Amos Race

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.

Shear fatigue micromechanics of the cement-bone interface: An in vitro study using digital image correlation techniques.

Loss of fixation at the cement–bone interface is known to contribute to aseptic loosening, but little is known about the mechanical damage response of this interface. An in vitro study using cement–bone specimens subjected to shear fatigue loading was performed, and the progression of stiffness changes and creep damage at the interface was measured using digital image correlation techniques. Stiffness changes and creep damage were localized to the contact interface between cement and bone. Interface creep damage followed a three‐phase response with an initial rapid increase in creep, followed by a steady‐state increase, concluding in a final rapid increase in creep. The initial creep phase was accompanied by an increase in interface stiffness, suggesting an initial locking‐in effect at the interface. Interface stiffness decreased as creep damage progressed. Power law models were reasonably successful in describing the creep and stiffness damage response and were a function of loading magnitude, number of loading cycles, and contact area at the interface. More microcrack damage occurred to the cement when compared to the bone, and the damage was localized along the interface. These findings indicate that damage to the cement–bone interface could be minimized by improving cement–bone contact and by strengthening the fatigue resistance of the cement.

Cement-implant interface gaps explain the poor results of CMW3 for femoral stem fixation: A cadaver study of migration, fatigue and mantle morphology.

Background The Norwegian Arthroplasty Register reported that CMW3 cement performed poorly for femoral stem fixation. Methods We implanted collared, satin-finished stems (Ra = 0.35 µm) into cadaver femora using CMW3 and with Simplex as control. Cement mantle function was quantified by stem migration after 300,000 cycles of “stair climbing”. Cement cracks and interface gaps were quantified in transverse sections. Results The variances of the CMW3 migrations were substantially higher than for the control (p < 0.001): subsidence for CMW3: –32 (SD 42) µm, and for Simplex: –7 (SD 9) µm (p = 0.2); retroversion for CMW3: 0.60° (SD 0.25), and for Simplex: 0.37° (SD 0.04) (p = 0.08). Crack length-densities were similar. CMW3 had significantly more non-apposed stem/cement interface: 52% (SD 17) versus 33% (SD 8) (p = 0.04). Migrations could be predicted by the fraction of non-apposed stem/cement interface (retroversion: R2=0.80, p < 0.001; subsidence: R2 = 0.46, p = 0.02) but not by cement cracks or non-apposed cement-bone interface. Interpretation We found that increased stem/cement non-apposition resulted in increased stem migration. Early migration is known to correlate with risk of revision. Thus, the higher stem-revision risk for CMW3 cement reported by the Norwegian Arthroplasty Register may have been due to inferior and variable stem/cement apposition.

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.

The effect of low-viscosity cement on mantle morphology and femoral stem micromotion: a cadaver model with simulated blood flow.

Background Limited data exist on the performance of low-viscosity cement in clinically realistic cadaver models. Methods Paired stem/cement/femur constructs were generated with low-viscosity and standard-viscosity cements. The constructs were created and tested under simulated in vivo conditions, for which novel techniques were developed during this study. Mantle function was quantified by stem/cortex micromotions over 105cycles of “stair-climbing”. Mantle morphology was determined from transverse sections. Results Penetration of low-viscosity cement was greater proximally but less distally (p = 0.02). Low-viscosity cement resulted in more stem retroversion (p = 0.04), but there was no difference in subsidence (p = 0.4). Low-viscosity cement mantles had greater fractions of non-apposed interface (p = 0.006). Fraction of non-apposed interface predicted stem retroversion (R2 = 0.64, p = 0.002). Interpretation Low-viscosity cement resulted in inferior cement mantles. Early micromotion was reduced by better interface apposition. The greater stem retroversion of low-viscosity cement would probably lead to higher revision rates. Early stem migration is due to interface non-apposition. Techniques should be developed to reduce non-apposition of cemented interfaces.

The Addition of a Hydroxyapatite Coating Changes the Immediate Postoperative Stability of a Plasma-Sprayed Femoral Stem

Nonbiologic and mechanical effects of hydroxyapatite coatings have received little evaluation. Hydroxyapatite coatings give porous metal the appearance of decreased roughness. We hypothesized that this apparent decrease in surface roughness would result in diminished initial implant stability. We measured the initial stability of titanium plasma sprayed press-fit femoral stems with and without HA. Stems were implanted into cadaver and synthetic femora and subjected to aggressive stair-climbing loads. Migrations (retroversion and subsidence) and cyclic motions were recorded. Hydroxyapatite coating significantly reduced retroversion (P = .0007) and cyclic subsidence (P = .0086). Scanning electron microscopy imaging revealed that HA coating appeared to have reduced roughness on a millimeter scale but increased roughness on a micrometer scale. We concluded that HA coating improves initial stability through mechanical means, before biological action.

Functional interface micromechanics of 11 en-bloc retrieved cemented femoral hip replacements

Background and purpose Despite the longstanding use of micromotion as a measure of implant stability, direct measurement of the micromechanics of implant/bone interfaces from en bloc human retrievals has not been performed. The purpose of this study was to determine the stem-cement and cement-bone micromechanics of functionally loaded, en-bloc retrieved, cemented femoral hip components. Methods 11 fresh frozen proximal femurs with cemented implants were retrieved at autopsy. Specimens were sectioned transversely into 10-mm slabs and fixed to a loading device where functional torsional loads were applied to the stem. A digital image correlation technique was used to document micromotions at stem-cement and cement-bone interfaces during loading. Results There was a wide range of responses with stem-cement micromotions ranging from 0.0006 mm to 0.83 mm (mean 0.17 mm, SD 0.29) and cement-bone micromotions ranging from 0.0022 mm to 0.73 mm (mean 0.092 mm, SD 0.22). There was a strong (linear-log) inverse correlation between apposition fraction and micromotion at the stem-cement interface (r2 = 0.71, p < 0.001). There was a strong inverse log-log correlation between apposition fraction at the cement-bone interface and micromotion (r2 = 0.85, p < 0.001). Components that were radiographically well-fixed had a relatively narrow range of micromotions at the stem-cement (0.0006–0.057 mm) and cement-bone (0.0022–0.029 mm) interfaces. Interpretatation Minimizing gaps at the stem-cement interface and encouraging bony apposition at the cement-bone interface would be clinically desirable. The cement-bone interface does not act as a bonded interface in actual use, even in radiographically well-fixed components. Rather, the interface is quite compliant, with sliding and opening motions between the cement and bone surfaces.

Application of circular statistics in the study of crack distribution around cemented femoral components

Cemented stem constructs were loaded in cyclic fatigue using stair climbing loading and the resulting fatigue damage to the cement mantle was determined in terms of angular position of crack and crack length. Techniques from circular statistics were used to determine if the distribution of micro-cracks was uniform. With a designated orientation of 0 degrees -90 degrees -180 degrees -270 degrees indicating lateral-anterior-medial-posterior anatomic directions, the overall distribution of cracks was not uniform (p<0.05) with a mean crack direction in the postero-medial (249 degrees) quadrant of the mantle. The crack angular distribution for proximal (postero-medial; 251 degrees) and distal (antero-medial; 112 degrees) regions of the cement mantle was also different (p<0.025). These findings suggest that the location of cement damage depends on anatomic position and appears to correspond with the tensile stress field in the cement mantle.

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.

A novel hydrogel-coated polyester mesh prevents postsurgical adhesions in a rat model.

BACKGROUND The specific aim of this study was to determine the whether a novel, hydrogel-coated polyester mesh (Scout) can be used to reduce the incidence and severity of adhesion formation in vivo. METHODS An established rat model of post-surgical adhesion formation was used in which adhesions are generated through surgical trauma to the surfaces of the cecum and the adjacent abdominal wall. Thirty-seven rats were randomly allocated either to a control group (no intervention; n=14 rats) or to one of two treatment groups in which the abraded surfaces were separated with either the Scout material (n=11 rats) or an FDA-approved form of expanded polytetrafluorethylene (PTFE) (PRECLUDE Vessel Guard; n=12 rats). Animals were euthanized 7 d after surgery and gross necropsy examinations were performed. Mechanical testing was used to measure the strength of any adhesions that were identified, and histology was used to characterize within the adhesion tissue and on the surface(s) of the barrier materials. RESULTS Five animals were excluded because of surgical failure (1 control; 2 PRECLUDE Vessel Guard; 2 Scout). Adhesions were seen in 10 of 13 control animals (77%). There were no adhesions in any of the animals treated with either PRECLUDE Vessel Guard or Scout material. Histology demonstrated mild cellular adhesion to both the PRECLUDE Vessel Guard and the Scout material. Although there was a sub-acute to chronic inflammatory response to the surgical trauma, there was no evidence of delamination, shearing, or degradation of either the Scout material or PRECLUDE Vessel Guard. CONCLUSIONS The hydrogel-coated Scout material was as effective as the approved predicate material in this model. Both materials were well tolerated. Further testing of the Scout material is now warranted.