ASPECTO METODOLÓGICO

Boone et al. [137] believed that the hip maximum range of motion during normal daily activities is 130° (-10° to 120°) which is during a hip flexion, Therefore in both CM and IM respectively with 157° and 141° range of motion there should be low risk of impingement in normal daily activities. However if surgeon does not ideally fix the prosthesis, patient performs some excessive hip flexing activities either voluntary or involuntary etc., the risk of impingement will be increased significantly. Surprisingly impingement have been observed in 70% of all kinds of the hip prostheses [126,127,131] as it was shown in Figure 3.20 and explained in 3.2.3. In this regard, with small change on design of the socket, IM tries to reduce the impact of impingement as well as EL problem, which is our main concern in this study.

IM socket consists of a flat surface, which is conformed with neck during impingement. There are 2 small inside and outside 1mm chamfers as well to eliminate a point contact with sharp edges during impingement (Figure 5.2). However CM only get the benefit of 2.5 mm chamfer which reduces the stress and contact pressure during impingement and EL in comparison with the older prostheses with sharper edges which was explained in 4.1.1 in depth. Although 2.5 mm radius chamfer has been selected with most of the manufacturer nowadays ([204, 205, 206, 207]), the nature of the CM design not only disables providing the flat surface on the edge but also encounters with the optimum chamfer radiuses which explained in 4.1.1.1. In this study Abaqus analyses the IM ring with the 2.5 mm radius chamfer of BDZ [207]. The neck of the ball contacts the CM rim with 5 different loads which are 1kN, 2kN, 2.8kN, 3.4kN and 6kN and the same loads are applied to IM when the neck is impinged with the ring of the IM.

129 7.1.1. CM and IM under 1kN force during Neck-Rim/Ring impingement

When 1 kN force is applied to the neck in y-axis, the neck is displaced vertically and presses the chamfer of the CM rim, or the flat surface of the IM ring. Distribution of the load on the flat surface of the IM ring shows the reduction of contact pressure by 63% in comparison with CM one. Figure 7.1a and Figure 7.1b are showing the contour results of contact pressures in CM and IM respectively due to the neck impingement.

a b

Figure 7.1. Contour results of Neck-Rim/Ring impingements on CM and IM under 1kN load a) Maximum 11.87 GPa Contact pressure on CM b) Maximum 4.4 GPa Contact pressure on IM

7.1.2. CM and IM under 2kN force during Neck-Rim/Ring impingement

Walking speed of 1 km⁄h could apply 280% of Human Body Weight (HBW) to the hip [95]. If the average patient HBW is 70 kgs, hip will be loaded under about 2kN force. With this assumption, the neck loads the rim of CM and the ring of IM with 2kN. The result shows excellent behaviour of the IM ring under 2 kN force with reduction of contact pressure by 59%. The contour result is illustrated in Figure 7.2a for CM rim and Figure 7.2b for IM ring.

a b

Figure 7.2. Contour results of Neck-Rim/Ring impingements on CM and IM under 2 kN load a) Maximum 15.6 GPa Contact pressure on CM b) Maximum 6.3 GPa Contact pressure on IM

130 7.1.3. CM and IM under 2.8 kN force during Neck-Rim/Ring impingement

Stair climbing may increase the force on the hip by 400% of HBW [208, 209], which can be a source of Neck-Rim/Ring impingement during high step climbing. In terms of occurrence almost 2800N force is applied to the neck which is pushed to the rim of CM or the ring of IM. Fortunately the result showed that the IM ring reduces the impact of neck impingement in comparison with the conventional rim of CM. Therefore contact pressure is reduced by 57%. This is illustrated in Figure 7.3a and Figure 7.3b.

a b

Figure 7.3. Contour results of Neck-Rim/Ring impingements on CM and IM under 2.8 kN load a) Maximum 17.4 GPa Contact pressure on CM b) Maximum 7.5 GPa Contact pressure on IM

7.1.4. CM and IM under 3.4 kN force during Neck-Rim/Ring impingement

If walking speed is increased from 1 km⁄h to 5 km⁄h, the median peak force would be increased from 280% HBW to 480% HBW [95, 209, 210]. In this regard a 3.4 kN force will be applied to the neck when it impinges the rim of CM or the ring of IM. The result in this test also shows the reduction of 56% of contact pressure by IM ring. Figure 7.4a and Figure 7.4b compare the contact pressures made by neck to the CM rim and the IM ring, respectively.

a b

Figure 7.4. Contour results of Neck-Rim/Ring impingements on CM and IM under 3.4 kN load a) Maximum 18.9 GPa Contact pressure on CM b) Maximum 8.3 GPa Contact pressure on IM

131 7.1.5. CM and IM under 6kN force during Neck-Rim/Ring impingement

Bergmann G et al. [95] reported the maximum applied force to the hip was due to the stumbling when the force was raised to 870% HBW. Figure 7.5a and Figure 7.5b compare CM and IM contact pressures during stumbling when IM ring shows 52% reduction in association with CM rim. Fortunately the hip joint can withstand up to 1500% HBW before fracture [209].

a b

Figure 7.5. Contour results of Neck-Rim/Ring impingements on CM and IM under 6 kN load a) Maximum 23.1 GPa Contact pressure on CM b) Maximum 11.1 GPa Contact pressure on IM

7.1.6. Discussion

All results showed significant reduction of contact pressure by the IM ring in comparison with the CM rim. The result was predictable in general term from the IM design, because once the given forces press the neck to the CM rim, relatively small part of the neck loads small area of the rim chamfer, which is more similar to point-to-point contact. Conversely when the same forces push the neck to the IM ring, relatively wider part of the neck loads the conformed flat surface of the IM ring (Figure 7.1b, 7.2b, 7.3b 7.4b and 7.5b demonstrates this graphically as well). This is more like surface-to- surface contact than point contact. Hence the distribution of the loads on the surfaces can reduce the contact pressure effectively. In terms of practicality of this idea, crowning technique may be needed for manufacturing. Table 7.1 and Figure 7.6 summarise the results of this study. The level of improvement shown give added confidence in the benefits of the new design.

132

Table 7.1. Comparison of the maximum contact pressure due to the impingement of the neck to CM rim and IM ring under 1kN, 2kN, 2.8kN, 3.4kN and 6kN

Load (kN)

Contact pressure (GPa)

CM IM 1 11.87 4.4 2 15.9 6.34 2.8 17.43 7.5 3.4 18.95 8.32 6 23.1 11.11

Figure 7.6. Maximum contact pressure of CM (blue bars) and IM (Red bars) during Neck-Rim/Ring Impingement under 1kN, 2kN, 2.8kN, 3.4kN and 6kN