METHOD AND APPARATUS FOR PREPARAING A SELF-CURING TWO COMPONENT POWDER/LIQUID CEMENT

Abstract: The method of preparing a self curing two component powder liquid cement is particularly useful for the preparation of bone cement in orthopedics. The methods consists in flooding (3) the interspaces (2) between the powder component particles (1), said interspaces (2) being evacuated, with a polymerizable liquid component (11) and mechanically homogenizing the resulting cement mass (15) by means of the internal mixing device (7). The method is preferably performed in a syringe type container (4) whereby the liquid component (11) in the ampule (13) is injected through the piston (5) into the powder component (1).

Description: 0 METHOD AND APPARATUS FOR PREPARING A SELF-CURING TWO COMPONENT POWDER/LIQUID CEMENT This invention relates to a method and apparatus for pre- paring a cement composed of a powder and a liquid component which polymerize when brought into contact with each other. The invention is especially useful in connection with a cement which is used for medical purposes, particularly as bone cement or denture base material. Background of the Invention Many modern day bone cements of the two component powder/liquid type are known which, when thoroughly mixed together, undergo polymerization thereby forming a hard and more or less durable cement mass. While the present invention can be used to prepare a variety of such cements, it is especially useful in the preparation of so-called bone cement used to anchor and support artificial joint components and other prostheses in natural bone. Accordingly, it will be described here specifically in that context. The currently preferred bone cement is polymethyl- methacrylate or so-called PMMA. PMMA is comprised of a powdered polymer and a liquid monomer. Upon mixing, these components polymerize within minutes so as to form a firm rigid bond between the prosthesis and the surrounding bone structure in which the prosthesis is placed. The present procedure for preparing PMMA bone cement is to thoroughly mix the powder and liquid component in or¬ der to start polymerization whereby the cement mass turns to a putty or dough consistency.- The partially cured cement is then applied to the bone structure to be treated, e.g. into the medullary canal of the femur which is receiving a femoral shaft of a hip joint prosthesis. All known methods for mixing bone cement have se¬ rious drawbacks, the most essential being: - poor mixing, which depends on the indivi- dual mixing technique ; - high exotherm, due to the considerable amount of liquid component necessary to produce an applica¬ ble cement mass by conventional mixing techniques ; - creation of porosities by inclusion and entrapment of air bubbles as well as by evaporation of excess monomer resulting in significant degradation of the mechanical properties of the cured cement. Summary of the Invention Accordingly, the present invention aims to provide an improved method and apparatus for preparing a two compo¬ nent cement which is independent from the individual mixing technique. Another object of the invention is to provide such a method which by its reduced amount of liquid component necessary to produce an applicable cement mass lowers con¬ siderably the maximum temperature reached during polymeriza- tion. Still another object of the invention is to provide such a method which eliminates porosities in the cured ce¬ ment by exclusion of air and by prevention of monomer eva¬ poration. Brief description of the drawing In the drawing : Fig. 1 is a perspective view of the powder component partic¬ les in a tightly packed state as used for the invention; Fig. 2 is a sectional view of the syringe type container for the powder component used in the apparatus according to the invention; Fig. 3 shows the liquid component injection through the pis¬ ton of the syringe type container in a sectional view; Fig. 4 shows the liquid component injection through the _ u. - plug of the syringe type container in a sectional view; Fig. 5 is a sectional view of the syringe type container showing the internal mixing process step; Fig. 6 is a sectional view of the syringe type container with an additional nozzle showing the extrusion process step ; Fig. 7 is a sectional view of the ampule used for the in¬ vention; Fig. 8 is a diagram showing the dynamics of the flooding process of the invention. Detailed ■description of the invention As illustrated in Figure 1 the powdered component of the cement 1 is packed as tightly as possible in the contai.ner 4 (Figure 2) and the container is evacuated. Thus void space 2 contains no, or very little air. When allowed to enter the container 4, the liquid component will flood (arrow 3) void space 2 between the powder particles. This is the ini¬ tial phase of the mixing process producing a powder-liquid mixture without any, or with very little air inclusion. Figure 2 shows a pre-packaged powder component 1 in the con¬ tainer 4. Container 4 is of a syringe type with piston 5. The extrusion end 18 of the container is closed with a plug 6. An internal mixing device 7 protrudes through the plug 6 with its shaft 8. The container is evacuated in the packaging pro- cess. For increased shelf life the outer package 9 is also evacuated and sealed. It may be made of aluminum foil for example. It may also serve as the sterile outer package. Figure 3 shows the liquid component injection. This is done prior to application upon removal of the container 4 from the package 9. A needle 10 is inserted through the elasto¬ mer part of piston 5. Liquid component of the cement, which predominantly consists of methylmethacrylate 11 enters the container 4 and floods as indicated by arrows 3 the pow¬ der component particles 1. Flooding front 12 advances as desc¬ ribed in more detail later (reference is made to Figure 8 des¬ cription). Liquid component 11 may be ejected as indicated by arrow 14 from its container 13 either by a piston if 13 is a syringe, or simply sucked out by the vacuum in the con¬ tainer 4 if the container 13 is an ampule as shown in Figure 7. Figure 4 shows the liquid i jection through the plug 6 with the aid of the needle 10. Figure 5 shows additional homogenization of the mixture 15 by means of the internal mixing device 7. Since flooding alone may not produce a mixture uniform enough for the cri- tical medical applications such as bone cement, means for additional mechanical mixing is provided in that an axially collapsable mixing device 7 is enclosed and sealed within the container 4. It may be made of a suitable metal, or plastic. Its shaft 8 protrudes through the plug 6 which ensures air tight seal of the container 4. The shaft 8 may be gripped in the chuck of say a power drill and turned as indicated by arrow 16. It may also be inserted in the specially pro- vided mixer 17. The end of the shaft 8 may be adapted for simple coupling to the mixer 17. Time allowed for homoge- nization depends on the rate of the polymerization and is typically a few minutes. Since handling of the cement is minimized and no settling time is needed for air bubbles to come out of the mixture as it is the case in the conventio¬ nal mixing, a major advantage of this procedure arises - mixing time may be increased many-fold. There is no mono¬ mer evaporation since the container 4 remains sealed through¬ out the injection and homogenization phases. Figure 6 shows extrusion 21 of the cement following homo¬ genization. For that a ' piston rod 20 is pushed in following removal of the plug 6. Extrusion may be done through the con¬ tainer end 18 or through an additional nozzle 19 attached to the container end 18. Figure 7 shows a preferred embodiment of the liquid component container 13. It is in the form of a glass ampule. Narrowed end 22 is shaped so as to allow needle attachment onto the conus 24. The cap 25 is broken-off at the neck 26. The needle 10 is attached to the conus 24. The ampule is then turned upside-down and the needle 10 is inserted into the container 4 through either the piston 5 or the plug 6. The cap 28 on the other narrowed end 23 of the ampule 13 is then broken- off at the neck 27 allowing the liquid 11 to flood the pow¬ der component particles 1 in the container 4 as shown in Figures 3 and 4. Figure 8 shows the dynamics of the flooding process, where¬ by pressure drop in the injection needle 10 is neglected. Flooding front 12 position x in the powder column 1 is pro- portional to the square root ot time. The constant of pro¬ portionality a depends on the powder column permeability, the liquid viscosity and the column cross-section. The front 12 will arrive to the end of the column xc in finite time tc and with finite speed. "

Claims: C l a i m s -L The method of preparing a self curing two component pow¬ der liquid cement characterized in that the powder com¬ ponent containing a polymerization catalyst is evacuated at least partially and the interspaces (2) between the powder particles (1) are flooded with the liquid compo¬ nent (11) containing a polymerizable monomer, comonomer, prepolymer or mixtures thereof, thereby forming a poly¬ merizable cement mass (15). 2. The method according to claim 1, characterized in that the polymerizable cement mass (15) is further homogenized by mechanical means (7), which preferably are collapsable. 3. The method according to claim 1 or 2 , characterized in that the weight ratio of powder/liquid is between 3,6 and 2,4 , preferably between 3,6 and 3,0. 4. The method according to one of the claims 1 to 3, charac¬ terized in that it is carried out in a container (4), preferably in form of a syringe, containing the evacua¬ ted powder (1) by introducing the liquid component (11) through the plug (6) or the piston (5) of the container (4). 5. The method according to one of the claims 1 to 4 charac¬ terized in that the powder component consists of partic¬ les (1) of approximately uniform size and preferably of approximately spherical form. 6. The method according to one of the claims 1 to 5, charac¬ terized in that the majority of the powder particles (1) have a diameter in the range of 10 to 100 μ . 7. The method according to one of the claims 1 to 6, charac¬ terized in that the evacuated interspaces (2) between the powder particles (1) comprise 25% to 35%, preferably bet¬ ween 26% to 30% of the total volume of the powder. 8. The method according to one of the claims 1 to 7, charac¬ terized in that the powder particles (1) are coated with a polymerization catalyst, preferably with a peroxide. 9. The method according to one of the claims 1 to 8, charac- terized in that additional components, preferably radio- opaquers and/or antibiotics are used in a geometrically similar form to the powder particles (1). 10. Apparatus for carrying out the method according to one of the claims 1 to 9, characterized in that said appa¬ ratus comprises: A. means (4) for confining the powder component (1) and maintaining it in an evacuated condition, B. means (10;5,6) for introducing the liquid component. (11), and C. means (18,19;5,20) for expelling the two component cement mass (15) from said apparatus. 11. Apparatus defined in claim 10, characterized in that the means (4) for confining the evacuated powder com¬ ponent (1) are inflexible. 12. Apparatus according to one of the claims 10 or 11, characterized in that said apparatus comprises a syrin¬ ge (4) with piston (5) and plug (6) and an internal mixing device (7), which preferably is collapsable axi- ally with regard to the syringe longitudinal axis. 13. Means for performing the method according to one of the claims 1 to 9, characterized in that said means compri¬ ses an ampule (13) containing the liquid component (11) with two necks (26,27) at opposite ends of the ampule (13) for breaking-off and opening of said ampule (13), and preferably with a narrowed end (22) allowing for attachment of a needle (10).

Inventors: TEPIC SLOBODAN, TEPIC, SLOBODAN
Applicants: EXPERIMENTELLE CHIRURGIE LAB F, LABOR FUER EXPERIMENTELLE CHIRURGIE
Application year: 1985
Main classification: A61F2
IPC codes: A61B17, A61F2, A61L24, B01F3, B01F7, B01F13, B01F15