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

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...

Prevention of aseptic loosening of total knee arthroplasties (TKAs) remains an important clinical challenge. Understanding how changes in morphology at the implant-bone interface with in vivo service affects implant stability and strength could lead to new approaches to mitigate loosening. Enbloc TKA retrievals and freshly-cemented TKA tibial components were used to determine if the mechanical strength of the interface depended on the amount of cement-bone interlock and the morphology of the supporting bone under the cement layer. Implants were sectioned into small specimens of the cement-interface-bone from under the tibial tray. Micro-CT scans were used to document interlock morphology and architecture of the supporting trabecular bone. Axial compression tests were used to assess mechanical behavior. Postmortem retrievals had lower contact fraction (42±55%) compared to freshly-cemented constructs (121±61%) (p=0.0008). Supporting bone architecture parameters were not different for the two groups. Increased interface contact fraction and supporting bone volume fraction (BV/TV) were positive predictors of interface strength (r2=0.72, p=0.0001). For the same supporting bone BV/TV, postmortem specimens had weaker interfaces; they were also more compliant. Cemented TKA with in vivo service experience a loss of fixation strength and increased micro-motion due to the loss of cement-bone interlock.

Aseptic loosening of cemented tibial components in total knee arthroplasty (TKA) has been related to inadequate cement penetration into the trabecular bone bed during implantation. Recent postmortem retrieval work has also shown there is loss of interlock between cement and bone by resorption of trabeculae at the interface. The goal of this study was to determine if TKAs with more initial interlock between cement and bone would maintain more interlock with in vivo service (in the face of resorbing trabeculae) and have less micro-motion at the cement–bone interface. The initial (created at surgery) and current (after in vivo service) cement–bone interlock morphologies of sagittal implant sections from postmortem retrieved tibial tray constructs were measured. The implant sections were then functionally loaded in compression and the micro-motion across the cement–bone interface was quantified. Implant sections with less initial interdigitation between cement and bone and more time in service had less current cement–bone interdigitation (r2 = 0.86, p = 0.0002). Implant sections with greater initial interdigitation also had less micro-motion after in vivo service (r2 = 0.36, p = 0.0062). This work provides direct evidence that greater initial interlock between cement and bone in tibial components of TKA results in more stable constructs with less micro-motion with in vivo service.

With in vivo service there is loss of mechanical interlock between trabeculae and PMMA cement in total knee replacements. The mechanisms responsible for the loss of interlock are not known, but loss of interlock results in weaker cement-bone interfaces. The goal of this study was to determine the pattern of resorption of interdigitated bone using a series of 20 postmortem retrieved knee replacements with a wide range of time in service (3 to 22 years). MicroCT scans were obtained of a segment of the cement-bone interface below the tibial tray for each implant. Image processing methods were used to determine interface morphology and to identify supporting, interdigitated, resorbed, and isolated bone as a function of axial position. Overall, the amount of remaining interdigitated bone decreased with time in service (p=0.0114). The distance from the cement border (at the extent of cement penetration into the bone bed) to 50% of the interdigitated volume decreased with time in service (p=0.039). Isolated bone, when present was located deep in the cement layer. Overall, resorption appears to start at the cement border and progresses into the cement layer. Initiation of trabecular resorption near the cement border may be a consequence of proximity to osteoclastic cells in the adjacent marrow space.