Abstract:
A prosthetic implant to replace damaged or diseased bone, especially in the maxillofacial region, is produced by producing a digital representation of the region of interest by CAT scan, using the data to create a model by stereolithography, and using the data also to produce the implant by CNC machining. The implant is an entire replacement extracts excised surgically, without need for reconstructing bone or soft tissue.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an improved method of making prosthetic implants, methods of treatment incorporating the use of such implants, and to the prosthetic implants themselves. 
     The invention is of particular usefulness in relation to maxillofacial surgery and will be particularly described with reference to that field, but may also be utilised in the treatment of disease or damage in other parts of the body. 
     2. Prior Art 
     Maxillofacial surgery may be necessary to deal with congenital defect, accidental damage, or malignant tumours. Such surgery presents particular difficulties since the aim is to achieve a result which is not only functional in dealing with the particular problem addressed, but also ensures that a patient is left with a good level of ability to breath, eat and speak, while at the same time achieving a satisfactory aesthetic appearance. 
     Techniques are known in which diseased or damaged bone is excised and replaced. The replacement may be by way of grafting bone taken from other parts of the patient&#39;s body. More recently replacement bone has been achieved by attaching a titanium armature to sound bone to act as a support for grafted bone cells derived from bone in other parts of the patient&#39;s body. In either case, it is then necessary to attempt to reform the adjacent soft tissue over the resulting implant. 
     Another known technique is the use of microvascular free transfer osseofasciocutaneous flaps, in which a flap of bone and skin, for example from the forearm and optionally with attached muscle, is transferred to the mouth, with the blood vessels of the flap being connected to those of the head by microscopic surgery. A functional result may be achieved, but is non-anatomical. 
     Such techniques are extremely time consuming and difficult. A typical maxillofacial repair may require a surgical procedure lasting up to about 15 hours, and the procedure will involve opening a second surgical site (typically in the region of the iliac crest) to obtain bone or bone cells for grafting. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to enable reconstructive surgery of this nature to be carried out much more rapidly, thus markedly reducing the stress on the patient caused by the surgery, while reducing the load on the surgical team and also markedly reducing the costs of the surgery. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a method of making a prosthetic implant comprises the steps of obtaining a set of data defining the body parts of interest in three dimensions, using said set of data to create a three dimensional model of at least part of the body parts of interest, and using the three dimensional model to develop and fit to size a prosthetic implant which entirely replaces body parts which are missing or are to be excised from the patient. 
     The invention also provides a prosthesis made by the foregoing method. 
     From another aspect, the invention provides a method of treating damaged, diseased or missing body parts which comprises excising damaged and/or diseased body parts and selected adjacent parts, and replacing excised and/or missing parts entirely by a prosthetic implant secured to the sound adjacent bone structures. In the preferred form of the method, both bone and soft tissue are replaced by a single prosthetic implant made of a material onto the surface of which soft tissue is capable of growing in a manner to prevent the incursion of infection; such a material will typically be titanium. 
     The foregoing method in preferably carried out by obtaining a set of data defining the body parts of interest in three dimensions, using said set of data to create a three dimensional model of at least part of the body parts of interest, and using the model to develop and fit to size the prosthetic implant prior to surgery. 
     In preferred forms of the invention the prosthesis extends through a body surface such as skin or mucous membrane, for example in the palate or nasal cavity. 
     The prosthetic implant may be provided with mechanical attachment means for the releasable attachment of further prosthetic devices such as dentures. 
     The set of data defining the body parts of interest is preferably reduced by CAT scanning. The data resulting from the CAT scanning may be manipulated by computer, for example to derive from a CAT head scan a set of data defining three dimensionally only the bony structures of the skull. 
     The three dimensional model may conveniently be produced by stereolithography in a manner known per se by laser irradiation of a photoreactive polymer. 
     A further aspect of the invention provides a connector block comprising a body and a post extending from the body, the post being shaped for selective attachment to dental prostheses, and the body being formed with passages for rivets for attachment to a surgical plate. 
     Preferably, the body is rectangular, and the connector block is formed integrally from titanium. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which: 
     FIG. 1 is a flow chart illustrating the method of the present invention; 
     FIGS. 2 a  and  2   b  are schematic views in side and front views respectively of a skull having an area of damage in the lower jaw, 
     FIGS. 3 a  and  3   b  are similar view of the skull with the damaged area excised; 
     FIGS. 4 a  and  4   b  are similar views of the same skull with a prosthesis implanted; 
     FIGS. 5 a  and  5   b  are schematic cross-sections illustrating a healthy jaw and a jaw with an implant; 
     FIG. 6 is a schematic side view of an alternative mounting arrangement; and 
     FIGS. 7 a ,  7   b  and  7   c  are respectively side, end and plan views of a connector block embodying a further aspect of the invention. 
    
    
     Referring to FIG. 1, the process of the present invention is based upon the use of a CAT scan to derive a set of data defining in three dimensions the body part of interest, for example the skull. Accordingly, a conventional CAT scan provides data to a data processing step in which the data defining the bony structures are retained and the soft structure data discarded. 
     The processed data is then used to produce a replica of the patient&#39;s skull by stereolithography. There are techniques well known per se for the production of three dimensional models from digital data by stereolithography by laser irradiation of a bath of photoreactive polymer. In this way, a model of the patient&#39;s skull in its existing form is obtained. 
     The data from the CAT scan can also be processed to provide a further set of data defining in three dimensions a desired replacement part. This further data is then used to produce a replacement part by CNC machining from solid titanium. 
     At this stage, the surgical team have a true scale model of the existing skull plus a machined replacement for part of the skull. These can be used in the workshop (that is, in non-surgical, non-sterile conditions) to refine the surgical operation to be performed. In particular, the surgeon can plan the best positions to cut to obtain sound bone on which to mount the implant. The cutting and mounting can be performed experimentally on the model skull, and the shape of the machined implant can be refined in this process. 
     Optionally, as indicated in FIG. 1, during the workshop stage cutting jigs may be produced which are located with respect to well-defined points on the skull and provide a guide to enable the surgeon to cut the bone accurately in the planned planes. 
     Once the surgical plan and prosthetic implant have been refined in the workshop, the prosthesis is implanted surgically in the conventional manner. Typically, the prosthesis will be secured to sound bone by means of bone screws or expansion-type fixings. 
     An important feature of the present invention is that the prosthesis is of a material, typically titanium, which is compatible with passing through the surface of soft tissue without permitting the ingress of infection along the exposed surface of the implant. This allows the prosthesis to be a complete replacement for excised parts. 
     For example, in the case where part of the upper or lower jaw or the palate must be removed, the parts removed are replaced only by the implant, without attempting to separate and then reposition the soft tissue of the gum or palate. This is not only much less time consuming in surgery, but also makes the surgical site functional much more quickly post-operatively. 
     FIGS. 2 to  4  illustrate such a procedure schematically with reference to a damaged lower mandible. 
     As seen in FIG. 2 a lower jaw  10  has an area of damage  12  involving both the jaw and the teeth. FIG. 3 illustrates the damaged area cut back to sound bone at  14  and  16 . In FIG. 4, a solid implant  18  of titanium has been attached to the sound bone areas  14 ,  16  by bone screws  20 . The implant  18  is provided with posts  22  to which a denture may be directly mounted. It will be understood that the implant  18  extends into the interior of the patient&#39;s mouth, within which it will be visible, and the margin of the healthy, non-excised gum will grow onto the surface of the implant. 
     The stages of FIGS. 2 to  4  will be carried through first in the workshop on the model skull, and only thereafter on the patient surgically. 
     For simplicity of description, FIG. 4 shows the implant  18  being attached by simple bone screws  20 . In View of the loads typically placed on the mandible, it is preferable to obtain a more secure mechanical engagement. One such arrangement is illustrated in FIG.  5 . The implant  18  is secured (for example, rivetted or welded) to a plate  24  which in turn is attached to the sound bone areas  14 ,  16 , to lie along the underside of the mandible. The example shown makes use of a “Thorp” plate which has regularly spaced apertures  26 . The plate  24  is attached to the bone by fasteners which comprise a titanium cylinder  28  passed through one of the apertures  26  into a bore drilled in the bone, and a screw  30  engaging internally in the cylinder  28  to produce a wedging effect. This arrangement is less prone to loosen than bone screws, and copes well with bone regrowth. 
     FIG. 5 illustrates in more detail the relationship of the implant, bone and soft tissue. 
     FIG. 5 a  shown schematically a healthy jaw including mandible  60 , tooth  62  and soft tissue  64 . 
     In FIG. 5 b , it will be seen that the implant  18  entirely replaces excised bone and soft tissue, without any need to recreate bone or soft tissue. The remaining soft tissue  64  locates on the surface of the implant  18 , FIG. 5 b  also illustrates a denture  66  releasably secured to the implant  18  by engagement with a post  68  upstanding from the body of the implant  18 . Posts of this nature are known per se for securing dentures in oral reconstruction. 
     The implant  18 , in the example of FIG. 6, is secured to the plate by titanium rivets  32 . 
     In a modification (not shown), the implant may be made in a modular fashion, with the total volume to be replaced being provided by a number of interfitting parts which may, for example, be secured to a common mounting plate such as the plate  24  of FIG.  6 . This arrangement may simplify the surgical procedure n certain cases. 
     FIG. 7 shows a connector block  50  which may be used with the embodiments described above, or for other applications. 
     The block  50 , which is machined from solid titanium, has a rectangular body  52  with an upstanding post  54 . The top of the post  54  is formed into a part-sphere  56  for attachment of dentures, bridgework, etc. 
     The body  52  is formed with parallel, circular passages  58  which enable the connector block  50  to be connected to an apertured device such as a “Thorp” plate by rivets, as in FIG.  6 . 
     The part-sphere  56  is suitable for certain known types of connection. It may be replaced by alternative formations at the top of the post, for example for cooperation with screw-type connections. 
     Modifications may be made to the foregoing embodiments within the scope of the invention. 
     For example, the invention may be applied to disorders of growth such as the situation where one cheekbone fails to grow and is sunken with regard to the other cheekbone. In this case, an implant can be produced by the techniques described but based on data from the normal cheekbone, and secured to the defective bone as an onlay. 
     In another example, a tumour of leg tissue may be excised and the requisite volume filled by an implant secured to a leg bone. 
     In both of these cases, the tissues overlying the site would be separated to allow insertion of the implant, and then reclosed. The implant thus does not penetrate the body surface post-operatively, and biocompatible materials other than titanium may be used.