Peter A. Revell

The size and shape of particulate polyethylene wear debris in total joint replacements.

Abstract Osteolysis induced by wear particles has been recognized as one of the major causes of long-term failure in total joint replacements. However, little is currently known about the exact nature of particles, as the particles are too small to be characterized by light microscopy. In this study, ultra-high molecular weight polyethylene (UHMWPE) particles retrieved from ten cases (six cemented and four uncemented) for Freeman type conforming tibiofemoral total knee replacements (TKRs), three Charnley total hip replacements (THRs) and five Imperial College/London Hospital double cup surface hip replacements for aseptic loosening were extracted using a high-performance method with ultracentrifugation and characterized by scanning electron microscopy. The equivalent circle diameter (ECD) of all 18 cases ranged from 0.40 to 1.15 μm (Mean ± SE = 0.70 ± 0.05 μm, median = 0.67 μm). The aspect ratio was 1.50 to 2.04 (Mean ± SE = 1.75 ± 0.04, median = 1.73), and roundness was 1.24 to 2.34 (Mean ± SE = 1.61 ± 0.07, median = 1.65). The numbers of particles were 5.2 × 108 to 9.17 × 1010/g tissue (Mean ± SE = 1.42 × 1010 ± 5.41 × 109/g tissue, median = 7.04 × 109). The number of polyethylene (PE) particles/g tissue in TKRs was significantly larger than that in THRs (1.04 × 1010/g tissue and 2.16 × 109/g tissue respectively, median. p = 0.03, Mann-Whitney U test). Unstable fixation of the tibial PE component might account for the accumulation of a large number of PE particles in the interface tissue.

Wear particulate species and bone loss in failed total joint arthroplasties

The aim of this study was to evaluate the relative contribution of polyethylene, metal, and polymethylmethacrylate (cement) particles to the overall bone loss in aseptic loosening. Twenty-four interface tissues with adjacent bone were obtained during 17 revision total joint arthroplasties (11 hips and six knees). Osteoclasts and macrophages were identified immunohistochemically on the bone surface. The length of the bone surface in contact with these cell types was measured and analyzed with reference to the particulate species present within the fibrous interface. The presence of abundant polyethylene particles significantly increased the proportion of the bone surface in contact with macrophages but did not have a significant influence on that of osteoclasts. Osteoclastic bone resorption was significantly more extensive in the presence of metal particles. In contrast, the presence of cement particles did not have a significant influence on macrophage or osteoclast coverage of the bone surface. These results highlight the significance of polyethylene particles in macrophage recruitment and subsequent osteolysis and suggest a different mechanism of bone loss related to metal, namely mediation through osteoclastic activities. The relative contribution of cement particles was negligible and needs reevaluation in light of evidence provided by others.

Biomechanical assessment of bone ingrowth in porous hydroxyapatite

Porous hydroxyapatite (Endobon®) specimens were implanted into the femoral condyle of New Zealand White rabbits for up to 6 months. After sacrifice, specimens were sectioned for histology and mechanical testing, where the extent of reinforcement by bony ingrowth was assessed by compression testing and fixation was assessed by push-out testing. From histological observations, it was established that the majority of bone ingrowth occurred between 10 day and 5 weeks after implantation and proceeded predominantly from the deep end of the trephined defect, with some integration from the circumferential sides. At 3 months, the implants were fully integrated, exhibiting bony ingrowth, vascularization and bone marrow stroma within the internal macropores. After 5 weeks, the mean ultimate compressive strength of retrieved implants (6.9 MPa) was found to be greater than that of the original implant (2.2 MPa), and by 3 months the fully integrated implants attained a compressive strength of approximately 20 MPa. Push-out testing demonstrated that after 5 weeks in vivo, the interfacial shear strength reached 3.2 MPa, increasing to 7.3 MPa at 3 and 6 months.

Histomorphological and biomechanical characterization of calcium phosphates in the osseous environment

Abstract The standardization of characterization techniques is becoming increasingly important for bone replacement materials as it becomes apparent that, for the field to advance, testing must be developed to allow the biocompatibility or bioactivity of a new material to be assessed and directly compared with existing materials. Currently there are many forms of biocompatibility test for materials destined for the osseous environment, ranging from immersion in simulated body fluid to implantation into living bone. However, the variety of ways in which the data from these tests may be acquired and interpreted, as a result of changes in parameters such as surgical technique and mechanical test conditions, means that much of the published data within the field is not comparable. This paper will introduce the concept of biocompatibility by considering calcium phosphate bioceramics, and discusses some aspects of in vivo experimental design, including simple histomorphometry techniques, in addition to considering practical methods for the assessment of the biomechanical characteristics of an osseointegrated implant.

Presence of macrophages at the bone-cement interface of stable hip arthroplasty components

The bone cement interface of four clinically stable hip-joint prosthetic components was examined histologically for the presence of macrophages using routine staining and a histochemical technique for acid phosphatase. Macrophages were found in the absence of wear debris in all four cases. Because these cells are capable of bone resorption, their presence at a well-fixed interface must give cause for concern.

Expression profile of T cell associated molecules in the interfacial tissue of aseptically loosened prosthetic joints.

The involvement of T cells in the progression of inflammation in response to wear debris at the interface of aseptically loosened joints is currently undefined. This cell type has repeatedly been demonstrated to be a common component of the cellular membrane, the interface, which forms between the bone and implant of total joint replacements (TJRs) [1, 2]. Three further insights into the role of this cell type in the interface were investigated here. Immunostaining demonstrated CD4 expression in 80% of the 15 cases tested while CD8 expression was present in 60% of the cases. Polymerase chain reaction (PCR) detected IFN-γ mRNA expression in 75% of eight cases tested; in contrast IL-10 mRNA was only demonstrated in 50% of these same cases. Proteins extracted from another eight cases of revision tissue were analyzed using Western blotting for IL-17, fractalkine (Fkn) and CD40. IL-17 and Fkn were a consistent feature of all cases tested (8/8), while CD40 was undetectable in one case (7/8). These results show that T cells present in the interface are more commonly of the helper T cell phenotype, although cytotoxic T cells are also present. Helper T cells (Th) are responsible for the polarization of the immune response through their production of key mediators. The PCR results obtained in this study suggest that a Th1 response characterized by the production of IFN-γ predominates over the Th2, IL-10 mediated response. Furthermore the demonstration of the expression of IL-17, Fkn and CD40, all of which are Th1 associated molecules, supports this conclusion.

Viscoelastic and biological performance of low-modulus, reactive calcium phosphate-filled, degradable, polymeric bone adhesives

The aim of this study was to investigate the effect of reactive mono- and tricalcium phosphate addition on the mechanical, surface free energy, degradation and cell compatibility properties of poly(lactide-co-propylene glycol-co-lactide) dimethacrylate (PPGLDMA) thin films. Dry composites containing up to 70 wt.% filler were in a flexible rubber state at body temperature. Filler addition increased the initial strength and Young’s modulus and reduced the elastic and permanent deformation under load. The polymer had high polar surface free energy, which might enable greater spread upon bone. This was significantly reduced by filler addition but not by water immersion for 7 days. The samples exhibited reduced water sorption and associated bulk degradation when compared with previous work with thicker samples. Their cell compatibility was also improved. Filler raised water sorption and degradation but improved cell proliferation. The materials are promising bone adhesive candidates for low-load-bearing areas.

Microporosity Affects Bioactivity of Macroporous Hydroxyapatite Bone Graft Substitutes

This paper describes an investigation into the influence of microporosity on early osseointegration within porous hydroxyapatite scaffolds. Two batches of phase pure porous hydroxyapatite were produced with total porosities of approximately 80%, but with varied levels of microporosity such that the strut porosity of the two batches were 10 and 20%. Cylindrical specimens 4.5mm in diameter were implanted in the distal femur of 6 month New Zealand White rabbits and retrieved for histological and histomorphometric analysis at 1 and 3 weeks. Optical microscopy demonstrated variation in the degree of capillary penetration and bone morphology within scaffolds at 1 and 3 weeks, respectively. Moreover, histomorphometry demonstrated that there was significantly more bone ingrowth within HA80-2 scaffolds and that the rate of bone formation within these scaffolds was significantly faster. These results indicate that the bioactivity of porous hydroxyapatite scaffolds may be improved by increasing the level of microporosity within the ceramic struts. Introduction The combined affects of an ageing population and greater expectations in quality of life have resulted in an increasing global demand for orthopaedic implants for the replacement or augmentation of damaged bones and joints. In bone grafting current ‘gold standards’ include the use of autograft (living bone from the patient) or allograft (dead, sterilised bone from bone banks) but these methods are increasingly recognised as non-ideal due to limitations in supply and consistency [1]. Porous ceramics have been considered for use as bone graft substitutes in the treatment of bone defects for over 30 years [2]. In particular, calcium phosphates such as hydroxyapatite (HA) have been promoted as a result of their osteoconductive properties. However, while it is well recognised that both the rate of integration and the final volume of regenerated bone may be primarily dependent on various features of the macro-porosity, such as volume fraction, pore size and pore connectivity [3], recent in vitro studies have demonstrated bone cell sensitivity to the level of microporosity within the ceramic struts [4]. The aim of this study was to investigate the influence of microporosity on early osseointegration within porous HA (PHA) scaffolds. Materials and Methods Two batches of phase pure PHA were produced using a novel slip foaming technique [5], both had total porosities of approximately 80%, but varied in the volume fraction of porosity distributed 2 Title of Publication (to be inserted by the publisher) Figure 1 – (a) Porosity distribution within batches HA80-1 and HA80-2. Micropore morphology within the struts of batches (b) HA80-1 and (c) HA80-2. Bar = 50 μm between their macropore (>50μm) and micropore (<20μm) populations (Fig. 1). This variation was such that the strut porosity of batches HA80-1 and HA80-2 were 10 and 20%, respectively. Morphological characterisation of both the macroand microporosity was performed through a combination of immersion densitometry and image analysis of serial sections using a Zeiss Axioskop optical microscope linked to a KS300 image analyser. Macropore size, interconnection size and connectivity index, a measure of inter-pore connectivity, were all measured. Both the open and closed % of microporosity within the struts was quantified. Cylindrical specimens 4.5mm in diameter were implanted in the distal femur of 6 month New Zealand White rabbits and retrieved for histological and histomorphometric analysis at 1 and 3 weeks. The volume of new bone ingrowth was calculated using a Weibel grid and the mineral apposition rate (MAR) was determined through the administration of fluorochrome labels at 1 and 2 weeks and measurement of the inter-label distance using a Zeiss Axioskop optical microscope with a UV light source, linked to KS300 image analyser. Figure 2 – (a) Macropore interconnection data and (b) porosity distribution of open and closed microporosity within batches HA80-1 and HA80-2. 0 1 2 3 4 5 6 7 8 HA80-1 HA80-2 M ic ro po ro si ty (% ) Open Closed (b) (a) 0 10 20 30 40 50 60 70 80 90 HA80-1 HA80-2 P or os ity (% ) Microporosity Macroporosity (a) (b) (c) 0 100 200 300 400 500 600 Interconnection Size Connectivity Index HA80-1 HA80-2 Figure 3 – Variation in ingrowth morphology within batches (a) HA80-1 (Goldner’s trichrome) and (b) HA80-2 (Toluidine blue). Bar = 100 μm

Cancellous bone repair using bovine trabecular bone matrix particulates.

At 5 and 15 weeks post-surgery, biomechanical and histological analyses of cancellous bone defects filled with the bovine trabecular bone matrix (BBM) and hydroxyapatite (Hap) particulates of dimensions 106-150 microm were investigated. It was observed that at 5 weeks post-surgery the stiffness properties of the BBM filled defects were significantly higher than those observed in the Hap filled defects (p < 0.01) but comparable to those recorded in intact cancellous bone from the same anatomical position. Histologically, no significant differences were observed in the percentage of new bone contact with the particles. The biomechanical properties of the Hap filled defects mirrored those in intact cancellous bone only at 15 weeks post-surgery. BBM particles thus appeared to accelerate the early healing of osteotomies. It is therefore suggested that particles of this bioceramic be the subject of intense research for more usage in both periodontal osseous defects and orthopaedic fractures.

Changes in distant organs in response to local osteogenic growth factors delivered by intraosseous implants: a histological evaluation

Osteogenic growth factors are added to enhance osteointegration and os teogenesis of synthetic bone substitutes to improve clinical outcome. Reactions to par ticles of wear debris from implanted material could lead to bone resorption similar to resorption around a total joint prosthesis and also to inflammatory responses in distant organs. Porous hydroxyapati te (HA) scaffolds pretreated with insulin like growth factor (IGF) -I (0.5 (LD) or 3.0 (HD) μg/implant) and IGF-II (0.5 μg/implant) were implanted in vivo in rabbit femur to enhance their bioactivity and bone bonding properties. Heart, kidney, liver, lung, lymph nodes and spleen were collect ed during systematic postmortem examination at 1, 3 and 5 weeks postimplantation for quantitati ve and qualitative analysis. Local wound healing of the periprosthetic bone and the responses in th distant internal organs were characterised using light microscopy and electron microscopy. All tissues from implanted groups except heart and kidneys exhibited an increase in the cellularity at week 1 and 3. In the lung, there was also evidence of lympho-proliferation and aggregation in the IGF groups and presence of exudates in the IGF-1 HD group. The hypertrophy and hyperplasia appeared to be growth factor, dose and time dependent with IGF-1 LD <IG F-II <IGF-1 HD. At 5 weeks the tissues appeared as regenerated to the normal level wi th regression of the lymphocytic infiltration. We report for the first time, that local delivery of IGF-I and -II by intraosseous HA implant to enhance osteointegration produced no adverse persistent effect s in important internal organs.