JBJS | The Effect of the Orientation of the Acetabular and Femoral Components on the Range of Motion of the Hip at Different Head-Neck Ratios*

The Journal of Bone & Joint Surgery, Volume 82, Issue 3

Articles | March 01, 2000

The Effect of the Orientation of the Acetabular and Femoral Components on the Range of Motion of the Hip at Different Head-Neck Ratios*

DARRYL D. D'LIMA, M.D.†; ANDREW G. URQUHART, M.D.‡; KNUTE O. BUEHLER, M.D.§; RICHARD H. WALKER, M.D.†; CLIFFORD W. COLWELL, JR.†, M.D., LA JOLLA, CALIFORNIA

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Investigation performed at the Division of Orthopaedic Surgery, Scripps Clinic, La Jolla

The Journal of Bone & Joint Surgery. 2000; 82:315-21

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Abstract

Background: Prosthetic impingement due to poor positioning can limit the range of motion of the hip after total hip arthroplasty. In this study, a computer model was used to determine the effects of the positions of the acetabular and femoral components and of varying head-neck ratios on impingement and range of motion.

Methods: A three-dimensional generic hip prosthesis with a hemispherical cup, a neck diameter of 12.25 millimeters, and a head size ranging from twenty-two to thirty-two millimeters was simulated on a computer. The maximum range of motion of the hip was measured, before the neck impinged on the liner of the cup, for acetabular abduction angles ranging from 35 to 55 degrees and acetabular and femoral anteversion ranging from 0 to 30 degrees. Stability of the hip was estimated as the maximum possible flexion coupled with 10 degrees of adduction and 10 degrees of internal rotation and also as the maximum possible extension coupled with 10 degrees of external rotation. The effects of prosthetic orientation on activities of daily living were analyzed as well.

Results: Acetabular abduction angles of less than 45 degrees decreased flexion and abduction of the hip, whereas higher angles decreased adduction and rotation. Femoral and acetabular anteversion increased flexion but decreased extension. Acetabular abduction angles of between 45 and 55 degrees permitted a better overall range of motion and stability when combined with appropriate acetabular and femoral anteversion. Lower head-neck ratios decreased the range of motion that was possible without prosthetic impingement. The addition of a modular sleeve that increased the diameter of the femoral neck by two millimeters decreased the range of motion by 1.5 to 8.5 degrees, depending on the direction of motion that was studied.

Conclusions: There is a complex interplay between the angles of orientation of the femoral and acetabular components. Acetabular abduction angles between 45 and 55 degrees, when combined with appropriate acetabular and femoral anteversion, resulted in a maximum overall range of motion and stability with respect to prosthetic impingement.

Clinical Relevance: During total hip arthroplasty, acetabular abduction is often constrained by available bone coverage, while femoral anteversion may be dictated by the geometry of the femoral shaft. For each combination of acetabular abduction and femoral anteversion, there is an optimum range of acetabular anteversion that allows the potential for a maximum range of motion without prosthetic impingement after total hip arthroplasty. These data can be used intraoperatively to determine optimum position.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

The orientation of the prosthetic components in terms of acetabular abduction and anteversion and femoral anteversion is one of the major implant-related factors limiting the range of motion after total hip arthroplasty. Implant-design variables, such as the head-neck ratio¹ and the presence of a modular head with an extended sleeve, also have been implicated^8,14. In the current study, a three-dimensional computer simulation was used to analyze the interactions between head-neck ratios and prosthetic orientations in determining the range of motion of the hip.

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

†Division of Orthopaedic Surgery, Scripps Clinic, 10666 North Torrey Pines Road, MS126, La Jolla, California 92037. E-mail address for D. D. D'Lima: ddlima@scripps.edu. E-mail address for C. W. Colwell, Jr.: colwell@scripps.edu. Please address requests for reprints to C. W. Colwell, Jr.

‡1050 Isaac Streets Drive, 122, Oregon, Ohio 43616.

§2600 N.E. Ness Road, Suite 1, Bend, Oregon 97701.

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

‡1050 Isaac Streets Drive, 122, Oregon, Ohio 43616.

§2600 N.E. Ness Road, Suite 1, Bend, Oregon 97701.

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FIG1:: Fig. 1 Rendered image of computer simulation demonstrating the axes around which the prosthetic orientation is described. The curved arrows indicate the positive direction of rotation.

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FIG2:: Fig. 2 Combined contour maps for range of motion (for a prosthesis with a head diameter of twenty-eight millimeters). White zones correspond to prosthetic orientations that allow an excellent range of motion (greater than 110 degrees of flexion, 30 degrees of extension, 45 degrees of adduction-abduction, and 45 degrees of external rotation) in all directions; black zones correspond to those that result in a poor range of motion (less than 90 degrees of flexion, 15 degrees of extension, 30 degrees of adduction-abduction, and 30 degrees of external rotation) due to prosthetic impingement in at least one direction; and gray zones correspond to those that allow at least a borderline range of motion (between excellent and poor) in all directions.

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FIG3:: Fig. 3 Combined contour maps for intraoperative stability (for a prosthesis with a head diameter of twenty-eight millimeters). White zones correspond to prosthetic orientations that allow an excellent range of motion in all directions; black zones, to those that result in a poor range of motion due to prosthetic impingement in at least one direction; and gray zones, to those that allow at least a borderline range of motion in all directions.

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FIG4:: Fig. 4 Graph showing the maximum hip flexion possible with different combinations of prosthetic orientation and different head sizes. FAV = femoral anteversion, and AAV = acetabular anteversion.

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FIG5:: Fig. 5 Graph showing the maximum hip extension possible with different combinations of prosthetic orientation and different head sizes. FAV = femoral anteversion, and AAV = acetabular anteversion.

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FIG6:: Fig. 6 Graph showing the maximum hip abduction possible with different combinations of prosthetic orientation and different head sizes. FAV = femoral anteversion, and AAV = acetabular anteversion.

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FIG7:: Fig. 7 Graph showing the maximum hip adduction possible with different combinations of prosthetic orientation and different head sizes. FAV = femoral anteversion, and AAV = acetabular anteversion.

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FIG8:: Fig. 8 Graph showing the maximum external rotation of the hip possible with different combinations of prosthetic orientation and different head sizes. FAV = femoral anteversion, and AAV = acetabular anteversion.

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TABLE I: COMBINATIONS OF PROSTHETIC ORIENTATIONS THAT PERMIT TYING A SHOE AND STOOPING⁶

Acetabular Abduction (degrees)	Femoral Anteversion (degrees)	Acetabular Anteversion (degrees)
35	0	No position
35	10	=10
35	20	All positions
35	30	=10
45	All positions	All positions
55	All positions	All positions

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TABLE I: COMBINATIONS OF PROSTHETIC ORIENTATIONS THAT PERMIT TYING A SHOE AND STOOPING⁶

Acetabular Abduction (degrees)	Femoral Anteversion (degrees)	Acetabular Anteversion (degrees)
35	0	No position
35	10	=10
35	20	All positions
35	30	=10
45	All positions	All positions
55	All positions	All positions

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

A three-dimensional model of a generic hip-replacement prosthesis (Fig. 1) was initially generated with use of Pro/ENGINEER (Parametric Technology, Waltham, Massachusetts), a parametric computer-assisted-design software program. The femoral head diameter ranged from twenty-two to thirty-two millimeters. A diameter of 12.25 millimeters was arbitrarily chosen for the femoral neck. The femoral component was modeled with a 135-degree neck angle and a forty-four-millimeter head offset; offset was measured as the horizontal distance between the center of the head and a vertical line through the center of the distal portion of the stem. The acetabular component was modeled as a pure hemisphere, with an outer diameter of sixty millimeters and an inner diameter set to match each respective head size. The depth of the socket was therefore the same as the radius of the head for each condition.

The axes around which motion was described were determined according to recommendations made by the International Society of Biomechanics¹⁵. The origin of the hip axis is located at the center of the head. With the hip in the neutral position, the x axis points anteriorly, the y axis points superiorly, and the z axis points to the patient's right side. Flexion of the hip is described around the pelvic z axis, which is fixed to the pelvis; axial rotation of the hip, around the femoral y axis, which is fixed to the femur; and adduction and abduction of the hip, around a floating axis, which is mutually perpendicular to the hip flexion axis and the hip rotation axis. Abduction of the acetabular component is measured from the horizontal around the pelvic x axis, and anteversion (true anteversion) of the acetabular component is measured around the pelvic y axis (Fig. 1). The femoral component was assumed to have been inserted in direct alignment with the mechanical axis that corresponds to the femoral y axis (0 degrees of mechanical femoral varus or valgus), and anteversion of the femoral component was measured around the femoral y axis (Fig. 1).

The range of motion was measured by moving the femur in the desired direction until the neck visually impinged on the liner of the cup. Acetabular abduction angles of between 35 and 55 degrees were studied in combination with acetabular and femoral anteversion angles of between 0 and 30 degrees. Flexion, extension, abduction, adduction, and axial rotation were measured for various combinations of acetabular abduction, acetabular anteversion, and femoral anteversion. This was done for head-neck ratios corresponding to a 12.25-millimeter neck diameter associated with head diameters of twenty-two, twenty-six, twenty-eight, and thirty-two millimeters (range of head-neck ratios, 1.8 to 2.6). The range of motion associated with a component that had a modular head with a sleeve extension that increased the diameter of the neck by two millimeters (but did not change the offset of forty-four millimeters) also was measured.

To determine the effect of different combinations of prosthetic positions on implant impingement, contour maps were generated for a prosthesis with a twenty-eight-millimeter-diameter head and a 12.25-millimeter-diameter neck for three representative acetabular abduction angles (35, 45, and 55 degrees). The range of motion of the hip was mapped against femoral anteversion and acetabular anteversion on the x and y axes, respectively. Range-of-motion measurements included flexion, extension, adduction, abduction, and internal and external rotation, which were uncoupled from each other. Each range of motion was classified according to three zones: excellent, poor, and borderline (white, black, and gray, respectively, in Fig. 2). The contour maps then were blended so that each displayed a zonal classification (excellent, borderline, and poor) for the combined range of motion in all directions (flexion-extension, adduction-abduction, and rotation). All motions were recorded as the maximum range before impingement.

To determine the effect of each combination on the stability of the hip, two coupled ranges of motion were measured: the maximum flexion possible with the hip in 10 degrees of adduction and 10 degrees of internal rotation, and the maximum extension possible with the hip in 10 degrees of external rotation. These ranges of motion were chosen to simulate one of the intraoperative stability tests performed during total hip arthroplasty. Contour maps again were generated, as described, to determine which combinations resulted in optimum stability (that is, the maximum range of motion before impingement) (Fig. 3).

In addition, prosthetic orientations that permitted certain activities of daily living, such as tying a shoe with the foot on the ground and stooping, were recorded. According to the positions described by Johnston and Smidt⁶, a position of 129 degrees of flexion, 18 degrees of abduction, and 13 degrees of external rotation was chosen for tying a shoe and a position of 125 degrees of flexion, 25 degrees of abduction, and 15 degrees of external rotation was selected for stooping.

Results

Introduction | Materials and Methods | Results | Discussion | References

Overall, increasing acetabular abduction angles increased hip flexion, extension, and abduction and decreased adduction and axial rotation (Figs. 4, 5, 6, 7 and 8). Increasing acetabular or femoral anteversion increased hip flexion (Fig. 4) and decreased hip extension (Fig. 5), but to varying degrees at different angles of acetabular abduction. At 45 degrees of acetabular abduction, both femoral and acetabular anteversion increased flexion to the same degree. At acetabular abduction angles of more than 45 degrees, acetabular anteversion increased flexion more than femoral anteversion did; this effect was reversed at acetabular abduction angles of less than 45 degrees. Combined femoral and acetabular anteversion had an additive effect on hip flexion. Acetabular anteversion decreased hip abduction, whereas femoral anteversion alone did not have much effect (Fig. 6). On the other hand, femoral anteversion decreased hip adduction, whereas acetabular anteversion alone did not have much effect (Fig. 7). Acetabular and femoral anteversion were inversely related to external rotation of the hip, with the decrease in external rotation equaling the amount of acetabular or femoral anteversion (Fig. 8). Maximum internal rotation of the hip was always greater than 45 degrees for all combinations and hence was not included in the analysis, as prosthetic impingement does not seem to be a limiting factor for this parameter.

Femoral Head Size

The size of the head was related to the range of motion of the hip, as expected, but this relationship was not linear. An increase in head size of four millimeters, from twenty-two to twenty-six millimeters, resulted in a larger improvement in the range of motion than did a similar increase from twenty-eight to thirty-two millimeters (Figs. 4, 5, 6, 7 and 8). Also, the position of the component was related to the extent to which head size affected the total range of motion. Higher acetabular abduction angles magnified the changes in hip flexion, extension, and external rotation due to changes in the head-neck ratios. Femoral anteversion reduced the changes in flexion and extension and increased the changes in adduction and abduction due to changes in head size, but it had no such effect on rotation.

Range of Motion

With the cup in 35 degrees of abduction, a very narrow band of prosthetic orientations resulted in the potential for an excellent range of motion (Fig. 2). The femoral or the acetabular component, or both, had to be anteverted more than 10 degrees to result in at least a borderline range of motion. With the cup in 45 degrees of abduction, there was a wider margin for error. Judicious combination of femoral and acetabular anteversion could result in an excellent range of motion. For example, if the femoral component was anteverted less than 15 degrees, the acetabular component had to be anteverted more than 15 degrees to make up for the loss in the range of motion; on the other hand, if the femoral component was anteverted more than 25 degrees, the acetabular component had to be anteverted less than 20 degrees to stay in the excellent zone. With the cup in 55 degrees of abduction, the pattern was different; femoral anteversion of less than 15 degrees resulted in an excellent range of motion, whereas a combination of high anteversion of both the acetabular and the femoral component (greater than 15 degrees for each) resulted in a borderline or poor range of motion.

Stability

The combined contour maps for intraoperative stability (Fig. 3) demonstrated somewhat different results. At 35 degrees of acetabular abduction, there were no zones of excellent stability and femoral anteversion of less than 10 degrees resulted in zones of poor stability. At 45 degrees of acetabular abduction, there was a narrow zone of excellent stability. Less than 15 degrees of anteversion of one component had to be compensated for by anteversion of the other component to remain outside a zone of poor stability. At 55 degrees of acetabular abduction, the zone of excellent stability was wider. Zones of poor and borderline stability were restricted to either too little or too much anteversion of both components.

Modular Sleeve Extension

The use of a modular head component with an extended sleeve that effectively increased the diameter of the neck by two millimeters resulted in a 1.5 to 8.5-degree decrease in the range of motion, depending on the direction of motion that was tested.

Activities of Daily Living

Combinations resulting in a range of motion that was sufficient to permit tying a shoe with the foot on the ground and stooping were analyzed (Table I). For example, at 35 degrees of acetabular abduction and 10 degrees of femoral anteversion, at least 10 degrees of acetabular anteversion was necessary to permit tying a shoe or stooping.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

Prosthetic impingement determines the functional end point of the stable range of motion after a hip arthroplasty. Additional factors, such as osseous impingement and soft-tissue tension, can only decrease this range of motion. Therefore, optimum positioning of the components is necessary to avoid a decrease in the stable range of motion due to prosthetic impingement.

The optimum orientation of hip components remains controversial, with recommendations varying widely. Coventry³ suggested that more than 40 degrees of abduction and more than 15 degrees of anteversion of the cup prevents posterior dislocation due to impingement, Charnley² recommended no anteversion, Harris⁵ recommended acetabular abduction of 30 degrees and acetabular anteversion of 20 degrees, and Lewinnek et al.⁹ recommended acetabular abduction of between 30 and 50 degrees and acetabular anteversion of between 5 and 20 degrees. Seki et al.¹³, in a computer-simulation study, recommended acetabular abduction of between 30 and 50 degrees, acetabular anteversion of between 10 and 30 degrees, and femoral anteversion of 10 degrees; however, only one specific manufacturer's design with a single head size (twenty-six millimeters) was modeled, and only the range of hip flexion and extension was studied. Robinson et al.¹² reported the results of another computer-simulation study in which the range of motion and the contact area were measured. They suggested that, although flexion of the hip increased with acetabular abduction, acetabular anteversion, and femoral anteversion (similar to the findings in the present study), the contact area between the head and the liner decreased. Again, their range-of-motion analysis was limited to one head size and one specific manufacturer's design. Therefore, in the current study, an objective computer simulation of different combinations of component positions was performed, without use of any specific manufacturer's design but with use of a generic model with different head sizes (range of head-neck ratios, 1.8 to 2.6).

There is a complex interaction among abduction of the acetabular component, anteversion of the acetabular component, and anteversion of the femoral component in determining the maximum prosthetic range of motion. The results of the present study suggest that, for a twenty-eight-millimeter-diameter head with a 12.25-millimeter-diameter neck, acetabular abduction angles of between 45 and 55 degrees offer the widest excellent zones in terms of maximizing the range of motion; they also offer wider excellent zones in terms of stability according to the criteria used in this study. However, a previous computer-simulation study demonstrated a decrease in the contact area with increasing abduction of the cup, which may increase the potential for wear¹². The abduction angle of the cup may be constrained intraoperatively by osseous coverage. Femoral anteversion may be similarly dictated by the anatomy of the femoral canal, especially when the implant is inserted without cement. In the event that the chosen acetabular abduction angle is not optimum, the potential for prosthetic impingement can be minimized by selecting the appropriate degree of anteversion. Too little anteversion of either the femoral or the acetabular component decreases flexion, while too much anteversion reduces extension and adduction. Acetabular abduction angles of less than 45 degrees tend to decrease flexion and abduction; this can be countered by increasing acetabular or femoral anteversion, or both. Higher acetabular abduction angles increase flexion and abduction but may reduce extension, adduction, and external rotation, especially if combined with too much femoral or acetabular anteversion, or both. Therefore, for each combination of acetabular abduction and femoral anteversion, there is an optimum range of acetabular anteversion that may minimize impingement and thus give the patient the potential for a maximum range of motion.

Head size is known to influence range of motion, wear, and dislocation^1,4,7,8. An increase in head size increases the range of motion and stability but also increases volumetric wear^10,11. Neck extensions in modular components sometimes necessitate the addition of a sleeve or flange to the head, effectively decreasing the head-neck ratio as well as the range of motion. This phenomenon was recently found to be associated with increased radiographic wear¹⁴.

Surgeons face a complex decision in selecting the appropriate design features. Knowledge of the effect of these features, singly and in combination with other factors, can lead to better selection of the implant, with positive effects with regard to the postoperative range of motion and decreased impingement, dislocation, and wear.

The computer model of the hip that is presented in this study is relatively easy to reproduce. Any combination of positions of the acetabular and femoral components and head-neck ratios can be studied. Although our model uses generic design features, specific design parameters also can be implemented either with use of data provided by the manufacturer or by digitizing the surface of an actual prosthesis with a high-resolution digitizing stylus. The effect of design features such as polyethylene liner lips and offset thus can be investigated, and the findings can be used to assist the surgeon in choosing the appropriate implant.

References

Introduction | Materials and Methods | Results | Discussion | References

Chandler, D. R.; Glousman, R.; Hull, D.; McGuire, P. J.; Kim, I. S.; Clarke, I. C.; and Sarmiento, A.: Prosthetic hip range of motion and impingement. The effects of head and neck geometry. Clin. Orthop.,166: 284-291, 1982.166284 1982 [PubMed]

Charnley, J.: Total hip replacement by low-friction arthroplasty. Clin. Orthop.,72: 7-21, 1970.727 1970 [PubMed]

Coventry, M. B.: Late dislocations in patients with Charnley total hip arthroplasty. J. Bone and Joint Surg.,67-A: 832-841, July 1985.67-A832 1985

Elfick, A. P.; Hall, R. M.; Pinder, I. M.; and Unsworth, A.: Wear in retrieved acetabular components: effect of femoral head radius and patient parameters. J. Arthroplasty,13: 291-295, 1998.13291 1998 [PubMed]

Harris, W. H.: Advances in surgical technique for total hip replacement: without and with osteotomy of the greater trochanter. Clin. Orthop.,146: 188-204, 1980.146188 1980 [PubMed]

Johnston, R. C., and Smidt, G. L.: Hip motion measurements for selected activities of daily living. Clin. Orthop.,72: 205-215, 1970.72205 1970 [PubMed]

Kelley, S. S.; Lachiewicz, P. F.; Hickman, J. M.; and Paterno, S. M.: Relationship of femoral head and acetabular size to the prevalence of dislocation. Clin. Orthop.,355: 163-170, 1998.355163 1998 [PubMed]

Krushell, R. J.; Burke, D. W.; and Harris, W. H.: Range of motion in contemporary total hip arthroplasty. The impact of modular head-neck components. J. Arthroplasty,6: 97-101, 1991.697 1991 [PubMed]

Lewinnek, G. E.; Lewis, J. L.; Tarr, R.; Compere, C. L.; and Zimmerman, J. R.: Dislocations after total hip-replacement arthroplasties. J. Bone and Joint Surg.,60-A: 217-220, March 1978.60-A217 1978

Livermore, J.; Ilstrup, D.; and Morrey, B.: Effect of femoral head size on wear of the polyethylene acetabular component. J. Bone and Joint Surg.,72-A: 518-528, April 1990.72-A518 1990

Maxian, T. A.; Brown, T. D.; Pedersen, D. R.; and Callaghan, J. J.: Adaptive finite element modeling of long-term polyethylene wear in total hip arthroplasty. J. Orthop. Res.,14: 668-675, 1996.14668 1996 [PubMed]

Robinson, R. P.; Simonian, P. T.; Gradisar, I. M.; and Ching, R. P.: Joint motion and surface contact area related to component position in total hip arthroplasty. J. Bone and Joint Surg.,79-B(1): 140-146, 1997.79-B(1)140 1997

Seki, M.; Yuasa, N.; and Ohkuni, K.: Analysis of optimal range of socket orientations in total hip arthroplasty with use of computer-aided design simulation. J. Orthop. Res.,16: 513-517, 1998.16513 1998 [PubMed]

Urquhart, A. G.; D'Lima, D. D.; Venn-Watson, E.; Colwell, C. W., Jr.; and Walker, R. H.: Polyethylene wear after total hip arthroplasty: the effect of a modular femoral head with an extended flange-reinforced neck. J. Bone and Joint Surg.,80-A: 1641-1647, Nov. 1998.80-A1641 1998

Wu, G., and Cavanagh, P. R.: ISB recommendations for standardization in the reporting of kinematic data. J. Biomech.,28: 1257-1261, 1995.281257 1995 [PubMed]

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FIG1:: Fig. 1 Rendered image of computer simulation demonstrating the axes around which the prosthetic orientation is described. The curved arrows indicate the positive direction of rotation.

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TABLE I: COMBINATIONS OF PROSTHETIC ORIENTATIONS THAT PERMIT TYING A SHOE AND STOOPING⁶

Acetabular Abduction (degrees)	Femoral Anteversion (degrees)	Acetabular Anteversion (degrees)
35	0	No position
35	10	=10
35	20	All positions
35	30	=10
45	All positions	All positions
55	All positions	All positions

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TABLE I: COMBINATIONS OF PROSTHETIC ORIENTATIONS THAT PERMIT TYING A SHOE AND STOOPING⁶

Acetabular Abduction (degrees)	Femoral Anteversion (degrees)	Acetabular Anteversion (degrees)
35	0	No position
35	10	=10
35	20	All positions
35	30	=10
45	All positions	All positions
55	All positions	All positions