Computer-Aided Engineering Applications in Biomechanics

A KNO,'iLEDGE-BAS-':D APPROACH �O ':iHE CAD/CA�J OF CEMENTLESS TOTAL HIP REPLACEMENT

G.R.HarvGyl,3, R.A.H.Harvey2, D.rr.H.Harvey3.

1. The Queen's University of 3elfast, U.K. 2. Cuckfield Hospital, West Sussex, U.K. 3. Hedenco Limited, London. �.K.

ABSTRACT

Object oriented programming developed in artificial intelligence research leads to a

fundamental departure from the original way of modelling objects. It allows libraries of standard and custom-made components to be b�ilt up, have thoir dimensi;nal parameters altered at will, and at the same ti�e have associated procedures operate o� other co�patible objects.

Many of the problems arising from the malpositioning of hip �eplacement components can be considered using interactive computer graphics and artificial intelligence.

IN IRO DU CTION

Total hip replacement, the obvious solution for the over-65. sedentary patient, has had a high failure rate when applied to the "high risk" groups of younger, active and sometimes overweight patients.

In the elderly, conventional cemented total hip replacement has a more than 90% success, lasting at least 15 years m . but in patients under 50 [2) failure rates of 40% after five years have been reported. Nearly all the failures can be attributed to the breakdown of the acrylic cement used to fix the components to the bone.

Revision of a failed implant to another cemented total hip is extremely difficult, costly and frequently unsuccessful. Further revisions then become necessary.

Given the adverse effects of cement and given that it acts simply as a grouting agent between the prosthesis and the bone, why not dispense with it and use a carefully computer designed prost�esis which fits securely into an accurately cut bone? It was in Britain (where cement fixation '{as first widely used). that the complications of using cement were first pointed out and the earliest reports of cementless fixation appeared [3J .

r1ET:{CDOLOGY

Many of the problems of malpositioning of hip components can be considered using

interactive computer graphics and artificial intelligence.

Computer graphics (CAD) can be used to position components for pre-operation decision-making in total l1.ip replacement. This includes tl1e selec-:;ion or design of a suitable prcst�esis. its insertion and positioning on a visual display unit and demonstrates ho·" intelligent CAD can give a clearer understanding of the surgical steps involved in �ip replacenlent.

A computer �raphics facility is partic­ularly suited to prosthesis layout where a number of arrangements are tried until an acceptable solution is founc.

Radiographs of the diseased joint are taken; one ir. the anterior-posterior (AP) direction and the other in the medilll­lateral direction (ML). A metal sphere of !mown diameter can be attac:'led to the patient for computer screen scaling purposes. Additional sectional contour data can be obtaihed using a CT scan of the femur.

UsinS a back-lit digitising tablet (Fig. 1) the affected joint is outlined on the VDU anc the contour data then used to create a three-dioensional model of the proximal femur and acetabulum on the VDU and stored in memory.

Up to 50 sizes of iEplant are available in Burope and the U.S.A. and these c�n be stored in the computer memory to ferm a library so that a suitable 'off-the-shelf' prosthesis may be found.

In th e event of failure to find a standard arthroplasty from memory a custom­desizned prostheSiS can be developed. Fig.2 shows a custom designed pros thes is vit� a fluted steD and micromadrephoric surface for biological fixation (also kno'wn as bone ingrm,rth fixation).

lJlSCUSSION An important influence on modern

computer designed arthroplasties is the effect of CAD software developoent now emerging fro:n artificilll intelligence (AI) research. Already object-oriented programm­ing techniq ue s are beginnin� to bring impressive benefits to the applications programEer and to the bio-engineering desi�ner at the workstation. And as a

IEEE ENGINEERING IN MEDICINE & BIOLOGY SOCIETY 10TH ANNUAL INTERNATIONAL CONFERENCE--0657 CH2566-8/88/0000--0657 $1.00 @ 1988 IEEE

Fig. 1.

Fig. 2.

result true 'knowledge engineering' is being applied to the more substantial shared databases which now characterise the work of serious computer-aided design.

Object-oriented programming simplifies the writ:L,g of applications software, allm,s libraries of standard components to be built up. and makes it quite impossible to call up 'square pegs' to fit round holes.

Every object within the programming environment contains complete intelligence about itself (down to component part-numbers. if necessary) and enough procedures to allo\., it to be redefined. mirrored, rotated,

collapsed, magnified or whatever - with subsequent changes in other associated objects being effected automatically.

Admittedly. it is no trivial task to ensure that all possible interactions between an object and its associated objects are catered for in the original procedures. But this is the responsibility of the programmer who first constructed the object, and who subsequently maintaiIls it .

CONCLUSION

Using an object-oriented approach, a user identifies the item (object) and then sends messages telling it to erase, move and display and knows how to execute the requested operations. Similarly, a deSigner can alter performance characteristics or parameters of components and see the implications of modification reflected throughout the whole deSign.

Object-oriented software therefore. takes CAD beyond the sheer physical geometry. The deSigner can stack up differ­ent property cases, say for finite-element analysis, which can be explored more readily, allowing different failure modes to be examined, and permitting the bio­engineering designer a greater control over the ' injury potential' of his produ�t.

RSFERENCES

[� Total hip replacement, NIH Consensus Development Conference S�mmary. Vol. 4 No.4, 1982.

� D.K. Collis, "Cemented total hip replac­ement in patients who are less than fifty years old" Journal Bone Joint Surgery (Am.) 1984 66:353 - 359.

� P.A. Ring, "Complete replacement arthroplasty of the hip by the Ring prosthesis". Journal Bone Joint Surgery 1968. 50B : 720 - 31.

ACKNOl.vLEDGEI1 ENT

The authors wish to acknowledge the assistance received from the NATO Collabor­ative Research Grants Programme under Grant No. 0138/87.

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