Biomechanical study of the atlantoaxial joint after artificial atlanto-odontoid joint arthroplasty.

OBJECTIVE: To investigate the stability and three-dimensional movements of the atlantoaxial joint after artificial atlanto-odontoid joint (AAOJ) arthroplasty by comparing with a conventional method. METHODS: After anterior decompression, 24 human cadaveric spinal specimens of C0-C3 were randomly divided into two groups: Group A receiving artificial AAOJ arthroplasty; Group B experiencing anterior transarticular screw (ATAS) fixation. Two groups underwent flexibility test in intact and instrumented states. Rotational angle of the C0-C3 segments was measured to study the immediate stability and function of anterior decompression with AAOJ arthroplasty compared with the intact state and ATAS fixation. RESULTS: Compared with the intact state, anterior decompression with AAOJ arthroplasty resulted in a significant decrease in the range of motion (ROM) and neutral zone (NZ) during flexion, extension and lateral bending (P less than 0.05); however, with regard to axial rotation, there was no significant difference in ROM and NZ (P larger than 0.05). Compared with anterior decompression with ATAS fixation, anterior decompression with AAOJ arthroplasty during flexion, extension and lateral bending, significant difference was found in ROM and NZ (P larger than 0.05); however, as for axial rotation, there was a significant increase in ROM and NZ (P less than 0.05). CONCLUSION: The self-designed AAOJ has an excellent biomechanical performance, which can restore excellent instant stability and preserve the movement of the atlantoaxial joint.

[Percutaneous vertebroplasty with EH composite material: an experiment study].

OBJECTIVE: To investigate the appropriate ratio of liquid/powder and use of radiopaque agent in EH composite material for percutaneous vertebroplasty (PVP). METHODS: (1) EH composite material was divided into 6 groups. The material without contrast with the liquid/powder ratios 8:9, 8:8, and 8:7 was classifieds as groups I, II, and III; and the EH composite material with the liquid/powder ratios 8:9, 8:8, and 8:7 and with the addition of 20% barium sulfate by weight was classified as the groups IV, V, and VI. The curing temperature was measured. The bone cement of different groups was made into cylinders to be X-rayed to observe the opacity. Universal tester was used to examine the strength and stiffness. (2) The vertebrae (T8 approximately L5) were isolated from the cadaver of an elder female patient with osteoporosis. Universal tester was used to examine the strength and stiffness of the vertebral bodies (VBs). Osteoporotic vertebral compression fracture (OVCF) model was made. PVP procedure was mimicked by puncturing through the bilateral pedicle of vertebral arch into the anterior 1/3 of the vertebral bodies and the EH composite materials of the groups II and V were injected into the VBs Then the temperatures of the geometric center (CT) and spinal canal posterior wall (PT) of the VBs were measured in a water bath with the temperature of 37 degrees C. Twenty-four hours later the vertebrae underwent X-ray examination to observe the opacity and underwent examination of strength and stiffness with universal tester. RESULTS: (1) The sticking periods (?) of the groups IV, V, and VI were significantly longer than those of the corresponding groups I, II, and III respectively by about 60 s, and the highest temperature of the groups IV, V, and VI were significantly lower than those of the corresponding groups I, II, and III respectively. Addition1 of barium sulfate increased the opacity of the bone cement, but did nor significantly influence the strength of the bone cement. The properties of the group V was the best. (2) The bone cement was easy to be injected into the VBs. The peak PT was not beyond 50 degrees C. After the injection of the bone cement of the groups II, the strength and stiffness of the VB were (1501.6 +/- 5.0) N/mm and (285.6 +/-) N/mm, both significantly higher than those before the injection [(547.5 +/- 3.1) N/mm and (104.1 +/- 1.3) N/mm]; and after the injection of the bone cement of the groups V, the strength and stiffness of the VB were (1355.0 +/- 4.5) N/mm and (257.7 +/- 1.9), both significantly higher than those before the injection [(543.8 +/- 2.7) N/mm and (103.4 +/- 1.1) N/mm]. The opacity of the VBs injected with the bone cement of the group V was better than those injected with the bone cement of the group II. CONCLUSION: The EH (8/8) with 20% barium sulfate is a proper and effective filling material for the treatment of OVCF.

[Reconstruction of the mandibular model using a three-dimensional laser scanner].

OBJECTIVE: To construct a three-dimensional (3D) mandibular model using a 3D laser scanner, and explore a new method for reconstructing the finite element geometry model. METHODS: A mandible specimen was scanned with the 3D laser scanner to form the point clouds of the mandibular surface, which were subsequently aligned for reconstruction of the mandibular model. RESULT: A 3D model of the mandible surface was reconstructed, which could be used for finite element simulation. CONCLUSION: The 3D laser scanning system can be used to reconstruct the 3D model with irregular geometry for finite element simulation.

[Mechanical strength and in vivo degradation of human hair keratin-polylactic acid composite rods].

OBJECTIVE: To test the mechanical strength and observe the in vivo degradation of self-made human hair keratin-polylactic acid (HHK-PLA) composite rods designed for use in internal fixation. METHODS: Twenty such HHK-PLA composite rods were tested for shear strength, bending strength and bending modulus using the material testing system MTS-858 Mini Bionix. A total of 36 samples of HHK-PLA composite rods designed for internal fixation of bone fracture were randomly implanted in dorsal subcutaneous tissue of 18 SD rats, and weight losses of these rods were measured 1, 2, 4, 8, 12, 16, 20, 24 and 28 weeks after the implantation to evaluate the in vivo degradation of the material. RESULTS: The initial shear strength of HHK-PLA rod was 241 MPa, bending strength 358 MPa, and bending modulus 13 GPa. The test demonstrated a slower rate of degradation in SD rats in earlier period following implantation than in later period. CONCLUSION: HHK-PLA composite rods have good initial mechanical strength and tolerable degradation in vivo, and may be used potentially for internal fixation of the weigh-bearing bones of the limbs.

[Six degree-of-freedom acquisition and analysis of jaw opening and closing with motion capture system].

OBJECTIVE: To explore the six degrees of freedom of jaw opening and closing movement with motion capture and analysis system to establish a quantitative method for studying mandibular movement and a digital basis for virtual reality study of mandibular movement. METHODS: In a male adult with normal dentition without temporomandibular joint disorders, 3 fluorescent markers were pasted in the upper dentition and 4 in the lower dentition. Six cameras of the motion capture system were arranged in a semi-circular fashion. The subject sat in front of the camera at an 80-cm distance with the Frankfort plane kept parallel to the horizontal plane. The degree-of-freedom (3 linear displacement and 3 angular displacement) of jaw opening and closing movement was obtained by collecting the marker motion. RESULTS: Six degrees of freedom of jaw opening and closing were obtained using the motion capture system. The maximum linear displacements of X, Y and Z axes were 5.888 089 cm, 0.782 269 cm, and 0.138 931 cm, and the minimum linear displacements were -3.649 83 cm, -35.961 2 cm, -5.818 63 cm, respectively. The maximum angular displacements of X, Y and Z axes were 0.760 088°, 2.803 753°, and 0.786 493°, with the minimum angular displacements of -2.526 18°, -0.625 94°, and -25.429 8°, respectively. Variations of linear displacements during jaw opening and closing occurred mainly in the Y axis, and those of angular displacement occurred mainly in the Z axis. CONCLUSION: The six degree-of-freedom of mandibular movement can be accurately obtained with the motion capture system to allow quantitative examination of the mandibular movement.

[Application of three-dimensional laser scanning-based maxillofacial soft tissue reconstruction in orthodontic treatment].

OBJECTIVE: To establish a convenient and rapid method for constructing a digital model of the maxillofacial soft tissue based on three-dimensional laser surface scanning to allow direct and accurate observation of the soft tissue changes in the course of orthodontic treatment. METHODS: The point cloud data of three-dimensional laser scanning of the maxillofacial region were acquired from a healthy woman with Angle Class I occlusion, who maintained a horizontal Frankfort plane during scanning with the scanner placed at a distance of 80 cm. The scanning was repeated twice after wearing the dental cast for an Angle Class I occlusion. The three-dimensional digital model of the maxillofacial soft tissue was constructed based on the point cloud using GeoMagic10.0 software. RESULTS: The high-resolution three-dimensional model of the maxillofacial soft tissue reconstructed allowed accurate observation of the distinct facial anatomical landmarks and represented directly the soft tissue changes in the process of orthodontic treatment by merging the models. Using the analytic tool provided by the software, this model also allowed direct quantitative measurement of the nasolabial angle and the distances from the esthetic plane to the upper lip, labral inferior, and mentolabial sulcus, which were 111.86°, -3.57 mm, -2.54 mm, and 3.95 mm before orthodontic treatment as compared to 114.31°, -2.73 mm, -1.06 mm, and 3.46 mm during treatment, and 116.53°, -0.15 mm, 0.64 mm, and 3.11 mm after the treatment, respectively. CONCLUSION: Three-dimensional laser surface scanning enables accurate and rapid construction of the digital model of the facial soft tissues, which may provide valuable assistance in orthodontic treatment.

[Effect of microwave plasma parameters on the surface modification of denture base resins].

OBJECTIVE: To explore the working parameters of the microwave plasma for surface modification of denture base resins. METHODS: Using orthogonal test method, 80 specimens of denture material were divided into 16 groups for testing. At the gas flow of 13.7 cm3/min, the specimens were tested under the working condition of different microwave powers (100, 150, 200 and 250 W), different gasses (oxygen, air, argon, oxygen and argon mixture), different gas pressure (0.4, 0.6, 0.8, and 1 kPa), and varying processing time (1, 2, 5, and 8 min). Each group of specimens was treated with the microwave plasma and the contact angles were confirmed by scanning electron microscopy. RESULTS: The pictures of instantaneous droplet morphology were obtained from each group of samples, and the values of contact angle were measured. The contact angle was 68.86 degrees in the untreated group, and the angles of several treated groups were 48.15 degrees , 40.7 degrees and 32.76 degrees, showing a statistically significant variation of the contact angle after the treatment. CONCLUSION: Processing time of 8 min, microwave power at 100 W, total pressure in the chamber at 0.8 kPa, and the presence of oxygen constitute the optimal condition of microwave plasma for surface modification of denture base resins. Under these conditions, the smallest contact angle of the denture base resins is 34.58 degrees, suggesting a good effect of surface modification.

Biomechanical evaluation of stability and three-dimensional movements of the atlantoaxial joint after artificial atlanto-odontoid joint arthroplasty.

OBJECTIVE: To investigate the stability and three-dimensional movements of the atlantoaxial joint after artificial atlanto-odontoid joint (AAOJ) arthroplasty. METHOD: Ten sets of AAOJ implanted in bony specimens from 10 adults were used to test the pull-out strength of the atlas-axis components with a MTS858 Mini Bionix machine. Another twelve human cadaveric specimens including C(0)-C(4) were used to evaluate the three-dimensional movements of C(1)-C(2) under five different conditions in sequence, that is, the complete specimen, anterior decompression, posterior transarticular screws fixation, AAOJ arthroplasty and fatigue test. RESULT: There were significant differences between atlas and axis components in the maximum pull-out strength and trajectory length, however the yield length was not significantly different. The maximum pull-out strength of the atlas and axis was positively correlated with trajectory length (r(1)= 0.880, P < 0.05) and yield length (r(2)= 0.606, P < 0.05), respectively. After AAOJ arthroplasty, the range of movement (ROM) with respect to rotation and the neutral zone of the atlantoaxial joint were close to normal (P > 0.05), but the ROM in flexion-extension and lateral bending was significantly smaller compared with the specimens which underwent anterior decompression (P < 0.05). No abrasion and abnormal mobilization were observed after 2000 cycles of flexion, extension, lateral bending and axial rotation in the fatigue test. CONCLUSIONS: The self-designed AAOJ has excellent biomechanical performance, and AAOJ arthroplasty can restore excellent instant stability and preserve the movement of the atlantoaxial joint.

[Finite element analysis of the biomechanics of human mandible in response to impact force].

OBJECTIVE: To explore the biomechanical mechanism of impact force-induced mandibular fractures and its finite element analysis. METHODS: Three mandibular impact fracture models were prepared using intact human mandibular specimens and simulated maxillary models according to the Hanau principle of articulator and a MTS-858 biological material testing machine. Mandibular impact was delivered in the direction of the chin level at the mandibular postural position (MPP) on MTS. The computerized mandibular model was then established from 3-dimensional laser scanning images for finite element analysis using ANSYS7.0. RESULTS: The 3 mandibular specimens were fractured at the chin, where the maximum force was 2151.10-/+ 125.18 N with response time of 17.3-/+2.3 ms. Impact simulation with ANSYS mimicking stress changes in the internal jaw suggested the chin as place where maximum stress occurred. According to the stress curve, the maximum stress of 3201.35 kPa occurred at the point 1.92 cm from the upper edge of the chin. CONCLUSION: The combination of mandibular impact experiments and finite element analysis allows quantification of several parameters of the jaw and provides clues for understanding the biomechanical mechanism of mandibular impact fractures.