Do capsular pressure and implant motion interact to cause high pressure in the periprosthetic bone in total hip replacement?

Roles of inflammatory cell infiltrate in periprosthetic osteolysis

Classically, particle-induced periprosthetic osteolysis at the implant-bone interface has explained the aseptic loosening of joint replacement. This response is preceded by triggering both the innate and acquired immune response with subsequent activation of osteoclasts, the bone-resorbing cells. Although particle-induced periprosthetic osteolysis has been considered a foreign body chronic inflammation mediated by myelomonocytic-derived cells, current reports describe wide heterogeneous inflammatory cells infiltrating the periprosthetic tissues. This review aims to discuss the role of those non-myelomonocytic cells in periprosthetic tissues exposed to wear particles by showing original data. Specifically, we discuss the role of T cells (CD3+, CD4+, and CD8+) and B cells (CD20+) coexisting with CD68+/TRAP- multinucleated giant cells associated with both polyethylene and metallic particles infiltrating retrieved periprosthetic membranes. This review contributes valuable insight to support the complex cell and molecular mechanisms behind the aseptic loosening theories of orthopedic implants.

Mechanical instability induces osteoclast differentiation independent of the presence of a fibrous tissue interface and osteocyte apoptosis in a rat model for aseptic loosening

Background and purpose - Insufficient initial fixation or early micromotion of an implant is associated with a thin layer of fibrous tissue at the peri-implant interface. It is unknown if bone loss is induced by the fibrous tissue interface acting as an active biological membrane, or as a membrane that will produce supraphysiologic fluid flow conditions during gait, which activates the mechanosensitive osteocytes to mediate osteoclast differentiation. We investigated whether mechanically induced osteolysis is dependent on the fibrous tissue interface as a biologically active scaffold, or if it merely acts as a conduit for fluid flow, affecting the mechanosensitive osteocytes in the peri-prosthetic bone.Methods - Using a rat model of mechanically instability-induced aseptic loosening, we assessed whether the induction of osteoclast differentiation was dependent on the presence of a peri-implant fibrous interface. We analyzed the amount of osteoclast differentiation, osteocyte apoptosis, pro-resorptive cytokine expression and bone loss using immunohistochemistry, mRNA expression and micro-CT.Results - Osteoclast differentiation and bone loss were induced by mechanical instability but were not affected by the presence of the fibrous tissue membrane or associated with osteocyte apoptosis. There was no increased mRNA expression of any of the cytokines in the fibrous tissue membrane compared with the peri-implant bone.Interpretation - Our data show that the fibrous tissue membrane in the interface plays a minor role in inducing bone loss. This indicates that the peri-implant bone adjacent to loose bone implants might play an important role for osteoclast differentiation.

Mechanically Induced Periprosthetic Osteolysis: A Systematic Review

BACKGROUND

Peri-prosthetic bone loss can result from chemical, biological, and mechanical factors. Mechanical stimulation via fluid pressure and flow at the bone-implant interface may be a significant cause. Evidence supporting mechanically induced osteolysis continues to grow, but there is no synthesis of published clinical and basic science data.

QUESTIONS/PURPOSES

We sought to review the literature on two questions: (1) What published evidence supports the concept of mechanically induced osteolysis? (2) What is the proposed mechanism of mechanically induced osteolysis, and does it differ from that of particle-induced osteolysis?

METHODS

A systematic review was performed of the PubMed and Web of Science databases. Additional relevant articles were recommended by the senior authors based on their expert opinion. Abstracts were reviewed and the manuscripts pertaining to the study questions were read in full. Studies showing support of mechanically induced osteolysis were quantified and findings summarized.

RESULTS

We identified 49 articles of experimental design supporting the hypothesis that mechanical stimulation of peri-prosthetic bone from fluid pressure and flow can induce osteolysis. While the molecular mechanisms may overlap with those implicated in particle-induced osteolysis, mechanically induced osteolysis appears to be mediated by distinct and parallel pathways.

CONCLUSIONS

The role of mechanical stimuli is increasingly recognized in the pathogenesis of peri-prosthetic osteolysis. Current research aims to elucidate the molecular mechanisms to better target therapeutic interventions.

The Peritubercle Lucency Sign is a Common and Early Radiographic Finding in Slipped Capital Femoral Epiphysis

BACKGROUND

A rotational mechanism of slipped capital femoral epiphysis (SCFE) in which the epiphyseal tubercle acts as a fulcrum has been recently described. However, there is limited radiographic evidence supporting this theory. We aimed to investigate whether a radiographic lucency around the epiphyseal tubercle (peritubercle lucency sign) could be observed before or at the diagnosis of a subsequent slip in patients initially presenting with unilateral SCFE.

METHODS

We evaluated 70 patients with unilateral SCFE who were followed until a contralateral slip was diagnosed, and 180 patients with unilateral SCFE who had no contralateral slip until skeletal maturity between 2000 and 2017. We revised anteroposterior and lateral radiographs from the initial presentation, surveillance, and diagnosis of a contralateral SCFE. The presence or absence of the peritubercle lucency sign in the contralateral initially uninvolved hip was recorded for every radiograph. The presence of hip pain, and the presence of classic radiographic parameters for SCFE diagnosis (epiphyseal tilt and Klein's line) were recorded when the peritubercle lucency sign became noticeable.

RESULTS

The peritubercle lucency sign was observed in 84% [59/70; 95% confidence interval (CI), 73%-92%] of the patients who developed contralateral slip. Most patients (46/59=78%; 95% CI, 65%-87%; P<0.001) had the peritubercle lucency sign visible within 9 months after the initial diagnosis of unilateral SCFE. The sign preceded the definitive diagnosis of the contralateral slip at a median of 9 weeks (interquartile range, 0 to 10 wk). At the time of first observation of the peritubercle lucency sign, 36% (25/70; 95% CI, 25%-48%) of the patients were asymptomatic and 49% (34/70; 95% CI, 36%-61%) of the hips showed no abnormalities in the tilt angle or the Klein line. Among patients who did not experience a contralateral slip (n=180), the sign was observed in 2 patients (1.1%; 95% CI, 0.2%-4.4%). The sensitivity was 84% and specificity was 99%.

CONCLUSIONS

A peritubercle lucency is an early imaging sign, present in >80% of contralateral slips following an initial presentation of unilateral SCFE. The presence of the peritubercle lucency sign may be helpful for contralateral hip surveillance. Future studies are necessary to establish the clinical validity of the peritubercle lucency sign and whether it may serve as a predictor of contralateral involvement.

LEVEL OF EVIDENCE

Diagnostic level III.

GSK-3β inhibition suppresses instability-induced osteolysis by a dual action on osteoblast and osteoclast differentiation

Currently, there are no medications available to treat aseptic loosening of orthopedic implants. Using osteoprotegerin fusion protein (OPG-Fc), we previously blocked instability-induced osteoclast differentiation and peri-prosthetic osteolysis. Wnt/β-catenin signaling, which regulates OPG secretion from osteoblasts, also modulates the bone tissue response to mechanical loading. We hypothesized that activating Wnt/β-catenin signaling by inhibiting glycogen synthase kinase-3β (GSK-3β) would reduce instability-induced bone loss through regulation of both osteoblast and osteoclast differentiation. We examined effects of GSK-3β inhibition on regulation of RANKL and OPG in a rat model of mechanical instability-induced peri-implant osteolysis. The rats were treated daily with a GSK-3β inhibitor, AR28 (20 mg/kg bw), for up to 5 days. Bone tissue and blood serum were assessed by qRT-PCR, immunohistochemistry, and ELISA on days 3 and 5, and by micro-CT on day 5. After 3 days of treatment with AR28, mRNA levels of β-catenin, Runx2, Osterix, Col1α1, and ALP were increased leading to higher osteoblast numbers compared to vehicle-treated animals. BMP-2 and Wnt16 mRNA levels were downregulated by mechanical instability and this was rescued by GSK-3β inhibition. Osteoclast numbers were decreased significantly after 3 days of GSK-3β inhibition, which correlated with enhanced OPG mRNA expression. This was accompanied by decreased serum levels of TRAP5b on days 3 and 5. Treatment with AR28 upregulated osteoblast differentiation, while osteoclastogenesis was blunted, leading to increased bone mass by day 5. These data suggest that GSK-3β inactivation suppresses osteolysis through regulating both osteoblast and osteoclast differentiation in a rat model of instability-induced osteolysis.

The implication of the osteolysis threshold and interfacial gaps on periprosthetic osteolysis in cementless total hip replacement

Osteolysis around joint replacements may develop due to migration of wear particles from the joint space into gaps between the interface bone and the implant where they can accumulate in high concentrations to cause tissue damage. Osteolysis may appear in various postoperative times and morphological shapes which can be generalized into linear and focal. However, there are no clear explanations on the causes of such variations. Patients' degree of sensitivity to polyethylene particles (osteolysis thresholds), the local particle concentration and the access route provided by the interface gaps have been described as determining factors. To study their effects, a 2D computational fluid dynamics model of the hip joint capsule in communication with an interfacial gap and the surrounding bone was employed. Particles were presented using a discrete phase model (DPM). High capsular fluid pressure was considered as the driving force for particle migration. Simulations were run for different osteolysis thresholds ranging from 5×10⁸ to 1×10¹² particle number per gram of tissue and fibrous tissue generation in osteolytic lesion due to particles was simulated for the equivalent of ten postoperative years. In patients less sensitive to polyethylene particles (higher threshold), osteolysis may be linear and occur along an interfacial gap in less than 5% of the interfacial tissue. Focal osteolysis is more likely to develop in patients with higher sensitivity to polyethylene particles at distal regions to an interfacial gaps where up to 80% of the interfacial tissue may be replaced by fibrous tissue. In these patients, signs of osteolysis may also develop earlier (third postoperative year) than those with less sensitivity who may show very minor signs even after ten years. This study shows the importance of patient sensitivity to wear particles, the role of interfacial gaps in relation to morphology and the onset of osteolysis. Consequently, it may explain the clinically observed variation in osteolysis development.

Micromotion-induced peri-prosthetic fluid flow around a cementless femoral stem

Micromotion-induced interstitial fluid flow at the bone-implant interface has been proposed to play an important role in aseptic loosening of cementless implants. High fluid velocities are thought to promote aseptic loosening through activation of osteoclasts, shear stress induced control of mesenchymal stem cells differentiation, or transport of molecules. In this study, our objectives were to characterize and quantify micromotion-induced fluid flow around a cementless femoral stem using finite element modeling. With a 2D model of the bone-implant interface and full-factorial design, we first evaluated the relative influence of material properties, and bone-implant micromotion and gap on fluid velocity. Transverse sections around a femoral stem were built from computed tomography images, while boundary conditions were obtained from experimental measurements on the same femur. In a second step, a 3D model was built from the same data-set to estimate the shear stress experienced by cells hosted in the peri-implant tissues. The full-factorial design analysis showed that local micromotion had the most influence on peak fluid velocity at the interface. Remarkable variations in fluid velocity were observed in the macrostructures at the surface of the implant in the 2D transverse sections of the stem. The 3D model predicted peak fluid velocities extending up to 2.2 mm/s in the granulation tissue and to 3.9 mm/s in the trabecular bone. Peak shear stresses on the cells hosted in these tissues ranged from 0.1 to 12.5 Pa. These results offer insight into mechanical stimuli encountered at the bone-implant interface.

Osteolysis around total knee arthroplasty: a review of pathogenetic mechanisms

Aseptic loosening and other wear-related complications are some of the most frequent late reasons for revision of total knee arthroplasty (TKA). Periprosthetic osteolysis (PPOL) pre-dates aseptic loosening in many cases, indicating the clinical significance of this pathogenic mechanism. A variety of implant-, surgery- and host-related factors have been delineated to explain the development of PPOL. These factors influence the development of PPOL because of changes in mechanical stresses within the vicinity of the prosthetic device, excessive wear of the polyethylene liner, and joint fluid pressure and flow acting on the peri-implant bone. The process of aseptic loosening is initially governed by factors such as implant/limb alignment, device fixation quality and muscle coordination/strength. Later, large numbers of wear particles detached from TKA trigger and perpetuate particle disease, as highlighted by progressive growth of inflammatory/granulomatous tissue around the joint cavity. An increased accumulation of osteoclasts at the bone-implant interface, impairment of osteoblast function, mechanical stresses and increased production of joint fluid contribute to bone resorption and subsequent loosening of the implant. In addition, hypersensitivity and adverse reactions to metal debris may contribute to aseptic TKA failure, but should be determined more precisely. Patient activity level appears to be the most important factor when the long-term development of PPOL is considered. Surgical technique, implant design and material factors are the most important preventative factors, because they influence both the generation of wear debris and excessive mechanical stresses. New generations of bearing surfaces and designs for TKA should carefully address these important issues in extensive preclinical studies. Currently, there is little evidence that PPOL can be prevented by pharmacological intervention.

Emerging ideas: Instability-induced periprosthetic osteolysis is not dependent on the fibrous tissue interface

BACKGROUND

Stable initial fixation of a total joint arthroplasty implant is critical to avoid the risk of aseptic loosening and premature clinical failure. With implant motion, a fibrous tissue layer forms at the bone-implant interface, leading to implant migration and periprosthetic osteolysis. At the time of implant revision surgery, proresorptive signaling cytokines are expressed in the periimplant fibrous membrane. However, the exact role of this fibrous tissue in causing periprosthetic osteolysis attributable to instability remains unknown.

QUESTIONS/HYPOTHESES

We propose an alternative mechanism of periprosthetic osteolysis independent of the fibrous tissue layer, where pressurized fluid flow along the bone-implant interface activates mechanosensitive osteocytes in the periprosthetic bone, causing the release of proresorptive cytokines and subsequent osteoclast differentiation and osteolysis.

METHOD OF STUDY

An animal model for instability-induced osteolysis that mimics the periprosthetic bone-implant interface will be used. In this model, a fibrous tissue membrane is allowed to form in the periprosthetic zone, and pressurized fluid flow transmitted through this membrane reliably creates osteolytic lesions in the periprosthetic bone. In this study, half of the rats will have the fibrous tissue present, while the other half will not. We will determine whether the fibrous tissue membrane is essential for the release of proosteoclastic cytokines, leading to osteoclast differentiation and periprosthetic bone loss, by measuring the volume of bone resorption and presence of proresorptive cytokines at the bone-implant interface.

SIGNIFICANCE

We will determine whether the fibrous tissue membrane is crucial for osteoclastogenic signaling in the setting of periimplant osteolysis. In the future, this will allow us to test therapeutic interventions, such as specific cytokine inhibitors or alterations in implant design, which may translate into new, clinically relevant strategies to prevent osteolysis.

Fluid-structure interactions in micro-interlocked regions of the cement-bone interface

Experimental tests and computational modelling were used to explore the fluid dynamics at the trabeculae-cement interlock regions found in the tibial component of total knee replacements. A cement-bone construct of the proximal tibia was created to simulate the immediate post-operative condition. Gap distributions along nine trabeculae-cement regions ranged from 0 to 50.4 μm (mean = 12 μm). Micro-motions ranged from 0.56 to 4.7 μm with a 1 MPa compressive load to the cement. Fluid-structure analysis between the trabeculae and the cement used idealised models with parametric evaluation of loading direction, gap closing fraction (GCF), gap thickness, loading frequency and fluid viscosity. The highest fluid shear stresses (926 Pa) along the trabecular surface were found for conditions with very thin and large GCFs, much larger than reported physiological levels (~1-5 Pa). A second fluid-structure model was created with a provision for bone resorption using a constitutive model with resorption velocity proportional to fluid shear rate. A lower cut-off was used, below which bone resorption would not occur (50 s(-1)). Results showed that there was initially high shear rates (>1000 s(-1)) that diminished after initial trabecular resorption. Resorption continued in high shear rate regions, resulting in a final shape with bone left deep in the cement layer, and is consistent with morphology found in post-mortem retrievals. Small gaps between the trabecular surface and the cement in the immediate post-operative state produce fluid flow conditions that appear to be supra-physiologic; these may cause fluid-induced lysis of trabeculae in the micro-interlock regions.