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CLINICAL RESEARCH

Does Patient-specific Functional Pelvic Tilt Affect Joint Contact Pressure in Hip Dysplasia? A Finite-element Analysis Study

Kitamura, Kenji MD1; Fujii, Masanori MD, PhD1; Ikemura, Satoshi MD, PhD1; Hamai, Satoshi MD, PhD1; Motomura, Goro MD, PhD1; Nakashima, Yasuharu MD, PhD1

Author Information
Clinical Orthopaedics and Related Research: August 2021 - Volume 479 - Issue 8 - p 1712-1724
doi: 10.1097/CORR.0000000000001737
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Abstract

Introduction

Hip dysplasia is common, and it increases the likelihood that osteoarthritis will develop [16]. Its anatomic features include insufficient acetabular coverage of the femoral head and shallow acetabular concavity [32], with substantial individual variations in acetabular version and deficiency type [33]. These deformities lead to structural instability, reduced load bearing on the joint surface area, and abnormal distribution of joint stress on the articular cartilage [6, 17], sometimes resulting in premature hip degeneration [10].

Individual-specific, finite-element modeling is useful for predicting the mechanical behavior of normal and dysplastic hips [1] and is used for simulating and planning periacetabular osteotomy in patients with hip dysplasia [26, 49]. Previous studies involving finite-element analyses of hip contact mechanics have used either the supine position or the standardized pelvic position, which is based on the anterior pelvic plane coordinate system, as a reference [17, 26, 45, 49]. However, recent studies have revealed that the sagittal pelvic tilt varies widely among candidates for hip preservation surgery and suggested that assessments in the supine or standard pelvic position may overlook changes in acetabular version and coverage in the weightbearing position [38, 39, 42]. Data from another study using an individual-specific, finite-element analysis suggested that the sagittal pelvic tilt alters the loading environment and stress distribution of the hip and may accelerate degeneration in the dysplastic hip [20]. To date, the effects of individual and postural variations in physiologic pelvic tilt on the mechanical environment of the dysplastic hip are unknown. Individual-specific, finite-element analyses that account for physiologic pelvic tilt may provide valuable insight into the contact mechanics of dysplastic hips, which can lead to improved diagnosis and treatment in this patient population.

We therefore used finite-element analysis to ask whether there are differences between patients with hip dysplasia and patients without dysplasia in terms of (1) physiologic pelvic tilt, (2) the pelvic position and joint contact pressure, and (3) the morphologic factors associated with joint contact pressure.

Patients and Methods

Patients

Between September 2016 and December 2019, 82 patients with symptomatic hip dysplasia underwent transposition osteotomy of the acetabulum [9]. Supine and standing AP pelvic radiographs and CT images were obtained preoperatively. The inclusion criterion for this study, which was met by 70 patients (70 hips), was the presence of hip dysplasia defined as a lateral center-edge angle [46] ≥ 0° and < 20° on AP pelvic radiographs acquired with the patient in the supine position. Among patients with bilateral hip dysplasia, the operated-on side was investigated. Based on our exclusion criteria, we omitted 11 patients with advanced osteoarthritis (Tönnis grade ≥ 2 [41]), one with a major femoral head deformity, 19 with previous hip surgeries on either joint, one who previously underwent surgery for spinal disease, and six with images of insufficient quality for analysis. Thus, 32 patients (32 hips) were ultimately eligible for this study (Table 1). The mean age of the patients (all of whom were female) was 39 ± 10 years.

Table 1. - Patient demographic and radiographic data
Parameters Hip dysplasia (n = 32 hips) Control (n = 16 hips) p value
Age in years 39 ± 10 36 ± 7 0.08
Sex
 Female 100 (32) 100 (16) > 0.99
BMI in kg/m2 22 ± 3 21 ± 3 0.34
Height in cm 157 ± 6 159 ± 6 0.65
Weight in kg 55 ± 9 53 ± 8 0.46
Laterality, right hip 63 (20) 50 (8) 0.41
Tönnis classification < 0.001
 Grade 0 59 (19) 100 (16)
 Grade 1 41 (13) 0 (0)
Lateral center-edge angle in ° 11 ± 5 32 ± 4 < 0.001
Data presented as mean ± SD or % (n).

For the control group, we reviewed AP pelvic radiographs and CT images of 33 female volunteers from previous studies [15, 20] who underwent radiographic examinations after being confirmed as having no history of disease or articular symptoms in their hips by medical interviews. Seven patients with frank or borderline hip dysplasia (lateral center-edge angle < 25°) and 10 patients without suitable images were excluded. Ultimately, 16 individuals (16 randomly selected hips; that is, left or right) were included as controls (Table 1). The mean age of the participants was 36 ± 7 years.

CT Evaluations

For all participants, pelvic CT images were acquired from the top of the iliac crest to the distal femur while in the supine position (matrix 512 × 512, field of view 272-619 mm, and slice thickness 1 or 2 mm). After downloading CT data in the Digital Imaging and Communications in Medicine format, two authors (KK, MF) performed the following measurements using the 3-D Template image-processing software (Kyocera Medical Corp). The X and Y axes corresponded to the transverse and sagittal axes, respectively, on an axial CT slice, while the Z axis corresponded to the longitudinal axis of the scanner. First, the pelvic position was standardized by aligning the coordinate system of the CT scanner with that of the anterior pelvic plane as defined by the bilateral anterosuperior iliac spines and midpoint between the pelvic tubercles [3, 25]. Next, the sagittal pelvic tilt on supine and standing radiographs was reproduced on coronal digitally reconstructed radiographs by matching the vertical-to-horizontal ratio of the pelvic foramen (Fig. 1A) [34]. The sagittal pelvic tilt was measured as the angle formed by the anterior pelvic plane and Z axis (anterior pelvic plane [APP] angle), with a positive value representing anterior tilting of the pelvis (Fig. 1B) [38]. The change in pelvic tilt between the supine and standing positions was calculated by subtracting the APP angle in the supine position from that in the standing position (ΔAPP angle); hence, a positive value represented anterior change in pelvic tilt from the supine to the standing positions.

F1
Fig. 1:
A-B (A) These images show the method of matching the sagittal pelvic tilt using radiographs and CT images. On an AP pelvic radiograph, the vertical diameter of the pelvic foramen between the bilateral sacroiliac joints and pubic symphysis (V), divided by the maximum horizontal diameter of the pelvic foramen (H), was calculated. On a digitally reconstructed radiograph created from CT images, the pelvis is rotated sagittally until the ratio of the foramen matches that on the AP pelvic radiograph. (B) The standard, supine, and standing pelvic positions are shown. We defined the standard pelvic position as that where the CT scanner coordinate system matches the anterior pelvic plane coordinate system. The sagittal pelvic tilt in the supine and standing positions was quantified as the angle formed by the anterior pelvic plane and Z axis (anterior pelvic angle).

Morphologic parameters were measured on AP pelvic radiographs and CT images with reference to the standing position. The radiographic parameters included the lateral center-edge angle, Tönnis angle, head extrusion index, crossover sign, posterior wall sign, anterior wall index, and posterior wall index [37, 40]. The CT parameters included the lateral, anterior, and posterior center-edge angle; acetabular roof obliquity; acetabular anteversion; and acetabular inclination angle [31].

Finite-element Models

We used Mechanical Finder software version 10 (Research Center for Computational Mechanics Inc) to create linear finite-element models that described the shape and density distribution of each bone visible on the CT images [44]. Three-dimensional finite-element models of the hemipelvis, proximal femur, and articular cartilage were created using a previously described method (Fig. 2A) [20]. The articular cartilage of the acetabulum and femoral head was modeled with a constant thickness of 1.8 mm as a homogeneous and isotropic material [35, 49]. The mean numbers of finite elements and shell elements did not differ between the hip dysplasia and control group models (1,372,541 versus 1,339,334 [p = 0.31] and 65,501 versus 67,977 [p = 0.11], respectively). To allow for bone heterogeneity, the distribution of bone mineral densities (ρ in g/cm3) of each model component was computed using a standardized equation for determining the bone mineral density from Hounsfield units (HU) as follows [20, 30, 44]: ρ (g/cm3) = [HU + 1.4246] × [0.001/1.058 [ HU value > -1], ρ (g/cm3) = 0 [HU value ≤ -1]. Next, the elastic finite-element modulus was evaluated using the average bone mineral density value of the element, as described by Keyak et al. [19]. We set the Poisson ratio of the bone at 0.3. We set the elastic modulus and the Poisson ratio of the articular cartilage at 15 megapascals (MPa) and 0.45 MPa, respectively [26, 49].

F2
Fig. 2:
A-B (A) A representative finite-element model with distribution of the elastic modulus in a patient with hip dysplasia is shown. The bone model was produced with a 2-mm tetrahedral element and a 0.4-mm triangular shell element on its surface. The cartilage of the acetabulum and femoral heads was created with a constant thickness of 1.8 mm and discretized using a locally refined 0.5-mm to 2.0-mm tetrahedral element in the weightbearing region of the acetabular cartilage. Three nodal shell elements, each with a thickness of 0.0005 mm, were placed on the surface of acetabular cartilage to visualize the contact pressure on the acetabular cartilage. (B) The loading scenario was based on the single-leg stance, with the hip contact force acting on the nodal point at the hip’s center. During loading, the iliac crest and pubic area were completely fixed, and the distal femur was kept free only in the Z direction while restrained in the X and Y directions. Tied- and sliding-contact constraints were set on the cartilage-to-bone and cartilage-to-cartilage interfaces, respectively. Frictional shear stress between the contacting articular surfaces was ignored.

Boundary and Loading Conditions

Using the finite-element models, we performed a nonlinear contact analysis to calculate the joint contact area and contact pressure on the acetabular cartilage. Loading was performed for three pelvic positions reproduced by rotating the pelvis in the sagittal plane around the center of the femoral head: standardized (referring to the anterior pelvic plane), supine, and standing (Fig. 1B). The finite-element models of the femur were standardized by referring to the coordinate system described by the International Society of Biomechanics [47]. The definitions of tied- and sliding-contact constraints were set on the cartilage-to-bone and cartilage-to-cartilage interfaces, as previously described [5]. The iliac crest and pubic area were completely fixed, and the distal femur was kept free only in the Z direction while restrained in the X and Y directions. The loading scenario was based on a single-leg stance, with the hip contact force acting on the node of the hip’s center (Fig. 2B) [4]. A consistent body weight of 500 N was defined for all participants to avoid any scaling effect of the body weight on the absolute contact pressure value. The total joint contact force was set at 1158 N, and the components of the X, Y, and Z axes were set at 150 N, 71 N, and 1146 N, respectively. The loaded nodes were allowed to move only in the direction of the applied load.

Ethical Approval

Ethical approval for this study was obtained from the Graduate School of Medical Sciences, Kyushu University (approval number 30-137) for this retrospective study. All participants in both groups provided written informed consent to participate in this study and were informed of the radiation exposure required.

Statistical Analysis

Using the data described above, we performed the following analyses: (1) comparison of radiographic and CT parameters between the hip dysplasia and control groups, (2) assessment of the relationship between the pelvic position and finite-element model parameters, and (3) assessment of the morphologic factors associated with joint contact pressure.

Two board-certified orthopaedic surgeons (KK, MF) performed all measurements. To test the intraobserver reliability, we repeated the measurements in a blind test on randomly selected hips more than 2 weeks later. Intraobserver reliabilities, evaluated using the kappa value and intraclass correlation coefficient (ICC), were good or excellent for both observers (kappa value 0.77 to 0.89 and 0.89 to 1.00; ICC 0.80 to 0.98 and 0.89 to 0.98). To test measurement reproducibility, two independent observers (KK, MF) performed measurements on 20 randomly selected hips in a blind test. Interobserver reliabilities evaluated using the kappa value and ICC were good or excellent (kappa value 0.77 to 0.89; ICC 0.88 to 0.98).

We used a t-test or the Wilcoxon rank sum test to compare continuous parameters between the hip dysplasia and control groups, depending on the normality (Shapiro-Wilk test) and homoscedasticity (f-test). We used a chi-square test to compare categorical parameters between the two groups, while we used a paired t-test or a Wilcoxon signed rank test with a Bonferroni correction to compare continuous parameters among the three pelvic positions. The Tukey-Kramer honestly significant difference test or the Steel-Dwass test was used for multiple comparisons. Differences were defined as significant when the p value was < 0.05. Correlations between two continuous parameters were evaluated using the Pearson or the Spearman correlation coefficient, as appropriate. Statistical analyses were performed using JMP® version 14.0 (SAS Institute).

Results

Physiologic Pelvic Tilt

Although the APP angle varied widely among individuals (Fig. 3), it was greater in patients with hip dysplasia than in participants in the control group when in the standing position (3° ± 6° versus -2° ± 8°; mean difference 5° [95% CI 1° to 9°]; p = 0.02), but did not differ between the two groups when in the supine position (8° ± 5° versus 5° ± 7°; mean difference 3° [95% CI 0° to 7°]; p = 0.06) (Table 2). The pelvis tilted posteriorly when changing from the supine to the standing position in both groups, with a similar ΔAPP angle between the two groups (-6° ± 5° versus -7° ± 4°; mean difference 1° [95% CI -2° to 5°]; p = 0.33) (Table 2). Although the ΔAPP angle was less than 5° in 44% (14 of 32) of patients with hip dysplasia, the pelvis tilted posteriorly by more than 5° in 56% (18 of 32) of patients and the posterior change was greater than 10° in 22% (7 of 32) of patients (Fig. 3). Most radiographic and CT parameters differed between the dysplastic and control hips; the exception was the crossover sign (Table 2).

F3
Fig. 3:
A-B These bar charts show the distribution of the APP angles in the supine and standing positions, as well as the postural change in the APP angle, in (A) patients with hip dysplasia and (B) control participants.
Table 2. - Radiographic and CT parameters in the hip dysplasia and control groups
Parameter Hip dysplasia (n = 32 hips) Control (n = 16 hips) Mean difference (95% CI) or difference of medians p value
Radiographic parameters
 Lateral center-edge angle in °a 11 (1-17) 31 (25-38) -19 < 0.001
 Tönnis angle in °a 21 (11-31) 5 (-10 to 9) 16 < 0.001
 Head extrusion indexa 37 ± 6 16 ± 5 21 (21-22) < 0.001
 Positive crossover signb 28 (9) 2 of 16 hips 0.21
 Positive posterior wall signa 72 (23) 4 of 16 hips 0.002
 Anterior wall indexa 0.29 (0.05-0.53) 0.45 (0.25-0.74) 0.16 < 0.001
 Posterior wall indexa 0.83 (0.54–1.20) 1.04 (0.90-1.16) -0.21 0.002
CT parameters in °a
 Lateral center-edge angle 13 ± 5 32 ± 5 -18 (-21 to -15) < 0.001
 Anterior center-edge angle 38 ± 9 52 ± 8 -14 (-20 to -9) < 0.001
 Posterior center-edge angle 94 (73-129) 105 (90-125) -10 0.03
 Acetabular roof obliquity 19 (10–31) 4 (-13 to 9) 16 < 0.001
 Acetabular inclination angle 48 ± 3 41 ± 3 7 (5-9) < 0.001
 Acetabular anteversion angle 23 ± 4 20 ± 4 3 (0-5) 0.04
APP angles in °c
 Supine APP angle 8 ± 5 (-4 to 19) 5 ± 7 (-9 to 14) 3 (0-7) 0.06
 Standing APP angle 3 ± 6 (-11 to 14) -2 ± 8 (-16 to 12) 5 (1-9) 0.02
 ΔAPP angle -6 ± 5 (-17 to 5) -7 ± 4 (-15 to 0.7) 1 (-2 to 5) 0.33
aValues are presented as the mean ± SD or the median (range).
bValues are presented as % (n).
cValues are presented as the mean ± SD (range).

Pelvic Position and Joint Contact Pressure

The mean contact area was smaller (standing position; 490 ± 128 mm2 versus 919 ± 121 mm2; mean difference 430 mm2 [95% CI 352 to 507]; p < 0.001) (Table 3) and the median (range) maximum contact pressure was higher (standing position; 7.3 MPa [4.1 to 14] versus 3.5 MPa [2.2 to 4.4]; difference of medians 3.8 MPa; p < 0.001) (Table 4) in dysplastic hips than in controls at all pelvic positions (Fig. 4). The mean contact area was smaller (Table 3) in the standing position than in the supine position in both the hip dysplasia (490 ± 128 mm2 versus 581 ± 117 mm2; mean difference -91 mm2 [95% CI -152 to -30]; p < 0.001) and control groups (919 ± 121 mm2 versus 1094 ± 100 mm2; mean difference -175 mm2 [95% CI -255 to -95]; p < 0.001). The median (range) maximum contact pressure was greater (Table 4) in the standing position than in the supine position in both the hip dysplasia (7.3 MPa [4.1 to 14] versus 5.8 MPa [3.5 to 12]; difference of medians 1.5 MPa; p < 0.001) and control groups (3.5 MPa [2.2 to 4.4] versus 2.5 MPa [2.0 to 3.2]; difference of medians 1.0 MPa; p < 0.001). The increase in the maximum contact pressure from the supine to the standing position was negatively correlated with the ΔAPP angle in both the hip dysplasia (r = -0.87; p < 0.001) and control groups (r = -0.68; p = 0.004). When the dysplastic hips were classified according to the ΔAPP angle (Fig. 5), the mean increase in the maximum contact pressure when shifting from the supine to the standing position was the greatest (3.9 MPa) in the group with large posterior tilt (ΔAPP angle < -10°), followed by the mild posterior tilt group (-10° ≤ ΔAPP angle < -5°) (1.7 MPa) and unchanged group (-5° ≤ ΔAPP angle < 5°) (0.1 MPa). Meanwhile, the mean increase in the maximum contact pressure when changing from the supine to the standing position did not differ between control subgroups (Fig. 5). The mean contact area (Table 3) and median maximum contact pressure (Table 4) of the standard and standing pelvic positions did not differ in either the hip dysplasia or control groups. However, the differences in the maximum contact pressure between the standard and standing positions varied from -3.3 MPa to 2.9 MPa in dysplastic hips and from -1.4 MPa to 1.3 MPa in controls and was negatively correlated with the standing APP angle in both the hip dysplasia (r = -0.60; p < 0.001) and control groups (r = -0.55; p = 0.028). When the dysplastic hips were classified according to the standing APP angle, the mean difference in the maximum contact pressure was the greatest in patients with a standing APP angle < -5° (Fig. 6).

Table 3. - Comparison of the mean contact area (in mm2) among the three pelvic positions
Group Standard positiona Supine position Standing position
Mean ± SD Mean ± SD Mean difference (95% CI)b p valueb Mean ± SD Mean difference (95% CI)c p valuec
Hip dysplasia (n = 32) 484 ± 96 581 ± 117 96 (43-150) < 0.001 490 ± 128 5 (-51 to 62) > 0.99
Control (n = 16) 954 ± 113 1094 ± 100 140 (63-217) < 0.001 919 ± 121 -35 (-119 to 49) 0.93
p valued < 0.001 < 0.001 < 0.001
aStandard pelvic position was defined as that where the CT scanner coordinate system matches the anterior pelvic plane coordinate system.
bSupine versus standard position.
cStanding versus standard position.
dHip dysplasia versus control group.

Table 4. - Comparison of the median maximum contact pressure (in megapascals) among the three pelvic positions
Group Standard positiona Supine position Standing position
Median (range) Median (range) Difference of mediansb p valueb Median (range) Difference of mediansc p valuec
Hip dysplasia (n = 32) 7.4 (4.3-15) 5.8 (3.5-12) -1.1 < 0.001 7.3 (4.1-14) -0.1 > 0.99
Control (n = 16) 3.2 (2.2-4.5) 2.5 (2.0-3.2) -0.7 < 0.001 3.5 (2.2-4.4) 0.1 0.84
p valued < 0.001 < 0.001 < 0.001
aStandard pelvic position was defined as that where the CT scanner coordinate system matches the anterior pelvic plane coordinate system.
bSupine versus standard position.
cStanding versus standard position.
dHip dysplasia versus control group.

F4
Fig. 4:
This figure shows the distribution of the joint contact pressure on the acetabular cartilage of the right hip in representative patients in the hip dysplasia group (lateral center-edge angle of 16°) and control group (lateral center-edge angle of 31°). The joint contact pressure was higher in the dysplastic hip than in the control hip, as reflected in the color distribution. A color image accompanies the online version of this article.
F5
Fig. 5:
The box plots show the changes in the maximum contact pressure when shifting from the supine to the standing positions among the three subgroups classified according to the change in the APP angle: hips with ΔAPP angle < -10°, -10° ≤ ΔAPP angle < -5°, and -5° ≤ ΔAPP angle < 5 °. ap < 0.05 for dysplastic versus control hips; bp < 0.05 for the different dysplastic hip subgroups.
F6
Fig. 6:
These box plots show the differences in the maximum contact pressure between the standard and standing positions among the three subgroups classified according to the standing APP angle: hips with APP angle < -5°, -5° ≤ APP angle < -5°, and APP angle ≥ 5°. ap < 0.05 for dysplastic versus control hips; bp < 0.05 for different subgroups in dysplastic hips. cp < 0.05 for different subgroups of control participants.

Morphologic Factors Associated with Joint Contact Pressure

The standing APP angle was negatively correlated with the maximum contact pressure in the standing position in patients with hip dysplasia (r = -0.46; p = 0.008) (Table 5). In terms of anatomy, the following radiographic parameters correlated with the maximum contact pressure: lateral center-edge angle (ρ = -0.58; p < 0.001), Tönnis angle (r = 0.50; p = 0.003), extrusion index (r = 0.53; p = 0.002), and anterior wall index (r = -0.45; p = 0.01). Among CT measurement parameters, lateral center-edge angle (r = -0.70; p < 0.001), anterior center-edge angle (r = -0.59; p < 0.001), acetabular roof obliquity (r = 0.57; p < 0.001), and acetabular inclination angle (r = 0.55; p = 0.001) correlated with the maximum contact pressure (Table 5). None of these parameters correlated with the maximum contact pressure in control individuals.

Table 5. - Correlations of the maximum contact pressure with radiographic and CT parameters in the standing position\
Parameters Hip dysplasia Control
Correlation coefficienta p value Correlation coefficienta p value
Radiographic parameters
 Lateral center-edge angle -0.58 < 0.001 -0.19 0.48
 Tönnis angle 0.50 0.003 0.16 0.55
 Extrusion index 0.53 0.002 0.01 0.99
 Anterior wall index -0.45 0.01 -0.01 0.99
 Posterior wall index -0.11 0.55 0.29 0.28
CT parameters
 Lateral center-edge angle -0.70 < 0.001 -0.21 0.43
 Anterior center-edge angle -0.59 < 0.001 -0.15 0.58
 Posterior center-edge angle -0.16 0.38 0.17 0.52
 Acetabular roof obliquity 0.57 < 0.001 0.16 0.55
 Acetabular inclination angle 0.55 0.001 -0.19 0.47
 Acetabular anteversion angle 0.22 0.23 0.27 0.31
Sagittal pelvic tilt
 Standing APP angle -0.46 0.008 -0.24 0.36
aPearson or Spearman correlation coefficients.

Discussion

Recent studies have revealed that substantial individual and postural variations in the physiologic pelvic tilt affect the acetabular orientation and coverage in patients with hip dysplasia [38, 39, 42]. Thus, assessments while the patient is in the supine or standard pelvic position may not consider the functional orientation of the acetabulum and femur, and a weightbearing position may be more appropriate for assessing the biomechanical environment of the hip and planning acetabular orientation surgery. To date, the effect of physiologic pelvic tilt on the joint contact pressure in hips with dysplasia has not been well described. In this study, we found that postural change in the pelvic tilt affects the mechanical environment of the hip; the joint contact area was smaller and the contact pressure was greater in the standing position than in the supine position. We also found that the discrepancy in the maximum contact pressure between the standard and standing positions ranged from -3.3 MPa to 2.9 MPa; this discrepancy correlated with sagittal pelvic tilt in the standing position. Therefore, future studies that incorporate the effect of patient-specific pelvic tilt in the standing position on the biomechanical environment of the hip may lead to further understanding of the pathogenesis of this patient population and better define appropriate acetabular reorientation maneuver and indications for joint preservation surgery.

Limitations

Our study has several limitations. First, certain restrictions were introduced in the specification of loading and boundary conditions. Given the function of muscles and capsular ligaments surrounding the hip and joint instability, the actual joint reaction force on the hip may not be in the direction through the hip’s center. However, based on the assumption that the hip contact force acts on the hip center, Chegini et al. [5] validated that the acetabular cartilage injury coincided with a high von Mises stress zone. Hence, it may be acceptable to apply load at the hip’s center. Moreover, only one loading condition (a single-leg stance) was investigated; other conditions corresponding to daily activities and the gait cycle were not assessed. However, a recent study found that acetabular coverage measured in the standing position is a suitable surrogate for coverage measured during gait [43]; therefore, we posit that our observation of the single-leg stance scenario represents the loading conditions during walking. Furthermore, we defined a consistent body weight for all participants, which does not take into account the effect of individual body weight on contact pressure. In this study, a consistent body weight was adopted to avoid the scaling effect of body weight on the absolute value of contact pressure and to focus on the effect of individual differences in morphology and physiological pelvic tilt on contact pressure.

Second, we did not model patient-specific cartilage or the labrum because they were not clearly identifiable on plain CT images. Previous studies demonstrated the similarity of peak contact pressures between constant thickness cartilage models and patient-specific cartilage models [26] and the validity of finite-element models without a labrum [1, 22]. However, the labrum may play a larger role in load transfer and joint stability [18], and further studies ought to determine the effect of the absence of the labrum. Third, articular cartilage was assumed to be isotropic and linear elastic in this study, although it is actually an anisotropic, nonlinear, biphasic material with time-dependent mechanical behavior. However, for static loading such as single-leg stances, this is a reasonable assumption given that the time-dependent behavior can be ignored [2].

We further acknowledge that 50 patients (61%) were not eligible for this study and only female patients met the criteria, which could have resulted in a selection bias. The excluded patients did not differ from the included patients in terms of the demographic and radiographic parameters assessed in this study, suggesting that the risk of a potential bias was low. However, further research is needed to address the impact of differences in hip morphology between sexes on the generalizability of our observations. Additionally, the sample size was relatively small, which may have limited our ability to identify potential differences. Our post hoc power analysis, however, showed adequate power (> 99% power at p = 0.05) to detect a difference in joint contact pressure between hip dysplasia and control groups. Lastly, 41% of the hip dysplasia group had Tönnis Grade 1 osteoarthritis, whereas all the controls had Tönnis Grade 0; thus, the effect of this difference on joint contact pressure was a concern. Nevertheless, in patients with hip dysplasia, the maximum contact pressure did not differ between hips with Tönnis Grades 0 and 1, suggesting that the effect of mild osteoarthritis was negligible.

Physiologic Pelvic Tilt

The sagittal pelvic tilt varied widely between individuals in our study, and the pelvis tilted posteriorly from the supine to the standing position in both the hip dysplasia and control groups. Similar to our observation, previous studies found that the pelvis tilted posteriorly by 6° to 8° in patients with hip dysplasia and by 4° to 5° in normal individuals when shifting from the supine to the standing position [21, 24, 38]. Moreover, other studies found anterior pelvic tilt to be a feature of patients with hip dysplasia [11, 36] and suggested that such patients show a compensatory increase in anterior pelvic tilt [27, 48]. We also observed that the pelvis was more anteriorly tilted in dysplastic hips than in control hips in the standing position; however, the reported mean APP angle of normal participants ranged widely (from -8° to 7°) in the standing position [21, 24, 29]; thus, we cannot conclude that patients with hip dysplasia have a pelvic tilt pattern that is distinct from that of people who do not have dysplasia. The posterior change in pelvic tilt was more than 10° in 22% of patients with hip dysplasia, and these patients showed a marked increase in joint contact pressure (3.9 MPa) in the weightbearing position. AP pelvic radiography while bearing weight is a convenient method to evaluate the individual’s range and direction of pelvic rotation and should be added to the routine assessment of hip dysplasia.

Pelvic Position and Joint Contact Pressure

Consistent with previous studies [28], the current study showed that the joint contact pressure was approximately twice as high in dysplastic hips as in controls. Recent studies demonstrated that posterior change in the pelvic tilt when shifting from the supine to the standing position resulted in increased acetabular version and decreased anterosuperior acetabular coverage [11-13]. Shear stress forces and overload pressure both concentrate on the anterosuperior acetabulum in dysplastic hips, and the intraarticular pathologic findings generally originate from the anterosuperior labrochondral junction [10, 17]. Therefore, a decrease in the anterosuperior acetabular coverage can exacerbate joint degeneration [20]. We found that postural change in the pelvic tilt affects the mechanical environment of the hip. The joint contact area was smaller and the contact pressure was greater in the standing position than in the supine position. Moreover, the increase in the joint contact pressure correlated with the posterior pelvic tilt when shifting from the supine to the standing position. Previous studies have highlighted the usefulness of joint contact pressure for reflecting the status of the load transfer mechanism and even the progress of hip osteoarthritis [6, 14]. We recommend assessing postural change in sagittal pelvic tilt when diagnosing hip dysplasia and planning preservation hip surgery, as the morphologic and biomechanical evaluation using images acquired when in the supine position may overlook patient-specific change in the acetabular orientation and hip contact mechanics in a weightbearing position. Further studies are needed to elucidate the effect of patient-specific functional pelvic tilts on pain onset, occurrence and distribution of intraarticular lesions, the degeneration process of dysplastic hips, and the clinical result of joint preservation surgery.

Morphologic Factors Associated with Joint Contact Pressure

Similar to previous reports that used mathematical and discrete element models [8, 13, 28], we observed an association between a decrease in anterior and lateral acetabular coverage of the femoral head and increased joint contact pressure in a weightbearing position. Although the acetabular anteversion angle was not associated with joint contact pressure in this study, Daniel et al. [7] reported that the acetabular anteversion angle affected joint contact stress in hip dysplasia, especially during staircase walking. Besides these anatomical parameters, an increase in posterior pelvic tilt in the standing position was associated with increased joint contact pressure. Periacetabular osteotomy treats symptomatic hip dysplasia by improving the containment of the femoral head and correcting abnormal hip biomechanics in three dimensions [12, 23]. Morphology-based [7] and biomechanics-based planning methods (including individual-specific, finite-element modeling) have been described to reorient the acetabulum [26, 49]. However, these studies performed morphologic and biomechanical analyses based on the anterior pelvic plane coordinate system without considering the individual variety of physiologic pelvic tilt in the weightbearing position. In our study, the discrepancy in the maximum contact pressure between the standard and standing positions ranged from -3.3 MPa to 2.9 MPa; this discrepancy may affect the estimation of the ideal acetabular reorientation that minimizes the joint contact pressure. Specifically, in patients with posterior pelvic tilt in the standing position, anterior rotation of the acetabular fragment may be beneficial for correcting the anterolateral acetabular deficiency and decreasing joint contact pressure. Given the substantial effect of sagittal pelvic tilt on acetabular coverage and joint contact pressure, future investigations aiming to reorient the acetabulum should use biomechanics-based planning that accounts for patient-specific functional pelvic tilts in the weightbearing position.

Conclusion

We found that individual and postural variations in physiologic pelvic tilt affect the contact pressure in the hip. The sagittal pelvic tilt in the standing position varied from that in the supine and standard pelvic positions in a substantial number of patients with hip dysplasia. Based on our findings, future studies concerning the pathogenesis of hip dysplasia and joint preservation surgery should not only include the supine or standard pelvic position, but also need to incorporate the effect of the patient-specific pelvic tilt in the standing position on the biomechanical environment of the hip. Further studies are needed to elucidate the effect of patient-specific functional pelvic tilts on the degeneration process of dysplastic hips, the acetabular reorientation maneuver, and the clinical result of joint preservation surgery.

Acknowledgments

We thank Mitsugu Todo PhD, of the Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan; Takeshi Utsunomiya MD, Kyohei Shiomoto MD, Ryosuke Yamaguchi MD, Taishi Sato MD, and Shinya Kawahara MD, of the Department of Orthopaedic Surgery, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan; as well as Toshihiko Hara MD, of the Department of Orthopaedic Surgery, Aso Iizuka Hospital, Iizuka, Japan, for their invaluable advice for this study.

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