Agitating – In vacuum chamber
Reexamination Certificate
1998-06-10
2003-07-15
Cooley, Charles E. (Department: 1723)
Agitating
In vacuum chamber
C366S243000
Reexamination Certificate
active
06592247
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device for mixing and delivering orthopaedic bone cement or the like.
2. Description of the Prior Art
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 dictates that 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. Some of these are in the form of hand-held mixing bowls. The substances to be mixed are placed in the bowl to which a vacuum is applied. The substances are mixed by means of a rotating paddle extending into the bowl which is rotated manually by means of a handle extending through the lid of the bowl. In some applications, the use of such a mixing bowl, an example of which is disclosed in WO 93/10892, is favoured. Many 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 such a vessel; a bowl is easy to use and it is important that the nurses feel confident since timing is very crucial and the mixture must be ‘right first time’. Many 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.
However, in some applications it is preferable or necessary to apply the mixed cement to the bone by means of a syringe. Indeed some surgeons, particularly in Europe, prefer syringe-type application to “hand packing”. If the cement is mixed in a bowl, it must then be transferred to a dispensing syringe which can be messy and time consuming and may expose the mixture to more air entrapment. This problem has been overcome by combining 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.
However, a device such as disclosed in EP-A-0178658 may be inadequate in that ‘dead spots’, i.e. areas where the components are not sufficiently mixed, occur, particularly at the outer edges of the mixing chamber.
Further, the mixing paddle of EP-A-0178658 is rotated by a rotary electric drive motor. This makes the device costly and space consuming and requires specialist and time-consuming installation. The device is not easily portable and its use is, therefore, not particularly flexible.
U.S. Pat. No. 4,758,096 also discloses a bone cement mixer in which the cement is mixed in the dispensing vessel. In this device, the mixing is effected manually by means of a “masher” plate-type agitator. The masher plate is attached to a shaft attached to a handle. The agitator is movable in the chamber both axially and rotatably to permit mixing of the cement by the user moving the handle vertically and rotatably. However, such a mixing operation is difficult and inefficient and does not result in thorough mixing of the cement, leaving areas of unmixed powder.
SUMMARY OF THE INVENTION
The present invention aims to overcome the above-mentioned problems.
According to one aspect of the invention, there is provided a bone cement mixing apparatus comprising a mixing chamber, a mixing element rotatably mounted in said mixing chamber, and drive means for causing rotation of said mixing element, wherein said drive means comprises a handle axially movable relative to said mixing chamber, and a gear mechanism between said handle and said mixing element such that said gear mechanism translates axial movement of said handle into rotation of said mixing element.
According to another aspect of the invention, there is provided a method of mixing bone cement in a mixing chamber, by a mixing element rotatably mounted in said chamber containing the material to be mixed; said method comprising axially moving a drive handle, relative to said chamber, wherein said handle is connected to said mixing element via a gear mechanism, such that axial movement of said handle is translated via said gear mechanism into rotation of said mixing element.
The preferred type of gear mechanism for use in the practice of the present invention is a “barley twist” mechanism wherein a downwards stroke of the handle moves a threaded rod axially through a threaded gear housing in the form of a drive bush coupled to the mixing element imposing a rotational force in a first direction on the mixing element. On the upward stroke of the handle, the mixing element is caused to rotate in the opposite direction. Rotating the element alternately in different directions during mixing provides very thorough mixing. The “barley twist” mechanism allows this with a simple push-pull action of the handle rather than the user having to rotate the handle backwards and forwards alternately which can be difficult and tiring, particularly when mixing cement of high viscosity.
Although the present invention may be applied to any bone cement mixing chamber, it is preferably used in combination with a dispensing syringe. Thus, the cylinder of the syringe forms the body of the mixing chamber and a plunger is slidably mounted at one end of the cylinder for causing ejection of the mixed cement.
As discussed above, the quality of the cement is greatly improved if it is mixed under vacuum and, therefore, in a preferred embodiment, the invention comprises means for creating a vacuum within the chamber.
For thorough mixing, the mixing element is preferably in the form of a paddle arrangement mounted on a shaft which in the preferred embodiment includes an axial bore through which the threaded rod of the barley twist gear mecha
Brown Tim
Foster David
Alix Yale & Ristas, LLP
Cooley Charles E.
Summit Medical Ltd.
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