Agitating – In vacuum chamber
Reexamination Certificate
2000-02-16
2001-04-03
Soohoo, Tony G. (Department: 1723)
Agitating
In vacuum chamber
C366S194000, C366S197000, C366S241000, C366S219000
Reexamination Certificate
active
06210031
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to stirring and extruding of orthopedic bone cement to an application site.
DESCRIPTION OF THE PRIOR ART
Cement used as a casting and grouting material to implant prosthetic devices into live bone is made from a very fine cement powder, typically polymethyl methacrylate, mixed with a monomer liquid, typically methyl methacrylate, to form a flowable bone cement mixture. Physical mixing of the dry cement powder and liquid is required in order to make a flowable cement. It is not sufficient to merely bring the liquid into contact with the cement powder because the liquid will not flow into the powder uniformly. During mixing the monomer liquid should be distributed equally throughout the mixture so that the mixture is uniform and possesses a uniform viscosity, consistent with the manufacturer's specifications.
Bone and dental cements are stirred or mixed using pre-packaged amounts or doses of dry bone cement powder and monomer liquid prepared by the manufacturer of the cement. The amounts of powder and liquid are measured to provide a cement mixture having desired properties when the powder and liquid are uniformly mixed together. Failure to mix the liquid and powder together uniformly means that part of the mixture contains an excess of monomer liquid and is runny and part of the mixture contains a deficiency of monomer liquid and is more viscous than desired or, in some cases, retains unwetted dry powder.
Bone cement is conventionally mixed in a closed bone cement mixer. The mixer has a mixing chamber, a cover to close the chamber, and a mixing element or stirrer movable within the mixing chamber to mix the powder bone cement and monomer liquid together physically and form a flowable bone cement. Cement is mixed by placing the required amounts of bone cement powder and monomer liquid in the mixing chamber, which may be the interior of a syringe cartridge seated in the mixer. A cover is placed on the top of the mixer to close the mixing chamber. The cover supports a mixing element or stirrer which is inserted into the cement powder when the cover is placed on the mixing chamber. Vacuum may be applied to the interior of the chamber to withdraw gas. The mixing element is manually rotated in the chamber for a predetermined amount of time to mix the powder and liquid and form a flowable bone cement. Insertion of the stirrer into the bone cement powder before monomer liquid is poured onto the powder compacts the powder and makes uniform mixing difficult.
Prior to mixing, bone cement powder is typically poured into the empty mixing chamber and monomer liquid is poured into the chamber on top of the powder. Alternatively, the monomer liquid may be poured into the mixing chamber before bone cement powder is poured into the chamber. When several doses of bone cement are mixed, powder and monomer liquid may be poured into the mixer alternately. When the dry bone cement powder is poured into the mixing chamber, it forms a body having a height. The body is comprised of loosely compacted particles within the chamber. Before physical mixing of the bone cement the monomer liquid is not uniformly distributed along the height of the powder body. When the liquid is poured on top of the powder, the liquid can form a puddle which seals the top of the powder and prevents underlying air between particles of the bone cement powder from flowing out of the powder. Trapped air can form inclusions in mixed bone cement.
The problems of uneven monomer liquid distribution just described can be quite significant where the height of the body of bone cement powder is greater than its diameter. This is typically the case when the cement is mixed in a tall, narrow bone cement cartridge of the type used to extrude mixed cement to an application site. Because of this problem, some conventional mixers use wide bowl-type mixing chambers rather than narrow cartridges. Cement mixed in a bowl-type mixing chamber must be transferred from the bowl-type chamber to a cartridge for extrusion to a prepared application site. The transfer from the bowl-type mixing chamber to a cartridge takes time, involves loss of cement and, most seriously, includes a likelihood of trapping air inclusions in the body of mixed cement. Air inclusions weaken bone cement.
After the bone cement powder and liquid are poured into the mixing chamber, the ingredients are physically mixed together by moving the stirrer in the mixing chamber. However, when mixing in a bone cement cartridge, it can be difficult to produce uniform distribution of monomer liquid along the height of the body. Where there is excess monomer liquid, the mixed bone cement will be less viscous than the manufacturer's standard. Where there is an excess of powder, the resulting mixture will be more viscous than the standard and, indeed, may contain regions where there is no monomer liquid.
It is quite important that monomer liquid be evenly distributed throughout the bone cement mixture. Bone cement with an excess of monomer liquid sets up slowly requiring increased operating room time and the risk of prosthesis displacement during protracted set-up. Such cement also possesses reduced strength. Further, during the increased set-up period there is a risk that blood or other bodily fluid will displace runny cement from adjacent the formed bone surface at an implant site and thereby weaken fixation between the hardened cement and bone.
Bone cement mixed with a deficiency of monomer liquid has a high viscosity and may be difficult to flow properly to the application site. This cement does not flow readily into the irregularities of a prepared bone surface to form a reliable joint. Further, bone cement with a deficiency of monomer liquid sets up relatively rapidly, reducing the already short amount of time available to the surgeon to perform an implantation procedure.
The implantation of a bone prosthesis using a bone cement mixture in which part of the bone cement has an excess of monomer liquid and is relatively runny and part of the bone cement has a deficiency of monomer liquid and is relatively viscous, causes additional problems in the operating room because the surgeon is not able to predict with accuracy the proper length of time for the cement to set and form a joint securing an implanted prosthetic element to a prepared live bone application site. Setting must occur before any stress may be placed on the prosthetic element. Premature stressing of the implanted element will displace the carefully placed element and will impair fixation. For instance, the initial seating of a prosthetic hip ball in the adjacent hip socket stresses the implanted stem on the ball and must be delayed until the bone cement securing the stem in the proximal end of the femur has set. When bone cement is not mixed uniformly, the surgeon must delay seating the ball in the socket for a period of time greater than the specified time for setting the cement. A surgeon implanting a prosthetic element using the bone cement cannot rely on the set-up time specification provided by the bone cement manufacturer. The specification assumes uniform mixing of the cement.
The problems described above are well recognized. A conventional method for achieving greater uniformity of monomer liquid distribution when using a syringe cylinder as a mixing chamber requires pouring cement powder and monomer liquid alternately, in small amounts, into the chamber until the total amounts to be mixed have been poured into the chamber. The powder and liquid are then mixed. While this method can achieve improved uniformity of monomer liquid distribution, it has serious disadvantages. First, it consumes valuable extra time, and the duration of the time consumed results in a mixture which is not homogeneous in terms of the starting time of its mixing and, hence, its setting time. Second, the layering of monomer liquid which results from this method can isolate regions of loosely packed dry powder containing air which will not be effectively removed by application of vacuum during
Hooker, P.C. Thomas
Soohoo Tony G.
LandOfFree
Bone cement device and package does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Bone cement device and package, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Bone cement device and package will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2515826