Funnel chest is a common developmental deformity of the thorax, which is clinically manifested by the abnormal backward curvature and depression of the lower and middle sternum of the anterior chest wall and the rib cage on both sides, resembling a funnel. The pathogenesis of funnel chest is widely discussed, such as developmental disorders of the sternum and rib cartilage, abnormal development of the diaphragm, airway obstruction and genetic factors. The traditional methods include Wada sternal reversal and Ravitch sternal lift, but they are gradually replaced by minimally invasive incisional plate implantation sternal lift (Nuss surgery) due to large trauma, bleeding and poor postoperative results. After continuous refinement and improvement, the Nuss procedure has now become a common surgical procedure for the treatment of funnel chest. However, with the popularity of clinical applications and the impact of the learning curve, complications associated with the Nuss procedure have been increasingly reported. Serious complications such as cardiac perforation are even life-threatening to patients. How to preoperatively assess the severity of funnel chest, shape the support plate more precisely, reduce postoperative plate displacement and pain, and scientifically evaluate the postoperative orthopedic effect have all become problems that clinicians must face. The rapid development of computer technology, image processing technology, medical physics, and optics has provided powerful support for medical diagnosis and treatment, and the computer simulation technology developed on this basis has provided a useful way for clinicians to shorten the learning curve and reduce complications of Nuss surgery. In this paper, we discuss the potential application value of computer simulation technology in the treatment of funnel chest with our own clinical practice experience.
1. Materials and methods
1.1 Clinical data
Seventy-six patients with detailed clinical data and follow-up results of funnel chest were collected from April 2007 to August 2010, all of whom underwent Nuss surgery and received regular postoperative follow-up. Among them, 55 cases were male and 21 cases were female, with an average age of 9.4 years.
1.2 Analysis method
The Haller index was used to judge the degree of funnel chest deformity, postoperative pain was assessed by the VAS visual analog score, and whether the plates were displaced according to the frontal and lateral chest films at the follow-up.
2. Results
All patients underwent imaging CT examination to assess the degree of deformity, and the Haller index ranged from 4.2 to 25.6. 18 patients had double plates intraoperatively. In addition to visual assessment of the orthopedic effect, objective assessment by chest radiograph or CT was also performed during the follow-up. In four cases, the families of the children refused imaging review and follow-up because of fear of potential radiological damage. In the postoperative pain assessment, all patients had varying degrees of pain, of which 82.7% had severe pain. There were 3 cases of postoperative plate displacement and no cases of scoliosis.
3. Discussion
The author has completed nearly 80 cases of Nuss and its modifications in the past three years, and has developed a better assessment system in terms of plate length, number and plasticity by using CT 3D imaging technology for preoperative funnel degree assessment. However, there are still deficiencies in patient pain assessment and orthopaedic appearance prediction, and there are no relevant studies and reports in China. Computer simulation technology is safe, reproducible, and highly accurate, and this paper will discuss its application in the treatment of funnel chest from the following aspects.
3.1 Objective evaluation system of funnel chest
Funnel chest can be diagnosed immediately based on visual inspection of the thorax, but the degree of its chest wall deformity and postoperative results need objective indicators to assess. From the initial caliper measurement of anterior-posterior/lateral chest diameter and measurement of funnel effusion developed into X-ray and CT imaging after thoracic spacing, funnel chest index, Haller index calculation, etc. The above indicators can be used to assess the degree of deformity of most funnel chests. In clinical practice, the author mainly uses the Haller index as an objective index for judging the degree of deformity. However, the preoperative definition of the shape and position of the orthopedic plate and the prediction of the postoperative corrective appearance, especially for asymmetric and complex funnel chest, are closely related to the surgeon’s clinical experience, and the shape and placement of the plate are often subjective and uncertain to some extent. Therefore, it is often necessary to repeatedly change the shaping pattern and reposition the plates due to unsatisfactory orthopedic results. A standard objective assessment system is needed to avoid subjective bias in the correction of funnel chest and to shorten the learning curve as much as possible. Finite elements, one of the common methods of numerical computation, have been widely used in engineering image processing and imaging algorithms. Its integration into the medical field is an emerging cross-cutting research direction that has been developed recently. Finite element meshing is a crucial step in the numerical simulation analysis, which directly affects the accuracy of the subsequent numerical calculation and analysis results. Generally speaking, the increase of the number of meshes will improve the computational accuracy, but at the same time, the computational scale will also increase, so the number of meshes should be determined by weighing two factors together.
The optical non-contact 3D form measurement technique is a high accuracy, high efficiency and non-contact high speed inspection method based on finite element algorithm. glinkowski et al. thus designed and developed a digital stripe projection optical measurement system specifically for the degree of funnel chest deformity. The system uses a projector as the active projection light source, and the pattern, period and phase shift of the streak structured light are designed in the computer before signal acquisition. The stripe pattern is projected onto the patient’s body by the projector, and the stripe bends with the height of the funnel chest surface, and the degree of bending represents the three-dimensional information of the degree of funnel deformity. The CCD camera surrounding the patient acquires the human thoracic shape signal at rest and obtains the dense point cloud data. The data is input into the computer through a digital converter for data streamlining, merging and 3D reconstruction, and finally a 3D digital model of the funnel chest can be obtained. This method not only facilitates the change of stripe characteristics, but also provides fast and accurate phase shift of the stripe raster controlled by sinusoidal signal and binary Gray code, and the intensity of the stripe raster is not easily affected by the surrounding environment. The system can collect up to 4 million point clouds with an accuracy of 0.2-0.4mm, and the data acquisition time is only 0.7s, so it is also advantageous for children with poor control. Another significant feature is that the system is completely free of radioactive intake, which reduces the potential for radiological damage and facilitates continuous, multiple assessments of postoperative correction at follow-up. The optical 3D measurement method resulted in an index for assessing the degree of funicular deformity, I3ds, which was calculated as the ratio of the shortest distance between the anterior and posterior chest walls at the level of the most severe funicular depression to the width of the chest wall at that level, and the Haller index was verified by simultaneous CT measurements. 1.2775 (X=I3ds, Y=Haller Index).
3.2. postoperative pain assessment
Postoperative pain is one of the most common complications after Nuss surgery, and if not actively managed, patients often develop secondary respiratory complications and have the potential to lead to plate displacement and acquired scoliosis. Pain is currently managed clinically with various routes of analgesia, but there are no active defensive strategies to reduce pain.Nagasao et al [8], in a study using the finite element analysis software ANSYS to assess the effect of the number of orthopedic plates on postoperative pain in Nuss surgery, used 6, 18, and 36 beam units to construct three-dimensional models of the rib, sternum, and vertebral body, respectively, and rib cartilage according to its morphological The rib cartilage was constructed by 3-10 beam units according to its morphological complexity. The results of the analysis showed that the frequency of epidural controlled analgesia (PCEA) required for pain was significantly higher in the former group compared to the latter group, and the time to return to activity after surgery was longer than that of the double-plate group. In the analysis of specific stress-generating sites, there was no significant difference between the two groups except for the stresses on the fourth and fifth ribs, which were statistically different (single plates were greater than double plates). The reason for the different levels of pain produced by the different numbers of plates may be related to the fact that the double plate disperses the downward stress generated by the sternal lift, while reducing the torsional force generated by the potential rotation of the plate, thus showing a lesser overall pain in the double plate group than in the single plate group. With further support from prospective randomized controlled studies, the double plate orthopedic strategy may not only be used for the treatment of complex funnel chest, but also as a measure to reduce postoperative pain.
3.3 Plate and thoracic force analysis
Currently, most of the steel plates used in Nuss surgery are imported from abroad, but it is believed that similar domestic alternatives will enter the clinic sooner or later. This will inevitably bring about differences in orthopaedic results due to different materials and manufacturing processes. Krauze et al. investigated the stresses on plates of different materials, lengths and thicknesses after orthopedic surgery by means of finite element analysis and Young’s modulus. The results suggest that after the thickness of the titanium alloy steel plate of the same specification is increased from 2.5 mm to 3.5 mm, the required orthopedic distance in the central area of the steel plate is significantly reduced, but there is no significant change in the local pressure. Like the 3.5 mm thickness titanium alloy steel plate, the 3.5 mm thickness chromium-nickel-molybdenum alloy steel plate also has the characteristic of requiring a small degree of shaping, but the ultimate pressure of the contact point between the two materials and the sternum cannot exceed 895 MPa and 690 MPa, respectively; otherwise, exceeding the tensile limit of the material will easily cause deformation of the supporting steel plate and lose its long-term shaping effect. Chang et al. of Chang Gung University, Taiwan, used finite element analysis combined with the 3D visualization system software AMIRA to collect data from three children with symmetrical funnel chest through 16-row CT and conducted a study to simulate the stresses on various parts of the thorax after Nuss surgery. The results suggested that the cartilage of the third to seventh ribs was subjected to greater stresses at the union with the sternum and ribs, but the pressure on the third to seventh ribs was most pronounced in the dorsal region near the spine, and the average value of its local maximum pressure was even as high as 35.5 MPa. The authors attributed the possible postoperative scoliosis of Nuss to the abnormal stresses here. This mathematical geometric model based on CT scans was used for finite element analysis and evaluation of the Nuss procedure, which could provide the clinician with preoperative numerical information such as support plate shaping, rib gap placement site, stress measurement, number of plates, etc. On the other hand, the system has shortcomings due to the inability to achieve maximum fit due to factors such as CT imaging pixels and scan thickness. However, with the development of CT imaging technology, this system has potential clinical applications.
3,4. surgical simulation
Nagasao et al. used finite element algorithms and computer-aided imaging software to simulate the Nuss procedure in 18 patients with funnel chest, including 10 children and 8 adults. Nuss procedure and predicted their postoperative mechanical indices of each rib stress due to altered thoracic profile. The researchers used six points in the model, including the midpoint of the sternum, the glabella, and the rib cartilage belonging to the ribs intersecting with the supporting plate on both sides, as the evaluation points for the comparison before and after the Nuss simulation. The mean correlation coefficient was as high as 0.997, and even the simulated effect in one case was exactly the same as the real situation. In a study of the predicted stress values of the 12 ribs after the Nuss procedure, the authors found that the adult group produced a greater degree of stress in the ribs than the pediatric group, and that the former occurred mainly in the third to seventh ribs, whereas the latter was limited to the plate support ribs (mostly the fifth rib). This result coincided with the clinical time to return to activity after surgery: 2.3 ± 1.2 days in the children’s group and 5.1 ± 1.6 days in the adult group, suggesting that the adult group endured greater pain after the Nuss procedure. The reason for this phenomenon may be related to the fact that in children, the rib cartilage has greater elasticity and can absorb most of the stress due to sternal lift, whereas in adults, the rib cartilage is more rigid due to ossification and cannot absorb the stress due to sternal lift and transmit it to the adjacent ribs. In this study, the authors also calculated the stresses in cortical bone, cancellous bone, and cartilage as a result of the stretching that occurs after orthopedic treatment by using the Young’s modulus principle. The reasons for the different degrees of pain in children and adults due to different bone densities after Nuss surgery were explained from the point of view of material mechanics.
With the increasing use of computer simulation technology in clinical practice, clinicians can use the simulation results to shape the orthopedic plate before Nuss surgery, assess the reasonableness of the number of plates used and their placement, and predict the extent of potential complications such as pain, reducing the risks associated with the lack of clinical experience. It also allows patients and their families to fully understand the severity of the funnel chest and preview the postoperative orthopedic appearance before surgery, increasing their confidence in healing their patients. The establishment of more assessment systems without radioactive intake will allow for better assessment of surgical outcomes and follow-up.