The precision of radiostereometric measurements. Manual vs. digital measurements.

The precision of digital vs. manual radiostereometric measurements in total hip arthroplasty was evaluated using repeated stereoradiographic exposures with an interval of 10-15 min. Ten Lubinus SP2 stems cemented into bone specimens and 12 patients with the same stem design were used to evaluate the precision of stem translations and rotations. The precision of translations and rotations of the cup and femoral head penetration was studied in 12 patients with whole polyethylene cups.The use of a measurement method based on digitised radiographs improved the precision for some of the motion parameters, whereas many of them did not change. A corresponding pattern was observed for both the intra- and interobserver error. Of the wear parameters, the most pronounced improvements were the 3D wear and in the proximal-distal direction, although the anterior-posterior precision was also improved. The mean errors of rigid body and elliptic fitting decreased in all evaluations but one, consistent with a more reproducible identification of the markers centres and the edge of the femoral head.Increased precision of radiostereometric measurements may be used to increase the statistical power of future randomised studies and to study new fields in orthopaedics requiring higher precision than has been available with RSA based on manual measurements.     

The effects of femoral head size on the deformation of ultrahigh molecular weight polyethylene acetabular cups

Late loosening of cemented acetabular cups is increasingly being recognized as a clinical problem. One of the factors which may contribute to loosening is high localized deformation and stress at the cement-bone interface, the magnitude of which depends on the size of the total hip replacement (THR) femoral head. The effects of varying the femoral head size, from 22 to 32 mm, on strain values measured on the surface of the cup were investigated using experimental stress analysis techniques. The largest absolute strains were recorded when loading with the 22 mm head size. Peak strain values decreased to a minimum with the 26 mm head size and increased steadily with head sizes beyond 26 mm. The selection of an acetabular cup size and corresponding femoral head size in a total hip arthroplasty should not be an arbitrary one, but should be based on scientific studies which indicate minimum states of stress within the cup and cement mantle, as well as clinical evidence that the combination of components shows a reduced incidence of failure. This study experimentally quantifies the states of stress on the surface of the acetabular cup and points to the possible existence of an optimum component size to minimize surface stress.     

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.     

The cementless artificial hip cup--implant components with reliable long-term stability?

A hip joint simulator was developed to analyse the mechanism of loosening of cementless artificial hip cups. The machine induces vibrating motions and asymmetrical tilt shock loadings of the artificial cup. On measuring the primary stability of threaded cups, the simulation tests performed on pelvis substitute models, and animal and human acetabula failed to show any loosening of threaded cups fixed in place with a screw-in torque of more than 10 Nm. Instable cups became loose all the sooner, the lower the preload between the cup and femoral head. This demonstrates the importance of both the screw-in torque of the cup and the training status of the periarticular muscles.     

A new type of model-based Roentgen stereophotogrammetric analysis for solving the occluded marker problem

Roentgen stereophotogrammetric analysis (RSA) measures micromotion of an orthopaedic implant with respect to its surrounding bone. A problem in RSA is that the markers are sometimes overprojected by the implant itself. This study describes the so-called Marker Configuration Model-based RSA (MCM-based RSA) that is able to measure the pose of a rigid body in situations where less than three markers could be detected in both images of an RSA radiograph. MCM-based RSA is based on fitting a Marker Configuration model (MC-model) to the projection lines from the marker projection positions in the image to their corresponding Roentgen foci. An MC-model describes the positions of markers relative to each other and is obtained using conventional RSA. We used data from 15 double examinations of a clinical study of total knee prostheses and removed projections of the three tibial component markers, simulating occlusion of markers. The migration of the tibial component with respect to the bone, which should be zero, for the double examination is a measure of the accuracy of algorithm. With the new algorithm, it is possible to estimate the pose of a rigid body of which one or two markers are occluded in one of the images of the RSA radiograph with high accuracy as long as a proper MC-model of the markers in the rigid body is available. The new algorithm makes RSA more robust for occlusion of markers. This improves the results of clinical RSA studies because the number of lost RSA follow-up moments is reduced.     

Ceramic acetabular cups for hip endoprostheses. 7: How do position of the center of rotation and the CCD angle of the shaft modify range of motion and impingement?]

The range of motion (ROM) of total hip prostheses is influenced by a number of parameters. An insufficient ROM may cause impingement, which may result in subluxation, dislocation or material failure of the prostheses. In a three-dimensional CAD simulation, the position of the centre of rotation and the CCD angle of the stem were investigated. Displacement of the centre of rotation of the femoral head may be due to wear (PE cups) or to the design of the prosthesis (ceramic cups). Stems of widely differing design have been developed and implanted. The results of the present study demonstrate that the ROM is clearly reduced by increasing penetration of the femoral head. At an inclination angle of 45 degrees, a depth of penetration of 2 mm restricts flexion by about 15 degrees, and a depth of penetration of 3 mm by about 30 degrees. At smaller angles of inclination the ROM is reduced and flexion and abduction are associated with an increased risk of impingement. With steeper acetabular cup inclinations, the risk of impingement decreases, but dislocation, the risk of rim fractures (ceramic cups), and wear and penetration rates (PE cups) increase. The CCD angle of the stem should be oriented to the anatomical situation. At high CCD angles (> 135 degrees), flexion is clearly limited, in particular when there is penetration of the femoral head. For modern total hip arthroplasty, prosthetic systems characterised by precise positioning of components, minimum wear, slightly recessed inserts, and appropriate CCD angles should be used.     

Ceramic cups for hip endoprostheses. 6: Cup design, inclination and antetorsion angle modify range of motion and impingement]

The present investigation focuses on total hip replacement using ceramic acetabular components. The relationship between the position of the cup and the range of motion (ROM) was investigated. A limited range of motion may cause impingement, which is defined as contact between the femoral neck and the rim of the acetabular cup. Impingement may result in wear, chipping, fracture or dislocation of the femoral head. Joint movements were simulated in a three-dimensional CAD program. The results obtained underscore the importance of correct positioning and design of the cup for achieving a ROM as close to the physiological situation as possible. With ceramic cups, the inclination angle should not be more than 45 degrees, and the antetorsion angle between 10 and 15 degrees. If the cup is too vertical, the risk of dislocation and fracture of the ceramic increases. If, on the other hand, the angle of inclination is too small, flexion and abduction will be greatly limited. The study shows that acetabular components with non-recessed ceramic inserts should not be used. Slight recession of the insert helps to avoid impingement. The ROM is reduced and the risk of impingement appreciably increased when mushroom-shaped femoral heads (XL heads) or ceramic inserts protected by a polyethylene ring are used.     

Radiological evaluation of polyethyelene wear of cementless total hip endoprosthesis]

During the last years increasingly cementless hip endoprostheses have been implanted. Radiological wear measurement of cemented hip endoprostheses for the material couples polyethylene cup-ceramic- or metal head has been established in the literature. However, for cups encased by metal (screwing or pressfit cups), this method of measurement is not applicable. Therefore, a method has been developed to measure wear on radiographs on cementless spherical implants. The data were compared to those, obtained from conventional wear measurements on cemented hip cups. The results indicate that both techniques generate comparable results, thus validating the new technique as being suited for cementless cup implants.     

Investigation of explanted hip and acetabulum hip prostheses]

Retrieved ceramic femoral heads and acetabular cups were investigated. On the basis of the case studies, the reasons for revision are discussed. Wear patterns and wear rates were found to differ from those observed in hip simulating testing. Monolithic ceramic cups showed a high wear rate. Owing to their limited range of motion, ceramic "mushroom heads" are associated with impingement that leads to a high risk of cup loosening, high wear rates and in vivo fractures. The combination of ceramic "mushroom heads" and cups is not recommended. An evaluation of complications shows that some can be explained by patient behaviour--e.g. Japanese sitting position, horse riding. Designers need to develop new concepts offering a larger range of motion, for example, with head diameters of 32 and 36 mm that reduce the risk of impingement, subluxation and dislocation, while increasing the range of motion. The potential of ceramic/ceramic coupling has been known since the 70s, and ceramic concepts for total hip replacement are currently experiencing a renaissance, although further developments are still possible.     

Ultrastructure of murine trisomy 1 optic cup

The optic cups of two gestational day 11 trisomy (ts) 1 mouse embryos and a normal littermate control were examined using transmission electron microscopy (TEM). One trisomic embryo had a small lens with a lens stalk; the other was aphakic. The resolution available with TEM allowed detailed evaluation of cell organelles, spatial relationships, and the intra- and extracellular structural environment of the optic cup in normal and abnormal mouse embryos. Differences between the normal littermate and the trisomic optic cups, as well as between the two ts 1 structures, included the following: 1) melanin granules in the retinal layer and intraretinal space as well as in the pigment layer, 2) neither pseudostratified nor cuboidal neuroepithelium in trisomic optic cups, 3) increasing cell lysis with severity of eye defect, 4) fusion between retinal and pigment layer cells and cells from the pigment layer and head mesoderm. This investigation not only confirmed some of the abnormal morphology found in light microscopic studies of ts 1 at this gestational age but also identified other anomalies in the ts 1 eye that may play a part in the dysgenesis of this organ. The roles of larger than normal intercellular lacunae, disorganized microtubules, and the connections between different cell types in the ts 1 optic cup require further investigation.     

A parametric study of acetabular cup design variables using finite element analysis and statistical design of experiments.

To isolate the primary variables influencing acetabular cup and interface stresses, we performed an evaluation of cup loading and cup support variables, using a Statistical Design of Experiments (SDOE) approach. We developed three-dimensional finite element (FEM) models of the pelvis and adjacent bone. Cup support variables included fixation mechanism (cemented or noncemented), amount of bone support, and presence of metal backing. Cup loading variables included head size and cup thickness, cup/head friction, and conformity between the cup and head. Interaction between and among variables was determined using SDOE techniques. Of the variables tested, conformity, head size, and backing emerged as significant influences on stresses. Since initially nonconforming surfaces would be expected to wear into conforming surfaces, conformity is not expected to be a clinically significant variable. This indicates that head size should be tightly toleranced during manufacturing, and that small changes in head size can have a disproportionate influence on the stress environment. In addition, attention should be paid to the use of nonmetal backed cups, in limiting cup/bone interface stresses. No combination of secondary variables could compensate for, or override the effect of, the primary variables. Based on the results using the SDOE approach, adaptive FEM models simulating the wear process may be able to limit their parameters to head size and cup backing.     

The influence of different surface treatments on the primary stability of cementless acetabular cups: an in vitro study]

INTRODUCTION: Long-term stability of cementless acetabular cups depends on osseointegration, which requires primary stability of the implant. The aim of this study was to determine the influence of different surface treatments on the primary stability of press-fit acetabular cups. Mechanical lever-out tests were performed to quantify the stability in vitro. MATERIALS AND METHODS: A hemispherical press-fit cup design with a flattened pole was used and different surface modifications were applied: smooth, corundum-blasted, titanium plasma spray, rough titan plasma spray, and titanium plasma spray with a rim. The outer diameter of all cups was kept constant. Polyurethane foam was selected as the test material and cup insertion was performed with a maximal force of 6000 N. The excess length between the cup and the surface of the foam blocks was measured. The maximum lever-out force was measured and the lever-out torque was calculated. RESULTS: The excess length of cups with a smooth surface was significantly less (p<0.001) than for the other cups, with no significant differences among the other surface modifications. The lever-out torque for cups with a smooth surface was significantly less (p<0.001) than for the other cups, with no significant differences among the other surface modifications. CONCLUSION: Only the cup with a smooth surface showed significant differences for excess length and lever-out torque. The other surface modifications exhibited the same stability. As long as a rough surface is chosen, cup design seems to have a greater influence on stability than surface modification. Although the study did not mimic real in vivo conditions and the lever-out-torques cannot be transferred to clinical situations, initial stability before bony ingrowth occurred could be clearly analysed.     

Model-based Roentgen stereophotogrammetry of orthopaedic implants

Attaching tantalum markers to prostheses for Roentgen stereophotogrammetry (RSA) may be difficult and is sometimes even impossible. In this study, a model-based RSA method that avoids the attachment of markers to prostheses is presented and validated. This model-based RSA method uses a triangulated surface model of the implant. A projected contour of this model is calculated and this calculated model contour is matched onto the detected contour of the actual implant in the RSA radiograph. The difference between the two contours is minimized by variation of the position and orientation of the model. When a minimal difference between the contours is found, an optimal position and orientation of the model has been obtained. The method was validated by means of a phantom experiment. Three prosthesis components were used in this experiment: the femoral and tibial component of an Interax total knee prosthesis (Stryker Howmedica Osteonics Corp., Rutherfort, USA) and the femoral component of a Profix total knee prosthesis (Smith & Nephew, Memphis, USA). For the prosthesis components used in this study, the accuracy of the model-based method is lower than the accuracy of traditional RSA. For the Interax femoral and tibial components, significant dimensional tolerances were found that were probably caused by the casting process and manual polishing of the components surfaces. The largest standard deviation for any translation was 0.19mm and for any rotation it was 0.52 degrees. For the Profix femoral component that had no large dimensional tolerances, the largest standard deviation for any translation was 0.22mm and for any rotation it was 0.22 degrees. From this study we may conclude that the accuracy of the current model-based RSA method is sensitive to dimensional tolerances of the implant. Research is now being conducted to make model-based RSA less sensitive to dimensional tolerances and thereby improving its accuracy.     

An optical method for determining deformation and form changes in polyethylene surfaces of explanted acetabular cups of hip endoprostheses]

AIM: Most methods used for the determination of volumetric wear of polyethylene cups are based on the assumption that the head of the prosthesis penetrates the cup in "cylindrical" fashion. The new accurate optical method is independent of this disputable assumption. METHOD: The articulating surface of the cup is scanned with light and a data set of 60,000 pixels obtained in this way is stored in a computer. Data obtained from used cups were compared with those obtained from unused cups. The volumetric wear was calculated directly by threefold integration. To assess the changes in surface shape, the data are fitted by an ellipsoid whose long axis defines the mean direction of load. A total of 18 retrieved and 3 unused cups of different types were studied. RESULTS: The unused acetabular cups deviated only slightly from ideal hemispheres. The surfaces showed rotational symmetry, and an undulation having an amplitude of 0.1 mm between dome and equator. For all explanted cups, the assumption of cylindrical penetration of the head into the polyethylene was shown not to represent the true situation. The cup expands in all directions, and the volumetric wear is underestimated by 50% with the traditional methods. The data suggest that long-term survival may be jeopardized when the main direction of loading is centered on the dome of the cup. Ceramic heads were associated with smaller rates of volumetric wear. CONCLUSION: The new optical method is characterised by short measuring times, precision and simple application. Analysis of the wear patterns of polyethylene components using this technique may contribute to a further understanding of the complex mechanisms of aseptic loosening.     

Epoxy Embedding of Thin-Layer Material in Commercially Available Vinyl Cups for Light and Electron Microscopy

Epoxy embedding of cell cultures, impression smears and settled cell suspensions is performed in small vinyl cups, in which the entire sequence of culture, fixation, dehydration and embedding is feasible. The cup is readily stripped from the polymerized block to allow selected areas to be marked, photomicrographed by light, and thin sections of the selected parts cut for electron microscopy. The vinyl cups were obtained from Fabri-Kal Corp., Kalamazo, Mich. 49001 in sheets of 66 cups, each cup measuring 15 mm in diameter and 10 mm deep.     

Microstructure analysis and wear behavior of titanium cermet femoral head with hard TiC layer

Titanium cermet was successfully synthesized and formed a thin gradient titanium carbide coating on the surface of Ti6Al4V alloy by using a novel sequential carburization under high temperature, while the titanium cermet femoral head was produced. The titanium cermet phase and surface topography were characterized with X-ray diffraction (XRD) and backscattered electron imaging (BSE). And then the wear behavior of titanium cermet femoral head was investigated by using CUMT II artificial joint hip simulator. The surface characterization indicates that carbon effectively diffused into the titanium alloys and formed a hard TiC layer on the Ti6Al4V alloys surface with a micro-porous structure. The artificial hip joint experimental results show that titanium cermet femoral head could not only improve the wear resistance of artificial femoral head, but also decrease the wear of UHMWPE joint cup. In addition, the carburized titanium alloy femoral head could effectively control the UHMWPE debris distribution, and increase the size of UHMWPE debris. All of the results suggest that titanium cermet is a prospective femoral head material in artificial joint.     

Effects of implant design parameters on fluid convection, potentiating third-body debris ingress into the bearing surface during THA impingement/subluxation

Aseptic loosening from polyethylene wear debris is the leading cause of failure for metal-on-polyethylene total hip implants. Third-body debris ingress to the bearing space results in femoral head roughening and acceleration of polyethylene wear. How third-body particles manage to enter the bearing space between the closely conforming articulating surfaces of the joint is not well understood. We hypothesize that one such mechanism is from convective fluid transport during subluxation of the total hip joint. To test this hypothesis, a three-dimensional (3D) computational fluid dynamics (CFD) model was developed and validated, to quantify fluid ingress into the bearing space during a leg-cross subluxation event. The results indicated that extra-articular joint fluid could be drawn nearly to the pole of the cup with even very small separations of the femoral head (<0.60mm). Debris suspended near the equator of the cup at the site of maximum fluid velocity just before the subluxation began could be transported to within 11 degrees from the cup pole. Larger head diameters resulted in increased fluid velocity at all sites around the entrance to the gap compared to smaller head sizes, with fluid velocity being greatest along the anterosuperolateral cup edge, for all head sizes. Fluid pathlines indicated that suspended debris would reach similar angular positions in the bearing space regardless of head size. Increased inset of the femoral head into the acetabular cup resulted both in higher fluid velocity and in transport of third-body debris further into the bearing space.     

A 42-year-old patient presenting with femoral head migration after hemiarthroplasty performed 22 years earlier: a case report

Introduction

Treatment of femoral neck fractures in young adults may require total hip arthroplasty or hip hemiarthroplasty using a bipolar cup. The latter can, however, result in migration of the femoral head and poor long-term results.

Case presentation

We report a case of femoral head migration after hemiarthroplasty performed for femoral neck fracture that had occurred 22 years earlier, when the patient (a Japanese man) was 20 years old. He experienced peri-prosthetic fracture of the femur, subsequent migration of the prosthesis, and a massive bone defect of the pelvic side acetabular roof. After bone union of the femoral shaft fracture, the patient was referred to our hospital for reconstruction of the acetabular roof. Intra-operatively, we placed two alloimplants of bone from around the transplanted femoral head into the weight-bearing region of the acetabular roof using an impaction bone graft method. We then implanted an acetabular roof reinforcement plate and a cemented polyethylene cup in the position of the original acetabular cup. Eighteen months post-operatively, X-rays showed union of the transplanted bone.

Conclusions

Treatment of femoral neck fractures in young adults is usually accomplished by osteosynthesis, but it may be complicated by femoral head avascular necrosis or by infection or osteomyelitis. In such cases, once an infection has subsided, either hip hemiarthroplasty using a bipolar cup or total hip arthroplasty may be required. However, if the acetabular side articular cartilage is damaged, a bipolar cup should not be used. Total hip arthroplasty should be performed to prevent migration of the implant.     

Life cycle assessment and eco-efficiency analysis of drinking cups used at public events

Background, aim, and scope  

At the request of the Public Waste Agency for the Flemish Region, the Flemish Institute for Technological Research performed a life cycle assessment (LCA), according to the International Organization for Standardization (ISO) 14040 procedures (ISO 1997, 1998, 2000, and ISO 2006), followed by an eco-efficiency analysis of four alternative types of drinking cups for use at public events. The results of the LCA study served as input for the eco-efficiency analysis in which the costs related to the four cup systems were studied and combined with the environmental impacts. The objective of this study was to gain insight into the current environmental impacts and costs related to existing systems for drinking cups at public events in Flanders (Belgium) in order to outline a well-founded policy with regard to this subject. Since the results of this comparative study are publicly available, a critical review was performed according to ISO 14040 (review by interested parties, using a review panel) after each stage (goal and scope, data inventory, impact analysis/interpretation, eco-efficiency analysis) during the study.     

Deformation of the Durom Acetabular Component and Its Impact on Tribology in a Cadaveric Model—A Simulator Study

Background

Recent studies have shown that the acetabular component frequently becomes deformed during press-fit insertion. The aim of this study was to explore the deformation of the Durom cup after implantation and to clarify the impact of deformation on wear and ion release of the Durom large head metal-on-metal (MOM) total hips in simulators.

Methods

Six Durom cups impacted into reamed acetabula of fresh cadavers were used as the experimental group and another 6 size-paired intact Durom cups constituted the control group. All 12 Durom MOM total hips were put through a 3 million cycle (MC) wear test in simulators.

Results

The 6 cups in the experimental group were all deformed, with a mean deformation of 41.78±8.86 µm. The average volumetric wear rate in the experimental group and in the control group in the first million cycle was 6.65±0.29 mm³/MC and 0.89±0.04 mm³/MC (t = 48.43, p = 0.000). The ion levels of Cr and Co in the experimental group were also higher than those in the control group before 2.0 MC. However there was no difference in the ion levels between 2.0 and 3.0 MC.

Conclusions

This finding implies that the non-modular acetabular component of Durom total hip prosthesis is likely to become deformed during press-fit insertion, and that the deformation will result in increased volumetric wear and increased ion release.

Clinical Relevance

This study was determined to explore the deformation of the Durom cup after implantation and to clarify the impact of deformation on wear and ion release of the prosthesis. Deformation of the cup after implantation increases the wear of MOM bearings and the resulting ion levels. The clinical use of the Durom large head prosthesis should be with great care.