Within the medical field, fixation devices are used in different ways to assist in the reconstruction of damaged body tissue. A fixation device may be used to directly secure tissue in close approximation to neighboring tissue to effect healing such as is the case with meniscal fixation devices, or fracture fixation pins, screws, or wires. Other types of fixation devices are intended to provide mechanical stability and load sharing during the healing process, as when a graft is secured in a bone tunnel for ACL reconstruction. Fixation devices may also be used in conjunction with other device hardware such as plates, rods, or various other connecting members known in the art as part of an implant assembly, such as with a spinal screw fixing a plate to the vertebral body, spinal pedicle screws connected to posterior rod assemblies to name just a couple examples. Other fixation devices are used to anchor suture to bone so the suture can be used to secure injured soft tissues.
Fixation devices typically have an elongate body, and one or more engaging feature(s) for retaining the device within body tissue or as part of a device assembly. The fixation device may either be inserted into body tissue directly, through a preformed hole with or without the aid of a tap, or as part of a device assembly within the bone cavity, such as with a screw/sheath assembly. Oftentimes fixation devices require the application of torsional forces from an insertion tool at one end of the implant to secure their application into body tissue, as with screw-type implants. Insertion tools are typically formed from an elongate shank having a mating feature formed on a distal end thereof for mating with a corresponding mating element formed on or in the head of the fixation device. One common type of driver tool includes a hexagonal-shaped or square-shaped socket for receiving a corresponding hexagonal-shaped or square-shaped head of a fixation device.
Certain conventional fixation devices and their drivers have some drawbacks. Device heads with hexagonal or square shaped cross-sections, for example, tend to have a relatively low stripping strength, meaning that under relatively small torque loads the drive head may be permanently damaged and torque transfer thus inhibited. If the head shape decreases the amount of material on the fixation device head or anchor head that interfaces with the driver, then the amount of material that needs to yield or be “stripped” from the drive head is reduced, thus reducing the stripping strength of the head.
Conventional fixation device heads also tend to have a relatively low failure torque, which can result in shearing of the head during insertion or stripping of the head elements necessary to transfer torque to the device. This type of failure can also be caused by the geometry of the head, which can reduce the overall cross-sectional area of the drive head. Fixation devices were historically constructed of implantable metals and alloys which afforded sufficiently high tensile and torsional strength to withstand the rigors of insertion, but the implant remained in the body for prolonged periods of time. Polymer, ceramic, or composite material systems, both biodegradable and non-biodegradable, have been developed for similar applications, but typically have lower tensile and torsional strength than metal counterparts, thus increasing the risk of device failure during application of high tortional torque loads during implantation in the body, as described above. More recently, biodegradable composite material systems have been developed that incorporate filler materials within the polymer matrix, such as calcium phosphate particles, which are osteoconductive. These filled systems may have further reduced tensile or torsional properties compared to unfilled polymer systems. Thus there is a need for an improved drive head for implantable fixation devices that has higher torsional resistance to strippage or shearing off.
One option to increase the failure torque of a fixation device is to increase the size of the head. Large device heads, however, require a large driver tool, which in turn requires a relatively large bone tunnel to be formed in the bone. This is particularly undesirable where the bone tunnel is to be formed in the cancellous bone, and where the procedure is minimally invasive and must traverse through a cannula or arthroscope. Accordingly, most fixation devices are adapted for use with a relatively small driver tool, and thus they have a relatively small drive head, which can result in a low failure torque and a low stripping strength, particularly in harder bone applications. A drive head of improved torsional strength is desirable to reduce the risk of deformation during insertion. Additionally, a drive head more resistant to deformation upon application of torsion may make a revision procedure easier, as there are some instances where torque driven devices need to be backed out and perhaps even reinserted.
Accordingly, there remains a need for fixation devices having improved physical properties, and in particular having a high failure torque and a high stripping strength.