Endoprosthesis, in particular an artificial hip joint

Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone

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Details

606 77, A61F 236

Patent

active

060713127

DESCRIPTION:

BRIEF SUMMARY
The invention is directed to a hip joint prosthesis comprising a stem and a joint part.
Joint endoprostheses replace joints that have been damaged through degenerative, inflammatory, traumatic or tumorous changes and thus make it possible for the patient to lead a substantially normal life.
As is well known, the ideal joint implant should enable physiologic proximal-metaphyseal load transfer in the weight-bearing bone portion. For adequate fixation, especially for the primary fixation of cementless implants, conventional implants, especially in the hip area, have a prosthesis stem which extends far into the marrow space of the weight-bearing bone.
It is further known that the required physiologic introduction of load is changed as a result of the conventional long-stem prosthesis such that distal diaphyseal introduction of force is benefitted instead of proximal-metaphyseal introduction of force (Menge, M. 1994, Die metaphysare Prothesenverankerung--ein neues Konzept fur die Femurprothese [Metaphyseal Prosthesis Fixation--a new concept for femur prosthesis]). As a result of this redirection of force into the distal region of the weight-bearing bone (femur), there is a reduction of mechanical stress on the bone in the proximal region, referred to as stress-shielding. This results in the frequently observed resorption of bone in the proximal region, especially of the calcar (calcar atrophy). In addition, an increased introduction of force is caused distally in the stem tip region with resulting cortical hypertrophy (Cohen, C., Rushton, N., 1995, Bone Remodelling in the Proximal Femur After Charnley THA), (Kuiper, J. H., 1993, Three-Dimensional Optimization of Hip Prosthetic Design). These mechanisms lead to loosening of the prosthesis in the bone bearing portion and accordingly to failure of the implant.
It is further known that the loss of bone matter in cases of a replacement operation makes the fixation of the replacement prosthesis considerably more difficult and often requires the use of expensive special implants.
It is further known that known copolymers, for example, those produced from D,L-lactide or L-lactide or from glycolide or from a mixture of these materials, can be used as bioresorbable materials. Further, self-reinforced materials have been developed which are far superior to previous materials with respect to their mechanical properties. These materials are already used in the manufacture of bone screws, bone pins and bone plates, see, e.g., Tormala, P. 1993, Ultra-high Strength, Self-Reinforced Absorbable Polymeric Composites for Applications in Different Disciplines of Surgery, Clin. Mat. pp. 35-40.
It is further known that the resorption of large amounts of bioresorbable material in the body can lead to undesirable foreign body reactions.
The object of the invention is to minimize the frequency of prosthesis replacement operations arising from loosening of the prosthesis; these operations are complicated in technical respects relating to organs, expensive, and burdensome to the patient.
This object is fundamentally met by the upper stem part and the lower stem art being connected by means of a connection-element manufactured from bioresorbable material.
A particularly advantageous embodiment form has a second connection element made from non-resorbable material, for example, high quality steel, and arranged in the region of the first connection element, made from bioresorbable material.
The advantages achieved by the invention consist in that this type of prosthesis, due to its long stem, ensures sufficient is primary stability appropriate for current prosthetic variants in the period immediately following the operation. Through the progressive decomposition of the bioresorbable intermediate piece in the weeks following the operation, a continuous or steady mechanical decoupling of the distal stem part from the proximal stem part is achieved, resulting in a reduced mechanical loading of the distal stem part. Bony ingrowth of the proximal stem part occurs simultaneously. This process then

REFERENCES:
patent: 3833002 (1974-09-01), Palma
patent: 4973333 (1990-11-01), Treharne
patent: 5201738 (1993-04-01), Scott et al.
patent: 5201771 (1993-04-01), Belykh et al.
patent: 5735901 (1998-04-01), Maumy et al.

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