CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of PCT/EP2018/058843, filed Apr. 6, 2018, which claims the benefit of European Patent Application No. 17165580.6 filed on Apr. 7, 2017, the disclosures of which are incorporated herein in their entirety by reference.

FIELD OF THE INVENTION

The invention relates to an external fixator device and a method of its use in treating bone fractures and in orthopedic interventions, such as corrective osteotomies.

BACKGROUND

Fractured bones and corrective osteotomies are surgically treated by means of one of the three generic medical device families: (i) plates and screws; (ii) intramedullary nails, and (iii) external fixators. Selection of external fixation by trauma and orthopedic surgeons is greatly influenced by their background, attitude, place of practice and general economic conditions in addition to the nature of the medical problem to be treated. This greatly complicates any estimates of what the number of cases being treated by any of the methods might be worldwide, but all three are universally considered as fundamentally important surgical aids.

In many cases, internal fixation by plates or nails is simply not possible in most of the undeveloped world, thus making external fixation the only potentially viable, modern alternative to complement conservative treatments by fracture splinting or casting. Unfortunately, the costs of external fixators produced in the developed world are also prohibitive for most of the undeveloped countries.

The main drawback of external fixation is the rather uncertain and uneven progress of bone healing when compared to internal fixation. Passage of bone pins or wires through soft tissue surrounding the bone and the skin increases the risk of infection when the fixator is kept on the patient for a long period of time, e.g. several months.

Much of the uncertainty can be explained in view of rather recent research findings, which have elucidated different biological phases of fracture healing that call for different mechanical conditions at the fracture site. In the very early stages, in the first week or two, as the repair is initiated, differentiating factors emanating from the surrounding bone need to provide signals to the proliferating cells in the fracture zone to actually turn themselves into bone forming cells. In absence of any movement across the fracture, or the osteotomy, mass transport of the signals out of the bone and throughout the zone of repair is limited to diffusion, which may not suffice when the gaps of the fracture are in excess of some hundreds of micrometers. Formation of fibrous tissue in the gap, which is what tends to quickly fill any tissue defects, may slow down, if not totally frustrate formation of bone proper, leading to delayed unions or non unions. Movement, and particularly compression of the gap promotes convective mass transport of the biological factors out of the bone and within the gap, driving the differentiation process towards bone formation.

After this early period, however, once the early organic matrix for bone has been produced and the mineralization starts to set in, excessive movement will prevent bridging of the gaps between the nuclei of mineralization and disrupt the healing process. Thus, at this stage of the healing process, in order to facilitate a safe and thorough process of mineralization, the movement across the gap should be reduced as much as possible calling for as stiff a construct of the external fixator as possible.

In the final stages of fracture union, presence of the stiff external fixator, however, may hinder remodeling of healed bone by reducing the full physiological loading it will need to support once the fixator is removed, so again, a change in fixator stiffness, back to low, is called for.

There are numerous ways in which this modulation can be carried out with conventional external fixators, but most of them require tedious and complex measures. Thus, there is a need to provide an external fixator which overcomes the problems associated with the state of the art.

DESCRIPTION OF THE INVENTION

The present inventors have provided a novel fixator which is designed to allow insertion of bone pins in parallel fashion, but also at an oblique angle to each other. The stiffness of such a construct with no more than two bone pins parallel to each other is much higher than of that with all pins parallel.

Modulation of the overall stiffness can be effected by locking either only two, or all three of the pins inserted in a bone segment. For the maximum effect of modulation the external frame that holds the bone pins together needs to be as stiff as possible. It also needs to lock the pins very stably against the bending and axial forces.

The fixator of this invention fulfills the mechanical requirements outlined above, but it also solves the problem of production at costs affordable to even some of the undeveloped world. Its deployment in the developed world can reduce the overall costs of the medical treatment not only because of the lower price of the hardware, but because, and more importantly, if used according to the proposed method, it can shorten the time of healing process and avoid unnecessary further surgeries.

The invention discloses an external fixator clamp with a shape and function of a yoke, adapted to clamp a bone pin to a support element that bridges the distance from one to the other segment of the fractured bone. Clamps according to this invention require very small amount of materials for production of clamp components that are also of simple shapes and well suited for efficient manufacturing. The strength of the external fixator construct with 6 pins and 6 clamps has been demonstrated to match or exceed that of a double rod conventional frame using 12 clamps for 6 pins.

The clamp and an external fixator comprising said clamp can be used in human and veterinary medicine.

In one aspect, an external fixator according to the present invention comprises:(i) at least one external support element (1),(ii) a plurality of bone pins (2),(iii) at least one fixing element (100) for fixing a bone pin to the support element, and(iv) optionally at least one connecting element to connect several external support elements to each other.

A further aspect of the present invention is directed to a method for external fixation of a bone, comprising applying the fixator to a subject in need thereof. The method is particularly suitable for the treatment of bone fractures and in orthopedic interventions, such as corrective osteotomies.

The external support element may be a tube or a solid rod. It may have a straight or curved shape depending on the mechanical requirements of the fracture to be fixed. The device may comprise one or more external support elements. If several support elements are present, they may be connected to each other by connecting elements such as clamps or bars.

The bone pins are designed to be inserted on one side into the bone to be fixed. Usually, at least three bone pins are used per bone segment, i.e. on either side of the fracture to be fixed.

The stiffness of the construct may be modulated by using a fixing element, in the following designated as a clamp, which can be tightened or loosened depending on the stage of the bone healing process.

In the device of the present invention, each bone pin may be fixed to a single support element at a single fixing point. The bone pins can be placed either perpendicularly to the support element or at an oblique angle to the support element. They may be inserted through fixing elements aiming at the bone from either side of the support element (in the following, this element of the fixator is designated as “tube”, but it is clear that a “solid rod” may serve the same function). The diameter of the tube is usually on the scale of cross sectional dimensions of the bone being treated, thus fixing the pins to opposite sides of the tube, gives the construct an added three dimensional stability.

In a preferred construct two bone pins are inserted parallel to each other and perpendicular to the tube on each side of the fracture. More preferably, these bone pins are fixed to opposite sides of the tube. Further, third bone pins are inserted into the bone at an oblique angle to the other pins and to the tube on each side of the fracture.

At the time of surgery, the third bone pin is not clamped tight to the tube. Selectively clamping the bone pins allows for modulation of the construct stiffness. In this preferred configuration, with two parallel bone pins perpendicular to the frame and one oblique bone pin per bone segment, locking the oblique bone pin in addition to the perpendicular bone pins will increase the overall stiffness of the device 2 to 3 times, depending on the tube intrinsic stiffness. This factor of modulation also depends on the stiffness of the bone pins and the bone itself, but it is invariably much higher than what can be achieved by, for example, additionally locking the third bone pin in a more conventional configuration with all three bone pins being parallel to each other, which brings only a fractional increase in construct stiffness.

Properly modulating the construct stiffness between biologically distinct phases of bone healing can dramatically reduce the time of healing and improve the strength of the healed defect. Particularly, and in contrast to the well-accepted notion of dynamization promoted by the company Orthofix in the context of external fixation, an initial period of one to two weeks with low stiffness, followed by an increased stiffness of the construct, leads to an outcome superior to what can be obtained by keeping either low or high stiffness throughout the healing process, or to switching from initially high to low stiffness (J Bone Joint Surg Am. 2012 Nov. 21; 94(22):2063-73. doi: 10.2106/JBJS.K.01604. Improved healing of large segmental defects in the rat femur by reverse dynamization in the presence of bone morphogenetic protein-2. Glatt V, Miller M, Ivkovic A, Liu F, Parry N, Griffin D, Vrahas M, Evans C).

In this context, the term “reverse dynamization” is used to describe stepping up from low to high stiffness of the external fixator construct during the course of the healing process. Correspondingly, the term “dynamization” is used to describe stepping down from high to low stiffness. According to the present invention, a dynamization of the construct may be deployed towards the end of the treatment, not at the start. Changing the stiffness of the construct according to the present invention is a simple, ambulatory intervention—the clamps holding one of the bone pins on each side of the fracture that were not fixed tight at the time of surgery are now clamped tight.

