Vertebroplasty is a minimal invasive surgical procedure for treatment of vertebral compressive fractures, whereby cement is injected percutaneously into a vertebral body. Cement viscosity is believed to influence injectability, cement wash-out and leakage. Altering the liquid to powder ratio can affect the viscosity, level of cohesion and extent cement fill within the vertebral body and the ultimately strength and stiffness of the cement-vertebra composite. The association of these combined factors remains unclear. The aim of this study was to determine the relationship between cement viscosity and the potential augmentation of strength and stiffness in a model simulating in-vitro prophylactic vertebroplasty of osteoporotic vertebral bodies. Samples of synthetic bone (Sawbone) representing osteoporotic bone were manually injected with 1mL of calcium phosphate cement using a 11G cannulated needle. Calcium phosphate cement was produced by mixing alpha-tricalcium phosphate, calcium carbonate and hydroxyapatite with an aqueous solution of 5 wt% disodium hydrogen phosphate. Three liquid to powder ratio (LPR) representing different viscosity levels were used; i.e. 0.5mL/g (low viscosity), 0.45mL/g (medium viscosity) and 0.35mL/g (high viscosity). Cement filled samples were then placed in an oven (37oC) for 20 min and then immersed in Ringer's solution (37oC) for 3 days. Samples of synthetic bone without cement injection were used as controls. Potential for leakage and wash-out was determined using gravimetric analysis. Extent of cement fill was determined using computer tomography (CT). Samples were tested under axial compression at a rate of 1 mm/min and the strength and stiffness determined. Statistical significance against controls was determined using a one-way analysis of variance (p<0.05). Low viscosity cement showed more cement leakage (p=0.512) and increased cement wash-out after 3 days in Ringer's solution (p=0.476). Qualitative assessment of cement fill within the vertebral body using CT imaging supported the wash-out results. The strength (p<0.05-0.01) and stiffness (p<0.01) of samples significantly increased by cement injection in comparison to control, the extent of this increase was greater with increasing cement viscosity. Linear correlation analysis showed a definite association between the mechanical properties and viscosity of injected cement and was dependent on the amount of cement retained within the synthetic bone post-setting

Background. Aseptic loosening of cemented femoral stems results from migration of wear particles along the bone-cement interface, producing a foreign body reaction. After cement insertion, blood back pressure can disrupt the bone-cement interface, enabling this spread of wear particles. Our study investigates whether altering timing and speed of stem insertion can reduce this risk. Methods. We inserted mock “C-Stem” femoral components (De Puy-Synthes), using Smartset HV cement (De Puy-Synthes) into artificial femora, fitted with proximal and distal pressure transducers. Cement insertion began two, three or four minutes after mixing. Cement pressures were then allowed to settle for one minute and the stems were then inserted over durations of 25, 60 or 90 seconds. Results. Quicker insertion led to high peak pressures; however insertion over 90 seconds at 4 and 5 minutes achieved more sustained pressures above blood back pressure. Slower insertion particularly improved proximal pressurisation. Furthermore when the stem was inserted over 90 seconds at 5 minutes then the cement pressures remained high after full insertion of the stem. Conclusions. Commonly, femoral stems are inserted rapidly. Our study demonstrates that by inserting the femoral stem more slowly, cement pressure can be maintained above blood back pressure for longer. We believe that after slower stem insertion, the higher cement viscosity enables the surgeon to maintain cement pressure, via the stem, sufficient to resist blood back pressure without inadvertently over inserting the stem. This safe and simple modification of cementing technique generates higher and more sustained cement pressures. Level of evidence. 3. Disclosures. Depuy-Synthes kindly provided the equipment used in the study. None of the authors received any payment or other benefits

Interfacial defects between the cement mantle and a hip implant may arise from constrained shrinkage of the cement or from air introduced during insertion of the stem. Shrinkage-induced interfacial porosity consists of small pores randomly located around the stem, whereas introduced interfacial gaps are large, individual and less uniformly distributed areas of stem-cement separation. Using a validated CT-based technique, we investigated the extent, morphology and distribution of interfacial gaps for two types of stem, the Charnley-Kerboul and the Lubinus SPII, and for two techniques of implantation, line-to-line and undersized.

The interfacial gaps were variable and involved a mean of 6.43% (sd 8.99) of the surface of the stem. Neither the type of implant nor the technique of implantation had a significant effect on the regions of the gaps, which occurred more often over the flat areas of the implant than along the corners of the stems, and were more common proximally than distally for Charnley-Kerboul stems cemented line-to-line. Interfacial defects could have a major effect on the stability and survival of the implant.

The role of vacuum mixing on the reduction of porosity and on the clinical performance of cemented total hip replacements remains uncertain. We have used paired femoral constructs prepared with either hand-mixed or vacuum-mixed cement in a cadaver model which simulated intra-operative conditions during cementing of the femoral component. After the cement had cured, the distribution of its porosity was determined, as was the strength of the cement-stem and cement-bone interfaces.

The overall fraction of the pore area was similar for both hand-mixed and vacuum-mixed cement (hand 6%; vacuum 5.7%; paired t-test, p = 0.187). The linear pore fractions at the interfaces were also similar for the two techniques. The pore number-density was much higher for the hand-mixed cement (paired t-test, p = 0.0013). The strength of the cement-stem interface was greater with the hand-mixed cement (paired t-test, p = 0.0005), while the strength of the cement-bone interface was not affected by the conditions of mixing (paired t-test, p = 0.275). The reduction in porosity with vacuum mixing did not affect the porosity of the mantle, but the distribution of the porosity can be affected by the technique of mixing used.