PATELLA PROSTHESES

Abstract: A patella prosthesis is a mushroom shaped member whose head (1) is symmetrical or contoured to correspond to a human patella. The stem (2) is for insertion in a drilling in the resectioned surface which is shaped to receive the inferior surface (4) of the head (1). The preferred material is polyethylene or high purity alumina ceramic and there may be a metal backing (5) with a porous surface over the inferior side (4). Hydroxyapatite coating may also be applied here.

Description: Patella prostheses This invention relates to surgical devices and is concerned with a prosthetic patella replacement for use as a surgical implant. The surgical management of arthritis of the knee involves external supports such as bandages, braces and walking aids. Operative intervention may include anaes¬ thetic, intra-articular steroid injection or radio-isotope synovectomy, arthroscopic washout, debridement, menisectomy, chondroplasty or synovectomy, esteotomy or an arthroplasty. Joint arthroplasty may be an excision arthroplasty or interpositional or fascial arthroplasty or a prosthetic arthroplasty. Prosthetic arthroplasty of the knee may be undertaken as a uni-compartmental arthroplasty, a bi- compartmental arthroplasty, a patello-femoral arthroplasty or a total knee arthroplasty. The total knee arthroplasty may be subdivided into a hinged or linked arthroplasty although this has been superseded by the total condylar arthroplasty which may be unconstrained, semi-constrained or constrained according to the freedom of motion conferred upon the arthroplasty by its design (Goodfellow and O'Connor 1986) . Patello-Femoral replacement was not initially performed as a part of total knee arthroplasty (Sneppen et al 1978) . The anterior surface of the femur was left uncovered articulating directly with the patella. This resulted in unacceptable knee pain and poor function. Therefore modifi¬ cations were introduced which resurfaced the anterior surface of the femur (Insall et al 1976) . This still resulted in unacceptable anterior knee pain and poor function. Therefore a patella prosthesis was developed which was initially a polyethylene dome shape fitting flush on top of the patella after the articular surface was resected (Moreland Thomas and Freeman 1979) . A small keel or peg fitted down in the cancellous bone of the patella to stabilise the prosthesis onto the bone. This patella dome was cemented in situ with acrylic cement (Insall, Lachiewich and Burstein 1982, Wright et al 1990). As un-cemented prostheses became popular in the early 1980's the design of the patella was changed in most implants (Hungerford and Krackow 1985) . The basic concept was unaltered. However a metal back was attached to the implant to reduce deformation of the polyethylene, to allow initial fixation without acrylic cement and to allow bone ingrowth into the porous metal backing of the patella thus giving long term stability within the bone (Kenna and Hungerford 1984) . However the recent reports now emerging throughout the world refer to great problems with such a design. As the polyetheylene thickness is reduced to allow for the metal backing, the polyethylene becomes liable to disintegrate under the stresses to which it is submitted. The wear rate progresses through the thin polyethylene at an earlier stage, the metal backing may then be exposed and cause squeaking or scratching of the implant and metallic debris may be added to the plentiful polyethylene debris produced. The manufacture of the metal backing has been difficult, leading to disruption of the pegs from its reverse side. The metal backing and the polyethylene may become dissociated, and manufacture of a harder carbon fibre impregnated polyethylene patella dome has led to excessive wear, release of carbon fibre into the knee and disruption of the polyethylene due to faulty manufacture. Patella fracture or dislocation may also occur and this may be more common with the thicker designs of patella implant or where insufficient patella bone is resected (Scott et al 1978. ochizuki and Schurman 1979, Clayton and Thirupathi 1982, Ranawat 1986, Simison, Noble and Hardinge .1986, Lynch, Rorabeck and Bourne 1987) . The incidence of patello-femoral complications foll¬ owing knee arthroplasty is now the most common complication and has been reported as being up to 64% of cases (Sneppen et al 1978, Clayton and Thirupathi 1982, Merkow, Soudry and Insall 1985, Scuderi et al 1989)-. Another factor which has received little attention is the shape of the articulating surface in relation to patello-femoral stability in the lateral plane. It is accepted that the anterior flange of the femoral component must be long enough to prevent lateral tracking or soft tissue impingement (Sneppen et al 1978, Thorpe, Bocell and Tullos 1990) . Hungerford suggested an anatomical shape of patella button with the maximal thickness placed laterally. However the use of this implant fell into disrepute with the difficulties of insertion and high complication rate (Kenna and Hungerford 1984, Hungerford and Krackow 1985). Recently a design has been released with a central post and a surroundng shelf at a lower level. This was produced to aid patello-femoral stability by increasing the lateral resis¬ tance to patella subluxation from the patello-femoral groove. The incidence of patello-femoral complications has led many surgeons to abandon patella resurfacing (Scott et al 1978, Picetti, McGann and Welch 1990). There has been little concern with the mechanics of the human patella in relationship to the articular geometry. Thus the patella is a torque converter between the pull vertically of the quadriceps muscle and the patella tendon acting on the tibial to extend the knee joint. The patella articulates at its thickest inferior pole in knee extension thus maintaining the point of articulation within the depth of the patella groove distally where it is more stable, but also shifting the quadriceps tendon anteriorly thus in¬ creasing the moment around the knee and generating a greater extension force on the tibia near full extension. This produces an advantageous mechanical advantage to the quadriceps and also confers greater stability to the patello-femoral joint. The human patella during knee flexion articulates upon its medial and lateral edges, so allowing the patella to sink between the femoral condyles and a greater range of knee flexion for a given quadriceps flexibility. The articulation on the patella shifts to the superior portion which is thinner and allows sinkage between the condyles. But it also allows less patella flexion during knee flexion and hence further knee flexion for a given quadriceps flexibility (Ritter and Campbell 1987) . The aim of the invention is to design an implant using materials which will be subject to less wear, less subject to disintegration under stress, and less likely to become disrupted from any metal backing. It should also allow a greater thickness of the implant to prolong its longevity, but without increasing the thickness of the whole patella- implant complex and without weakening the construction. It should further be possible to allow un-cemented insertion of the implant, and further objects are to enable reconst¬ ruction of the anatomical patella to have a shape which allows sinkage between the condyles during knee flexion, to minimise the patella flexion during knee flexion, to increase the extensor moment during knee extension, and to increase patello-femoral stability. According to one aspect of the present invention there is provided a patella prosthesis comprising a generally mushroom shaped member whose stem is received in a counter- sunk hole in the surface of a re-sectioned patella articular surface. The preferred materials are polyethylene and high purity alumina ceramic. A metal backing with a porous surface may be provided on the inferior side of the prosthesis. Also, a hydroxyapat- ite coating may be applied to this side, either to the main material or to the metal backing if that is provided. In a simple form, the prosthesis is symmetrical about the axis of the mushroom-shaped member. However, it will generally be preferred to have the head of this member asymmetrical and shaped to follow closely the contours of the human patella. According to another aspect of the present invention there is provided a method of manufacturing a patella prosthesis comprising scanning a patient's patella to make a record of the contours and then constructing a patella prosthesis using said record to reproduce the contours on a workpiece. Such a patella prosthesis will of course be as defined above. According to a further aspect of the present invention there is provided a method of implanting a patella prosthe- sis comprising carrying out a patella articular resection, drilling the surface thus formed, and implanting a patella prosthesis as defined above with the stem inserted in the drilling. For a better understanding of the invention some embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an inferior view of a patella dome, Figures 2 and 3 are lateral views of a patella dome such as that of Figure l, Figure 4 is a coronal view of a patella dome for improved torque conversion, Figure 5 is a sagittal view of the dome of Figure 4, Figure 6 is a coronal view of another torque converter patella dome, and Figure 7 is a sagittal view of the dome of Figure 6. All the domes are generally of squat mushroom shape with a head 1 and a stem 2. They differ primarily in the contours of the head and in the materials in which they are made. Typically, the diameter of the head might be 20-30 mm, the depth of the head from crown to the end of the slope 6- 10 mm, the depth of the shallow cylindrical portion of the head (or edges 3) 3mm, the diameter of the stem 10 mm and its length 4 mm. In Figures 1 and 2, the dome of the patella prosthesis is of conventional symmetrical shape. It can be manufactured from polyethylene or from high purity alumina ceramic. The latter is particularly resistant to wear and so the production of wear debris and disruption is minimised. After patella articular surface resection the surface is further drilled to allow the stem 2 to be countersunk, thus providing an implant with a thicker articular surface whilst not increasing the overall thicknes of the construct. These two versions are fixed with acrylic cement. A third version has a hydroxyapatite coating to the narrow cylindrical edge 3 and inferior side 4 of the head 1 and to the stem 2, thus allowing for tissue integration and uncemented use. A fourth version, shown in Figure 3, is generally similar in overall dimensions but has an incorpor¬ ated metal backing 5 with a porous metal surface (which may not extend over the stem 2) and hydroxyapatite coating, for uncemented use. A second type of patella implant, shown in Figures 4 and 5 and in Figures 6 and 7, is modified from the first type described above by having an eccentric dome more akin to the shape of the human patella. This I refer to as the 'Torque Converter Patella Dome' (TPD) . Again it can be manufactured from polyethylene or high purity alumina ceramic, with or without a hydroxyapatite coating to the sides and base, or (as in Figures 6 and 7) with a ceramic articulating surface having a metal backing with a porous surface and a hydroxyapatite coating. A third type of patella implant is for patients who have isolated patello-femoral arthritis at an early age and in whom total condylar knee arthroplasty can be avoided by patella resurfacing in isolation. The three dimensional anatomy of a patient's patella is taken from a scan of the patella, either from the pictures of a computerised tomog¬ raphy scan or alternatively a magnetic resonance imaging scan of the in situ arthritic patella. What I refer to as an 'Elite Custom Patella Dome' (ECPD) is then constructed by computer assisted manufacturing techniques for implantation in the patient. Each implant is therefore a customised model of the patient's original patella. In materials and appear¬ ance there will not be any great difference between this type and the second type of Figures 4 to 7 and therefore they are not illustrated.

Claims: CLAIMS 1. A patella prosthesis comprising a generally mush¬ room-shaped member whose stem is received in a countersunk hole in the surface of a resectioned patella articular surface. 2. A patella prosthesis according to Claim 1, characterised in that the prosthesis is of polyethylene. 3. A patella prosthesis according to Claim 1, characterised in that the prosthesis is at least substan- tially of high purity alumina ceramic. 4. A patella prosthesis according to Claim 1, 2 or 3, characterised in that a metal backing with a porous surface is provided on the inferior side of the prosthesis. 5. A patella prosthesis according to any preceding claim characterised in that a hydroxyapatite coating is applied to the inferior side of the prosthesis. 6. A patella prosthesis according to any preceding claim, characterised in that it is symmetrical about the axis of the mushroom shaped member. 7. A patella prosthesis according to any one of claims 1 to 5, characterised in that the head of the mushroom shaped member is asymmetrical and shaped to follow closely the contours of a human patella. 8. A method of manufacturing a patella prosthesis comprising scanning a patient's patella to make a record of the contours and then constructing a patella prosthesis using said record to reproduce the contours on a workpiece. 9. A method as claimed in Claim 1, wherein the patella prosthesis is as claimed in Claim 7. 10. A method of implanting a patella prosthesis, the method comprising carrying out a patella articular resec¬ tion, drilling the surface thus formed, and implanting a patella prosthesis as claimed in any one of Claims 1 to 7, or manufactured as claimed in Claim 9, with the stem inserted in the drilling.

Inventors: JOHNSON DAVID PAUL, JOHNSON, DAVID, PAUL
Applicants: JOHNSON DAVID PAUL, JOHNSON, DAVID, PAUL
Application year: 1992
Main classification: A61F2
IPC codes: A61F2, A61L27