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.     

Effect of acetabular component anteversion on dislocation mechanisms in total hip arthroplasty

Quantifying soft-tissue tension around the hip joint during total hip arthroplasty remains difficult. In this study, a three-dimensional computer-aided design model was developed to clarify how component position in total hip arthroplasty contributes to the primary cause of posterior dislocation in cases of flexion, adduction and internal rotation. To better understand the influences of anteversion angle of the acetabular component, its effects on the primary causes of dislocations and the range of motion were investigated. Three different primary dislocation mechanisms were noted: impingement of the prosthetic femoral neck on the cup liner; impingement of the osseous femur on the osseous pelvis; and spontaneous dislocation caused by soft-tissue traction without impingement. Spontaneous dislocation could be detected by calculating hip forces at any thigh position using the computer-aided design model developed. In computer analysis, a transition from prosthetic impingement rate to osseous impingement rate occurred with increasing anteversion angle of the acetabular component. Spontaneous dislocation was detected at angles > 10° of anteversion of the acetabular component when flexion occurred with extreme adduction and internal rotation. This study demonstrated the possibility of spontaneous dislocation that results not from prosthetic or bony impingement but from muscle traction with increased range of motion.     

Useability of an image based nagivation system in reconstruction of leg alignment in total knee arthroplasty - results of a propsective study]

Accurate leg alignment is one important factor for long term survival in total knee arthroplasty (TKA). The aim of this study was to determine the accuracy of a CT-based navigation system in restoration of the leg axis. 80 TKA (P.F.C.-Sigma--DePuy) were implanted either using a CT-based navigation-system or the conventional technique. There were no significant differences between the groups according to the preoperative leg deformity. Accuracy of implantation was determined on postoperative long-leg coronal and lateral x-rays. A postoperative leg axis between 3 degrees varus and 3 degrees valgus was obtained in 35/40 patients (88%) in the computer assisted and 28/40 patients (70%) in the conventional operated group. A higher precision in the sagittal plane was seen for the tibial (computer assisted: 3.9 +/- 2.2 vs. conventional TKA 1.3 +/- 5.8) and the femoral component (computer assisted: 3.5 +/- 2.3 vs. conventional TKA 6.9 +/- 3.6). The use of the CT-based VectorVision-System allows a significant improvement in the accuracy of implantation in TKA. The CT-based module has the advantage of a precise preoperative planning and a useful documentation and control tool for each important step. On the other hand there are additional costs and time consuming logistics.     

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.     

Importance of maintaining the basic stress pathway above the acetabular dome during acetabular reconstruction

The basic stress pathway above the acetabular dome is important for the maintenance of implant stability in press-fit acetabular reconstruction of total hip arthroplasty. However, information on the basic stress pathway and its impact factors remains unclear. The objective of this study was to investigate the effects of the orientations and positions of the acetabular component on the basic stress pathway. The basic stress pathway above the acetabular dome was defined as two parts: 3D basic trabecular bone stress distribution and quantified basic cortical bone stress level, using two subject-specific finite element normal hip models. The effects were then analysed by generating 32 reconstructed acetabular cases with different cup abduction and anteversion angles within a range of 35–50° and 10–25°, respectively, and 12 cases with different hip centre heights within a range of 0–15 mm above the acetabular dome. The 3D trabecular stress distribution decreased remarkably in all cases, while the 80% of the basic cortical bone stress level was maintained in cases when the acetabular component was positioned at 10° or 15° anteversion and 40° or 45° abduction angles. The basic stress pathway above the acetabular dome was disturbed when the superior displacement of the hip centre exceeded 5 mm above the anatomical hip centre. Positioning the acetabular component correctly contributes to maintain the stress balance between the acetabular cup and the bone during acetabular reconstruction, thus helping restore the normal hip biomechanics and preserve the stability of the implants.     

The influence of acetabular cup material on pelvis cortex surface strains, measured using digital image correlation

Acetabular cup loosening is a late failure mode of total hip replacements, and peri-prosthetic bone deterioration may promote earlier failure. Preservation of supporting bone quality is a goal for implant design and materials selection, to avoid stress shielding and bone resorption. Advanced polymer composite materials have closer stiffness to bone than metals, ceramics or polymers, and have been hypothesised to promote less adverse bone adaptation. Computer simulations have supported this hypothesis, and the present study aimed to verify this experimentally.A composite hemi-pelvis was implanted with Cobalt Chromium (CoCr), polyethylene (UHMWPE) and MOTIS^®carbon-fibre-reinforced polyether etherketone (CFR-PEEK) acetabular cups. In each case, load was applied to the implanted pelvis and Digital Image Correlation (DIC) was used for surface strain measurement. The test was repeated for an intact hemi-pelvis. Trends in implanted vs. intact bone principal strains were inspected to assess the average principal strain magnitude change, allowing comparison of the potential bone responses to implantation with the three cups.The CFR-PEEK cup was observed to produce the closest bone strain to the intact hip in the main load path, the superior peri-acetabular cortex (+12% on average, R²=0.84), in comparison to CoCr (+40%, R²=0.91) and UHWMPE cups (?26%, R²=0.94). Clinical observations have indicated that increased periacetabular cortex loading may result in reduced polar cancellous bone loading, leading to longer term losses in periprosthetic bone mineral density. This study provides experimental evidence to verify previous computational studies, indicating that cups produced using materials with stiffness closer to cortical bone recreate physiological cortical bone strains more closely and could, therefore, potentially promote less adverse bone adaptation than stiffer press-fitted implants in current use.     

A kinematic model for calculating cup alignment error during total hip arthroplasty

Reduced range of motion, prosthetic impingement, and joint dislocation can all result from misalignment of the acetabular component (i.e. cup alignment) in patients undergoing total hip arthroplasty. Most methods for acetabular component alignment are designed to provide 45-50 degrees abduction and 15-25 degrees of operative anteversion (also known as flexion) with respect to the anterior pelvic plane coordinate system. Yet in most cases, this coordinate system is not assigned properly, due to differences in patient anatomy and improper positioning in the operating room. This misalignment can result in an error in the cup alignment, which can cause the above-mentioned consequences. This work presents a complete mathematical formulation for the analysis of the inaccuracies related to the anterior pelvic plane axes (APPA) definition and their effect on final cup orientation. We do this by introducing a method taken from Kinematics of Mechanisms, and by representing the errors in the APPA as three concurrent axes of rotation, followed by the version and abduction rotations which are defined relative to the previous rotations. We also present a sensitivity analysis of the results by introducing differential changes between sequential coordinate frames, which simulates the errors in the APPA and their effect on cup orientation. Finally, we demonstrate a computational method which provides corrected version and abduction angles to achieve the desired cup orientation, given that the actual measurement errors are known.     

Primary stability of cement free hip cup prostheses--morphometric comparative analysis of two cup types]

An uncemented implanted acetabular cup must ensure primary stability and postoperative osteointegration. For both conditions the immediate postoperative contact between bone and the structured surface of the cup is of decisive importance. Two different types of acetabular cups manufactured by ESKA (standard model and Kapuziner model) were implanted in the innominate bones of elderly deceased persons. After plastination, thin serial sections were obtained. On the basis of X-rays of these sections, contact between bone and the reticulate surface of the cups was quantified. The quality of the contact was analysed densitometrically.     

Design, construction and modularity of pressure-fit acetabular cups]

To enable a comparison of different pressfit acetabular cups objective criteria are essential. The aim of this study is to describe the design features of this type of cup and to analyse currently available cups. 30 implants were systematically measured and analysed. The mean surface roughness (Ra) was determined and configurations established with the light section technique. For further evaluation the cups were transversely sectioned. The cups are made of pure titanium, titanium alloy or polyethylene coated with titanium. Five implants take the form of monoblocks. The configuration is predominately (n = 25) flattened spherical. The size of eight cups corresponds to the outer diameter, 19 cups have a larger outer diameter (overdimensioning), 3 cups have a smaller outer diameter (underdimensioning). The magnitude of overdimensioning is, on average, 1.9%. 9 cups are provided with plugs, hollow cylinders, fins or rings as outer stabilizers. Surface roughness achieved with corundum blasting is 6.8 microns. Titanium porous-coated implants have a surface roughness of 21-32 microns. 24 cups have polyethylene inserts, most of which are snap-fixed with equatorial lips. For 16 cups, full-ceramic inserts are available. 4 cups have a metal insert. Titanium implants with structured or HAC-coated surfaces have become the accepted standard for cementless acetabular cup implantation. Together with ceramic, metal, or modified polyethylene inserts they meet the requirement for permanent osteo-integrative stability.     

A novel acetabular alignment guide for THR using selective anatomic landmarks on the pelvis

This paper presents a novel approach for acetabular alignment during the implant of a prosthetic hip joint in a natural pelvis. The alignment instrument uses selective anatomic bony landmarks on the pelvis, which are accessible in surgery, to guide the placement of the acetabular component in the appropriate orientation. A closed form solution, involving both a forward and reverse analysis, is presented to relate the parameters of the device with the abduction and anteversion angles. Using mathematical models, this device should allow the surgeon to place the acetabular component with an orientation between 10.9 degrees and 19.1 degrees anteversion and 35.7 degrees and 44.3 degrees abduction with 95% confidence in a male/left specimen for the commonly accepted target of 15 degrees anteversion and 40 degrees abduction. This device is currently being used successfully by one of the authors in THR surgery.     

The influence of friction and interference on the seating of a hemispherical press-fit cup: a finite element investigation.

The formation of gaps in the polar region of acetabular cups is seen as a drawback of press-fit fixation of non-cemented acetabular cups. Recent findings indicate a link between long-term polar gaps and the gaps present directly after implantation. In this study the process of press-fitting is simulated with a linear-elastic two-dimensional axisymmetric finite-element model. The aim of this paper is to investigate the possible importance of friction and interference on the formation of these gaps. A range of cup-bone friction coefficients (mu = 0.1-0.5) is assigned to the cup-bone interface in order to represent the unknown amount of friction occurring during press-fitting. The cup is modeled with a radius of 27 mm, whereas the radius of the cavity is varied between 26.50 and 26.75 mm, thus, creating 0.50 and 0.25 mm radial interference fits. The difference in cavity radius represents the discrepancy between the radius of the last-reamer-used and radius of the cavity it creates. The subchondral plate is considered as being completely removed during reaming. The effects of impact blows via the surgeon's mallet during surgery are modeled as a series of four load pulses, in which peak force is gradually increased from 0.5 to 4.0 kN. The effects of load removal as well as those of load application are investigated. On load application, the cup penetrates into the cavity, and on load removal, the cup rebounds. Depending on the friction, interference and load applied, the position of the cup after the load pulse is somewhere between its position at peak force and its position at the beginning of the pulse. Although the simplifications and conditions involved in the creation of the model necessitate caution when interpreting the results for all clinical cases, it is found that the seating of hemispherical cups in trabecular bone could be more satisfactory for intermediate values of friction (mu = 0.2-0.3) and smaller interference fits (0.25 mm).     

Simulator for migration processes and wear in the artificial hip acetabulum for radiologic measurement]

For assessing migration of cups, standard X-rays or stereo radiological images (SRI) are available. In addition, software is also available for measurements. The accuracies of the various systems are established statistically, in part combined with phantoms, and compared. To date, no known phantom is available for the simulation of acetabular cup migration with account being taken of the position of the pelvis in the X-ray beam. Such an appliance covering 8 different parameters has now been developed, the cup can be moved horizontally, vertically and in the loading direction. Angular accuracy is +/- 0.5 degree, and wear of a magnitude of 0.25 mm can be simulated. Two degree elevation of the pelvis, left or right, can be simulated. The position of the pelvis around the horizontal axis permits continuous variation. This appliance can simulate migratory movements of the acetabular cup within a pelvis, and wear within the cup. In addition, the spatial position of the pelvis can be varied. The X-ray images can be used to investigate the accuracy of evaluation strategies.     

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.     

The effect of geometry and abduction angle on the stresses in cemented UHMWPE acetabular cups – finite element simulations and experimental tests

Background  

Contact pressure of UHMWPE acetabular cup has been shown to correlate with wear in total hip replacement (THR). The aim of the present study was to test the hypotheses that the cup geometry, abduction angle, thickness and clearance can modify the stresses in cemented polyethylene cups.     

In-vivo 3-Dimensional gait symmetry analysis in patients with bilateral total hip arthroplasty

Although three-dimensional (3D) asymmetry has been reported in unilateral THA patients during gait, it is not well understood whether asymmetric hip kinematics during gait persist in bilaterally operated THA patients. The purpose of this study was to compare the in vivo 3D kinematics and component placement between bilateral and unilateral THA patients during gait. Eight bilateral and thirty-three unilateral THA patients were evaluated for both hips during treadmill gait using a validated combination of 3D computer tomography-based modeling and dual fluoroscopic imaging system (DFIS). The in vivo 3D kinematics of the unilateral THA group was first assessed. The magnitudes of kinematics and component placement difference between implanted hips in the bilateral THA group and between the implanted and non-implanted hips in the unilateral THA group were compared. The study results showed asymmetric gait kinematics in the unilateral THA group. Although the magnitude of kinematics differences between sides for both the bilateral and unilateral THA groups did not change significantly for hip rotations (p > 0.05), the bilaterally operated THA group has significantly lower magnitude of hip gait translation difference. Significant reduction in the magnitude of the acetabular cup adduction, stem adduction, and combine hip anteversion and adduction difference was observed in the bilateral THA group (p < 0.05). Our findings demonstrated that despite significant improvements of component placement and reduced magnitude of hip gait translation difference between implanted hips in the bilateral THA group, asymmetric hip kinematic rotations persisted in patients with bilateral THA during gait.     

The safe-zones for combined cup and neck anteversions that fulfill the essential range of motion and their optimum combination in total hip replacements

Reduction of the range of motion (ROM) until prosthetic impingement of a total hip replacement may lead to frequent impingement, subluxation and dislocation especially for patients with good hip movement. The ROM until prosthetic impingement can be calculated using the technical ROM (theta) and the cup and neck positions by a previously created mathematical formula. A larger (theta) with proper cup and neck positions results in a larger ROM. However there was only one paper written in English, which revealed the optimum theoretical combination of cup and neck anteversions. ROM of more than 110 degrees flexion, 30 degrees internal-rotation at 90 degrees flexion, 30 degrees extension and 40 degrees external-rotation were defined as the criteria for essential ROM for ADL. The safe-zones for combined cup anteversion (betaanat) and neck anteversion (b) were defined as the areas that fulfill all the criteria of ROM without prosthetic impingement. The safe-zones were created for 35 degrees , 45 degrees and 55 degrees cup abductions (alpha) and for 120 degrees and 135 degrees (theta). The safe-zones for combined (betaanat) and (b) were much larger for a 135 degrees (theta) than a 120 degrees (theta). Their safe-zones showed that (b) should be reduced if (betaanat) is increased and choosing a lower (alpha) requires that the sum of (betaanat) and (b) should be higher and vice versa. A (theta) of more than 135 degrees is recommended as it further increases the size of the safe-zone and provides a larger ROM, and the optimum values of combined cup and neck anteversions can be estimated by the formula: (alpha) + (betaanat) + 0.77(b) = 84.3.     

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.     

Initial stability of press-fit acetabular cups--an in-vitro study]

AIM OF THE STUDY: Mechanical lever-out tests were performed in vitro to investigate the initial stability of press fit acetabular cups. METHOD: Five different uncemented, hemispherical press-fit cups were implanted in a standardized manner into Sawbones, Polyurethane foam blocks. Each cup was levered-out by using a 250 mm stainless steel rod, which was connected to the acetabular cup. Loads were then applied to the rod causing the cup to be diplaced. Lever-out forces were recorded by a computer. RESULTS: The results of the lever-out forces ranged between 39,2 and 50,8 Nm. The highest initial stability was achieved by two Titanium cups with a Titanium plasmaspray coating, a flattened pole and a sharp equatorial edge. According to our trials the equatorial rim of the polyurethane cavity is the crucial area for the implant's initial stabilty. There the highest amount of attrition was observed. CONCLUSION: To guarantee a high reproducibilty of the tests it is essential to pay particular attention to the quality of the polyurethane foam blocks, to the exactness of the reaming procedure and to a defined cup insertion. However as our trials were carried out under optimized labaratory conditions one must be careful not to over-interpret its results. Intraoperatively primary stability is also influenced by the quality of the bone.     

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.