Implant-cement Interface Research Articles

Bone remodeling in the resurfaced femoral head: Effect of cement mantle thickness and interface characteristics

Metal-on-metal hip resurfacing prostheses were re-introduced during the last 10–15 years. These prostheses have the potential to better restore normal function with limited activity restriction, being an option for younger and more active patients. Resurfacing procedures have demonstrated high failure rates in national registers [1,2]. Multiple factors may affect early and long-term HR performance. The influence of femoral cement mantle thickness and different interface characteristics between the prosthesis components on the long-term performance of resurfacing prostheses is still unknown. In the present work, a model was used to predict bone remodeling with different mantle thicknesses and interface characteristics. A very thin cement mantle (0.25mm) increased bone resorption at the superior femoral head, while greater thickness (1 or 3mm) had a lesser effect. In all cases, bone apposition was predicted around the stem and at the stem tip. Bone formation and resorption were observed clinically in good agreement with the predictions calculated in simulations. Computed results showed that 1-mm cement mantle thickness combined with a bonded bone–cement interface and a debonded implant–cement interface was an appropriate configuration. Bone remodeling results and computed equivalent strains were correlated. In conclusion, we have been able to demonstrate the importance of choosing an adequate cement mantle thickness. Additionally, computational studies should consider realistic interface characteristics between the components in order to perform simulations closer to reality.

Cement-Implant Interface Contamination: Possible Reason of Inferior Clinical Outcomes for Rough Surface Cemented Stems

Background:Shape-closed cemented implants rely on a stronger bond and have displayed inferior clinical outcomes when compared to force-closed designs. Implant contamination such as saline, bone marrow and blood prior to cement application has the potential to affect the cement-implant bond. The consequences of implant contamination were investigated in this study.Methods:Fifty Titanium alloy (Ti-6Al-4V) dowels were separated into ten groups based on surface roughness and contaminant, and then cemented in polyvinyl chloride tubes. Push-out testing was performed at 1mm per minute. The roughness of the dowel surface was measured before and after the testing. The dowel surface and cement mantel were analyzed using a Scanning Electron Microscopy (SEM) to determine the distribution and characteristics of any debris and contaminants on the surface.Results:Contaminants largely decreased stem-cement interfacial shear strength, especially for rough surfaces. Saline produced the greatest decrease, followed by blood. The effect of bone marrow was less pronounced and similar to that of oil. Increasing surface roughness increased the interfacial bonding strength, even with contaminants. There was a non-significant increase in mean bonding strength for smooth surfaces with bone marrow and oil contamination. SEM showed that contaminants influence the interfacial bond by different mechanisms. More debris was found on rough samples following testing.Conclusions:The results of this study underscore the importance of keeping an implant free from contamination, and suggest if contamination does occur, a saline rinse may further decrease the stability of an implant. The deleterious effects of contamination on rough surface cement bonding were considerable, and indicate that contamination at the time of surgery may, in part, contribute to inferior clinical outcomes for rough surfaced cemented stems.

The Effect of Bone Lavage on Femoral Cement Penetration and Interface Temperature During Oxford Unicompartmental Knee Arthroplasty with Cement

Aseptic loosening is the most common cause for revision unicompartmental knee arthroplasty and is associated with failure of the bone-cement or cement-implant interface. The purpose of the present study was to analyze different bone lavage techniques for the bone-cement and cement-implant interfaces of the femoral component and to study the effect of these techniques on cement penetration and on interface temperature. In an experimental cadaver study, Oxford unicompartmental knee arthroplasty was performed in twenty-four matched-paired knees to study the effect of pulsed lavage compared with syringe lavage on femoral cement penetration and interface temperature. Interface temperature, cement penetration pressure, and ligament tension forces were measured continuously during the procedure, and cement penetration was determined by performing sagittal bone cuts. Cleansing the femoral bone stock with use of pulsed lavage (Group B) led to increased femoral cement penetration (mean, 1428 mm²; 95% confidence interval, 1348 to 1508 mm²) compared with syringe lavage (Group A) (mean, 1128 mm²; 95% confidence interval, 1038 to 1219 mm²) (p < 0.001). Interface temperature was higher in Group B (mean 22.6°C; 95% confidence interval, 20.5°C to 24.1°C) than in Group A (mean, 21.0°C; 95% confidence interval, 19.4°C to 23.0°C) (p = 0.028), but temperatures never reached critical values for thermal damage to the bone. Pulsed lavage leads to an increased femoral cement penetration without the risk of heat necrosis at the bone-cement interface.

Influence of time in-situ and implant type on fixation strength of cemented tibial trays — A post mortem retrieval analysis

BackgroundLoosening of the tibial tray is cited as the most common cause of failure in total knee arthroplasty but the mechanism remains unclear. Post mortem specimens provide a unique opportunity to investigate the clinical condition. MethodsTwenty two cemented components were serially retrieved in situ at autopsy from a university clinic. They were investigated for mechanical stability by pull-out, which was related to cement morphology and bone quality from CT scans, and to polyethylene wear by score analysis. Implants were grouped into three types: a particular fixed bearing design (n=8), a particular rotating platform design (n=5) and other mixed designs (n=9). FindingsTrends were observed for pull-out force to decrease with time in situ and increase with cement penetration but was unrelated to bone density or polyethylene wear. For the fixed bearing implants decreasing pull-out strength was related to an increasing proportion of failure at the bone–cement interface. For the mixed designs the opposite was observed. The rotating platform implants failed at the implant–cement interface. InterpretationThe analysis demonstrated that interface failure is dependent on the implant design, but that both the stem and the bone interfaces weaken with time in situ. Published findings for laboratory implantations have demonstrated that greater cement penetration improves fixation and this was reflected for clinical samples in this study.

Aseptic Tibial Debonding as a Cause of Early Failure in a Modern Total Knee Arthroplasty Design

We observed isolated tibial component debonding from the cement in one modern primary TKA design (NexGen LPS 3° tibial tray; Zimmer, Warsaw, IN, USA). This failure mechanism is sparsely reported in the literature. We (1) assessed survivorship of this tibial tray with special emphasis on debonding; (2) described clinical and radiographic features associated with tibial failure; and (3) compared patient and radiographic features of the failures with a matched cohort. A total of 1337 primary TKAs were performed with a cemented NexGen LPS 3° tibial tray over an 11-year period. Twenty-five knees (1.9%) were revised for tibial debonding. BMI and radiographic alignment in the tibial debonding group were compared with a matched control group. Implant survivorship was assessed using tibial debonding as the end point. Survival free of revision from tibial debonding was 100% at 1 year and 97.8% at 5 years. The tibial failures shared a typical radiographic pattern with debonding at the cement-implant interface and subsidence into varus and flexion. We found no link between limb alignment or individual component alignment and failure because 22 of the 25 failures occurred in well-aligned knees. Our standardized followup of patients undergoing TKA at routine intervals allowed us to discover a higher rate of revision resulting from tibial debonding. We have discontinued the use of this particular tibial tray for primary TKA and surveillance for patients undergoing TKA continues to be warranted.

A probabilistic modelling scheme for analysis of long-term failure of cemented femoral joint replacements

Reliable prediction of long-term medical device performance using computer simulation requires consideration of variability in surgical procedure, as well as patient-specific factors. However, even deterministic simulation of long-term failure processes for such devices is time and resource consuming so that including variability can lead to excessive time to achieve useful predictions. This study investigates the use of an accelerated probabilistic framework for predicting the likely performance envelope of a device and applies it to femoral prosthesis loosening in cemented hip arthroplasty. A creep and fatigue damage failure model for bone cement, in conjunction with an interfacial fatigue model for the implant-cement interface, was used to simulate loosening of a prosthesis within a cement mantle. A deterministic set of trial simulations was used to account for variability of a set of surgical and patient factors, and a response surface method was used to perform and accelerate a Monte Carlo simulation to achieve an estimate of the likely range of prosthesis loosening. The proposed framework was used to conceptually investigate the influence of prosthesis selection and surgical placement on prosthesis migration. Results demonstrate that the response surface method is capable of dramatically reducing the time to achieve convergence in mean and variance of predicted response variables. A critical requirement for realistic predictions is the size and quality of the initial training dataset used to generate the response surface and further work is required to determine the recommendations for a minimum number of initial trials. Results of this conceptual application predicted that loosening was sensitive to the implant size and femoral width. Furthermore, different rankings of implant performance were predicted when only individual simulations (e.g. an average condition) were used to rank implants, compared with when stochastic simulations were used. In conclusion, the proposed framework provides a viable approach to predicting realistic ranges of loosening behaviour for orthopaedic implants in reduced timeframes compared with conventional Monte Carlo simulations.

A Model-Free Feature-Based Bi-Planar RSA Method for Kinematic Analysis of Total Knee Arthroplasty

Fluoroscopic imaging is commonly used for assessing relative motions of orthopaedic implants. One limiting factor to in vivo model-based roentgen stereophotogrammetric analysis of total knee arthroplasty is the need for 3D models of the implants.The 3D models of the implant components must be reverse-engineered, if not provided by the company, which makes this method impractical for a clinical study involving many types or sizes of implants. This study introduces a novel feature-based methodology that registers the features at the implant-bone or implant-cement interface of the components that have elementary shapes. These features include pegs with hemispherical heads, and straight, circular or curved edges located on flat faces of the box of the femoral component or the stem geometry of the tibial component. Software was developed to allow easy registration of these features through a graphical user interface. The accuracy and precision of registration for multiple flexion angles from 0 to 120 deg was determined with reference to registered poses of the implants through experiments on bone replica models and also on a cadaver specimen implanted with total knee prostheses. When compared to an equivalent bi-planar model-based registration, the results were comparable: The mean accuracy of this feature-based method was 1.45 deg and 1.03 mm (in comparison to 0.95 deg and 1.32 mm for the model-based approach), and the mean precision was 0.57 deg and 0.26 mm (in comparison to 0.42 deg and 0.44 mm for the model-based approach).The methodology and the developed software can easily accommodate different design of implants with various fixation features. This method can facilitate in vivo kinematic analysis of total knee arthroplasty by eliminating the need for 3D models of the implant components.

A Device to Control Implant and Bone-Cement Temperatures in Cemented Arthroplasty

At present, most of the orthopaedic implants used in articular reconstruction are fixed to host bone using acrylic bone-cement. Bone-cement polymerization leads to an exothermic reaction with heat release and consequent temperature rise. The increase of temperature in the bone beyond the tolerated limits can develop osteocyte thermal necrosis and ultimately lead to bone resorption at the cement-bone interface, with subsequent loosening of the implant. Another issue that plays an important role in implant loosening is debonding of the cement from the implant initiated by crack formation at the interfacial voids. It is well established that low porosity enables better fatigue cement properties. Moderate preheating of the implant is expected to reverse the direction of polymerization, and has the ability to reduce interfacial void formation and improve interfacial shear strength. To increase the implant temperature at the initial cementing phase in order to reduce interfacial void formation, and subsequently, cool the implant in the latter cement polymerization phase to prevent the possibility of bone thermal necrosis, a new automated electronic device was designed to be use in cemented joint replacements. The developed device was specifically designed for the knee arthroplasty, namely for tibial-tray cementing. The device controls the heat flux direction between the tibial-tray and the atmosphere through the “Peltier effect,” using Peltier tablets. The device is placed on the tibial-tray during the cementing phase and starts to heat it in a first phase, promoting the polymerization that initiates at the warmer cement-implant interface. In a second phase, the heat flux in the Peltier tablets is inverted to extract the heat generated during cement polymerization. The device efficiency was evaluated by cementing several tibial-trays in bovine fresh bone and measuring the tray and cement temperatures. The temperature results in the implant and in the cement showed that the device increases and maintains the implant temperature above room temperature at the initial cementing phase, while in the subsequent phase it cools the tibial-tray and cement. Significant differences were found for peak cement temperatures between the tests performed with and without the device. The device showed its capacity to promote the beginning of cement polymerization at the implant interface contributing towards improving interfacial shear strength and in reducing the peak cement temperature in the subsequent polymerization process, thus contributing to the prevention of the bone thermal necrosis effect.

Aseptic loosening of the patellar component at the cement–implant interface

We present four cases of aseptic loosening at the implant-cement interface following patellar resurfacing. All patients initially had good results, but then presented with onset of a new anterior knee pain. The radiographs including flexed lateral and skyline view of the knee were normal in all the cases. After carefully ruling out infection, aseptic loosening at the cement–implant interface was diagnosed on further investigation. Aseptic loosening of the patellar button at the implant-cement interface can be difficult to diagnose with standard knee radiographs. During flexed lateral radiograph of the knee and the skyline view radiograph of the patellofemoral joint, the patella is compressed on the femur and thereby reducing the loose patellar button. This phenomenon has not been previously described.Patients presenting with new onset of knee pain after an initial good results following patellar resurfacing require further investigation to exclude loosening at the cement–implant interface as plain radiographs can be misleading.

Does high‐flexion total knee arthroplasty promote early loosening of the femoral component?

High-flexion knee replacements have been developed to accommodate a large range of motion (RoM > 120°). Knee implants that allow for higher flexion may be more sensitive to femoral loosening as the knee load is relatively high during deep knee flexion, which could result in an increased failure potential at the implant-cement interface of the femoral component. A 3D finite element knee model was developed including a posterior-stabilized high-flexion knee replacement to analyze the stress state at the femoral implant-cement interface during a full squatting movement (RoM ≤ 155°). During deep flexion (RoM > 120°), tensile and shear stress concentrations were found at the implant-cement interface beneath the proximal part of the anterior flange. Particularly, the shear stresses at this interface location increased during high flexion, from a peak stress of 4.03 MPa at 90° to 6.89 MPa at 140° of flexion. Tensile stresses were substantially lower, having a peak stress of 0.72 MPa at 100° of flexion. Using data from earlier interface strength experiments, none of the interface beneath the anterior flange was predicted to fail in the normal flexion range (RoM ≤ 120°), whereas the prediction increased to 2.2% of the interface during deeper knee flexion. Thigh-calf contact reduced the knee forces, interface load, and failure risk beyond 140-145° of flexion. Based on the more critical stresses at the femoral fixation site between 120° and 145° of flexion, we conclude that the femoral component has a higher risk of loosening at high-flexion angles.

Mixed-mode failure strength of implant–cement interface specimens with varying surface roughness

Aseptic loosening at the implant–cement interface is a well-documented cause of failure in joint arthroplasty. Traditionally, the strength of the implant–cement interface is determined using uni-axial normal and shear loading tests. However, during functional loading, the implant fixation sites are loaded under more complex stress conditions. For this purpose, the strength of the implant–cement interface under mixed-mode tensile and shear loading conditions was determined in this study using interface specimens with varying interface roughness. For the lowest roughness value analyzed (Ra=0.89μm), the interface strength was 0.40–1.95MPa at loading angles varying between pure tension and shear, whereas this was 4.90–9.90MPa for the highest roughness value (Ra=2.76μm). The interface strength during pure shear (1.95–9.90MPa) was substantially higher than during pure tension (0.58–6.67MPa). Polynomial regression was used to fit a second-order interpolation function through the experimental interface strength data (R2=0.85; p<0.001), relating the interface strength (S [MPa]) to the interface loading angle (α [degrees]) and interface roughness (Ra [μm]): S(α,Ra)=0.891Ra2+0.001α2−0.189Ra−0.064α−0.060.Finally, an interface failure criterion was derived from the interface strength measurements, describing the risk of failure at the implant–cement interface when subjected to a certain tensile and shear stress using only the interface strength in pure tensile and shear direction. The findings presented in this paper can be used in numerical models to simulate loosening at the implant–cement interface.

Failure mechanism of the all-polyethylene glenoid implant

Fixation failure of glenoid components is the main cause of unsuccessful total shoulder arthroplasties. The characteristics of these failures are still not well understood, hence, attempts at improving the implant fixation are somewhat blind and the failure rate remains high. This lack of understanding is largely due to the fundamental problem that direct observations of failure are impossible as the fixation is inherently embedded within the bone.Twenty custom made implants, reflecting various common fixation designs, and a specimen set-up was prepared to enable direct observation of failure when the specimens were exposed to cyclic superior loads during laboratory experiments. Finite element analyses of the laboratory tests were also carried out to explain the observed failure scenarios.All implants, irrespective of the particular fixation design, failed at the implant–cement interface and failure initiated at the inferior part of the component fixation. Finite element analyses indicated that this failure scenario was caused by a weak and brittle implant–cement interface and tensile stresses in the inferior region possibly worsened by a stress raiser effect at the inferior rim.The results of this study indicate that glenoid failure can be delayed or prevented by improving the implant/cement interface strength. Also any design features that reduce the geometrical stress raiser and the inferior tensile stresses in general should delay implant loosening.

Glenoid loosening after total shoulder arthroplasty: An in vitro CT‐scan study

Glenoid fixation failure has only been grossly characterized. This lack of information hinders attempts to improve fixation because of a lack of methodologies for detecting and monitoring fixation failure. Our goal was twofold: to collect detailed data of glenoid fixation fracture, and to investigate computed tomography (CT)-scanning as a tool for investigations of fixation failure. Six cadaver scapulas and six bone-substitute specimens were cyclically loaded and CT-scanned at clinical settings after 0, 1,000, 5,000, 10,000, 30,000, 50,000 and 70,000 load cycles. The fixation status was evaluated by inspection of the scans. After 70,000 cycles, the specimens were sectioned, and the fixation inspected by microscopy. The results of the microscopy analysis were compared to the CT-scan analysis. Fracture of the glenoid fixation initiated at the edge of the glenoid rim and propagated towards and around the keel of the implant. The entire process from initiation to complete fracture took place at the polyethylene implant-cement interface, while the cement, the adjacent bone, and the cement-bone interface remained intact. Thus, strengthening the polyethylene-cement interface should improve glenoid fixation. Microscopy results validated the CT methodology, suggesting that the CT technique is reliable.

A numerical study of failure mechanisms in the cemented resurfaced femur: Effects of interface characteristics and bone remodelling

Failure mechanisms of the resurfaced femoral head include femoral neck fracture in the short-term and stress shielding and implant loosening in the long-term. In this study, finite element simulations of the resurfaced femur considering a debonded implant-cement interface, variable stem-bone interface conditions, and bone remodelling were used to study load transfer within the resurfaced femur and to investigate its relationship with known failure mechanisms. Realistic three-dimensional finite element models of an intact and resurfaced femur were used. Various conditions at the interface between the stem of the prosthesis and the bone were considered. Loading conditions included normal walking and stair climbing. For all stem-bone contact conditions, the tensile stresses in the cement mantle varied between 1 MPa and 5.4 MPa, except near the distal rim of the resurfacing component where they reached 5.4-7MPa. In the case of full stem-bone contact, high von Mises stresses (114-121MPa) were generated in the implant at the stem-cup junction. These stresses were considerably reduced (maximum von Mises stress, 76 MPa) where a gap was present at the stem-bone interface. Resurfacing led to strain shielding of the bone of the femoral head (20-75 per cent strain reductions) and periprosthetic bone resorption (50-80 per cent bone density reductions) for all interface stem-bone contact conditions. In the lateral femoral head and the proximal femoral shaft around the trochantric region, bone density reductions varied between 10 per cent and 50 per cent. Bone apposition was observed in the inferior-medial part of the femoral head and proximal femoral neck region. For full stem-bone contact, more load was transferred through the stem to the surrounding bone, exacerbating strain shielding. Although femoral hip resurfacing conserves bone stock at the primary operation, strain shielding and periprosthetic bone resorption might lead to eventual loosening over time. Post-operatively, the resurfacing procedure generated elevated strains (0.50-0.75 per cent strain) in the proximal femoral neck-component junction irrespective of the variation in interface conditions, indicating an initial risk of femoral neck fracture. Subsequent to bone remodelling, this strain concentration was considerably reduced (0.35-0.50 per cent strain), lowering the risk of neck fracture. In order to reduce the potential risk of neck fracture, patients should avoid activities which might induce high loading of the hip during the early post-operative period to allow the bone around the proximal femoral neck to remodel and heal.

Role of Surgical Position on Interface Stress and Initial Bone Remodeling Stimulus around Hip Resurfacing Arthroplasty

Valgus alignment of femoral resurfacing components has been advocated to reduce proximal femur loading and thus minimize the risk for femoral neck fractures. However, such reduction in loading may exacerbate undesirable stress shielding. This study examined the effect of extreme implant orientations (±15°) and stem canal overreaming on initial bone remodeling stimulus using finite element models. The changes in implant-cement interface stresses due to implant alignment were also evaluated. The valgus model showed increased initial bone resorption stimulus, which extended distally and peripherally around the femoral neck. The peak implant-cement interface shear stress for the varus model was 10.9 MPa, exceeding the interface shear strength. Overreaming of the stem canal eliminated distal tip loading, but proximal stress shielding was still unavoidable. These data show bone loading and interface fixation trends emanating from valgus and varus implant positions that will be of interest to practicing physicians.

Optimizing the bone cement-implant interface by hydrolysis-resistant conditioning of the metal surface

The hydrolytic degradation of the implant-cement interface has to be seen as the main reason for aseptic loosening of cemented total hip replacements. Therefore, a new method of conditioning the metallic surface was developed in order to achieve a hydrolytic-resistant bound stability between the implant and bone cement. Preliminary experimental data on test bodies are presented here. The metallic surface of 6 pairs of cylindrical test bodies each (CoCr-alloy, circular testing surface with O 6 mm) were conditioned by the method of silicoating/silanisation to gain a covalent coupling with the applied bone cement. In order to examine the initial stability and the hydrolytic resistance of the metal-cement compound, these pairs of surface-conditioned test bodies (SCT) as well as a reference series of surface-unconditioned test bodies (SUT) were immersed for 0, 30, 90, 150 days (d) in moisture environment (physiological saline solution, 37 degrees C) after coupling with bone cement. The adhesive strength of the test bodies-(bone cement-compounds) were determined by tensile tests on an universal testing machine (Typ Z030, Zwick, Ulm) with gimbal suspension. At time 0 d (that was without immersion of the test bodies) the mean maximum tensile bond strength of the SCT-cement-compounds was 39.5 MPa (SD +/- 4.7 MPa) and that of the SUT-cement-compounds 37.1 MPa (SD +/- 7.3 MPa) (p = 0.575). After immersion the tensile bond strength of the SUT-cement-compounds significantly decreased to an average of 13.5 MPa (SD +/- 2.7 MPa) (30 d), 10 MPa (SD +/- 1.7 MPa) (90 d) and 12.3 MPa (SD +/- 1.4 MPa) (150 d) (p < 0.01). In contrast, the SCT-cement-compounds showed a nearly unchanged high mechanical stability with tensile bond strength values of 37.0 MPa (SD +/- 4.9 MPa) after 30 d, 36.1 MPa (SD +/- 5.0 MPa) after 90 d und 30.2 MPa (SD +/- 4.7 MPa) after 150 d (p > 0.01). With reservation as to further in vitro and in vivo investigations the increased hydrolytic stability of the metal-cement-bound of surface-conditioned CoCr-alloy test bodies promises an improvement of the long-term stability of cement total joint replacements.

Assessment of radiolucent lines in cemented shoulder hemi-arthroplasties: study of concordance and reproducibility

We report on the concordance and reproducibility of the evaluation of radiolucent lines in the humeral component of shoulder arthroplasty. Thirty-two shoulder prostheses were assessed independently, on two occasions, by five observers. The level of inter- and intra-observer agreement was calculated using the kappa statistic. Intra-observer agreement: the overall kappa values ranged from 0 to 0.6, meaning poor, fair and moderate agreement levels. Inter-observer agreement: when the anteroposterior (AP) views were analysed, the values obtained for the bone-cement interface ranged from 0.290 to 0.539, meaning a poor-to-moderate agreement. For the cement-implant interface, the values ranged from 0.064 to 0.684, meaning a poor-to-good agreement. When radiolucent lines of the humeral component were analysed, inter-observer agreement proved to be as low as that obtained when total hip or knee components were analysed. Intra-observer agreement showed better results.

Metallic Shedding, Surface Finish Changes, and Extensive Femoral Osteolysis in the Loose Spectron EF Stem

The addition of a proximal rough surface finish in the satin finish Spectron stem (Spectron EF) reportedly does not alter its clinical performance. However, we have revised 15 Spectron EF stems because of aseptic loosening and extensive femoral osteolysis at early and intermediate-term followup, raising a question as to its safety. We sought to determine which interface has aseptic failure, the presence of massive femoral osteolysis, and the consequences of loosening at the implant-cement interface in the implant's surface finish. Fifteen patients with aseptic loosening of a proximal rough surface-finished Spectron EF stem at intermediate-term followup (Group A) were compared with seven patients having revisions of the same stem for infection or recurrent dislocation but without loosening (Group B). Radiographs for Group A showed debonding at the cement-implant interface and stem subsidence in 13 hips, and severe femoral osteolysis in seven, including two pathologic periprosthetic fractures. During revision surgery, all 13 subsided stems showed a metallic stained pseudomembrane and metallic shedding with polishing of the stem surface. None of the patients in Group B who had revision surgery had these changes. Aseptic failure of the Spectron EF stem was characterized by debonding, subsidence, and metallic shedding with concomitant massive femoral osteolysis and metallosis, which is similar to conditions reported for loose, rough cemented stems, but has not been reported for the satin finish Spectron stem. Therapeutic study, Level III (retrospective comparative study). See the Guidelines for Authors for a complete description of levels of evidence.

The effect of three-dimensional shape optimization on the probabilistic response of a cemented femoral hip prosthesis

Probabilistic analyses allow the effect of uncertainty in system parameters on predicted model performance measures to be determined. Furthermore, using performance functions to describe a failure event, the probability of failure can be quantified. The effect of three-dimensional prosthesis shape optimization on the probabilistic response and failure probability of a cemented hip prosthesis system is investigated. Random variables include joint and muscle loading, cortical and cancellous bone and PMMA bone cement elastic properties, and strength parameters describing failure of the bone cement and the prosthesis–bone cement interface. Several performance functions describing the bone cement and prosthesis–cement interface are used to compute the probability of failure. When evaluated deterministically, most performance functions indicated a safe design, with the exception of interface tensile failure. However, when evaluated probabilistically, finite probabilities of failure were computed, some significant. The most likely mode of failure before shape optimization was prosthesis–bone cement interface tensile failure with a predicted probability of failure of 97.9%. Deterministic prosthesis shape optimization reduced the probability of failure for all performance functions and reduced prosthesis–bone cement interface tensile failure by 31.7%. Probability sensitivity factors indicate that the uncertainty in the joint loading, cement strength, and implant–cement interface strength have the greatest effect on the computed probability of failure. Implant shape optimization results in a more robust implant design that is less sensitive to uncertainties in joint loading, which cannot be easily controlled, and more sensitive to cement and interface properties, which are easier to modify.

Analysis of 16 Retrieved Proximally Cemented Femoral Stems

Sixteen proximally cemented, collared, and distally splined, Bridge Hip femoral stems with a matte proximal surface and smooth distal surface were retrieved because of loosening. Electron microscopy, with correlated elemental analysis, identified titanium particulate embedded in the internal surface of the cement mantle. Data supported the observations that loosening of the femoral stems was related to proximal debonding at the cement-implant interface, loosening at the proximal cement-bone interface, and inherent rotational instability. Cement-implant interface debonding resulted in the proximally matte femoral stem surface abrading with the opposing cement mantle, resulting in particulate and osteolysis in some cases. Careful consideration of implant design and clinically relevant biomechanical testing protocols should be considered before the clinical introduction of future proximally cemented femoral stems.