Design of New Micro-controlled Dynamic Interlocking Intramedullary Nail for Tibia
|School||Southern Medical University,|
|Keywords||Tibet mini-pigs tibia medullary cavity three-dimensional reconstructionof CT micro-controlled dynamic intramedullary nail computer assistancetechnology|
Reserch backgroundThe static intramedullary nailing techniques have been mainly used for stabilization of tibial fractures at present. Although it provides stable fixation, it can produce stress-shielding effect which can lead to delayed uninon and nonunion. The orthopedists make the static intramedullary nailing to be dynamic to treat with the high incidence of nonunion, and make a consensus. The way to dynamization is to remove the distal or proximal locking screws. After removing the locking screws, it produces the effects of dynamic and compression which can accelerate the bone healing. But the risk of rotation or abbreviation deformity occurs after removing the locking screws, some cases have to receive fixation again, which increases the new burden and prolongs treatment period for the patients. So some surgeons try to design dynamic intramedullary nailing to deal with the risk and accelerate the bone healing for the patients, which can reduce the physical and psychological burden for the patients.Now more and more people realize the importance of dynamic intramedullary nailing, so they try to reform the structure of traditional nailing or design sliding nail holes instead of locking holes, or combine the new structure and sliding nail holes to produce dynamization. Although some new dynamic intramedullary nailings have been made, they can not work so well as we expect because of their defects. The new nails have common shortages of immature complicated structure and operating procedure, so it is hard to apply in clinic service, a kind of practical dynamic nail is desired. On the other hand, animal model is very important for the design of new dynamic intramedullary nailing, it needs appropriate long bone medullary cavity. People choose rabbits or sheep for the study previously. But the medullary cavity of these animal is not close to the human s, it is hard to meet the needs in design of new intramedullary nailings, so the more suitable animal model needs to be searched.To enhance the cure rates of long bone fractures and benefit the suffering patients, we try to design a new micro-controlled dynamic intramedullary nail with bioresorbable material based on the morphology of tibia medullary cavity which is measured in Tibet mini-pigs, and establish the animal model of fixation of tibial fracture by dynamic intramedullary nail.Object1. Measuring and exploring the morphology of tibia medullary cavity in Tibet mini-pigs by three methods of X-ray, three-dimensional reconstruction of CT and specimen measurement, then we compare the advantages and disadvantages between three methods.2. Using UG software to design a new micro-controlled dynamic intramedullary nail with bioresorbable material by based on the morphology of tibia medullary cavity in Tibet mini-pigs.To explore the relation between in intramedullary nail designing and medullary morphology.3. To choose a kind of bioresorbable material used to implant in nail to make fixation stable in early stage and produce dynamization in mid-term stage.4. To evaluate the stability of nail and the matching between intramedullary nail and medullary cavity by establishing animal model of tibial frature fixation using the new intramedullary nail in Tibet mini-pigs.5. To evaluate the dynamic and compressing effect on bone healing by the new dynamic intramedullary nail in tibial fractures through comparing experimental group treated by new micro-controlled dynamic intramedullary nail with control group treated by common intramedullary nail.Method 1. Subjects and data Acquisition1.1Experimental Data:From Octorber2010to January2011,40tibia specimens were collected from the Animal Experimental Center of Southern Medical University for X-ray, CT scan, and osteotomy. From February2011to August2011, we analysed the bioresorbable speciality, proportion of ingredients and biomechanics of PLGA/HA material. In November2011,5Tibet mini-pigs were used to establish the animal model of tibial fracture fixation by new dynamic intramedullary nail. In May2012,10Tibet mini-pigs(180d,28.5Kg) were randomly assigned to groups of experimental group (n=5) and control group(n=5), the tibial fractures of experimental group were treated with new micro-controlled dynamic intramedullary nail comparing with control group with common intramedullary nail. The outcome of fractures healing was evaluated after operation.1.2Device Hardware:Siemens FD-X digital radiography, Siemens Somatom PLUS16-row CT, Micro-CT(SCANCO μCT80, Medical AG,Switzerland), Footscan gait analysis system (RSscan company, Belgium). Software:PACS image system, mimics software, UG3D modeling software, footscan SOFTWARE18.104.22.168Scanning requirements:CT scanning condition:120kV,300mAs, Scanning parallleters:the width of collimating device is0.75mm;thickness is0.75mm;slice thickness is0.75mm, Scanning area:the total length of tibia. Micro-CT:5.5KV,145μA,50μm, Scanning area:the fracture healing region of tibia.2. Experimental methods2.1Morphology measurements of cavity in Tibet mini-pigs2.1.1X-ray:40tibial specimens were marked at first, then received anteroposterior and lateral film by Siemens FD-X digital radiography. Data was saved in DICOM format to morphology measurements of tibia medullary cavity in Tibet mini-pigs after taking X-ray.2.1.2CT scan:40marked tibial specimens were received sagittal, coronal plane and cross section by Siemens Somatom PLUS16-row CT. Data was saved in DICOM format,then input into Mimics software, and thus reconstructed the formation of three-dimensional solid model to the morphology measurements of tibia medullary cavity in Tibet mini-pigs after taking X-ray.2.1.3Specimen measurement:The dry tibial specimens were measured through vernier caliper by osteotomy in sagittal, coronal and cross planes. The levels of sagittal and coronal osteotomy were anatomy axis of tibia. The levels of cross osteotomy were tibial tubercle, narrow part and expand parts of cavity.According to the literatures, measured parameters were total length of medullary cavity(from tibial tubercle to medial malleolus), coronal and sagittal width of narrow point of cavity, tibial medullary cavity arc.2.2Design of intramedullary nailWe designed the new micro-controlled dynamic intramedullary nail and sighting device through UG and Mimics software based on the morphology of tibia medullary cavity in Tibet mini-pigs. The indexes of nail included nail length, diameter, arc and the diameters of locking screw and bioresorbable material. The new micro-controlled dynamic intramedullary nail had the characteristic of stable fixation at early stage before material absorbed and dynamization at mid-term stage of bone healing.2.3Tests of bioresorbable materialTo test the degradation period, Biomechanical stability, degradation rate of PLGA/HA compound material.2.4. Animal model and matching analysisUsing Mimics and UG software to observe the matching of intramedullary nail and tibial cavity before operation.10tibia fracture fixation model with intramedullary nail were made under general anesthesia. After general anesthesia, we exposed the entry point of tibia, input the nail,filled bioresorbable material, then set up locking screws and nail cap. X-ray was taken to evaluate the fixation and the matching of intramedullary nail and cavity.10tibia were received anteroposterior and lateral film by Siemens FD-X digital radiography after operation. The end of nail, cavity integrity, distance of two sides of nail apart from bone cortex were detected through X-rays.2.5Experiment of tibia fracture fixation with intramedullary nail2.5.1Animal group and experiment:10adult female Tibet mini-pigs were randomly assigned to groups of experimental group (n=5) and control group (n=5), the tibial fractures of experimental group were treated with new micro-controlled dynamic intramedullary nail comparing with control group with common intramedullary nail. Fractures healing was evaluated after operation, animals were killed at20weeks postoperatively.2.5.2Postoperative evaluation of intramedullary nail fixation X-ray:Bone healing and inner intramedullary nail was observed periodically by X-rays taken at Od,4w,8w,12w,16w,20w after operation. Gait analysis:Gait analysis was taken to measure the left foot pressure distribution at4w,8w,12w,16w,20w after operation. The mean of10different value from10points was collected for the pressure data. Micro-CT:Animals were killed to measure vBMD and BV/TV of tibia fracture healing region by micro-CT at20w postoperatively.2.6Statistical analysisUsing statistical software SPSS13.0for date analyzing, data records was depicted by the mean±standard deviation (X±s). The Medullary cavity parameters were analyzed by Explore analysis and compared by Repeated Measures ANOVA. The independent sample t test was used to compare with two sets of distance of two sides of nail apart from bone cortex, foot pressure value, Micro-CT data. When P<0.05, there was significance.Result1. Data of measurements of tibial medullary cavity in Tibet mini-pigsThe length of medullary cavity measured by X-ray, CT and specimen were (113.31±9.56) mm,(112.22±8.11) mm and (112.09±10.46) mm, which showed no significant difference between three methods (P=0.814). Measurements of coronal width of narrow point between three methods had significant differences, they were (8.53±0.99)mm,(8.07±0.77) mm,(8.06±0.73) mm, respectively (P=0.016). With regard to the outcomes of sagittal width of narrow point, a significant difference was also observed between three methods [(6.73±1.05)mm,(6.20±0.89) mm,(6.15±0.90) mm; P=0.011]. However, no significant differences were found in the measurements of tibial medullary cavity arc (25.66°±3.36°,24.49°±3.21°,25.12°±3.18°, P=0.276).2. Shape of tibial medullary cavityExpansion in both ends with triangle in the proximal and oval in the distal part was the primary morphology characteristics of tibia medullary cavity in Tibet minipigs. Most of tibial plateau and metaphysis was in lateral region of tibia, the lateral cortex was more convex to the anatomy axis than the medial cortex. It looked like a "S" shape from sagittal plane with backward lean of upper part and arc shape convex backward of lower part.3. Size of intramedullary nailNail size:length90-130mm, diameter6-9mm, arc15°～25°, diameter of locked screw and bioresorbable material both were4.0mm.4. Components of bioresorbable material and proportionThe PLGA/HA compound material was choosed to be the bioresorbable material, because of its suitable absorption decay time and stable biomechanics. The rate of absorption in intramedullary nail was slower than in the cavity and subcutaneous area. The appropriate ratio between PLA and PGA was7:3.5. Effects of fixation and matching of intramedullary nail10tibia fracture fixation model of dynamic intramedullary nail were established with good result of nine case of nail ends did not beyond knee joint line and no cavity rupture. no significant differences were found in distances of two sides of nail apart from bone cortex[medial side(0.81±0.21)mm, lateral side (0.76±0.23)mm, P=0.352]. The postoperative X-rays showed good result of fixation and good match between intramedullary nail and cavity, and the tibia fractures got clinic healing at12weeks postoperatively.6. Outcomes of experimentThere were significant differences between both posterior limbs in experiment group [left (3.57±0.28) N、right (10.37±0.27) N, P=0.000] and control group [left (3.49±0.32)N、right(10.47±0.41)N, P=0.000] at8weeks postoperatively. Mini-pigs in experimental group regained approximately normal gait and load at16weeks postoperatively [left (10.54±0.29) N、right (10.59±0.27) N, P=0.804], compared with significant difference in control group[left (8.46±0.25) N, right (10.53±0.17) N, P=0.000]. X-rays of experimental group showed bony union comparing clinic union and one nonunion case in control group at20weeks. Micro-CT showed higher level of vBMD [experimental group (555.47±23.49)mg/mm3, control group (494.45±16.62) mg/mm3, P=0.001] and BV/TV (experimental group0.84±0.04, control group0.78±0.03,P=0.022) in experimental group.Conclusion1. The precision of CT is higher than X-ray and the results of CT measurement is more close to specimen measurement. Therefore, data measured by CT is recommend for the study.2. The shape of tibia medullary cavity in Tibet mini-pigs is similar to the human’s, suited to design new intramedullary nail and establish fixation model. Design of intramedullry nail is mainly based on the length of medullary cavity, width of narrow point and arc of cavity.3. PLGA/HA compound material containing proper proportion of PLA and PGA suites to be the bioresorbable material for early fixation of locking screws in intramedullary nail.4. The new micro-controlled tibia intramedullary nail designed by cavity measurements and computer assistance technology makes good match with cavity and stable fixation in tibia fracture.5. The new dynamic intramedullary nail is better than the common static nail in promoting bone healing and reducing complication.