Agitating – In vacuum chamber
Patent
1994-08-22
1996-02-27
Jenkins, Robert W.
Agitating
In vacuum chamber
366288, B01F 716
Patent
active
054943493
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a device for mixing orthopaedic bone cement or the like.
Orthopaedic bone cement is used throughout the world to secure hip, knee and other metallic prostheses in an appropriate anatomical position. The bone cement is produced by thoroughly mixing together two components, usually methylmethacrylate monomer liquid and polymethylmethacrylate powder. The mixing is usually carried out using a simple bowl and spatula. The surgeon then removes the required amount of cement and manipulates it by hand before inserting it into a preformed cavity or applying it to a resected bony surface where the prosthesis is to be positioned. Cement may either be applied by hand or may be put into a syringe and applied thereby. However, this simple mixing method has two major drawbacks.
Firstly, free methylmethacrylate fumes are emitted from the mixture. It is desirable to remove these fumes, or prevent them from escaping into the atmosphere, since they have an unpleasant odour and may be harmful to operating room personnel. The fumes are known to cause nausea and giddiness and are generally objectionable, particularly to the nurses who actually carry out the mixing. Recently there has also been concern that long term exposure to these fumes results in a more serious health risk. Current employment law relating to occupational health means medical staff must now be protected against the exposure to hazardous substances.
Secondly, a very high mixing efficiency is required to produce a homogenous cement material. During the mixing process air is naturally introduced into the mixture since air is inherently existent within the powder and also in and around the mixing vessel. Air bubbles are also produced by the "boiling off" of monomer which occurs during the mixing process. The introduction of air produces a weak cement and, since the joint must usually support a heavy load, it is important to reduce the amount of air in the mixture as much as possible in order to improve the mechanical strength of the cement material.
In order to eliminate as much air as possible from the mixture mixing is now preferably carried out under vacuum. This considerably reduces the amount of air in the mixture. Mixing in a conventional bowl and spatula system can produce a product with a porosity value of approximately 20 to 25%. In a vacuum mix, the porosity is often reduced to levels below 5%.
Several devices for mixing the cement in a vacuum are presently available. The most successful of these combine a mixing chamber with a syringe. For example, EP-A-0178658 discloses a device for mixing bone cement comprising a mixing container connected to a feed device. A vacuum source is connected to the feed device for mixing the substances under vacuum. This device has proved to be a very efficient mixing and transfer system and eliminates the need to transfer the mixed cement from the mixing bowl to a syringe, which can be messy and time consuming and may expose the mixture to more air entrapment.
However, these combined mixer/syringe devices are expensive. Further, they are restricted to applying the cement by a syringe, whereas the majority of surgeons prefer to "hand pack" the cement. Bowl mixing also tends to be preferred by nurses who are used to the convenience of mixing in this vessel; a bowl is easier to use and it is important that the nurses feel confident since timing is very crucial and the mixture must be `right first time`. Mixer/syringe devices may, therefore, have more potential for failure. Surgeons also tend to prefer bowl mixers because they can easily take samples of the cement from the bowl at any time to determine the progress of polymerisation as it is crucial that the mixture does not begin to set before it is applied.
Thus, the mixer/syringe devices have practical and economical limitations and a bowl mixer is a more attractive option.
Bowl mixing devices have been proposed which comprise a mixing bowl with a lid. A mixing paddle extends from the lid into the bowl and is rotated by means of a handle o
REFERENCES:
patent: 3640510 (1972-02-01), Lea
patent: 4079917 (1978-03-01), Popeil
patent: 4185072 (1980-01-01), Puderbaugh
patent: 4721390 (1988-01-01), Lidgren
patent: 4961647 (1990-10-01), Coutts
Patent Abstracts of Japan vol. 1, No. 118 (M-78)(3800) 8 Oct. 1977.
Patent Abstracts of Japan No. 53,088,259 (Tokyo Denki) 8 Mar. 1978.
International Search Report from European Patent Office, Mar. 9, 1993.
Jenkins Robert W.
Summit Medical Ltd.
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