Can we determine anterior hip coverage from pelvic anteroposterior radiographs? A study of patients with hip dysplasia

doi:10.21203/rs.3.rs-2633529/v1

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Research Article

Can we determine anterior hip coverage from pelvic anteroposterior radiographs? A study of patients with hip dysplasia

https://doi.org/10.21203/rs.3.rs-2633529/v1

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Journal Publication

published 24 Jun, 2023

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Purpose: Insufficient coverage causes hip joint instability and results in hip pain. Anterior hip coverage can be determined on both pelvic anteroposterior (AP) radiographs and false profile (FP) radiographs. Four parameters are commonly used to determine the anterior coverage on pelvic AP radiographs: the crossover index, crossover sign, anterior wall index (AWI), and rule of thirds. This study aims to clarify the relationship between these 4 parameters on AP radiographs and the anterior center edge angle (ACEA) on FP radiographs.

Methods: In this study, 53 patients who underwent periacetabular osteotomy for hip dysplasia at our center between July 2020 and October 2020 were retrospectively reviewed. Four parameters on AP radiographs and the ACEA on FP radiographs before surgery and 6 months after surgery were measured and compared for each hip.

Results: Upon examining the 53 hips in this study, there was no correlation between either the crossover index and the ACEA (P=0.66) or the crossover sign before surgery. The postoperative correlation between the crossover index and the ACEA was weak (r=0.36, P=0.007), and that between the crossover sign and the ACEA was moderate (r=0.41, P=0.003). There was a weak correlation between the AWI and ACEA both before (r=0.288, P=0.036) and after (r=0.349, P=0.011) the operation. Evaluation of the anterior coverage by the rule of thirds was also not consistent when determining the anterior coverage with the ACEA.

Conclusion: Anterior coverage on AP radiographs is largely inconsistent with ACEA on FP radiographs, especially before the surgery. It is recommended to take FP radiographs routinely for determining anterior hip coverage.

Hip dysplasia

Anterior coverage

Periacetabular osteotomy

Pelvic anteroposterior radiographs

False profile radiograph

Hip dysplasia is a hip deformity that is characterized by insufficient acetabular coverage of the femoral head, which causes hip joint instability and results in hip pain. However, the lack of anterior hip coverage can cause symptoms around the hip joint as well as lack of lateral coverage. [1–3] Following the increasing understanding of the 3D morphology of the hip joint, an increasing emphasis has been placed on the importance of anterior hip coverage by not only joint surgeons[1, 4, 5] but also sports medicine surgeons[6–8] and pediatric orthopedic surgeons[2, 9], especially for borderline hip dysplasia. [2, 7, 10–12]

An accurate measurement of anterior hip coverage is crucial to determine whether patients with hip dysplasia need hip preservation surgery. It is also a crucial factor in estimating the outcome of hip preservation surgery. [2]Although 3D measurements are the most accurate method for determining the 3D morphology of the acetabulum, pelvic tilt caused by the supine position in CT scanning reduces its ability to reproduce the weightbearing state of the hips,[13–20] which would influence the measurements of anterior coverage.[15, 17, 21–23] Furthermore, few surgeons can measure 3D acetabular coverage directly on an imaging system in the hospital, which is even more impossible during surgery, so 2D measurements of anterior coverage are still widely used in clinical practice.

Radiographs taken in the standing position can reproduce the weight-bearing status of a hip joint. A standing false profile (FP) radiograph is one of the most intuitive methods to see the supporting part of the acetabulum and to measure the anterior coverage of the hip joint. It is widely accepted by hip surgeons that the anterior center edged angle (ACEA) measured on FP radiographs can be utilized in determining the anterior coverage of the hip joint, although there is one study that found that the ACEA (measured by a different method) may have limited ability to predict three-dimensional coverage of the femoral head in patients with developmental dysplasia of the hip.[24] An association between ACEA and clinical outcomes has been revealed in previous studies,[2, 7, 25–29] It presents stronger power to predict a surgery. [2]

Methods of estimating anterior hip coverage by identifying the anterior acetabular rim on a standing pelvic AP graph are also used in clinical practice. In particular, standard FP radiographs are not available. For instance, during the surgery. Currently, the parameters that are commonly used to determine anterior coverage on pelvic AP radiographs include the crossover index, crossover sign[30], anterior wall index (AWI)[31], and rule of thirds[32] (Fig. 1).

The results of a previous study show that different parameters of anterior coverage present different abilities to predict clinical results. [2] The correlation between the parameters is far from strong. [30, 33, 34]

Therefore, this study aims to define the correlation between the crossover index and the ACEA, the correlation between the crossover sign and the ACEA, the correlation between the anterior wall index (AWI) and the ACEA, and the correlation between the rule of thirds and the ACEA.

After being approved by the Institutional Review Board of Chinese PLA hospital, patients who underwent isolated periacetabular osteotomy (PAO) for hip dysplasia at our center between July 2020 and October 2020 were retrospectively reviewed in this study. Patients enrolled in the study must have a follow-up for over 6 months. Patients with severe hip deformities who did not have enough anatomic landmarks for accurate measurement were excluded.

Pelvic standing AP radiographs and standard FP radiographs[24] were taken for each patient before surgery and at 6 months after the surgery when the patients could walk with full body weight. Because standing radiography can recreate the state of the hip during body weight loading, the pelvic tilt was not normalized.[35]

On the FP radiographs, the ACEA was measured by the angle between the line joining the midpoint of the femoral head to the anterior rim of the acetabular sourcil and its vertical line. The normal reference value for the ACEA is 20–40°. An ACEA less than 20° is defined as deficient anterior coverage, an ACEA greater than 40° is defined as excessive anterior coverage, and an ACEA between 20–40° is defined as normal anterior coverage.[24]

The parameters for estimating the preacetabular anterior coverage on standing pelvic AP radiographs were measured. These parameters included the crossover index, the crossover sign[30], the anterior wall index[31], and rule of thirds[32] (Fig. 1).

The positive crossover sign is defined as crossover of the anterior and posterior rims of the acetabulum that can be identified on the AP radiograph. The crossover index is the ratio of the width of the anterior acetabulum (a) and the width of the posterior wall of the acetabulum (b) [22] (Fig. 1A). Because there is a high probability of a small crossover at the lateral acetabulum and at the anterior and posterior margins, in clinical practice, a crossover index with a value greater than 15% is generally defined as a positive crossover sign[30] (Fig. 1B).

The anterior wall index refers to the percentage of the area covered by the anterior rim of the acetabulum in the radius of the femoral head[36] (Fig. 1C). According to Professor Tannast[32, 37], the rule of thirds categorizes the pelvic anterior coverage by the ratio of the width of the anterior acetabular wall that covers the femoral head. That is, an AWI between 0 and 33% is defined as undercoverage, between 33% and 66% is defined as normal, and greater than 66% is defined as overcoverage (Fig. 1D).

All measurements were performed by two experienced hip surgeons who were able to perform periacetabular osteotomy independently.

To explore the relationship between the 4 parameters above and the ACEA in patients with hip dysplasia, we analyzed (1) the correlation between the crossover index and the ACEA, (2) the agreement of the classification of hips by the crossover sign and by the ACEA, (3) the correlation between the AWI and the ACEA, and (4) the agreement of the classification of hips by rule of thirds and by the ACEA. Bost preoperative and postoperative data were analyzed.

Statistical methods: SPSS 26 (IBM, Armonk, NK) statistical software was used for data analysis. The Kolmogorov–Smirnov (K-S) test was used to determine the normal distribution of all of the measurement data. Pearson and Spearman correlation analysis was used to examine the correlation between the crossover index and the ACEA. Kendall's tau correlation analysis was conducted, and the result of the crossover sign was compared to the result of ACEA. The correlation between the anterior wall index and the ACEA was determined by Pearson correlation analysis. When hip joints were classified as undercovered, normal and overcovered by the rule of thirds, Kendall's tau correlation analysis was used to make comparisons between the classification and results of ACEA one by one. The alpha value was set to 0.05.

To date, 60 hips of 60 patients meet our inclusion criteria. Severe patients (7 hips) with deformities were excluded. Of the 7 patients, 1 had multiple epiphyseal dysplasia (MED), 2 had severe coxa plana, 1 had severe hip joint deformation and subluxation caused by poliomyelitis, and 3 had severe hip subluxation due to cerebral palsy. None of our included hips had a treatment history. A total of 53 cases (53 hips) were included in the study, including 6 males and 47 females. The mean age of the patients at surgery was 29.3 ± 8.6 (14–46).

Among all of the measurement data of the 53 hips in this study, the K-S test results showed that the preoperative and postoperative ACEA and the preoperative and postoperative AWI were normally distributed.

Preoperatively, the average ACEA was 9.0 ± 16.1° (-24.8°~37.0°). Anterior undercoverage was detected in 41 hips, and normal coverage was detected in 12 hips. Postoperatively, the average ACEA was 34.1 ± 9.3° (7.3°-60.4°). Anterior undercoverage was detected in 3 hips, 40 hips had normal anterior coverage, and 10 hips were overcovered anteriorly.

Preoperatively, the average crossover index was 0.05 ± 0.10 (0.00 ~ 0.37). The crossover index had no correlation with the ACEA (P = 0.66). Postoperatively, the mean crossover index was 0.03 ± 0.11 (0.00 ~ 0.57). The crossover index was correlated with the ACEA (P = 0.007). The correlation coefficient was 0.36.

Preoperatively, 9 hips had positive crossover signs. According to the ACEA classification, none of them had excessive anterior coverage. Positive crossover signs were found in 4 hips after surgery, among which 3 hips had excessive anterior coverage according to the ACEA classification. Using the ACEA classification as the standard, the sensitivity was 0.30, and the specificity was 0.75. Postoperatively, a correlation was observed between the crossover sign and the ACEA after surgery (P = 0.003), and the correlation coefficient was 0.41.

As shown in Fig. 2, a preoperative AWI of 0.18 ± 0.13 (0.00 ~ 0.56) was correlated with the ACEA (P = 0.036), but the correlation was weak (r = 0.288). The postoperative AWI was 0.23 ± 0.12 (0.00 ~ 0.56), which was correlated with the ACEA (P = 0.011). The correlation was also weak (r = 0.349).

In this study, the average AWI of all of the patients with a normal ACEA (20–40°) was 0.21 ± 0.11 (0.00 ~ 0.44).

According to the classification of rule of thirds, anterior undercoverage was detected in 48 hips, and normal anterior coverage was detected in 5 hips preoperatively. Among the 53 hips, 41 hips agreed with the anterior coverage according to ACEA classifications (grey cells), and the accuracy was 77.36%. Nine hips (16.98%) were underestimated, and 3 hips (%) were overestimated. (Table 1) There was no correlation with the anterior coverage classified by the ACEA (P = 0.14).

Table 1

Comparison of the classification of hips by the rule of thirds and by the ACEA in preoperative patients with hip dysplasia
	Deficient by the ACEA	Normal by the ACEA	Sum
Deficient by the rule of thirds	39(73.58%)	9(16.98%)	48(90.56%)
Normal by the rule of thirds	3( 5.66%)	2( 3.77%)	5( 9.43%)
Sum	42(79.25%)	11(20.75%)	53(100%)

Postoperatively, the rule of thirds classification showed deficient anterior coverage in 42 hips and normal anterior coverage in 11 hips. No overcovered hip was detected. Among the 53 hips, only 10 hips agreed with the classification according to the ACEA (grey cells), and the accuracy was only 18.87%. The anterior coverage was underestimated in 43 hips (81.13%). (Table 2) There was no correlation between the anterior coverage classification by the rule of thirds and by the ACEA.

Table 2

Comparison of the classification of hips by the rule of thirds and by the ACEA in postoperative patients with hip dysplasia
	Deficient by the ACEA	Normal by the ACEA	Excessive by the ACEA	Sum
Deficient by the rule of thirds	3(5.66%)	33(62.26%)	6(11.32%)	42(79.24%)
Normal by the rule of thirds	0	7(13.21%)	4(7.55%)	11(20.75%)
Excessive by the rule of thirds	0	0	0	0
Sum	3(5.66%)	40(75.47%)	10(18.86%)	53(100%)

For hip dysplasia, the evaluation of the anterior coverage of the hip joint is as important as that of the lateral coverage. Currently, the ACEA measured on FP radiographs is one of the most widely used parameters for evaluating anterior coverage. It has been proven to be related to clinical results.[2, 7, 25–29] Clinically, radiologists are not always familiar with FP radiographs. Sometimes it seems redundant to take an additional FP radiograph. Thus, hip surgeons often use pelvic AP radiographs to estimate the anterior coverage.

The crossover index has been proven to be related to the ACEA in patients with Femoroacetabular impingement (FAI). [30] However, to our knowledge, no study has included patients with hip dysplasia. In this study, the crossover index had no correlation with the ACEA preoperatively. However, after the deficiency was corrected in the surgery, there was a weak correlation. The result is similar to what has been described in patients with FAI.

In our study, when the crossover sign was used to detect anterior overcoverage in preoperative patients with hip dysplasia, none of the results agreed with the results from the ACEA. Postoperatively, some crossover sign results agreed with the results from ACEA, but there was an unsatisfactory specificity (75.0%) and a worse sensitivity (30.0%). Therefore, the positive crossover sign in the clinic might only be a reminder of excessive anterior coverage. It is not recommended to use only the crossover sign as the decisive parameter. The first impression from the positive crossover sign must be confirmed by other measurements, such as ACEA.

The reason why the agreement between excessive anterior coverage detected by ACEA and detected by the crossover index/the crossover sign is not high originated from their different methodologies. Essentially, the “crossover” is determined by the relationship between the anterior rim and the posterior rim of the acetabulum. A positive crossover sign means that the anterior acetabular wall is partially larger than the posterior acetabular wall. Positive results may be caused by not only excessive coverage of the acetabulum but also deficient posterior coverage. Deficient posterior walls are frequently seen in patients with hip dysplasia, [38] which greatly affects the determination of the anterior coverage. Therefore, using the crossover sign to determine the anterior coverage of the acetabulum might lead to overestimation of the anterior coverage. This also explained why only a postoperative correlation between the crossover sign and the ACEA existed. The corrected posterior wall decreases the error caused by the deficient posterior wall, and the value of the crossover sign in determining anterior coverage increases.

When we measured the AWI, we used the location of the anterior rim of the acetabulum on the femoral head (rather than the relationship between the anterior wall and the posterior wall of the acetabulum used in the crossover sign and crossover index) on the pelvic AP radiograph to determine the anterior coverage. The AWI measurement avoids the effect of a deficient posterior wall of the acetabulum. It is weakly but significantly correlated with the ACEA. When comparing the preoperative and postoperative data, it was found that the preoperative correlation was lower (Fig. 3). On the preoperative scatter plots of the AWI and ACEA, there are some points far from the regression line. When we traced these points, some common characteristics were found in the cases that were represented by these points. In all these cases, the lateral contours of the anterior acetabulum spines (AIIS) are clearly visible on pelvic AP radiographs, as demonstrated in Fig. 3A. On the FP radiograph, it could be seen that the anterior edge of the articular surface was located only slightly anterior to the top of the femoral head center. The supporting area of the articular surface was not sufficient (Fig. 3B). The prominent AIIS instead of the real anterior rim of the socket formed the seemingly normal anterior wall on the pelvic AP radiograph (Fig. 3A). In this case, the anterior coverage of the hip joint was greatly overestimated on the pelvic AP radiograph because of the effect of the AIIS. This can also explain why there is only a moderate correlation between AWI on pelvic radiographs and CT. [39] In the same patient, after the acetabulum was reoriented in PAO, the supporting area of the acetabulum rotated laterally and anteriorly. As the anterior hip coverage significantly increased, the ACEA increased significantly as well (Fig. 3D). However, the AWI did not lead to any changes on the pelvic AP radiographs (Fig. 3C).

For the above reasons, the weakness of estimating the anterior coverage on pelvic AP radiographs lies in the fact that some of the anterior rims identified on AP radiographs are not the real anterior rims of the socket. “Anterior walls” on AP radiographs are enlarged by the AIIS. This interference factor affected our determination of the anterior coverage evaluation to a certain extent.

The rule of thirds classification, which was verified by Professor Tannast, is based on the concept of AWI. Therefore, the method has problems similar to AWI. In this study, we did not replicate results from Professor Tannast. The rule of thirds was verified not by direct measurements of the ACEA or CT data but by the software Hip²Norm (University of Bern, Bern, Switzerland).[32, 37] Essentially, Hip²Norm is also based on measurements from a pelvic AP radiograph (Fig. 4). The principle is calculating the anterior coverage on the horizontal plane by the projection of the anterior and posterior walls of the pelvis on the sphere of the femoral head in the statistical model.[40, 41] Because Hip²Norm is also based on the AP radiograph, the mechanism is the same as AWI and the rule of thirds. The underlying logic of all 3 methods is the same, so a perfect correlation between them does not mean that they reproduce anterior hip coverage perfectly.

The AIIS greatly affected the identification of the anterior rim of the joint surface on pelvic AP radiographs. Only when the AIIS is not prominent does the pelvic anterior wall, which we can see on a pelvic AP radiograph, agree with the anterior wall of the socket. Without the AIIS’ interference, we can see that the rule of thirds will slightly underestimate the anterior coverage with trigonometric calculation. The results of the calculations show that when the ACEA ranges from 20° to 40°, the AWI value approximately ranges between 0.06 and 0.23 rather than ranging from 0.33–0.66, which would be recommended by the rule of thirds (Fig. 5). In this study, the AWI was 0.21 ± 0.11 (0.00 ~ 0.44) for all patients with an ACEA between 20 and 40°, which was also significantly lower than the reference value of 0.33–0.66. The anterior coverage of some hips was underestimated (preoperative 9 hips, accounting for 16.98% of the total and postoperative 34 hips, accounting for 81.13%).

When the AIIS is prominent, the anterior rim identified on a pelvic AP radiograph is located laterally and inferiorly to the anterior border of the joint surface. Because it is impossible to visually distinguish the acetabular surface from the AIIS on the AP radiograph, measuring the anterior wall on a pelvic AP radiograph will result in overestimation of the anterior coverage. For these patients, there is a possibility of obtaining deficient postoperative anterior coverage if we correct the acetabulum based only on pelvic AP radiographs. If FP radiographs can be obtained before surgery, the correction of the anterior coverage can be more accurate (Fig. 3). For patients with hip dysplasia, a certain error rate is obtained when estimating the anterior coverage on pelvic AP radiographs.

Therefore, it is risky to evaluate anterior coverage using pelvic AP radiographs instead of FP radiographs.

There are some limitations in this study: 1. Because the accuracy of anterior coverage measured from CT data in the supine position was greatly affected by pelvic tilt,[13, 17] the gold standard of anterior coverage should be anterior coverage measured on standing CT data. Because we do not perform hip CT on standing position right now, after careful consideration, we finally decided to use ACEA, a 2D parameter, as a standard of all the parameters. There are several reasons that we took this approach. (1) ACEA is taken in the patient’s standing position. It can reproduce the weight-bearing state of the hip joint in the standing position. (2) ACEA has been confirmed in clinical practice and widely accepted by most doctors in clinical practice.[2, 7, 25–29] The acetabulum is a three-dimensional structure, whereas the ACEA on an FP radiograph is only a two-dimensional parameter. We will try to solve this problem in subsequent studies.

Determining the anterior coverage based on pelvic AP radiographs should be done with caution. It is recommended to take FP radiographs routinely for determining anterior hip coverage.

AP view: pelvic anteroposterior view; FP view: false profile view; ACEA: anterior center edge angle; AWI: anterior wall index; PAO: periacetabular osteotomy; FAI: femoroacetabular impingement; K-S test: Kolmogorov–Smirnov test; MED: multiple epiphyseal dysplasia; AIIS: anterior acetabulum spines

Acknowledgements

Not applicable.

Authors’ contributions

HC, ZZ and WS conceived and designed the study, supervised the data collection and analysis, and wrote the manuscript. NR and YL assisted with the study design, oversaw participant recruitment. DL, JZ and HZ managed the program, provided counseling to participants. All authors have read and approved the final submitted manuscript.

Funding

No funding source was involved in the conduction of this study. Publication costs are funded by our institution.

Availability of data and materials

Data available upon request. The contact should be made via the first author [email protected].

Ethics approval and consent to participate

All experimental protocols were approved by the Institutional Review Board of Chinese PLA hospital (2021KY042-KS001). Written informed consent was obtained from all adult patients or minors patient’s parent and/or legal guardian for study participation. All methods were carried out in accordance with relevant guidelines and regulations.

Consent for publication

The authors have received written consent from participants to publish individual patient data (images).

Competing interests

No author is involved in conflict of interest. No author has received any funding.

Author details

Senior Department of Orthopaedics, the Fourth Medical Center of PLA General Hospital, Beijing 100048, China

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published 24 Jun, 2023

Read the published version in BMC Musculoskeletal Disorders →

Editorial decision: Major revision
17 May, 2023
Reviewers agreed at journal
04 May, 2023
Reviews received at journal
20 Apr, 2023
Reviewers agreed at journal
10 Apr, 2023
Reviewers invited by journal
06 Apr, 2023
Editor assigned by journal
24 Mar, 2023
Editor invited by journal
17 Mar, 2023
Submission checks completed at journal
17 Mar, 2023
First submitted to journal
27 Feb, 2023

You are reading this latest preprint version

Can we determine anterior hip coverage from pelvic anteroposterior radiographs? A study of patients with hip dysplasia

Status:

Journal Publication

Version 1

Abstract

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Introduction

Patients And Methods

Results

Discussion

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Abbreviations

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Additional Declarations

Status:

Journal Publication

Version 1