Swellable bone anchors

Our research group developed a new swellable polymeric material system that was targeted towards a range of applications requiring fixation in bone, such as suture anchors. The material system developed was a cross-linked Poly(methylmethacrylate-acrylic acid) random copolymer. Implants made from this copolymer exhibited an ability to absorb body fluids and swell in a controlled manner. In the applications mentioned above, the swelling of these anchors is constrained by the surrounding bone, and consequently an interfacial pressure developed at the bone-implant interface. This interfacial pressure, in turn, increased the frictional resistance and improved significantly the fixation strength of the implants. In our experience with these anchors, two major problems were found: (1) it took relatively long time for the swelling anchors to reach their equilibrium state and therefore immediate fixation of smooth swelling anchors was poor; (2) excessive swelling weakened the mechanical properties of the anchors and this lead to a decrease in holding strength. Therefore, it was necessary to: (1) design a built-in immediate fixation mechanism; (2) reduce the time required for the anchor to reach equilibrium, and (3) optimize the material composition for proper amount of swelling. In this thesis, swelling behavior of P(MMA-AA) was studied in order to understand its excessive swelling in vivo, while not in vitro. Then, the study of optimal swelling strain of anchors was performed in foam of various densities (Such foam is commonly used to simulate the mechanical properties of cancellous bone). In the end, fixation strengths of swellable anchors were evaluated on human cadaveric specimens in the shoulder and spine areas and compared to the fixation strengths of commercially available anchors.