8th CONFERENCE on. DYNAMICAL SYSTEMS THEORY AND APPLICATIONS December 12-15, Łódź, Poland

Transcription

1 8th CONFERENCE on DYNAMICAL SYSTEMS THEORY AND APPLICATIONS December 12-15, Łódź, Poland MINIMALY INVASIVE PECTUS EXCAVATUM REPAIR PROCEDURE NUMERICAL STUDY Jan Awrejcewicz, Bartosz Łuczak Abstract: This peper deals with numerical analysis of dynamics of the human thorax with Nuss implant. Three thorax models are considered. The first model is designed to recognize stress distribution in a healthy human rib cage. The second and third ones taken into account model a chest after Nuss pectus excavatum (funnel chest) repair procedure. As a result of our study we tried to give guidelines for phisicans how to correctly apply, from engineering point of view, the Nuss implant. 1. Introduction Pectus Excavatum, well known as funnel chest, is one of the most common major congenital anomalies, occurring in approximately one in every 400 births [3, 4]. The sternum has a posterior (backwards) curve that creates an asymmetrical, deeply sunken chest. With this deformity some physical symptoms are closely connected: (i) inability to take deep breath; (ii) shortness of breath; (iii) chest pain; (iv) variety of respiratory complications. Psychological symptoms overlooked follow: (i) mild self conscious behavior; (ii) loss of motivation; (iii) anxiety and other social problems. These uncomfortable, even painful, symptoms are difficult to measure with heart and lung function tests. This has made it difficult for many physicians to see Pectus Excavatum (PE) as anything but a cosmetic problem. The symptoms are, however, very real and worrying for the PE patient, and are consistently described by all suffer PE patients. They interfere greatly with leading a normal life as

2 the child grows older. Pectus can have a profound detrimental effect on quality of life. The depressed breastbone interferes with left lung expansion and with blood exchange between the heart and lungs. The heart may hit against the breastbone during exertion. The deformity varies in severity from mild to severe, where mild cases may respond to exercise and posture programs, whereas more severe cases require surgical correction. Often applied Nuss technique is also known as the Minimally Invasive Repair of Pectus Excavatum (MIRPE) [4]. The operation for correction starts with general anesthesia and the placement of an epidural catheter for the management of pain after the operation. Traditional repair (Ravitch method) of PE has required a long and complex procedure with multiple rib resections and a large sternal scar. The Nuss procedure avoids any cartilage resection and sternal osteotomy by placing a carefully preformed convex steel bar under the sternum through bilateral thoracic incisions, and then the bar is turning over to elevate the deformed sternum and costal cartilages to a desired position (Fig. 1). Depending on severity of PE one or two these bars are inserted in the manner shown in Fig. 1. Figure 1. Nuss repair procedure The bar is left in position for two or more years, depending on the age of a patient and a severity of the deformity, when re-modeling of the deformed cartilages and sternum has occurred [4]. Longterm follow-up (over 10 years) shows that the Nuss procedure provides excellent results with less than 5% recurrence of the deformity after the bar is removed [4]. Recall that Nuss implant is left in a human organism for two or even more years. It can happen that during such a long period of time a patient may participate in a road or another traumatic accident (ex. in contact sport). Therefore, an investigation of a rib cage responses to impact loads is carried out. A thorax finite element model, which has the biofidelity in the geometry and characteristics, can be useful for evaluation of chest impact tolerance with Nuss implant and in the future design of a novel safer implant.

3 2. Materials and methods The computational capability of the ANSYS finite element program based on the finite element method has been employed in modelling of a human thorax with implant. It is necessary to take into account the complex geometry of thorax in order to analyze its mechanical properties and this software makes it possible to apply both the non-linear geometry and non-linear modeling of mechanical systems. Finite element thorax model used in analysis has been recently developed by the authors [1, 2]. Antrophometric data of thorax is obtained from CT scans measurements and from drawings of cross-sections found in atlases of the human anatomy [3]. The thorax model has been validated for frontal impact [1, 2], and the numerical results has been compared with cadavers test data [5]. Figure 2. Thorax model with implants Note that the ribcage is difficult to model due to the complex construction of residual parts. Validated thorax model has been modified for investigation of implant by placing a model convex steel bar under the sternum. Fig. 2 shows the thorax model with implants. In the posterior end of each ribs the cylindrical support has been implemented to simulate costovertebral joints. Loads has been applied in the center of sternum. This initial conditions are the same for all of the numerical tests. The materials properties of the thorax model are shown in Table 1. Table 1. Material properties of thorax model Model component Density (kg/m 3 ) Coefficient Sternum E= MPa, υ=0,3 Costal Cartilages E= 24,5 MPa, υ=0,4 Ribs E= MPa, υ=0,3 Implant (steel 316L) E= 2, MPa, υ=0,29 E, Young s modulus; υ, Poisson ratio;

4 3. Numerical Results Three thorax models are considered. The first one is designed to investigate stress distribution in a healthy human rib cage. The second and third ones model a chest after Nuss pectus excavatum (funnel chest) repair procedure with one and two bars implanted, respectively. Main aim of numerical study is to analyze stress distribution (Fig. 3.) and deformation (Fig. 4.) using the mentioned three models. Careful analysis of numerical results yields an answer suggesting a type of surgical treatment for PE patients. a) b) c) Figure 3. Equivalent stresses in thorax: a health thorax, b one bar, c two bars. Deformation of thorax are shown in Fig. 4. Notice that deformation of thorax with pectus bar is smaller (6 [mm]) than deformation of healthy ribcage (25 [mm]). Stiffness of implant may disturb a correct thorax movement in both inhale and exhale time. However, after Nuss repair procedure thorax life capacity are expanded, and the PE patient may not feel discomfort of limited thorax movable, in particular during exertion. a) b) c) Figure 4. Deformations in thorax: a health thorax, b one bar, c two bars.

5 a) b) Figure 5. Equivalent stresses in implants: a one bar, b two bars. 280 Equivalent Stress [MPa] Force [N] Figure 6. Equivalent stresses in implants. one implant two implants 4. Conclusions Our investigation has been performed in order to analyze stress distribution in the human thorax after Nuss pectus excavatum repair procedure. Two cases of Nuss PE surgical treatment have been tested i.e. with implantation of one bar and with implantation of two pectus bars. It is easy to recognize that stress distribution in thorax skeleton with implant differ from stress distribution in healthy human ribcage (see Fig. 3.). Notice that stress distribution is violated by the implant and the ribs, which the implant is leant to and transfer large majority of load. In the health thorax each load put to the sternum is evenly distributed among ribs 1-7.

6 Comparing cases shown in Fig. 4. a-c, one may observe that the sternum displacement in the model with implant is smaller. However, comparing Fig.4. b and Fig. 4. c one may conclude that there is no significant difference between stiffness of thorax with one and two pectus bars. Maximum equivalent stress in implant (denoted in Fig. 5 by red color) reaches value 111 [MPa] in case of one implant (and reaches maximum value 80 [MPa] in model with two pectus bars for the same load conditions. In Fig. 6, changes of maximum equivalent stress value in implant as the function of load are reported. Notice that maximum stress values received in case of two bars implantation are approximately 25% smaller than the stress obtained in the first analyzed case. Realize that exceeding stress of 450 [MPa] may trigger permanent deformation of implant. From the mechanical point of view implantation of two pectus bars give evenly stress distribution in thorax skeleton and it seems that this method is safer for a patient. PE patient after Nuss repair procedure with two bars implantation may actively practice sport or other activity without risk of either rib or sternum fracture or even permanent deformation of implant. References The research has been financially supported by the Minister of Education under grant No. 4T07A References 1. Awrejcewicz J., Łuczak B., The finite model element of the human rib cage, Special Issue of Journal of Computational Methods in Science and Engineering (to appear). 2. Awrejcewicz J., Łuczak B. Numerical model of a thorax, Proceedings of the 7th Conference on Dynamical Systems - Theory and Applications, Eds: J. Awrejcewicz, A. Owczarek, J. Mrozowski, Łódź, Poland, December 8-11, 2003, Bochenek A., and Reicher M., (1997) Human Anatomy I, Medicine Doctor Press PZWL, Warsaw in Polish. 4. Correia M., Bernardo E. J. and Fernandes E. J., (1997) Surgery of chest wall deformities, European Journal of Cardio-thoracic Surgery 12 pp Kroell C. K., Schneider, D. C. and Nahum, A. M., (1971) Impact Tolerance and Response of The Human Thorax, SAE Paper no Awrejcewicz Jan, Łuczak Bartosz Department of Automatics and Biomechanics, Technical University of Lodz, 1/15 Stefanowski St., Lodz, Poland awrejcew@p.lodz.pl, bartlucz@p.lodz.pl