Stress and strain state for some types of hip joint stems.

Sticlaru, Carmen ; Davidescu, Arjana ; Crainic, Nicolae 等

1. INTRODUCTION

The first modern total hip prostheses, implanted by the English surgeon Philip Wiles, in 1938, had a short femoral component. The metal ball sat on a short shaft, which passed through the thighbone's neck and was firmly anchored by a plate on the outside of the thighbone.

In 1978 two Swiss surgeons (Jacob and Huggler) constructed a similar total hip device. Their total hip had a femoral component with a "thrust plate" (fig. 1). The thrust plate was an oval metal plate resting on the femoral neck; the surgeon removed the diseased femoral head carefully before creating an even area for support of the thrust plate. The idea was that the body weight should pass through this trust plate directly onto the femoral neck and upper part of the thighbone, in theory at least. No bone cement is used.

The obvious advantage of this construction is that the marrow cavity of the thighbone is left intact. If this total hip device should fail and must be revised, the revision operation will be easy, say the authors, practically as simple as in hips never operated before. (Buergi et al., 2005). This operation is more radical than the surface replacement; in that operation most of the femoral head is still retained.

Since its introduction on the market in 1980, the device has been redesigned two times, always a sign that something was wrong with the previous models.

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In the January 2005, four Swiss surgeons presented results of 102 operations with the latest model of the thrust plate total hip (Buergi et al., 2005). This model has been on the market since 1992. The patients were considerably improved by the operation, and the six years survival of the prosthesis was 96%. There were, however, 2% of postoperative infections.

There is a rather restricted number of Swiss, German, Indian, and Australian surgeons performing this total hip replacement. The total hip device is something between surface replacement and an ordinary total hip joint. The obvious candidate is a young patient.

2. STABILITY OF THE TOTAL HIP

In the healthy hip joint the femoral head is continually in close and stabile contact with the socket during all movements.

The stability of the healthy hip joint is provided by numerous supporting structures around the hip joint, including a thick joint capsule, a system of joint ligaments built in the joint capsule, and a ligament inside the hip joint itself. These joint structures create a passive resistant force on the hip joint that keeps the femoral head in close contact with the hip joint socket during all movements.

Moreover, the 19 muscles surrounding the hip joint provide further dynamic stability to the hip joint. Every surgeon who tried to extract the femoral head from the hip joint in a patient with a broken femoral neck (collum femurs fracture) knows how difficult task it is.

During total hip replacement a portion of these supporting structures (muscles, ligaments, capsule) is cut (divided) for easier access to the hip joint. Even if the surgeon tries to restore muscle and soft tissue balance by suturing together the cut ligaments, muscles, and joint capsule after the total hip replacement, there is usually some imbalance of soft tissues left.

Figure 2.a.: In the healthy hip joint ligaments, joint capsule and muscles around the joint (and one ligament inside the joint) provide continuous close contact between the femoral head and joint socket. The femoral head is large.

Figure 2.b.: During the total hip joint operations a portion of the muscles, joint ligaments and joint capsule were severed to gain access to the hip joint. Even if these structures were sutured back after insertion of the artificial hip, the force which keeps the ball component in close contact with the cup component has been impaired. The size of the ball component is smaller than the removed head (Pennock et al., 2002).

The stress shielding is a mechanism that protects the skeleton from the natural stresses that the everyday life puts on it. Total hip device exerts such stress shielding effect on the skeleton around it.

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The skeleton is very economical system. Where the weight load on the skeleton is large, the skeleton grows more bone tissue in the loaded area; the net result is a more closely packed and stronger skeleton that has the strength to sustain the increased load.

Femoral shaft component placed within a thighbone makes such "two materials composite" where the femoral component sustains the greater part of the load.

The shaft component of a total hip device is much stiffer than the skeleton and will take the greater part of the body weight load. Consequently, the shaft component is "overloaded", whereas the skeleton around the shaft is "unloaded".

Figure 3.a presents the image of the stresses from the body weight on the lower limb skeleton as a steady flow of impulses that starts in the lower back. In the normal healthy skeleton, the stresses flow symmetrically from there downwards through hip joints, thighbones, knee joints, lower leg bones, and feet into the floor.

Figure 3.b presents that the situation changes when there is a total hip joint device. The much stiffer shaft component of the total hip takes over the majority of the load stresses.

Unfortunately, the thickening of the skeleton is often painful. The patients with cementless shafts of total hip devices often claim about the pain in the thigh, especially during the first years after the surgery (Pennock et al., 2002).

The surgeons believe that stress shielding is harmful because the weaker skeleton may fracture. The manufacturers are developing shaft components that have less "shielding effect". Because the stress shielding effect depends on the difference between the stiffness of the shaft component and the stiffness of the thighbone, the manufacturers try to produce shaft components with stiffness values more close to the stiffness of the thighbone. "Diminished shielding effect" of the femoral shaft component is one of the selling arguments of new models of total hip devices (McGrory et al., 1995).

3. FINITE ELEMENT ANALYSIS

The introduction of finite element analysis (FEA) into orthopedic biomechanics allowed continuum structural analysis of bone and bone-implant composites of complicated shapes.

FE analysis was performed on four versions of the stem which are characterised by different geometrical forms. Models were generated, analysed and post processed using the ANSYS finite element package (version 11). They were based on drawings realized in ProEngineer. A full three-dimensional model for it is shown in figure 4. These are the models which will be discussed in this paper. In any case, it was desirable to produce as realistic model as possible for the narrow stem.

Figure 5 shows the meshed (a, b, c, d), constrained and loaded three-dimensional models (e) of the stem.

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In figures 6, 7 are presented some aspects depickted from FE analyses. Figure 6 shows the stress state for the stem--it can be seen that the maximum value appear for the stem without collar (fig. 6 a). It is important for the the stress state (to obtain minimum values) that the stem has a collar for support the stem in the femur, in possible cases.

4. CONCLUSION

There have been many models of total hip device on the market; the number is > 1 000. Moreover, new models appear steadily and are advertised as decisive improvement. This paper improves these studies with some FEA aspects for the presented stems. This result is consistent with FE analyses of prostheses cemented into the femur which indicate that the stress is greatest in the middle of the shaft (Mathiasa et al., 1998). The stress in the shaft is believed to be acceptable for two reasons. Firstly, this part of the stem closely resembles conventional designs which have proved successful in the past. Secondly, the fatigue limit for stainless steel in bending is approximately 370 MPa for a maximum tensile strength of 870 MPa (Hute, 1995). This figure depends on parameters such as surface finish and the method of machining. This fatigue limit continues to increase for steels with tensile strengths of up to 1100 MPa and then starts to level out. As the tensile strength of the steel used for the prosthesis is 1460 MPa, the stresses encountered here will be below this fatigue level. The values for the strain state for the femur are also in the acceptabil limits and are positioned in the lower part of the femur.

5. REFERENCES

Buergi ML et al. (2005). Radiological Findings and Clinical Results of 102 Thrust-Plate Femoral Hip Prostheses: A Follow-up of 2 to 8 Years, Journal Arthroplasty 2005; 20: 108-17, ISSN (electronic): 1532-8406

Hutte (1995). Enginer Book, Fundamentals, Editura tehnica, Bucuresti, 1995; ISBN 973-31-01-913

McGrory et al. (1995). Effect of femoral offset on range of motion and abductor muscle strength after a a total hip arthoplasty, J Bone Joint Surg-Br 1995; 77-B:865-9

Mathiasa K. J., Leahya J. C., Heatona A., Deansb W. F. & D. Hukinsa W. L. (1998). Hip joint prosthesis design: effect of stem introducers, Medical Engineering & Physics Volume 20, Issue 8, October 1998, pp 620-624 ISSN: 1350-4533

Pennock J et al. (2002). Morse type tapers. Journal Arthroplasty 2002; 17: 773-8, ISSN (electronic): 1532-8406