Early Changes in Cement-Bone Fixation Using a Novel Rat Knee Replacement Model

Peri-operative zoledronic acid attenuates peri-prosthetic osteolysis in a rat model of cemented knee replacement

Progressive osteolysis can occur at the cement-bone interface of joint replacements and the associated loss of fixation can lead to clinical loosening. We previously developed a rat hemiarthroplasty model that exhibited progressive loss of fixation with the development of cement-bone gaps under the tibial tray that mimicked patterns found in human arthroplasty retrievals. Here we explored the ability of a bisphosphonate (zoledronic acid, ZA) to attenuate cement-bone osteolysis and maintain implant stability. Sprague-Dawley rats (n = 59) received a poly(methylmethacrylate) cemented tibial component and were followed for up to 12 weeks. Treatment groups included peri-operative administration of ZA (ZA group), administration of ZA at 6 weeks postop (late ZA group), or vehicle (Veh group). There was a 60% reduction in the rate of cement-bone gap formation for the ZA group (0.15 mm3/week) compared to Veh group (0.38 mm3/week, p = 0.016). Late ZA prevented further progression of gap formation but did not reverse bone loss to the level achieved in the ZA group. Micromotion from five times body weight toggle loading was positively correlated with cement-bone gap volume (p = 0.009) and negatively correlated with the amount of cement in the metaphysis (p = 0.005). Reduced new bone formation and enduring nonviable bone in the epiphysis for the ZA group were found. This suggests that low bone turnover in the epiphysis may suppress the early catabolic response due to implantation, thereby maintaining better fixation in the epiphysis. This preclinical model presents compelling supporting data documenting improved maintenance of the cement-bone fixation with the use of peri-operative bisphosphonates.

Potential for supraphysiologic fluid shear stresses in a rat cemented knee replacement model

The mechano-biologic environment associated with aseptic loosening of cemented joint replacements is not fully understood. The goal of this study was to use a preclinical rat knee arthroplasty model to explore the changes in cement-bone morphology and micromotion that occur with in vivo service. Narrow gaps between cement and bone under the tibial tray were present at early time points, and with even small magnitude micromotion, resulted in large micromotion-to-gap width ratios. These data were then used to develop models of fluid flow in the cement-bone gaps to estimate potential for high fluid shear stress (FSS). Modeling results revealed supraphysiologic (>4 Pa) FSS were possible, particularly for cases in which eccentric loading applied to the implant and if the fluid in the gap consisted of marrow or synovial fluid. The early, high FSS environment, could cause fluid-induced periprosthetic osteolysis locally, resulting in progressive loss of cement-bone fixation.

Experimental study of the effects of hypoxia simulator on osteointegration of titanium prosthesis in osteoporotic rats

Background

Poor osseointegration is the key reason for implant failure after arthroplasty,whether under osteoporotic or normal bone conditions. To date, osseointegration remains a major challenge. Recent studies have shown that deferoxamine (DFO) can accelerate osteogenesis by activating the hypoxia signaling pathway. The purpose of this study was to test the following hypothesis: after knee replacement, intra-articular injection of DFO will promote osteogenesis and osseointegration with a 3D printed titanium prosthesis in the bones of osteoporotic rats.

Materials and methods

Ninety female Sprague–Dawley rats were used for the experiment. Ten rats were used to confirm the successful establishment of the osteoporosis model: five rats in the sham operation group and five rats in the ovariectomy group. After ovariectomy and knee arthroplasty were performed, the remaining 80 rats were randomly divided into DFO and control groups (n = 40 per group). The two groups were treated by intraarticular injection of DFO and saline respectively. After 2 weeks, polymerase chain reaction (PCR) and immunohistochemistry were used to evaluate the levels of HIF-1a, VEGF, and CD31. HIF-1a and VEGF have been shown to promote angiogenesis and bone regeneration, and CD31 is an important marker of angiogenesis. After 12 weeks, the specimens were examined by micro-computed tomography (micro-CT), biomechanics, and histopathology to evaluate osteogenesis and osseointegration.

Results

The results of PCR showed that the mRNA levels of VEGF and CD31 in the DFO group were significantly higher than those in the control group. The immunohistochemistry results indicated that positive cell expression of HIF-1a, VEGF, and CD31 in the DFO group was also higher. Compared with the control group, the micro-CT parameters of BMD, BV/TV, TB. N, and TB. Th were significantly higher. The maximal pull-out force and the bone-to-implant contact value were also higher.

Conclusions

The local administration of DFO, which is used to activate the HIF-1a signaling pathway, can promote osteogenesis and osseointegration with a prosthesis in osteoporotic bone.

Progressive loss of implant fixation in a preclinical rat model of cemented knee arthroplasty

Aseptic loosening of total knee arthroplasty continues to be a challenging clinical problem. The progression of the loosening process, from the initial well-fixed component, is not fully understood. In this study, loss of fixation of cemented hemiarthroplasty was explored using 9-month-old Sprague-Dawley rats with 0, 2, 6, 12, 26 week end points. Morphological and cellular changes of cement-bone fixation were determined for regions directly below the tibial tray (epiphysis) and distal to the tray (metaphysis). Loss of fixation, with a progressive increase in cement-bone gap volume was found in the epiphysis (0.162  mm³ /week), but did not progress appreciably in the metaphysis (0.007 mm³ /week). In the epiphysis, there was an early and sustained elevation of osteoclasts adjacent to the cement border and development of a fibrous tissue layer between the cement and bone. There was early formation of bone around the cement in the metaphysis, resulting in a condensed bone layer without osteoclastic bone resorption or development of a fibrous tissue layer. Implant positioning was also an important factor in the cement-bone gap formation, with greater gap formation for implants that were placed medially on the tibial articular surface. Loss of fixation in the rat model mimicked patterns found in human arthroplasty where cement-bone gaps initiate under the tibial tray, at the periphery of the implant. This preclinical model could be used to study early biological response to cemented fixation and associated contributions of mechanical instability, component alignment, and periprosthetic inflammation.

Similitude of cement-bone micromechanics in cemented rat and human knee replacement

A preclinical rat knee replacement model was recently developed to explore the biological and mechanobiological changes of trabecular resorption for cement-bone interdigitated regions. The goal here was to evaluate the relevance of this model compared with human knee replacement with regards to functional micromechanics. Eight nonsurvival, cemented knee replacement surgeries were performed, the interdigitated gap morphology was quantified, and interface micromotion between cement and bone was measured for 1 to 5 bodyweight loading. Computational fluid dynamics modeling of unit cell geometries with small gaps between trabeculae and cement was used to estimate fluid flow. Gap width (3.6 μm) was substantially smaller compared with cement-bone gaps reported in human knee replacement (11.8 μm). Micromotion at the cement-bone border was also decreased for the rat knee replacement (0.48 μm), compared with human (1.97 μm), for 1 bodyweight loading. However, the micromotion-to-gap width ratio (0.19 and 0.22 for, rat and human), and estimated fluid shear stress (6.47 and 7.13 Pa, for rat and human) were similar. Replicating the fluid dynamic characteristics of cement-bone interdigitated regions in human knee replacements using preclinical models may be important to recapitulate trabecular resorption mechanisms due to proposed supraphysiologic fluid shear stress. Statement of clinical significance: local cement-bone micromotion due to joint loading may contribute to the process of clinical loosening in total joint replacements. This work shows that while micromotion and gap morphology are diminished for the rat knee model compared to human, the motion-to-gap ratio, and corresponding fluid shear stress are of similar magnitudes.