Polymethylmethacetate (PMMA) is a bone cement used in over 725,000 primary hip arthroplasties in 2018. Cement integrity is affected by external factors, including temperature, mixing technique and moisture uptake, which can influence cement microstructure. Changes in the cement microstructure may ultimately threaten the survivorship of the implant.

The introduction of enhanced recovery and various local anaesthetic infiltration techniques have been adopted in an attempt to facilitate early mobilisation and reduce length of stay. Our study aims to investigate if the mechanical properties of PMMA are altered with exposure to Ropivacaine LA.

Cements were cured in three separate states (air, serum and serum with LA) and the mechanical properties tested at 24 hours and 28 days. Using Refobacin bone cement provided by ZimmerBIOMET, cylindrical molds (12×6mm) were constructed with a split-mold. The LA used was 2mg/ml Ropivacaine hydrochloride solution. Using pilot data, this study was powered to 80% and a sample size of 10 per group (n=60) was calculated.

Cement samples were subjected to compressive loading using a universal testing apparatus (Zwick/Roell). Yield-strength and modulus values were extracted from the respective stress versus strain curves. Significant differences were determined by one-way anova for each time point, and Bonferroni post-hoc testing to determine significance between actual groups.

At 24-hours there were no significant differences in strength or modulus between groups. At 28-day strength and modulus increased in all groups. Compared to the air group, both serum and LA groups show a significant decrease in compressive strength. The modulus for the LA group is significantly less stiff compared to the air group.

The results suggest that the initial exposure to LA has a significant impact on the physical properties of the PMMA. We propose increased awareness of the potential effects this may have on the longevity and survivorship of cemented implants.

The aseptic loss of bone after hip replacement is a serious problem leading to implant instability. Hydroxyapatite coating of joint replacement components produces a bond with bone and helps to reduce loosening. However, over time bone remodeling at the implant interface leads to loss of hydroxyapatite. One possible solution would be to develop a coating that reduces hydroxyapatite and bone loss. Hydroxyapatite can be chemically modified through the substitution of ions to alter the biological response. Zinc is an essential trace element that has been found to inhibit osteoclast-like cell formation and decrease bone resorption. It was hoped that by substituting zinc into the hydroxyapatite lattice, the resultant zinc-substituted hydroxyapatite (ZnHA) would inhibit ceramic resorption and the resorption of bone. The aim of this work was to investigate the effect of ZnHA on the number and activity of osteoclasts.

Discs of phase pure hydroxyapatite (PPHA), 0.37wt% ZnHA and 0.58wt% ZnHA were produced, sintered at 1100 degrees Celsius and ground with 1200 grit silicon carbide paper. They were cultured in medium containing macrophage colony stimulating factor and receptor activator of nuclear factor kappa B ligand (RANKL) for 11 and 21 days. A control disc of PPHA cultured in medium containing no RANKL was also used. On the required dates the discs were removed and the cells stained for actin with phalloidin-TRITC and the cell nuclei with 4',6-Diamidino-2-phenylindole dihydrochloride. Cells with 3 or more nuclei were classed as osteoclasts and counted using ImageJ. On day 21 after the cells had been counted, the cells were removed and the discs coated in platinum before viewing with a scanning electron microscope. Resorption areas were then measured using ImageJ.

The addition of zinc was observed to significantly decrease the number of differentiated osteoclasts after 21 days (p<0.005 for 0.58wt% ZnHA compared to PPHA and p<0.01 for 0.37wt% ZnHA compared to PPHA). The area of resorption was also significantly decreased with the addition of zinc (p<0.005 for the comparison of 0.58wt% ZnHA with PPHA)

The work found that zinc substituted hydroxyapatite reduced the number and subsequent activity of osteoclasts.

School of Mechanical Engineering, University of Birmingham, Birmingham, UK

This study investigated the effects on friction of changing the dimensions of a ball-and-socket Total Disc Arthroplasty (TDA).

A generic ball-and-socket model was designed and manufactured based on the dimensions and geometry of a metal-on-metal Maverick (Medtronic, Minneapolis, USA) device. Keeping the radial clearance similar to the Maverick, the ball and socket dimensions varied between 10 to 16 mm and 10.015 to 16.015 mm, respectively, in order to enable the comparison between different dimensions. The implants were made out of Cobalt Chrome Molybdenum alloy, with a surface roughness of 0.05 μm.

A Bose spine simulator (Bose Corporation, ElectroForce Systems Group, Minnesota, USA) was used to apply an axial compressive force to the TDA. Axial rotation of ±2° was then applied at various frequencies and the resulting frictional torque measured. The tests were performed under an axial load of 50, 600 and 1200 N and frequencies of 0.5, 1.0, 1.5 and 2.0 Hz, for four different samples of radii 10, 12, 14 and 16 mm (48 combinations in total).

The results showed variation of frictional torque in different frequencies for all four samples under constant axial load. It was observed that the frictional torque had the lowest value for the implant with ball radius of 16mm. It might be concluded that the implant with larger ball radius may create less friction and hence offer a longer life.

Introduction: Fracture of cortical long bone can be treated with open reduction and internal fixation. Although the lag-screw technique would provide a stronger compression across the fracture site, this may not translate directly into a significantly better mechanical stability of the construct. In narrow long bone such as the metacarpus, it may be technically difficult to over-drill the near cortex. In addition, the fracture configuration as well as the presence of soft tissue attachment and neurovascular bundle in such smaller bones may give rise to technical difficulty in fixation with all the screws from the same side.

Hypothesis: We therefore propose to test the hypothesis that there is no difference in the mechanical stability of the construct, in terms of three points bending and axial loading, between fixation with lag screw (L) and cortical screw (1C) in long bone oblique fracture.

Secondly, we proposed that fixation with alternate cortical screws from both sides of the cortices (2C) may confer a stronger mechanical stability than fixation with all screws from one side (1C).

Method: A 12 cm long oblique osteotomy was created along the shaft of tibial saw bone to produce an artificial long oblique fracture. The two fragments were held together and precompressed similarly with reduction clamp. Lag screw technique (L), unilateral (1C) and bilateral (2C) simple cortical screws techniques were used to fix the fracture with a total of four 4.5mm cortical screw in each construct. The test piece was placed into position and compressed under displacement control using a Bose ElectroForce^® 3300 Series test instrument (Bose Corporation ElectroForce Systems Group, Minnesota USA). The displacement was applied at a rate of 0.42mm/s. The displacement and load were measured every 0.0586 seconds and recorded using Win-Test Software (Bose Corporation ElectroForce Systems Group, Minnesota USA).

Result: In three-points bending, fixation with unilateral cortical screws (1C) provided the highest mean stiffness values of the three techniques with an average of 27.72 N/mm (standard deviation STD 4.90 N/mm). The lag screw technique (L) had slightly lower mean stiffness values with an average of 26.29 N/mm (STD 3.46 N/ mm). Fixation with opposing screws (2C) had a lowest mean stiffness averaging 18.31 N/mm (STD 14.49 N/ mm). In axial compression, unilateral cortical screws (1C) provided the highest average stiffness at 290.33 N/mm (STD 89.84 N/mm) The opposing pre-compression technique had an average stiffness of 241.24 N/ mm (STD 121.30 N/mm) and the lag screw technique 198.94 N/mm (STD 58.33N/mm).

Conclusion: In conclusion, fixation with unilateral cortical screws (1C) provided a better mechanical stability than lag screw. Fixation with bilateral cortical screws (2C) did not provide a more stable construct than unilateral screw fixation.

Introduction: For years traditional intramedullary nails (IMNs) have been used with great success to treat long bone fractures, however, based upon our clinical observations, we hypothesise that design changes incorporated into newer femoral IMNs reduces fracture stability resulting is a higher incidence of non-union.

AIMS: To biomechanically test the factors that may reduce fracture stability.

Materials and methods: The fracture fixation model consisted of custom made stainless steel IMNs of different wall thicknesses and outer diameters, cylinders manufactured from stainless steel, aluminium or HDPE of differing inner diameters and wall thicknesses, and 5mm rods made from stainless steel or titanium. The dimensions of the cylinders were chosen to resemble those commonly observed in the distal femur. The test nails and cylinders were connected using a single rod. Axial loading was undertaken up to 2KN (constant rate of 0.5KN/sec) and repeated a minimum of three times. The effects of various factors such as IM nail wall thickness and outer diameter, the alloy from which the rods were manufactured, and, the diameter, wall thickness and material properties of the cylinders were studied.

Results: The factors that most affected stability were the diameter, wall thickness and the material properties of the cylinders, with the least stable configuration being a HDPE cylinder with a diameter of 75mm and a wall thickness of 3mm. By reducing the diameter of the cylinder to 50mm combined with increasing the wall thickness to 5mm, stability increased considerably even when HDPE was used. The stability of each fracture fixation system was further reduced by using titanium rods.

Discussion: In clinical practice, new femoral IMNs permit longer cross screws to be inserted in the distal femur where the diameter is greatest and the cortical bone is thinnest. Since cancellous bone offers little resistance, screws effectively span from one cortex to the other gaining limited purchase in the bone. As a result, the newer IMN systems are more likely to displace regardless of the direction and force applied. This effect is exaggerated by using titanium. Overall the combination of screw length, choice of alloy and cortical thickness could easily explain our unsatisfactory clinical observations.