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. 2022 Jan 11;480(4):818–828. doi: 10.1097/CORR.0000000000002106

Small Random Angular Variations in Pelvic Tilt and Lower Extremity Can Cause Error in Static Image-based Preoperative Hip Arthroplasty Planning: A Computer Modeling Study

Aidin Eslam Pour 1,✉, Jean Yves Lazennec 2, Kunj P Patel 1, Manan P Anjaria 1, Paul E Beaulé 3, Ran Schwarzkopf 4
PMCID: PMC8923586  PMID: 35014975

Abstract

Background

Many THA simulation models rely on a limited set of preoperative static radiographs to replicate sagittal pelvic tilt during functional positions and to recommend an implant orientation that minimizes the risk of prosthetic impingement. However, possible random changes in pelvic or lower extremity angular motions and the effect of coronal and axial pelvic tilt are not included in these preoperative models.

Questions/purposes

(1) Can prosthetic impingement occur if the pelvic tilt or lower extremity alignment randomly varies up to ± 5° from what is measured on a single preoperative static radiographic image? (2) Do changes in coronal and axial pelvic tilt or lower extremity alignment angles have a similar effect on the risk of prosthetic impingement?

Methods

A de-identified pelvis and lower-body CT image of a male patient without previous THA or lower extremity surgery was used to import the pelvis, femur, and tibia into a verified MATLAB computer model. The motions of standing, pivoting, sitting, sit-to-stand, squatting, and bending forward were simulated. THA implant components included a full hemispherical acetabular cup without an elevated rim, polyethylene liner without an elevated rim, femoral head (diameter: 28 mm, 32 mm, 36 mm, or 40 mm), and a triple-taper cementless stem with three different neck shaft angles (127°, 132°, or 135°) with a trapezoidal neck were used in this model. A static model (cup anatomical abduction 40°, cup anatomical anteversion 20°, stem anatomical anteversion 10°) with a predefined range of sagittal pelvic tilt and hip alignment (0° coronal or axial tilt, without random ± 5° change) was used to simulate each motion. We then randomly varied pelvic tilt in three different pelvic planes and hip alignments (flexion, extension, abduction, adduction, rotation) up to ± 5° and assessed the same motions without changing the implant’s anatomical orientation. Prosthetic impingement as the endpoint was defined as mechanical abutment between the prosthetic neck and polyethylene liner. Multiple logistic regression was used to investigate the effect of variation in pelvic tilt and hip alignment (predictors) on prosthetic impingement (primary outcome).

Results

The static-based model without the random variation did not result in any prosthetic impingement under any conditions. However, with up to ± 5° of random variation in the pelvic tilt and hip alignment angles, prosthetic impingement occurred in pivoting (18 possible combinations), sit-to-stand (106 possible combinations), and squatting (one possible combination) when a 28-mm or a 32-mm head was used. Variation in sagittal tilt (odds ratio 4.09 [95% CI 3.11 to 5.37]; p < 0.001), axial tilt (OR 3.87 [95% CI 2.96 to 5.07]; p < 0.001), and coronal tilt (OR 2.39 [95% CI 2.03 to 2.83]; p < 0.001) affected the risk of prosthetic impingement. Variation in hip flexion had a strong impact on the risk of prosthetic impingement (OR 4.11 [95% CI 3.38 to 4.99]; p < 0.001).

Conclusion

The combined effect of 2° to 3° of change in multiple pelvic tilt or hip alignment angles relative to what is measured on a single static radiographic image can result in prosthetic impingement. Relying on a few preoperative static radiographic images to minimize the risk of prosthetic impingement, without including femoral implant orientation, axial and coronal pelvic tilt, and random angular variation in pelvis and lower extremity alignment, may not be adequate and may fail to predict prosthetic impingement-free ROM.

Clinical Relevance

Determining a safe zone for THA implant positioning with respect to impingement may require a dynamic computer simulation model to fully capture the range of possible impingement conditions. Future work should concentrate on devising simple and easily available methods for dynamic motion analysis instead of using a few static radiographs for preoperative planning.

Introduction

Many computer simulations have been designed to investigate hip impingement and provide recommendations for implant alignment in THA. The sagittal pelvic tilt and the hip-spine relationship were recognized as the risk factors for postoperative THA dislocation, which resulted in the wider use of computer simulations for preoperative planning [10, 12, 21, 26-28, 30, 32, 47, 51]. Most of these commercial computer simulations concentrate on the orientation of the acetabular implant and sagittal pelvic tilt and try to predict a range for acetabular cup abduction and anteversion that prevents prosthetic impingement. However, the femoral implant’s orientation was not included in many of these models. In addition, these simulation models only considered the sagittal pelvic tilt in a few different positions, such as standing and sitting. These assessments are based on static radiographic images and ignore axial and coronal pelvic tilt. Patients can have pelvic coronal and axial tilt, which might affect their hip motion (Fig. 1A-B).

Fig. 1.

Fig. 1

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(A) A preoperative EOS® image (ATEC spine group) shows coronal tilt of the pelvis. As seen in this view, the left iliac wing is higher than the right iliac wing in both standing and sitting positions. The coronal tilt is more pronounced in the sitting position. (B) A postoperative EOS® image shows substantial axial tilt of the pelvis. The left obturator foramen is much larger than the left obturator foramen in both standing and sitting positions. The axial tilt is pronounced in the standing and sitting positions. A color image accompanies the online version of this article.

The assumption of these models is that the patient’s body posture does not change or changes minimally compared with a static image. Maintaining body posture and balance during regular activities of daily living is complex and depends on many factors, such as muscle fatigue, visual input, mental status, and mood changes [2, 3, 7, 31, 46]. This complex process can result in minor changes in posture, pelvic tilt, or lower extremity alignment. It is unknown whether a combination of these minor changes in posture can combine to cause prosthetic or bony impingement and potential dislocation, edge loading, and excessive polyethylene wear. Hip impingement is a three-dimensional (3D) and dynamic event. It is not reasonable to expect that patients keep the same posture and follow the same pattern of motion shown by their static radiographs during activities of daily living.

We therefore asked: (1) Can prosthetic impingement occur if the pelvic tilt or lower extremity alignment randomly varies up to ± 5° from what is measured on a single preoperative static radiographic image? (2) Do changes in coronal and axial pelvic tilt or lower extremity alignment angles have a similar effect on the risk of prosthetic impingement?

Methods and Materials

Computer Model Development

We developed a MATLAB 2020b model (Simscape-Multibody, MathWorks) to investigate THA implant ROM during standing, pivoting, sitting, sit-to-stand, squatting, and bending forward. The study was exempt from institutional board review because no human participant was included in the study and only a de-identified CT scan was shared with our research team. An electronic medical record query was made by the department of radiology to find adult patients with high-quality CT scans of bilateral lower extremities, without any artifacts caused by motion or metallic objects, without any previous THA or lower extremity surgery, any previous spine surgeries, or any substantive radiographic abnormalities or implants. A de-identified pelvis and lower-body CT image of a 64-year-old male patient was the first CT scan that met all the inclusion criteria as described above and had the needed high-quality images; this CT image was used to import the pelvis, femur, and tibia into the model. THA implant components (a full hemispherical acetabular cup without an elevated rim [best-fit outer diameter 56 mm, thickness 4.8 mm], polyethylene liner without an elevated rim [inner diameter 28 mm, thickness 10.3 mm; inner diameter 32 mm, thickness 8.3 mm; inner diameter 36 mm, thickness 6.3 mm; or inner diameter 40 mm, thickness 4.3 mm], femoral head [diameter of 28 mm, 32 mm, 36 mm, or 40 mm], and a triple-taper cementless stem with three different neck shaft angles [127°, 132°, or 135°] with a trapezoidal neck) were designed in SolidWorks (Dassault Systèmes, SolidWorks Corp) and imported into the MATLAB model as a computer-aided design (CAD) file. The CAD models were produced based on commercially available primary acetabular cup and liner and femoral stems. This model is designed to generate large datasets regarding 3D THA motions using different head sizes, stem-neck angles, and shapes, considering pelvic tilt and lower extremity alignment. It also provides a visual image of hip motion during different activities of daily living.

The femoral stem-neck axis line passes through the center of the femoral stem neck and the center of the femoral head and exits at the polar axis of the head. Our model captures the position and motion of the polar axis inside the liner during activities of daily living with less than a 1° error in the polar coordinate system (Fig. 2), as verified in silico. When the polar axis is aligned with the acetabular liner (Fig. 3), the polar axis faces the center of the liner (coordinates of the polar axis = 0,0). The polar axis moves toward the edge of the liner with different hip motions. A motion map can be created for each stem, which includes each tested motion: 127° (Fig. 4) and 135° (Fig. 5). In the motion maps for the tested motions, 127° (Fig. 4) and 135° (Fig. 5), the red line shows the point at which prosthetic impingement occurs between the trapezoidal femoral neck and polyethylene liner. The coordinates of the closest position of the polar axis to the edge of the liner during each motion were captured. Prosthetic impingement was defined to occur when the closest point reached or crossed the red line. To verify the MATLAB model in silico, an independent model was written in SolidWorks (separate project). The variables that were compared between the two models included the orientation of the implants relative to the reference planes (anterior, horizontal, and vertical pelvic planes) and relative to each other (in standing, pivoting, sitting, sit-to-stand, squatting, and bending forward); there were no differences (by statistical analysis) between the measurements in the two models.

Fig. 2.

Fig. 2

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The polar coordinate system is shown in this figure. A color image accompanies the online version of this article.

Fig. 3.

Fig. 3

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This figure shows how the joint’s motion is mapped. When the head and liner are parallel, the polar axis is centered on the polyethylene liner. Motion of the polar axis can be captured during motion. A color image accompanies the online version of this article.

Fig. 4.

Fig. 4

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A-B These graphs show the motion map for a 28-mm head with 127° neck stems (A) without and (B) with ± 5° of variation. As seen in these figures, when only ± 5° of variation is introduced in the model, impingement would occur. The motion patterns are different compared with a stem with 135° neck angle. x- and y-axes represent distance (meters). A color image accompanies the online version of this article.

Fig. 5.

Fig. 5

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A-B These figures show the motion map for a 28-mm head with 135° neck stems (A) without and (B) with ± 5° of variation. As seen in these figures, when only ± 5° of variation is introduced in the model, impingement would occur. The motion patterns are different from those of a stem with a 127° neck angle. x- and y-axes represent distance (meters). A color image accompanies the online version of this article.

Implant Orientation

The anterior pelvic plane was used as the reference plane to calculate the anatomic acetabular implant orientation (Fig. 6). We measured anatomic femoral anteversion from the posterior femoral condylar plane, and we measured functional femoral anteversion relative to the vertical plane.

Fig. 6.

Fig. 6

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This figure shows the anterior pelvic plane. A color image accompanies the online version of this article.

Model Without Random Variation

To imitate current preoperative planning models that concentrate on sagittal tilt, axial and coronal pelvic tilts were considered 0°, with no random variation in the lower extremity alignment in the model (Fig. 7A-C). The motions were then simulated for the static model. Sample videos are provided for sit-to-stand motion without implant impingement (Supplementary Video 1; http://links.lww.com/CORR/A721) and with implant impingement (Supplementary Video 2; http://links.lww.com/CORR/A722). Because there is no agreement regarding hip ROM during different activities of daily living [5, 16, 20, 34, 53, 55], we chose an average hip ROM value and sagittal pelvic tilt with ± 30° for the simulation based on the reported range [8, 17, 24, 39, 40, 49] (Table 1). The anatomic acetabular implant orientation was 40° for abduction and 20° for anteversion. The femoral stem was placed in 10° of anteversion. We first simulated six different motions with modification of just the sagittal pelvic tilt up to ± 30° in the anterior or posterior direction (in 1° increments) with different head diameters and stem-neck angles (732 possible combinations for each motion; total possible combinations = 4392). The model tests each combination one at a time, calculates the polar coordinate system for the polar axis position, and determines if it crosses the red line representing prosthetic impingement and records the data.

Fig. 7.

Fig. 7

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A-C This figure shows the (A) coronal pelvic tilt, (B) axial pelvic rotation, (C) and sagittal pelvic tilt. A color image accompanies the online version of this article.

Table 1.

Pelvic tilt and hip alignment for tested motions for the static model (without ± 5° random variation)a

Pelvic tilt Hip alignment during motion/position
Hip position Body motion or position Sagittal tilt (range) Coronal tilt Axial tilt Flexion Rotation Abduction
Extension Standing -5° (-35° to 25°) 0° 0° 5° 0° 0°
Pivoting 5° (-15° to 35°) 0° 0° 5° 50° 0°
Flexion Sitting -15° (-45° to 15°) 0° 0° 80° 0° 0°
Sit-to-stand 10° (-15° to 35°) 0° 0° 80° -5° 0°
Squatting 5° (-25° to 35°) 0° 0° 95° 0° 5°
Bend over 55° (25° to 85°) 0° 0° 20° 0° 5°
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a

Anterior sagittal pelvic tilt is positive and posterior sagittal pelvic tilt is negative; for rotation, a positive number shows external rotation and a negative number shows internal rotation.

Model with Random Variation

To introduce variation in pelvic tilt and lower extremity ROM, we introduced ± 5° of variation for pelvic tilt (sagittal, coronal, and axial) and lower extremity alignments while keeping the anatomic cup and stem orientation similar to that of the static model (Table 2). Sample videos are provided for the sit-to-stand motion with a 32-mm femoral head and without risk of implant impingement (Supplementary Video 3; http://links.lww.com/CORR/A723) and with a 28-mm head with risk of implant impingement (Supplementary Video 4; http://links.lww.com/CORR/A724). After the introduction of ± 5° of random variation in pelvic tilt and hip ROM, our model produced 5,314,683 different combinations per head diameter for each tested motion (total possible combinations = 127,552,392).

Table 2.

Pelvic tilt and hip alignment for tested motions for the dynamic model (with variation)a

Pelvic tilt Hip alignment during motion/position
Hip position Body motion or position Sagittal tilt (range) Coronal tilt Axial tilt Flexion Rotation Abduction
Extension Standing -5° (-35° to 25°) 0° ± 5° 0° ± 5° 5° ± 5° 0° ± 5° 0° ± 5°
Pivoting 5° (-15° to 35°) 0° ± 5° 0° ± 5° 5° ± 5° 50° ± 5° 0° ± 5°
Flexion Sitting -15° (-45° to 15°) 0° ± 5° 0° ± 5° 80° ± 5° 0° ± 5° 0° ± 5°
Sit-to-stand 10° (-20° to 35°) 0° ± 5° 0° ± 5° 80° ± 5° -5° ± 5° 0° ± 5°
Squatting 5° (-25° to 35°) 0° ± 5° 0° ± 5° 95° ± 5° 0° ± 5° 5° ± 5°
Bend over 55° (25° to 85°) 0° ± 5° 0° ± 5° 20° ± 5° 0° ± 5° 5° ± 5°
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a

Anterior sagittal pelvic tilt is positive and posterior sagittal pelvic tilt is negative; for rotation, a positive number shows external rotation and a negative number shows internal rotation.

Primary and Secondary Study Goals, Predictors and Outcomes

Our primary study goal was to investigate the effect of adding ± 5° of random variation in pelvic tilt and hip ROM (relative to one fixed value from the static radiographic image) on the prosthetic impingement without changing the anatomical implant orientation. Our secondary goal was to investigate the magnitude of the effect of the axial and coronal pelvic tilt and hip alignment change on the prosthetic impingement.

To achieve these goals, the predictors for our analysis included sagittal, coronal, and axial pelvic tilt and hip flexion, abduction, and rotation angles during activities of daily living. Other predictors included the different head diameters (28 mm, 32 mm, 36 mm, and 40 mm) and stem-neck angles (127°, 132°, and 135°). The primary outcome variable was prosthetic impingement that resulted from up to ± 5° of random variation in pelvic tilt and hip ROM. The secondary outcome was the effect of change in coronal and sagittal pelvic change or hip ROM on hip impingement compared with the sagittal pelvic tilt. The cup and stem orientation were not modified during the modeling (cup abduction: 40°, cup anteversion: 20°, and femoral anteversion: 10°).

Ethical Approval

This study was exempt from institutional review board approval because no humans were included in the study.

Statistical Analysis

All continuous variables are described using the mean, mean difference, SD, and 95% confidence interval. Normal distribution of the values was checked using the Shapiro-Wilk normality test for each series of measurements. A univariate logistic regression model was used to analyze each of the predicting variables seperately, which showed a significant effect of the predicting variables on prosthetic impingement. A multiple logistic regression analysis was used to investigate the effect of variation in pelvic tilt and hip alignment (predictors) on prosthetic impingement (primary outcome). The regression model’s output included both the odds ratio and 95% CI. The Hosmer-Lemeshow goodness-of-fit test was used to test our logistic regression model. Multicollinearity was tested in Stata (StataCorp LP) using a collinearity test. There was no multicollinearity (individual variation inflation factor for variables = 1; average model variation inflation factor =1). Because of the very large sample size (127,552,392) and because significance was reached, we did not perform a sample size analysis. The data were analyzed using Stata 16.2 MP (StataCorp LP).

Results

Changes in Pelvic Tilt or Lower Extremity Angles Lead to Prosthetic Impingement

Even with a 28-mm head, no prosthetic impingement was observed in any of the tested functional positions when no random variation was included in the model, as shown in the sample motion map (without variation) of a stem with 127° neck angle (Fig. 4A) or a stem with 135° neck angle (Fig. 5A). After introducing up to ± 5° of variation in pelvic tilt or hip alignment, we found that prosthetic impingement occurred during pivoting when a 28-mm head and a 127° neck-angle stem were used (18 possible combinations) (Table 3), as shown in the sample motion map (with variation) of a stem with 127° neck angle (Fig. 4B) or a stem with 135° neck angle (Fig. 5B). Impingement during the sit-to-stand and squatting motions occurred with a 135° neck when a 28-mm head (106 possible combinations) or a 32-mm head (one possible combination) was used (Table 3). For impingement, it was not necessary to have 5° of variation in all variables at the same time. Even small variations of 2° to 3° in all variables, when combined, could cause prosthetic impingement.

Table 3.

Prosthetic impingement occurrence in different tested motions based on the head diameter and stem neck-shaft anglea

Stem neck angle Head diameter
127° 132° 135° 28 mm 32 mm 36 mm 40 mm
Standing
Pivoting 18 18
Sitting
Sit-to-stand 106 105 1
Squatting 1 1
Bending forward
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a

Despite the prediction of no prosthetic impingement by the static model without variation, prosthetic impingement occurs with introduction of up to ± 5° variation in hip angles and pelvic tilt; each number represents one possible combination with variation that could cause prosthetic impingement.

Changes in Pelvic Tilt or Lower Extremity Alignment Angles Affect Impingement Risk Differently

After controlling for confounding and interaction among variables such as prosthetic femoral head diameter and prosthetic femoral neck angle (Table 4), we found that more anterior sagittal tilt in activities such as sit-to-stand or squatting or more posterior pelvic tilt in pivoting can increase the risk of impingement (odds ratio 4.09 [95% CI 3.11 to 5.37]; p < 0.001). More axial tilt toward the operative hip in sit-to-stand or squatting motions or away from the operative hip in pivoting can increase the risk of impingement (OR 3.87 [95% CI 2.96 to 5.07]; p < 0.001). More coronal tilt toward the implant in sit-to-stand and squatting motions or away from the implant in pivoting can increase the risk of impingement (OR 2.39 [95% CI 2.03 to 2.83]; p < 0.001). For hip ROM, variation in flexion had a strong impact on the risk of prosthetic impingement (OR 4.11 [95% CI 3.3 to 5]; p < 0.001). On the other hand, increase in hip abduction in sit-to-stand and squatting motions (OR 0.29 [95% CI 0.23 to 0.36]; p < 0.001) and increase in hip external rotation in sit-to-stand and squatting motions (OR 0.27 [95% CI 0.22 to 0.35]; p < 0.001) had a strong protective effect against prosthetic impingement.

Table 4.

Results of logistic regression with prosthetic impingement as the endpoint

Logistic regression OR Standard errora p value 95% CI
Sagittal pelvic tilt 4.09 0.57 < 0.0001 3.11-5.37
Axial pelvic rotation 3.87 0.53 < 0.0001 2.96-5.07
Coronal pelvic tilt 2.39 0.21 < 0.0001 2.03-2.83
Hip flexion 4.11 0.41 < 0.0001 3.3-5
Hip abduction 0.29 0.03 < 0.0001 0.23-0.36
Hip internal-external rotation 0.27 0.03 < 0.0001 0.22-0.35
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a

Number of observations = 127,552,392; LR chi square (9) = 2325.36; Prob > chi square = 0.00001; Pseudo R2 = 0.72; OR = odds ratio.

Discussion

To decrease the rate of postoperative prosthetic impingement and potential edge loading and excessive polyethylene wear and THA dislocation, current preoperative planning for THA may include computer modeling to recommend an acetabular cup and femoral stem orientation based on a few static radiographic images. As minor variation in posture, pelvic tilt, or hip ROM relative to measurements in static radiographic images is possible, we investigated the potential impact of this minor angular variation. We showed that random angular variation of as little as ± 5° in pelvic tilt and lower extremity alignment relative to what is measured on a static radiographic image can result in hip prosthesis impingement. Furthermore, axial and coronal pelvic tilt can affect the chances of prosthetic impingement. This demonstrates the need for simple and easily available methods for dynamic analysis of motions instead of relying on a few static radiographic images for preoperative planning. This becomes even more important when operating on a patient with an abnormal hip-spine relationship and very small pelvis and proximal femoral geometry that can limit the use of certain femoral stems or acetabular implants that could accommodate larger femoral head sizes.

Limitations

Our study has limitations. We did not include bone-on-bone impingement or soft tissue in this model because the purpose of the study was to investigate prosthetic ROM, which is predictable and independent of bony anatomy. Bone-on-bone impingement depends on the patient’s anatomy, removal of osteophytes, and restoration of hip length and offset. Our model is not a finite-element analysis and it does not investigate the rigidity of the implants or soft tissue as it does not affect the relative position of the head and liner and prosthetic ROM/impingement. We used one pelvis and lower extremity CT image from one male patient, which was the first sample with the highest quality CT scan without anatomical abnormalities that met our criteria. However, regardless of the anatomic shape and size of the pelvis, which is individualized and affected by sex, the anatomic and functional orientations of the acetabular and femoral implants are always measured relative to the anterior pelvic plane, horizontal or vertical planes, or distal femoral condylar axis. We acknowledge that bony coverage and anatomy may influence the surgeon’s decision regarding implant size or offset to prevent implant or bony impingement; however, these considerations do not affect the relative motion of the head and liner. It is a valid argument that the pelvis and lower extremity motions based on numbers for pelvic tilt or hip alignment used in this study (Table 1) may not represent real-life motion. However, considering the wide range chosen for pelvic tilt and hip alignment angles that have been reported [8, 17, 24, 39, 40, 49] in addition to introducing the random variation (5,314,683 combinations/motion), we believe that our model comes closer to representing real-life motion than just one static radiographic image. We did not add more than ± 5° of variation because the goal of the study was to present the collective effect of small variations in pelvis and limb position and measurements. Choosing a larger variation would result in billions of combinations, which would make data collection and analysis impossible. This would also increase the number of combinations that could cause prosthetic impingement and further validate our findings. We have also performed the simulation with only one combination of cup and stem orientation (cup abduction: 40°, cup anteversion: 20°, and stem anteversion: 10°). Although it is possible to repeat the test with different implant orientations, it is not practical because of the large amount of data it creates for analysis (127,552,392 combinations for analysis per set of prosthesis head sizes and stem-neck angles). Again, this would only further support the findings of this study by introducing more variation and showing increased possibilities for prosthetic impingement.

Changes in Pelvic Tilt or Lower Extremity Angles Lead to Prosthetic Impingement

Our findings support that relying on one pelvic measurement from static radiographs, ignoring the axial and coronal tilt and femoral anteversion and the possible variation during activities of daily living, can increase the risk of encountering prosthetic impingement during functional hip range of motion. We are not aware of any previous study that investigated the combined effects of small variations in the pelvic tilt in all three planes and hip alignment on prosthetic impingement. The current study was possible mainly because of the recent development of simulation models for THA. Many of the recent commercialized models are designed to investigate patient-specific implant orientation per each patient’s specific sagittal pelvic tilt but not to generate large datasets by modifying many variables for hypothetical situations, as in our model [4, 9, 13, 18, 36, 52]. All THA computer simulation models need to consider 5° to 10° of variation in the pelvis and lower extremity angles to provide prosthetic impingement-free ROM. Real-life motions have some random variations, and these variations will make the recommended safe zone smaller, which will be more important in patients who have an abnormal hip-spine relationship. Similar to previous publications, we found that larger prosthetic head sizes can provide a better head-neck ratio and reduce the risk of impingement for patients [1, 19, 22, 29]. Although it has been reported that using prosthetic heads with larger diameter can increase the risk of metallosis or implant fracture, future studies should investigate these tradeoffs [6, 11, 50].

Changes in Pelvic Tilt or Lower Extremity Alignment Angles Affect Impingement Risk Differently

Many of the previously published models concentrated on the sagittal pelvic tilt and ignored coronal and axial pelvic tilt [13, 14, 23, 33, 35-37, 41, 43-45, 48, 54, 55]. The effect of different spine conditions, especially scoliosis, on the pelvis is 3D and thus will affect not only the sagittal pelvic tilt but also coronal and axial pelvic orientation [15, 25, 38, 42]. Even without these spinal pathologies, patients may not repeat the motions exactly in the same manner each time and lean more toward one side while rising from a chair or reaching for an object, thus increasing pelvic orientation more than expected or accounted for in a static model. Our findings support our hypothesis that not only the sagittal pelvic tilt but also leaning toward the operative hip in bending forward and away from the operative hip in pivoting (coronal and axial tilt) can increase the risk of prosthetic impingement, further supporting that even minor activity variations, which can be expected by our patients, can contribute to the possibility of prosthetic impingement. Our model also suggests that increases in hip abduction or external rotation angles in bending forward decreased the risk of prosthetic impingement. A possible clinical application of these findings entails a possible modification to current patient hip precautions after surgery. For example, patients could be instructed to have their hip in some degree of abduction while performing a sit-to-stand maneuver, such as getting in and out of a sitting position; this added hip position may help lower the risk of prosthetic impingement. These activity recommendations require future clinical studies and evaluation.

Conclusion

Even a few degrees of variation in pelvic tilt or lower extremity angles, measured on static preoperative radiographic images, can increase the risk of prosthetic impingement and potential hip dislocation, edge loading, and polyethylene wear. Suggesting an accurate personalized safe zone for THA requires dynamically collected data, over a period of time, of daily and leisure activities and considering the average/range for each patient for each motion. Considering that this may not be practical due to current technology gaps and resource limitations, computer simulation models need to consider random variations in pelvis and lower extremity alignment to attempt to imitate real-life motions, as these motions have random variations as well. Simulation modeling based on a few static images without consideration of axial and coronal pelvic tilt or random variations in lower extremity alignment overestimates the stability of the hip and may be inadequate for establishing a functional safe zone.

Acknowledgments

We would like to thank James Ashton-Miller PhD, Ronald Zernicke PhD, Lawrence Dorr MD, and David Lewallen MD for all their contributions and support as mentors for the NIAMS K08 grant to study hip-spine relation and THA dislocation.

Footnotes

One of the authors (AEP) is funded by National Institute of Arthritis, Musculoskeletal and Skin Diseases (NIAMS) K08 grant to investigate THA dislocation and the hip-spine relationship. One of the authors (PEB) certifies the receipt of personal payments or benefits, during the study period, in an amount of less than USD 10,000 from Matortho and Zimmer-Biomet and USD 10,000 to USD 100,000 from Microport and Corin as well as institutional support from Medacta and DePuy/Johnson & Johnson. One of the authors (RS) certifies receipt of personal payments or benefits, during the study period, in an amount of less than USD 10,000 from Intelijoint and in an amount of USD 100,000 to USD 1,000,000 from Smith and Nephew.

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.

Ethical approval for this study was waived by Yale University.

This work was performed at the Yale University, New Haven, CT, USA.

Contributor Information

Jean Yves Lazennec, Email: lazennec.jy@wanadoo.fr.

Kunj P. Patel, Email: pkunj@umich.edu.

Manan P. Anjaria, Email: manjaria@umich.edu.

Paul E. Beaulé, Email: pbeaule@toh.ca.

Ran Schwarzkopf, Email: ran.schwarzkopf@nyulangone.org.

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