Absorbable Biomaterials

INTRODUCTION
Historically, the use of implants in orthopedic surgery has originated from fracture repair and joint replacement applications. During the late 1920s, stainless-steel bone implants such as Kirshner nails and Steinman pins were popularized for the surgical treatment of fractures (1). With the introduction of new surgical materials such as cobalt alloys, polyethylene and poly(tetrafluoroethylene) [Teflon], surgeons and engineers began working toward the design and fabrication of artificial joints. The advent of new high strength implant materials allowed researchers such Dr. John Charnley to begin pioneering work in total hip replacement surgery in the late 1930s (1,2). As advances in chemistry, metallurgy, and ceramics progressed throughout the years, a large variety of implants have entered the orthopedic market. Today, orthopedic implants are composed of specialized metals, ceramics, polymers, and composites that possess a large range in properties. Although these materials have been successfully fabricated into a variety of implants, one common issue has remained. Once the device has performed its required function and is no longer needed, it remains as a bystander in the now healthy tissue. The issue is that the long-term presence of an implant in the body can result in implant-related complications such as loosening, migration, mechanical breakdown and fatigue, generation of wear particles, and other negative effects (3-6). With prolonged patient life spans and higher activity levels, more and more people are now outliving the lifetime of their implants.
The potential for long-term implant problems has driven researchers to look to a unique category of materials that are capable of being completely resorbed by the body. These bioresorbable or biodegradable materials are characterized by the ability to be chemically broken down into harmless byproducts that are metabolized or excreted by the body. Materials of this type offer a great advantage over conventional nonresorbable implant materials. Bioresorbable implants provide their required function until the tissue is healed, and once their role is complete, the implant is completely resorbed by the body. The end result is healthy tissue with no signs that an implant was ever present. As the implant is completely gone from the site, long-term complications associated with nonresorbable devices do not exist.

ORTHOPEDIC APPLICATIONS OF RESORBABLE IMPLANTS
The ability of a resorbable implant to provide temporary fixation followed by complete resorption is a desirable property for a large variety of surgical applications. In relation to orthopedic surgery, this behavior is particularly useful based on the goal of restoring physiological function to the tissues and joints of the skeleton. In general, orthopedic surgery is often compared with carpentry in that the surgeon's instruments often consist of hammers, drills, and saws. Similar to carpentry, specialized screws, plates, pins, and nails are used to fix one material to another. In orthopedics, this fixation can be categorized into two main areas: bone-to-bone fixation and soft tissue-to-bone fixation. Bone fixation is used in the treatment of complex fractures and in reconstructive procedures of the skeleton.
The implants used in these surgeries are designed to maintain the position of the bone fragments, to stabilize the site, and to allow for eventual fusion of the fracture. As a result of the fracture healing process, the bone is remodeled so effectively that it is often difficult to locate the initial injury. With nonresorbable implants, the long-term presence of the device only serves as a source for potential complications. Resorbable implants, on the other hand, alleviate this concern by fully resorbing and allowing the bone to completely remodel into its normal physiological state.
In addition to bone fixation, soft tissue fixation is also an excellent application of resorbable implants. This type of reconstruction is often the result of trauma to joints such as the knee and shoulder. Typically developing from sports injuries or accidents, the goal is to restore stability to the joint by replacing or reconstructing the ligament or tendon interface to bone. In the knee, for example, the reconstruction of a torn anterior cruciate ligament (ACL) is a common sports medicine procedure. This type of surgical reconstruction consists of replacing the torn ACL with a bone-tend-on-bone graft taken from the patient's patella and fixing the graft across the joint. During the procedure, the bony portion of the ACL graft is fixed in bone tunnels drilled into the tibia and femur. In order to stabilize the graft and aid in the formation of a stable bone-to-ligament interface, interference screws are used to fix the graft to the site. Once bone has been incorporated into the graft, the device is no longer needed.
Another example of soft tissue reconstruction is the repair of a tear in the rotator cuff tendon of the shoulder. This type of injury requires reestablishing the tend-on-to-bone interface. To facilitate this process and restore stability to the shoulder, implants called suture anchors are used to provide a means to affix the torn tendon to the bone of the humerous. Just as the name describes, these implants function by providing an anchor in bone that allows the attached suture to tighten down on the tendon and pull it in contact with bone. As healing progresses, a stable interface develops and joint function is restored. Similar to other fixation applications, once the interface has fully healed, the implant is no longer needed.

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