Experimental Study Of The Posterior Wall Of Acetabular Middle Column In Morphology And Biomechanics

Background:In China, with the industrial development, the incidence of acetabular fractures due to traffic accidents has increased rapidly in the past decade. High-energy injuries cause pelvis and acetabular fractures, especially comminuted fracture with bone defects. Professor Zhang Chuncai, the leader of MO group, based on the systematic concept of Letournel and Judet in two columns, put forward the theory of acetabular three-column Innovatively, which on the basis of the evolution of acetabulum, the integrity of the pelvis and the head-mortar counterparts. We defined the acetabular roof and the top of the corresponding weight-bearing areas of bone as acetabular middle column. Middle column is a special part of acetabulum which includes weight-bearing articular surface. Commitment to integrity and stability of the acetabulum is an important basis for the physiological function, once been involved, the fractures may result great changes in the contact area and stress between the femoral head and acetabulum, thereby it would greatly increase the long-term probability of occurrence of traumatic arthritis. Although fractures involving acetabular weight-bearing articular surface was clearly stated in the middle of the last century, and it is important factor in the diagnosis and treatment of acetabular fractures, there is very few of basic or clinical research for acetabular roof in more than half a century, resulting in a sharp contrast with its important role. In clinical work, fractures of acetabular middle column often occurred, especially in the posterior wall of acetabular middle column which is the weak area. But there is still lack of systematic research about posterior wall of acetabular middle column in anatomical measurement, classification method, diagnostic criteria, surgical techniques and internal fixation material. Therefore, posterior wall of acetabular middle column need to be further studied in the structure and function, and should be treated differently from generalized posterior wall on the point of view of the morphologic and biomechanics.Part I Anatomic measurements and quantitative analysis of the posterior wall of acetabular middle columnObjective:To quantitatively analyze the shape characteristics and clinical significance by anatomical measurement of the posterior wall of acetabular middle column, thus to provide a new basis for the theory of acetabular three-column and offer a theoretical reference for clinical treatment.Methods:15formalin-preserved cadaveric pelvises (8males and7females) were used for this investigation。The whole acetabular posterior walls were marked with "angle" sector method and the thickness was measured with caliper in different levels of the different split points. The measurement results were validated and analyzed.Results:1、At5mm away from the acetabular rim, the average thickness of posterior wall of acetabular middle column fluctuated between6.47±0.61mm and7.43±0.71mm; the average thickness of posterior wall of acetabular posterior column fluctuated between5.62±0.51mm and6.33±0.61mm; the average thickness of acetabular roof fluctuated between7.71±0.74mm and8.27±0.99mm. Comparatively, no statistically significant difference was found between the average thickness of posterior wall of acetabular middle column and posterior wall of acetabular posterior column (P>0.05), but the average thickness of acetabular roof was significantly larger than posterior wall of acetabular middle column (P<0.05); At10mm away from the acetabular rim, the average thickness of posterior wall of acetabular middle column fluctuated between8.81±0.67mm and13.35±0.89mm; the average thickness of posterior wall of acetabular posterior column fluctuated between7.02±0.63mm and7.66±0.69mm; the average thickness of acetabular roof fluctuated between14.46±0.97mm and17.05±1.35mm. Comparatively, the average thickness of posterior wall of acetabular middle column was significantly larger than posterior wall of acetabular posterior column(P<0.05),and the average thickness of acetabular roof was significantly larger than posterior wall of acetabular middle column (P<0.01); At15mm away from the acetabular rim, the average thickness of posterior wall of acetabular middle column fluctuated between12.08±0.78mm and19.84±1.03mm; the average thickness of posterior wall of acetabular posterior column fluctuated between10.17±0.76mm and11.12±0.77mm; the average thickness of acetabular roof fluctuated between23.23±1.12mm and26.01±1.53mm. Comparatively, the average thickness of posterior wall of acetabular middle column was significantly larger than posterior wall of acetabular posterior column (P<0.01), and the average thickness of acetabular roof was significantly larger than posterior wall of acetabular middle column (P<0.01)2、At5mm away from the acetabular rim, the average thickness of posterior wall of acetabular middle column fluctuated between6.47±0.61mm and7.43±0.71mm. At 10mm away from the acetabular rim, the average thickness of posterior wall of acetabular middle column fluctuated between8.81±0.67mm and13.35±0.89mm. At15mm away from the acetabular rim, the average thickness of posterior wall of acetabular middle column fluctuated between12.08±0.78mm and19.84±1.03mm. Comparatively, with the increase of the measured distance, the thickness of posterior wall of acetabular middle column had a significant increase (P<0.05); At5mm away from the acetabular rim, the average thickness of posterior wall of acetabular posterior column fluctuated between5.62±0.51mm and6.33±0.61mm. At10mm away from the acetabular rim, the average thickness of posterior wall of acetabular posterior column fluctuated between7.02±0.63mm and7.66±0.69mm. At15mm away from the acetabular rim, the average thickness of acetabular roof fluctuated between23.23±1.12mm and26.01±1.53mm. Comparatively, with the increase of the measured distance, the thickness of posterior wall of acetabular posterior column had a significant increase (P<0.05); At5mm away from the acetabular rim, the average thickness of acetabular roof fluctuated between7.71±0.74mm and8.27±0.99mm. At10mm away from the acetabular rim, the average thickness of acetabular roof fluctuated between14.46±0.97mm and17.05±1.35mm. the average thickness of acetabular roof fluctuated between23.23±1.12mm and26.01±1.53mm. Comparatively, with the increase of the measured distance, the thickness of acetabular roof had a significant increase (P<0.01)3、At5mm away from the acetabular rim, the average thickness of posterior wall of acetabular middle column fluctuated between6.77±0.59mm and7.67±0.67mm in males and between6.13±0.61mm and7.11±0.55mm in females, the average thickness of posterior wall of acetabular posterior column fluctuated between5.94±0.47mm and6.64±0.57mm in males and between5.25±0.44mm and6.01±0.49mm in females. At10mm away from the acetabular rim, the average thickness of posterior wall of acetabular middle column fluctuated between9.39±0.65mm and14.38±0.86mm in males and between8.15±0.71mm and12.17±0.94mm in females, the average thickness of posterior wall of acetabular posterior column fluctuated between7.25±0.63mm and8.13±0.81mm in males and between6.76±0.64mm and7.12±0.55mm in females. At15mm away from the acetabular rim, the average thickness of posterior wall of acetabular middle column fluctuated between12.50±0.78mm and20.47±1.13mm in males and between11.60±0.94mm and19.12±0.99mm in females, the average thickness of posterior wall of acetabular posterior column fluctuated between10.54±0.78mm and11.55±0.71mm in males and between9.75±0.65mm and10.63±0.83mm in females. However, no statistically significant difference was found in each group between males and females.Conclusion:By means of anatomy and morphology measurements, we found that the lip of posterior wall of acetabular middle column was short and thick, while the lip of posterior wall of acetabular posterior column was long and thin. The thickness of entire acetabular posterior edge revealed an increasing tendency from the posterior wall of acetabular posterior column to the posterior wall of acetabular middle column to acetabular roof. And this trend became more obvious with increasing distance away from acetabular rim.We could see, at5mm away from the acetabular rim, no statistically significant difference was found between the average thickness of posterior wall of acetabular middle column and posterior wall of acetabular posterior column (P>0.05), but at10mm and15mm away from the acetabular rim, the average thickness of posterior wall of acetabular middle column was significantly larger than posterior wall of acetabular posterior column (P<0.05). And in each group the average thickness of acetabular roof was significantly larger than posterior wall of acetabular middle column (P<0.01). The morphology of bone structure depends on its mechanical environment. So the posterior wall of acetabular middle column could not only maintain the stability of hip joint but also bear the load. And because of its weakness, fractures of posterior wall of acetabular middle column occurred more common in clinic. In addition, Our study could provide anatomic reference for clinical treatment of acetabular fractures and avoid internal fixation into the joint. And it could provide a new theoretical basis for the new division of the region of acetabulum(theory of acetabular three-column). Part II Experimental biomechanics study about fracture of posterior wall of acetabular middle columnObjective:Through the experimental research of fractures of posterior wall of acetabular middle column, we discuss the importance and particularity of posterior wall of acetabular middle column and its effect to a sort of fracture which related to the roof of acetabulum. From the aspect of biomechanics we try to find evidence to provide theoretical basis of three column classification of acetabulum and clinical work.Methods:Six formalin-preserved cadaveric pelvises were used for this investigation. The fractures model of posterior wall of acetabular middle column and acetabular posterior column were created. The first group of same fixation constructs (two interfragmentary screws and a locking reconstruction) in different fractures were compared; The second group of different fixation constructs(two interfragmentary screws and a locking reconstruction; ATMFS) in fractures of posterior wall of acetabular middle column were compared. Pelvises were axial loaded with six cycles of1500N. Dislocation of superior and inferior fracture site was analyzed with the digital image correlation technology. Comparison between the experimental groups was performed by statistical analysis.Results:1-In the first group, the same fixation constructs (two interfragmentary screws and a locking reconstruction) in different fractures (posterior wall of acetabular middle column and posterior wall of acetabular posterior column) were compared. All vector displacements were below the clinically tolerable maximum value of2mm. Therefore each tested fixation technique was able to stabilize fractures of the cadaveric pelvises up to a vertical load of1500N. The mean vector dislocation at superior fracture site of posterior wall of acetabular middle column was1.538±0.050mm and1.486±0.059mm at inferior one; The mean vector dislocation at superior fracture site of posterior wall of acetabular posterior column was0.924±0.067mm and0.833±0.045mm at inferior one. And there was significant difference between two sides (P<0.05)2、In the second group, the different fixation constructs (two interfragmentary screws and a locking reconstruction; ATMFS) in fractures of posterior wall of acetabular middle column were compared. All vector displacements were below the clinically tolerable maximum value of2mm. Therefore each tested fixation technique was able to stabilize fractures of the cadaveric pelvises up to a vertical load of1500N. The mean vector dislocation at superior fracture site of posterior wall of acetabular middle column on the side of screws and LCP was1.519±0.053mm and1.519±0.053mm at inferior one; The mean vector dislocation at superior fracture site of posterior wall of acetabular middle column on the side of ATMFS was1.552±0.040mm and1.552±0.040mm at inferior one. And no statistically significant difference was found between the two types of fixation (P>0.05)3、In each group, the mean vector dislocation at superior fracture site was significantly larger than inferior one.4、In each group, the dislocation at Y-axis was significantly larger than X-axis.Conclusion:We found although the posterior wall of acetabular middle column and posterior wall of acetabular posterior column belonged to posterior wall of acetabulum in general, the stress on the posterior wall of acetabular middle column was significantly larger than the posterior wall of acetabular posterior column. So we should discriminate them in clinic. In addition, the different fixation constructs (two interfragmentary screws and a locking reconstruction; ATMFS)) in fractures of posterior wall of acetabular middle column achieved rigid internal fixation (all vector displacements were below the clinically tolerable maximum value of2mm). but two groups conducted only eight cycles and the mean vector dislocation of fractures of posterior wall of acetabular middle column was close to2mm in our study. In reality the displacement of fractures of posterior wall of acetabular middle column might be progressively increased because of long-term compressive stress coupled with the synergies of muscle contraction, even more than2mm. These might be the reasons of loss of reduction, femoral head subluxation and traumatic arthritis in clinical follow-up process.