Study of the stress-strain state of the humeral models in supracondylar comminuted fractures in children and adolescents with different options of percutaneous fixation

Background. Fracture of the distal epimetaphysis of the humerus in children and adolescents is one of the most common injuries, accounting for 16–50 % of all bone fractures. Currently, there are two main configurations to fix supracondylar fractures in children and adolescents: the crossed pin design and the design with 2 or 3 lateral pins diverging in the coronal plane. The disadvantage of lateral fixation is an increased risk of loss of reposition, which can lead to cubitus varus. A significant problem when using a crossed fixation structure is iatrogenic damage to the ulnar nerve (2–8 %). The purpose was to compare the level of stresses in the humeral model with a supracondylar comminuted fracture with various options of percutaneous fixation under the influence of different loads. Materials and methods. A basic finite-element model of the humerus was developed, based on which a model of a supracondylar comminuted fracture was created. Two options of osteosynthesis were modeled: with two pins located crosswise (cross fixation) and a bundle of three pins (lateral fixation). The stress-strain state of the models was studied under the influence of tensile, bending and twisting loads. Results. The presence of a comminuted epicondylar fracture of the humerus leads to asymmetric changes in the zone of stress distribution in the epicondyles above and below the fracture line when performing cross fixation with two pins. In lateral fixation with three pins under the influence of tensile load, the tension in the medial epicondyle is reduced to a minimum and its level is doubled on the lateral epicondyle and especially in the bone fragment. This is related to the one-sided conduction of a bundle of pins. At the same time, the medial epicondyle remains unfixed and, accordingly, the loads on it are practically not transferred. The bone regenerate is too soft to prevent the movement of the distal fragment. However, a more rigid fixation of the lateral epicondyle than in the construction with two pins across, causes an increase in the stress level in the lateral epicondyle and bone fragment. The total size of the cross-sectional area of the pin bundle with lateral fixation ensures a twice lower stress level in them, compared to cross fixation. Conclusions. Mathematical modeling of the humerus with a supracondylar comminuted fracture made it possible to prove the advantage of cross fixation with two pins over lateral fixation with a bundle of pins in all load variants. This is especially determined by the stress level of the bone fragment, the stress on which increases many times under twisting and bending loads due to an asymmetric location of pins in lateral fixation.