External fixator

Details External fixator External fixator

1996-04-12

1998-09-08

Buiz, Michael

Surgery

Instruments

Orthopedic instrumentation

606 57, 606 59, A61B 1756

Patent

active

058039244

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to an external fixator.
Fractures of bone heal by the formation, at the site of the fracture, of a bone-like reparative tissue called callus. Bone fracture treatment attempts to hold the bone fragments in anatomical alignment while the natural process of healing is taking place. Fracture surgeons employ various methods of operative treatment such as the use of plates and screws, intra-medullary nails and external fixators to hold fracture fragments in normal alignment.
External fixation refers to the rigid anchorage of bone fragments to an external device (an outrigger) such as a metal bar, via the medium of transfixing pins. By these means, the fractured bone may be kept relatively immobile while the fracture heals, allowing use of the limb etc.
Typically, a single or unilateral outrigger, essentially a metal splint, is located outside the body, and bone fragments are fastened to it through the medium of pins. The outrigger is usually a rigid metal bar incorporating fixed or sliding pin-holding clamps. With this type of fixator, it is difficult to maneuver bone fragments into new positions once pins have been inserted. Furthermore, pin sites are usually limited to two or three per fracture fragment.
Modular fixators have been developed. In this variety, one or more outrigger members are linked together like a piece of scaffolding by way of special clamps. However, although the modular fixator is more versatile in its applicability, it is often less stable than the rigid fixator with consequent risk of fracture displacement and mal-alignment. Too much movement at the fracture site is deleterious to healing, often resulting in non-union or delayed union.
On the other hand, the rigid fixator prevents the micro-movement at the fracture site which is necessary for normal healing. The result is diminished callus formation and, as a consequence, delayed union and non-union. Hence, rigid fixators have been modified to allow some degree of axial movement at the fracture site, i.e. in the direction of the bone axis. Simple telescoping devices are the usual methods but the current designs have been shown by several workers to be unreliable in performing the function.
Pin loosening is a common complication of external fixation of fractures. The aetiology is not entirely clear but may be due to problems with pin anchorage in bone. According to some workers a less than secure anchorage causes bone resorption. It has been suggested that pin anchorage in bone may be improved by pin design but this has not been supported by results from pull-out tests. Other workers have suggested radial pre-load techniques, in which the pin is fitted into an undersized hole, and a pre-load technique in which the pin is bent resiliently before clamping to the external frame, but these methods do not prevent bone resorption at the pin-bone interface.
Loosening may also be due to mechanical failure at the pin-bone interface. Excessive stresses generated in the bone by axial and bending forces may cause local bone failure. Stresses generated at the pin-bone interface may be reduced by increasing the stiffness of the pins. In practice this is done by using large diameter pins or by increasing the number of pins. However, these methods do not prevent pin loosening. Furthermore, there is a limit to the size of pins, otherwise fracture of the bone will occur at the pin site.
Conical pins have been used, but it appears from theoretical calculations that these may increase shear stress and stresses at the pin-bone interface.
One aim of the present invention is to provide an external fixator which will allow a controlled degree of micro-movement at the fracture site. Micro-movement (generally less than 0.5 mm) is intended to simulate the movement experienced within unfractured bone.
Another aim of the invention is to provide a transfixing pin which will be held securely in the bone.
A first aspect of the invention provides an outrigger for an external fixator, the outrigger comprising first and second members for attac

REFERENCES:
patent: 2398915 (1946-04-01), Bell
patent: 4448482 (1984-05-01), Lathlaen
patent: 5320622 (1994-06-01), Faccioli et al.
Leaflet on Orthofix.RTM. Modulsystem showing prior art systems.
Leaflet on Isodyn.RTM. External Fixator showing prior art systems.

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