New implant designs for fracture fixation in osteoporotic bone.
Screws are one of the limiting factors for fixation of implants, particularly in poor bone quality. A class of new implants with an implant-bone-interface optimized regarding load transition by increasing the peripheral area might improve the anchorage of implants in osteoporotic bone. However, the shape of these implants requires new technologies for insertion. The goal of the work presented here was to analyze the relevant parameters regarding implant geometry and to demonstrate the effect of new procedures for their insertion. The investigation was divided into three parts: 1) implant design optimisation, 2) efficiency of cortical bone ablation, and 3) implant insertion technology. Finite element analysis (FEA) was performed to investigate the influence of the number of lobes, the radius of the outer curvature and additional milling to remove any sharp changes of section around the lobe. Opening of the cortical bone with an Er:YAG laser was studied using calf cortex from 2 to 7 mm thickness. The effect of a) pulse energy and pulse duration, b) cortical thickness, c) wet or dry boundary conditions on volume and geometry of ablated bone, time required to penetrate the cortical bone and local bone tissue damage was quantified. Pneumatic and ultrasound based insertion were compared in the third experiment. The cortical bone was prepared in the following ways: a) no opening, b) predrilling of three holes (1 mm diameter each) and c) exact pre-cutting of the whole contour. Increasing the radius of the outer curvature from 2 to 5 mm reduces the peak stresses during loading in all planes in the implant as well as in the adjacent cortical bone by about 30-40%. An increase in the number of lobes from two to three decreases the mean peak stress by about 46% (alpha < 0.001) and the range between the minimal and maximal peak stresses for different loading directions by about 83%. Penetration of cortical bone with an Er:YAG laser was possible up to a cortical thickness of 6 mm with fewer than 100 pulses. The ablation rate per pulse increased more with increasing duration than with increasing energy. Signs of bone damage such as melting were only visible when high pulse energies and durations were used. Insertion of the prototype was possible with all devices, but only when the whole contour was cut out of the cortical bone. However, the use of the ultrasound vibrator led to heating up of the tissue fluid and subsequently to water evaporation and tissue damage. Insertion of the prototype was possible with both pneumatic vibrators, but only when the whole contour was cut out of the cortical bone. New implant designs may lead to reduced stress peaks in the surrounding bone and might be inserted with the help of new insertion technologies, namely laser cutting of cortical bone and pneumatic vibration. Further studies are required to optimize these technologies prior to clinical use.