The dog as a preclinical model to evaluate interface morphology and micro-motion in cemented total knee replacement.

OBJECTIVES: This study investigated cemented fixation of the tibial component from a canine total knee replacement preclinical model. The objective was to determine the local morphology at the material interfaces (implant, cement, bone) and the local relative micro- motion due to functional loading following in vivo service. METHODS: Five skeletally mature research dogs underwent unilateral total knee replacement using a cemented implant system with a polyethylene (PE) monobloc tibial component. Use of the implanted limb was assessed by pressure-sensitive walkway analysis. At 60 weeks post-surgery, the animals were euthanatized and the tibia sectioned en bloc in the sagittal plane to create medial and lateral specimens. High resolution imaging was used to quantify the morphology under the tray and along the keel. Specimens were loaded to 50% body weight and micro- motions at the PE-cement and cement-bone interfaces were quantified. RESULTS: There was significantly (p = 0.002) more cement-bone apposition and interdigitation along the central keel compared to the regions under the tray. Cavitary defects were associated with the perimeters of the implant (60 ± 25%). Interdigitation fraction was negatively correlated with cavitary defect fraction, cement crack fraction, and total micro-motion. CLINICAL SIGNIFICANCE: Achieving good interdigitation of cement into subchondral bone beneath the tibial tray is associated with improved interface morphology and reduced micro-motion; features that could result in a reduced incidence of aseptic loosening. Multiple drill holes distributed over the cut tibial surface and adequate pressurization of the cement into the subchondral bone should improve fixation and reduce interface micromotion and cavitary defects.

Direct evidence of "damage accumulation" in cement mantles surrounding femoral hip stems retrieved at autopsy: cement damage correlates with duration of use and BMI.

The "damage accumulation" phenomenon has not been quantitatively demonstrated in clinical cement mantles surrounding femoral hip stems. We stained transverse sections of 11 postmortem retrieved femoral hip components fixed with cement using fluorescent dye-penetrant and quantified cement damage, voids, and cement-bone interface gaps in epifluorescence and white light micrographs. Crack density (Cr.Dn), crack length-density (Cr.Ln.Dn), porosity, and cement-bone interface gap fraction (c/b-gap%) were calculated, normalized by mantle area. Multiple regression tests showed that cement damage (Cr.Ln.Dn. & Cr.Dn.) was significantly positively correlated (r(2)=0.98, p<0.001) with "duration of use" and body mass index ("BMI") but not cement mantle "porosity". There were significant interactions: "duration of use"*"BMI" was strongly predictive (p<0.005) of Cr.Dn.; and "duration of use"*"porosity" was predictive (p=0.04) of Cr.Ln.Dn. Stem related cracks accounted for approximately one fifth of Cr.Dn and one third of Cr.Ln.Dn. The mean c/b-gap% was 13.8% but it did not correlate (r(2)=0.01, p=0.8) with duration of use. We concluded that duration-dependent fatigue damage accumulation occurred during in vivo use. BMI strongly influenced cement crack length and the rate of new crack formation over time. Voids did not increase the rate of crack initiation but appeared to have promoted crack growth over time. Although not progressive, substantial bone resorption at the cement-bone interface appeared to be common.

Using 'subcement' to simulate the long-term fatigue response of cemented femoral stems in a cadaver model: could a novel preclinical screening test have caught the Exeter matt problem?

Previously, cement was formulated with degraded fatigue properties (subcement) to simulate long-term fatigue in short-term cadaver tests. The present study determined the efficacy of subcement in a 'preclinical' test of a design change with known clinical consequences: the 'polished'-to-'matt' transition of the Exeter stem (revision rates for polished stems were twice those for matt stems). Contemporary stems were bead blasted to give Ra = 1 microm (matt finish). Matt and polished stems were compared in cadaver pairs under stair-climbing loads (three pairs of size 1; three pairs of size 3). Stem micromotion was monitored during loading. Post-test transverse sections were examined for cement damage. Cyclic retroversion decreased for polished stems but increased for matt stems (p < 0.0001). The implant size had a substantial effect; retroversion of (larger) size-3 stems was half that of size-1 stems, and polished size-3 stems subsided 2.5 times more than the others. Cement damage measures were similar and open through-cracks occurred around both stems of two pairs. Stem retroversion within the mantle resulted in stem-cement gaps of 50-150 microm. Combining information on cyclic motion, cracks, and gaps, it was concluded that this test 'predicted' higher revision rates for matt stems (it also implied that polished size-3 stems might be superior to size-1 stems).

Novel methods to study functional loading micromechanics at the stem-cement and cement-bone interface in cemented femoral hip replacements.

We have developed a technique to directly observe the micromechanics of the stem-cement and cement-bone interfaces of cemented femoral stems under physiologically relevant loading conditions. Thick transverse sections of a stem-cement-femur construct were fixed to the base of a test frame. Ante- and retro-verting torques were applied to the femoral stem by screwing the stem (via a pair of through holes) to an axle, which was turned using a lever arm actuated by the test frame cross-head. The surface of each transverse section was serially digitally imaged during loading. The displacements of the stem, cement and bone were determined using digital image correlation. These data were then used to calculate the relative displacements across the interfaces. This method provides a path to more thorough understanding of load-transfer from femoral stem to femur.

Vacuum-mixing cement does not decrease overall porosity in cemented femoral stems: an in vitro laboratory investigation.

The role of vacuum mixing on the reduction of porosity and on the clinical performance of cemented total hip replacements remains uncertain. We have used paired femoral constructs prepared with either hand-mixed or vacuum-mixed cement in a cadaver model which simulated intra-operative conditions during cementing of the femoral component. After the cement had cured, the distribution of its porosity was determined, as was the strength of the cement-stem and cement-bone interfaces. The overall fraction of the pore area was similar for both hand-mixed and vacuum-mixed cement (hand 6%; vacuum 5.7%; paired t-test, p = 0.187). The linear pore fractions at the interfaces were also similar for the two techniques. The pore number-density was much higher for the hand-mixed cement (paired t-test, p = 0.0013). The strength of the cement-stem interface was greater with the hand-mixed cement (paired t-test, p = 0.0005), while the strength of the cement-bone interface was not affected by the conditions of mixing (paired t-test, p = 0.275). The reduction in porosity with vacuum mixing did not affect the porosity of the mantle, but the distribution of the porosity can be affected by the technique of mixing used.

Prevention of mesh-dependent damage growth in finite element simulations of crack formation in acrylic bone cement.

Peak stress levels predicted in finite element analysis (FEA) usually depend on mesh density, due to singular points in the model. In an earlier study, an FEA algorithm was developed to simulate the damage accumulation process in the cement mantle around total hip replacement (THR) implants. It allows cement crack formation to be predicted, as a function of the local cement stress levels. As the simulation is driven by mesh-dependent peak stresses, predicted crack formation rates are also likely to be mesh dependent. The aim of this study was to evaluate the mesh dependence of the predicted crack formation process, and to present a method to reduce the mesh dependence. Crack-propagation experiments were simulated. Experimental specimens, representing transverse slices of cemented THR reconstructions, were subjected to cyclic torsional loading. Crack development around the corners of the stem was monitored. The experiments were simulated using three meshes with increasing levels of mesh refinement. Crack locations and orientations were accurately predicted, and were virtually independent of the level of mesh refinement. However, the experimental crack propagation rates were overestimated considerably, increasing with mesh refinement. To eliminate the effect of stress singularities around the corners of the stem, a stress averaging algorithm was applied in the simulation. This algorithm redistributed the stresses by weighted spatial averaging. When damage accumulation was computed based on averaged stresses, the crack propagation rates predicted were independent of the level of mesh refinement. The critical distance, a parameter governing the effect of the averaging algorithm, was optimized such that the predicted crack propagation rates accurately corresponded to the experimental ones. These results are important for the validity and standardization of pre-clinical testing methods for orthopaedic implants.

The influence of surface roughness on stem-cement gaps.

We have compared the interface morphology at the stem-cement interface of standard Charnley stems with a satin finish (Ra = 0.75 microm) with identical stems which had been grit-blasted over their proximal third (Ra = 5.3 microm) to promote a proximal bond. The stems were cemented into cadaver femora using conventional contemporary cementing techniques. After transverse sectioning, we determined the percentage of the perimeter of the stem which had a gap at the interface. There were substantial gaps (mean 31.4 +/- 17.1%) at the stem-cement interface in the grit-blasted region. This fraction was significantly (paired t-test, p = 0.0054) higher than that found around the contralateral satin-finished stems (mean 7.7 +/- 11.7%). Although studies of isolated metal-cement interfaces have shown that the bond strength can increase with surface roughness it cannot be assumed that this effect will be observed under clinical conditions.

Elastic and viscoelastic properties of the human pubic symphysis joint: effects of lateral impact joint loading.

The human pelvis is susceptible to severe injury in vehicle side impacts owing to its close proximity to the intruding door and unnatural loading through the greater trochanter. Whereas fractures of the pelvic bones are diagnosed with routine radiographs (x-rays) and computerized tomography (CT scans), non-displaced damage to the soft tissues of pubic symphysis joints may go undetected. If present, trauma-induced joint laxity may cause pelvic instability, which has been associated with pelvic pain in non-traumatic cases. In this study, mechanical properties of cadaveric pubic symphysis joints from twelve normal and eight laterally impacted pelves were compared. Axial stiffness and creep responses of these isolated symphyses were measured in tension and compression (perpendicular to the joint). Bending stiffness was determined in four primary directions followed by a tension-to-failure test. Loading rate and direction correlated significantly (p<0.05) with stiffness and tensile strength of the unimpacted joints, more so than donor age or gender. The impacted joints had significantly lower stiffness in tension (p <0.04), compression (p<0.003), and posterior bending (p<0.03), and more creep under a compressive step load (p<0.008) than the unimpacted specimens. Tensile strength was reduced following impact, however, not significantly. We concluded that the symphysis joints from the impacted pelves had greater laxity, which may correlate with post-traumatic pelvic pain in some motor vehicle crash occupants.

Mixed-mode failure response of the cement-bone interface.

Mechanical failure of the cement-bone interface can contribute to clinical loosening of cemented total hip replacements. The conditions that cause loosening are poorly understood, in part, due to a lack of information on the mechanical behavior of the cement bone interface. The purpose of this study was to determine the mechanical behavior of the cement-bone interface due to mixed-mode (combined tension and shear) loading and to develop a failure model for the cement bone interface. Laboratory tests of machined cement-bone test specimens were performed with mixed-mode loading conditions (loading angles of 22.5 degrees, 45 degrees, and 67.5 degrees) to determine the mechanical response in the pre-yield and post-yield state. After accounting for the quantity of interdigitated bone as a covariate, the mixed-mode data were combined with previous tension (0 degrees) and shear data (90 degrees) to develop a failure model for the cement bone interface. The strength of the interface was positively correlated with the quantity of interdigitated bone (r2 = 0.70, 0.53, 0.49, for 22.5 degrees, 45 degrees, and 67.5 degrees, respectively). There was a significant increase in failure strength (P < 0.001) with increasing mixed-mode angle. When all data were incorporated into an elliptical failure criterion, the average error between the actual and predicted strength was 33%. These results can now be incorporated into constitutive models of the cement bone interface to determine the initiation and progression of interface failure in cemented total hip replacements.

Fatigue fracture of the stem-cement interface with a clamped cantilever beam test.

A clamped cantilever beam test was developed to determine the fatigue crack propagation rate of the CoCr alloy/PMMA cement interface at high crack tip phase angles. A combination of finite element and experimental methods was used to determine the fatigue crack growth rates of two different CoCr alloy/PMMA cement surfaces. A crack tip phase angle of 69 deg was found, indicating that loading at the crack tip was mixed-mode with a large degree of in-plane shear loading. The energy required to propagate a crack at the interface was much greater for the plasma-sprayed CoCr surface when compared to the PMMA-precoated satin finish (p < 0.001). Both interface surfaces could be modeled using a Paris fatigue crack growth law over crack propagation rates of 10(-4) to 10(-9) m/cycle.

Acetabular fracture patterns: associations with motor vehicle crash information.

BACKGROUND: Motor vehicle crashes are the most common cause of acetabular fractures, which have been associated with significant morbidity and mortality. METHODS: To date, medical and collision information has been collected on 83 acetabular fracture patients treated at the University of Alabama at Birmingham's Level I trauma center. The fractures were grouped according to the Judet-Letournel classification scheme and investigated for correlation with age, sex, vehicle type, impact direction, and seat-belt use. RESULTS: The database included 41 women and 42 men with a combined average age of 32.8 years. Femoral shaft axis loading fractures correlated significantly with male sex, trucks, and frontal impacts. Greater trochanter loading fractures occurred statistically more frequently in side impacts. Women received a significant higher percentage of off-axis loading fractures, which were associated more in angled frontal impacts. CONCLUSION: Acetabular fracture type strongly correlated with impact direction, supporting the fracture mechanisms proposed by Judet and Letournel.

Mechanical strength of the cement-bone interface is greater in shear than in tension.

The objective of this study was to determine the relative mechanical properties of the cement-bone interface due to tensile or shear loading. Mechanical tests were performed on cement-bone specimens in tensile (n = 51) or shear (n = 55) test jigs under the displacement control at 1 mm/min until complete failure. Before testing, the quantity of bone interdigitated with the cement was determined and served as a covariate in the study. The apparent strength of the cement-bone interface was significantly higher (p < 0.0001) for the interface when loaded in shear (2.25 MPa) when compared to tensile loading (1.35 MPa). Significantly higher energies to failure (p < 0.0001) and displacement before failure (p < 0.01) were also determined for the shear specimens. The post-yield softening response was not different for the two test directions. The data obtained herein suggests that cement-bone interfaces with equal amounts of tensile and shear stress would be more likely to fail under tensile loading.

Mixed-mode fracture toughness of the cobalt-chromium alloy/polymethylmethacrylate cement interface.

Mechanical debonding of the stem/cement interface has been implicated in the failure process of cemented femoral hip components. The nature of this failure process remains poorly understood due, in part, to limited understanding of how interfacial debonding occurs in response to a wide range of loading conditions. The purpose of this investigation was to determine the fracture toughness of the cobalt-chromium alloy/polymethylmethacrylate interface under mixed-mode loading conditions. The hypothesis was that the critical energy release rate was dependent on the phase angle of the crack tip and that the fracture response would be significantly different for a smooth compared with rough interface surface. A novel in-plane shear test fixture was developed with use of a combination of finite element and experimental fracture-mechanics tests. A wide range (-65-60 degrees) of phase angles was determined with the in-plane shear test and a clamped cantilever-beam test. Sixty experimental tests were performed for cobalt-chromium alloy bars with a plasma-sprayed coating or a precoat of polymethylmethacrylate over a satin-finished surface. For the specimens with the plasma-sprayed coating, critical energy release rates (500-700 J/m2) were not a function of the phase angle of the crack tip. In contrast, critical energy release rates (15-80 J/m2) were found to be strongly affected by the phase angle for the specimens precoated with polymethylmethacrylate. The critical energy release rate for specimens with the plasma-sprayed surface was significantly (p < 0.01) greater than for those precoated with polymethylmethacrylate. The critical energy release rate increased markedly with the phase angle of the crack tip for the specimens precoated with polymethylmethacrylate. The results suggest that the failure response of a stem with a plasma-sprayed surface may be insensitive to the loading angle of the crack tip, whereas a stem precoated with polymethylmethacrylate may be more likely to debond under tensile opening loading.

Expression of Epstein-Barr virus gp350 as a single chain glycoprotein for an EBV subunit vaccine.

There is currently no commercially available vaccine for Epstein Barr virus (EBV)-related disease in humans. Since the EBV glycoprotein gp350/220 is the primary target for EBV-neutralizing antibodies following natural infection in humans and some forms of gp350/220 have been shown to protect against EBV-related disease in animal models, it is a likely candidate for an EBV subunit vaccine. We have made gp350/220 gene constructs that facilitate gp350 secretion from CHO cells and created splice site mutations in the gene that effectively prevent production of the gp220 species. Recombinant CHO cell gp350 (MSTOP gp350) is recognized by several different anti-gp350/220 monoclonal antibodies, and is also competent to bind to the cellular EBV receptor, CD21, suggesting that the recombinant protein is conformationally similar to wild-type EBV gp350/220. The MSTOP gp350 antigen raises high antibody titers in rabbits and these antibodies neutralize wild-type EBV. These properties make MSTOP gp350 a realistic candidate for a subunit vaccine against EBV-related disease.

Pre-yield and post-yield shear behavior of the cement-bone interface.

Aseptic loosening of cemented total hip replacements is thought to involve mechanical failure of the cement-bone interface. However, the mechanical response of the interface, particularly the post-yield behavior, is not well understood. The purpose of this study was to determine the constitutive behavior of the cement-bone interface for loading in shear using a combination of experimental and finite element methods. A total of 55 cement-bone specimens (5 x 10 x 15-20 mm) from the proximal femur of human cadavers were loaded to failure under displacement control with use of a custom shear test jig. Finite element models of the test specimens were made and included provision for a two-parameter nonlinear interface model at the cement-bone interface. The experimental tests revealed a complicated load versus displacement response with an initial linear region and a reduction in slope until the ultimate strength (2.25+/-1.49 MPa) was reached, followed by an exponential decrease in load with increasing displacement until the entire interface debonded. Failure most often occurred at the cement-bone interface, where the cement penetrated into the bone with bone remaining in the cement in 30 specimens and with bone remaining in the cement and cement spicules remaining in the bone in 22 specimens. The adjacent bulk bone and cement did not appear to be permanently deformed. Finite element models of the test specimens revealed that failure initiated at the base of the test specimen before the peak load had been reached. The two interface parameters, interface strength (2.71+/-1.90 MPa) and interface-softening exponent (4.96+/-3.47 1/mm), could be determined directly from the experimental data and provided a good fit with the experimental structural response for a wide range of interface strengths. These results show that the cement-bone interface does not fail abruptly when the shear strength is reached but absorbs a substantial amount of energy with post-yield strain-softening behavior.

Cemented femoral stem performance. Effects of proximal bonding, geometry, and neck length.

The effects of proximal bonding, distal stem geometry, and femoral neck length on cement and interface stresses were determined to understand better their role in clinical performance. The effects of stem design were compared with the effects of environmental variables, patient weight, and patient activity. Finite element models were used to determine peak cement and interface stresses, and an experimental layout was used to separate design and environmental effects. Bonding reduced cement mantle stresses by 35% to 60%, to levels below the cement fatigue strength. A flat sided implant provided more torsional resistance, reducing shear stresses at the proximal cement-prosthesis interface by 22% to 73% with respect to a distal round implant. Neck length had minimal effects on stresses compared with bonding or implant geometry. Cement-bone interface stresses were more sensitive to patient activity than to the design variables. Therefore, claims that a strong cement and prosthesis bond may be harmful to the bone-cement interface are unjustified based on these results. The best combination of design variables was a proximally bonded, flat sided implant with neck length left to the surgeon's discretion. This combination was most effective at protecting the cement mantle and prosthesis interface and perhaps the cement-bone interface by minimizing stresses associated with cement debris generation.

Fracture toughness of CoCr alloy-PMMA cement interface.

An unsymmetric cantilever geometry was used experimentally to determine the critical energy release rate values for cobalt chromium alloy-polymethylmethacrylate cement (CoCr alloy-PMMA) interfaces with satin finished, grit blasted, and plasma sprayed surface treatments applied to the CoCr alloy. Critical energy release rates of 0.013, 0.181, and 0.583 N/mm were found for the satin finish, grit blasted, and plasma sprayed CoCr alloy-PMMA interfaces, respectively. A finite element model of the experimental test specimen was used to determine the crack tip phase angles (-8.73 degrees to -27.1 degrees) that indicated that the tensile load applied to the specimens resulted in a tensile (mode I) and in-plane shear (mode II) loading at the crack tip. The experimental data suggest that a satin finish CoCr alloy-PMMA interface has minimal resistance to crack propagation when compared to grit blasted or plasma sprayed surface treatments.

Modeling the tensile behavior of the cement-bone interface using nonlinear fracture mechanics.

The tensile mechanical behavior of the cement-bone interface where there was a large process (plastic) zone at the interface was modeled using a nonlinear fracture mechanics approach. A finite element method was employed, which included a piecewise nonlinear interface, to investigate the behavior of experimental cement-bone test specimens and an idealized stem-cement-bone (SCB) structure. The interface model consisted of a linear elastic region with high stiffness until the yield strength was reached, followed by an exponential softening region, until zero stress. The yield strength and rate of exponential softening after yielding at the cement-bone interface were shown to have a marked effect on the structural stiffness of the SCB model. The results indicate that both yield strength and postyield behavior should be included to characterize the mechanics of the cement-bone interface fully.

Tensile strength of the cement-bone interface depends on the amount of bone interdigitated with PMMA cement.

An experimental investigation was performed to (1) determine the general mechanical behavior and in particular, the post-yield behavior of the cement-bone interface under tensile loading, (2) determine where interface failure occurs, and (3) determine if the mechanical properties of the interface could be related to the density of bone at the interface and/or the amount of cement-bone interdigitation. Seventy-one cement-bone test specimens were machined from human proximal femurs that had been broached and cemented using contemporary cementing techniques. The amount of cement-bone interdigitation was documented and the quantitative computed tomography equivalent mineral density (QCT density) of the bone with cement was measured. Specimens were loaded to failure in tension under displacement control and exhibited linear elastic behavior with some reduction in stiffness until the peak tensile stress was reached (1.28 +/- 0.79 MPa). A substantial amount of strain softening (negative tangent stiffness) with an exponential-type decay was found after the peak stress and continued until there was complete debonding of the specimens (at 0.93 +/- 0.44 mm displacement). Interfacial failure most often occurred at the extent of cement penetration into the bone (56% of specimens) or with small spicules of cement left in the bone (38% of specimens). The results showed that the post-yield tensile behavior contributes substantially to the energy required to cause failure of the cement-bone interface, but the post-yield behavior was not well correlated with the amount of interdigitation or density of bone. Linear regression analysis revealed a moderate (r2 = 0.499, p < 0.0001) positive relationship between the tensile strength of the cement-bone interface and the quantity of bone interdigitated with the cement.

Analysis of the kinematics of the scaphoid and lunate in the intact wrist joint.

In conclusion, this study combined three different technologies to simultaneously monitor scaphoid, lunate, and global wrist motion in three dimensions and concurrently collect data on the pressure distribution in the radiocarpal and ulnocarpal joints. This information was collected dynamically in real time while the wrist was moved in reproducible, physiologic cycles of motion. The scaphoid and lunate flex and extend as well as pronate and supinate while the wrist moves in the plane of flexion and extension. There is minimal radial and ulnar deviation of these carpal bones during this motion. During wrist radial and ulnar deviation, the scaphoid and lunate both flex and extend as well as deviate radially and ulnarly. The pressures in the wrist also change as the wrist moves. Pressures in the wrist are not evenly distributed and, during some movements, are localized to specific areas. The data also support the concept that there is a hysteresis effect on both the carpal bones and the pressure distribution patterns while the wrist is moving. The results of this study can provide baseline data to compare with other studies that evaluate various pathologic abnormalities of the wrist joint.