- Email: [email protected]
Three-Dimensional Hip Anatomy in Osteoarthritis
The Journal of Arthroplasty Vol. 24 No. 6 2009
Three-Dimensional Hip Anatomy in Osteoarthritis Analysis of the Femoral Offset Elhadi Sariali, MD,*y Alexandre Mouttet, MD,z Gilles Pasquier, MD,§ and Ernesto Durante§
Abstract: Two hundred twenty-three patients with osteoarthritic hips were analyzed using computed tomography and a specific image processing software (HIP-PLAN®) to determine 3-dimensional morphological data of the hip focusing on femoral offset (FO). Mean FO was found to be 42.2 ± 5.1 mm, 2.2 mm greater than the 2-dimensional FO values reported in the literature. The FO was found to be above 45 mm in 31% of patients and greater than 50 mm in 12%. The error associated with the use of conventional plane x-rays to measure FO was found to be 3.5 ± 2.5 mm, the x-ray technique generally underestimating the measure of FO. The sum of acetabular and femoral anteversion was found to be out of the safe zone regarding dislocation risk in 47% of patients. Keywords: 3-dimensional, hip anatomy, femoral offset, prosthesis design, modular neck. © 2009 Elsevier Inc. All rights reserved.
the femoral head, and anteversions were determined. The results of the study define requirements for prosthesis designs that restore patient hip anatomy more closely. This should improve functional outcomes and reduce mechanical complications such as dislocation. This study allowed the errors associated with the use of the conventional plane x-rays technique to be quantified.
Leg length and offset restoration is known to improve function after total hip arthroplasty and to minimize the risk of complications such as dislocation and limp [1,2]. Usually, femoral offset (FO) is measured on plain x-rays; however, femoral anteversion and external rotation lead to errors in the real offset because they correspond to the projection on the frontal view in a 2-dimensional (2-D) plane. To our knowledge, there is no 3-dimensional (3-D) analysis of FO in the literature among patients undergoing hip arthroplasty. The goal of the study was to determine 3-D morphological data of the hip in a cohort of white patients undergoing hip arthroplasty for current osteoarthritis. Accurate measures of FO, height of
Materials and Methods Two hundred twenty-three hips with primary idiopathic osteoarthritis have been analyzed using computed tomography (CT) scanning with images processed using the HIP-PLAN® (Symbios, Switzerland) software. The software allowed image postprocessing for reorienting the pelvis or the femur to a standardized orientation to analyze hip anatomy.
From the *Medical Engineering Department, Leeds University, United Kingdom; yHopítal Pitié Salpétrière, Paris, France; zPolyclinic Saint-Roch, France; and §Centre Hospitalier de Roubaix, France. Submitted July 7, 2007; accepted April 18, 2008. No benefits or funds were received in support of the study. Reprint requests: Elhadi Sariali, MD, 154 Rue de Picpus 75012 Paris. © 2009 Elsevier Inc. All rights reserved. 0883-5403/08/2406-0024$36.00/0 doi:10.1016/j.arth.2008.04.031
Tomography Spiral CT Protocol Patients were scanned in the supine position with neutral rotation of the lower limb. Spiral CT included the pelvis from the iliac crest until the femoral
990
Three-Dimensional Hip Anatomy in Osteoarthritis Sariali et al
991
Fig. 1. Determination of the frontal plane and the APP plane.
isthmus, which corresponds to the narrowest section of femoral diaphysis. Six CT slices were also performed on the knee. Description of Software HIP-PLAN The software shows simultaneously 3 windows with frontal, sagittal, and axial view. It allows the operator to modify the perspective on the hip, performing translation in the 3 directions and rotation around the 3 axes.
During the CT scan, patient position is not perfectly controlled, often giving asymmetry of the pelvis that makes calculation inaccurate (Fig. 1). Thus, image processing, allowed the pelvis to be reoriented to remove unwanted pelvic rotations. The bicondylar axis of the knee was determined on an axial view to calculate anteversions. The acetabulum analysis was achieved in 2 planes: the anterior pelvic plane (APP plane) as defined by the anterior superior iliac spines and
Fig. 2. Determination of the femoral head center and the acetabular center.
992 The Journal of Arthroplasty Vol. 24 No. 6 September 2009
Fig. 3. Determination of the femoral frame to evaluate the intramedullary anatomy.
pubic tubercles as references, and the frontal plane using the craniopodal axis as suggested by Murray [3] (Fig. 1). For determination of the APP plane, the volume was reoriented until the frontal view matched with the APP plane (Fig. 1). The position of the acetabulum was determined using a hemisphere that was superimposed upon it on the 3 views (Fig. 2). The software provided values of acetabulum anteversion and inclination angles in the APP plane and also in the frontal plane, which corresponds to the value according to the method of Murray [3]. The femoral analysis was performed in a femoral frame, which was determined to maximize the view of the femoral metaphyseal canal, on both the frontal and the sagittal view. The origin of the femoral frame was the center of a
cross-section through the base of the femoral neck as used by Billing [4] and Murphy et al [5]. The medial-lateral axis was chosen parallel to the femoral metaphyseal axis on a cross-section through the base of the neck, and the craniopodal axis was parallel to the femoral diaphyseal axis (Fig. 3). A sphere was superimposed on the femoral head (Fig. 2) to determine the coordinates of its center. Afterward, 2 measurements were made: the FO as the distance between the center of the femoral head and the femur axis, and the height of the femoral head center from the tip of the great trochanter (Fig. 4). The value of the femoral neck angle (FNA) was measured in the plane defined by the neck's axis and the femoral axis. These values cannot be measured accurately on 2-D x-rays because they vary with hip rotation. The femoral anteversion was calculated according to the technique described by Murphy et al [5]. Studied Parameters The following measurements were made:
Fig. 4. The FO and the height of the femoral head center from the tip of the great trochanter were calculated in the femoral frame.
1. the vector perpendicular to the APP plane, which gives its direction; 2. the vector perpendicular to the frontal view of the femur frame; 3. coordinates of acetabulum center, inclination, and anteversion of the acetabulum in the APP plane and in the frontal plane. Anteversion in the frontal plane was calculated according to the method of Murray [3] and was used as the reference value for the acetabular anteversion; 4. the FO and its projection on the frontal plane that corresponds to its 2-D measure on plan x-rays; 5. the height between the tip of the greater trochanter and the femoral head center measured in the femur frame;
Three-Dimensional Hip Anatomy in Osteoarthritis Sariali et al
993
Table 1. Reproducibility and Precision of the Measurements in Millimeters or Degrees
Correlation score Precision
Femur frame origin
Acetabulum inclination
Acetabulum anteversion
Femoral head center
FO
FNA
Distance great trochanter, head center
Femoral anteversion
0.98 1.2 mm
0.92 3.5°
0.93 1.4°
0.99 0.7 mm
0.98 1 mm
0.94 2°
0.91 1 mm
0.96 2.2°
6. the femoral anteversion as described by Murphy et al [5]; 7. the FNA measured in the plane determined by the neck's axis and the femur axis.
analyzed using a Student t test. For abnormally distributed variables or normally distributed variables with different variances, we used the MannWhitney test. A P value of less than .05 was considered to be significant.
Statistical Methods Method Accuracy Determination Intraobserver variations were determined from 30 randomly selected sets of CT scan measured initially and after 1 week by the same user. Intraclass correlation was then measured and evaluated using the grouping recommended by Landis and Koch [6] for the κ statistics. Scores between 0.61 and 0.8 represented substantial agreements, and those greater than 0.81 represented almost perfect agreements. For the determination of reproducibility of the APP plane and the femoral frontal plane, a perpendicular vector was used to calculate the intraclass correlation of its coordinates, and also the angle between the 2 vector measurements. The precision of all the parameters, calculated as the average plus twice the SD of the differential between the 2 measurements, was also determined. Comparison of Variables Distribution of variables was tested for normality using the Ryan-Joiner and Shapiro-Wilk tests. For normally distributed variables, when 2 groups had the same variances, differences between them were
Results Method Accuracy Femoral Frame. For the direction vector, there were perfect agreement and good precision because the intraclass correlation score was 0.9, 0.99, and 0.99 for the x, y, and z coordinates, respectively. The precision was 2°. Anterior Pelvic Plane. For the direction vector, there were perfect agreement and good precision because the intraclass correlation score was 0.99, 0.92, and 0.97 for the x, y, and z coordinates, respectively. The precision was 2°. All measurements had a correlation score ranging between 0.91 and 0.99. Results are shown in Table 1. Parameter Values for Acetabulum The APP plane was anteverted an average of 4° ± 5°, facing forward and downward. The mean acetabular inclination angle was 48° ± 8° in the APP plane and 45.8 ± 6.4° in the frontal plane. The acetabular inclination was significantly lower (2.2°) when measured in the frontal plane (P b
Fig. 5. Distribution of acetabular and femoral anteversions.
994 The Journal of Arthroplasty Vol. 24 No. 6 September 2009
Fig. 6. Distribution of the sum of acetabular and femoral anteversions. Only 53% of the patients are in the safe zone defined by Jolles [10].
.0000001). According to the method of Murray [3], the mean acetabular anteversion angle was 26 ± 6.6° in the APP plane and 21.9° ± 6.6° in the frontal plane (Fig. 5). The acetabular anteversion was significantly lower (4.2°) when measured in the frontal plane (P b .0000001). The acetabular offset was 90 ± 6 mm. Parameter Values for Femur The femoral anteversion was 21.9° ± 9.4° (Fig. 5). The sum of acetabular and femoral anteversion was 43° ± 16.8°, with 40% of patients having a value lower than 40° and 7% of patients having a value higher than 60°. Therefore, only 53% of patients had a value inside the safe zone defined by Jolles et al [7] regarding the dislocation risk (between 40° and 60°) (Fig. 6). The mean FO was 42.2 ± 5.1 mm, with 31% of patients greater than 45 mm and 12% of patients
Fig. 8. Difference between the 3-D measure of FO and the 2-D measure of its projection on the frontal plane. Because of femoral anteversion and stiff rotation, 2-D measures are 3.5 mm lower.
greater than 50 mm (Fig. 7). The mean value of the projection of the FO on the frontal view was found to be 38.7 ± 5.9 mm. The difference between the 3-D measure of the FO and its 2-D projection measure on the frontal view was highly significant, ranging between 0 and 13.7 mm, with a mean value found to be 3.5 ± 2.6 mm. Therefore, the 2-D x-rays measurements had an error of 8.6 mm (Fig. 8). The tip of the great trochanter was higher than the center of the femoral head by 9.5 ± 5 mm. The FNA was 129.5° ± 5.5°. There was a correlation between the FNA and the height of the femoral head center from the tip of the great trochanter (correlation coefficient, 0.72). The linear correlation could be described by a linear equation (y = 136.5 + 0.8x), where x is negative if the femoral head center is under the tip of the great trochanter. There was no correlation between FO and femoral anteversion (correlation coefficient = 0.18), and between FO and femoral head center height (correlation coefficient = 0.4) (Fig. 9).
Discussion
Fig. 7. Distribution of FO. Thirty-one percent of patients had an offset higher than 45 mm.
To our knowledge, there are a few CT scan analyses of hip anatomy in the literature [8-11], with none investigating the 3-D measure of FO among patients undergoing hip arthroplasty. The HIP-PLAN software was found to have an excellent reproducibility and precision. It allowed preoperative 3-D analysis, especially for anteversion and offset. The FO values found in our study are close to the anatomical data reported by Noble et al [8] in their cadaveric study of 200 femora (43 ± 6.8 mm). Femoral offset is usually analyzed clinically using plane x-rays; thus, the measurements correspond to the projections on the frontal plane. The anatomical
Three-Dimensional Hip Anatomy in Osteoarthritis Sariali et al
995
Fig. 9. Plot of femoral head center using FO (mm) and height from the great trochanter edge (millimeters). Circles correspond to 1 SD and 2 SDs.
femoral anteversion is approximately 22° and could reach more than 50° in some patients, so there is a distortion between the real offset and the value of its projection on the frontal view, because the latter is lower, with the difference found to be ranging from 0 to 13.7 mm (average, 3.5 ± 2.6) in this study. The authors postulate that FO is likely underestimated on x-rays because of the femoral anteversion and the stiff rotation frequently observed in degenerative hips with osteoarthritis. This distortion between the x-ray values and real values of the offset has also been reported by De Thomasson et al [12] who found that FO was underestimated when planned on x-rays in up to 40% of cases. Husmann et al [9] reported also lower values for FO when measured on x-rays (40 ± 0.5 mm). Asayama et al [1] reported that functional manifestation of abductor weakness began to appear when the FO decreased by 12% (5 mm) and that patients decompensated functionally for a decrease of more than 28% (11.7 mm). Hence, the under-
estimation related to the use of x-rays could affect the clinical outcome. The mean FO was found to be 42.2 mm, with 31% of the patients having a value greater than 45 mm, which is unlikely to be restored with a current nonmodular commercially available implant. This rate of patients is similar to the results of Bourne and Rorabeck [2] who found that offset was not restored in 60% of patients when using a 135° neck-shaft angle and in 32% of patients if using a 131° neckshaft angle. After femoral neck osteotomy is performed, it is important to reconstruct the hip by positioning the center of the femoral head at the same position as before the hip destruction by osteoarthritis to restore the offset and the lower limb length. Thus, anatomical landmarks are needed to control these parameters. Two independent 3-D parameters, the height of the femoral head center from the tip of the great trochanter and the FO, were defined to an accuracy of 1 mm. These 2 parameters were selected because
996 The Journal of Arthroplasty Vol. 24 No. 6 September 2009 they are accessible during the procedure; thus, we could quantify the quality of reconstruction. Recommendations for the values of the implant characteristics depend on the author [13-16]. If the implant anteversion was identical to the anatomical preoperative data, it was found that only 53% of the patients were in the safe zone defined by Jolles et al [7] for dislocation risk. Forty percent had an increased risk for a posterior dislocation and 7% for an anterior one. Cup anteversion was required to increase by 6° to an average cup anteversion of approximately 28° to displace all the patients into the safe zone defined by Jolles. Acetabular anteversion depends on the plane where it is calculated. The sagittal angle between the APP and the frontal plane was found to be in the order of 4° ± 5°, giving a difference between the values of acetabular anteversion in these 2 planes of 4.2 ± 6. Thus, the value of acetabular anteversion in the APP depends on the APP angle precision and varies by 1° when the APP angle varies also by 1°. This sagittal APP angle could be calculated on x-rays but is less accurate than our technique. In fact, Eckman et al [17] showed that a radiographic calculation using lateral pelvic radiographs could be accurate (SD, 1.38; precision, 2.8°); however, failure to correctly observe a landmark can introduce large errors (12.4°). Iliac spinous are difficult to spot, which explains the inaccuracy of APP determination in clinical practice as shown by Spencer et al [18] who found a precision of 19.2° (SD, 9.6°). No correlation was found in this study between FO and femoral anteversion, which confirms the results of Krishnan et al [19] who highlighted that extramedullary anatomy was probably independent of the intramedullary one. The determination of FNA was less accurate than the height of femoral head center from the great trochanter edge. This was thought to be related to the fact that the femoral neck is not an exact cylinder; thus, its axis determination is not accurate. For preoperative planning, FNA should not be used; rather, it would be better to use the height of femoral head center from the great trochanter edge because the later is more accurate and could be detected during surgical procedure. The tip of the greater trochanter was, on average, 9.5 mm higher than the femoral head center; this is opposite to the common misconception that, among normal people, these 2 points are on the same perpendicular line to the femur axis. These results are similar to those reported by Krishnan et al [19] (8 mm). Thus, we should be cautious when using the tip of the great trochanter as a height reference. In fact, if it is used during the
operative procedure, a limb lengthening of approximately 9.5 mm could occur. CT scans of the hip give much more information about preoperative anatomy such as offset, accurate values of anteversions, torsional abnormalities, and FNA. Jurik et al [20] proved that a spiral CT is equivalent to 4 radiographies of the pelvis, which is similar to what is done for a patient undergoing a hip arthroplasty where an anteroposterior and a lateral view of the hip and an anteroposterior view of the pelvis are taken. Shuetz et al [21] reported that new multiple detector helical CT reduce x-ray dose to 0.3 mGy, which is 10 times less than the usual spiral CT. It would be safer to use multiple detector helical CT for hip surgery because we could get more information, achieve 3-D planning, and get reconstructed numerical radiographies with a lowdose index equivalent to a lung radiography.
Conclusion The study defined a very accurate means to measure 3-D hip anatomy especially for the determination of offset and the anteversion, allowing optimized designs of hip prosthesis to be determined. The results suggest that current nonmodular commercially available implants may not be able to restore the natural FO in the hip joint. Anatomical stems including modular neck and head may be useful to achieve preoperative anatomical goals. The software HIP-PLAN is an accurate tool to analyze the preoperative hip anatomy.
Acknowledgment The authors thank Mr Todd Stewart from the university of Leeds for his invaluable help in correcting the grammar and syntax errors.
References 1. Asayama I, Chamnongkich S, Simpson KJ, et al. Reconstructed hip joint position and abductor muscle strength after total hip arthroplasty. J Arthroplasty 2005;20:414. 2. Bourne RB, Rorabeck CH. Soft tissue balancing: the hip. J Arthroplasty 2002;4(Suppl 1):17. 3. Murray DW. The definition and measurement of acetabular orientation. J Bone Joint Surg Br 1993;75: 228. 4. Billing L. Roentgen examination of the proximal femur end in children and adolescents. A standardized technique also suitable for determination of the collum anteversion and epiphyseal angles. A study of
Three-Dimensional Hip Anatomy in Osteoarthritis Sariali et al
5. 6.
7.
8.
9.
10.
11.
12.
13.
slipped epiphysis and coxa plana. Acta Radiol 1954; 110 Supplementum. Murphy SB, Simon SR, Kijewski PK, et al. Femoral anteversion. J Bone Joint Surg Am 1987;69:1169. Landis JR, Koch G. The measurement of observer agreement for categorical data. Biometrics 1977;33: 159. Jolles BM, Zangger P, Leyvraz PF. Factors predisposing to dislocation after primary total hip arthroplasty. Multivariate analysis. J Arthroplasty 2002;17:282. Noble PC, Alexander JW, Lindhal LJ, et al. The anatomic basis of femoral component design. Clin Orthop Relat Res 1988;235:148. Husmann O, Rubin PJ, Leyvraz PF, et al. Threedimensional morphology of the proximal femur. J Arthroplasty 1997;12:444. Rubin PJ, Leyvraz PE, Aubaniac JM, et al. The morphology of the proximal femur: a three dimensional radiographic analysis. J Bone Joint Surg Br 1992;74:28. Argenson JN, Ryembault E, Flecher X, et al. Threedimensional anatomy of the hip in osteoarthritis after developmental dysplasia. J Bone Joint Surg Br 2005; 87:1192. De Thomasson E, Mazel C, Guingand O, et al. Value of preoperative planning in total hip arthroplasty. Rev Chir Orthop 2002;88:229. Ranawat GS, Maynard MJ, Desmukh RG. Cemented primary total hip arthroplasty. Master techniques in
14. 15.
16.
17.
18.
19.
20.
21.
997
orthopaedic surgery: the hip. Philadelphia: Lippincott-Raven; 1998. p. 217. Terry Canale S. Campbell's operative orthopaedics. 9th ed. St. Louis: Mosby; 1998. Callaghan JJ, Rosenberg AG, Rubash HE, editors. The adult hip. Philadelphia: Lippincott-Raven; 1998. p. 1142. Morrey BF, Chao E, Cooney W, et al, editors. Joint replacement arthroplasty. New york: Churchill Living-stone; 1991. p. 647. Eckman K, Hafez MA, ED F, et al. Accuracy of pelvic flexion measurements from lateral radiographs. Clin Orthop Relat Res 2006;451:154. Spencer JM, Day RE, Sloan KE, et al. Computer navigation of the acetabular component: a cadaver reliability study. J Bone Joint Surg Br 2006;88:972. Krishnan SP, Carrington RWJ, Mohiyaddin S, et al. Common misconceptions of normal hip joint relations on pelvic radiographs. J Arthroplasty 2006;21: 409. Jurik AG, Jensen LC, Hansen J. Total effective radiation dose from spiral CT and conventional radiography of the pelvis with regard to fracture classification. Acta Radiol 1996;37:651. Schuetz A, Kolokytas O, Wisianowski C, et al. Multiple detector helical CT of the chest can reduce x-ray dose to 0,3 mGy. 85th Scientific Assembly and Annual Meeting. November 28 to December 3. Chicago: McCormick Place; 1999.
Three-Dimensional Hip Anatomy in Osteoarthritis Analysis of the Femoral Offset Elhadi Sariali, MD,*y Alexandre Mouttet, MD,z Gilles Pasquier, MD,§ and Ernesto Durante§
Abstract: Two hundred twenty-three patients with osteoarthritic hips were analyzed using computed tomography and a specific image processing software (HIP-PLAN®) to determine 3-dimensional morphological data of the hip focusing on femoral offset (FO). Mean FO was found to be 42.2 ± 5.1 mm, 2.2 mm greater than the 2-dimensional FO values reported in the literature. The FO was found to be above 45 mm in 31% of patients and greater than 50 mm in 12%. The error associated with the use of conventional plane x-rays to measure FO was found to be 3.5 ± 2.5 mm, the x-ray technique generally underestimating the measure of FO. The sum of acetabular and femoral anteversion was found to be out of the safe zone regarding dislocation risk in 47% of patients. Keywords: 3-dimensional, hip anatomy, femoral offset, prosthesis design, modular neck. © 2009 Elsevier Inc. All rights reserved.
the femoral head, and anteversions were determined. The results of the study define requirements for prosthesis designs that restore patient hip anatomy more closely. This should improve functional outcomes and reduce mechanical complications such as dislocation. This study allowed the errors associated with the use of the conventional plane x-rays technique to be quantified.
Leg length and offset restoration is known to improve function after total hip arthroplasty and to minimize the risk of complications such as dislocation and limp [1,2]. Usually, femoral offset (FO) is measured on plain x-rays; however, femoral anteversion and external rotation lead to errors in the real offset because they correspond to the projection on the frontal view in a 2-dimensional (2-D) plane. To our knowledge, there is no 3-dimensional (3-D) analysis of FO in the literature among patients undergoing hip arthroplasty. The goal of the study was to determine 3-D morphological data of the hip in a cohort of white patients undergoing hip arthroplasty for current osteoarthritis. Accurate measures of FO, height of
Materials and Methods Two hundred twenty-three hips with primary idiopathic osteoarthritis have been analyzed using computed tomography (CT) scanning with images processed using the HIP-PLAN® (Symbios, Switzerland) software. The software allowed image postprocessing for reorienting the pelvis or the femur to a standardized orientation to analyze hip anatomy.
From the *Medical Engineering Department, Leeds University, United Kingdom; yHopítal Pitié Salpétrière, Paris, France; zPolyclinic Saint-Roch, France; and §Centre Hospitalier de Roubaix, France. Submitted July 7, 2007; accepted April 18, 2008. No benefits or funds were received in support of the study. Reprint requests: Elhadi Sariali, MD, 154 Rue de Picpus 75012 Paris. © 2009 Elsevier Inc. All rights reserved. 0883-5403/08/2406-0024$36.00/0 doi:10.1016/j.arth.2008.04.031
Tomography Spiral CT Protocol Patients were scanned in the supine position with neutral rotation of the lower limb. Spiral CT included the pelvis from the iliac crest until the femoral
990
Three-Dimensional Hip Anatomy in Osteoarthritis Sariali et al
991
Fig. 1. Determination of the frontal plane and the APP plane.
isthmus, which corresponds to the narrowest section of femoral diaphysis. Six CT slices were also performed on the knee. Description of Software HIP-PLAN The software shows simultaneously 3 windows with frontal, sagittal, and axial view. It allows the operator to modify the perspective on the hip, performing translation in the 3 directions and rotation around the 3 axes.
During the CT scan, patient position is not perfectly controlled, often giving asymmetry of the pelvis that makes calculation inaccurate (Fig. 1). Thus, image processing, allowed the pelvis to be reoriented to remove unwanted pelvic rotations. The bicondylar axis of the knee was determined on an axial view to calculate anteversions. The acetabulum analysis was achieved in 2 planes: the anterior pelvic plane (APP plane) as defined by the anterior superior iliac spines and
Fig. 2. Determination of the femoral head center and the acetabular center.
992 The Journal of Arthroplasty Vol. 24 No. 6 September 2009
Fig. 3. Determination of the femoral frame to evaluate the intramedullary anatomy.
pubic tubercles as references, and the frontal plane using the craniopodal axis as suggested by Murray [3] (Fig. 1). For determination of the APP plane, the volume was reoriented until the frontal view matched with the APP plane (Fig. 1). The position of the acetabulum was determined using a hemisphere that was superimposed upon it on the 3 views (Fig. 2). The software provided values of acetabulum anteversion and inclination angles in the APP plane and also in the frontal plane, which corresponds to the value according to the method of Murray [3]. The femoral analysis was performed in a femoral frame, which was determined to maximize the view of the femoral metaphyseal canal, on both the frontal and the sagittal view. The origin of the femoral frame was the center of a
cross-section through the base of the femoral neck as used by Billing [4] and Murphy et al [5]. The medial-lateral axis was chosen parallel to the femoral metaphyseal axis on a cross-section through the base of the neck, and the craniopodal axis was parallel to the femoral diaphyseal axis (Fig. 3). A sphere was superimposed on the femoral head (Fig. 2) to determine the coordinates of its center. Afterward, 2 measurements were made: the FO as the distance between the center of the femoral head and the femur axis, and the height of the femoral head center from the tip of the great trochanter (Fig. 4). The value of the femoral neck angle (FNA) was measured in the plane defined by the neck's axis and the femoral axis. These values cannot be measured accurately on 2-D x-rays because they vary with hip rotation. The femoral anteversion was calculated according to the technique described by Murphy et al [5]. Studied Parameters The following measurements were made:
Fig. 4. The FO and the height of the femoral head center from the tip of the great trochanter were calculated in the femoral frame.
1. the vector perpendicular to the APP plane, which gives its direction; 2. the vector perpendicular to the frontal view of the femur frame; 3. coordinates of acetabulum center, inclination, and anteversion of the acetabulum in the APP plane and in the frontal plane. Anteversion in the frontal plane was calculated according to the method of Murray [3] and was used as the reference value for the acetabular anteversion; 4. the FO and its projection on the frontal plane that corresponds to its 2-D measure on plan x-rays; 5. the height between the tip of the greater trochanter and the femoral head center measured in the femur frame;
Three-Dimensional Hip Anatomy in Osteoarthritis Sariali et al
993
Table 1. Reproducibility and Precision of the Measurements in Millimeters or Degrees
Correlation score Precision
Femur frame origin
Acetabulum inclination
Acetabulum anteversion
Femoral head center
FO
FNA
Distance great trochanter, head center
Femoral anteversion
0.98 1.2 mm
0.92 3.5°
0.93 1.4°
0.99 0.7 mm
0.98 1 mm
0.94 2°
0.91 1 mm
0.96 2.2°
6. the femoral anteversion as described by Murphy et al [5]; 7. the FNA measured in the plane determined by the neck's axis and the femur axis.
analyzed using a Student t test. For abnormally distributed variables or normally distributed variables with different variances, we used the MannWhitney test. A P value of less than .05 was considered to be significant.
Statistical Methods Method Accuracy Determination Intraobserver variations were determined from 30 randomly selected sets of CT scan measured initially and after 1 week by the same user. Intraclass correlation was then measured and evaluated using the grouping recommended by Landis and Koch [6] for the κ statistics. Scores between 0.61 and 0.8 represented substantial agreements, and those greater than 0.81 represented almost perfect agreements. For the determination of reproducibility of the APP plane and the femoral frontal plane, a perpendicular vector was used to calculate the intraclass correlation of its coordinates, and also the angle between the 2 vector measurements. The precision of all the parameters, calculated as the average plus twice the SD of the differential between the 2 measurements, was also determined. Comparison of Variables Distribution of variables was tested for normality using the Ryan-Joiner and Shapiro-Wilk tests. For normally distributed variables, when 2 groups had the same variances, differences between them were
Results Method Accuracy Femoral Frame. For the direction vector, there were perfect agreement and good precision because the intraclass correlation score was 0.9, 0.99, and 0.99 for the x, y, and z coordinates, respectively. The precision was 2°. Anterior Pelvic Plane. For the direction vector, there were perfect agreement and good precision because the intraclass correlation score was 0.99, 0.92, and 0.97 for the x, y, and z coordinates, respectively. The precision was 2°. All measurements had a correlation score ranging between 0.91 and 0.99. Results are shown in Table 1. Parameter Values for Acetabulum The APP plane was anteverted an average of 4° ± 5°, facing forward and downward. The mean acetabular inclination angle was 48° ± 8° in the APP plane and 45.8 ± 6.4° in the frontal plane. The acetabular inclination was significantly lower (2.2°) when measured in the frontal plane (P b
Fig. 5. Distribution of acetabular and femoral anteversions.
994 The Journal of Arthroplasty Vol. 24 No. 6 September 2009
Fig. 6. Distribution of the sum of acetabular and femoral anteversions. Only 53% of the patients are in the safe zone defined by Jolles [10].
.0000001). According to the method of Murray [3], the mean acetabular anteversion angle was 26 ± 6.6° in the APP plane and 21.9° ± 6.6° in the frontal plane (Fig. 5). The acetabular anteversion was significantly lower (4.2°) when measured in the frontal plane (P b .0000001). The acetabular offset was 90 ± 6 mm. Parameter Values for Femur The femoral anteversion was 21.9° ± 9.4° (Fig. 5). The sum of acetabular and femoral anteversion was 43° ± 16.8°, with 40% of patients having a value lower than 40° and 7% of patients having a value higher than 60°. Therefore, only 53% of patients had a value inside the safe zone defined by Jolles et al [7] regarding the dislocation risk (between 40° and 60°) (Fig. 6). The mean FO was 42.2 ± 5.1 mm, with 31% of patients greater than 45 mm and 12% of patients
Fig. 8. Difference between the 3-D measure of FO and the 2-D measure of its projection on the frontal plane. Because of femoral anteversion and stiff rotation, 2-D measures are 3.5 mm lower.
greater than 50 mm (Fig. 7). The mean value of the projection of the FO on the frontal view was found to be 38.7 ± 5.9 mm. The difference between the 3-D measure of the FO and its 2-D projection measure on the frontal view was highly significant, ranging between 0 and 13.7 mm, with a mean value found to be 3.5 ± 2.6 mm. Therefore, the 2-D x-rays measurements had an error of 8.6 mm (Fig. 8). The tip of the great trochanter was higher than the center of the femoral head by 9.5 ± 5 mm. The FNA was 129.5° ± 5.5°. There was a correlation between the FNA and the height of the femoral head center from the tip of the great trochanter (correlation coefficient, 0.72). The linear correlation could be described by a linear equation (y = 136.5 + 0.8x), where x is negative if the femoral head center is under the tip of the great trochanter. There was no correlation between FO and femoral anteversion (correlation coefficient = 0.18), and between FO and femoral head center height (correlation coefficient = 0.4) (Fig. 9).
Discussion
Fig. 7. Distribution of FO. Thirty-one percent of patients had an offset higher than 45 mm.
To our knowledge, there are a few CT scan analyses of hip anatomy in the literature [8-11], with none investigating the 3-D measure of FO among patients undergoing hip arthroplasty. The HIP-PLAN software was found to have an excellent reproducibility and precision. It allowed preoperative 3-D analysis, especially for anteversion and offset. The FO values found in our study are close to the anatomical data reported by Noble et al [8] in their cadaveric study of 200 femora (43 ± 6.8 mm). Femoral offset is usually analyzed clinically using plane x-rays; thus, the measurements correspond to the projections on the frontal plane. The anatomical
Three-Dimensional Hip Anatomy in Osteoarthritis Sariali et al
995
Fig. 9. Plot of femoral head center using FO (mm) and height from the great trochanter edge (millimeters). Circles correspond to 1 SD and 2 SDs.
femoral anteversion is approximately 22° and could reach more than 50° in some patients, so there is a distortion between the real offset and the value of its projection on the frontal view, because the latter is lower, with the difference found to be ranging from 0 to 13.7 mm (average, 3.5 ± 2.6) in this study. The authors postulate that FO is likely underestimated on x-rays because of the femoral anteversion and the stiff rotation frequently observed in degenerative hips with osteoarthritis. This distortion between the x-ray values and real values of the offset has also been reported by De Thomasson et al [12] who found that FO was underestimated when planned on x-rays in up to 40% of cases. Husmann et al [9] reported also lower values for FO when measured on x-rays (40 ± 0.5 mm). Asayama et al [1] reported that functional manifestation of abductor weakness began to appear when the FO decreased by 12% (5 mm) and that patients decompensated functionally for a decrease of more than 28% (11.7 mm). Hence, the under-
estimation related to the use of x-rays could affect the clinical outcome. The mean FO was found to be 42.2 mm, with 31% of the patients having a value greater than 45 mm, which is unlikely to be restored with a current nonmodular commercially available implant. This rate of patients is similar to the results of Bourne and Rorabeck [2] who found that offset was not restored in 60% of patients when using a 135° neck-shaft angle and in 32% of patients if using a 131° neckshaft angle. After femoral neck osteotomy is performed, it is important to reconstruct the hip by positioning the center of the femoral head at the same position as before the hip destruction by osteoarthritis to restore the offset and the lower limb length. Thus, anatomical landmarks are needed to control these parameters. Two independent 3-D parameters, the height of the femoral head center from the tip of the great trochanter and the FO, were defined to an accuracy of 1 mm. These 2 parameters were selected because
996 The Journal of Arthroplasty Vol. 24 No. 6 September 2009 they are accessible during the procedure; thus, we could quantify the quality of reconstruction. Recommendations for the values of the implant characteristics depend on the author [13-16]. If the implant anteversion was identical to the anatomical preoperative data, it was found that only 53% of the patients were in the safe zone defined by Jolles et al [7] for dislocation risk. Forty percent had an increased risk for a posterior dislocation and 7% for an anterior one. Cup anteversion was required to increase by 6° to an average cup anteversion of approximately 28° to displace all the patients into the safe zone defined by Jolles. Acetabular anteversion depends on the plane where it is calculated. The sagittal angle between the APP and the frontal plane was found to be in the order of 4° ± 5°, giving a difference between the values of acetabular anteversion in these 2 planes of 4.2 ± 6. Thus, the value of acetabular anteversion in the APP depends on the APP angle precision and varies by 1° when the APP angle varies also by 1°. This sagittal APP angle could be calculated on x-rays but is less accurate than our technique. In fact, Eckman et al [17] showed that a radiographic calculation using lateral pelvic radiographs could be accurate (SD, 1.38; precision, 2.8°); however, failure to correctly observe a landmark can introduce large errors (12.4°). Iliac spinous are difficult to spot, which explains the inaccuracy of APP determination in clinical practice as shown by Spencer et al [18] who found a precision of 19.2° (SD, 9.6°). No correlation was found in this study between FO and femoral anteversion, which confirms the results of Krishnan et al [19] who highlighted that extramedullary anatomy was probably independent of the intramedullary one. The determination of FNA was less accurate than the height of femoral head center from the great trochanter edge. This was thought to be related to the fact that the femoral neck is not an exact cylinder; thus, its axis determination is not accurate. For preoperative planning, FNA should not be used; rather, it would be better to use the height of femoral head center from the great trochanter edge because the later is more accurate and could be detected during surgical procedure. The tip of the greater trochanter was, on average, 9.5 mm higher than the femoral head center; this is opposite to the common misconception that, among normal people, these 2 points are on the same perpendicular line to the femur axis. These results are similar to those reported by Krishnan et al [19] (8 mm). Thus, we should be cautious when using the tip of the great trochanter as a height reference. In fact, if it is used during the
operative procedure, a limb lengthening of approximately 9.5 mm could occur. CT scans of the hip give much more information about preoperative anatomy such as offset, accurate values of anteversions, torsional abnormalities, and FNA. Jurik et al [20] proved that a spiral CT is equivalent to 4 radiographies of the pelvis, which is similar to what is done for a patient undergoing a hip arthroplasty where an anteroposterior and a lateral view of the hip and an anteroposterior view of the pelvis are taken. Shuetz et al [21] reported that new multiple detector helical CT reduce x-ray dose to 0.3 mGy, which is 10 times less than the usual spiral CT. It would be safer to use multiple detector helical CT for hip surgery because we could get more information, achieve 3-D planning, and get reconstructed numerical radiographies with a lowdose index equivalent to a lung radiography.
Conclusion The study defined a very accurate means to measure 3-D hip anatomy especially for the determination of offset and the anteversion, allowing optimized designs of hip prosthesis to be determined. The results suggest that current nonmodular commercially available implants may not be able to restore the natural FO in the hip joint. Anatomical stems including modular neck and head may be useful to achieve preoperative anatomical goals. The software HIP-PLAN is an accurate tool to analyze the preoperative hip anatomy.
Acknowledgment The authors thank Mr Todd Stewart from the university of Leeds for his invaluable help in correcting the grammar and syntax errors.
References 1. Asayama I, Chamnongkich S, Simpson KJ, et al. Reconstructed hip joint position and abductor muscle strength after total hip arthroplasty. J Arthroplasty 2005;20:414. 2. Bourne RB, Rorabeck CH. Soft tissue balancing: the hip. J Arthroplasty 2002;4(Suppl 1):17. 3. Murray DW. The definition and measurement of acetabular orientation. J Bone Joint Surg Br 1993;75: 228. 4. Billing L. Roentgen examination of the proximal femur end in children and adolescents. A standardized technique also suitable for determination of the collum anteversion and epiphyseal angles. A study of
Three-Dimensional Hip Anatomy in Osteoarthritis Sariali et al
5. 6.
7.
8.
9.
10.
11.
12.
13.
slipped epiphysis and coxa plana. Acta Radiol 1954; 110 Supplementum. Murphy SB, Simon SR, Kijewski PK, et al. Femoral anteversion. J Bone Joint Surg Am 1987;69:1169. Landis JR, Koch G. The measurement of observer agreement for categorical data. Biometrics 1977;33: 159. Jolles BM, Zangger P, Leyvraz PF. Factors predisposing to dislocation after primary total hip arthroplasty. Multivariate analysis. J Arthroplasty 2002;17:282. Noble PC, Alexander JW, Lindhal LJ, et al. The anatomic basis of femoral component design. Clin Orthop Relat Res 1988;235:148. Husmann O, Rubin PJ, Leyvraz PF, et al. Threedimensional morphology of the proximal femur. J Arthroplasty 1997;12:444. Rubin PJ, Leyvraz PE, Aubaniac JM, et al. The morphology of the proximal femur: a three dimensional radiographic analysis. J Bone Joint Surg Br 1992;74:28. Argenson JN, Ryembault E, Flecher X, et al. Threedimensional anatomy of the hip in osteoarthritis after developmental dysplasia. J Bone Joint Surg Br 2005; 87:1192. De Thomasson E, Mazel C, Guingand O, et al. Value of preoperative planning in total hip arthroplasty. Rev Chir Orthop 2002;88:229. Ranawat GS, Maynard MJ, Desmukh RG. Cemented primary total hip arthroplasty. Master techniques in
14. 15.
16.
17.
18.
19.
20.
21.
997
orthopaedic surgery: the hip. Philadelphia: Lippincott-Raven; 1998. p. 217. Terry Canale S. Campbell's operative orthopaedics. 9th ed. St. Louis: Mosby; 1998. Callaghan JJ, Rosenberg AG, Rubash HE, editors. The adult hip. Philadelphia: Lippincott-Raven; 1998. p. 1142. Morrey BF, Chao E, Cooney W, et al, editors. Joint replacement arthroplasty. New york: Churchill Living-stone; 1991. p. 647. Eckman K, Hafez MA, ED F, et al. Accuracy of pelvic flexion measurements from lateral radiographs. Clin Orthop Relat Res 2006;451:154. Spencer JM, Day RE, Sloan KE, et al. Computer navigation of the acetabular component: a cadaver reliability study. J Bone Joint Surg Br 2006;88:972. Krishnan SP, Carrington RWJ, Mohiyaddin S, et al. Common misconceptions of normal hip joint relations on pelvic radiographs. J Arthroplasty 2006;21: 409. Jurik AG, Jensen LC, Hansen J. Total effective radiation dose from spiral CT and conventional radiography of the pelvis with regard to fracture classification. Acta Radiol 1996;37:651. Schuetz A, Kolokytas O, Wisianowski C, et al. Multiple detector helical CT of the chest can reduce x-ray dose to 0,3 mGy. 85th Scientific Assembly and Annual Meeting. November 28 to December 3. Chicago: McCormick Place; 1999.