| I. Background and PurposeAs increasing of injury in traffic accidents and on the job, the rate of incidence in pelvic fracture was increaseing year by year.Recently,the fracture,which caused the mortality rate of5%-20%and the disability of50%-60%,has accounted for1%-3%in the total number of cases.And the acetabular fracture was the most one in the pelvic fracture.The present result shown the operation treatment was superior to conservative treatment.Both-column acetabular fracture had a high incidence in complicated-acetabular fractures. The main methods of treatment contained anatomical reduction,firmly fixation and postoperative rehabilitation exercise.However, there was no one mode of fixation unanimously accepted, because of the deep anatomic location and irregular shape in acetabulum, and important blood vessel and nerve accompanied the acetabulum,in which the bone was thin.For these reasons, no one of the all internal fixed methods can be accepted universally even now. The fixation of anterior specially-shaped reconstruction plate combinated with trans-plate quadrilateral screws we designed in treatment of acetabular fractures, especially the quadrilateral fracture,has the advantage of a single incision, safety procedured on operation and ground moving early.Letournal and Judet pointed out that the excellent and good rate was about75-80%after a long-term follow-up of large numbers acetabular fractures being fixed by current operation method.Professor Cai Xian-hua adopted anterior specially-shaped reconstruction plate combinated with trans-plate quadrilateral screws to fix complicated acetabular fratures from2004to2010,and the results were followed up on average of41.7months.According to the standard of modified Merled’Aubigne and Postell score,the excellent and good rate were more than83.3%, significantly higher than domestic results being reported.With the further development of clinical and research,it is urgent to clarify the acetabular stability after fracture fixed by anterior specially-shaped reconstruction plate combinated with trans-plate quadrilateral screws, possibility the patient can get up early to take the seat or activities,and the property of stress distribution and displacement in pelvis or internal fixation.Stability of internal fixation can be well reflected by traditional detection method of biomechanics in specimens,but the three dimensional finite element model can well reflect the distribution of stress and displacement because of its higher simulation.The two methods can be used for biomechanical study of acetabular fractures after fixation, and providing biomechanical data for postoperative evaluation.In order to provide better guide for the clinical treatment, the mechanical characteristics of both-column acetabular fracture fixed by anterior specially-shaped reconstruction plate combinated with trans-plate quadrilateral screws were explored in our experiment,. Ⅱ.Methods1. The method biomechanics in specimensWe selected six formalin-preserved cadaveric pelvis for this investigation. Six normal human specimens which were including the forth lumbar vertebra and proximal1/3femoral shaft were obtained from the department of anatomy of the Southern Medical University, without known metabolic bone disease or tumors. The trabecular appearance and bony quality were examined and bone abnormalities were ruled out by a standard anteroposterior X-ray. Soft tissue was removed, except the sacrum, sacroiliac joint, the hip joint capsule, ligaments of Sacral spine, sacral tubercle and obturator internus. We selected reconstruction plate,1/3tube and titanium screws(3.5mm) of AO, and ZWICKZ100material machine(Germany ZWICKZ, a precision of0.1%, Department of Mechanical Room in Wuhan University of Technology).Only one hip was used for testing from each hip surgery. The specimen was fixed in the position of standing and sitting in pelvis, and given the loading repeatedly from400to700newton. In sitting position:the lateral displacement of posterior column fracture in inner wall was measured by multifunctional digital dial gauge measuring, the overall axial displacement of pelvis was measured by beam sensor of ZwickZ100electronic universal testing machine; In standing position:the lateral displacement of posterior column fracture in inner wall and the longitudinal displacement of anterior inferior iliac spine were measured by multifunctional digital dial gauge measuring. Their stiffness was calculated. Firstly, we tested the six normal human pelvis (Control groupA) respectively in standing and sitting simulations as a control group, then we made highly both-column fractures by wire saw on one hip, and fixed the fracture by anterior specially-shaped reconstruction plate combinated with trans-plate quadrilateral screws(The experimental groupB) or the anterior construct plate-1/3tube buttress-plate fixation (Control group C) on randomly. Then, we tested the fracture pelvic again in positioning and sitting position respectively.2.The method of biomechanics on finite element modelingThe age of one adult male volunteers was32, and the X-ray examination was used to exclude the injury of pelvis, tumor, deformity and other lesions. CT was used to scan pelvis on1mm of thickness for getting the two-dimensional original image.The instrument was provided by the CT center of Wuhan General Hospita of Guangzhou Military Region (Biograph16HR, Siemens company, Germany). We applied ANSYS9.0software to establish the left half pelvis finite element model using the image which had been deal with by freeform software. Then using remeshing of modle, we established the both-column acetabular fracture. After using vernier gauges to measure the physical size of3.5series screw, titanium plate (AO Johnson Sindith equipment),we established three-dimensional finite element model of anterior specially-shaped reconstruction plate combinated with trans-plate quadrilateral screws by using the measured value through Proe3.0, Freeform and ANSYS9SoftwareAccording to different materials, we set up unit properties.By restraining the unilateral acetabular, half sacrococcyx and pubic symphysis,we simulated interaction condition of femoral head to acetabulum on single foot standing (500N). Then we calculated nephogram of displacement and stress distribution, and marked the maximum stress and the maximum displacement region.III.results1.Biomechanical results of cadaverAfter acetabular fracture been fixed in group B and group C, the fracture blocks of anterior column and posterior column in specimens after internal fixation were firmly fixed immediately and got anatomic reduction. Soft tissue (the ligament of sacrotuberous,the ligament of sacrospinous and obturator internus) was taut. CT and X ray showed the fracture has been got anatomical reduction.In group B, the resist of manual twisting, internal shift and separation in posterior column were stronger than that in group C, The screws and plates did not crack in process of testing.After the removal of loading,the acetabular and fixation returned to an initial form or state.In standing position:As the load adding from400N to700N, the horizontal displacement in inner wall and the longitudinal displacement in the anterior were increasing in line. The displacement of specimens in group B appeared less than1mm and obviously less than that in group C.The displacement of specimens in group B under the400N loading appeared more than lmm.That mean it started to appear dislocation of the femoral head under the400N. The inner wall (the horizontal displacement were more than1mm):400N1case,500N1case,600N1case,700N2cases; The anterior(the longitudinal displacement were more than1mm):400N1case,500N2cases,600N3cases,700N3cases. Statistical analysis showed that, Compareing with the longitudinal displacement of A group and B group, there was no difference (P>0.05), In group B to group C or group A to group C,the differences were significant or very significant (p<0.01or p<0.05).In the600N under physiological load, statistical analysis:the differences between group B to group C were significant(P=0.012),while the differences between B group and A was not significant (P=0.089). According to the standard of Thomas,acetabulum appeared partial hip instability and stiffness instability in group B and group C. showed, while the former was less than the latter.In sitting position:As the load adding from400N to700N, the lateral displacement in posterior column were increasing in line.The lateral displacement of the posterior column-inner wall in group A> group B and group C absolutely were within0.5mm, without statistically significant difference (P>0.05); The axial stiffness of pelvic were increasing in line, It appeared group A<group B<group C.There were significant difference beteewn group B-group C or group A and group C (p<0.01). While the difference in group B was close group A (p>0.05). On the loading of600N, the fracture got anatomical reduction, there was significant difference beteewn group B and group C in axial stiffness of pelvic, the former was close to normal pelvis.2.Biomechanical results of finite element modelHowever the modeling validity needs to be validated, the finite element model can overcome the defect that samples of traditional cadaveric biomechanical experiment is limited and not easy to repeating-test.The experimental finding that stress was mainly from the surface near the sacrum,and transmit form sacroiliac joint and the greater sciatic notch to the acetabulum.The displacement distribution of Iliac bone presented to median sacral spine at maximum displacement, and distally diffused.Until the iliac wing gradually,the dispacement reduced to0.These were consistent with the theoretical and experimental results, and close to the pelvic modeling.The stress of fixation(anterior specially-shaped reconstruction plate combinated with trans-plate quadrilateral screws) was concentrated at the screw and plate in the fracture of greater sciatic notch.The maximum value was0.838*109Pa, especially in the joint parts of titanium plate and screw.the stress was less than the limit of titanium material.The proximal first trans-plate quadrilateral screws born high stress.Area across the square fracture line, trans-plate quadrilateral screws appeared stress concentration area.The stress distribution of both column acetabular fracture by anterior specially-shaped reconstruction plate combinated with trans-plate quadrilateral screws well distributed, the maximum displacement was0.0474mm.On the physiological load,the fracture got anatomical reduction.Ⅳ. conclusion 1.In standing position:According to the standard of Thomas,acetabulum appeared partial hip instability and stiffness instability in group B and group C. showed, while the former was less than the latter.Statistical analysis showed that the fracture blocks of anterior column and posterior column in specimens after internal fixation were firmly fixed immediately. Its stability was close to whole pelvis. Comprehensive analysis showed that the patients after fixation can take a sitting early.Their hip were more stable.2.1n sitting position:The frature of group B and group C got anatomical reduction, It was desirable to take seat after operation. Statistical analysis showed that the stability of hip after operation in group B were stronger, and was close to whole pelvis. Comprehensive analysis showed that the patients after fixation can take a seat early. It was more safe and reliable.3.There was no significant difference beteewn group C and group A(p<0.01). the patients fixed by the anterior construct plate-1/3tube buttress-plate fixation (group C)can take a seat more safe than taking a sitting after operation.4.Biomechanical results of finite element model showed that the stress distribution of both column acetabular fracture by anterior specially-shaped reconstruction plate combinated with trans-plate quadrilateral screws was well distributed, the maximum displacement was0.0474mm.On the physiological load,the fracture got anatomical reduction. The stress of fixation was concentrated at the screw and plate in the fracture of greater sciatic notch.The stress was less than the limit of titanium material. Trans-plate quadrilateral screws can be applied to fix the square area and posterior column of complex acetabular fracture. |
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