Trabecular resorption from interdigitated regions between cement and bone has been found in postmortem‐retrieved knee replacements, but the viability of interdigitated bone, and the mechanism responsible for this bone loss is not known. In this work, a Sprague–Dawley (age 12 weeks) rat knee replacement model with an interdigitated cement–bone interface was developed. Morphological and cellular changes in the interdigitated region of the knee replacement over time (0, 2, 6, or 12 weeks) were determined for ovariectomy (OVX) and Sham OVX treatment groups. Interdigitated bone volume fraction (BV/TV) increased with time for Sham OVX (0.022 BV/TV/wk) and OVX (0.015 BV/TV/wk) group, but the rate of increase was greater for the Sham OVX group (p = 0.0064). Tissue mineral density followed a similar increase with time in the interdigitated regions. Trabecular resorption, when it did occur, started at the cement border with medullary‐adjacent bone in the presence of osteoclasts. There was substantial loss of viable bone (~80% empty osteocyte lacunae) in the interdigitated regions. Pre‐surgical fluorochrome labels remained in the interdigitated regions, and did not diminish with time, indicating that the bone was not remodeling. There was also some evidence of continued surface mineralization in the interdigitated region after cementing of the knee, but this diminished over time. Statement of clinical significance: Interdigitated bone with cement provides mechanical stability for success of knee replacements. Improved understanding of the fate of the interdigitated bone over time could lead to a better understanding of the loosening process and interventions to prevent loss of fixation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2163–2171, 2019

Background Loss of mechanical interlock between cement and bone with in vivo service has been recently quantified for functioning, non-revised, cemented Total Knee Arthroplasties (TKA). The cause of interlocking trabecular resorption is not known. The goal of this study was to quantify the distribution of PE debris at the cement-bone interface and determine if polyethylene (PE) debris is locally associated with loss of interlock. Methods Fresh, non-revised, postmortem-retrieved TKAs (n=8) were obtained en bloc. Lab-prepared constructs (n=2) served as negative controls. The intact cement-bone interface of each proximal tibia was embedded in Spurr’s resin, sectioned, and imaged under polarized light to identify birefringent PE particles. PE wear particle number density was quantified at the cement-bone interface and distal to the interface, then compared to local loss of cement-bone interlock. Results The average PE particle number density for postmortem retrieved TKAs ranged from 8.6 (1.3) to 24.9 (3.1) particles/mm2 (SE) but was weakly correlated with years in service. The average particle number density was twice as high distal (> 5mm) to the interface compared to at the interface. The local loss of interlock at the interface was not related to presence, absence, or particle density of PE. Conclusions PE debris can migrate extensively along the cement-bone interface of well-fixed tibial components. However, the amount of local bone loss at the cement-bone interface was not correlated with the amount of PE debris at the interface, suggesting that the observed loss of trabecular interlock in these well-fixed TKAs may be due to alternative factors.