Table of Contents:
Interference screws are commonly used in the fixation of bone-tendon-bone (btb) grafts from the patellar tendon. Currently interference screws are made from either metal (titanium) or biodegradable materials. There are some disadvantages to implants made of these materials. Metal screws are implanted permanently and can lead to complications at the implant site or have to be removed sometimes in a second operative procedure. In the case of biodegradable screws the degradation process is still a problem and inflammatory reactions in surrounding tissues have been described. Implants made from bovine cortical bone can be a biological alternative. They are biocompatible and can be integrated at the site of the implantation in the long term. In the first part of this study we evaluated the rotational strengths of screws made of bovine cortical bone (cb-screws). In a second part of the study the initial fixation strengths and the stiffness of btb-transplants fixed with different interference screws in porcine tibiae were evaluated.
Material and methods:
In this study two types of screws, made of bovine cortical bone, were tested. The first types of screws were manufactured at our hospital out of pieces of bovine cortical bone. The bone and the screws were soaked in acetone and autoclaved. The second type of screw was already manufactured and treated by the Tutoplast® protocol by a company (Tutofix®Screw, Tutogen Medical GmbH, Germany). To evaluate the maximum torsional strengths cb-screws with different diameters (7mm, 8mm, 9mm) as well as 8mm Tutofix® screws were mounted in methyl methacrylate resin. Torque was then applied manually with a hand-held torque screw driver and the maximum force at failure was recorded. For the second part of the study porcine knees were used. The graft was the patellar tendon harvested with a bone block from the tuberositas tibiae. The bone block was fitted and then fixed in the bone tunnel with either standard titanium interference screws (8mm diameter), with cb-screws (8mm diameter) or an 8mm Tutofix® screw. In a first group the specimens were loaded single cycle in a universal testing machine to failure, in a second group the specimens underwent first 500 cycles at submaximal loads between 40 and 400 N and were then loaded to failure.
The mean failure torque was 1.25N/m for the 7mm screws, 2.59N/m for the 8mm screws and 3,09N/m in the 9mm group. The Tutofix® screws broke at an average of 2,14N/m. In the first group of btb-grafts fixed with titanium screws the mean maximal load to failure was 1018N, with 8mm cb-screws 841N and 718N for the Tutofix® screws. Following cyclic loading the mean maximal load to failure was 858N in the titanium group, 887N in the group with the cb-screws and 678N for the Tutofix® screws.
This study shows that screws made from bovine cortical bone are strong enough to be used as interference screws in the ACL-reconstruction. Evaluating the initial fixation strength and the stiffness of btb-transplants, fixed with these screws, it can be concluded, that cb-screws can offer an alternative to the commonly used interference screws (metal or biodegradable materials) in the reconstruction of a damaged anterior cruciate ligament. Further improvement of the design of the screws and the instruments should furthermore improve the biomechanical properties of cb-screws. Future research is needed to validate the safety of these xenografts and to monitor their effects after implantation in the human body.