ANALYSIS AND DESIGN OF CEMENTLESS HIP JOINTS USING

CAD/CAM

G. R. Harvey, The Queen's University, Belfast. U.K.

R.A. H. Harvey, Cuckfield Hospital, West Sussex. U.K.

D. R. H. Harvey, Medenco Ltd., London. U.K. ABSTRACT

During the past ten years researchers have been seeking alternative ways to fix the implant in the 'active 50-yea{-old' wa .e earner.

Obtaining the initial fxat ion and stress transfer necessary for bone in rowth without using cement has proved difjcult for both surgeons and designers.

Cement acts as as a space filler as well as a method of fixation. Therefore, in cemented femoral components, only three or four sizes are required to fit 95% of the population.

Without cement, individual anatomical variation of the upper end of the femur has made it extremely difficult to obtain adequate fit and stability of the implant. This paper describes a procedure for the design and analysis of cementless hip joints using CAD/CAM.

INTRODUCTION

The use of cementless prostheses involves bringing the prosthesis into contact with the bone to enable ingrowth to take place.

Even if ingrowth occurs, t he smoothness of stress transfer from implant to the bone is not as uniform as with cement, and 'hot spots' can occur w,here the bone is subjected., to high stress. Low spots' are also a possibility where bone is understressed. Overstressed bone can cause pain. This is felt to be responsible for the rather common occurrence of thigh pain in cementless total hips.

METHODS AND RESULTS

Scaled radio raphs and contour data generated from CT and hfRI by edge detection techniques are fed directly into a CAD/CAM pro ramme. This programme can generate either a 2b or 3D representation of thearoximal end of the femur.

The rosthesis esigner next considers the shape , k r m , material, surface texture, short and long-term effects of bone remodelling and implant stiffness for the proposed cementless prosthesis.

Proof testing plays an important role in ensuring the reliability of a prosthesis stem but it is essential for the designer to develop an adequate data base for materials and optimised surface f inishes to ensure long-l i fet ime implants.

When a biomaterial is implanted in vivo into bone, the bone-biomaterial interface IS subjected to deformations that depend on the loading environment and. the rigidity of the two materials [l]. Ideally, . i f the elastic properties of bone and biomaterial are strictly identical, both materials remain in close contact during the loading; if the biomaterial is stiffer than bone, only the interface areas subjected to compression stay in contact while shear forces occur on the remaining part of the interface (Fig. 1).

A B

Figure 1 Representation of bone-biomaterial interface u.nder com ressive loading for (A ) a stiff material and (g) a material having the same rigidity as bone.

These shear stresses can theoretically be respons ib le for a f ib rous membrane interposition - an effect independent from the surface chemistry of the biomaterial [2].

The biological performence of biomaterials such. as titanium alloys implies the chemical and physical potential of the material response and t h e physiological and pathological potential of the host response.

TIME EFFECT

To discuss the implications of these implant-related design factors on bqne remodelling, it is important to distin uish between short-term and long-term effects Short term effects relate to the effects on t h e healing process around the implant; long-term effects, to the effects associated with bone remodelling subsequent to healing.

1228 Annual International Conference of the IEEE Engineering in Medicine and Biology Society. Vol. 12, NO. 3, 1990 CH2936-3/90/0000-1228 $01.00 Q 1990 IEEE

Shofl-Term Effects The preferred effect of an implant in the short term is to minimize the healing period. This does , not, however, necessarily lead to a universal implant stiffness requirement for all applications. For example, although a belief exists that more compliant that is, 1owe.r stiffness) implants would lead to

raster heal ing around jo int replacement corn onents, insufficient stiffness and flexural rigid?ty would result in delayed unions in fracture healing. Therefore, it is concluded that there is a question to be answered: Is there a preferred stiffness for an implant that would enhance the rate of bone healing? Although this be s the fundamental question of regulation of c e f mitosis during wound healing in general simple studies on wound healing rates as a function of implant stiffness would give an indication of whether such a preferred stiffness exists. The question must also be considered in terms of the geometry of the bone-implant T t e m in ,relation to the loading directions.

us, a series arrangement of bone and implant should be considered to be distinct from the situation in which bone and implant are arranged essentially in parallel with each other relative to the direction of loading (Fig 2). Lona-Term Effects As noted earlier, it is a Known fact that relatively stiff implants can lead to enhanced bone loss due to remodelling, particularly for wel l -bonded bone- implant systems in which implant and bone are arranged in parallel configurations relative to major loadin directions. Remodelling causes enhanced bone Yoss with fixation lates on the periosteal surface, with intramedulkry rods in long bones and with stemmed hip joint implants. The major concern with bone loss is that the implant must bear more load and hence is, more liable to fatigue fracture. Therefore, it is essential that the extent of bone loss be appreciated for the proper design of implants in situ fracture.

FINITE ELEMENT ANALYSIS

Until modern computer technology became available, the task of accurately designing and stressing corn lex shaped components was near1 impossiple. The shear magnitude of ca l cuhons required often led to compromise or even avoidance of analysis altogether. Mistakes would often result and high safety factors would be included to overcome the shortcomings. Designers were not able to realise the high specific pro erty advanta es of prostheses materials a n 8 therefore con8dence in new alloy materials was low.

Today, modern computer methods take the tedium out of design calcu1ation.s and allow more complex analysis to be carried out. For more com lex arthroplasties, finite element analysis (FzA) is required. FEA has been around for many years and has found its way into almost every branch of engineering. Some packages have been integrated with CAD,/CAM packages. By illustratin stresses in the form of colour-coded contours, far e amounts of information can be quickly assimifated.

Contact of implan: with:

Cortlcolis

Figure 2 Proposed cementless prosthesis showing re ions of high and low stress bone remojelling.

DISCUSSION

design during

The accurate desi n and analysis of cementless hip joint im Qants is potentially of g rea t va lue in tRe case of t he active-50-year-old' wage earner.

Radiographic studies of human clinical cases with cemented and cementless hip implants suggest that major changes in bone architecture around the proximal (loss of bone) and distal (bone sclerosis) ends of the implant occur 2 to 3 years after implantation.

Implants that are too compliant, in certain situations, .can lead ,to detrimental remodelling, especially in the series arrangement of implant and bone.

ACKNOWLEDGEMENT

This research was sponsored in art by NATO Grant No. 0138/87 and by Jedenco Ltd. Biomedical Stability and Design Project.

REFERENCES

1. Zweymuller, K.: A cementless titanium hip endoprosthesis s stem. Basic research , and clinical results. yn The AAOS Instructional Course Lectures, Vol 35. St. Louis, C.V.Mosby, 1986. 2. Engh, ,C.A., and Bobyn, J.D.: Biological fixation in total hip arthroplasty. Slack, 1985, pp. 1-261.

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