Changes in biomechanical properties of chemotherapy bone cement after a year in saline storage

Introduction: Acrylic based bone cements are a versatile treatment modality in Orthopaedic surgery due to their wide variety of uses and tolerance to high degree of customization. Bone cement can be used to repair and stabilize pathologic fractures and may potentially prevent recurrence post tumor resection. Chemotherapeutic bone cements are favorable because they can potentially minimize systemic side effects and the need for radiation. Cements can be combined with soluble fillers such as polyethylene glycol (PEG) to optimize drug elution. Even though studies have measured the mechanical properties of bone cement in dry state, the exact change in the mechanical properties of bone cement after drug/soluble filler elution is largely unknown. This study investigates the change in mechanical properties of commercially available bone cements modified with PEG fillers after one year of storage in drug elution medium. Methods: Confidence Ultra, Vertebroplastic, and Palacos cement were used and mixed with varying amounts (0–50%) of PEG and chemotherapy agents (methotrexate or doxorubicin). Bone cement specimens were stored in a saline solution for one year after which they were tested in compression at 1 mm/min until failure. Results: The modulus and compression strength of bone cements decreased with increase in soluble filler composition. Although soluble fillers were shown to weaken the mechanical properties of the bone cement, Palacos and Vertebroplastic cements retained their mechanical properties better than Confidence. Discussion: When using chemotherapeutic bone cements, combining soluble fillers enhances drug elution at the expense of mechanical properties. However, the results showed that mechanical properties of different commercially available bone cements behave differently with similar percentages of soluble filler and drug added making it difficult to predict changes in mechanical properties of bone cement intraoperatively. This elucidates the need for well characterized bone cement optimized for chemotherapy drug delivery

Biomechanical analysis of four external fixation pin insertion techniques

Having multiple external fixation pin designs and insertion techniques has led to debate as to which combination creates the stiffest construct. This study sought to biomechanically evaluate construct strength using self-drilling (SD) and self-tapping (ST) pins inserted with either bicortical or unicortical fixation. SD and ST 5.0 mm stainless steel pins were used in combination with bicortical self-drilling (BCSD), bicortical self-tapping (BCST), unicortical self-drilling (UCSD), and unicortical selftapping (UCST) techniques. Pre-drilling for the self-tapping pins was completed with a 4.0 mm drill bit using ¾ inch polyvinyl chloride (PVC) pipe as the insertional medium. The PVC pin constructs were then loaded to failure in a cantilever bending method using a mechanical testing system. Ten trials of each technique were analyzed. BCSD insertion technique had the highest maximum failure force and stiffness of all tested techniques (P<0.0001). SD pins were significantly stronger to bending forces than ST pins in both the unicortical and bicortical setting (P<0.0001). Three point bending tests of the 5.0 mm SD and ST threaded area showed that threaded portion of the SD pins had a 300 N greater maximum failure force than the ST pins. Biomechanical analysis of external fixation pin insertion techniques demonstrates that bicortical fixation with SD pins achieved the greatest resistance to bending load. Despite both pins being 5.0 mm and constructed from stainless steel, ST and SD behaved differently with regard to maximum failure force and stiffness. This study demonstrates that insertion technique and pin selection are both important variables when attempting to achieve a stiff external fixation construct