There are many reports on cup placement, inclination, anteversion, and position. This article strictly defines the area of cup placement based on data from 185 healthy hip centers and Ranawats triangle. Linear wear rate (LWR) was measured in 55 total hip arthroplasty (THA) cases and categorized as low or high. The relationships between these categories and cup size, inclination, anteversion, position, age, follow-up period, bone graft, cup osteolysis, and stem osteolysis were investigated.
In total hip arthroplasty (THA), preoperative templating to determine lateral and anterior inclination of the cup is important to avoid postoperative impingement, dislocation, high wear rate, and loosening. There are many reports in the literature regarding cup inclination. Charnley1 recommended 0° of anterior inclination, Coventry2 recommended 40° of lateral and 15° of anterior inclination, and Harris3 recommended 30° of lateral and 20° of anterior inclination. There also are many reports in the literature on avoiding impingement.4,5 Based on these studies, cup inclination angle typically ranges from 30° to 50° degree lateral and 0° to 30° degree anterior.
Although there are several references for cup position such as the contralateral femoral head center, the THA template, and intraoperative findings, a standard point of reference for the femoral head remains undecided. In patients with hemi-osteoarthritis of the hip joint, the position of the femoral head on the healthy side can be used as a point of reference. However, in patients with bilateral osteoarthritis and high dislocation due to acetabular dysplasia, there is no physical point of reference apart from the height of the teardrop.
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Figure 1: Approximate femoral head center and true acetabular regions according to Ranawat. The lowest point of the teardrop is the intersection of D and E. Key: A=20% height of pelvis, B=approximate femoral head center, C=true acetabular region, D=Kohlers line, E=Shentons line.
Ranawat et al6 proposed using the true acetabular region as the area of cup position and the approximate femoral head center (AFHC) as the point of reference (Figure 1). In their method, the approximate femoral head center was determined by placing the cup at an inclination of approximately 45° lateral and 25° anterior. The cup size was selected to fit the anteroposterior acetabular diameter and to contact the medial wall of the acetabular floor in depth. The inferior cup edge was placed at the level of the inferior acetabular margin. In Asian patients, the approximate femoral head center appears to be more proximal and more medial relative to the proper head center. Thus, it is necessary to define an acceptable area for the position of the femoral head center.
Pagnano et al7 reported cases with 15 mm higher cup placement required femoral side revision, regardless of lateral placement. They also suggested a zone for THA that consists of Ranawats true acetabular region with a superior border 1 cm proximal and a lateral border 1 cm lateral from the approximate femoral head center. In their study of 34 patients, Russotti and Harris8 used a minimum of 35 mm proximally above the inter-teardrop line as a high hip center. Hirakawa et al9 suggested the best position for the femoral head center was <35 mm vertically from the inter-teardrop line and 25 mm laterally from the teardrop.
This study measured the standard femoral head center of healthy hip joints to define a permissible range for acetabular cup inclination and position and to investigate the ultra high molecular weight polyethylene (UHMWPE) linear wear rate.
Materials and Methods
Measurement of Hip Center in Normal Hips
To determine the standard femoral head center (SFHC), 185 hip centers were measured in 105 healthy individuals without hip disease. Participants included 72 women (122 hips) and 33 men (63 hips). Mean age of participants was 61 years (range, 20-87 years).
An Epson digital scanner (ES-8500; Seiko Epson, Nagano, Japan) was used to scan anteroposterior views of bilateral hip radiographs (X-p). The size of the scanned files was standardized according to the real size of the X-p, and the femoral head centers were measured using Rhinoceros computer-assisted design (CAD) software (Robert McNeel and Associates, Seattle, Washington).
Figure 2: Coordinate system to measure the center of hip joints. The X-axis is the line between the bilateral lowest point of the teardrop; the Y-axis is perpendicular to the X-axis; and the coordinates of the hip center are represented as (X, Y). Figure 3: Radiograph demonstrating measurement of linear wear with A=circle with the same diameter as the cup: adjust the cup of X-p to this circle, B=circle of the head just after surgery, and C=circle of head before this study. The distance between the centers of B and C were measured using CAD software.
The coordinate system was defined such that the starting point of the coordinate system would be the lowest point of the teardrop on the AP view. The X-axis was formed by the line between the bilateral lowest points of the teardrop, and the Y-axis was perpendicular to the X-line. The coordinates of the hip center were expressed as (X, Y) (Figure 2). When the teardrops could not be identified, the teardrop was defined as the set of points 5 mm lateral to the intersection of Kohlers line and Shentons line according to the procedure described by Ranawat et al.6
To develop a formula for the hip center in terms of the pelvic height, simple regression analysis was performed between the pelvic height (PH) and each X and Y coordinate using StatView version 5.0 software (SAS Institute Inc, Cary, North Carolina). The safety zone of the hip center for THA was defined as a rectangular zone formed by the standard femoral head center and Ranawats approximate femoral head center (0.1×pelvic height, 0.1×pelvic height).6
Measurement of Hip Center After Arthroplasty
A total of 55 hip joints in 37 patients who underwent primary THA for secondary osteoarthritis were analyzed. Patients included 8 men (10 hips) and 29 women (45 hips). Mean patient age was 49.3 years (range, 18-78 years). Mean follow-up was 61 months (range, 24-127 months). A cementless cup and stem (Omnifit; Stryker Orthopaedics, Mahwah, New Jersey) were used in all cases.
Cups were placed such that the inferior margin of the acetabular cup was at the inferior edge of the primary acetabulum. The lateral inclination and the anteversion of the cup were set to 45° and 25°, respectively, using a cup positioner. In cases in which the acetabulum had a massive bony defect at the superior margin of the acetabulum, a bulky bone graft was performed using the resected femoral head and the cup was fixed with cancellous screws.10
Anteroposterior radiographs of the bilateral hip joints, taken within 1 month of surgery and immediately before this study, were scanned using an Epson digital scanner. Coordinates of the hip centers were measured as described above, and lateral inclination and anteversion of each cup were measured using the method described by Ackland et al.11 A simple regression analysis was performed to standardize the coordinate of the hip center with respect to pelvic height using StatView statistical software.
Evaluation of Linear Wear Rate, Acetabular Bone Graft, and Osteolysis
The linear wear rate of UHMWPE was measured using the modified radiographic procedure described by Griffith et al.12 The inclination and scale of pelvis of X-p were adjusted to match bilateral anterosuperior iliac spines and the curves of iliac crest of each X-p using Photoshop version 7.0 software (Adobe Systems Inc, San Jose, California). The scale of the pelvis also was adjusted according to the cup size using Rhinocerous CAD software.
Using the same coordinate system as described above, the coordinates of the cup center, the head center, and the pelvic height were measured. Polyethylene wear was defined as the difference in the head center in each radiograph (Figure 3). A linear wear rate <0.19 mm/year was defined as low and a linear wear rate >0.19 mm/year was defined as high according to the average linear wear rate of the Omnifit system reported by DLima et al.13
Figure 4: Scattergrams demonstrating the result of single regression analysis of the standard femoral head center (SFHC) and approximate femoral head center (AFHC) by pelvic height (PH), X-axis: PH. Formulas based on single regression analysis and R2 are shown. We defi ne the rectangular zone formed by AFHC and SFHC. The average femoral head center would be A(X, Y) if THA was performed within this zone.
An acetabular bone graft was defined as a graft whose acetabular coverage was >30% of the acetabular cups.14,15 The union of the grafted bone to the host bone, as evidenced by trabecular bridging of the host-graft interface and structural integrity, also was evaluated. Osteolysis around the cups and stems was investigated.
The Mann-Whitney, Fisher exact, and Pearson correlation tests were used for statistical analysis. Values <.05 were considered statistically significant.
Mean coordinates were (37±5.7, 16.7±4.3 mm) for the standard femoral head center and (22.2±1.3, 22.2±1.3 mm) for Ranawats approximate femoral head center. The following formula was obtained for the standard femoral head center: (X, Y)=(0.167×pelvic height mm, 0.07×pelvic height mm) from simple regression analysis with the starting point defined as the level of the teardrop (Figure 4).
Mean cup lateral inclination and anteversion were 48.5° (range, 23°-68°) and 13.1° (range, 0°-28°), respectively. The cup position coordinates were (18-43, 9-32 mm), with the mean being (29.4, 18.3 mm) when the starting points were defined as the level of the teardrop. The following formula was obtained by simple regression analysis: (X, Y)=(0.145×PH [mm], 0.09×PH [mm]). The relationships between the approximate femoral head center or standard femoral head center and cup position are shown in Figure 4.
All of the grafted bones were taken on the bone bed of the acetabulum in which trabecular formation could be observed from grafted bone to the host bone. Mean linear wear rate was 0.287 mm/year (range, 0-1.049 mm/year).
A rectangular zone with a diagonal line connecting the standard femoral head center and Ranawats approximate femoral head center was defined (Figure 4). Cup positions were in this zone in 18 hips of 18 patients (13 women and 5 men); these hips comprised group 1. Cup positions were outside this zone in 37 hips of 37 patients (32 women and 5 men); these hips comprised group 2.
There were no significant differences between groups 1 and 2 for follow-up, pelvic height, cup size, cup lateral inclination, and cup anteversion. However, the difference in operative age between groups 1 and 2 was statistically significant (Mann-Whitney U test, P=.045).
Bone grafts were performed in 3 hips in group 1 and 13 hips in group 2. Cup osteolysis was identified in 6 hips in group 1 and 14 hips in group 2. Stem osteolysis was detected in 6 hips in group 1 and 15 hips in group 2. There were no significant differences in gender, bone graft, and osteolysis (Table 1).
There was a significant difference in linear wear rate (Mann-Whitney U test, P <.0001) between the low and high linear wear rate groups. There were no significant differences between the 2 groups in operative age, follow-up, pelvic height, cup size, cup lateral inclination, and cup anteversion. There also were no significant differences in gender, bone graft, and osteolysis (Table 2).
Correlations were analyzed between bone graft, cup osteolysis, stem osteolysis, linear wear rate, age, pelvic height, cup size, cup lateral inclination, cup anteversion, and cup position. Significant differences were observed between bone graft and cup size, stem osteolysis and cup size, and cup osteolysis and operative age (Table 3).
Scattergrams with simple regression analyses of linear wear rate, cup lateral inclination, and cup anteversion are shown in Figure 5. There were no significant differences in cup lateral inclination and anteversion between the low and high linear wear rate groups (Pearsons correlation test).
A scattergram of the cup position (X- and Y-coordinates) is shown in Figure 6, in which the rectangular zone, whose diagonal was a line connecting the approximate femoral head center and standard femoral head center points, is indicated. Hip joints belonging to the low and high linear wear rate groups were distributed equally in this scattergram.
Figure 5: Scattergrams and simple regression analysis, Y-axis: linear wear rate (LWR); X-axis: degree of lateral inclination and anteversion. Pearson correlation test revealed no statistically significant difference between LWR and lateral inclination or anteversion, but a correlation was found between LWR and lateral inclination.
We proposed that a standard femoral head center could be calculated by simple regression analysis based on pelvic height, and we also proposed a new strict zone as a guide for cup placement in preoperative planning. The standard femoral head center is more lateral and distal than the approximate femoral head center, so we defined the area formed by these points. This area can easily be used in calculations by pelvic height. In this study, the area was assumed to be a rectangular zone; however, we do not actually know whether it is triangular, circular, or another shape. The assumption that the zone is rectangular simplifies calculations.
We investigated whether our calculation method based on pelvic height was suitable and which area formula would be most effective. Pagnano et al7 recommended the cup be placed up to 15 mm higher than the approximate femoral head center. For a pelvic height of 203.5 mm (average in our study), the approximate femoral head center would be defined as (0.1×PH, 0.1×PH) mm, and a 15-mm higher position would yield an increase in the ratio of 15/203.5=0.073. Thus, 15 mm higher than the approximate femoral head center would be 0.1+0.073=0.173 (ratio).
Russotti and Harris8 used a minimum of 35 mm proximally above the inter-teardrop line as the high hip center, and 35-mm proximal placement yields an increase in the ratio of 35/203.5=0.172. Hirakawa et al9 suggested the best position for the femoral head center was <35 mm vertically from the inter-teardrop line and 25 mm laterally from the teardrop. Twenty-five millimeters laterally means 25/203.5=0.123. It appears that in these cases, our calculation method based on the pelvic height would be suitable and not cause a great error.
In our study, cup placements were never >15 mm higher (>.173) or 35 mm proximal placement (>.172). Our study did not prove the conclusion of Hirakawa et al,9 although our formula was generally useful in such cases. We had >25-mm lateral cup placement, which could vary 0.123 laterally, but there was no significant difference in this zone.
We could not establish a correlation between cup position and linear wear rate. However, linear wear rate appears to be associated with age, gender, body weight, and femoral head size. These factors may, to a certain extent, be more important than cup position. We might define this extent as the permissible area for normal hip center after THA from the low linear wear rate group, ie, the area marked by X=0.091~0.210, Y=0.043~0.160. For example, for a pelvic height of 203.5 mm, the permissible area was defined as (18.5~42.7, 9.9~32.5) mm.
Figure 6: Scattergram between the femoral head center X ratio and the femoral head center Y ratio. The intersection with the dotted line is the center of approximate femoral head center, and the intersection with the broken line is the center of the standard femoral head center. Our defi ned area was X=0.1~0.165, Y=0.075~0.1. Linear wear rate was seen almost throughout the field of this diagram.
Russotti and Harris8 suggested proximal positioning of the acetabular component without lateral displacement can provide an acceptable result in cemented THA. Their average lateral displacement was 36 mm (range, 25-55 mm). Considering the Asian physique, our defined zone (ratio: 0.091~0.210; calculated range for pelvic height of 203.5 mm: 18.5~42.7 mm) was within this range.
There was no statistically significant relationship between the cup inclination and linear wear rate. Hirakawa et al9 suggested that with all-polyethylene monolithic cemented cups, better long-term results were obtained at a cup inclination angle of <40° and medial position of the cup. Kligman et al16 reported an increased wear rate per year correlated with a discrepancy of >18.3° between the contralateral acetabular angle and acetabular cup inclination.
In our study, we did not evaluate the contralateral acetabular angle because only bilateral cases were included. However, Sharp et al17 reported the acetabular angle ranged from 38° to 45°. Thus, our lateral inclination of approximately 48° was almost within the normal area. Our low linear wear rate data suggest a lateral inclination from 35° to 65° is permissible. However, placing the cup at >45° lateral inclination would cause a dynamic failure, so we try to avoid such placement.
Anteversion angle did not have a significant influence on linear wear rate. Our low linear wear rate data suggest an anteversion ranging from 0° to 30° is permissible. It is also important to avoid the impingement between cup and stem.
To keep the cup placement in the strict zone, a bulky bone graft sometimes is required, especially in dysplasia cases. In such cases, there is debate regarding whether a bone graft should be performed. Mulroy and Harris14 advocated against using shelf autografts in their report investigating cemented acetabular component. Other reports on THA results using uncemented cups despite acetabular bone grafts showed no significant difference,15,18 and our results corroborated these reports. However, such cases required long-term nonweight-bearing gait and occasionally long-term admission.
In younger patients, it is important to keep acetabular bone stock for further operation and also to reduce leg-length discrepancy. In older patients, it is difficult to maintain a nonweight-bearing gait and to improve bone quality when bone atrophy occurs. Thus, for early weight bearing, we used a slight high placement as indicated by our formula and checked that there was adequate bone coverage on the cup, at least 60% according to Mulroy14 and Morsi.15
Three significant correlations were identified in our study: bone graft and cup size, stem osteolysis and cup size, and cup osteolysis and age. A smaller cup seemed to be more suitable in cases with bone graft rather than without bone graft; stem osteolysis occurred in the case of a smaller cup. Rapid linear wear rate was reported to be the most important risk factor for the durability of the cup. Griffith et al12 reported the incidence of heavy wear was directly related to the patient age; heavy wear occurred in 12% of patients <50 years but only in 1.5% of patients >60 years. Our results corroborated this finding. However, sometimes there is no other choice but to perform THA on young patients.
These results suggest a larger cup could reduce the extent of grafted bone and stem osteolysis. The acetabulum should be reamed to a depth where it contacts the medial wall and the anteroposterior width should be reamed as deep as possible to be able to place as large a cup as possible medially without breaking any walls.
There was a pitfall in the fine adjustment of the positioning of the femoral head. At larger lateral inclinations of the cup, the femoral head attained a more medial and superior (ie, less lateral and inferior) position. Johnston et al19 followed a mathematical approach to determine optimal placement of the acetabular cup. They suggested placement of the center of the acetabulum as far medially, inferiorly, and anteriorly as anatomically possible is of prime importance in reducing the loads at the hip.
On the basis of our findings and existing data, we propose that the strict zone for the femoral head center is the rectangular zone demarcated by 2 points, namely, the approximate femoral head center and the standard femoral head center, and that the permissible zone is determined by the lower linear wear rate.
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Drs Ito, Tokunaga, and Endo are from the Department of Reconstructive and Transplant Medicine, Division of Orthopedic Surgery, Niigata University Graduate School of Medical and Dental Sciences, Niigata, and Drs Takano and Yuasa are from the Department of Orthopedic Surgery, Akita Red Cross Hospital, Akita, Japan.
Drs Ito, Tokunaga, Endo, Takano, Yuasa have no relevant financial relationships to disclose.
Correspondence should be addressed to: Masayuki Ito, Department of Orthopedic Surgery, Niigata Civilization Hospital, 463-7, Shumoku, Central Ward, Niigata City, 950-1197 Japan.