There are currently a variety of metallic screws available for attaching bone plates to fracture and surgery repair sites; spinal, cranial, and maxillo-facial plates have all been fastened using metal screws. The disadvantage of metal devices is that they are permanent and must be removed through a second surgical procedure. If they remain in the body, tissue atrophy and subsequent loosening of the screw may occur. Synthetic absorbable biocompatible polymers are well known in the art. Such polymers are typically used to manufacture medical devices, which are implanted in body tissue and absorb over time. Synthetic absorbable biocompatible aliphatic polyesters include homopolymers, copolymers (random, block, segmented and graft) of monomers such as glycolic acid, glycolide (d, l, meso and mixtures thereof), lactic acid, lactide, .epsilon.-caprolactone, trimethylene carbonate and p-dioxanone. Numerous U.S. Pat. Nos. describe these polymers including 5,431,679; 5,403,347; 5,314,989; 5,431,679; 5,403,347; and 5,502,159.
With the advent of absorbable polymers has come a new generation of fasteners (screws, pins, etc.) that are designed to be gradually absorbed by the body as their functional use declines. Screws made of an absorbable material have an advantage in that they remain only for the period of time required for healing. There are a number of threaded fasteners disclosed in the prior art which utilize absorbable materials.
U.S. Pat. No. 5,169,400, hereinafter Muhling et al., discloses a bone screw made of resorbing plastic material comprising an internal tool insertion channel that is coaxial with the screw shaft and extends along most of the length of the shaft, where the cross-section of the insertion channel is non-circular and matches the cross-section of the insertion tool.
U.S. Pat. No. 5,470,334, hereinafter Ross et al., discloses a bioabsorbable interference bone fixation screw having an internal coaxial channel into which a rotatable driver may be inserted for screw application. The channel defines a number of radial forces receiving surfaces for transmitting forces from the driver to the screw.
U.S Pat. No. 5,695,497, hereinafter Stahelin, discloses a biodegradable screw and suitable screwdriver. The screw consists of an internal coaxial channel, with a plurality of lobe areas arranged uniformly about the channel area, which is radially symmetrical about the longitudinal axis in regular rotary steps.
All of these patents relate to screws having an internal coaxial channel that is used for insertion of the device. The disadvantage of having an internal coaxial channel becomes evident when considering a screw with a shaft diameter of 1-2 millimeters or less. Including an internal channel in such a small fastener would leave little remaining material, thus greatly compromising the strength of the device. Such a design is therefore not desirable when the goal is to maximize the strength of a very small threaded fastener, such as would be required in cranio-facial surgical applications where small plates and fasteners are used. However, a very small fastener without an internal channel is difficult to handle during a surgical procedure. A feature is therefore required that will enable the surgeon to grasp and deploy the fastener with ease.
What is needed in this art is a novel solid threaded fastener consisting of an integral component that is used for inserting the fastener and is then detached and discarded while also providing the surgeon means to remove the screw if repositioning is required. The current invention discloses such a device, and an instrument for deploying such a device.