Journal of Orthopaedic Surgery 2005;13(1):46-51
Does the self-centering mechanism of bipolar hip endoprosthesis really work in vivo? H Tsumura, N Kaku, T Torisu Department of Orthopedic Surgery, Oita University, Oita, Japan
ABSTRACT Purposes. To examine radiographically the component motion in a bipolar prosthesis and to determine whether the self-centering mechanism really works in vivo. Methods. 38 patients with 41 bipolar hip endoprostheses (30 for coxarthrosis and 11 for osteonecrosis of femoral head) were included in this study. Two radiographs of each case were taken to evaluate the self-centering mechanism. The first anteroposterior radiograph of both hip joints was taken at the maximum abduction while the patient standing on the endoprosthetic leg. The second radiograph was taken after the patient returned to neutral position while standing on 2 legs. In the present study, the order in which the radiographs were taken differed from
previously reported studies. The radiographs were analysed using the method similar to that of Drinker and Murray. The adductive motion from abduction to a neutral position is within the range of inner bearing oscillation. Results. The outer head alignment changed from 23 degrees to 12 degrees in the patients with osteonecrosis. However, the valgus position of the outer head (36 degrees) remained unchanged in the patients with coxarthrosis standing on 2 legs in the neutral position. Conclusion. The self-centering mechanism of the bipolar endoprosthesis functioned in the patients with osteonecrosis, but did not work in the coxarthrosis group. Key words: hip prosthesis; radiography; range of motion, articular
Address correspondence and reprint requests to: Dr Hiroshi Tsumura, Medical Doctor, Department of Orthopedic Surgery, Faculty of Medicine, Oita University, 1-1 Idaigaoka, Hasama-machi Oitagun, Oita 879-5593, Japan. E-mail:
[email protected]
Vol. 13 No. 1, April 2005
Self-centering mechanism of bipolar hip endoprosthesis
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INTRODUCTION C
Bipolar hip arthroplasty was independently introduced by Bateman1 and Giliberty2 in 1974 as an alternative procedure to the use of Moore and Thompson unipolar devices. The femoral stem and head are the basic features of the bipolar hip endoprosthesis, which articulates at the polyethylene bearing insert and the polished metal outer head. The motion is likely to occur at the interface between the metal femoral head and the polyethylene insert as well as at the interface between the outer head and the acetabular. The basic concept of the bipolar hip endoprosthesis of both authors is the same; however, some distinct differences existed between their designs. In Bateman’s initial endoprosthesis, the centre of rotation of the inner head is located at the same point as the centre of rotation of the outer head, whereas in Giliberty’s endoprosthesis, the centre of rotation of the inner head is located distally to the centre of rotation of the outer head. This geometrical configuration of Giliberty’s endoprosthesis generates a varus-producing moment that causes a varus fixation of the outer head.3 The second-generation bipolar hip endoprosthesis was based on the clinical outcomes of the firstgeneration and were modified to have a self-centering mechanism. This mechanism is intended to generate a valgus-producing moment by setting the centre of rotation of the inner head proximal to the centre of rotation of the outer head. In addition to the above studies, couple reports have discussed in vivo component disassembly4,5; however, few have mentioned the self-centering mechanism. To determine whether the self-centering mechanism functions in vivo as intended, we took dynamic radiographs in a unique order and examined the bipolar component motion.
MATERIALS AND METHODS A total of 47 patients (40 females and 7 males) with 57 bipolar hip endoprostheses participated in this study. 41 and 16 bipolar hip endoprostheses were used respectively in the patients with coxarthrosis and in the patients with osteonecrosis of the femoral head (Ficat stage II or III). Reaming of the acetabulum was performed in all hips of the coxarthrosis group, but not performed in any hips of the group with femoral head osteonecrosis, leaving the acetabular cartilage intact. A combination of modular titanium stem and integral bipolar cup (Smith and Nephew, Memphis [TN], US) was used as the bipolar hip endoprosthesis
A
D
B
Figure 1 Angle measurements of bipolar hip endoprosthesis (Drinker and Murray6) were calculated as follows: (i) motion of the stem to the pelvis was measured by the change in the abduction angle of the leg between line A (from the acetabular index) and line B; (ii) motion of the outer head was measured by the change in the angle between line A and line C; (iii) motion of the stem to the outer head was determined by the change in the angle between line B and line C; and (iv) outer head alignment was expressed as the angle between line C and line D (the line connecting the bilateral inferior margin of the teardrops).
in all patients. The integral bipolar cup endoprosthesis has an integral inner femoral head, the diameter of which is 22 mm. To evaluate the self-centering mechanism, 2 radiographs of each endoprosthesis case were taken. The first anteroposterior radiograph of both hip joints was taken while the patient standing on the endoprosthetic leg and abducting the contralateral leg as much as possible. The second radiograph was taken after the patient returned to neutral position and standing on 2 legs. Analysis of the radiographs was performed using a method similar to that of Drinker and Murray.6 Motion of the stem to the pelvis was measured by the change in the abduction angle between line A (from the acetabular index) and line B. The motion of the outer head was measured by the
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Journal of Orthopaedic Surgery
H Tsumura et al. Table 1 Details of the patients
Patient profiles
Age (years) Male:female Body weight (kg) Follow-up period (years) Outer head size (mm) * †
Table 2 Angle changes of the leg and the stem to the outer head of the 2 groups
Coxarthrosis group (n=30) Mean, (SD)
Osteonecrosis group (n=11) Mean, (SD)
p value
59 (10) 2:26 54.9 (8.6) 2.0 (1.3)
54 (11) 3:7 56.9 (10.5) 1.9 (1.1)
0.26*† 0.10 0.44* 0.51*
41.7 (3.7)
45.5 (3.9)
0.01*
Abduction angle of the leg Motion of the stem to the outer head *
Abduction position Neutral position †
Osteonecrosis group (n=11) Mean, (SD) 11.8°i (3.1°i)
p value
12.5°i (6.5°i)
11.0°i (1.6°i)
