| BackgroundSevere pelvic deficiency presents a difficult problem in hip arthroplasty. Specially, the extent of preoperative acetabular bone deficiency is more serious than AAOS classification C or Paprosky classification?.The goals of restoring or preserving pelvic bone stock, placing the acetabular component in the correct anatomic position, optimizing joint stability, equalizing leg lengths, and achieving stable fixation are not as readily achieved in such a situation. Up to now, different ways of treating this problem have been proved a high failure rate at intermediate term follow up. The main reason for acetabular revision components failure is aseptic loosening, and the reason for this type of cup loosening is not come from wear debris of polyethylene but from the instability of the cup, including the insufficient of initial stability and the loss of long term mechanism of component stability. Instability in local will lead to loosening of the whole cup or flange fracture. So the key point to improve the clinical results of acetabular revision is how to get more reliable initial and long term stability of revision components. Transacetabular screw fixation is a very useful technique in total hip arthroplasty, using as an assistant fixation of acetabular component. Transacetabular screw can close the crevice generated during cup implantation, increase the initial instability of acetabular cup, and provide the cup rotation stability. This study proposed that transacetabular screws should be safely implanted not only into ilium, but also into pubis and ischium for whole acetabular cups fixation. As an simple technique, this article was focus on the possibilty and vlaue of transacetabular screws fixation. And in acetabular revisions with severe bone defects or reconstructions after periacetabular tumor resections, the acetabular cup fixed by screws with modified technique could be contribute to get rigid initial and long term stability. ObjectiveIn order to overcome the inherent limitations of traditional technique, we promoted that transacetabular screws could be safely implanted into ilium, pubis and ischium. This model of cup fixation was called as the fixation for the whole circumference. We have done three parts of action to do the research. First part is the anatomic study, a computer aided calculating system was done to determine the entry zones and screw directions for transacetabular screws. Second part is the biomechanical study, a simulated model with gait load to measure the displacement and strain distribution of the contour reconstruction cup with or without the transacetabular screw fixation. Third part is the clinical study, we reviewed a series cases with cage reconstruction. Risk analysis to know whether the fixation method with or without transacetabular screw could be used as a intraoperative factor to predict cage failure.MethodsFirst part:Twenty 3D hemi-pelvis models were constructed by CT data, and each was divided into four parts:posterior ilium, anterior ilium, pubis and ischium. Each part was undertaken parallel osteotomy and the contour coordinates of each section were recorded. The acetabular cup in each model was defined as a hemisphere with 52mm diameter. An uniform lattice at equal intervals of 2°in longitude-latitude on the acetabular cup were regarded as entry points. A computer aided program was developed to simulate screw trajectories in all directions through every entry point, then select out all the useful entry points and corresponding trajectories. Then the optimum direction of screw trajectory of each entry point was also calculated. Topographic maps of entry zone were delineated in the acetabular quadrant system to present the results.Second Part:Ten real hemi-pelvis cadavers were used for biomechanical test. The revision acetabular reconstruction setting was simulated by removal of specimen material inside and superiorly the acetabulum to create bone loss model like AAOS typeC. Each Specimen was used twice. Once the contour reconstruction ring was fixed only by flange screws in ilium and ischium. Another time was fixed both by flange screws and transacetabular screws in dome. The resultant force in a pelvic reference frame were plotted to simulate heelstrike, midstance and toe off stance phase, along with the corresponding hip fextion/extension angles. The strain of acetabular rim both in superior measure point near the root of ilial flange and in inferior measure point near the root of ischial flange was measured. Also the relative displacement between bone and ring under load was also measured to show the initial stability.Third part:A consecutive series of 40 patients received acetabular revision between 2002 and 2010 in our department were reviewed, including 16 male and 24 female patients. The mean age was 60.3 years and the mean follow-up time was 39.2 months. According to AAOS classification, there were 29 type C and 11 type D cases; while according to Paprosky classification, there were 29 type III A and 11 type?B cases. All the patients received regular clinical evaluations, Harris score estimations and radiological assessments about the acetabular stability and breakage of the ring or the screw. The survival analysis was applied when grouped by gender, age, fracture classification, superior or lateral migration of the cup, abduction angle, fixation method (including simple flange fixation and coupled with transacetabular screw fixation), bone grafting technique and cup type, to explore the relationships between reconstruction cup survival rate and the above factors.ResultsAccording the bone anatomy of pelvis, the posterior ilium was the most favorable area for the implantation of long screws, secondly the anterior ilium, thirdly the pubis and minimally the ischium. The entry zone in posterior ilium has the longitude from-148°to 29.2°, latitude from 7.5°to 87.3°?and a angle for trajectory direction was-85.3°to-53.8°,?angle from 46.3°to 60.2°. The distribution of entry zone of anterior and posterior ilium was overlapped in the extent. And the anatomy of ischium was not permitted transacetabular screws to reach the distant 1/3 part of the ischium ramus.Pressure strain was created both in superior and inferior measure site. In group one which reconstruction ring was fixed only by flange screws, under load in heelstrike, midstance and toe off stance phase, transverse and longitudinal strain in superior measure site was-19.3±13.3,-41.8±27.3,-53.7±29.and-44.3±20.2,-96.1±36.9,-133.6±69.4, in inferior measure site was-23.2±15.7,-46±24.9,-56.2±24 and-23±18.1,-48.2±25.8,-62.9±32.7. In group two which reconstruction ring was fixed both by flange and transacetabular screws, under the same load, transverse and longitudinal strain in superior measure site was -10.7±9.38,-30.4±23.1,-32.7±20.8 and -15.9±10.4,-39.2±24.6,-54.5±30, in inferior measure site was -15.9±10.3,-34.2±21.8,-45.4±21.9 and -15.9±10.4,-39.2±24.6,-54.5±30. The mean values of displacement in group one and group two were 0.40±0.26mm,0.64±0.27mm,0.89±0.28mm and 0.37±0.22mm, 0.61±0.19mm,0.89±0.33mm respectively, with no significant difference in statistic test between this two group.At an average of 39.2 months after revision,2 patients received rerevision because of cup lossening and 2 other patients showed radiological lossening. The statistical analysis indicated the migration of the ring, the abduction angle and the fixation technique were high correlated with cup failure, especially when the superior migration was over 8.5mm or the abduction angle was larger than 53.5°Using the screw fixation both in the flange and in the acetabular dome could help to reduce loosen risks. Additionally, there were no infection and 2 other dislocations, the later were successfully treated by conservative treatment and the final Harris score was74.5±15.4.ConlusionTransacetabular technique is a useful technique for cup fixation in acetabular revisions with severe bone defects or reconstructions after periacetabular tumor resections. The transacetabular screws were permitted to deeper insertion in ilium, pubis and ischium when the trajectories were not perpendicular to the cup wall. Biomechanical test showed though the contour cup initial stability could not be improved by transacetabular screw technique, but the max strain in root part of the flange will be reduced using transacetabular screws in ilium, pubis and ischium together, and the strain distribution will also be improved. From the clinical results, we get to know that there was still high short term to midterm failure risk for acetabular ring on acetabular revision cases with serious bone defects. But a lower migration of the ring, a decreased abduction angle and the transacetabular fixation technique would help to reduce such failure. |
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