No abstract

Background. Pectus excavatum is characterized by retraction of the sternum and anterior ribs of different depth and width. The formation, its prediction, calculation of chest deformity, and their study when planning thoracoplasty using the Nuss procedure for this pathology is an important problem of orthopedics and thoracic surgery. The purpose of the work was to calculate the coefficient of restoration of the chest shape by the ratio of the pectus excavatum depth and the chest size in the frontal plane before and after mathematical modeling of thoracoplasty using the Nuss procedure. Methods. To assess displacement of ribs depen-ding on depth deformity of chest h, two models were built. The first model is a flat frame on supports, the elements of which consist of cartilaginous parts of ribs and sternum. For this model, the dependence of the force F was determined, which is necessary to correct the depth of chest deformity. The second model is a curved bar that simulates a rib, to one of the ends of which a support load is applied, calculated during the analysis of the first model. For this model, the displacement of the plate fixation point under the action of a given force was determined. To obtain more accurate results, a finite element study was performed on a chest model. Results. The correction of pectus excavatum depth without fixing plate to ribs was simulated. The displacements of rib sections in the place of plate fixation at different depths of pectus excavatum was assessed: h = 2 cm, h = 3 cm, h = 4 cm, h = 5 cm. The analysis of calculation results showed that after correction of the depth of chest deformity, its size in the frontal plane decreases. So, at the maximum deformation depth h = 5 cm, the deviation of the rib sections at the plate fixation point occurred by 2.4 cm. Conclusions. The relationship between the pectus excavatum depth and chest size in the frontal plane was established when modeling the newly formed chest form during for Nuss procedure. The coefficient of restoring the chest shape was mathematically calculated, which is 2 (2∆ = h), where h is the depth of pectus excavatum. The practical significance of the coefficient is that when planning thoracoplasty and shaping plate, the distance between its lateral ends, which corresponds to the chest shape and adjoin ribs, must be reduced by ½ h (where h is the depth of pectus excavatum) before correcting the pectus excavatum full adherence to the ribs in the postoperative period.

Surgical treatment of a congenital pectus excavatum (CPE) includes using of a plate that holds the correct anatomical shape of the chest for a long time. Before implanting the plate behind a sternum, it is bent so that after correcting the deformation, it reproduces shape of the ribs on both sides and fits close to them. Displacement and rotation are the most frequent complications after plate implantation. Therefore, mechanical properties of the plate, such as: strength, ability to withstand the load during surgery and in the long term period — is one of the main guarantees to obtain the good result. Objective. To study the mechanical properties of the plate which is used for thoracoplasty at CPE under conditions comparable to real loads in order to gain maximal correction. Methods. Experimental bending tests were performed on 5 standard plates of 2.5 mm thick, 13 mm width and 200 mm length, made from titanium Ti6–AL4–V (according to ISO 5832-3). All plates were bent with a special surgical instrument according to the anatomical shape of a chest, using method of surgery by D. Nuss. All plates supports were placed on a distance from the fixing screws. The force of bending load was applied to the middle of the plate, gradually increasing from 150 to 600 H with a step 50 H. At each value of the loading we measured the height of the plate edge elevation. Results. Under load conditions of 600 H, the elevation of a plate edge did not exceed 3 mm and was equal to (2700 ± 177) μcm. In a case of twice lower loads (300 H), value of lifting edge of the plate decreased in three times and was (700 ± 85) μcm. Changes that happened under loading were the same for all five types of the plate. Conclusions. It is proved that the plates can fully withstand corrective loads with a minimal deformation after thoracoplasty. The discrepancy bet­ween the plate shape in the lateral parts to the restored thorax is caused by the changing of configuration of the last one and increasing in the anteroposterior size. Key words. Plate, mechanical pro­perties, thoracoplasty.

The most common method of treating of the congenital funnel-shaped chest is thoracoplasty method by D. Nuss. During this surgery, a significant mechanical effect is created on the ribs, sternum, spinal column, which act instantly and continuously for a long time and create new biomechanical conditions for the «chest – rib – spine» system. Objective. To construct a functional model of the chest with a spinal column, which takes into account the movements in the costal-vertebral joints, it allows modeling the funnel-shaped deformation in conditions close to the reality, its operative correction, predicting the results and choosing the optimal parameters of thoracoplasty. Methods. Normal and funnel-shaped chest models based on the articular connection of the ribs to the spine were created using SolidWorks. The main calculations were made using the ANSYS program. To estimate the stress-strain state (SSS), stresses are selected by Mises. Results. The created dynamic mathematical model of the chest makes it possible to conduct a reliable analysis of the biomechanical interaction of the plate with the chest, to analyze the stress-strain state of the constructed models in the norm, with and without taking into account the movements in the costal-vertebral joints. In addition, it allows to simulate the operation by D. Nuss and to study the biomechanical changes in conditions close to reality, occurring in the «chest – rib – spine» system, to determine the areas of maximum loads and safety boundaries. Conclusions. The reproduction of articular ribs rotation in the dynamic model changes the picture of the SSS distribution. In the case of modeling the correction of funnel-shaped deformation of the chest by the method by D. Nuss, the largest zone of stress concentration was found on the outer posterior surface of the sixth pair of ribs. The most tense vertebrae were ThV– ThVI, but the maximum values did not exceed the permissible values. In the case of a lower plate conduction, the correction is achieved with better SSS values in the higher elements of the «chest – ribs – spine» system.

No abstract