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Despite the widespread use of cement as a means of fixation of implants to bone, surprisingly little is known about the micromechanical behavior in terms of the local interfacial motion. In this work, we utilized digital image correlation techniques to quantify the micromechanics of the cement-bone interface of laboratory-prepared cemented total hip replacements subjected to nondestructive, quasistatic tensile and compressive loading. Upon loading, the majority of the displacement response localized at the contact interface region between cement and bone. The contact interface was more compliant ( p ¼ 0.0001) in tension (0.0067 AE 0.0039 mm/MPa) than compression (0.0051 AE 0.0031 mm/MPa), and substantial hysteresis occurred due to sliding contact between cement and bone. The tensile strength of the cement-bone interface was inversely proportional to the compliance of the interface and proportional to the cement/bone contact area. When loaded beyond the ultimate strength, the strain localization process continued at the contact interface between cement and bone with microcracking (damage) to both. More overall damage occurred to the cement than to the bone. The opening and closing at the contact interface from loading could serve as a conduit for submicron size particles. In addition, the cement mantle is not mechanically supported by surrounding bone as optimally as is commonly assumed. Both effects may influence the longevity of the reconstruction and could be considered in preclinical tests. ß

Background Aseptic loosening continues to be a shortand long-term complication for patients with cemented TKAs. Most studies to this point have evaluated tibial component fixation via radiographic changes at the implant-bone interface and quantification of component migration; direct assessment of morphologic features of the interface from functioning TKAs may provide new information regarding how TKAs function and are fixed to bone. Questions/purposes In a postmortem retrieval study, we asked: (1) What are the morphologic features at the cement-trabecular bone interface in retrieved tibial components? (2) Do constructs with greater time in service have less cement-trabecular bone interlock? (3) Do constructs with more estimated initial interlock sustain more interlock with in vivo service? Methods Fourteen postmortem retrieved tibial components with time in service from 0 to 20 years were sectioned and imaged at high resolution, and the current contact fraction, estimated initial interdigitation depth, current interdigitation depth, and loss of interdigitation depth were quantified at the cement-bone interface. Estimated initial interdigitation depth was calculated from the initial mold shape of the cement mantle that forms around the individual trabeculae at the time of surgery. Loss of interdigitation depth was the difference between the initial and current interdigitation depth. Results There was resorption of trabeculae that initially interlocked with the cement in the postmortem retrievals as evidenced by the differences between current interdigitation and the estimated original interdigitation. The current contact fraction (r 2 = 0.54; p = 0.0027) and current interdigitation depth (r 2 = 0.33; p = 0.033) were less for constructs with longer time in service. The current contact fraction for implants with 10 or more years in service (6.2%; 95% CI, 4.7%-7.7%) was much less than implants with less than 10 years in service (22.9%; 95% CI, 8.9%-37%). Similarly, the current interdigitation depth for implants with 10 or more years in service (0.4 mm; 95% CI, 0.27-0.53 mm) was much less than implants with less

The cement-bone interface plays an important role in load transfer between cemented implant systems and adjacent bone, but little is known about the micromechanical behavior of this interface following in vivo service. Small samples of postmortem-retrieved cement-bone specimens from cemented total hip replacements were prepared and mechanically loaded to determine the response to tensile and compressive loading. The morphology of the cement-bone interface was quantified using a CT-based stereology approach. Laboratoryprepared specimens were used to represent immediate postoperative conditions for comparison. The stiffness and strength of the cementbone interface from postmortem retrievals was much lower than that measured from laboratory-prepared specimens. The cement-bone interfaces from postmortem retrievals were very compliant (under tension and compression) and had a very low tensile strength (0.21 AE 0.32 MPa). A linear regression model, including interface contact fraction and intersection fraction between cement and bone, could explain 71% (p < 0.0001) of the variability in experimental response. Bony remodeling following an arthroplasty procedure may contribute to reduced contact between cement and bone, resulting in weaker, more compliant interfaces. ß

Predicting fracture risk for patients with metastatic femoral lesions remains an important clinical problem. Mirels' criterion remains the most formalized radiographic scoring system with good sensitivity (correctly identifying clinical fractures) but relatively poor specificity (correctly identify cases that d8o not fracture). A series of patients with metastatic femoral lesions had Computed Tomography (CT) scans, were followed prospectively for 4 months, and categorized into fracture (n ¼ 5), nonfracture (n ¼ 28), or stabilized (n ¼ 11) groups. CT based-Finite Element (FE) modeling was used to predict fracture for these cases using axial compression (AC), level walking (LW), and aggressive stair ascent (ASA) loading conditions. The FE predicted fracture force was greater for the non-fracture compared to the fracture group for all loading cases. The ability of the FE models to predict fracture cases (sensitivity) was similar for the groups (Mirels, AC, LW: 80%, ASA: 100%). The ability of the models to correctly predict the nonfracture cases (specificity) was improved for AC (71%) and LW (86%) loading conditions, when compared to Mirels specificity (43%), but poorer for the ASA (21%) conditions. The results suggest that FE models that assess fracture risk using LW conditions can improve fracture prediction over Mirels scoring in a clinical population. Keywords: fracture prediction; metastatic; femur; finite element; Mirels score Prophylactic stabilization is sometimes needed for patients with metastatic femoral lesions, but correctly identifying cases that are at risk of fracture remains a clinical challenge. This determination is based on the clinician's assessment of radiographs and prior experience. Mirels' criterion remains the most formalized radiographic scoring system. [1][2][3][4] It has good sensitivity (correctly identifying patients that do go on to fracture) but poor specificity (predicts that patients will fracture, when in fact they don't). Clearly, improved efforts are needed to avoid unnecessary fracture in those at high risk and unnecessary surgery in those at low risk.Computed Tomography (CT)-based finite element (FE) modeling has been used to assess fracture risk of the femur for fragility fractures 5-7 and shows improvement in fracture prediction over standard bone density measures.8 FE modeling has also been used to estimate fracture risk for femoral metastatic lesions, but these comparisons have been made using cadavers with laboratory created defects.9,10 CT-based finite element modeling has the potential to improve fracture risk assessment, but this has not been performed on a patient population with metastatic disease.In this study, we performed non-linear FE analysis for a series of clinical cases with disseminated tumors to the femur and known clinical outcomes (fracture, no fracture, or prophylactic stabilization). We asked three research questions: (1) Does a CT-based FE modeling approach discriminate bone strength between the fracture and no fracture patient groups? (2) Do...

We have compared the interface morphology at the stem-cement interface of standard Charnley stems with a satin finish (Ra = 0.75 μm) with identical stems which had been grit-blasted over their proximal third (Ra = 5.3 μm) 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 vertebral column of the Atlantic white‐sided dolphin, Lagenorhynchus acutus, reflects the radical reorganization of the cetacean column for locomotion in water. Both posterior thoracic and anterior caudal vertebrae have been “lumbarized,” and discontinuities occur within the caudal series at the synclinal point and fluke base. Morphology changes subtly as body size increases. Neural process height increases more rapidly, and centrum length more variably, than other vertebral parameters. As a result, large animals have disproportionately tall neural processes, short necks, long mid‐body regions, and short flukes. Vertebral columns of large animals also show greater complexity (range, irregularity, and polarization) of centrum length than do those of smaller animals. Comparisons among dolphins reveal that complexity trends with respect to differentiation of parts run counter to the trend with respect to number of parts, a relationship predicted by Williston in 1914.

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