseal fracture of the humerus[21].
The entry point and pin pathway had a major influence on the total stiffness of lateral pins. One pin entering through the ONC greatly enhances the stiffness of divergent lateral pins in all aspects. When entering from the ONC, there is more pin-cortical bone contact area, rendering pins more stability. This finding was consistent with that of Gottschalk et al,[3]. The distal lower pins of the two lateral-entry configurations (2LP, 2LD) in our model, with only a small or no ONC contact area, may have to penetrate and re-enter the bone through their course along the olecranon fossa giving them a shorter bone tunnel length and therefore less stability. Our study was however not the first to emphasize on the effect of lateral pin pathways. Using synthetic bone model, one study compared the influence of the distal lateral pin when the position of the other, proximal lateral pin was fixed. They came to the conclusion that, a lateral pin placed parallel to the metaphyseal flare of the lateral humeral cortex, in combination with a second diverging pin crossing the fracture site at the medial edge of the coronoid fossa, provides the optimum fixation[22]. It was however not clear whether the second pin was a capitellar entry one.
Adding a third pin to 2-divergent lateral pins in any configuration significantly enhances stiffness of lateral pins in all directions. From our study, we observed that a third, middle pin travelling all the way within the bony cavity of the lateral column increases stability of lateral pins in the sagittal plane, while that going through the olecranon fossa increases stability in the coronal and rotational planes. We believe the increased stability demonstrated is due to the more reinforcement along the fracture line by the additional pin, longer bone tunnel length and more pin-cortical bone contact regions. The best stiffness was however generated by the 3-crossed pin configuration (3CP, 2-divergent lateral and 1-medial pins), which had a mid-ONC distal lateral pin. Stiffness values were far more than any other 3-pins configuration in all loading directions. Our finding differed from that of Feng C et al., who stated that an additional pin did not bring about increased stiffness in their model[6]. This difference may be partly due to the difference in testing models, fracture patterns and entry and exit points.
Despite the novelty of this model, our study had some limitations. First, the model was based on the computed tomography images of a 6-year-old boy, which may not represent the diverse pediatric population. However, since the majority of SHFs occur around this age, it was therefore used for our model. Secondly, we only looked at the most commonly encountered typical, transverse fracture pattern[17], which may not represent all clinical circumstances of pediatric supracondylar humeral fractures. Models of different age, gender and fracture pattern must be developed and tested, as well as clinical observations should be carried out to further verify the findings in this study.