Biomechanical evaluation of swelling copolymeric bone anchor in vivo

Introduction: Bone anchors are essential components in orthopedic surgeries such as reattaching torn soft tissue to bone, fixation of implants and internal fracture fixation. Conventional bone anchors, made of materials such as titanium, stainless steel, or PEEK are typically threaded devices that achieve fixation through shear resistance of the bone between the threads. In lower density bone such as at the shoulder or in lower density bone, poor fixation is achieved resulting in unacceptable high rates of surgical failure. To address this problem, a novel swelling copolymeric anchor made of methyl methacrylate (MMA) and acrylic acid (AA) has been introduced [1]. These anchors achieve their fixation to bone by absorbing interstitial fluids and expanding by a controlled amount. Swelling produces radial stresses at the interface resulting in frictional resistance, an efficient resistance to pull-out forces, particularly in low density bone. The radial stresses produced are also expected to stimulate osteointegration. Previous studies investigated the fixation strength and mechanical properties of the swelling anchor in artificial bone in saline solution [2,3,4]. The primary goal of this work is to evaluate the mechanical and biological behavior of these swelling bone anchors in an animal model in vivo

Materials and

Methods: Following approval from the Institutional Animal Care and Use Committee, twelve sheep underwent general anesthesia for the insertion of swelling cylindrical implants into the vertebral bodies of the lumbar spine (L2 to L5). The surgical procedure involved a left lateral retroperitoneal transpsoatic approach, which was guided by fluoroscopy. Each animal received four press-fitted implants consisted of two swelling implants used for pull-out tests, one non-swelling implant use as control, and one implant used for histology. All the porous anchors were cylindrical (8mm Dia and length.) and made of a solid core (5 mm dia.) and a porous sleeve around it. The swelling anchors utilized for mechanical testing featured a 2mm threaded metal rod at their center, used for pull-out tests (Fig. 1). Six animals were sacrificed twelve weeks after implantation surgery, while the remaining six were sacrificed after twenty-four weeks. At time of sacrifice, vertebral bodies were harvested and immediately subjected to micro-CT imaging followed by pull-out test conducted on a tensile testing machine (Mark 10TM). Micro-CT images were analysed by DragonflyTM software.

Results, Conclusions, and Discussions: No inflammatory response or fibrous tissue encapsulation was observed at implantation site indicates good biocompatibility. After conducting pull-out test, the solid core came out separating from the porous sleeve leaving it fixed to the bone (Figure 2). It was also observed that the mechanical properties of the swelling anchors decreased (80.76% degradation rate after removal) compared to the non-swelling control group (0% degradation rate). This reduction is derived from over swelling of the anchors in vivo. The staining results of the histology samples evident higher presence of osteoid and early bone formation in the porous sleeve compared to the non-swelling anchors. Pull-out test results also showed that non-swelling anchors had higher fixation strength compared to the swelling anchors (Table 1). In conclusion, strong integration between bone-implant interface was produced and observed. However, the composition ratio of MMA and AA needs to be optimized in further studies to limit degradation of the material properties along with controlling the swelling ratio.

Acknowledgements (Optional): Funding for this study, provided by the Coulter-Drexel Translational Research Partnership Program is gratefully acknowledged.