Stem-cement Interface Research Articles

Fluid flow around model femoral components of differing surface finishes: in vitro investigations.

We studied fluid flow at the stem-cement interface of bonded and debonded, polished and rough model femoral components. In a first series of experiments, fluid flow along the interface between bone cement and well-fixed model femoral components, differing in surface finish, and in shape, was measured. Fluid migration along the bone-cement interface of rough stems (Ra 3 microm) was greater than that on polished stems (p < 0.001). This was true of cylindrical and conical tapered stems. On stems with the same surface finish, shape did not influence fluid migration. In a second series of experiments, fluid flow along the stem-cement interface of 5 highly polished and 10 rough-finished (5 of Ra approximately 1.5 microm and 5 of Ra approximately 3 microm), debonded, tapered circular stems was measured. None of the rough stems could prevent fluid flow along the stem-cement interface. Polished tapered stems sealed the interface and, after 48 hrs of continuous pressure, no fluid flow was observed. This difference in the ability to seal the stem-cement interface between rough and polished stems was significant (p < 0.001). The difference in fluid migration along the stem-cement interface of rough and polished stems which we observed offers a plausible explanation of the occurrence of osteolysis distal to the articulation of cemented THR in the presence of cement mantle defects. It may also explain why osteolysis is uncommon with polished double-tapered stems.

Interface gap after implantation of a cemented femoral stem in pigs.

We studied the interface gap around cemented femoral stems. Fresh pig femora were used. Bone cement mixed under vacuum or at atmospheric pressure was injected into the femoral canal and a cobalt chrome stem was then implanted. The femora were sectioned transversely from the minor trochanter and distally by using a high-pressure water cutter. Most of the interfaces had intimate contact. However, in all specimens, small gaps were found at the bone-cement and cement-stem interfaces. The gaps at the interfaces between the bone and cement and the cement and stem were measured, using a computerized video digital system. They occupied about 10% of the circumference at the bone-cement interface and about 15% of the circumference at the cement-stem interface, irrespective of the mixing procedures. Most gaps were less than 100 mu at the interfaces. In conclusion, cemented implants in the animal model showed that small gaps are found at the interfaces directly after implantation. These gaps may be weak points and initiate debonding when loading the prostheses.

The influence of the time-dependent properties of bone cement on stress in the femoral cement mantle of total hip arthroplasty.

An empirically determined formula for the creep behavior of bone cement was incorporated into a validated computer model of a cemented femoral total hip arthroplasty component. The stress patterns in the cement mantle were observed over a period of one week, in one instance where the stem-cement interface was rigidly bonded, and in a second, where it was allowed to slip. Principal stresses and maximum shear stresses were shown to decrease rapidly after loading in both situations, suggesting that the stresses generated were not high enough to cause immediate failure, although they may be significant in the long term.

Effects of prosthesis surface roughness on the failure process of cemented hip implants after stem–cement debonding

Retrieval studies suggest that the loosening process of the cemented femoral components of total hip arthroplasties is initiated by failure of the bond between the prosthesis and the cement mantle. Finite element (FE) analyses have demonstrated that stem–cement debonding has stress-producing effects in the cement mantle. High interface friction, which corresponds to a degree of surface roughness, reduces these stresses. In experiments, however, debonded rough stems produced more cement damage than polished ones; in the Swedish Hip Register polished stems were clinically superior with respect to stems with a mat surface finish. The purpose of the present study was to investigate this contradiction. For this purpose, global and local FE models with debonded stem–cement interfaces were used to study the effects of prosthesis surface roughness on the cement stresses on a global scale and microscale, respectively. Similar to earlier numerical studies, the global FE model predicted that an increased surface roughness of the stem reduced the stresses in the cement mantle. The local model provided insight in the load-transfer mechanism on a microscale and could explain the experimental and clinical findings. The local cement peak stresses around the asperities of the surface roughness profile increased with increasing surface roughness and decreased again beyond a particular roughness value. Cement abrasion is caused by localized stresses in combination with micromotion. From this study it can be concluded that to minimize cement abrasion, debonded stems should either have a polished microstructure to minimize the local cement stresses or have a profiled macrostructure to minimize micromotions at the stem–cement interface. © 1998 John Wiley & Sons, Inc. J Biomed Mater Res, 42, 554–559, 1998.

Effects of prosthesis surface roughness on the failure process of cemented hip implants after stem-cement debonding.

Retrieval studies suggest that the loosening process of the cemented femoral components of total hip arthroplasties is initiated by failure of the bond between the prosthesis and the cement mantle. Finite element (FE) analyses have demonstrated that stem-cement debonding has stress-producing effects in the cement mantle. High interface friction, which corresponds to a degree of surface roughness, reduces these stresses. In experiments, however, debonded rough stems produced more cement damage than polished ones; in the Swedish Hip Register polished stems were clinically superior with respect to stems with a mat surface finish. The purpose of the present study was to investigate this contradiction. For this purpose, global and local FE models with debonded stem-cement interfaces were used to study the effects of prosthesis surface roughness on the cement stresses on a global scale and microscale, respectively. Similar to earlier numerical studies, the global FE model predicted that an increased surface roughness of the stem reduced the stresses in the cement mantle. The local model provided insight in the load-transfer mechanism on a microscale and could explain the experimental and clinical findings. The local cement peak stresses around the asperities of the surface roughness profile increased with increasing surface roughness and decreased again beyond a particular roughness value. Cement abrasion is caused by localized stresses in combination with micromotion. From this study it can be concluded that to minimize cement abrasion, debonded stems should either have a polished microstructure to minimize the local cement stresses or have a profiled macrostructure to minimize micromotions at the stem-cement interface.

Fatigue of bone cement with simulated stem interface porosity.

Cracks in bone cement have been observed in carefully examined post-mortem preparations of cemented stems. These cracks were probably caused by fatigue, and frequently appeared to initiate at pores. Ubiquitous porosity, occurring preferentially at, or near, the stem, is most likely caused by polymerization shrinkage. Preparation of air-free cement has only a marginal influence on the interface porosity, but pre-heating the stem in order to reverse the direction of polymerization can reduce or eliminate it. To estimate the impact of interface porosity on the fatigue strength of bone cement, test plates for this study were cast in a steel mold without release foils, and with one side of the mold warmer. Sample plates so prepared from chilled, partial vacuum-mixed PALACOS, have one face essentially pore-free and the other porous, the extent and morphology of the porosity being very similar to that observed on the stem-cement interface. Four-point bending fatigue strength, determined after 60 d conditioning in Ringer's solution at 37 degrees C, was only 20 MPa (at 10(6) cycles, with the porous side under tension) compared to 30 MPa for conventionally prepared, pore-free material. This corresponds to a 10-100 fold reduction in cycles to failure in the range of stresses predicted to occur in vivo.

Surface roughness of debonded straight-tapered stems in cemented THA reduces subsidence but not cement damage

Although stress analyses have shown that the mechanical endurance of cemented femoral THA reconstructions is served by stems that firmly bond to their cement mantles, retrieval studies suggest that this may be difficult to achieve. Clinical studies with roentgen stereophotogrammetric analyses have shown that stems may gradually debond from their cement mantle. Accepting the fact that stem debonding is unavoidable, stem subsidence and cement stresses can be reduced by increasing stem–cement friction, as indicated by finite element stress analyses. Hence, it can be hypothesized that debonded stems with high surface roughness values would damage the cement mantle to a lesser extent as compared to polished ones. To confirm this hypothesis, tapered stems with polished and rough surface finishes were implanted in cement mantles and cyclically loaded for 1.7 million times. It was investigated how surface roughness affected the damage in the cement mantle, and how it was related to prosthetic subsidence. The polished taper subsided considerably more than the rough one (630 vs. 270 μm at the end of the experiments). In addition, it was found that the polished taper displayed step-wise subsidence, which is probably due to the interaction of stick–slip processes at the interface, associated with creep of the acrylic cement. The rough taper subsided monotonously. Scanning Electron Microscopic (SEM) analysis of the taper–cement structures showed that the rough taper was completely debonded from the cement mantle, creating a gap at the interface, and that many large cement cracks and particles were created. Around the polished taper, only a few cracks were found and the taper–cement interface seemed undamaged. It was concluded that an increased surface roughness does not necessarily lead to a reduction in cement damage. On the contrary, compared to polished ones, debonded rough stems may produce more cement cracks and acrylic cement debris, and provide routes to transport these wear products. Hence, after failure of the stem–cement interface, straight–tapered stems with an increased surface roughness accelerate the failure process due to inferior fail–safe features. Consequently, in vivo subsidence patterns at the stem–cement interface should be considered in combination with the surface finish of the implant. An amount of post-operative subsidence of a rough stem may be much more damaging for the reconstruction than the same amount for a polished stem.

Effect of modular head seating on the cement-stem interface strength of femoral prostheses

This study tested the hypothesis that modular head seating in situ reduces the cement-stem interfacial strength. Femoral prostheses were implanted in eight pairs of cadaveric femurs; one femur of each pair received a stem for which the modular head was seated by dropping a weight from a given height (treated); the contralateral femur received a nonseated stem (control). Transverse sections were cut from four standardized locations on each implanted femur, and the yield shear stress and ultimate shear stress of the interface were determined from push-out tests. There was no significant difference in cement-stem interfacial strength between seated and nonseated prostheses. These results suggest that seating modular heads in situ has no deleterious effect on the acute interfacial strength of cemented femoral implants.

Influence of stem geometry on mechanics of cemented femoral hip components with a proximal bond.

Nonlinear, three-dimensional, finite element models of cemented femoral hip components with a proximal stem-cement bond were developed with use of a Charnley stem geometry and a modified Charnley stem geometry that had a cylindrical cross section over the distal two-thirds of the stem (Distal-Round). Peak tensile stresses in the proximal cement mantle increased 63 and 74% for the Charnley and Distal-Round stems, respectively, when the proximal stem-cement interface was debonded. The shear stresses over the stem-cement interface with a proximal bond were 29% larger for the Distal-Round stem than for the Charnley stem. After the proximal stem-cement interface was debonded, the peak tensile stresses in the cement mantle were 15% larger for the Distal-Round stem than for the Charnley stem. The results illustrate that stresses within the proximal cement mantle could be substantially reduced for both Charnley and Distal-Round stems through use of a proximal stem-cement bond. However, the risk of debonding may be higher for the Distal-Round stem because of increased shear stresses, and once debonded the risk of further loosening due to failure of the cement mantle would also be higher for the Distal-Round stem.

The effects of cement-stem debonding in THA on the long-term failure probability of cement

The damage accumulation failure scenario is one of the most prominent ones of cemented THA reconstruction, and involves the accumulation of mechanical damage in materials and interfaces due to repetitive dynamic loading, eventually resulting in gross loosening. This study addresses this scenario by combining finite element techniques with the theory of continuum damage mechanics, to analyze the damage accumulation process in the cement mantle. It was investigated how damage accumulation was affected by stem-cement debonding, and what the effects of a layer with poor bone quality around the cement mantle were. For the unbonded stem, it was determined if clinical migration rates can be explained by failure of the cement mantle, and whether cement failure promotes the formation of a pathway for debris at the stem-cement interface. It was found that stem-cement debonding not only elevated the initial stress levels with a factor of about two to three as demonstrated in earlier studies, but remained to have an impact on the failure process of the cement mantle. Stem-cement debonding accelerated the failure process by a factor of four, and promoted the formation of a pathway for debris at the stem-cement interface, particularly when the bone support to the cement mantle was reduced. The amount of subsidence was only substantial when the damaged cement mantle was surrounded by a layer of bone with reduced stiffness. This study supports the hypothesis that the survival of cemented THA is enhanced by a firm and lasting bond between the stem and the cement mantle, although this may be difficult to realize clinically.

Cement debonding process of total hip arthroplasty stems.

Retrieval studies have indicated that debonding of the stem cement interface in total hip arthroplasty precedes clinical failure of femoral components. This study addressed the mechanisms that play a role in the debonding process by analyzing how debonding is likely to proceed in the course of time. It was investigated whether debonding is an immediate process or if it is likely to develop slowly with time, which interface stress components contribute particularly to its progression, and whether the mechanical integrity of the cement mantle is likely to be compromised by the debonding process. To answer these questions, a 3-dimensional finite element model of a femoral total hip arthroplasty reconstruction was developed and used to simulate the debonding process. The results showed that debonding was governed by the shear stress component at the interface. Debonding started in the tip region and the proximal, medial anterior region. These debonded regions expanded until the whole interface was de bonded. Cement stresses slowly increased at the end of the debonding process to a level twice as high as the initial one. The probability of debonding, as measured by an interface failure index, remained constant as debonding progressed. This indicates that, for this particular design, much less surface area is required for load transfer than is provided by the stem, and the debonding process does not necessarily accelerate quickly once debonding is initiated.

Effects of cement creep on stem subsidence and stresses in the cement mantle of a total hip replacement.

In cemented total hip prostheses, the role of creep of the acrylic cement (polymethyl methacrylate, [PMMA]) in increasing or decreasing the chance of failure of the cement mantle is a subject of ongoing controversy. In the present study we used a three-dimensional finite-element model of a cemented stem to assess the influence of cement creep on subsidence of the stem, and on the stress and strain in the cement under cyclic load, both in the short and long term. The cement layer was assigned the shear and bulk creep moduli of Zimmer regular PMMA cement, which were obtained experimentally. The stem-cement interface was modeled either as (1) completely bonded, (2) completely debonded with friction, or (3) completely debonded and frictionless. Under the cyclic load some cement creep occurred with all three bonding conditions, allowing additional subsidence of the stem and a decrease in the stress components within the cement. During the unloaded period the full recovery of the preload conditions could be reached with the completely bonded and with the frictionless interfaces. With the frictional interface there was residual cement creep, residual stresses within the cement, and residual subsidence of the stem during the unloaded period; however, the reduction of the stress was at most 13% and the subsidence was about 0.46 mm. The much larger subsidence of debonded stems that is often observed clinically might be attributed to the factors which were not included in the present model, such as circumferential bone remodeling.

Effects of cement creep on stem subsidence and stresses in the cement mantle of a total hip replacement

In cemented total hip prostheses, the role of creep of the acrylic cement (polymethyl methacrylate, [PMMA]) in increasing or decreasing the chance of failure of the cement mantle is a subject of ongoing controversy. In the present study we used a three-dimensional finite-element model of a cemented stem to assess the influence of cement creep on subsidence of the stem, and on the stress and strain in the cement under cyclic load, both in the short and long term. The cement layer was assigned the shear and bulk creep moduli of Zimmer regular PMMA cement, which were obtained experimentally. The stem-cement interface was modeled either as (1) completely bonded, (2) completely debonded with friction, or (3) completely debonded and frictionless. Under the cyclic load some cement creep occurred with all three bonding conditions, allowing additional subsidence of the stem and a decrease in the stress components within the cement. During the unloaded period the full recovery of the preload conditions could be reached with the completely bonded and with the frictionless interfaces. With the frictional interface there was residual cement creep, residual stresses within the cement, and residual subsidence of the stem during the unloaded period; however, the reduction of the stress was at most 13% and the subsidence was about 0.46 mm. The much larger subsidence of debonded stems that is often observed clinically might be attributed to the factors which were not included in the present model, such as circumferential bone remodeling. © 1997 John Wiley & Sons, Inc.

The influence of the stem-cement interface in total hip replacement—a comparison of experimental and finite element approaches

Experimental and finite element investigations were carried out on axisymmetric models of the femoral component of a total hip replacement. In one instance, the interface between the stem and the surrounding bone cement was assumed to be rigidly bonded; in a second, it was allowed to slip. For the latter case, a friction coefficient of 0.2 was determined experimentally. The predictions of the finite element models demonstrated excellent agreement with the results from the experimental tests at all sites where comparisons were made, thus validating these models. The effect of stem-cement slip was shown to reduce the maximum shear stress in the cement mantle by approximately 30 per cent.

Mechanical effects of stem cement interface characteristics in total hip replacement.

Stem cement debonding is 1 of the most common forms of fixation failure and is thought to be a prelude to gross loosening of a total hip reconstruction. However, the immediate consequences of debonding remains a matter of controversy. The dynamic effects of stem cement debonding in total hip reconstruction were analyzed using 3-dimensional finite element techniques. Stem cement interface conditions were assumed as completely bonded or unbonded, with or without friction. The dynamic effects were accounted for, as presented by the stance and swing phases of the gait cycle. It was found that both cyclic micromotions at the stem cement interface and stresses in the cement mantle were effectively reduced by friction. The friction cases produced failure probabilities of the cement mantle that were relatively close to the one generated by the bonded stem. The probability of mechanical failure of the cement bone interface decreased after debonding and decreased more with reduced stem cement friction. These results show that, although a firm and lasting bond between stem and cement may be desirable for preventing cement failure, the mechanical effects of a debonded stem are less detrimental than were assumed earlier. For straight tapered stem shapes subjected to the loading conditions described, a polished stem may be desirable for the cement bone interface mechanics.

Axisymmetric finite element analysis of a debonded total hip stem with an unsupported distal tip.

A tapered femoral total hip stem with a debonded stem-cement interface and an unsupported distal tip subjected to constant axial load was evaluated using two-dimensional (2D) axisymmetric finite element analysis. The analysis was performed to test if the mechanical condition suggest that a "taper-lock" with a debonded viscoelastic bone cement might be an alternative approach to cement fixation of stem type cemented hip prosthesis. Effect of stem-cement interface conditions (bonded, debonded with and without friction) and viscoelastic response (creep and relaxation) of acrylic bone cement on cement mantle stresses and axial displacement of the stem was also investigated. Stem debonding with friction increased maximum cement von Mises stress by approximately 50 percent when compared to the bonded stem. Of the stress components in the cement mantle, radial stresses were compressive and hoop stresses were tensile and were indicative of mechanical taper-lock. Cement mantle stress, creep and stress relaxation and stem displacement increased with increasing load level and with decreasing stem-cement interface friction. Stress relaxation occur predominately in tensile hoop stress and decreased from 1 to 46 percent over the conditions considered. Stem displacement due to cement mantle creep ranged from 614 microns to 1.3 microns in 24 hours depending upon interface conditions and load level.

POROSITY REDUCTION IN BONE CEMENT AT THE CEMENT-STEM INTERFACE

The fatigue failure of bone cement, leading to loosening of the stem, is likely to be one mode of failure of cemented total hip replacements. There is strong evidence that cracks in the cement are initiated at voids which act as stress risers, particularly at the cement-stem interface. The preferential formation of voids at this site results from shrinkage during polymerisation and the initiation of this process at the warmer cement-bone interface, which causes bone cement to shrink away from the stem. A reversal of the direction of polymerisation would shrink the cement on to the stem and reduce or eliminate the formation of voids at this interface. We have investigated this by implanting hip prostheses, at room temperature or preheated to 44 degrees C, into human cadaver femora kept at 37 degrees C. Two types of bone cement were either hand-mixed or vacuum-mixed before implantation. We found that the area of porosity at the cement-stem interface was dramatically reduced by preheating the stem and that the preheating temperature of 44 degrees C determined by computer analysis of transient heat transfer was the minimum required to induce initial polymerisation at the cement-stem interface. Temperature measurements taken during these experiments in vitro showed that preheating of the stem caused a negligible increase in the temperature of the bone. Reduction of porosity at the cement-stem interface could significantly increase the life of hip arthroplasties.

Reinforcement of PMMA bone cement with a continuous wire coil – a 3D finite element study

The changes in the mechanical response of a bone cement reinforcement, comprised of a continuous stainless steel coil imbedded within the PMMA bone cement matrix surrounding the distal tip of the total hip arthroplasty, was investigated. To achieve this, a 3D finite element model depicting two and one half rotations of the coil imbedded within the cement at the distal tip was constructed. Ideally, the wire coil should reduce the radial, and to a greater extent, the hoop stresses developing within the cement and at the cement-stem interface. As a means of comparison, a control model of only bone cement was also built. For the radial stresses, the control had about 4.5 times the compressive stress of the reinforced models (0.039 (+/-0.00065) MPa vs. 0.0087 (+/-0.0012) MPa) at the cement-stem interface. The tensile hoop stresses were also 4.5 times higher (4.272 (+/-0.0147) MPa and 0.95 (+/-0.0052) MPa) for the control than for the reinforced models. This indicates that the wire coil reinforcement is effective in reducing the cement mantle's radial and, more importantly, the hoop stresses which may lead to the failure of both the cement and the implant as a whole.

Coulomb frictional interfaces in modeling cemented total hip replacements: A more realistic model

Loosening of cemented femoral hip stems could be initiated by failure of the cement mantle due to high cement stresses. The goals of this study were to determine if realistic stem-cement interface characteristics could result in high cement stresses when compared to a bonded stem-cement interface and to determine if stem design parameters could be chosen to reduce peak cement stresses. Three-dimensional finite-element models of cemented femoral hip components were studied with bonded or realistic Coulomb friction stem-cement interfaces. The results showed that the use of a non-bonded, non-linear Coulomb friction interface resulted in substantially different stress fields in the cement when compared to a bonded stem-cement interface. Tensile stresses in the proximal cement mantel for the Coulomb friction interface case (10.8 MPa) were greater than the fatigue strength of the cement. In contrast, the tensile stresses in the cement mantle were not greater than the fatigue strength for the bonded case (7.5 MPa). Failure of the cement mantle in the proximal femur could therefore be initiated by a lack of a bond at the stem-cement interface. The effect of different cross-sectional stem geometries (medial radii of 3.0, 4.9 and 5.5 mm and antero-posterior widths of 9.8 and 13.7 mm) and different elastic moduli (cobalt chromium alloy and titanium alloy) for the stem material were also evaluated for models with a Coulomb friction interface. Changes in the stem cross-section and elastic modulus had only limited effects on the stress distributions in the cement. Of the parameters evaluated in this study, the characteristics of the stem-cement interface had the largest effect on cement mantle stresses.

Prediction of clinical outcome of THR from migration measurements on standard radiographs. A study of cemented Charnley and Stanmore femoral stems

We report the theoretical basis of a method to measure axial migration of femoral components of total hip replacements (THR). The use of the top of the greater trochanter and a lateral point on the collar of the stem, allowing for variations of up to 10 degrees rotation of the femur in any direction between successive radiographs, gave a maximum error of 0.37 mm. At a more realistic 5 degrees rotational variation, the error was only 0.13 mm. These data were confirmed in an experimental study using digitisation of points and special software. We also showed that the centre of the femoral head, the stem tip, and the lesser trochanter provided less accurate landmarks. In a second study we digitised a series of radiographs of 51 Charnley and 57 Stanmore THRs; the mean migration rates were found to be identical. We then studied 46 successful stems with a minimum follow-up of eight years and 46 stems which had failed by aseptic loosening at different times. At two years, the successful stems had migrated by a mean of 1.45 +/- 0.68 mm, but the failed cases had a mean migration of 4.32 +/- 2.58 mm (p < 0.0001). Of the successful cases 76% had migrated less than 2 mm, while in the failed group 84% had migrated more than 2 mm. For any particular case migration of more than 2.6 mm at two years had only a 5% chance of continuing success and would therefore merit special follow-up. Only 24% of the eventually successful stems showed migration at the stem-cement interface, but this had happened in every failed stem. We conclude that it would be possible to evaluate a new cemented design of femoral stem over a two-year period by the use of our method and to compare its performance against the reported known standard of the Charnley and Stanmore designs.