Does the self-centering mechanism of bipolar hip endoprosthesis really work in vivo?

Transcription

1 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 , Japan. htsumura@med.oita-u.ac.jp

2 Vol. 13 No. 1, April 2005 Self-centering mechanism of bipolar hip endoprosthesis 47 INTRODUCTION Bipolar hip arthroplasty was independently introduced by Bateman 1 and Giliberty 2 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 disassembly 4,5 ; 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 D A C B Figure 1 Angle measurements of bipolar hip endoprosthesis (Drinker and Murray 6 ) were calculated as follows: ( 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; (i motion of the outer head was measured by the change in the angle between line A and line C; (ii 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

3 48 H Tsumura et al. Journal of Orthopaedic Surgery Patient profiles Age (years) Male:female Body weight (kg) Follow-up period (years) Outer head size (mm) * Mann-Whitney U test Fisher s exact probability test Table 1 Details of the patients Coxarthrosis group (n=30) Mean, (SD) 59 (10) 2: (8.6) 2.0 (1.3) 41.7 (3.7) Osteonecrosis group (n=11) Mean, (SD) 54 (11) 3: (10.5) 1.9 (1.1) 45.5 (3.9) p value 0.26 * * 0.51* 0.01* Table 2 Angle changes of the leg and the stem to the outer head of the 2 groups Abduction angle of the leg Motion of the stem to the outer head * Mann-Whitney U test Coxarthrosis Osteonecrosis group (n=30) group (n=11) Mean, (SD) Mean, (SD) 12.7 i ( i ( i ( i ( 1. 6 p value < 0.87 * 0 <0.001* Abduction position Neutral position Table 3 Outer head alignment of the 2 groups * NA not applicable Repeated measures ANOVA Coxarthrosis Osteonecrosis group (n=30) group (n=11) Mean, (SD) Mean, (SD) 35.7 i ( i ( i ( i ( 3. 9 p value NA* <0.001 change in the angle between line A and line C, and the motion of the stem to the outer head was determined as the change in the angle between line B and line C. The outer head alignment was expressed as the angle between line C and line D, which is the line connecting the bilateral inferior margin of the teardrops (Fig. 1). Of the 57 endoprosthesis cases, 16 hips were unable to achieve the abduction position while standing on one leg. The Trendelenburg test results for these hips were positive and they were excluded from the analysis. Ultimately, 38 patients (33 female and 5 male) and a total of 41 bipolar hip endoprostheses were examined in this study. The details of the patients in these 2 groups are shown in Table 1. The mean age of the patients was 59 years (95% confidence interval [CI]; range, years) in the coxarthrosis group and 54 years (95% CI; range, years) in the osteonecrosis group. The mean follow-up period was 1.9 years in the coxarthrosis group and 2.0 years in the osteonecrosis group. The mean diameter of the outer head was 41.7 mm (95% CI; range, mm) in the coxarthrosis group and 45.5 mm (95% CI; range, mm) in the osteonecrosis group. The gender data were analysed using Fisher s exact probability test, and the data of outer head alignment were analysed using repeated measures analysis of variance. Other statistical analyses were performed with the non-parametric Mann-Whitney U test using Statistical Package for the Social Science (version 12.0J; SPSS Inc., Chicago [IL], US). RESULTS Figures 2 and 3 show the typical radiographs of a bipolar hip endoprosthesis in vivo. The motion behaviour of the outer head in a patient with coxarthrosis without articular cartilage is shown in Fig. 2, whereas Fig. 3 shows the motion behaviour in a patient with osteonecrosis with intact articular cartilage. In Fig. 2, the outer head moved as the pelvis was rotating, but in Fig. 3, the angle between the outer head and the stem did not change while the pelvis was rotating internally. This movement indicated that the self-centering mechanism did not work in coxarthrosis group, but it worked in the osteonecrosis group. The outer head did not impinge on the neck of the stem during this motion. The mean abduction angle of the leg in the coxarthrosis group was 12.7 (95% CI; range, ) and 11.8 (95% CI; range, ) in the osteonecrosis group. The leg abduction angle was not significantly different between the 2 groups (p=0.87). The mean angle of the stem motion to outer head during adduction from abduction position to neutral position was 12.5 (95% CI; range, ) in coxarthrosis group and 1.0 (95% CI; range, ) in osteonecrosis group (Table 2). The motion of the stem to outer head was significantly larger in coxarthrosis group than in osteonecrosis group (p<0.001). In the osteonecrosis group, the mean outer head alignment changed from 22.6 to 11.8 during adduction; however, in the coxarthrosis group, the outer head

4 Vol. 13 No. 1, April 2005 Self-centering mechanism of bipolar hip endoprosthesis 49 (a) (b) Figure 2 The motion behaviour of the outer head in a coxarthrotic patient without articular cartilage. The following radiographs show that the self-centering mechanism does not work: (a) anteroposterior radiograph of both hip joints at maximum abduction while standing on one leg, (b) anteroposterior radiograph in the neutral position while standing on 2 legs. (a) (b) Figure 3 The motion behaviour of the outer head in an osteonecrotic patient with intact articular cartilage. The following radiographs show the successful self-centering mechanism: (a) anteroposterior radiograph of both hip joints at maximum abduction while standing on one leg, (b) anteroposterior radiograph in the neutral position while standing on 2 legs. remained in valgus position (35.6 ) even in a neutral posture (Table 3). The motion of outer head in relation to the pelvis was significantly smaller in the coxarthrosis group than in the osteonecrosis patients (p<0.001). DISCUSSION Many investigators have studied the component motion of bipolar hip endoprosthesis. Krein and Chao 7 experimentally investigated the motion behaviour of several bipolar hip endoprostheses with and without the self-centering mechanism. Wetherell et al. 8 observed the function of the self-centering mechanism on cadaveric acetabula. These studies demonstrated that the self-centering mechanism could effectively generate a valgus-producing moment both theoretically and experimentally. Lung et al. 9 reported that some bipolar endoprostheses functioned as unipolar prostheses when the loading was increased to 2800 N. This change in functionality could be

5 50 H Tsumura et al. Journal of Orthopaedic Surgery because of the increased friction that occurred at the endoprostheses interfaces. Pickard et al. 10 examined the tribological condition of acetabular tissue after bipolar hip replacement. They reported that the friction coefficients of acetabular tissue in the majority of the bipolar hip replacement patients were significantly higher than the normal control cartilage. They concluded that the motion of the bipolar hip endoprosthesis should occur at the inner bearing. 10 The bipolar hip endoprosthesis motion is therefore a complex mechanism, as reported in the literature. 11,12 Drinker and Murray 6 performed motion studies 2 years and 3.4 years postoperatively in 13 patients who had been given Bateman s bipolar hip endoprosthesis for avascular necrosis. They found that patients had less inner bearing motion as time increased, and also a marked reduction in inner bearing motion on bearing weight, compared with the supine unloaded position. A report by Phillips 13 showed that the prosthesis functioned bipolarly with movement occurring primarily at the inner surface in 80% of the arthritis group, whereas in 75% of the fracture group, the prosthesis functioned unipolarly. He concluded that the motion behaviour of the prosthesis depended on the condition of the acetabular cartilage. In the above studies, the prosthesis motion in vivo was analysed using dynamic radiography or fluoroscopy, but the self-centering mechanism was not reported. In the present study, dynamic radiographs were used; however, the order that the radiographs were taken differed from the previous studies. We proposed that our method was unique and the order of the radiographs was important to evaluate the self-centering mechanism in vivo. In the opposite direction (from neutral position to abduction), it is uncertain as to whether the outer head motion is caused by the self-centering mechanism or by the engagement from the contact between the femoral neck and the rim of outer head. The motion during adduction from abduction to the neutral position is within the range of oscillation of the inner bearing and should therefore avoid contact with the femoral neck. The present study revealed that the outer head in the osteonecrotic patients with intact articular surface aligned in the direction of the applied load. Hence, we concluded that the self-centering mechanism of the bipolar endoprosthesis in the osteonecrosis group functioned successfully; however, the self-centering mechanism did not work in the coxarthrosis group. The mean diameter of the outer head in the osteonecrosis group was larger than that in the coxarthrosis group. Biomechanically, the frictional torque (coefficient of friction times the radius) of a large outer head should be greater than a smaller one. Furthermore, a large outer head tends to resist the effect of the self-centering mechanism. However, because the self-centering mechanism was most effective in our osteonecrosis group, the difference in the diameter of the outer head did not affect its motion in this study. Indeed, the increased acetabular friction in the coxarthrosis group appeared to be a more important biomechanical factor than the size of the bipolar head. Burton et al. 14 reported that reaming of the acetabulum resulted in a decrease in outer component motion and a relative increase in inner component articulation. Our study showed that the outer head motion decreased under high frictional coefficients at the outer head surface. The self-centering mechanism also could not become effective under high frictional coefficients at this outer head surface. The component disassembly, which was reported in Giliberty s endoprosthesis, 4,5 did not occur in our patients with coxarthrosis. However, the patients will have to be followed up carefully to monitor the integrity of the endoprosthesis. ACKNOWLEDGEMENTS The authors wish to thank Dr John Egan for his thoughtful review and Miss Ikuko Hirao for proofreading the manuscript. REFERENCES 1. Bateman JE. Single-assembly total hip prosthesis: preliminary report. Orthop Digest 1974;2: Giliberty RP. A new concept of bipolar endoprosthesis. Orthop Rev 1974;3: Giliberty RP. Hemiarthroplasty of the hip using a low-friction bipolar endoprosthesis. Clin Orthop 1983;175: Bhuller GS. Use of the Giliberty bipolar endoprosthesis in femoral neck fractures. Clin Orthop 1982;162: Morita H, Himeno S, Tsumura H, Kobayashi E, Mochida M, Takehi I, et al. Clinical study after bipolar hip prosthesis replacement [in Japanese]. Joint Surgery 1990;17: Drinker H, Murray WR. The universal proximal femoral endoprosthesis. A short-term comparison with conventional hemiarthroplasty. J Bone Joint Surg Am 1979;61:

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