AU657574B2 - Dynamic external fixator - Google Patents

Description

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ORIGINAL

NAMIE OF APPLICANT ACTUAL INVENTOR ADDRESS for SERVICE INVENTION TITLE DETAILS OF ASSOCIATED PROVISIONAL APPLICATIONS Dr. Ninoc Patel Dr. Minoo Patel 21 Ranfurlie Drive, Glen Waverley 3150.

Dynamic External Fixator Nos PK 8476 19 Septem~ber 1991.

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1_ -1- DYNAMIC EXTERNAL FIXATOR The present invention relates to an improved dynamic external fixator for treating bone fractures in humans and animals, especially near the joints.

External fixation is a procedure whereby long pins I are screwed into the bone through tiny incisions and 0 the external portions of the pins are connected by to an assembly comprising a rod or rods to provide the stability across the fracture site, necessary for the bone to heal. The pins are divided into groups or sets of two or occasionally three or even more There is generally one set of pins on either side of the fracture site, but essentially in 00 0 the one and the same bone. Fractures into ends of bones, the parts that form joints with corresponding surfaces of other bones, especially the radius or the tibia, but essentially any bone, create special problems. Since at the ends of bones there is no space to insert the pins in the small end piece of bone, which normally forms a joint surface, the pins have to be inserted in such a way that one set of pins is across the joint. Thus, for fractures of the distal end of the radius bone, for example, the proximal set of pins in the radius and the distal set is in the metacarpals.

-2- Known fixators originally consisted of a rigid external rod or rods which were connected across the sets of pins. When connected across a joint this resulted in a rigidly immobilised joint for six to ten weeks, and like a plaster cast, resulted in stiffness in the joint which in many cases would never become fully mobilised.

Joint surfaces which have been involved in fractures Goo* 444 "°are known to heal better and more smoothly if there 10 is movement in the joint while healing occurs.

This has led to more recent developments where fixators have become 'dynamic',that is, allowing 0 44 movement of the joint while still fixing the fracture.

One of the earliest dynamic fixators was reported by Clyburn in the Journal of Bone and Joint Surgery in 1987. This device uses a universal ball and socket joint in the external connecting rod coupled with a pistoning and shock absorbing device. The procedures involve the location of the centre of rotation of the wrist joint which roughly lies over the capitate. The result is a complicated and hence expensive device which involves a complex method of use not favoured by all surgeons, particularly in surgeries where time is vital.

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-3- A number of later devices now available use a combination of one or more universal joints. They suffer from a number of common deficiencies a)they are generally expensive.

b)they do not maintain the necessary distraction (pulling the crushed fractured bone pieces apart into anatomical position) at the fracture site when allowing movement.

c)they do not offer the required resistance to o 0 motion whilst allowing movement.

d)they have the inability to account for the complexities of joint movement, particularly around the wrist, where there is a very complex axis of o. movement between ten different bones. Attempts to overcome this problem have resulted in the use of multiple universal joints with the attendant problem of instability as outlined above.

d)they require the use of special clamps to hold sets of pins together. This gives less freedom in the positioning of of the pins and violating basic biomechanical principles.

Accordingly, it is an object of this invention to provide an improved dynamic fixator which overcomes one or more of the aforementioned disadvantages of known fixator systems.

-i 1 i; 1 i i Thus, one broad form of the invention which may be preferred provides a dynamic fixator including a connecting rod for external connection across a (bone) joint between pins fixed to the bones on opposite sides of the said joint, said rod being formed in two parts separated by a gap bridged by a flexible joint comprising a coiled spring co-axial with said rod and connected at S oa each end to a respective said rod end.

o oa 10 Preferably an end of each part of said rod is inserted into a respective end of said spring.

°i Preferably the ends of said spring are welded to the 0 respective said end parts.

a a. Preferably said spring is a helical spring and a plurality of turns of said spring extend across said o. 0 gap.

o t Preferably said spring is about 8 cm in length and said gap is 4 cm in length.

Preferably said spring is formed of wire of about 1.0 mm diameter and is tightly wound with an internal diameter substantially corresponding to the diameter of said rod.

,1 i. In order that the invention may be more readily understood a particular embodiment will now be described with reference to Figs. I, II, III, IV, and V of the accompanying drawings Figs. I and II are side elevations (not to scale) of a prior art universal ball and socket joint in a dynamic fixator, SFig.III shows a dynamic fixator according to the 10 invention, Fig.IV shows the fixator of Fig.III in a bent or flexed position, and Fig.V is an end view of the fixator of Figs.III and IV on an enlarged scale.

Firstly, referring to prior art shown in Figs. I and II it will be. readily appreciated that S. C A B Swhere A is the length of one rigid part of the fixator, B is the length of the other rigid part of 20 the fixator, and the C is the distance between the free ends of the fixator when it is in a non-linear S(bent or flexed) configuration. The tendency with the 'ball and socket' fixators is thus to compress the bony fragments during movement of the joint which is a distinct disadvantage in a fixator whose prime function is to hold the bony fragments distracted.

Referring now to Figs. III, IV and V, the rod 10 of a fixator according to the embodiment is a two part rod, the parts being parted at a joint 11 and -6connected together by a spring 12. The ends of the separate parts of the rod 10 at the spring 12 are inserted into respective ends of the spring 12 and permanently attached thereto by welding or other means, or any other way.

The longer of the two parts of rod 10 is 22.0 cm and the shorter part is 8.0 cm with a separating gap of cm. The rod has a diameter of 4.0 mm. The spring 12 has a length of 8.0 cm and a wire diameter of mm and is wound in tight coils with an inner diameter of 4.0 mm. Therefore the ends of the rod 0 are a press-fit into the respective ends of spring So 12 and are inserted therein a distance of 2.0 cm in r each case. The rods and spring formed of surgical Oo 15 steel or other suitable material; the rod of a material having the required strength and rigidity and the spring of a material having sufficient structural integrity to provide some resistance to :o bending movement as well as sufficient flexibility to allow motion of a joint whilst holding the fracture surfaces apart.

boo& As will be evident in Figs. III and IV, where the distance between the free ends of the rod 10 is rr represented by D, whilst D A' B' from II and IV D C This is because the spring not only allows movement but it opens up and resists collapse thereby preventing the fractured bone pieces from being crushed.

It will be evident to persons involved in Orthopaedics that the spring joint provides considerable advantages over existing devices. The coiled spring acts as a series of tiny universal Si -7joints when bent or flexed and allows movements in all three planes around multiple adjacent triplanar centres. In other words there is no single centre of rotation as there can be a number of centres around which the spring bends or flexes, or the predominant centre can move constantly during motion. This closely replicates the movement of the wrist joint, which is made of the surfaces of as many as 10 bones and has a complex movement. The spring obviates the need to calculate the single 'average' centre of rotation of the wrist to coincide with the single (i centre of rotation of the 'ball and socket' e •universal joint. The coiled spring whether S.compressed or distracted always tries to restore or 15 back to its original position. Therefore when o initially applied it prevents the bone fragments at the fracture site from collapsing and later allows movement while still maintaining the distraction.

o. In practice, the fixator as described in the invention may be used with conventional Schanz screws (pins) and existing clamps to provide a complete dynamic fixator assembly. In the case of the wrist fractures it is preferred to use a "modification of the existing A.O. method by using 25 3.5 mm Schanz screws in the horizontal coronal plane on the lateral aspect of the forearm. The screws would initially be connected by standard solid rods to maintain distraction. After about 2-3 weeks the solid rods would be replaced by the spring jointed rod of the invention which would allow early mobilisation of the joint. However this dynamic assembly can be used with ANY assembly, existing or otherwise, for the purposes of dynamisation.

Whilst a particular embodiment of the rod has been described hereinabove, it should be emphasised that i- C 4

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04 0 00 0 e oi oo o 0 0 0000 a 0 0 o00o 0010 a ao o o *a B 0 0 o a 0 0 I r t e I €1 0.0 Sa a a a a So a« +e o o aoa there is no fixed specification in terms of size and materials. Clearly the diameter and length of the rod and dimensional specifications of the spring can be modified to suit different applications and to suit any existing rigid rod system of external fixation.

An important point to be understood here is that the all external fixators are the same except the external rods and their characteristics. The pins inserted into the bone and the clamps connecting these pins to the external rod are all essentially very similar to each other, and can be inter-changed in their usage with each other and with external rods. This new dynamic external fixator is based on 15 a new external rod; it is this external rod which makes the fixator static or dynamic. Surgeons may, and will, use this dynamic external rod with all sorts of pins and clamps. This is totally fair. What is unfair is the incorporation of the coiled spring 20 dynamising device.

It is this coiled spring design incorporation in the external rod, that makes this a unique invention.

The fixator rod can thus be modified to be used in any situation where movement is required in a multiaxial plane around varying points of rotation whilst maintaining required compression or distraction.

Since modifications within the spirit and scope of the invention may be readily effected by persons skilled in the art and craft of Orthopaedic surgery, and instrument design and manufacture, it is to be understood that the invention is not limited to the -o i.

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particular embodiment described, by way of example, hereinabove.

Dated November 15, 1994 DR. MINOO PATEL Applicant.

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