Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Patent
1994-10-21
1998-10-20
Weiss, John G.
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
623901, 623 22, A61F 228
Patent
active
058240837
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a femoral prosthesis component which is to be anchored without using cement and a method of producing it.
In the fields of surgery and orthopaedics with respect to the locomotor system, the artificial joint replacement has become a standard surgical intervention and is today one of the most frequently carried out operations of all. The long-term results of replaced joints have been quite variable and the life time of, for example, replaced hip joints ranges from few weeks up to 27 years (Draenert and Draenert 1992).
It has been found out by scientific research that different factors are responsible for the loosening of an endoprosthesis component, such as infections, insufficient surgical skills, choice of the wrong implant and excessive strain. Nevertheless, so far many cases of loosening could not be explained in a satisfying manner. It has only been detected that certain combinations of factors frequently lead to a loosening, such as a massive cement-free implant used in combination with the bone of a rheumatic. Such types of prostheses which were to interlock within the bone and were implanted together with bone cement showed (Draenert 1988) that bone cement as a filling material between metal and bone cannot fulfil an anchoring function but is pulverised.
The problem involved in the anchoring of prosthesis components could in the end be attributed to the phenomenon of bone deformability. This explained why an easily deformable bone of a rheumatic is deformed by a metal prosthesis anchored without cement such that rapid loosening ensued. On the other hand, it could be shown that a fragile or soft spongiosa as well as a normal spongiosa (cancellous bones) can be stiffened by means of polymethylmethacrylate (PMMA) bone cement and thus gets extremely rigid (Draenert and Draenert 1992). A thus stiffened bone structure could be found with all those implants which had successfully been used for 10 to 20 years and could be histologically examined. On the other hand, quite compact femora could be provided with prosthesis components without using bone cement as an anchoring means, and these prosthesis components have successfully been implanted for about 10 years (Draenert and Draenert 1992). However, in these cases, the results could not often be reproduced.
It is an object of the present invention to provide a femoral prosthesis component which can be anchored without using cement and with which, after its implantation, good long-term results can be expected.
SUMMARY OF THE INVENTION
This object is achieved by the present invention.
In connection with the invention, the problem has been investigated how the strength of a bone influences the life time of an implant. By means of histological studies it could clearly be proven that soft, deformable bones only show a stable anchoring if the implants used have a low mass.
The present invention is based on the following findings regarding the anchoring of prostheses: Every bone exhibits an individual shape and an individual strength; therefore, both factors must be taken into account when selecting the prosthesis. A solid, compact bone is a good indication for metal-bone anchoring without using bone cement. Two factors are above all important in this context: 1. to obtain the best possible primary stability of the anchorage and 2. to provide the largest possible surface for the transmission of forces between the prosthesis and the bone. It has, however, to be considered that the various compartments of the bone, such as epiphysis, metaphysis and diaphysis, have completely different shapes and strengths. There were early trials to adapt the prosthesis shaft to the medullary cavity, cf. EP-A-0 038 908; however, it was rapidly found out that one single implant design could not be adjusted to the variety of different bone shapes (Noble et al., 1988); moreover, there was no possibility of determining the strengths of a bone and considering them when designing a prosthesis.
In connection with the present in
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Snow Bruce E.
Weiss John G.
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