| Objective:1.The purpose of this research was to develop a new minimally invasive anatomical internal fixation system for various types of calcaneal fractures,which has the characteristics of high strength,low-profile,reliable fixation,easy to insert through tarsal sinus incision.2.To construct models of calcaneal fracture fixed with new anatomic internal fixation system,conventional locking plate and cannulated screws respectively,then to analyze and compare the strength and stability of the three fixation methods by finite element method.Method:1.Selecting standard calcaneus of male and female 120 cases of normal adult calcaneus CT data were collected.There were 60 males and females,and the ratio of left and right feet was 1:1.Three-dimensional reconstruction was performed by using Mimics software,and the following anatomical parameters were measured: length of calcaneus,height of calcaneal neck,calcaneal posterior articular surface and calcaneal body,width of anterior calcaneus,calcaneal neck,anterior and middle articular surface and calcaneal body,cephalad inclination angle and interior inclination angle of sustentaculum tali,Gissane's angle and bohler's angle.The most significant statistical difference between male and female in the above anatomical parameters was selected out,and the measurement results were arranged in ascending order and divided into three sections of 0-33%,33%-66%,and 66%-100%.The median of each interval was analyzed,then three standard calcaneus were selected correspondingly and imported into the three-dimensional model processing software Geomagic Wrap 2017 in STL format.Bone defects were repaired,smoothed and preserved after optimization treatment.2.Design of new anatomical plate for calcaneus The design of new type of anatomical plate for calcaneus is based on the standard length of the calcaneus for men and women.The bottom of the plate consists of four parts:(1)Head of plate: The anterior edge of the plate is 2 mm from the calcaneocuboid joint,and the upper and lower edges are consistent with the upper and lower edges of the calcaneal head.(2)Neck of plate: At the border between the calcaneal neck and the calcaneal body,the plate bends backward and upward along the Gissane's angle.(3)Body of plate: the upper and lower edges of the plate are parallel to the posterior articular surface and extend upward to the apex of the articular surface,covering the the entire calcaneal thalamus.(4)Tail of plate: At the border between bodyand tuberosity of calcaneus,the plate bends downward and covers the posterior and upper parts of the calcaneal body.According to the above design,three kinds of plate bottoms were drawn on the lateral wall of the standard calcaneus using Imageware 13.2 software,and the control points were adjusted to make the steel plate bottoms adhere to the bone surface morphology.The bottom of the designed plate was imported into Solidworks 2017 software in IGES format and processed by bottom stretching,cutting and edge rounding.Screw holes were added in four parts of calcaneus in turn:(1)at the head of plate,round locking screw holes and Kirschner wire hole were arranged longitudinally and parallel to the calcaneocuboid joint surface;(2)at the body of plate,round locking screw holes were added at the proximal end and common screw holes were added at the distal end of the plate.The screw holes were arranged consistent with the radian of calcaneal thalamus;(3)at the tail of plate,the round locking screw holes were added and arranged with the radian of the plate.3.Finite element analysis of calcaneal fracture model fixed with plate(1)Sanders IIIAB calcaneal fracture model was made by Solidworks2017 software.Three parts of the posterior articular surface were displaced by 2 mm,then the fracture model was fixed with the minimally invasive anatomical plate.(2)The model was imported into Hypermesh software and the properties of plate,screw and calcaneus were given respectively,then the calcaneus was loaded vertically,Loading the calcaneus vertically,increasing the load gradually to 750 N at 150 N each time,then the following analysis were made:(1)displacement distribution,stress distribution and peak stress of fracture fragment;(2)stress distribution and peak stress of plate.(3)Fixation models of conventional locking plate and cannulated screws were established.Stress and displacement distributions of the models were analyzed by repeating the finite element method mentioned above,and the strength and stability of three kinds of internal fixtures were compared.Result:1.Surface Anatomical Measurement of Calcaneus The anatomical measurements of 120 cases of calcaneus CT data showed that there was no significant difference in the anatomical parameters between the left and right calcaneus of the same sex.However,There were significant differences in all parameters except Gissane's angle,Bohler's angle,cephalad inclination angle and interior inclination angle of sustentaculum tali between different sexes,among which the length and width of calcaneus were the most significant.The width of male calcaneus was(41.84 +1.89)mm,which was(37.94 +1.93)mm in female.The total length of male calcaneus was(81.14 +4.22)mm,which was(73.70 +3.78)mm in female.The length of calcaneus was used as a reference to design the plate.It was divided into three intervals: 0-33%(small),33%-66%(medium)and 66%-100%(large).The median among small interval ofmale,medium and large intervals of female is close,about 75 mm.However,the median of small interval of female is less than 70 mm,and the median of medium and large intervals are more than 80 mm,and there is significant statistical difference among them.In summary,three cases of calcaneus with lengths of 70 mm,75 mm and 80 mm were selected as standard models for plate design.2.Design of Minimally Invasive Anatomical Plate and Screw Holes According to the anatomical differences of three standard calcaneal lengths,we designed anatomical plates of 62 mm,67 mm and 72 mm respectively.The entity design of the plate is as follows:(1)The thickness of the plate is 1.5 mm,the edge of the plate was rounded with a radius of 0.5 mm,and the corner of the head and tail of the plate was rounded with a radius of 3 mm.(2)The distance between the front edge of the plate and the articular surface of the calcaneocuboid joint is 3 mm,and the distance between the back edge of the plate and the posterior edge of the calcaneus is 5 mm.The distance between the edge of the head,neck and body of the plate and the upper edge of the calcaneus is 3 mm.(3)The width of head of the plate is 15 mm.There are two round locking screw holes with diameter of 3.5 mm and one Kirschner wire hole with diameter of 2.0 mm.(4)The neck of the plate is10 mm wide and bends upward along the Gissane's angle without screw holes designed.(5)The body of the plate is 10-15 mm wide,and there are 2round locking screw holes with a diameter of 2.4 mm at the proximal end,and 5 elliptical ordinary screw holes with longitudinal diameter of 4 mm and transverse diameter of 3.5 mm at the distal end of the plate.(6)The tail of the plate is 10 mm wide with 3.5 mm round locking screw holes designed.(7)The edges of all screw holes are 3mm apart from each other,and the edge of the screw hole is 3mm away from the edge of the plate.3.Vertical Stress Analysis(1)The maximum displacement of the new minimally invasive anatomical plate and the conventional locking plate were located at the head of calcaneus,with displacement distances of 0.527 mm and 0.733 mm,respectively;the maximum displacement of the cannulated screws fixation was located on the posterior articular surface,with a displacement distance of 0.887 mm;(2)the peak stress of the minimally invasive anatomical plate was 157.06 Mpa;(2)the peak stress of the conventional plate was 381.70 Mpa;and the peak stress of the cannulated screws was 424.15 Mpa.(3)In the model of minimally invasive anatomical locking plate fixation,the maximum stress of calcaneus was located at the screw hole of the tuberosity,with a peak stress of 75.50 Mpa;in the model of conventional locking plate fixation,the maximum stress of calcaneus was located at the screw hole of the sustentaculum tali with a peak stress of 95.43 Mpa;in the model of cannulated screws fixation,the maximum stress was located in the cancellous bone under the posterior articular surface,with a peak stress of109.9 Mpa.CONCLUSION: The new anatomical plate has the advantages of high strength,stable fixation and adherence to the lateral wall of calcaneus,which is more reliable than the conventional locking plate and hollow screw fixation. |
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