WO1997028760A1

WO1997028760A1 - Pin or screw-like securing device for osteosynthesis

Info

Publication number: WO1997028760A1
Application number: PCT/CH1997/000040
Authority: WO
Inventors: Wilfried Schilli; Martin Schug
Original assignee: Institut Straumann Ag
Priority date: 1996-02-09
Filing date: 1997-02-10
Publication date: 1997-08-14
Also published as: AU2612797A

Abstract

The essential feature of the securing device, principally a bone screw, for use in osteosynthesis is that the surface of its shaft, which engages mono or bicortically in the bone fragments to be joined, is additionally roughened. The securing devide is designed directly to stabilise the fragments or secure joining components, e.g. a bone plate. The surface roughness may be produced by addition or preferably by material displacement and/or removal, especially by blasting with solid particles and/or etching. The securing means of the invention makes it possible for the first time, when also used as a small fragment screw, to obtain osseo-integration, obviating the need to revise the securing means. Titanium or a titanium-based alloy is particularly suitable material.

Description

Spike or screw fastener for osteosynthesis

Field of application of the invention

The invention relates to a surgically inserted thorn or screw-shaped fastening means for use in osteosynthesis for the union of fractured, repositioned bone parts, the fastening means being set mono- or bicortical and solely effecting or additionally serving to hold together the bone parts Fixing connection part. This concerns the maxillo-craniofacial osteosynthesis and the osteosynthesis of other small bone fragments, e.g. in hand and foot surgery. Furthermore, the invention relates to the fixation of permanent implants in all areas with a low fixation depth in the bone, e.g. implantable hearing aids, epitheses and small joints. In principle, all those areas of application come into consideration where long screws are not required or cannot be used because of increased risk.

State of the art

Without wishing to restrict the scope of the invention, the mandrel-shaped or screw-shaped fastening means - the most typical form is the small fragment screw - is initially referred to as a bone screw in order to shorten the use of language. A distinction is made between osteosynthesis and pressure osteosynthesis with an initial compression of the fragments using a tensioning device. The conventional bone screws used for this are with a See fine or coarse thread or a combined thread; otherwise the threaded part has a smooth surface, which is a quality criterion. There are bone screws with self-tapping threads. The bone screws consist of corrosion-free, biocompatible metal, whereby titanium has proven particularly useful in the field of small fragment supply. In order to reduce the strain on the patient and the surgical effort, there is an increasing need to be able to leave the mini-osteosynthesis systems consisting of pure titanium in situ on account of the improved biocompatibility of the material (cf. Edwards, TJC, David, DJ: A comparative study of miniplates used in the treatment of mandibular fractures. Plast. Reconstr. Surg. 97: 1150 (1996).

In addition to tissue compatibility, screwed-in bone screws must have good anchoring, both with regard to tearing out and unscrewing, the risk of loosening due to unscrewing becoming smaller with increasing number of screws when fixing a bone plate. Adequate anchorage of the screw is generally achieved if the screw has a greater length and is bicortical, i.e. is screwed continuously through the bone, with which the screw thread engages on both sides of the bone in the firmer corticalis.

Depending on the structure of certain bones and their anatomical position in the body, it is often impossible to use longer screws that penetrate the bicortical bone. In many cases, this is also prohibited due to the existing risk of injury to the structures which are located below the point of exit of the penetrating screw tip. Derar- The maxillofacial area is problematic, for example, so that often only relatively short, monocortical screws can be used here, which in any case may only have small dimensions. The use of small screws and the preferred use of commercially pure titanium are described, for example, in: Schwenzer, N. and Grimm, G. (ed.): Special surgery. Georg Thieme Verlag, Stuttgart et al., Vol. 2, 1990, pp. 562ff .; as well as in: Spiessl, B .: Osteosynthesis of the lower jaw. Springer-Verlag, Berlin and others, 1988, pp. 95ff.

In view of these requirements and restrictions, there have been numerous constructive attempts to improve the anchoring of short and small-diameter bone screws - small fragment screws. The materials used for the bone screws are primarily titanium or titanium-based alloys because of their advantageous biocompatibility and osseointegration (cf. Müller, ME, Allgöwer, M., Schneider, R., Willenegger, H.: Manual of Internal Fixation. Springer- Verlag, Berlin et al., 1990, pp. 94f.). So far, efforts have been directed to prevent premature loosening or tearing of screws sitting in the bone by special designs of the geometry of the threaded part. A special contour of the thread teeth is disclosed in CH-A-670 754. A screw is known from WO-A-94 16636, in which a modified outer diameter and a pitch decreasing towards the screw head is provided over the length of the threaded part (similar to WO-A-91 09572). The flank angle of the thread plays a special role in CH-A-681 595. Finally, in CH-A-681 957 and EP-A-0 491 211 proposes special geometries of the threaded part.

Certain improvements in primary stability (purely mechanical anchoring immediately after the operation) could be achieved with the aforementioned thread modifications and geometries of the entire thread portions or regions. In particular, the long-term stability of the anchoring should not and should not be improved with this. In parallel to the optimization of the screw geometry, attempts were then made to improve the engagement of screws in the bone by means of a dowel-like tool (DE-A-34 45 738) or by means of a wedge which can be pressed into the screw head (WO-A-88 03781). The effects of these measures remained modest, but they entail additional risks and considerable effort.

In the case of small fragment screws, it was previously assumed that these screws, in addition to good primary stability - due to the biocompatibility improved by the choice of titanium - would also allow a longer dwell time in the body. In addition, a minimally invasive removal of the screws is desired. To date, these requirements were considered to be best met by a polished screw surface [(cf. Müller, ME et al., P. 180; as well as the recognized teaching of the Arbeitsgemeinschaft für Osteosynthese (AO)]). The teaching known per se from larger joint or tooth implants, according to which a rough surface can additionally promote biological anchoring in the sense of osseointegration (secondary stability), remained for small questions. Thus, ment screws have so far been completely ignored (cf. WO-A-94 09717, EP-B-0 388 576, US-A-3 605 123, US-A-5 098 434).

The experts have refrained from transferring the teaching of larger implants to small fragment screws, although the problem of wound healing disorders and infections as a result of unstable osteosynthesis screws has been known for a long time and complication rates of up to 30% occur (see lecture registration by Wächter, R., Gutwald, R. and Schilli, W.: "Anchoring mini-osteosynthesis screws with a rough surface - experimental investigations and first clinical results" for the "47th Congress of the German Society for Oral and Maxillofacial Surgery with the American Association of Oral and Maxillofacial Surgeons ", October 1996; Ellis, III, E., Walker, L.: Treatment of mandibular angle fractures using two non compression miniplates. J. Oral Maxillofac. Surg. 52: 1031 (1994). Rather, screws have been traditionally used with smoother or even polished surface.

Object of the invention

In view of the hitherto incomplete state of development for small fragment screws, the invention is based on the problem of using a mandrel-shaped or screw-shaped fastening means, in particular in the diameter dimension of approximately <2.5 mm and, for example, a length of 2 to 3 times the diameter to create increased secondary stability. The screws must be able to be subjected to a functional load immediately after the operation, that is, in the critical initial phase of osteosynthesis, sufficient primary stability and accompanying Fracture healing also ensure a solid secondary stability, since usually no additional immobilization of the fracture can be considered. This means that it is also important to increase the pull-out strength, which among other things determines the quality of the secondary stability. With the aim of being able to leave mini-osteosynthesis systems biologically anchored in situ, there is also the need for a special osseointegration, which was hitherto neither possible nor desired for smooth-surface anchoring surfaces. It is also important not to reduce the mechanical strength values of the fastener in any way. Finally, the fastening means must be able to be produced in a technologically rational and cost-efficient manner.

Essence of the invention

Surprisingly, it has now been found that fasteners of the generic type, the surface of which coming into contact with the bone at least partially has an additional specific surface roughness generated, have significantly better secondary stability.

The above-mentioned surface roughness can be characterized by the fact that the spike or screw-shaped fastening means has:

1. Macro profiling in the form of a thread or at least in principle regular depressions and / or elevations (eg ribs, grooves, webs, knobs, honeycombs); 2. an in principle irregular macro roughness, which was generated, for example, by corundum blasting; and 3. an in principle irregular micro-roughness, which was generated for example by chemical surface treatment.

Exemplary embodiment

The mandrel-shaped or screw-shaped fastening means provided for use in osteosynthesis, for the union of fractured bone parts and those repositioned in a surgical procedure, has a head part and a shaft attached to it. The shaft can be provided entirely or partially with threads or other textures; several differently designed, separate, textured sections are also possible. A thread could have a self-tapping part. The shaft is designed to be inserted monocortically or bicortically in the bones to be healed.

The three structures, namely the macro-profiling and the macro-roughness and the micro-roughness, can be provided in a variety of mixed forms as well as superimposed or alternating alternately, both axially and radially on the shaft. It would be conceivable that the thread flanks and the core surface of the shaft are differently macro and micro-rough.

The fastening means either alone holds the bone parts together, or additional connecting parts are fixed by means of this fastening part. Such connecting parts are in particular extramedullary bone plates, as are used, for example, for fractures on extremities or on the skull. The the Repositioning - possibly under constant contact pressure - stabilizing implants are generally removed from the body again in a further surgical step after the bone has healed. Taking advantage of the material-related, advantageous biocompatibility and the now improved secondary stability, the previously necessary revision of the fastener can be omitted.

Contrary to the previous practice of making the surfaces of bone screws as smooth as possible - in some cases even highly polished - according to the invention, a surface roughness and thus enlargement is specifically produced on the shaft of the mandrel-shaped or screw-shaped fastening means located in the bone, as a result of which biological anchoring (secondary stability) can only be achieved in the bone. The surface roughness preferably consists of a macro and a micro roughness. Part of the macro roughness can be generated by material application, e.g. using the T_itanplasma-3_ρray method (TPS). The roughness can also advantageously be produced by mechanical and / or chemical and / or thermal treatment in the form of material displacement and / or material removal that goes deep into the base material.

The macro roughness is e.g. can be produced by single- or multi-phase grinding treatment of the relevant shaft surface; irradiation with solid particles can also be considered. Corundum has proven itself as an abrasive. The micro roughness is e.g. generated by chemical treatment as an etching process. In this respect, the principle

Process technology according to US-A-5 456 723 reference which leads to the so-called SLA surface (≤andblasted large grit acid etched). Particularly good results are achieved with a roughness value R _t of approx.> 10 μm (maximum peak-to-valley height).

The anchoring can also be further increased if the surface roughness is introduced in a direction-oriented manner. In order to produce this alignment, the irradiation is carried out at an adequate angle.

The significant successes in improving the screw anchoring have meanwhile been proven by in-vitro tests and by measurement values obtained in in-vivo studies.

I. Biomechanical in-vitro investigations of the primary stability of mini-titanium osteosynthesis screws with a smooth and rough surface (after registration of the lecture and lecture by Wächter, R. for the "34th Annual Meeting of the German Society for Plastic and Reconstructive Surgery eV ", May 1996 / October 10-12, 1996); Test conditions:

Application of mini-osteosynthesis screws made of pure titanium with three different surfaces (smooth, TPS-coated, SLA-treated) in defined bone specimens made of pig bone; Measurement of the maximum torque and the ratio of screwing in to loosening torque;

Result;

With smooth and modified screws with a rough surface, practically the same primary stabilities can be achieved. Anchoring of mini-osteosynthesis screws with a rough surface - first clinical results (lecture registration by Wächter, R., Gutwald, R. and Schilli, W., op. Cit.)

30 patients with mandibular fractures; osteosynthetic restoration with smooth, TPS-coated or SLA-treated monocortical mini-osteosynthesis screws; Comparison group: 20 patients with smooth, bicortical osteosynthesis screws; Measurement of the maximum loosening torque within the scope of the metal removal after 6 months; Result: Significantly better anchoring (secondary stability of the mini-osteosynthesis screws with a rough, modified surface compared to the mini-osteosynthesis screws with a smooth surface with practically identical primary stability; surface-treated, monocortical mini-osteosynthesis screws thus achieve the anchorage quality of bicorticals, smooth screws; es the data were determined according to the table below:

An additional in-vivo examination of the bony reaction (secondary stability) of mini-osteosynthesis screws with a smooth and rough surface serves to additionally confirm the results under I. and II.

Thanks to the invention, there is now a mandrel-shaped or screw-shaped fastening means available which, even with very small dimensions and the intervention in only one cortex layer - without completely protruding through the bone - has a significantly increased secondary stability compared to conventional bone screws. When using titanium or proven titanium-based alloys, the osseointegration is improved by the targeted lending of a rough surface structure - quasi superimposed on the macro-profiling. In addition, the versatile fastening means can be produced efficiently and inexpensively. Aids of any kind, such as dowels or wedges, are not required. Finally, the fastening means according to the invention open up the possibility of being able to leave it in situ, so that mini-osteosynthesis systems combined with the fastening means can also remain in the body.

Claims

claims

1. Spike or screw-shaped fastener for use in osteosynthesis as an operationally introduced means for the union of fractured, repositioned bone parts, the fastener having a shaft with a profile which is intended to engage mono- or bicortical, and the fastener ¬ means alone the cohesion of the bone parts and / or serves to fix a connecting part, characterized in that the surface of the fastening means engaging with the bone has at least partially an additionally generated surface roughness.

2. Fastening device according to claim 1, characterized in that the surface roughness is introduced into the depth of the base material by material displacement and / or material removal and / or is generated by additive application.

3. Fastening means according to claim 1 or 2, characterized in that the surface roughness is generated mechanically and / or chemically and / or thermally.

4. Fastening device according to one of claims 1 to 3, characterized in that the surface roughness is directional.

5. Fastening device according to one of claims 1 to 4, characterized in that the entire surface roughness is composed of a macro roughness and a micro roughness.

6. Fastening device according to claim 1, characterized in that the profiling is a thread or a thread-like structure.

7. Fastening device according to one of claims 1 to 6, characterized in that the additionally generated surface roughness is produced by irradiation with solid particles and subsequent chemical treatment or the reverse process sequence.

8. Fastening device according to claim 7, characterized in that corundum is used as the blasting agent and the chemical treatment is carried out as an etching process.

9. Fastening means according to one of claims 1 to 8, characterized in that the fastening means consists of titanium or a titanium-based alloy.

10. Fastening means according to claim 1, characterized in that the roughness has a roughness value R _t of approximately> 10 μm (maximum peak-to-valley height).