DUMMY DEVICE FOR MOULDING BONE CAVITY FORM

Abstract: There is disclosed a dummy device for moulding a bone cavity form, and a method of manufacture of a prosthetic device to be implanted in any given bone cavity and derived from the moulded dummy form. The moulded form corresponds at least partly with the shape of a bone cavity (12, 16, 14), which is to receive a prosthetic joint, with the cavity having an enlarged proximal end (12) and a narrower distal end (14), and the moulded form enabling the required profile of the prosthetic joint to be derived therefrom in order to be implanted in the bone cavity. The method uses a model (10) having a preformed proximal dummy portion (11) to fit in the proximal end (12) of the bone cavity, a distal end portion (13) to be received by the distal end (14) of the cavity, and a bag (15) to enclose at least the dummy head portion (11) of the model. After introduction of the model into the bone cavity, the space between the outer surface of the dummy portion (11) and the inner surface of the bag (15) is filled with a moulding composition which expends the bag into intimate contact with the wall surface of the cavity in the proximal end portion (12), and this composition is then allowed to set, after which the moulded form is removed from the cavity to form a moulded model from which the required profile of the prosthetic joint can be derived by following the external shape of the moulded model (22) with a profile detector (21) and causing a machine to shape the outer profile of a prosthetic joint (24) with a corresponding shape so that the prosthetic joint can be implanted into the bone cavity.

Description: DUMMY DEVICE FOR MOULDING BONE CAVITY FORM This invention relates to a dummy device for moulding a bone cavity form, and to a method of manufacture of a prosthetic device to be implanted in any given bone cavity. The invention has been developed primarily in connection with the implantation of replacement hip prostheses, though it should be understood that the principles of the invention may be applied generally to prosthetic joint implantation. It is known from EP0129531 to provide a method of manufacture of a prosthetic joint, in which a bone cavity is formed to receive the prosthetic joint, and then a flexible bag is introduced into the cavity and is filled with a mouldable composition which expands the bag to come into intimate contact with the wall of the cavity. The moulded form is then removed from the cavity, and the bag is stripped-off (since the moulding composition is chosen to be non-adherent to the material of the bag). The moulded form can then be installed in a profile detector apparatus which follows the profile of the moulded form and converts this into signals which control the operation of a forming machine in which a suitable standard size of prosthesis is mounted. The forming machine machines down the outer profile of the standard prosthesis to correspond with that of the moulded form, with suitable correction being built-in to take account of the removal of the outer skin (the bag) of the moulded form. The prosthesis can then be fitted snugly into the bone cavity, with all parts of the prosthesis engaging firmly with corresponding parts of the wall surface of the bone cavity. This minimises the risk of the prosthesis rocking in the cavity in service, so that tissue ingrowth can take place over a period of time, and also to reduce the generation of localised excessive pressure loadings which otherwise might create discomfort or actual pain to the patient . This known technique is a significant improvement over existing "cementless" implantation techniques, but there are at least two areas in which the technique could be modified to provide still further improvement over existing techniques . In particular, the necessity to use a moulding composition which is non-adherent to the bag involves an additional process step in the subsequent removal or unpeeling of the bag, and the need to compensate for this reduction in size of the profile in the setting-up of the operating parameters of the profile detector system and the machining of these standard prostheses. Further, the location of the distal end of the prosthesis in the bone cavity is not always a sufficiently tight fit. which can give rise to an unacceptable amount of rocking and / or flexing of the prosthesis in service. This arises because the distal end of the moulding bag is closed by a rubber plug secured to one end of a mounting rod, and this may be undersized relative to the diameter of the bone cavity formed at the distal end. and therefore the prosthesis manufactured from the moulded form (including the rubber plug) will have a similarly undersized distal end . With an insecure location of the distal end of the prosthesis, the latter is liable to rock in service, thereby applying load to the proximal end of the bone cavity, which is liable to wear and to be a cause of discomfort or pain to the patient. Accordingly, the invention provides an improved method of manufacture of a prosthetic joint to be implanted in a bone cavity of any particular form, and also an improved method of forming a moulded form from which can be derived the required profile of the prosthetic joint to be implanted in the cavity. According to one aspect of the invention there is provided a method of forming a moulded form corresponding at least partly with the shape of a bone cavity which is to receive a prosthetic joint and which cavity has an enlarged proximal end and a narrower distal end, and said moulded form enabling the required profile of the prosthetic joint to be derived therefrom in order to be implanted in the bone cavity: in which the method involves the use of a model having a pre-formed proximal dummy portion to fit in the proximal end of the bone cavity, a distal end portion to be received by the distal end of the cavity, and a bag to enclose at least the proximal dummy portion of the model, and the method comprising: introducing the proximal dummy portion and the distal end portion into the cavity until the model is seated in position : filling the space between the outer surface of the dummy portion and the inner surface of the bag with a moulding composition so as to expand the bag into intimate contact with the wall surface of the cavity in at least the proximal end portion of the cavity: allowing the composition to set into a moulded form corresponding at least with the enlarged proximal end portion of the cavity: and, removing the moulded form from the cavity to form a moulded model from which the required profile of the prosthetic joint can be derived. Preferably, the moulding composition is of such a nature that it is cold formable, and can form the required moulded form fairly rapidly to a sufficient degree of rigidity to enable the moulded form to be withdrawn from the cavity without any appreciable loss of shape. The composition may also be selected to be non- adherent with the material of the bag. to enable the bag to be stripped subsequently from the moulded form prior to monitoring of the outer profile of the moulded form. The invention is particularly applicable to the implantation of hip prostheses, and the dummy portion of the model may be selected from a suitable range of standard sizes, which will be supplied to suit the size of femoral rasp employed in the formation of the cavity. The rasp may be of the type disclosed in more detail in International Patent Application No PCT/GB89/01188. The distal end portion of the model also may be selected from a suitable range of standard sizes to fit snugly in the formed distal end portion of the cavity. When a prosthetic joint is manufactured from the derived profile of the moulded form, the distal end portion of the prosthetic joint also will fit snugly into the distal end portion of the cavity, thereby giving a rigid implantation of the prosthetic joint. However, it should be understood that the moulded form of the model does not need to conform exactly by its external profile with all aspects of the internal profile of the bone cavity into which the model is introduced (prior to expansion of the bag) , provided only that at least the proximal end portion of the moulded form conforms substantially in external profile with the internal profile of the enlarged proximal end portion of the bone cavity. Desirably, the distal end portion of the model is selected to have a close sliding fit in the distal end portion of the cavity, whereby the prosthetic device derived from the model can fit in the bone cavity in such a way that the distal end portion of the device provides lateral restraint which resists any tendency of the prosthetic device to rock about its proximal end portion, but also allows relative axial sliding movement in use, occasioned by loading of the femur of the patient. An intermediate portion of the moulded form which interconnects the proximal portion and the distal portion (and of the prosthetic device derived therefrom) does not need to be a close fit within its portion of the bone cavity, because its sole purpose is to form a rigid intermediate connection between the two portions. Indeed, it is preferred that the intermediate portion (of the model) should have lateral clearance with the adjacent internal wall of the bone cavity, so that the corresponding intermediate portion of the prosthetic device has similar lateral clearance, and whereby the femur of the patient is allowed to flex without coming into contact with the intermediate portion of the prosthetic device. This will ensure that axial loading is borne by the proximal end. The proximal dummy portion and the distal end portion are preferably supplied as separate components which can be releasably coupled together, to enable at least the distal end portion to be re-used. As mentioned above, the bag encloses at least the dummy portion, but it may be designed to enclose at least part of a stem portion forming the intermediate portion by which the distal end portion is connected to the dummy portion. The proximal portion, which will normally be discarded after use. is preferably made of suitable plastics material. Alternatively, it may be retained as a permanent repl ica. To facilitate the introduction of the moulding composition into the bag. it is preferred that the proximal dummy portion is formed with a passage, an inlet to the passage, and at least one surface outlet, to enable the moulding composition to be introduced under pressure into the bag via the interior of the proximal portion. According to a further aspect of the invention there is provided a method of manufacture of a prosthetic joint to be implanted in a bone cavity having an enlarged proximal end and a narrower distal end. and involving the use of a model having a preformed proximal dummy portion to fit in the proximal end of the bone cavity, a distal end portion to be received by the distal end of the cavity, and a bag to enclose at least the proximal portion of the model, and the method comprising: introducing the proximal portion and the distal end portion into the cavity until the model is seated in position; filling the space between the outer surface of the proximal portion and the inner surface of the bag with a moulding composition so as to expand the bag into intimate contact with the wall surface of the cavity in at least the proximal end portion of the cavity: allowing the composition to set into a moulded form corresponding at least with the enlarged proximal end portion of the cavity: removing the moulded form from the cavity to form a moulded model from which the required profile of the prosthetic joint can be derived: and. following the external shape of the moulded model form with a profile detector and causing a machine to shape the outer profile of a prosthetic joint with a corresponding shape so that the prosthetic joint can be implanted in the bone cavity. The invention also includes the model per se for use in either of the methods of the invention defined above. The invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a model for use in a method of forming a moulded form according to one aspect of the invention: Figure 2 is a detail view of part of the model shown in Figure 1 : Figure 3 is an exploded view of the component parts of the model shown in Figure 2: Figure 4 is a schematic illustration of a profile detector device for use in monitoring the outer profile of a moulded form made using the model shown in Figures 1 to 3; and . Figure 5 is a schematic illustration of a machine for manufacturing a prosthetic joint having a cutting head controlled by signals derived from the profile detector apparatus of Figure 4. Referring now to the drawings, there will be described in detail an embo ' diment of method of forming a moulded form corresponding with the form of a bone cavity which is to receive a prosthetic joint and having an enlarged proximal end and a narrower distal end, and which is to receive an implanted prosthetic joint, and there will also be disclosed an embodiment of method of manufacture of a prosthetic joint to be implanted in a bone cavity of any particular form. There is shown in Figures 1 to 3 a model which is designated generally by reference 10 and which has a preformed proximal dummy portion 11 to fit into the proximal end of a bone cavity, shown by dashed outline 12 in Figure 1. a distal end portion 13 to fit in the distal end 14 of the bone cavity, and a bag 15 (shown by dotted outline in Figure 1. and in solid lines in Figures 2 and 3) which encloses at least the dummy portion 11 of the model. In the illustrated arrangement, the internal profile of the bone cavity is designated by reference 16 (see dash dot outline in Figure 1) and is formed in a femur in order to receive an implanted prosthetic hip joint. The distal end portion 14 of the bone cavity 16 may be formed by any suitable reamer which will form a cylindrical passage at the end, and the proximal and enlarged end portion 12 of the bone cavity can be formed by any suitable means, such as particularly the femoral rasp disclosed in more detail in International Publication No W090/03764. The invention provides a method of forming a moulded form which corresponds with the form of the bone cavity 16 which is to receive a prosthetic joint, and then after extraction of the moulded form, its outer profile can be followed by a profile detector device, as shown schematically in Figure 4. and the signals derived from the monitoring of the profile are then used to control the operation of a machine having a cutting head which shapes the outer profile of a prosthetic joint to correspond with that of the moulded form, in order to fit in the bone cavity of the particular form, as shown schematically in Figure 5. However, as can be seen from Figure 1, and as will be described in more detail later, it is not essential that the moulded form conforms exactly to the internal profile of cavity 16, but it is important that there will be substantial conformity with the internal profile of enlarged proximal end cavity 12. and preferably also substantial conformity with distal portion 14 of the bone cavity. To form the moulded form which corresponds generally with the form of the bone cavity, the proximal portion 11 and the distal portion 14 of the model are introduced into the cavity 16 until the model is seated in position. The space between the outer surface of the portion 11 and the inner surface of the bag 15 is filled with a moulding composition so as to expand the bag 15 into intimate contact with the wall surface of the bone cavity in at least the proximal end portion 12 of the cavity. The composition is then allowed to set into a moulded form which corresponds at least with the enlarged proximal end portion of the cavity, and then the moulded form obtained thereby is removed from the cavity to present a moulded model from which the required profile of the prosthetic joint can be derived. It is preferred that the moulding composition is of such a nature that it is cold formable, and can form the required moulded form fairly rapidly to a sufficient degree of rigidity to enable the moulded form to be withdrawn from the cavity without any appreciable loss of shape. The moulding composition may be selected to be non- adherent with the material of the bag, to enable the bag to be stripped subsequently from the moulded form prior to monitoring of the outer profile of the moulded form. However, this is not essential to the invention, and the bag may remain adhering to the moulding composition and thereby form the outer skin of the moulded form whose profile can be monitored. Conveniently, the proximal portion 11 of the model may be supplied in a range of standard sizes to suit the size of femoral rasp employed in the formation of the cavity. Furthermore, the distal end portion 14 may be selected from a suitable range of standard sizes to fit snugly in the formed distal end portion of the cavity. It is important to provide a reasonably rigid location of the distal end of the prosthetic joint so as to prevent any rocking of the prosthesis and / or undue flexing in service. The proximal portion 11 and the distal end portion 14 are preferably supplied as separate components which can be releasably coupled together, to enable at least the distal end portion 13 to be re-used. As illustrated, the bag 15 encloses at least the proximal portion 11. but it may be designed to enclose further parts of the model down to, but not including the distal end portion 13. Thus, the bag may enclose in addition at least a part of a stem portion 17 which is detachably coupled with the lower end of the proximal portion 11, and which also has the distal end portion 13 detachably coupled therewith. As can be seen from Figure 1, there is a substantial lateral clearance 17a between the inner wall of adjacent portion of cavity 16 and the outer surface of stem portion 17. This clearance allows the femur of the patient to flex, without coming into contact with the prosthetic device derived from the moulded form of the mode l . The purpose of intermediate portion, or stem 17 is merely to serve as a rigid intermediate connection between proximal portion 11 and distal portion 13. with proximal portion 11. with the inflated bag 15 thereon, being a close fit within the enlarged proximal end portion 12 of the bone cavity. The corresponding proximal end portion of the prosthetic device derived from this moulded form of the model therefore also will fit snugly within the enlarged proximal end portion 12. The distal end portion of the prosthetic device derived from the shape of the selected distal end portion 13 of the model will have a suitable close sliding fit within the distal end portion 14 of the bone cavity, and this serves to anchor the proximal end portion of the prosthetic device against rocking movement on its seating, but the sliding contact permits flexing of the femur. The proximal portion 11 is made of suitable plastics material, and to facilitate the introduction of the moulding composition in to the bag 15, it is preferred that the proximal portion 11 has a central passage formed therein, an inlet to the passage, and at least one surface outlet, to enable the moulding composition to be introduced under pressure into the bag 15 via the interior of the proximal portion 11. In Figure 1. the outer surface of the proximal portion 11 is shown having a number of surface outlets 18, and the moulding composition will normally be pumped under pressure via an inlet 19 provided at the top of the proximal 11. The construction of the model is shown in more detail in Figures 2 and 3. The invention also includes a method of manufacture of a prosthetic joint to fit in the bone cavity of any particular form, and involving the use of the moulded form obtained from the model described above with reference to Figures 1 to 3. Figure 4 shows a profile detector apparatus schematically by reference 20 having a profile detector head 21 which is movable along the X and Y axes in order to monitor the outer profile of ' a moulded form 22 (shown schematically only) . and this movement of the head 21 is converted into suitable electrical signals which control the operation of a machining apparatus having a cutting head 23. as shown in Figure 5. which machines the outer profile of the prosthetic joint 24 to the required profile to fit into the bone cavity of the particular form. Referring now to Figures 2 and 3, the proximal portion 11 has a fenestrated core 27 which communicates with inlet 19, so that the moulding composition e.g. a suitable silicone, can be introduced under pressure into the bag 15. The distal end portion 13 is available in a range of diameters and lengths to suit the reamed isthmus, and has suitable provision 28 to seal the distal end of bag 15. The bag 15 also is provided with upper and lower beaded openings 15a and 15b to allow sealable attachment of the bag. The dummy portion 11 is supplied in a range of proximal inserts which are matched to the rasps used to prepare the endosteal cavity, and fastener 29 (see Figure 3) secures the bag 15 and prevents proximal leakage of silicone. Means 30 allows silicone to pass from the fenestrated core 27 into the bag 15. A neck portion 31 may be adjustably fixed to the head 15 to allow for anti-retroversion adjustment. Nut 32 clamps the assembly in combination with the distal end portion 13. A range of neck adjusters 33 is provided, each releasably connected to the neck 31 to give variable off¬ set .

Claims: CLAIMS 1. A method of forming a moulded form (22) corresponding at least partly with the shape of a bone cavity (12. 16. 14) which is to receive a prosthetic joint and which cavity has an enlarged proximal end (12) and a narrower distal end (14) , and said moulded form (22) enabling the required profile of the prosthetic joint (24) to be derived therefrom in order to be implanted in the bone cavity; in which the method involves the use of a model (10) having a preformed proximal dummy portion (11) to fit in the proximal end (12) of the bone cavity, a distal end portion (13) to be received by the distal end (14) of the cavity, and a bag (15) to enclose at least the dummy head portion (11) of the model, and the method comprising: introducing the proximal dummy portion (11) and the distal end portion (13) into the cavity until the model is seated in position: filling the space between the outer surface of the dummy portion (11) and the inner surface of the bag (15) with a moulding composition so as to expand the bag into intimate contact with the wall surface of the cavity in at least the proximal end portion (12) of the cavity; allowing the composition to set into a moulded form (22) corresponding at least with the enlarged proximal end portion (12) of the cavity: and, removing the moulded form from the cavity to form a moulded model from which the required profile of the prosthetic joint can be derived. 2. A method according to Claim 1, in which the moulding composition is of such a nature that it is cold formable. 3. A method according to Claim 1 or 2. in which the composition is selected to be non-adherent with the material of the bag. to enable the bag to be stripped subsequently from the moulded form prior to monitoring of the outer profile of the moulded form. 4. A method according to any one of Claims 1 to 3. in which the dummy portion of the model is selected from a range of standard sizes to fit into the enlarged proximal end portion of a cavity shaped to receive a prosthetic hip joint . 5. A method according to Claim 4. in which the distal end portion (13) of the model is selected from a range of standard sizes to fit snugly in the formed distal end portion (14) of the cavity. 6. A method according to Claim 4 or 5. in which a rigid stem portion (17) forms an intermediate connector between the dummy head portion ( 11) and the distal end portion ( 13) . 7. A method according to Claim 6, in which the stem (17) is releasably coupled with one or both of the dummy portion (11) and the distal end portion (13) . 8. A method according to any one of Claims 1 to 7. in which the proximal dummy portion (11) is formed with a passage, an inlet (19) to the passage, and at least one surface outlet (18) , to enable the moulding composition to be introduced under pressure into the bag (15) via the interior of the dummy portion (11) . 9. A method of manufacture of a prosthetic joint (24) to be implanted in a bone cavity having an enlarged proximal end (12) and a narrower distal end (14) , and involving the use of a model (10) having a preformed proximal dummy portion (11) to fit in the proximal end (12) of the bone cavity, a distal end portion (13) to be received by the distal end (14) of the cavity, and a bag (15) to enclose at least the proximal portion (11) of the model, and the method comprising: introducing the proximal portion (11) and the distal end portion (13) into the cavity until the model is seated in position ; filling the space between the outer surface of the proximal portion (11) and the inner surface of the bag (15) with a moulding composition so as to expand the bag into intimate contact with the wall surface of the cavity in at least the proximal end portion (12) of the cavity: allowing the composition to set into a moulded form corresponding at least with the enlarged proximal end portion (12) of the cavity: removing the moulded form from the cavity to form a moulded model (22) from which the required profile of the prosthetic joint (24) can be derived: and. following the external shape of the moulded model (22) with a profile detector (21) and causing a machine to shape the outer profile of a prosthetic joint (24) with a corresponding shape so that the prosthetic joint can be implanted in the bone cavity.

Inventors: ELLOY MARTIN, ELLOY, MARTIN
Applicants: THACKRAY C F LTD, CHAS F THACKRAY LIMITED
Application year: 1991
Main classification: A61F2
IPC codes: A61F2