Biodegradable osteosynthesis implant

Details Biodegradable osteosynthesis implant Biodegradable osteosynthesis implant

1999-10-15

2001-04-10

Philogene, Pedro (Department: 3732)

Surgery

Instruments

Orthopedic instrumentation

C606S076000, C623S016110, C623S011110

Reexamination Certificate

active

06214008

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a biodegradable osteosynthesis implant constructed of a polymeric biodegradable base material with active ingredients that aid regeneration of bone tissue in a fracture area.
2. Description of Prior Art
Metallic implants have been used in medicine for more than 200 years for fixation of broken bones. Success was achieved in the last decades with the selection of suitable compatible metal alloys to optimize the mechanical load-bearing capacity, and above all the tissue tolerance of these implants so that corrosion and rejection reactions rarely occur. Complications because of the metallic implants can arise in the course of X-ray and tomographic diagnostic processes. One main disadvantage associated with conventional implants is that metallic implants cannot remain in the human body over an unlimited time. It is therefore necessary to perform a second surgical procedure for removing the implant after the fracture has healed. This not only involves another stress for the patient, but also results in considerable costs for the second surgical procedure and the necessary second stay of a patient in a hospital. In addition, direct and indirect costs result from loss of work and after-treatment during the second healing of the wound. Therefore efforts have been made over a considerable period of time to replace the metallic materials with biodegradable polymers so that removal of the implant after healing of the fracture is no longer necessary.
The use of a biodegradable plate in connection with osteosynthesis was first described by D. E. Cutright and E. E. Hunsuck in the Journal of Oral Surgery, Volume 33, pages 28 to 34 (1972).
In developing novel biodegradable implants, it has been a goal to replace metallic implants. Thus the implant had to fix the bone fragments in place and support the mechanical forces acting on the bone.
However, implants made of absorbable polyesters have inferior mechanical load-bearing capabilities when compared to implants made of metal. Although it was possible using suitable production methods, for example injection molding methods, and using optimized material mixtures to produce implants, for example fiber-reinforced implants, which could resist great tensile stresses, the flexible strength of such workpieces is relatively low and far below that of metal implants. A range of applications of biodegradable osteosynthesis materials is limited to lightly stressed, rapidly healing fractures. Such fractures can occur, for example, on the skull, such as a roof of the skull, a cheek bone or an upper jaw.
SUMMARY OF THE INVENTION
As shown in
FIGS. 1
a
and
1
b
, an increase in the stability of the bone and a decrease in the stability of the implant are two dynamically occurring processes, which are expressed in two oppositely extending sigmoidal curves. Ideally, these curves are superimposed on each other in such a way that the stability of the implant decreases to the same extent as the stability of the bone increases again. The hypothetical course of the healing of the fracture with only the employment of a biodegradable osteosynthesis implant is represented in
FIG. 1
a
, such as occurs for example, in treatment of the above described fractures in the skull area. At the time equal to zero, the implant must be able to withstand the total mechanical stress, and the loss of mechanical stability, less a defined mechanical stability reserve, may advance no faster than it can be compensated by the increase in the stability of the healing bone tissue.
It can also be seen from
FIG. 1
a
that the loss of stability of the implant cannot be equated with its decomposition or loss of mass. The implant loses mechanical stability much faster by various physical and chemical processes, for example, by swelling, than it loses mass by abrasion and degradation.
Generally an implant not only must fix the bone fragments in place during the healing of the fracture, but must also support the mechanical forces acting on the bone. The implant can be relieved from supporting the mechanical forces acting on the bone to a large extent by means of an external support such as plaster, splints or an external fixation device, and merely needs to assure the fixation of the fragments. This quite decisively increases the application range of biodegradable polymer implants.
As shown in
FIG. 1
b
, the biodegradable and absorbable implant only needs to fix the bone fragments in a defined position with respect to each other until the bone material newly formed between them can take up this function again, and the additional fixation becomes unnecessary. But the mechanical stresses acting on the broken bone are supported by the external stabilizing device until the healing bone has regained a percentage of sufficient size of its mechanical properties, so that additional stabilization is no longer required. This does not mean that the bone must already have regained its full mechanical load-bearing capacity when the external stabilization means is removed. An affected area is commonly immobilized for an extended period of time, and the reduced muscle activity leads to loss of muscle and reduced mobility. To regain these, it is necessary to perform time-intensive and cost-intensive therapies after the removal of the external stabilizing means.
Polylactide (PLA), polyglycolide (PGA), poly((&egr;-caprolcatone) (PCL), poly (&bgr;-hydroxybutyrate) (PHB) or poly(p-dioxanone) (PDS) and their copolymers are decomposed in the body into the respective degradation products and can either flow into the body metabolism, or can be precipitated by the body through the urine or through breathing.
Polylactide (PLA) and polyglycolide (PGA) and their copolymers are decomposed by hydrolysis. Since the amorphous areas of partially crystalline PLA degrade more rapidly than the crystalline areas, irritation and inflammation because of the crystallites can occur after disintegration of the implant in the surrounding tissue, as described by E. Wintermantel and S-W Ha, “Biokompatible Werkstoffe und Bauweisen,” in Biocompatible Materials and Structural Forms, Springer publications, Berlin 1996. A second cause of complications can be from the acid hydrolysis products of the implant. The acid degradation products are removed from the interior of the implant essentially more slowly than from its surface, which leads to the amassing of acid decomposition products and an increasing acceleration of the decomposition by auto-catalytically acting carboxyl groups. If at a later time during the decomposition the remaining outer wall of the implant breaks, a sudden release of the acid products and therefore a sudden pH drop in the surrounding tissue can occur, which can also lead to inflammatory reactions.
The speed of bone healing can be accelerated by various growth factors (GFs). Soluble, low-molecular proteins, such as the insulin-like growth factors (IGFs) have been known for some time for their local action on the growth of cartilage and bones, as set forth by Canalis, E. and L. G. Raisz, in Endocr. Rev. 4; pages 62 to 77 (1983). The same authors have proven a positive effect of IGF on the bone-DNA synthesis in periosteal and non-periosteal bones.
One advantage of using IFG in the treatment of wounds and fractures is that up to now IFG has not shown any known relationship with oncogenes.
Two insulin-like growth factors are commercially offered. IGF-1 (also known as Somatomedin C) is a basic polypeptide having 70 amino acids, and having a molecular weight of 7649 D. Inter alia, IGF-1 stimulates the insertion of proteoglycan in the cartilage by means of chondrocytes, as set forth by Froger-Graillard et al., in Endocrinology 124; pages 2365 to 2372 (1989), and in addition the synthesis of DNS, RNS and proteins. The slightly acid polypeptide IGF-2, like IGF-1, has four domains and has a molecular weight of 7471 D. IGF-2 has 64 amino acids. IGFs are mainly dependent on growth hormones (Somatotropin; GH). IGF-1 is preponderantly active in adults, whi

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