Design And Application Of Extracortical Triplate Prosthesis For Children

Malignant bone tumor is one of the most common tumors of childhood. In the past, the case fatality rate and amputation rate of the patients were very high. However, neo-adjuvant chemotherapy and adjuvant chemotherapy have substantially improved survival, and limb-salvage surgery is common now[1]. Endoprosthetic replacements are used for the reconstruction during the limb-salvage surgery, which may provide reliable fixation. After the surgery, the reconstructed joint may endure larger mechanical pressure, reserve motor function, and recover the function of the limb rapidly. Endoprosthetic replacements have advantages such as early stability, early activity ability, and early weight bearing ability. Now, intramedullary stem fixation is the most common method of fixation for endoprosthesis. Nevertheless, bone grows and remodels at a high speed in children. With the growth and thickening of the bone, the marrow cavity is widened, which may cause the aseptic loosening of endoprosthesis. Aseptic loosening is one of the major complications of endoprosthetic replacements. There is no effective solution to this problem yet.Development of extracortical triplate prosthesis has led to excellent results in adults[2]. We hypothesizes that, because of the characteristic of the rapid bone growth in children, after the extracortical triplate fixation, there may be lots of bone formation between and around the plates, which can make the endoprosthesis fully osseo-integrated. The long-term fixation effect of this method may be better, which may reduce the rate of aseptic loosening. The purpose of this study is to investigate the feasibility and mechanism of extracortical triplate fixation in children.In this study, we carried out three experiments: 1. Accomplished the design of extracortical triplate prosthesis for children combining computer-aided design (CAD) technique. The software of Pro/E was used for the model building. In addition, to reduce the damage of blood supply of the periosteum caused by extrocortical fixation, different slots were designed on the plates to reduce the contact area(The size of the slots were: 2×3mm~2, 4×3mm~2, 8×3mm~2, 15×3mm~2, separately) ; 2. The finite element method (FEM) was adopted to understand the mechanical properties of each prosthesis. The results were considered as theoretical basis for animal experiments, and used for the optimizing of the design of the prosthesis; 3. The prostheses were manufactured and applied in the reconstruction after segmental resection of tibia in a young goat model.The results of X ray, histological observation and fluorescent labeling of bone show that after the reconstruction of segmental resection of tibia in a young goat model, there is significant new bone formation around the plates. In addition, the bone formation and remodeling are different around different kinds of slots. With the increase of slot size from 2mm to 8mm, the total cortical area and bone turnover rate around the slots increase significantly. But the total cortical area and bone turnover rate around slot of 15mm decrease. The results of FEM show that, although all the results are in the range of the work strength, with the increase of slot size, the stress and deformation increase significantly.After analysis of the results, we consider that: 1. After extrocortical triplate fixation, there are lots of bone formation and remodeling around the plates, which can make the endoprosthesis fully osseo-integrated; 2. It's very important to choose a proper design of slot, which may promote more bone formation. The impact on blood supply and the mechanical function of the prosthesis should both be considered as important factors during the design process; 3. extracortical triplate fixation may be an better alternative to reduce the rate of aseptic loosening.