Objectives

Up to 40% of unicompartmental knee arthroplasty (UKA) revisions are performed for unexplained pain which may be caused by elevated proximal tibial bone strain. This study investigates the effect of tibial component metal backing and polyethylene thickness on bone strain in a cemented fixed-bearing medial UKA using a finite element model (FEM) validated experimentally by digital image correlation (DIC) and acoustic emission (AE).

Aims

We present a case series of five patients who had revision surgery following magnetic controlled growing rods (MGCR) for early onset scoliosis. Metallosis was found during revision in four out of five patients and we postulated a mechanism for rod failure based on retrieval analysis.

Demineralised dentine matrix (DDM) contains a myriad of growth factors and matrix proteoglycans, the bioactivity of which can utilised in dental restorations and bone augmentations. This study aimed to develop a novel antimicrobial, bioactive dental cement to promote reparative dentinogenesis and prevent infections, improving the longevity of current dental restorations.

Nanocarriers containing DDM (extracted from non-carious dentine; 1–100 μg/mL), and triclosan (300 μg/mL) were made. Human dental pulp stem cells (hDPSCs) were treated with DDM nanocarriers (10 ng/mL-100 μg/mL) for 3, 9, 21 and 35 days. Cell proliferation and viability were assessed by cell counts, Caspase-Glo 3/7 (Promega) and MTT assays. qRT-PCR was used to examine the expression of osteogenic markers runx2 and osteocalcin at days 3, 9 and 21. A transwell chemotaxis/ migration assay was used to assess the ability of DDM nanoparticles to recruit hDPSC progenitors. Triclosan nanocarriers were tested using growth curves and zones of inhibitions for S. Anginosus and E. Faecalis. SEM and biomechanical testing was carried out on Vitremer (Henry Schein) dental cements containing loaded and empty nanocarriers.

DDM nanocarriers were able to significantly recruit hDPSCs and induce the expression of osteogenic markers in hDPSCs after 9 days. DDM Nanocarriers had no effect on cell proliferation or survival. Triclosan nanocarriers were able to inhibit the growth of S. Anginosus and E. Faecalis. Nanocarriers had limited effect on the biomechanical integrity of Vitremer cements.

Nanocarriers successfully delivered DDM to hDPSC, promoting their in vitro recruitment and osteogenic differentiation, and triclosan to endodontic bacteria inhibiting their growth. The nanocarriers were incorporated into cements with minimal physical artefacts, therefore a novel antimicrobial, bioactive dental cement was produced, which could be a useful tool for dental tissue engineering.

As many as 25% to 40% of unicompartmental knee replacement (UKR) revisions are performed for pain, a possible cause of which is proximal tibial strain. The aim of this study was to examine the effect of UKR implant design and material on cortical and cancellous proximal tibial strain in a synthetic bone model. Composite Sawbone tibiae were implanted with cemented UKR components of different designs, either all-polyethylene or metal-backed. The tibiae were subsequently loaded in 500 N increments to 2500 N, unloading between increments. Cortical surface strain was measured using a digital image correlation technique. Cancellous damage was measured using acoustic emission, an engineering technique that detects sonic waves (‘hits’) produced when damage occurs in material.

Anteromedial cortical surface strain showed significant differences between implants at 1500 N and 2500 N in the proximal 10 mm only (p < 0.001), with relative strain shielding in metal-backed implants. Acoustic emission showed significant differences in cancellous bone damage between implants at all loads (p = 0.001). All-polyethylene implants displayed 16.6 times the total number of cumulative acoustic emission hits as controls. All-polyethylene implants also displayed more hits than controls at all loads (p < 0.001), more than metal-backed implants at loads ≥ 1500 N (p < 0.001), and greater acoustic emission activity on unloading than controls (p = 0.01), reflecting a lack of implant stiffness. All-polyethylene implants were associated with a significant increase in damage at the microscopic level compared with metal-backed implants, even at low loads. All-polyethylene implants should be used with caution in patients who are likely to impose large loads across their knee joint.

Cite this article: Bone Joint J 2013;95-B:1339–47.

The most common mode of failure observed in cemented orthopaedic implants is aseptic loosening of the prosthesis over time. This occurs as a result of fatigue failure of the bone cement under different loading conditions. Although a great deal of research has been carried out on the fatigue crack development of poly(methyl methacrylate) (PMMA) bone cements, the effects of different loading frequencies at low and high stress intensities are not well understood. Therefore, the aims of this study are to determine the effects of loading PMMA bone cement at different stress intensities and loading frequencies, as seen in-vivo, and the effects of changing these parameters on fatigue crack propagation. To achieve these aims, disc compact tension (DCT) samples with chevron notches were made and Krak Gages (Russenberger Prufmaschinen, Neuhausen am Rheinfall, Switzerland) were attached to monitor crack growth. The bone cement used in this study was the Cemex System, which uses a cement gun to mix and apply the material into the cavity. From standard compression and bending tests it was found that the cement made using this system had an average compressive strength of 86.66±5.52MPa, an average bending modulus of 3696.06±121.13MPa and an average bending strength of 51.95±4.14MPa. These values are within the normal range of acrylic resin cements for implants and above the minimum requirements of the ISO5833:2002 standard. A program has been written that loads the DCT samples with a stress intensity of 0.2MPam^1/2, 0.6MPam^1/2 and 1.0MPam^1/2 at a frequency of 1Hz, 2Hz, 5Hz, 10Hz and 20Hz. The crack was allowed to grow 0.2mm at each frequency and the frequencies were increased (1Hz to 20Hz) then decreased in magnitude (20Hz to 1Hz) for each of the stress intensities.

This experimental design enables much more sensitive detection of small changes in crack growth rate than a conventional test where the crack grows through the entire range of δK at a single frequency. By repeatedly varying the loading within the same specimen the effects of variation between specimens can be removed, revealing significant differences in crack growth rate. The results provide important information on bone cement when loaded in conditions similar to those seen in-vivo and how frequency and stress intensities affect the fracture mechanics of PMMA.

Introduction: Acrylic bone cement (ABC) manufacturers vary their products by using different proportions of the principle ingredients to optimise handling time or mechanical properties. Surprisingly, there is limited research showing the effect of varying monomer/polymer and initiator/activator ratios (independent from other constituents) on thermal and mechanical properties of ABC.

Materials and Methods: The formula for CMW (DePuy) was reproduced using original ingredients obtained from different suppliers. The commercially available CMW monomer/polymer ratio is approximately (0.6 ml/gm). Six variants of CMW bone cement were prepared by varying the monomer/polymer ratio (0.4–1 ml/gm) and eight variants were prepared by varying the initiator/activator BPO/DMPT ratio (1.71–11.25). Specimens were stored in an incubator for 7 days at 37 °C. Thermal characteristics of the polymerisation reaction such as maximum polymerisation reaction temperature (Tmax) and setting time (Ts) were recorded using a thermocouple and Picolog digital data recorder. Compressive mechanical properties were measured using Zwick Roell All Round Testing System implementing ISO5833 recommendations. SPSS software was used to perform ANOVA and calculate Pearson correlation coefficient.

Results: Increasing monomer/polymer ratio resulted in prolongation of setting time (5.3–11.3 minutes) displaying a significant (p= 0.000) correlation (r=0.988); however, there was no significant correlation with Tmax (r=−0.123, p=0.792). Increasing the monomer/polymer ratio resulted in a significant reduction in yielding compressive strength (F=110.97, p=0.000) and modulus (F=16.1, p=0.000). Pearson correlation test showed that monomer/polymer ratio had a significant correlation with yielding compressive strength (r= −0.930, p=0.002) and a significant correlation with the corresponding modulus of elasticity (r= −0.827, p=0.022). An increase in the BPO/DMPT ratio did not display a significant (p= 0.172) correlation (r=−0.535) with Tmax; however, the setting time was prolonged by increasing the BPO/ DMPT ratio with a significant (p=0.002) strong positive correlation (r=0.903). Compression tests showed a significant (F=13.45, p=0.000) reduction in yielding compressive strength with a significant (p=0.04) inverse (r=−0.729) correlation with the BPO/DMPT ratio. Modulus of elasticity followed a similar pattern to a lesser degree displaying a significant (F=5.123, p=0.001) reduction in values which was moderately correlated (r=−0.619), though insignificant (p= 0.101) with BPO/ DMPT ratio.

Discussion and Conclusions: Varying the monomer/ polymer ratio independently from other constituents in acrylic bone cement significantly affects setting time and compressive mechanical properties. Setting time can be prolonged to increase handling time; however this will occur at the expense of a reduction in compressive stiffness and strength. Similarly, varying the BPO/DMPT ratio may result in optimised handling time; however, this will also cause a reduction in compressive strength and stiffness. These finding are paramount in clinical applications where compressive strength is essential e.g. percutaneous vertebroplasty.