Implant abutment

The present invention relates to a percutaneous implant abutment for bone anchored implant devices adapted to be anchored in the craniofacial region of a person, such as bone anchored hearing aids. Implant devices of this type normally comprise a screw-shaped bone anchoring element (fixture) for permanent anchorage in the bone tissue and an abutment sleeve for skin penetration. The complete structure can either be in one piece or the skin penetrating abutment could be connected to the fixture prior, during or after the implantation procedure by means of a screw connection or the like.

The invention can for instance be used in connection with hearing aid devices of the bone conduction type, e g bone anchored hearing implants such as Baha®, marketed by Cochlear Bone Anchored Solutions AB in Gδteborg, Sweden. The Baha® implant comprises an external unit which transforms sound to mechanical vibrations which are conducted via the abutment and the fixture into the bone of the skull. The vibrations are transmitted mechanically via the skull bone directly to the inner ear of a person with impaired hearing and allows for the hearing organ to register the sound. A hearing aid device of the BAHA® type is connected to an anchoring element in the form of an implanted titanium screw installed in the bone behind the external ear and the sound is transmitted via the skull bone to the cochlea (inner ear), i e the hearing aid works irrespective of a disease in the middle ear or not. The bone anchoring principle means that the skin is penetrated which makes the vibratory transmission very efficient.

This type of hearing aid device has been a revolution for the rehabilitation of patients with certain types of impaired hearing, but also as anti-stuttering means. It is very convenient for the patient and almost invisible with normal hair styles. It can easily be connected to the im-

planted titanium fixture by means of a bayonet coupling or a snap in coupling. One example of this type of hearing aid device is described in US Patent 4,498,461 and in SE 9702164-6 it is described a one-piece implant of this type, in which the fixture is integrated with a first coupling device. In WO 2005/037153 it is described how this type of hearing aid device can be used as an anti-stutte- ring device.

A well known problem with percutaneous implants is the infections and inflammation at the skin-implant interface. The infections are a result of bacterial colonization occurring at the area around the interface. There is generally a lack of integration of the skin to the implant which results in a gap between the two. This gap is unfortunately an ideal environment for the bacteria and if this zone is not properly managed, it is likely that an infection will occur. By creating an integration of the skin to the implant the adverse skin reactions associated with bone anchored percutaneous implants are expected to be reduced.

Creating integration between the skin and the implant requires that the implant is suitable for this purpose and that the soft tissue does not dissociate itself from the skin penetrating implant abutment by encapsulating the abutment in fibrous tissue.

In the field of dental implants it is previously known to use different types of abutments which penetrate the oral mucosa. However, it should be understood that there is a physiological difference between breaching the skin barrier compared to the oral mucosa. In the oral cavity the skin is not involved and there is another type of force situation. In contrast to dental implants the present invention relates to extraoral implants.

One object of the present invention is to provide an im-

plant abutment of the skin penetrating type in which specific adverse skin reactions are reduced.

It is recognized that bone anchored percutaneously implants are subjected to mostly shear forces, while percutaneously implants which are not bone anchored are subjected to several other types of forces, such as pull and torsion. Such different types of forces are also mostly involved in dental applications. Mostly shear forces are especially the case for implants with inherent movements such as bone anchored hearing implants due to the generation of vibratory movements.

It is also recognized that the effect that the shear forces has on the skin leads to tissue damage not only from a mechanical point of view but, more importantly, an indirect biological reaction which leads to foreign body reaction or dissociation from the material (encapsulation of the implant by fibrous tissue, etc). Some rections are acute and some are noticed after several weeks.

A further object of the invention is therefore to provide an implant abutment in which the shear forces between the implant abutment and the skin have been reduced. This would have a great impact on the wound healing and integration around bone anchored percutaneously implants.

According to the invention the shear modulus of the skin contacting part of the percutaneously implant abutment is reduced. Preferably the shear modulus should be less than 35 GPa.

Specifically, the shear modulus is reduced by a modification of the surface of the skin contacting part of the percutaneously implant abutment.

According to a preferred embodiment the surface of the skin contacting part of the percutaneously implant abut-

ment is coated with a biocompatible polymer with a thickness of 0.001-50 μm.

According to a another preferred embodiment the surface of the skin contacting part of the percutaneousIy implant abutment is coated with a ceramic material with a thickness of 0.001-50 μm.

According to even another preferred embodiment a surface enlargement treatment has been provided to the surface of the skin contacting part of the percutaneousIy implant abutment. Preferably a 10 % surface increase, compared to a conventional machined surface, is created resulting in a roughness value Sa of 0.5-10 μm.

It should be understood that there are percutaneousIy implants as such that are made of polymers (catheters etc) but they are not bone anchored and they are not exposed to the typical shear forces that are the case for implants with inherent movements such as bone anchored hearing implants due to the generation of vibratory movements.

In the following the invention will be described more in detail with reference to the accompanying drawings, in which

figure 1 illustrates an implant according to the invention anchored in the bone in the craniofacial region of a person,

figure 2 illustrates an implant according to the invention for bone anchorage,

figure 3 is a LM picture of the interface between the skin and the contacting part of the implant abutment, and

figure 4 is a SEM picture of the surface structure of the skin contacting part of the implant abutment.

Figure 1 illustrates a percutaneous implant 1 anchored in the bone in the craniofacial region of a person. The implant is specifically intended to be used for a bone anchored hearing aid or the like. The implant comprises a screw-shaped bone anchoring element (fixture) 2 for permanent anchorage in the bone tissue 3 and an abutment device 4 for skin 5 penetration. The complete structure can either be in one piece or the skin penetrating abutment 4 could be connected to the fixture prior, during or after the implantation procedure by means of a screw connection or the like. The screw-shaped anchoring element, the so- called fixture 2 is made of titanium which has a known ability to integrate with the surrounding bone tissue, so- called osseointegration. The fixture has a threaded part 2a which is intended to be installed into the skull bone and a flange 2b which functions as a stop when the fixture is installed into the skull bone. The apical part of the fixture has a known tapping ability with in this case three self-tapping edges 2c. A fixture of this type is described in the above-mentioned SE 0002627-8 and will therefore not be described in any detail here.

The skin penetrating part, the abutment 4 of the implant, comprises a substantially conical abutment sleeve. Conical abutment sleeves are previously known per se as separate components or as an integral part with the fixture, a one- piece implant. The abutment sleeve is provided with a first coupling part in order to cooperate with a second coupling part (not shown) by means of snap-in action or the like.

According to the invention the shear modulus of the skin contacting part of the percutaneousIy implant abutment 4 has been reduced. Preferably the shear modulus should be less than 35 GPa.

Specifically, the shear modulus is reduced by a modifica-

tion of the surface of the skin contacting part of the percutaneousIy implant abutment, illustrated by the structured abutment surface in figure 2. According to a preferred embodiment the surface of the skin contacting part of the percutaneousIy implant abutment is coated with a biocompatible polymer or a ceramic material with a thickness of 0.001-50 μm. The coating is applied in such a way that non-interconnected pores or crevices are created. Generally the coating should be applied in such a way that a structured surface such as a porous surface or a surface with indentations or a fibrous surface is obtained. A typical porous surface is illustrated by the SEM picture in figure 4.

The polymer coating is comparatively soft and decreases the shear stresses on the skin. Preferably a layer of a porous polymer is used for the coating with a thickness of about 30 nm. Such design is allowing the skin to heal into the polymer matrix.

Also a polymer containing a pharmaceutical drug that increases the production of extra-cellular matrix proteins in the soft tissue, such as collagen or keratin, might be used. The increased stability of the tissue increases the resistance to shear stress.

Also other types of materials might be used for increasing the skin tissue integration. Specifically, chemical substances such as pharmaceutical drugs and antioxidants, or biochemical substances such as proteins, biopolymers, growth factors, DNA, RNA or biominerals might be used. These substances are then associated to the implant with a purpose of increasing the amount of, or number of connections to extra cellular matrix proteins. Antibiotic, steroid or anti-inflammatory substances might also be used.

As an alternative to said coatings or substances, or in combination, a surface enlargement treatment can be provi-

ded to the surface of the skin contacting part of the per- cutaneously implant in order to increase the surface roughness. Such treatment can be achieved by using techniques that includes grit-blasting, polishing, micro-machining, laser treatment, turning, anodic oxidation, oxidation, chemical etching, sintering or plasma deposition of a titanium surface. Preferably such treatment should result in a 10 % surface increase, compared to a conventional machined surface and a roughness value Sa of 0.5-10 μm, measured by means of White Light Interferometry.

Figure 3 is a LM picture of the interface between the skin 5 and the contacting part of the implant abutment 4 of a polyurethane coated titanium material. The figure illustrates the situation after a healing period of 8 days and indicates a substantial integration of the abutment into the skin 5.

Figure 4 is a SEM picture of the surface structure of the skin contacting part of the implant abutment having an anodically oxidized surface.

It should be understood that only that part of the abutment surface which is in contact with the skin need to be modified. Other parts of the abutment such as the lower and upper end surfaces, i.e. the surfaces connected to the fixture and the coupling device respectively, might have a conventional, machined and/or polished surface.

Preferably the implant design includes a flange or a skirt perpendicular to the abutment orientation in order to mechanically increase the surface area and stability and thereby also reduce the shear stress on the implant-skin interface. Also the implant design might include one or more retention grooves or waists. Otherwise, however, the abutment should be designed without any sharp edges or corners in order to simplify the surface modification procedure.

The invention is not limited to the examples illustrated so far but can be varied within the scope of the following claims and for different extraoral applications. Specifically it should be understood that any combinations of the said surface modifications could be used, e.g. using composites, structured ceramic coatings, polymer/pharmaceutical drug coatings, anodized flange etc.