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Long-term results of rotational total hip arthroplasty: radiological analysis
J Orthop Sci (2004) 9:126–134 DOI 10.1007/s00776-003-0762-3
Long-term results of rotational total hip arthroplasty: radiological analysis Koji Akasaki Department of Orthopaedic Surgery, Kumamoto City Hospital, 1-1-60 Koto, Kumamoto 862-0909, Japan
Abstract We developed a rotational total hip prosthesis that has a 30 mm diameter metal-covered head with a polyethylene liner with which it can rotate around the neck of the stem. Long-term results of the rotational total hip arthroplasty with cement were evaluated in 55 hips of 52 patients. The diagnosis was degenerative osteoarthritis in all patients. The mean follow-up was 11.2 years (range 5–19 years). Eight of thirty 7 mm thick acetabular components were revised 7.6–14.3 years (mean 10.4 years) afterward. Two of twenty five 9.5 mm thick acetabular components and two femoral components were revised at 12 and 15 years, respectively. The mean polyethylene wear in the 9.5 mm thick acetabular components was significantly less than that in the 7mm thick components. The mean polyethylene wear inside the rotational head removed during the revision surgeries was 0.01 mm in diameter and 0.03 mm in depth per year, respectively. Fifty percent of the patients with 7 mm thick acetabular components, 9.5 mm thick components, and femoral components had surviving prostheses at 13.4, 15.2, and 16.3 years, respectively. It is possible that the rotational system reduces the stress against acetabular and femoral components, but the 30 mm diameter head caused high friction torque and required at least 9.5mm thickness in the acetabular component. Key words Total hip arthroplasty · Rotational head · Cement
wore out after several years. Based on these results, we began using the UHMWPE head instead of the polyester material head in our institute in 1972, but the UHMWPE head also wore away. In 1976 we developed the hemiarthroplasty prosthesis, which had a metalcovered outer head with a UHMWPE liner, which allowed it to rotate around the neck of the stem. We used the prosthesis mostly in patients with a femoral neck fracture.1 Based on these experiences, we developed the rotational total hip prosthesis with Mizuho-Ika (Tokyo) in 1983; it has a 30mm diameter metal-covered head with a UHMWPE liner, which allows it to rotate around the shaft of the neck (Fig. 1). The concept of the prosthesis is that the rotational system reduces rotational friction against the acetabular and femoral components, and the 30mm diameter large head increases the stability and reduces the linear wear between the head and acetabular component. We used this prosthesis for patients with degenerative osteoarthritis in our institute from 1983 to 1989. The purpose of this study was to evaluate the longterm results of the rotational THA with respect to radiological changes, polyethylene wear, effect of the rotational system, and survival data.
Introduction Total hip arthroplasty (THA) in its current form began in 1961,3 when the concept of low-friction arthroplasty using ultra-high molecular weight polyethylene (UHMWPE) was tested by Charnley. In 1970 Weber developed the rotational THA, which had a rotational polyester head in combination with a metal acetabular component.23 However, the rotational polyester head
Offprint requests to: K. Akasaki Received: September 18, 2003 / Accepted: December 16, 2003
Materials and methods Between October 1983 and December 1989 a series of 91 Japanese patients underwent 95 rotational THAs with cement at Kumamoto City Hospital. At the latest follow-up, 72 of the patients were still alive, 10 had died, and 9 had been lost to follow-up. Of the 72 patients who were alive, 52 (55 hips) returned for examination. The 20 remaining patients were not able to have follow-up radiographs and were interviewed by telephone only. Sixteen patients had no or slight pain, and three patients had moderate or severe pain on waking. One patient
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a,b
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c Fig. 1. a Rotational total hip prosthesis. b Outer diameter of the acetabular component is 44 or 49 mm. The metal-covered head with an ultra-high molecular weight polyethylene (UHMWPE) liner rotates around the shaft of the neck. The
length of the cobalt-chromium alloy (COP) curved stem is 9.5 cm (small), 10.5 cm (medium), or 11.5 cm (large). c Design for medium stem
had had revision surgery elsewhere. The follow-up period ranged between 5 and 19 years (mean 11.2 years). The total numbers of hips examined at each follow-up were 55 at 5 years, 43 at 7 years, 34 at 10 years, 23 at 15 years, and 10 at 18 years. There were 4 men and 48 women. The average age of the patients at the time of the surgery was 64.3 years (range 31–82 years). The preoperative diagnosis was degenerative osteoarthritis secondary to congenital dysplasia of the hip in all patients. The mean height of the patients was 149.3 cm (range 134–189cm), and the mean weight was 52.2 kg (range 35–76 kg). The opposite hips showed early-stage osteoarthritis (OA) in two patients, advanced-stage OA in two, and end-stage OA in three, as defined by the classification of Japanese Orthopaedic Association (JOA).21 THA was performed in three patients, arthrodesis in one patient, and osteotomies in one. Fifteen patients had slight degenerative changes, and the other 25 patients had no degenerative change. The rotational total hip prosthesis was developed in our institute together with Mizuho-Ika and was used for all patients (Fig. 1). The acetabular component was made of UHMWPE (molecular weight 5.5 ⫻ 106) with an outer diameter of 44 or 49 mm and was 7.0 or 9.5 mm thick. The acetabular components were surrounded by three 0.55 mm diameter stainless steel wires as radiological markers. The 30 mm diameter metal-covered head with the UHMWPE liner rotated around the neck of the stem. The UHMWPE inside the metal-covered
head was 3 mm thick. The shaft of the neck had a 15-mm diameter and was 35mm long. The length of the cobaltchromium alloy (COP) curved stem was 9.5cm (small), 10.5cm (medium), or 11.5cm (large). The 44 mm diameter acetabular components were used for 30 hips, and the 49 mm diameter components were used for 25 hips, depending on the size of the acetabulum. There was no statistical difference in age or body weight of the patients given 44 or 49 mm diameter acetabular components (unpaired t-test). The small stems were used for 12 hips, the medium stems for 17 hips, and the large stems for 26 hips, depending on the size of the femoral canal. All operations were done using the anterolateral approach. The acetabulum was prepared with reamers, and 1 cm diameter holes were placed in the ilium, ischium, and pubis. The cement was packed manually, and the acetabular component was inserted into place. The femoral canal was prepared by removing all loose cancellous bone using rasps. The cement was packed digitally, and the stem was inserted into place. Postoperatively, the patients were managed with bed rest for a few days, followed by partial weight-bearing with crutches, then progressing to full weight-bearing as tolerated. Postoperatively, 16 patients had engaged in moderate manual labor, 30 had done light labor, and 6 had been semisedentary. Radiographic evaluations were undertaken using anteroposterior radiographs that had been obtained
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at each follow-up. Radiolucent lines between the cement and the bone were measured on the three acetabular zones described by DeLee and Charnley,6 and the seven femoral zones delineated by Gruen et al.8 The angle of the acetabular component was measured as the angle formed by the interteardrop line and the rim line of the acetabular component. The wear of the acetabular component was determined using the method described by Griffith et al.7 The UHMWPE wear inside the rotational system removed during the revision surgeries was measured using a caliper with an accuracy of 0.05 mm. Subsidence of the femoral stem was defined as a decrease of at least 5mm between the lesser trochanter and the collar of the stem. Osteolysis was defined as any nonlinear radiolucency at the bone–cement interface that was at least 5 mm long. Loosening of the acetabular component was classified according to the classification of Hodgkinson et al.10: type 0, no demarcation; type 1, demarcation of the outer one-third only; type 2, demarcation of the outer and middle thirds; type 3, complete demarcation; type 4, socket migration (change of socket position as judged on serial radiographs). Loosening of the femoral component was classified according to the category of Harris et al.9 Definite loosening was defined as definite evidence of migration, a radiolucent line at the stem– cement junction, a discernible shift in the position of the femoral component or the cement mantle, or a fracture of the stem or the cement. Probable loosening was defined as a continuous radiolucent line along the entire bone–cement interface. Possible loosening was defined as a radiolucent line at the bone–cement interface that occupied more than 50% but less than 100% of the circumference of the stem. The Kaplan-Meier methods, with corresponding confidence intervals, was used to evaluate survival of im-
K. Akasaki: Results of rotational THA
plants with regard to type 4 socket demarcation, definite loosening of the femoral component, or revision. Statistical analyses were made using the unpaired t-test and the chi-square test. P values of less than 0.05 were considered significant.
Results Acetabular components The radiolucent lines that were ⬎1mm in width around the acetabular component are summarized in Fig. 2. The radiolucent lines increased especially in zone 1 after 5 years of surgery (Fig. 3). The mean position angle of 44mm diameter acetabular components was 47.5∞ (range 38∞–58∞) at surgery, 50.3∞ (range 38∞–68∞) 10 years after surgery, and 52.8∞ (range 40∞–60∞) at 15 years. The mean position angle of 49mm diameter acetabular components was 46.8∞ (range 38∞–54∞) at surgery, 48.8∞ (range 37∞–63∞) 10 years after surgery, and 50.0∞ (range 37∞–65∞) at 15 years. The angles in both 44 and 49 mm diameter components gradually increased, but there was no statistical difference between the two components during the follow-up period (unpaired t-test). There is no statistical difference in the acetabular angles between the 10 hips that underwent revision surgery and others until 7 years after surgery (unpaired t-test). The amount of polyethylene wear in acetabular components is summarized in Fig. 4. The mean wear in 9.5mm thick components was significantly lower than that in 7.0mm thick components at 5 years (P ⬍ 0.01) and 10 years (P ⬍ 0.05, unpaired t-test). The radiographic loosening was determined according to the classification of Hodgkinson et al.10 (Table 1). In 44 mm diameter components, 2 of 17 hips had type 4 demarcation (11.8%) at 10 years. At 15 years, one of six
Fig. 2. Incidence of radiolucent lines that were ⬎1 mm wide around the 7.0 mm and 9.5 mm thick acetabular components at 5, 10, and 15 years after surgery. The acetabular component was separated into three zones, as described by DeLee and Charnley6
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hips had type 4 demarcation (16.7%). The wires around the 44 mm diameter components broke in 11 hips (36.7%); 8 of the 11 components (72.7%) were classified as type 4 demarcation and were revised with 49 mm diameter components and acetabular reinforcement shells at a mean 10.4 years after surgery (range 7.6–14.3 years) (Table 2, Fig. 5). For the 49 mm diameter components, 1 of 13 hips had type 4 demarcation (7.7%) at 10 years; and at 15 years 1 of 7 hips had type 4 demarcation (14.3%). The wires around the 49 mm diameter components broke in four hips (16%); none of these components had type 4 demarcation or underwent revision surgery. The other two hips were revised with other components because of loosening of the type 4 socket
demarcation at 12 years and the definite loosening of femoral components at 15 years, respectively (Table 2). Femoral components The radiolucent lines that were ⬎1mm in width around the femoral components are summarized in Fig. 6. The radiolucent lines increased especially in zones 1, 2, and 7 after 10 years (Fig. 7). Eight hips showed cancellous condensation around the distal part of the stem, and six of the eight hips showed atrophy in the proximal area. Four hips had cortical hypertrophy (Fig. 3). Altogether, 3 of 34 hips had more than 7 mm of subsidence at 10 years; and at 15 years, 3 of 23 hips had more than 7mm of subsidence. Three hips had osteolysis (Fig. 7). Radiographic loosening was determined according to Harris et al.9 (Table 3). A total of 3 of 34 hips had definite loosening (8.8%) at 10 years; 2 of 21 hips had probable loosening (9.5%) and 3 of 21 had definite
a,b Fig. 3. Radiographs of rotational total hip arthroplasty (THA) with 9.5 mm thick acetabular component. a At 6 months after surgery. b At 18 years after surgery. Acetabular component shows type 1 demarcation. Femoral component shows no loosening and has cortical hypertrophy
Fig. 4. Mean amount of polyethylene wear in 7.0 mm and 9.5 mm thick acetabular components at 5, 10, and 15 years after surgery. Values are means ⫾ SD. *P ⬍ 0.01, **P ⬍ 0.05, ***not significant: unpaired t-test
Table 1. Socket demarcation (Hodgkinson’s classification) Socket demarcation
At 5 years
At 10 years
At 15 years
Thick 7.0 mm Type 0 Type 1 Type 2 Type 3 Type 4 Total
25 (83.3%) 6 (19.4%) 0 0 0 30
7 (41.2%) 5 (29.4%) 2 (11.8%) 1 (5.9%) 2 (11.8%) 17
2 (33.3%) 1 (16.7%) 2 (33.3%) 0 1 (16.7%) 6
Thick 9.5 mm Type 0 Type 1 Type 2 Type 3 Type 4 Total
22 (88%) 2 (8%) 1 (4%) 0 0 25
8 (61.5%) 2 (15.4%) 1 (7.7%) 1 (7.7%) 1 (7.7%) 13
2 (28.6%) 2 (28.6%) 1 (14.3%) 1 (14.3%) 1 (14.3%) 7
Results are the number of hips
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K. Akasaki: Results of rotational THA
c
a,b Fig. 5. Radiographs of rotational THA with the 7.0mm thick acetabular component. a At 6 months after surgery. b At 11 years after surgery. The acetabular component broke and was
revised with a 49 mm diameter component and acetabular reinforcement shell. c At 17 years after the first surgery the femoral component shows no loosening
Table 2. Revision of total hip arthroplasty Patient
Thickness of socket (mm)
Years of revision
Reason for revision
1
7.0
7.6
Loose socket
2
7.0
8.0
Loose socket
3
7.0
8.3
Loose socket
4
7.0
8.8
Loose socket
5
7.0
9.4
Loose socket
6
7.0
10.7
Loose socket
7
7.0
12.2
Loose socket
8
7.0
14.3
Loose socket
9 10
9.5 9.5
12.0 15.0
Loose socket and stem Loose stem
Methods of revision Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only PCA PCA
PCA, porous coated anatomic prosthesis
loosening (14.3%) at 15 years. Two hips underwent revision surgery of the femoral components at 12 and 15 years, respectively (Table 2). No hip had a dislocation. Rotational system In the rotational system, the inner diameter of the head was 15.0mm, and there was 1mm clearance between the lower edge of the rotational head and the upper edge of
Table 3. Stem loosening (Harris’ category) Stem loosening
At 5 years
At 10 years
At 15 years
None Possible Probable Definite Total
49 (89.1%) 4 (72.7%) 2 (36.4%) 0 55
26 (76.5%) 5 (14.7%) 0 3 (8.8%) 34
12 (57.1%) 4 (19.0%) 2 (9.5%) 3 (14.3%) 21
Results are the number of hips
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Fig. 6. Incidence of radiolucent lines that were ⬎1 mm wide around the femoral components at 5, 10, and 15 years after surgery. The femoral component was separated into seven zones, as described by Gruen et al.8 Table 4. UHMWPE wear inside the rotational head
Patient A B C D E F G H Mean
Period before revision (years)
Inner diameter (mm)
Clearance (mm)
7.6 8.3 8.8 9.4 10.7 12.0 12.2 14.3 10.4
15.05 15.05 15.05 15.05 15.10 15.05 15.15 15.10 15.08
0.9 0.8 0.8 0.7 0.7 0.6 0.4 0.5 0.67
UHMWPE, ultra-high molecular weight polyethylene
a,b Fig. 7. Radiographs of rotational THA with a 7.0 mm thick acetabular component. a At 6 months after surgery. b At 17 years after surgery. Acetabular component shows type 2 demarcation. Femoral component shows radiolucent line at zones 1–2, osteolysis at zones 3–5, and possible loosening
the neck collar. At the latest follow-up there was no radiologically detectable wear in the clearance between the head and the collar. In eight patients who underwent revision surgery for the acetabular component, the rotational heads were removed and measured for inner diameter and clearance between the head and the collar directly with a caliper (Table 4). The mean wear inside the side wall of rotational head was 0.01 mm/year, and the mean decrease in the clearance between the head and the collar was 0.03 mm/year. Survivorship analysis The Kaplan-Meier method, with corresponding confidence intervals, was used to calculate the probability of
revision during the follow-up, revision of the acetabular or femoral component, and radiographic evidence of type 4 socket demarcation or definite loosening of the femoral component (Fig. 8). The 80% survivals of the 7.0 mm thick acetabular component, the 9.5 mm thick component, and the femoral component were 8.4, 11.0, and 12.2 years, respectively. The 50% survivals of the 7.0 mm thick acetabular component, the 9.5 mm thick component, and the femoral component were 13.4, 15.2, and 16.3 years, respectively.
Discussion We reported, apparently for the first time, the long-term results of a THA that has a rotational head system. The concept of the prosthesis is that the rotational system reduces the rotational torque against the acetabular and femoral components, and the 30mm diameter large head increases stability and reduces linear wear inside the joint.
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K. Akasaki: Results of rotational THA
a,b
c Fig. 8. Kaplan-Meier survival curve, with 95% confidence intervals, for the 7.0 mm thick acetabular component (a), the 9.5 mm thick component (b), and the femoral component
(c) with regard to type 4 socket demarcation, definite loosening of the femoral component, or revision surgery as the endpoint
In the present study, the rotational system radiologically showed no detectable wear. However, for the eight patients with revision surgeries, the mean wear of the top wall and side wall inside the head was 0.03mm/ year and 0.01mm/year, respectively. The wear inside the rotational head was much less than that in the acetabular components. However, the wear suggests that the rotational head rotated or shared stress with the acetabular and femoral components and hence could reduce the incidence of loosening. The mean polyethylene wear in 9.5mm thick acetabular components was significantly less than that in 7.0mm thick components at 5 years (P ⬍ 0.01) and 10 years (P ⬍ 0.05). Oonishi et al. also reported that the acetabular component requires more than 9mm thickness to decrease the polyethylene wear.17 The wires around the 7.0mm thick acetabular components broke in 11 of 30 hips (36.7%), whereas those around the 9.5mm thick components broke in only 4 of 25 hips (16%). It is possible that the thin acetabular components caused creeping deformity, which led to enhance polyethylene wear. The mean polyethylene wear in 9.5mm thick acetabular components was 0.10mm/year at 10 years and 0.11mm/year at 15 years. The mean rates of UHMWPE wear in Charnley low-friction arthroplasties with the 22-mm head were reported to range from 0.07 to 0.15 mm/year, with most rates ranging from 0.12 to 0.15mm/year.4,5,13,24 Livermore et al. reported that the 28- and 32-mm heads had less linear wear than did the 22-mm head.13 The mean rates of linear wear in our study were similar to their rates for the 32-mm head and less than their rates for the 22-mm head in previous reports. Therefore, the rotational system might not decrease the acetabular wear by reducing the rotational stress on the acetabular components. The revision rate of 9.5 mm thick acetabular components was 0% at 10 years and 22.2% at 15 years. The rate for 7.0 mm thick components was 19.0% at 10 years and 53.3% at 15 years. The revision rate for 9.5mm thick components was significantly lower than that of 7.0mm thick components at 10 years (P ⬍ 0.01, chi-
square test). To examine the effect of the rotational head on loosening of the acetabular component, we first compared the revision rate for the acetabular component using the Müller prosthesis, which has a 32-mm head. The revision rate for the 9.5mm thick components in combination with a 30-mm head at 10 years in this study was lower than that for the Müller prosthesis at around 10 years.18,22 Sutherland et al. reported that the revision rate was 12%,22 and Reikerås reported it to be 8%.18 These results suggest that the rotational system decreases friction stress and the incidence of loosening in cases of 9.5mm thick components. However, the revision rate for 7.0mm thick components was higher than that in previous reports. In our study, the radiolucent lines were seen especially in zone 1 with a 7.0mm thick component after 5 years. This pattern is similar to that using the Charnley prosthesis.6 In addition, the wires around the acetabular component broke in all eight revision cases in which there were 7.0mm thick components (Fig. 5). In contrast, the wires in two revision cases with 9.5mm thick components did not break. One possibility is that the 7.0mm thick components had more creeping deformity around zone 1 than did the 9.5mm thick components, which caused the radiolucent lines and loosening. Another possibility is that the high friction torque caused by the large 30-mm head increased loosening. Charnley stated that it was extremely important to minimize friction torque during THA by using the smaller femoral head diameter.3 In combination with the large external diameter of the acetabular component, this should reduce the friction torque at the bone–cement interface. The 7.0mm thickness of the acetabular component in combination with the 30-mm (large) femoral head may be too thin to resist the friction torque at the bone– cement interface Therefore, the high friction torque could affect the bone–cement interface and increase the incidence of loosening. For the same reason, the revision rate of the 32-mm femoral head was reported to be higher than that of the 22-mm femoral head.15,18–20,22,24 In this study, even when
K. Akasaki: Results of rotational THA
using the 9.5 mm thick components in combination with a 30-mm rotational head, the revision rate was still higher than that using the Charnley prosthesis.19,20,24 These lines of evidence suggest that the rotational system decreases the friction stress and incidence of loosening, although the femoral head size has a more important effect on the friction stress against the acetabular component than does the rotational head system. One theoretical advantage of using a larger femoral head is that it decreases the linear wear and enhances the stability of the joint, even though it increases the friction torque. In this study, there were no hip dislocations during the follow-up period. The femoral component in this study was designed with a curve, compared with the Charnley prosthesis (Fig. 1). No stem had to be revised at 10 years, and only 2 of 23 stems (8.7%) were revised at 15 years. This result compares favorably with those in previous reports of the Müller curved-stem prosthesis.18,22 Sutherland et al. reported a revision rate of 20%22 and Reikerås one of 16.7%18 at about 10 years after surgery. When compared with the Charnley prosthesis using the firstgeneration cementing technique, the revision rate at 10– 15 years in this study was similar to that in previous reports, which ranged from 4% to 9%.2,11,12,16 The frequency and location of radiolucent lines were similar to those seen with the Charnley prosthesis.8 However, the revision rate was higher than that with the Charnley prosthesis using the improved cementing technique.14,24 The Müller curved-stem prosthesis was reported to have a higher incidence of loosening than the Charnley prosthesis.18,22 Possible explanations include the curve, the thin medial edge, and the small diameter in the Müller stem. Other factors, including the cementing technique, bone quality, and stem-insert position, are also important for stable fixation. In this study, it is possible that the rotational system reduced the rotational stress against the stem and decreased the incidence of radiolucency and loosening.
Conclusions The rotational system inside the head during THA seems to share the rotational torque against the acetabular and femoral components, thereby reducing the incidence of loosening. The 30 mm diameter head increased the stability and did not dislocate in all hips. The linear wear in the acetabular components were similar to that using a 32 mm diameter head. The 7.0 mm thick acetabular component was too thin to resist the friction torque caused by the large 30 mm diameter head, even though the rotational system reduced the stress. When compared to previous reports of the Charnley prosthesis, the revision rate of acetabular
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components was higher, suggesting that the femoral head size has a more important effect on reducing the friction stress on the acetabular component than does the rotational head system. It is possible to develop a lower-friction total hip prosthesis by improving the design and materials of the rotational head system or the bipolar system. Acknowledgments. The author sincerely thanks Dr. K. Takagi for his guidance with this work and Dr. K. Hirota for his help in the design of the prosthesis.
References 1. Akasaki K, Hirota K, Ise K, et al. Proximal migration in hip endoprosthesis with rotational system. Orthoped Traumatol 1984; 32:319–23 (in Japanese). 2. Brady LP, McCutchen JW. A ten-year follow-up study of 170 Charnley total hip arthroplasties. Clin Orthop 1986;211:51–4. 3. Charnley J. Arthroplasty of the hip: a new operation. Lancet 1961; 1:1129–32. 4. Charnley J, Cupic Z. The nine and ten year results of the lowfriction arthroplasty of the hip. Clin Orthop 1973;95:9–25. 5. Charnley J, Halley DK. Rate of wear in total hip replacement. Clin Orthop 1975;112:170–9. 6. DeLee JG, Charnley J. Radiological demarcation of cemented sockets in total hip replacement. Clin Orthop 1976;121:20–32. 7. Griffith MJ, Seidenstein MK, Williams D, et al. Socket wear in Charnley low friction arthroplasty of the hip. Clin Orthop 1978; 137:37–47. 8. Gruen TA, McNeice GM, Amstutz HC. “Modes of failure” of cemented stem-type femoral components: a radiographic analysis of loosening. Clin Orthop 1979;141:17–27. 9. Harris WH, McCarthy JC, O’Neill DA. Femoral component loosening using contemporary techniques of femoral cement fixation. J Bone Joint Surg Am 1982;64:1063–7. 10. Hodgkinson JP, Shelley P, Wroblewski BM. The correlation between the roentgenographic appearance and operative findings at the bone-cement junction of the socket in Charnley low friction arthroplasties. Clin Orthop 1988;228:105–9. 11. Johnston RC, Crowninshield RD. Roentgenologic results of total hip arthroplasty: a ten-year follow-up study. Clin Orthop 1983; 181:92–8. 12. Kavanagh BF, Dewitz MA, Ilstrup DM, et al. Charnley total hip arthroplasty with cement: fifteen-year results. J Bone Joint Surg Am 1989;71:1496–503. 13. Livermore J, Ilstrup D, Morrey B. Effect of femoral head size on wear of the polyethylene acetabular component. J Bone Joint Surg Am 1990;72:518–28. 14. Madey SM, Callaghan JJ, Olejniczak JP, et al. Charnley total hip arthroplasty use of improved techniques of cementing. J Bone Joint Surg Am 1997;79:53–64. 15. Morrey BF, Ilstrup MS. Size of the femoral head and acetabular revision in total hip-replacement arthroplasty. J Bone Joint Surg Am 1989;71:50–5. 16. Older J. Low-friction arthroplasty of the hip: a 10–12-year followup study. Clin Orthop 1986;211:36–42. 17. Oonishi H, Murata N, Kushitani S, et al. Polyethylene cup wear behavior of total hip prostheses. Orthop Ceramic Implants 1996;16:56–60. 18. Reikerås O. Ten-year follow-up of Müller hip replacements. Acta Orthop Scand 1982;53:919–22. 19. Ritter MA, Stringer EA, Littrell DA, et al. Correlation of prosthetic femoral head size and/or design with longevity of total hip arthroplasty. Clin Orthop 1983;176:252–7.
134 20. Schulte KR, Callaghan JJ, Kelley SS, et al. The outcome of Charnley total hip arthroplasty with cement after a minimum twenty-year follow-up. J Bone Joint Surg Am 1993;75:961–75. 21. Shima Y, Ueno R, Tamaki T, et al. Outcome measures of coxarthrosis. Nippon Seikeigekagakkai Zasshi (J Jpn Orthop Assoc) 1971;45:813–33 (in Japanese). 22. Sutherland CJ, Wilde AH, Borden LS, et al. A ten-year followup of one hundred consecutive Müller curved-stem total hip-
K. Akasaki: Results of rotational THA replacement arthroplasties. J Bone Joint Surg Am 1982;64:970– 82. 23. Weber BG. Die rotations-totalendoprothese des hüftgelenkes. Z Orthop 1970;107:304–15. 24. Wroblewski BM, Siney BA. Charnley low-friction arthroplasty of the hip. Clin Orthop 1993;292:191–201.
Long-term results of rotational total hip arthroplasty: radiological analysis Koji Akasaki Department of Orthopaedic Surgery, Kumamoto City Hospital, 1-1-60 Koto, Kumamoto 862-0909, Japan
Abstract We developed a rotational total hip prosthesis that has a 30 mm diameter metal-covered head with a polyethylene liner with which it can rotate around the neck of the stem. Long-term results of the rotational total hip arthroplasty with cement were evaluated in 55 hips of 52 patients. The diagnosis was degenerative osteoarthritis in all patients. The mean follow-up was 11.2 years (range 5–19 years). Eight of thirty 7 mm thick acetabular components were revised 7.6–14.3 years (mean 10.4 years) afterward. Two of twenty five 9.5 mm thick acetabular components and two femoral components were revised at 12 and 15 years, respectively. The mean polyethylene wear in the 9.5 mm thick acetabular components was significantly less than that in the 7mm thick components. The mean polyethylene wear inside the rotational head removed during the revision surgeries was 0.01 mm in diameter and 0.03 mm in depth per year, respectively. Fifty percent of the patients with 7 mm thick acetabular components, 9.5 mm thick components, and femoral components had surviving prostheses at 13.4, 15.2, and 16.3 years, respectively. It is possible that the rotational system reduces the stress against acetabular and femoral components, but the 30 mm diameter head caused high friction torque and required at least 9.5mm thickness in the acetabular component. Key words Total hip arthroplasty · Rotational head · Cement
wore out after several years. Based on these results, we began using the UHMWPE head instead of the polyester material head in our institute in 1972, but the UHMWPE head also wore away. In 1976 we developed the hemiarthroplasty prosthesis, which had a metalcovered outer head with a UHMWPE liner, which allowed it to rotate around the neck of the stem. We used the prosthesis mostly in patients with a femoral neck fracture.1 Based on these experiences, we developed the rotational total hip prosthesis with Mizuho-Ika (Tokyo) in 1983; it has a 30mm diameter metal-covered head with a UHMWPE liner, which allows it to rotate around the shaft of the neck (Fig. 1). The concept of the prosthesis is that the rotational system reduces rotational friction against the acetabular and femoral components, and the 30mm diameter large head increases the stability and reduces the linear wear between the head and acetabular component. We used this prosthesis for patients with degenerative osteoarthritis in our institute from 1983 to 1989. The purpose of this study was to evaluate the longterm results of the rotational THA with respect to radiological changes, polyethylene wear, effect of the rotational system, and survival data.
Introduction Total hip arthroplasty (THA) in its current form began in 1961,3 when the concept of low-friction arthroplasty using ultra-high molecular weight polyethylene (UHMWPE) was tested by Charnley. In 1970 Weber developed the rotational THA, which had a rotational polyester head in combination with a metal acetabular component.23 However, the rotational polyester head
Offprint requests to: K. Akasaki Received: September 18, 2003 / Accepted: December 16, 2003
Materials and methods Between October 1983 and December 1989 a series of 91 Japanese patients underwent 95 rotational THAs with cement at Kumamoto City Hospital. At the latest follow-up, 72 of the patients were still alive, 10 had died, and 9 had been lost to follow-up. Of the 72 patients who were alive, 52 (55 hips) returned for examination. The 20 remaining patients were not able to have follow-up radiographs and were interviewed by telephone only. Sixteen patients had no or slight pain, and three patients had moderate or severe pain on waking. One patient
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a,b
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c Fig. 1. a Rotational total hip prosthesis. b Outer diameter of the acetabular component is 44 or 49 mm. The metal-covered head with an ultra-high molecular weight polyethylene (UHMWPE) liner rotates around the shaft of the neck. The
length of the cobalt-chromium alloy (COP) curved stem is 9.5 cm (small), 10.5 cm (medium), or 11.5 cm (large). c Design for medium stem
had had revision surgery elsewhere. The follow-up period ranged between 5 and 19 years (mean 11.2 years). The total numbers of hips examined at each follow-up were 55 at 5 years, 43 at 7 years, 34 at 10 years, 23 at 15 years, and 10 at 18 years. There were 4 men and 48 women. The average age of the patients at the time of the surgery was 64.3 years (range 31–82 years). The preoperative diagnosis was degenerative osteoarthritis secondary to congenital dysplasia of the hip in all patients. The mean height of the patients was 149.3 cm (range 134–189cm), and the mean weight was 52.2 kg (range 35–76 kg). The opposite hips showed early-stage osteoarthritis (OA) in two patients, advanced-stage OA in two, and end-stage OA in three, as defined by the classification of Japanese Orthopaedic Association (JOA).21 THA was performed in three patients, arthrodesis in one patient, and osteotomies in one. Fifteen patients had slight degenerative changes, and the other 25 patients had no degenerative change. The rotational total hip prosthesis was developed in our institute together with Mizuho-Ika and was used for all patients (Fig. 1). The acetabular component was made of UHMWPE (molecular weight 5.5 ⫻ 106) with an outer diameter of 44 or 49 mm and was 7.0 or 9.5 mm thick. The acetabular components were surrounded by three 0.55 mm diameter stainless steel wires as radiological markers. The 30 mm diameter metal-covered head with the UHMWPE liner rotated around the neck of the stem. The UHMWPE inside the metal-covered
head was 3 mm thick. The shaft of the neck had a 15-mm diameter and was 35mm long. The length of the cobaltchromium alloy (COP) curved stem was 9.5cm (small), 10.5cm (medium), or 11.5cm (large). The 44 mm diameter acetabular components were used for 30 hips, and the 49 mm diameter components were used for 25 hips, depending on the size of the acetabulum. There was no statistical difference in age or body weight of the patients given 44 or 49 mm diameter acetabular components (unpaired t-test). The small stems were used for 12 hips, the medium stems for 17 hips, and the large stems for 26 hips, depending on the size of the femoral canal. All operations were done using the anterolateral approach. The acetabulum was prepared with reamers, and 1 cm diameter holes were placed in the ilium, ischium, and pubis. The cement was packed manually, and the acetabular component was inserted into place. The femoral canal was prepared by removing all loose cancellous bone using rasps. The cement was packed digitally, and the stem was inserted into place. Postoperatively, the patients were managed with bed rest for a few days, followed by partial weight-bearing with crutches, then progressing to full weight-bearing as tolerated. Postoperatively, 16 patients had engaged in moderate manual labor, 30 had done light labor, and 6 had been semisedentary. Radiographic evaluations were undertaken using anteroposterior radiographs that had been obtained
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at each follow-up. Radiolucent lines between the cement and the bone were measured on the three acetabular zones described by DeLee and Charnley,6 and the seven femoral zones delineated by Gruen et al.8 The angle of the acetabular component was measured as the angle formed by the interteardrop line and the rim line of the acetabular component. The wear of the acetabular component was determined using the method described by Griffith et al.7 The UHMWPE wear inside the rotational system removed during the revision surgeries was measured using a caliper with an accuracy of 0.05 mm. Subsidence of the femoral stem was defined as a decrease of at least 5mm between the lesser trochanter and the collar of the stem. Osteolysis was defined as any nonlinear radiolucency at the bone–cement interface that was at least 5 mm long. Loosening of the acetabular component was classified according to the classification of Hodgkinson et al.10: type 0, no demarcation; type 1, demarcation of the outer one-third only; type 2, demarcation of the outer and middle thirds; type 3, complete demarcation; type 4, socket migration (change of socket position as judged on serial radiographs). Loosening of the femoral component was classified according to the category of Harris et al.9 Definite loosening was defined as definite evidence of migration, a radiolucent line at the stem– cement junction, a discernible shift in the position of the femoral component or the cement mantle, or a fracture of the stem or the cement. Probable loosening was defined as a continuous radiolucent line along the entire bone–cement interface. Possible loosening was defined as a radiolucent line at the bone–cement interface that occupied more than 50% but less than 100% of the circumference of the stem. The Kaplan-Meier methods, with corresponding confidence intervals, was used to evaluate survival of im-
K. Akasaki: Results of rotational THA
plants with regard to type 4 socket demarcation, definite loosening of the femoral component, or revision. Statistical analyses were made using the unpaired t-test and the chi-square test. P values of less than 0.05 were considered significant.
Results Acetabular components The radiolucent lines that were ⬎1mm in width around the acetabular component are summarized in Fig. 2. The radiolucent lines increased especially in zone 1 after 5 years of surgery (Fig. 3). The mean position angle of 44mm diameter acetabular components was 47.5∞ (range 38∞–58∞) at surgery, 50.3∞ (range 38∞–68∞) 10 years after surgery, and 52.8∞ (range 40∞–60∞) at 15 years. The mean position angle of 49mm diameter acetabular components was 46.8∞ (range 38∞–54∞) at surgery, 48.8∞ (range 37∞–63∞) 10 years after surgery, and 50.0∞ (range 37∞–65∞) at 15 years. The angles in both 44 and 49 mm diameter components gradually increased, but there was no statistical difference between the two components during the follow-up period (unpaired t-test). There is no statistical difference in the acetabular angles between the 10 hips that underwent revision surgery and others until 7 years after surgery (unpaired t-test). The amount of polyethylene wear in acetabular components is summarized in Fig. 4. The mean wear in 9.5mm thick components was significantly lower than that in 7.0mm thick components at 5 years (P ⬍ 0.01) and 10 years (P ⬍ 0.05, unpaired t-test). The radiographic loosening was determined according to the classification of Hodgkinson et al.10 (Table 1). In 44 mm diameter components, 2 of 17 hips had type 4 demarcation (11.8%) at 10 years. At 15 years, one of six
Fig. 2. Incidence of radiolucent lines that were ⬎1 mm wide around the 7.0 mm and 9.5 mm thick acetabular components at 5, 10, and 15 years after surgery. The acetabular component was separated into three zones, as described by DeLee and Charnley6
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hips had type 4 demarcation (16.7%). The wires around the 44 mm diameter components broke in 11 hips (36.7%); 8 of the 11 components (72.7%) were classified as type 4 demarcation and were revised with 49 mm diameter components and acetabular reinforcement shells at a mean 10.4 years after surgery (range 7.6–14.3 years) (Table 2, Fig. 5). For the 49 mm diameter components, 1 of 13 hips had type 4 demarcation (7.7%) at 10 years; and at 15 years 1 of 7 hips had type 4 demarcation (14.3%). The wires around the 49 mm diameter components broke in four hips (16%); none of these components had type 4 demarcation or underwent revision surgery. The other two hips were revised with other components because of loosening of the type 4 socket
demarcation at 12 years and the definite loosening of femoral components at 15 years, respectively (Table 2). Femoral components The radiolucent lines that were ⬎1mm in width around the femoral components are summarized in Fig. 6. The radiolucent lines increased especially in zones 1, 2, and 7 after 10 years (Fig. 7). Eight hips showed cancellous condensation around the distal part of the stem, and six of the eight hips showed atrophy in the proximal area. Four hips had cortical hypertrophy (Fig. 3). Altogether, 3 of 34 hips had more than 7 mm of subsidence at 10 years; and at 15 years, 3 of 23 hips had more than 7mm of subsidence. Three hips had osteolysis (Fig. 7). Radiographic loosening was determined according to Harris et al.9 (Table 3). A total of 3 of 34 hips had definite loosening (8.8%) at 10 years; 2 of 21 hips had probable loosening (9.5%) and 3 of 21 had definite
a,b Fig. 3. Radiographs of rotational total hip arthroplasty (THA) with 9.5 mm thick acetabular component. a At 6 months after surgery. b At 18 years after surgery. Acetabular component shows type 1 demarcation. Femoral component shows no loosening and has cortical hypertrophy
Fig. 4. Mean amount of polyethylene wear in 7.0 mm and 9.5 mm thick acetabular components at 5, 10, and 15 years after surgery. Values are means ⫾ SD. *P ⬍ 0.01, **P ⬍ 0.05, ***not significant: unpaired t-test
Table 1. Socket demarcation (Hodgkinson’s classification) Socket demarcation
At 5 years
At 10 years
At 15 years
Thick 7.0 mm Type 0 Type 1 Type 2 Type 3 Type 4 Total
25 (83.3%) 6 (19.4%) 0 0 0 30
7 (41.2%) 5 (29.4%) 2 (11.8%) 1 (5.9%) 2 (11.8%) 17
2 (33.3%) 1 (16.7%) 2 (33.3%) 0 1 (16.7%) 6
Thick 9.5 mm Type 0 Type 1 Type 2 Type 3 Type 4 Total
22 (88%) 2 (8%) 1 (4%) 0 0 25
8 (61.5%) 2 (15.4%) 1 (7.7%) 1 (7.7%) 1 (7.7%) 13
2 (28.6%) 2 (28.6%) 1 (14.3%) 1 (14.3%) 1 (14.3%) 7
Results are the number of hips
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K. Akasaki: Results of rotational THA
c
a,b Fig. 5. Radiographs of rotational THA with the 7.0mm thick acetabular component. a At 6 months after surgery. b At 11 years after surgery. The acetabular component broke and was
revised with a 49 mm diameter component and acetabular reinforcement shell. c At 17 years after the first surgery the femoral component shows no loosening
Table 2. Revision of total hip arthroplasty Patient
Thickness of socket (mm)
Years of revision
Reason for revision
1
7.0
7.6
Loose socket
2
7.0
8.0
Loose socket
3
7.0
8.3
Loose socket
4
7.0
8.8
Loose socket
5
7.0
9.4
Loose socket
6
7.0
10.7
Loose socket
7
7.0
12.2
Loose socket
8
7.0
14.3
Loose socket
9 10
9.5 9.5
12.0 15.0
Loose socket and stem Loose stem
Methods of revision Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only Socket 9.5 mm thick and reinforcement only PCA PCA
PCA, porous coated anatomic prosthesis
loosening (14.3%) at 15 years. Two hips underwent revision surgery of the femoral components at 12 and 15 years, respectively (Table 2). No hip had a dislocation. Rotational system In the rotational system, the inner diameter of the head was 15.0mm, and there was 1mm clearance between the lower edge of the rotational head and the upper edge of
Table 3. Stem loosening (Harris’ category) Stem loosening
At 5 years
At 10 years
At 15 years
None Possible Probable Definite Total
49 (89.1%) 4 (72.7%) 2 (36.4%) 0 55
26 (76.5%) 5 (14.7%) 0 3 (8.8%) 34
12 (57.1%) 4 (19.0%) 2 (9.5%) 3 (14.3%) 21
Results are the number of hips
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Fig. 6. Incidence of radiolucent lines that were ⬎1 mm wide around the femoral components at 5, 10, and 15 years after surgery. The femoral component was separated into seven zones, as described by Gruen et al.8 Table 4. UHMWPE wear inside the rotational head
Patient A B C D E F G H Mean
Period before revision (years)
Inner diameter (mm)
Clearance (mm)
7.6 8.3 8.8 9.4 10.7 12.0 12.2 14.3 10.4
15.05 15.05 15.05 15.05 15.10 15.05 15.15 15.10 15.08
0.9 0.8 0.8 0.7 0.7 0.6 0.4 0.5 0.67
UHMWPE, ultra-high molecular weight polyethylene
a,b Fig. 7. Radiographs of rotational THA with a 7.0 mm thick acetabular component. a At 6 months after surgery. b At 17 years after surgery. Acetabular component shows type 2 demarcation. Femoral component shows radiolucent line at zones 1–2, osteolysis at zones 3–5, and possible loosening
the neck collar. At the latest follow-up there was no radiologically detectable wear in the clearance between the head and the collar. In eight patients who underwent revision surgery for the acetabular component, the rotational heads were removed and measured for inner diameter and clearance between the head and the collar directly with a caliper (Table 4). The mean wear inside the side wall of rotational head was 0.01 mm/year, and the mean decrease in the clearance between the head and the collar was 0.03 mm/year. Survivorship analysis The Kaplan-Meier method, with corresponding confidence intervals, was used to calculate the probability of
revision during the follow-up, revision of the acetabular or femoral component, and radiographic evidence of type 4 socket demarcation or definite loosening of the femoral component (Fig. 8). The 80% survivals of the 7.0 mm thick acetabular component, the 9.5 mm thick component, and the femoral component were 8.4, 11.0, and 12.2 years, respectively. The 50% survivals of the 7.0 mm thick acetabular component, the 9.5 mm thick component, and the femoral component were 13.4, 15.2, and 16.3 years, respectively.
Discussion We reported, apparently for the first time, the long-term results of a THA that has a rotational head system. The concept of the prosthesis is that the rotational system reduces the rotational torque against the acetabular and femoral components, and the 30mm diameter large head increases stability and reduces linear wear inside the joint.
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K. Akasaki: Results of rotational THA
a,b
c Fig. 8. Kaplan-Meier survival curve, with 95% confidence intervals, for the 7.0 mm thick acetabular component (a), the 9.5 mm thick component (b), and the femoral component
(c) with regard to type 4 socket demarcation, definite loosening of the femoral component, or revision surgery as the endpoint
In the present study, the rotational system radiologically showed no detectable wear. However, for the eight patients with revision surgeries, the mean wear of the top wall and side wall inside the head was 0.03mm/ year and 0.01mm/year, respectively. The wear inside the rotational head was much less than that in the acetabular components. However, the wear suggests that the rotational head rotated or shared stress with the acetabular and femoral components and hence could reduce the incidence of loosening. The mean polyethylene wear in 9.5mm thick acetabular components was significantly less than that in 7.0mm thick components at 5 years (P ⬍ 0.01) and 10 years (P ⬍ 0.05). Oonishi et al. also reported that the acetabular component requires more than 9mm thickness to decrease the polyethylene wear.17 The wires around the 7.0mm thick acetabular components broke in 11 of 30 hips (36.7%), whereas those around the 9.5mm thick components broke in only 4 of 25 hips (16%). It is possible that the thin acetabular components caused creeping deformity, which led to enhance polyethylene wear. The mean polyethylene wear in 9.5mm thick acetabular components was 0.10mm/year at 10 years and 0.11mm/year at 15 years. The mean rates of UHMWPE wear in Charnley low-friction arthroplasties with the 22-mm head were reported to range from 0.07 to 0.15 mm/year, with most rates ranging from 0.12 to 0.15mm/year.4,5,13,24 Livermore et al. reported that the 28- and 32-mm heads had less linear wear than did the 22-mm head.13 The mean rates of linear wear in our study were similar to their rates for the 32-mm head and less than their rates for the 22-mm head in previous reports. Therefore, the rotational system might not decrease the acetabular wear by reducing the rotational stress on the acetabular components. The revision rate of 9.5 mm thick acetabular components was 0% at 10 years and 22.2% at 15 years. The rate for 7.0 mm thick components was 19.0% at 10 years and 53.3% at 15 years. The revision rate for 9.5mm thick components was significantly lower than that of 7.0mm thick components at 10 years (P ⬍ 0.01, chi-
square test). To examine the effect of the rotational head on loosening of the acetabular component, we first compared the revision rate for the acetabular component using the Müller prosthesis, which has a 32-mm head. The revision rate for the 9.5mm thick components in combination with a 30-mm head at 10 years in this study was lower than that for the Müller prosthesis at around 10 years.18,22 Sutherland et al. reported that the revision rate was 12%,22 and Reikerås reported it to be 8%.18 These results suggest that the rotational system decreases friction stress and the incidence of loosening in cases of 9.5mm thick components. However, the revision rate for 7.0mm thick components was higher than that in previous reports. In our study, the radiolucent lines were seen especially in zone 1 with a 7.0mm thick component after 5 years. This pattern is similar to that using the Charnley prosthesis.6 In addition, the wires around the acetabular component broke in all eight revision cases in which there were 7.0mm thick components (Fig. 5). In contrast, the wires in two revision cases with 9.5mm thick components did not break. One possibility is that the 7.0mm thick components had more creeping deformity around zone 1 than did the 9.5mm thick components, which caused the radiolucent lines and loosening. Another possibility is that the high friction torque caused by the large 30-mm head increased loosening. Charnley stated that it was extremely important to minimize friction torque during THA by using the smaller femoral head diameter.3 In combination with the large external diameter of the acetabular component, this should reduce the friction torque at the bone–cement interface. The 7.0mm thickness of the acetabular component in combination with the 30-mm (large) femoral head may be too thin to resist the friction torque at the bone– cement interface Therefore, the high friction torque could affect the bone–cement interface and increase the incidence of loosening. For the same reason, the revision rate of the 32-mm femoral head was reported to be higher than that of the 22-mm femoral head.15,18–20,22,24 In this study, even when
K. Akasaki: Results of rotational THA
using the 9.5 mm thick components in combination with a 30-mm rotational head, the revision rate was still higher than that using the Charnley prosthesis.19,20,24 These lines of evidence suggest that the rotational system decreases the friction stress and incidence of loosening, although the femoral head size has a more important effect on the friction stress against the acetabular component than does the rotational head system. One theoretical advantage of using a larger femoral head is that it decreases the linear wear and enhances the stability of the joint, even though it increases the friction torque. In this study, there were no hip dislocations during the follow-up period. The femoral component in this study was designed with a curve, compared with the Charnley prosthesis (Fig. 1). No stem had to be revised at 10 years, and only 2 of 23 stems (8.7%) were revised at 15 years. This result compares favorably with those in previous reports of the Müller curved-stem prosthesis.18,22 Sutherland et al. reported a revision rate of 20%22 and Reikerås one of 16.7%18 at about 10 years after surgery. When compared with the Charnley prosthesis using the firstgeneration cementing technique, the revision rate at 10– 15 years in this study was similar to that in previous reports, which ranged from 4% to 9%.2,11,12,16 The frequency and location of radiolucent lines were similar to those seen with the Charnley prosthesis.8 However, the revision rate was higher than that with the Charnley prosthesis using the improved cementing technique.14,24 The Müller curved-stem prosthesis was reported to have a higher incidence of loosening than the Charnley prosthesis.18,22 Possible explanations include the curve, the thin medial edge, and the small diameter in the Müller stem. Other factors, including the cementing technique, bone quality, and stem-insert position, are also important for stable fixation. In this study, it is possible that the rotational system reduced the rotational stress against the stem and decreased the incidence of radiolucency and loosening.
Conclusions The rotational system inside the head during THA seems to share the rotational torque against the acetabular and femoral components, thereby reducing the incidence of loosening. The 30 mm diameter head increased the stability and did not dislocate in all hips. The linear wear in the acetabular components were similar to that using a 32 mm diameter head. The 7.0 mm thick acetabular component was too thin to resist the friction torque caused by the large 30 mm diameter head, even though the rotational system reduced the stress. When compared to previous reports of the Charnley prosthesis, the revision rate of acetabular
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components was higher, suggesting that the femoral head size has a more important effect on reducing the friction stress on the acetabular component than does the rotational head system. It is possible to develop a lower-friction total hip prosthesis by improving the design and materials of the rotational head system or the bipolar system. Acknowledgments. The author sincerely thanks Dr. K. Takagi for his guidance with this work and Dr. K. Hirota for his help in the design of the prosthesis.
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134 20. Schulte KR, Callaghan JJ, Kelley SS, et al. The outcome of Charnley total hip arthroplasty with cement after a minimum twenty-year follow-up. J Bone Joint Surg Am 1993;75:961–75. 21. Shima Y, Ueno R, Tamaki T, et al. Outcome measures of coxarthrosis. Nippon Seikeigekagakkai Zasshi (J Jpn Orthop Assoc) 1971;45:813–33 (in Japanese). 22. Sutherland CJ, Wilde AH, Borden LS, et al. A ten-year followup of one hundred consecutive Müller curved-stem total hip-
K. Akasaki: Results of rotational THA replacement arthroplasties. J Bone Joint Surg Am 1982;64:970– 82. 23. Weber BG. Die rotations-totalendoprothese des hüftgelenkes. Z Orthop 1970;107:304–15. 24. Wroblewski BM, Siney BA. Charnley low-friction arthroplasty of the hip. Clin Orthop 1993;292:191–201.