Acetabular Cup Implantation Research Articles

Assessing the Acetabular Cup Implant Primary Stability by Impact Analyses: A Cadaveric Study.

BackgroundThe primary stability of the acetabular cup (AC) implant is an important determinant for the long term success of cementless hip surgery. However, it remains difficult to assess the AC implant stability due to the complex nature of the bone-implant interface. A compromise should be found when inserting the AC implant in order to obtain a sufficient implant stability without risking bone fracture. The aim of this study is to evaluate the potential of impact signals analyses to assess the primary stability of AC implants inserted in cadaveric specimens.MethodsAC implants with various sizes were inserted in 12 cadaveric hips following the same protocol as the one employed in the clinic, leading to 86 different configurations. A hammer instrumented with a piezoelectric force sensor was then used to measure the variation of the force as a function of time produced during the impact between the hammer and the ancillary. Then, an indicator I was determined for each impact based on the impact momentum. For each configuration, twelve impacts were realized with the hammer, the value of the maximum amplitude being comprised between 2500 and 4500 N, which allows to determine an averaged value IM of the indicator for each configuration. The pull-out force F was measured using a tangential pull-out biomechanical test.ResultsA significant correlation (R2 = 0.69) was found between IM and F when pooling all data, which indicates that information related to the AC implant biomechanical stability can be retrieved from the analysis of impact signals obtained in cadavers.ConclusionThese results open new paths in the development of a medical device that could be used in the future in the operative room to help orthopedic surgeons adapt the surgical protocol in a patient specific manner.

Finite element model of the impaction of a press-fitted acetabular cup.

Press-fit surgical procedures aim at providing primary stability to acetabular cup (AC) implants. Impact analysis constitutes a powerful approach to retrieve the AC implant insertion properties. The aim of this numerical study was to investigate the dynamic interaction occurring between the hammer, the ancillary and bone tissue during the impact and to assess the potential of impact analysis to retrieve AC implant insertion conditions. A dynamic two-dimensional axisymmetric model was developed to simulate the impaction of the AC implant into bone tissue assuming friction at the bone-implant interface and large deformations. Different values of interference fit (from 0.5 to 2mm) and impact velocities (from 1 to 2m.s-1) were considered. For each configuration, the variation of the force applied between the hammer and the ancillary was analyzed and an indicator I was determined based on the impact momentum of the signal. The simulated results are compared to the experiments. The value of the polar gap decreases with the impact velocity and increases with the interference fit. The bone-implant contact area was significantly correlated with the resonance frequency (R 2=0.94) and the indicator (R 2=0.95). The results show the potential of impact analyses to retrieve the bone-implant contact properties.

Estimation of the acetabular cup implant stability using impact analysis

Event Abstract Back to Event Estimation of the acetabular cup implant stability using impact analysis Adrien Michel1, Romain Bosc1 and Guillaume Haiat1 1 CNRS, Laboratoire MSME UMR 8208, France Introduction: Press-fit procedures for the insertion of cementless hip prostheses aim at obtaining an optimal implant stability. Primary stability of the acetabular cup (AC) implant is an important factor for long term surgical success. The aim of this study is to determine the feasibility of the application of an acoustical method to retrieve the AC implant stability based on the analysis of the signal corresponding to the time variation of the force applied between the hammer and the ancillary. Materials and Methods: 1) In vitro Study: The first objective of this work was to investigate whether the AC implant primary stability can be evaluated using the signals obtained with an impact hammer in vitro. The experimental configuration is shown in Fig. 1. A hammer equipped with a force sensor was used to impact the AC implant in 20 bovine bone samples. For each sample, different stability conditions were obtained by changing the cavity diameter. For each configuration, the inserted AC implant was impacted four times with a maximum force comprised between 2500 and 4500 N. An indicator I was determined based on the partial impulse estimation and the pull-out force F was measured. 2) In silico study: The second aim of this study was to investigate the dynamic interaction occurring between the hammer, the ancillary and bone tissue during the impact and to assess the potential of impact analysis to retrieve AC implant insertion conditions. A dynamic two-dimensionnal axisymmetric finite element model was developed to simulate the impaction of the AC implant into bone tissue assuming friction at the bone-implant interface and large deformations. Different values of interference fit (from 0.5 to 2 mm) and impact velocities (from 1 to 2 m.s-1) were considered. For each configuration, the variation of the force applied between the hammer and the ancillary was analyzed and an indicator I was determined based on the impact momentum of the signal. 3) Cadaveric study: The third objective was to assess the potential of the method to retrieve the primary stability of acetabular cup (AC) implants inserted in cadaveric specimens. AC implants were implanted in 12 cadaveric hips. A hammer instrumented with a piezoelectric force sensor was used to measure the impacts signals between the hammer and the ancillary. The tangential pull-out force F was measured. Results: The implant stability and the value of the indicator I reached a maximum value for an interference fit equal to 1 mm for 18 out of 20 samples. When pooling all samples and all configurations, the implant stability and I were significantly correlated (R2=0.83, see Fig. 2)[1]-[4]. The value of the polar gap decreases as a function of the impact velocity and increases as a function of the interference fit. The bone implant contact area was significantly correlated with the resonance frequency (R²=0.94) and the indicator (R²=0.95). Concerning the cadaveric experiments, a significant correlation (R2 = 0.69) was found between the indicator I and the pull-out force F. Discussion and Conclusion: The AC implant primary stability can be assessed through the analysis of the impact force signals obtained using an impact hammer. This study opens new paths for the development of a medical device which could be used as a decision support system to assist the surgeon during the insertion of the AC implant.

Three finned press-fit cup: Does its initial fixation strength provide an adequate stability? Clinical midterm results of 685 implants

Introduction: One of the major causes of loosening of cementless acetabular cup implants is insufficient initial stability. A technical proposal to decrease the risk of suboptimal first stability is a circumferential finned design of the cup. This design aims to improve periacetabular bone contact and prevent rotational micromotion of the cup when optimal press-fit cannot be obtained. Materials and Methods: We retrospectively reviewed a group of 712 consecutive patients who underwent total hip arthroplasty from June 2006 to June 2014. In all patients, a titanium cup, characterized by three anti-rotational circumferential fins at the superior pole, was implanted. Results: Five hundred and ninety-two patients, for a total of 685 hips, were evaluated at a mean follow-up of 58 months (range 12-96 months). At 1-year follow-up, the average score increased to 82.90 (range 100-70) and at the final follow-up (58 months, range 12-96 months), it was 80.12 (range 100-66). In 22 cases (3%), screws to obtain a secure primary stability of the cup were used. Nineteen complications (2.6%) needing revision surgery were observed. Survivorship at 10 years was 98.7% (95% confidence interval [CI], 98.7-99.7%) with revision for aseptic cup loosening as an endpoint and 96.7% (95% CI, 98.3-95.1%) with revision for all causes of revision as the second endpoint. Discussion: In our group of patients, we did not observe the cases of very early cup loosening. The only two-cup revision, do to loosening of osteolysis, was observed 26 and 32 months before surgery. Conclusion: Our very low rate of additional screws represents an indirect sign of finned cup first stability. Three-finned cup design clinically confirmed to improve initial cup stability.

Ex vivo estimation of cementless acetabular cup stability using an impact hammer

Obtaining primary stability of acetabular cup (AC) implants is one of the main objectives of press-fit procedures used for cementless hip arthroplasty. The aim of this study is to investigate whether the AC implant primary stability can be evaluated using the signals obtained with an impact hammer.A hammer equipped with a force sensor was used to impact the AC implant in 20 bovine bone samples. For each sample, different stability conditions were obtained by changing the cavity diameter. For each configuration, the inserted AC implant was impacted four times with a maximum force comprised between 2500 and 4500 N. An indicator I was determined based on the partial impulse estimation and the pull-out force was measured.The implant stability and the value of the indicator I reached a maximum value for an interference fit equal to 1 mm for 18 out of 20 samples. When pooling all samples and all configurations, the implant stability and I were significantly correlated (R2 = 0.83).The AC implant primary stability can be assessed through the analysis of the impact force signals obtained using an impact hammer. Based on these ex vivo results, a medical device could be developed to provide a decision support system to the orthopedic surgeons.

In vitro evaluation of the acetabular cup primary stability by impact analysis.

The implant primary stability of the acetabular cup (AC) is an important parameter for the surgical success of press-fit procedures used for the insertion of cementless hip prostheses. In previous studies by our group (Mathieu, V., Michel, A., Lachaniette, C. H. F., Poignard, A., Hernigou, P., Allain, J., and Haiat, G., 2013, "Variation of the Impact Duration During the in vitro Insertion of Acetabular Cup Implants," Med. Eng. Phys., 35(11), pp. 1558-1563) and (Michel, A., Bosc, R., Mathieu, V., Hernigou, P., and Haiat, G., 2014, "Monitoring the Press-Fit Insertion of an Acetabular Cup by Impact Measurements: Influence of Bone Abrasion," Proc. Inst. Mech. Eng., Part H, 228(10), pp. 1027-1034), the impact momentum and duration were shown to carry information on the press-fit insertion of the AC within bone tissue. The aim of the present study is to relate the impact momentum recorded during the AC insertion to the AC biomechanical primary stability. The experimental protocol consisted in testing 13 bovine bone samples that underwent successively series of 15 reproducible mass falls impacts (5 kg, 5 cm) followed by tangential stability testing. Each bone sample was tested with different hole sizes in order to obtain different stability configurations. The impact momentum and the tangential primary stability reach a maximum value for an interference fit equal to around 1 mm. Moreover, a correlation between the impact momentum and the stability was obtained with all samples and all configuration (R2 = 0.65). The implant primary stability can be assessed through the measurement of the impact force signal analysis. This study opens new paths for the development of a medical device which could be used as a decision support system to assist the surgeon during the insertion of the AC implant.

Management of neglected acetabular fractures.

Management of neglected acetabular fractures is a difficult task. Osteosynthesis in such cases may not be an ideal solution because of the femoral head damage due to pressure by the fractured acetabular edge, avascular necrosis, difficulty in mobilizing the fragments due to callus formation, difficulty in indirect reduction of the fracture fragments and macerated acetabular fragments all contributing to inadequate fracture reduction. Majority of such fractures are now treated with total hip replacement. While treating such fractures with THR, problems associated with neglected acetabular fractures such as fracture non-union, hip dislocation, protrusio, cavitary bone defect or peripheral bone defect must be considered. 3D computed tomography scan provides a clear view about the acetabular and periacetabular bony anatomy. Impaction grafting and antiprotrusio cage or ring with a cemented acetabular cup can address most of the hip protrusio and cavitary bone defects. Segmental bone defect needs cortical strut-bone graft fixation and subsequent implantation of a cemented or uncemented acetabular cup implantation. Fracture non-union needs approximate reduction and fixation with plates followed by bone grafting and implantation of an acetabular cup. Despite these efforts, the outcome of THR in neglected acetabular fracture is considerable worse than after conventional hip replacement.

Variation of the impact duration during the in vitro insertion of acetabular cup implants

The acetabular cup (AC) is an implant impacted into a bone cavity and used for hip prosthesis surgery. Initial stability of the AC is an important factor for long term surgical success. The aim of this study is to determine the variations of the impact duration during AC implant insertion.Twenty-two bone samples taken from bovine femurs were prepared ex vivo for the insertion of an acetabular cup implant, following the surgical procedure used in the clinic. For each bone sample, ten impacts were applied using reproducible mass falls (3.5kg) in order to insert the AC implant. Each impact duration was recorded using a wide bandwidth force sensor.For all bone samples, the impact duration was shown to first decrease as a function of the impact number, then reaching a stationary value equal in average to 4.2±0.7ms after an average number of 4.1±1.7 impacts. The impact duration may be related to variations of the bone–implant interface contact rigidity because of an increase the amount of bone tissue in contact with the AC implant.Measurements of impact duration have a good potentiality for clinical application to assist the surgeon during the insertion of the AC implant, providing valuable information on the bone–implant interface contact properties.

Deformation of uncemented metal acetabular cups following impaction: experimental and finite element study

Predicting failure following the implantation of acetabular cups used in hip replacements is important in ensuring robust component designs. This study has developed 3D explicit dynamics finite element (FE) models to investigate the deformation of press-fit metal cups following insertion in the acetabular cavity. The cup deformation following insertion is clearly influenced by the forces encountered during insertion, the initial position of the cup in the cavity, the support provided by the underlying bone and the geometry of the cup itself. Experimentally validated explicit dynamics FE models were used to allow a physiologically relevant simulation of the impaction of cups, which is encountered in clinical practice, in comparison to previous studies that have used unrealistically high static forces to simulate a static press-fit insertion. Diametrical cup deformations were twice as large when the cup was tilted at 5° with respect to the cavity compared to when the poles of the cup and the cavity were aligned. The introduction of a non-uniform support to the cup increased deformations further by a factor of approximately 2.5. The greatest deformations established in the model were between 80 and 150 μm similar to typical cup–femoral head clearances. Increasing the thickness at the pole of the cup and reducing the cup diameter resulted in significantly smaller deformations being generated. These results suggest that small cup misalignments, which may not be noticeable in a clinical situation, may produce significant deformations after insertion especially when coupled with the non-uniform support found in the pelvis.

Die Verkürzungsosteotomie im Rahmen des alloarthoplastischen Gelenkersatzes bei hoher Hüftluxation im Erwachsenenalter

Total hip arthroplasty to create an articulating hip joint. Acetabular cup implantation in the original rotational center of the pelvis. Simultaneous femoral shortening osteotomy to prevent neurovascular damage and equalize leg length in patients with unilateral occurrence. Developmental dysplasia of the hip (DDH) in adults; type3 and 4 dislocation according to Crowe. Cerebrospinal dysfunction with permanent restriction of coordination ability, muscular dystrophies, and multiple malformations of the musculoskeletal system. Apparent disturbance of the bone metabolism. The Watson-Jones interval approach to the hip joint is used to avoid functional disorders of the hip abductors. After preparation of the proximal femur and femoral neck resection, adjustment of the non-regularly developed acetabular cavity with reduced anterior coverage takes place. The cup component is implanted and the interval between the vastus intermedius and the vastus lateralis below the lesser trochanter examined. Loss of periosteum of the femoral cortex due to blunt spreading is to be avoided. Following the femoral shortening osteotomy initially the preparation of the distal bone segment takes place to adjust the endofemoral canal for shaft implantation in the following preparation of the metaphyseal segment. Afterwards osteotomy-bridging implantation of a cementless, distal anchoring stem design is performed. Partial weight bearing of the leg with 20kp is necessary for 6weeks combined with therapy of existing contractures and active pelvic rotation training. Within 10 postoperative weeks full weight bearing is usually reached. After this period mobilization without crutches is possible. Inpatient rehabilitation is possible after bony recovery of the femoral osteotomy. From 1993 to 1999, the first 15 total hip arthroplasties were performed in adult patients with DDH; they were treated with simultaneous femoral shortening osteotomy and without additive osteosynthesis. During the midterm follow-up (4.3years), no failure of the femoral component was observed with complication-free osseous healing of the osteotomy. One cup revision was necessary in this period. The Merle d'Aubigné score increased from 8.2 preoperatively to 15.5points.

A Powered Technique for Inserting Acetabular Cups

This work focused to investigate the use of a powered automatic technique to insert acetabular cup implants. The use of a percussion hammer tool as a surgical instrument to insert acetabular cup implants was investigated in current work. Pull-out, lever-out and torque tests were carried out on cup implants inserted into test block specimens of polyurethane (PU) using specifically designed experimental setups as a means of comparing the current mallet and cup introducer (manual impaction) technique against the percussion hammer tool (powered impaction) technique. The experimental tests were based on calculating the maximum forces or moments of forces required to remove the cup implants from a test block specimen, which was representative of the acetabulum of the pelvic bone. It was found, the cup implants inserted using the powered impaction technique required a greater applied force, moment of force and torque in order to remove the cup from the cavity of the PU block specimen in the pull-out, lever-out and torque tests respectively. In terms of stability, the percussion hammer tool has the potential to improve the seating of cup implants within the cavity using a more precise and controlled technique, thus improving the over stability of the inserted cup implant.

Experimental Evidence of Impingement Induced Strains at the Interface and the Periphery of an Embedded Acetabular Cup Implant

After total hip arthroplasty, impingement of implant components may occur during every-day patient activities causing increased shear stresses at the acetabular implant-bone interface. In the literature, impingement related lever-out moments were noted for a number of acetabular components. But there is little information about pelvic load transfer. The aim of the current study was to measure the three-dimensional strain distribution at the macrostructured hemispherical interface and in the periphery of a standard acetabular press-fit cup in an experimental implant-bone substitute model. An experimental setup was developed to simulate impingement loading via a lever arm representing the femoral component and the lower limb. In one experimental setup 12 strain gauges were embedded at predefined positions in the periphery of the acetabular cup implant inside a tray, using polyurethane composite resin as a bone substitute material. By incremental rotation of the implant tray in steps of 10 and 30 deg, respectively, the strains were measured at evenly distributed positions. With the described method 288 genuine strain values were measured in the periphery of an embedded acetabular cup implant in one experimental setup. In two additional setups the strains were evaluated at different distances from the implant interface. Both in radial and meridional interface directions strain magnitudes reach their peak near the rim of the cup below the impingement site. Values of equatorial strains vary near zero and reach their peaks near the rim of the cup on either side and in some distance from the impingement site. Interestingly, the maximum of averaged radial strains does not occur, as expected, close to the interface but at an interface offset of 5.6 mm. With the described experimental setup it is now possible to measure and display the three-dimensional strain distribution in the interface and the periphery of an embedded acetabular cup implant. The current study provides the first experimental proof of the high local stresses gradients in the direct vicinity of the impingement site. The results of the current study help for a better understanding of the impingement mechanism and its impact on acetabular cup stability.

A Powered Technique for Inserting Acetabular Cups

This work focused to investigate the use of a powered automatic technique to insert acetabular cup implants. The use of a percussion hammer tool as a surgical instrument to insert acetabular cup implants was investigated in current work. Pull-out, lever-out and torque tests were carried out on cup implants inserted into test block specimens of polyurethane (PU) using specifically designed experimental setups as a means of comparing the current mallet and cup introducer (manual impaction) technique against the percussion hammer tool (powered impaction) technique. The experimental tests were based on calculating the maximum forces or moments of forces required to remove the cup implants from a test block specimen, which was representative of the acetabulum of the pelvic bone. It was found, the cup implants inserted using the powered impaction technique required a greater applied force, moment of force and torque in order to remove the cup from the cavity of the PU block specimen in the pull-out, lever-out and torque tests respectively. In terms of stability, the percussion hammer tool has the potential to improve the seating of cup implants within the cavity using a more precise and controlled technique, thus improving the over stability of the inserted cup implant.

Trabecular Metal Cup without Augments for Acetabular Revision in Case of Extensive Bone Loss and Low Bone-Prosthesis Contact

Current evidences in revision hip arthroplasty suggest to treat severe acetabular bone loss with dedicated implants, such as anti-protrusio cages, stemmed cups, modular systems supplied with iliac flanges and obturatory hook. However recent literature is reporting satisfactory outcomes with simple elliptical Trabecular Metal cups. Purpose of the study was to evaluate mid-term results of such a surgical procedure. All hip revisions performed from 2008 to 2009 with implantation of a TMT multi-hole acetabular cup without augmentations were retrospectively reviewed. The cases with low-degree acetabular bone loss (stage I and II according to GIR classification), with surgical report poorly describing the bone defect, with inadequate pre- and post-operative x-rays were ruled out. Twenty-five cases were identified, but four were lost to follow-up. The twenty-one patients were 71 year-old on average (from 60 to 82), with stage IV bone loss in 6 cases and stage III bone loss in 15 cases. Mean interval from surgery to evaluation was 20.9 months (from 13 to 30). The evaluation included bone-prosthesis contact estimation, component position, survivorship, complications, final Harris Hip Score, presence of periprosthetic radiolucencies. Host bone-prosthesis contact was estimated to be about 35%. Only three implant were subsequently reoperated (for infection, early migration, recurrent dislocation). The HHS among non-reoperated 18 patients was 81.96 on average (from 63.44 to 95.82). Six cases showed thin radiolucencies in one of the three Charnley zones, while three cases showed radiolucencies in two. None of these images was evolutive, thus they were not considered signs of loosening. The mid-term results of this series confirm the hypothesis that a porous tantalum acetabular cup is an effective option to deal with difficult acetabular revisions. Although no extra-acetabular fixation device is available, the very high surface friction guaranteed by the material and the supplemental stability provided by trans-acetabular screws seem to be sufficient to allow satisfactory reimplantation even in severely damaged pelves.

Biomechanical Evaluation of Acetabular Cup Implantation in Cementless Total Hip Arthroplasty

To report biomechanical properties of the Biologic Fixation System (BFX) acetabular cup impacted into a normal canine pelvis and to compare the effect of implant positioned to and beyond the medial acetabular wall. In vitro cadaveric study. Hemipelves of mature, large-breed dogs (n=6). For each dog, 1 hemipelvis was reamed to the depth of the acetabular wall (group A) and 1 was reamed an additional 6 mm after penetration of the medial cortex of the acetabulum (group B). The hemipelves were implanted with acetabular cups and loaded in compression through a matching femoral prosthetic component until failure. Specimen stiffness, and failure displacement, load, and energy were determined from load and displacement data and results between groups compared with a paired t-test. Mean failure load was greater in group A (3812 ± 391 N) than group B (2924 ± 316 N; P<.014). No other differences (P>.05) were observed between groups. Bone fracture (n=5) and cup displacement (1) occurred in group A whereas in group B there were 3 fractures and 3 cup displacements. Although medial placement of the BFX cup affected compressive failure loads, failure loads for both groups exceeded normal physiologic loads. Medial positioning of the acetabular cup does not appear to compromise acetabular implant-pelvic stability under normal physiologic loads. Because arthroplasty candidates often have abnormal acetabular architecture, mechanical properties of the cup placed in acetabula without a dorsal rim should be investigated.

Obesity Does Not Affect The Accuracy of Acetabular Cup Implantation in Total Hip Replacement

Post-operative radiographs of 111 patients who had undergone cemented Total Hip Replacement performed or supervised a single surgeon (GCF) were analysed. All surgeries were performed via an anterolateral approach with patients in the lateral position. Patients were stratified according to Body Mass Index (BMI) into three groups (healthy, overweight and obese) and acetabular cup angles measured. Mean values for cup abduction angle were similar for all three groups (44.5°, 46.8°, 44.0° respectively, p=0.335). Furthermore there were no differences in mean cup anteversion between groups (11.6°, 12.2°, 10.7 respectively, p=0.665). We conclude that obesity does not influence the accuracy of cup positioning in Total Hip Replacement.

The number of screws, bone quality, and friction coefficient affect acetabular cup stability

One of the major causes of loosening of cementless acetabular cup implants is insufficient initial stability. This study used three-dimensional finite element models of the pelvis and acetabular components to investigate the effects of the number of screws, bone quality, and friction coefficient of the acetabular cup on the initial stability under normal walking. A commercially available hemispherical acetabular cup with five screw holes was used as the default model. The stiffness of the pelvis and the friction coefficient of the cup were systematically varied, within a realistic range, to assess the initial stability of the acetabular cup. The simulations showed that the inserted screws provide only a localized reduction in the relative micromotion between the cup and pelvis therefore inserting several screw closed together might not be useful. Changes in the pelvic stiffness have a non-linear effect on the initial stability of acetabular cup and the subchondral bone provides good support for fixation of the cementless cup. The friction coefficient of the acetabular cup plays a limited role, comparing with the factor of bone quality, in resisting relative micromotion in the cup–pelvis interface.

Inter- and intraobserver assessment of periacetabular osteodensitometry after cemented and uncemented total hip arthroplasty using computed tomography

This study was initiated to evaluate the reproducibility of a novel method for measuring the periacetabular bone density after insertion of cemented and uncemented acetabular cups using CT in vivo. CT scans were obtained from 20 patients after cemented polyethylene cup implantation (ZCA, Zimmer, USA) and 20 patients after uncemented titanium alloy cup fixation (Cerafit, Ceraver, France). A manual segmentation of cancellous and cortical pelvic bone ventral, dorsal and cranial to the cup was undertaken. Values are given in Hounsfield units. Inter- and intraobserver studies were conducted using a special analysis software tool. To define the reproducibility of the method, all measurements were evaluated according to Bland and Altman. For both cemented and uncemented acetabular cups, reproducibility of bone density measurement for cortical and cancellous bone cranial, ventral and dorsal to the cup was high. There was no significant difference between the intraobsever study (two repeated measurements) and the interobserver study (two investigators), indicating the reproducibility of the method independent of the investigator. In conclusion, the periacetabular bone density measurement as conducted in this CT study is a new reproducible method for in vivo evaluation of cortical and cancellous pelvic bone after cemented and uncemented acetabular cup implantation. In vivo CT measurements will allow a thorough assessment of periacetabular stress-shielding phenomena.

Genauigkeit eines bildfreien Navigationssystemes für die Hüftpfannenimplantation – eine anatomische Studie / Accuracy of an Image-free Cup Navigation System – an Anatomical Study

The position of the acetabular cup is of decisive importance for the function of a total hip replacement (THR). Using the conventional surgical technique, correct placement of the cup often fails due to a lack of information about pelvic tilt. With CT-based and fluoroscopically-assisted navigation procedures the accuracy of implantation has been significantly improved. However, additional radiation exposure, high cost and the increased time requirement have hampered the acceptance of these techniques. The present anatomical study evaluates the accuracy of an alternative procedure--image-free navigation. This method requires little extra effort, does not substantially delay surgery, and needs no additional imaging. Press-fit cups were implanted in 10 human cadaveric hips with the help of the image-free navigation system, and the position of the cups was checked intraoperatively with a CT-based navigation system and postoperatively by computed tomography. All cups were implanted within the targeted safe zone with an average inclination of 44 degrees (range 40 degrees-48 degrees, SABW 2.7 degrees) and an average anteversion of 18 degrees (range 12-24 degrees, SABW 4.1 degrees). Analysis of accuracy of the image-free navigation software revealed only a small, clinically tolerable deviation in cup anteversion and cup inclination in comparison with the CT-based navigation system and the post operative CT scans. The evaluated image-free navigation system appears to be a practicable and reliable alternative to the computer-assisted implantation of acetabular cups in total hip arthroplasty.

Nonconsolidated Polyethylene Particles and Oxidation in Charnley Acetabular Cups

Nonconsolidated particles of ultra high molecular weight polyethylene are believed to be defects that adversely can affect the wear performance of total joint prostheses. The present study was done to determine the number, size, and distribution of these particles and to determine if their presence correlated with wear performance, as well as with other clinical and implant parameters. Forty retrieved and 7 new, never-implanted acetabular components were examined using light microscopy on thin cross sections. Particles were found in 92% of retrieved components and in all the new components. Particles in the retrieved components were either randomly distributed (32 components) or banded (with particles localized in regions approximately 1 mm below the outer surface of the component). No correlations were found between the number or area of particles and the wear performance or any of the clinical or implant variables. The presence of particles in the new implants was found to correlate with the length of time since the components had been radiation sterilized. For retrieved components, the density (and, therefore, the level of oxidative degradation) was high in the areas of banded particles. For new components, the density was higher the longer the time since sterilization. Nonconsolidated polyethylene particles are prevalent in total replacements but their source and cause are unknown. The results of this study show that they do not appear to affect or correlate with the length of implantation of acetabular cups. However, they still may be expected to adversely affect performance in cases where large numbers of particles are banded together near articulating surfaces of high stress environments such as found in the knee.