# Patent

## Title
Prosthesis with low stiffness factor

## Abstract

This is a division of application Ser. No. 07/620,012, filed Nov. 30, 1990, now U.S. Pat. No. 5,108,443, which is a continuation-in-part of U.S. patent application Ser. No. 07/406,586, filed Sept. 13, 1989, now U.S. Pat. No. 5,062,851.

## Background
1. Field of the Invention

The present invention relates to an anchoring element for supporting a joint mechanism, a method of applying an anchoring element within a bone, and a reconstructed joint.

A preferred embodiment of the invention will be described in detail in the following with reference to the reconstruction of a finger joint. However, the invention is not limited thereto. The invention can be used for other similar joints, such as toe joints, elbow joints, and so on. The invention can also be used in reconstructions after amputation or other defects.

2. Description of the Related Art

Finger-joint reconstructions, entailing prostheses, are primarily carried out on MCP joints which have become rheumatically changed. Prostheses known for this purpose include Swanson Silastic finger joints. Such joints consist of a material similar to silicon, are elongate in shape, and have ends with substantially circular cross sections. The ends can be fitted and anchored into the marrow cavity of each bone. The central portion of the prosthesis is elastically deformable and thus constitutes the actual joint mechanism.

The St. George prosthesis, a cemented prosthesis, is also known for finger-joint reconstructions.

A problem with these conventional prostheses is that the anchoring element supporting the actual prosthesis mechanism loosens (mainly due to bone resorption) with undesired displacement of the prosthesis in the direction of load.

Attempts have therefore been made in recent years to use titanium fixtures anchored in the marrow cavity of the bone with the object of becoming osseo-integrated as described by Hagert et al. "Metacarpophalangal Joint Replacement with Osseo-integrated Endoprostheses" in Scand. J. Plast. Reconstr. Surg. 20, pages 207-218, 1986. It is already known to permanently anchor oral and extraoral prostheses in bone tissue. This dental osseo-integration technique has been developed over the last 25 years by Professor Branemark and his colleagues, with excellent results in applying fixtures in the jawbone to hold teeth or arch attachments. However, the experiments performed by Hagert to apply this technique to the reconstruction of finger joints has not fulfilled expectations. The unacceptable results are evidently due to the entirely different conditions encountered when using this "dental technique" in the prosthetic reconstruction of finger-joints. For example, in the known techniques, the fixture is anchored at right angles to the longitudinal axis of the bone. In the finger joint, the fixture is placed along the axis of the bone. Of course, this creates totally different loads and stresses on the anchoring elements.

Today, the main problem in orthopedic prosthesis surgery is still loosening of the bone anchoring unit. However, with a success rate for dental implants of more than 90% over a 20 year period, a number of other problems arise which, so far, have been unnecessary to take into account. One of the major problems is increased wear on the joint mechanism. A different type of prosthesis design from that used hitherto is required if the osseo-integration method is to be applied. To enable the joint mechanism to be replaced without disturbing the bone anchorage, the prosthesis system must be divided into components where the joint-mechanism element can be separated from the actual bone-anchoring element. Further, if the two-stage method is to be used, it must be possible to connect the joint mechanism in the second stage if the patient, or at least the patient's reconstructed joint, is not to be kept immobilized. Two factors must therefore be taken into account: First, the joint mechanism is subject to sear and therefore must be replaceable. Second, to use the two-stage method, the joint mechanism must be replaceable.

It has now surprisingly been found that the problems and drawbacks of the techniques described above can be eliminated by the present invention.

The invention is based on extensive experimental biological analysis of the structure and function of joints in the course of a disease or in defective state after wear or inflammatory decomposition of bone tissue and extensive studies of the vascular supply to the bone marrow. It has been established that, in the synthetic replacement of destroyed articular cartilage and ligament, bone and marrow tissue must be treated as a structural and functional unit. It is absolutely necessary to account for the cooperation between the bone and marrow tissue, particularly over a long period of time, if the hard tissue is to function as a support element.

The invention is thus based on the realization that, when anchoring a prosthetic replacement for a joint surface and ligament to parts of the skeleton close to a joint, the interaction between bone marrow and bone tissue must be respected. This means that an attachment element in the marrow cavity of a long bone must be anchored by mechanical elements which will permit communication between marrow and bone tissue along, as far as technically possible, the entire length of the anchoring element, paying particular attention to the boundary layer between marrow and compact bone in the wall of the marrow cavity.

Thus, the invention is directed to an anchoring element for supporting a joint mechanism, wherein the anchoring element is substantially rotationally symmetrical, at least partially hollow, and includes a material which is compatible with the tissue of a bone. The anchoring element has a surface which can be at least partially osse-integrated with the tissue to achieve permanent endosteal anchorage in the longitudinal axis of the bone. The invention is also directed to a reconstructed joint.

The invention is also directed to a method of applying an anchoring element within a bone, comprising: cutting the bone close to its joint to expose the marrow cavity of the bone; forming a space in the marrow cavity close to the joint; applying a guide sleeve in the space; and screwing the anchoring element into the marrow cavity while using the guide sleeve to center the anchoring element.

Other features and advantages of the invention will become apparent from the following description of a preferred embodiment of the invention, with reference to the accompanying drawings.

FIG. 1 is a partial cross-sectional view of a reconstructed finger joint with an anchoring element in accordance with the invention;

FIG. 2 shows a fully reconstructed finger joint;

FIG. 3 illustrates how the anchoring element is located in the long bone;

FIG. 4 is a perspective view of the anchoring element;

FIG. 5 is a cross-sectional view along the line 5--5 of FIG. 4;

FIG. 6 is a partial cross-sectional view along the line 6--6 of FIG. 4, and illustrates a portion of the joint mechanism in accordance with the invention;

FIG. 7 illustrates the assembly of the joint mechanism to anchoring elements in accordance with the invention;

FIG. 8 is a partial cross-sectional view of the anchoring element of FIG. 4;

FIG. 9 is a partial cross-sectional view of another anchoring element in accordance with the invention; and

FIG. 10 is a partial cross-sectional view of another anchoring element in accordance with the invention.

In FIG. 1, the parts of a long bone on each side of a finger joint mechanism 1 are designated 2 and 3, respectively.

A rotationally symmetrical anchoring element 4 is formed of a hollow, substantially sleeve-like body 5 with external threading 6 and decreasing wall thickness. The open insertion end 7 of the body 5 tapers and is provided with slits 8 and 9 initiating from the open end 7.

The end 13 (FIG. 6) of the anchoring element 4 closest to the joint mechanism 1 is located in a guide sleeve 11. The guide sleeve 11 surrounds the joint end 13. The joint end 13 of the body 5 is thus locked by a wedge effect against the inner surface 14 of the guide sleeve 11 when the body 5 has been screwed sufficiently far into the bone tissue. The joint end 13 detachably receives connection parts 15 of the joint mechanism 1. The connection parts 15 may be centered dowels (FIG. 7) protruding from the joint mechanism 1. Such dowels cooperate with corresponding recesses 16 in the anchoring element 4 or a connection piece (not illustrated) arranged suitably between the guide sleeve 11 and the joint mechanism 1.

One or more holes may be directly connected with the radially outer surface of the anchoring element 4, with the edges of the holes toward the surface forming cutting edges. Self-tapping is thus achieved when the body 5 is screwed into the bone 2. The removed bone tissue 17 is taken up inside the anchoring element 4 as illustrated in FIG. 5.

Since the thickness of the wall of the anchoring element 4 gradually decreases toward the insertion end 7 and/or since the element 4 has the longitudinal slits 8 and 9, the requirement for good deformation is fulfilled, thus greatly reducing the risk of concentrated stress which causes particular problems in prior art designs. The arrows B--B in FIG. 1 denote the flexibility of the open end 7, i.e. its ability to adjust to the surrounding tissue. The same applies to the arrows A--A.

The slits 8 and 9 may suitably be provided with cutting edges and, since the hollow cavity takes up the shavings 17, as illustrated in FIG. 5, the anchoring element 4 is, in itself, its own preparation tool. At the same time, optimum conditions for normal anatomical and physiological situations are ensured such that disturbance of the remaining biological tissue, i.e. marrow and bone tissue, is minimized.

FIG. 2 shows the joint reconstruction achieved with the anchoring element 4 in place.

FIG. 3 illustrates how the anchoring element 4 is positioned in the bone 2. When in use, the anchoring element 4 is positioned in the boundary zone between marrow and bone tissue. The wall thickness of the sleeve-like body 5 decreases toward the slit insertion end 7, finishing in a cutting tapered edge 12. The edges of the longitudinal slits 8 may also be in the form of cutting edges. The shavings 17 produced when the anchoring element 4 is screwed into the marrow cavity are thus transferred to the hollow space inside the body 5 as illustrated in FIG. 5.

In operation, the part of the long bone 2 close to the joint is cut, thus exposing the marrow cavity. A probe (not illustrated) is then inserted into the marrow cavity to localize a suitable longitudinal axis for insertion of the anchoring element 4. The probe is used as a direction finder to center the anchoring element 4 as desired. A recess for the guide sleeve 11 is then carefully drilled in the exposed marrow cavity. After application of the guide sleeve 11, the sleeve-like body 5 is screwed down into the marrow cavity through the guide sleeve 11. The probe has of course been previously removed. The joint end 13 of the anchoring element 4 widens somewhat outwardly and upwardly as illustrated at 18 such that its diameter is slightly larger than the lower opening of the guide sleeve 11. Thus, a wedge effect will finally occur between the joint end 13 and the guide sleeve 11 (when the sleeve 11 is moved to the position illustrated in dotted lines in FIG. 6), so that both guide sleeve 11 and sleeve-like body 5 are positioned as desired.

The anchoring element 4 and the guide sleeve 11 consist of or are manufactured of titanium or are coated with titanium. The structure of the titanium surface is such that, at least over part of the relevant surface, integration is promoted. Advantageously, the surface may have irregularities consisting of micro-pitting, as described in Swedish patent 7902035-0, with pitting diameters of between 10 and 1000 nm, preferably 10-300 nm. Alternatively, the chemical composition of the titanium surface layer may be given certain positive tissue reactions, as described in Swedish patent 8505158-9, for instance.

FIG. 8 shows how the upper end 19 of the sleeve 11 facing the flange 20 of the element 4 is provided with a protruding edge 21 arranged to cooperate with threads or teeth 22 on the flange 20. The element 4 and sleeve 11 are thus non-rotatably locked when the body 5 has been screwed sufficiently far into the bone tissue.

An alternative locking effect between the sleeve 11 and the element 4 is achieved by the arrangement shown in FIG. 9, wherein axially arranged corrugations 23 are arranged to cooperate with the inner surface 14 of the sleeve 11. The embodiment illustrated in FIG. 9 is otherwise identical to the embodiment illustrated in FIG. 4.

As illustrated in FIG. 10, a further locking effect may be achieved by a special thread or radially directed teeth 24 at the upper rim or flange of the element 4, which is screwed into the tissue. The embodiment illustrated in FIG. 10 is otherwise identical to the embodiment illustrated in FIG. 4.

Although the invention has been described in relation to a preferred embodiment thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the invention be limited not by the specific disclosure herein, but only by the appended claims.

## Claims

1. A method of applying an anchoring element within a bone, said anchoring element comprising a sleeve-like body for engaging a marrow cavity of said bone, a flange attached to one end of the sleeve-like body, a guide sleeve for guiding the sleeve-like body into said marrow cavity, and means for locking the guide sleeve and the flange together, the method comprising the steps of: of said bone; said guide sleeve to center said anchoring element; and

2. The method of claim 1, further comprising, subsequent to said step (a), inserting a probe into said marrow cavity to localize a longitudinal axis, and removing said probe from said marrow cavity.

3. The method of claim 2, wherein said step (b) includes drilling said space in said marrow cavity close to said joint.

4. The method of claim 1, wherein said locking means comprises a protruding edge located on one end of said guide sleeve and teeth located on a surface of said flange facing said protruding edge, and said locking step (e) comprises moving said protruding edge and said teeth into contact.

5. The method of claim 1, wherein said locking means comprises axially arranged corrugations located on said sleeve-like body of said anchoring element for engaging an inner surface of said guide sleeve, and said locking step comprises moving said corrugations and said inner surface of said guide sleeve into contact.

6. The method of claim 1, further comprising the step of locking said flange in contact with said marrow cavity, said flange including radially directed teeth for contacting said marrow cavity.

7. A method for applying an anchoring element within a bone, said anchoring element comprising a sleeve-like body for engaging a marrow cavity of said bone, a flange attached to one end of the sleeve-like body, a plurality of radially extending teeth located on said flange for contacting said marrow cavity, and a guide sleeve for guiding said sleeve-like body into said marrow cavity, the method comprising the steps of: of said bone; central hollow region of the guide sleeve; and said radially extending teeth into contact with said marrow cavity.