Prosthetic implants

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.

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'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'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.

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'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'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'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. 5a shown schematically a healthy jaw including mandible 60, tooth 62
 and soft tissue 64.
 In FIG. 5b, 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. 5b 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.