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 Table of Contents  
ORIGINAL ARTICLE
Year : 2017  |  Volume : 3  |  Issue : 1  |  Page : 7-11

Comparison of two different medial reference points for measurements of the acetabular index


1 Department of Orthopaedics, Paediatric Orthopaedic Service, Kasturba Medical College, Manipal, Karnataka, India
2 Paediatric Orthopaedic Service, Ganjwala Orthopaedic Hospital, Ahmedabad, Gujarat, India

Date of Web Publication17-Feb-2017

Correspondence Address:
Hitesh Shah
Department of Orthopaedics, Kasturba Medical College, Manipal - 576 104, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2249-9008.200291

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  Abstract 

Introduction: Acetabular index (AI) is a commonly used quantitative measurement of acetabular inclination in plain radiographs. Repeated measurements of this index are used to determine dysplasia in children and for decision making about surgical management. Persistent acetabular dysplasia may be an indication for performing an acetabuloplasty. AI is commonly measured between the Hilgenreiner’s line (line that connects both triradiate cartilages) and the line joining lateral most ossified margin of the acetabulum and triradiate cartilage. Two different methods for measurement of AI with two different medial reference points at the triradiate cartilage have appeared in the literature. Aim: The specific aim of the study was to investigate the difference between AI measurements with two different methods and report on intraobserver and interobserver reliability of both the methods. Materials and Methods: Ninety-eight children with developmental dysplasia of the hip (DDH) (treated and untreated), younger than 9 years, were included in the study. Anteroposterior radiographs of the pelvis having acceptable pelvic rotation and pelvic tilt were included in the study. AI was measured using two different reference points for the affected and normal sides. AI was measured twice at 1-month interval by two investigators. The difference between the two measurement techniques was compared by the paired “t” test. Pearson’s correlation coefficient was calculated to test associations between the two measurement techniques. Results: The reproducibility of measurements of both the techniques was found satisfactory [intraclass correlation (ICC)-0.90 and 0.87]. Statistically significant difference (P value < 0.001) (5.7° for affected and 5.3° for normal side) between the indices measured by two techniques was noted. This difference was noted for all age groups. Significant positive correlations between both the techniques were noted in normal as well as dysplastic hips. Conclusion: Acetabular indices measured with two different medial points gave significantly different values. All subsequent assessment should be consistently carried out by the same method.

Keywords: Acetabular index, acetabuloplasty, developmental dysplasia of hip, medial reference point


How to cite this article:
Vijayan S, Ganjwala D, Shah H. Comparison of two different medial reference points for measurements of the acetabular index. Paediatr Orthop Relat Sci 2017;3:7-11

How to cite this URL:
Vijayan S, Ganjwala D, Shah H. Comparison of two different medial reference points for measurements of the acetabular index. Paediatr Orthop Relat Sci [serial online] 2017 [cited 2017 Dec 14];3:7-11. Available from: http://www.pors.co.in/text.asp?2017/3/1/7/200291


  Introduction Top


Developmental dysplasia of the hip is a condition in which the head of the femur is not making proper contact with the acetabulum. One of the features of this condition is dysplastic steep acetabulum. In many children, the acetabulum adapts to the head of the femur after reduction of the hip.[1] However, acetabular adaptation is variable. Some acetabula fail to adapt with time after reduction. Acetabular index (AI) is a widely used parameter to analyze the inclination of the acetabulum.[2] Dysplastic acetabula have higher AI that gradually diminishes as adaptation takes place. When dysplastic acetabulum fails to adapt adequately, acetabular osteotomy is indicated to correct acetabular inclination. AI is used as a main tool by the surgeons to decide whether operative procedure should be offered or not.[1],[3],[4] The AI is measured on anteroposterior radiograph of the pelvis with both the hips. To measure AI, a first line is drawn connecting both triradiate cartilages (Hilgenreiner’s line). A second line is drawn joining the lateral most ossified margin of the acetabulum and triradiate cartilage (medial reference point). The angle between these two lines is AI.[5],[6] We reviewed the literature and found two different medial reference points for measurement of AI. Some studies[7],[8] have used the upper most point of the triradiate cartilage (lowest point of ilium) to measure AI, whereas others[9],[10] have used the lateral border of the tear drop and the inferior point of the triradiate cartilage as medial reference point. It is also not clear whether both the methods give the same result or not. If they give different results, then the dilemma is whether the difference is significant or not. If the difference is significant then one should know what happens to this difference as the age increases. We tried to answer these questions.


  Aim Top


The aim of this study was to investigate the difference between AI measurements with two different methods. We also investigated intraobserver and interobserver reliability of both the methods.


  Materials and Methods Top


One hundred and ten radiographs of children younger than 9 years having developmental dysplasia of the hip (DDH) (treated and untreated) were evaluated. All radiographs were digitally recorded on an electronic picture archiving and communications system.

To avoid errors because of improper pelvis positioning, we included only radiographs with acceptable pelvic rotation and tilt. Quotient of the pelvic rotation was measured to evaluate the pelvic rotation. It was calculated by dividing the horizontal diameter of the obturator foramen of the right side by the diameter of the same on the left side [[Figure 1]a]. An index between 1.8 and 0.56 was considered acceptable.[5] Pelvic tilt index was used to determine the degree of pelvic tilt in the sagittal plane. It was calculated by the ratio between the vertical diameter, the obturator foramen, and the distance between the upper brim of the pubis and the Hilgenreiner’s line drawn horizontally connecting the lower margin of both the ilium in the triradiate cartilage complex [[Figure 1]b]. A ratio between 0.75 and 1.2 was considered acceptable.[5] Only those radiographs that had quotient of the pelvic rotation between 1.8 and 0.56 and pelvic tilt index between 0.75 and 1.2 were included in the study.[5] Out of 110 radiographs, 12 were excluded as positioning was not acceptable and 98 radiographs with acceptable pelvic rotation and pelvic tilt were included in the study; 65 radiographs were of unilateral (39: left, 26: right) DDH, whereas remaining 33 were of bilateral DDH. Thirty-four children were boys and 64 were girls. The mean age at the time of radiograph was 3.1 years (range 9 months to 8 years).
Figure 1: (a) Technique of measuring quotient of the pelvic rotation. The horizontal diameter of the obturator foramen of the right side (Qr) is divided by that of the left side (Ql). (b) Technique of measuring pelvic tilt. It was calculated by dividing vertical diameter of the obturator foramen (R) and the distance between the upper brim of the pubis and the Hilgenreiner’s line (T)

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The AI was measured by two different techniques. For both the methods, lateral point was the same − lateral most margin of the bony part of the acetabulum. Medial reference point differed in both the methods. For the first method, medial point was upper and lateral most point of the triradiate cartilage and the lower point of the ilium [[Figure 2]a and [Figure 2]b]. In the second method, the medial point was lower and lateral most point of the triradiate cartilage [[Figure 3]a and [Figure 3]b]. In both the methods, medial and lateral points were joined by a line. The angle between this line and Hilgenreiner’s line was measured in both the methods. The AI measured by the first method was labeled as AI-1, whereas the AI of the second method was labeled as AI-2. All angles were measured by image analysis software (Digimizer image analysis software, version 4.1.1.0; Medcale Software, Mariakerke, Belgium). The AI-1 and AI-2 were measured for affected as well as normal sides by two investigators (experienced paediatric orthopaedic surgeon and orthopaedic surgeon with 5 years of experience). Each radiograph was evaluated on two separate occasions blinded to each investigator at an interval of 4 weeks.
Figure 2: (a) Technique of measuring AI-1 by using upper most part of triradiate as medial reference point. Lateral point is lateral most margin of the bony part of the acetabulum. Both points are connected by a line. Angle between this line and Hilgenreiner’s line is AI-1. (b) Measurement of AI-1 on plain radiograph

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Figure 3: (a) Technique of measuring AI-2 by using lower most part of triradiate as a medial reference point. Lateral point is lateral most margin of the bony part of the acetabulum. Both points are connected by a line. Angle between this line and Hilgenreiner’s line is AI-2. (b) Measurement of AI-2 on plain radiograph

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The statistical analysis was performed with the Statistical Package for Social Sciences version 16 (IBM, Chicago, Illinois, USA). The interobserver and intraobserver reproducibility for AI-1 and AI-2 were assessed by computing interclass correlation coefficient. The mean difference between the two measurement techniques were compared by paired “t” test (two tailed). Pearson’s correlation coefficient was calculated and regression line was plotted to test associations between the two measurement techniques. A P value less than 0.05 was considered as being significant for all analyses.


  Results Top


The reproducibility of measurements of both the techniques was satisfactory for affected as well as normal hips [Table 1]. Values of AI-1 were less than the values of AI-2 and the difference was 5.7° for affected and 5.3° for normal side between the measurements by the two techniques [Table 2]. This difference was also significant for all age groups − from 1 to 8 years [Table 3]. AI values diminished as age advanced. This was observed with both the techniques and normal and affected sides [Table 4], [Figure 4]a and [Figure 4]b]. Positive linear correlation was noted between AI-1 and AI-2 in normal and affected hips.
Table 1: Interobserver and intraobserver reproducibility of radiographic measurements

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Table 2: Acetabular index – affected side and normal side

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Table 3: Acetabular index in dysplastic hip using two different techniques at various ages

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Table 4: Correlation between age and both the measurements

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Figure 4: Scattered diagram showing positive liner correlation between AI-1 and AI-2 in (a) dysplastic hips and (b) normal hips

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  Discussion Top


AI is the measurement to assess acetabular inclination in children.[10],[11] Decisions regarding further intervention are based on the age of the child as well as change in the value of the AI. Evaluation of the AI at regular intervals helps to understand acetabular development. If the AI does not improve by 4° in the first 6 months after closed or open reduction of DDH, the surgeon may need to consider surgical intervention for changing the acetabular inclination.[12]

There are two methods for measuring AI. Our study is the first of its kind that has compared two methods of measuring AI using two different medial reference points. Various studies have evaluated intraobserver and interobserver reliability for both the methods. They have shown poor-to-good interobserver reliability.[7],[8],[13],[14]

We found satisfactory interobserver and intraobserver reliability for both the methods. However, the acetabular indices measured by two different methods have difference of 5–6°. This difference is significant and constant. This difference becomes very important as decision for acetabular surgery is made when acetabulum fails to remodel by at least 4 degrees in the first 6 months of reduction. Change in the method is undesirable and can lead to wrong conclusion about acetabular adaption.

From our study we suggest that all studies related to AI should specify that which of the two techniques is used. When serial readings of AI are used for decision making, one should assess serial radiographs by the same method. Changing the method can lead to false judgment about acetabular adaptation.To measure the AI, two lines are used. One of these lines is Hilgenreiner’s line − a line joining triradiate cartilages of both the sides. Triradiate cartilage starts ossifying at around 9 years and therefore is difficult to draw after that age. Considering this fact, we included only children younger than 9 years in our study.

For accurate measurement of the AI, proper positioning of pelvis is required. Pelvic tilt or rotation affects the AI values.[5],[15] To avoid errors because of improper pelvis positioning, we included only those radiographs with acceptable pelvic rotation and tilt.

Limitation of the study

The current study is a cross-sectional study and majority of the children were less than 5 years. It is possible that as child grows, the triradiate cartilage gets thinner and difference between the two methods may diminish. Although we found a difference in the values measured by the two methods at various ages, longitudinal study wherein children are followed up to the age of 8 may give us better understanding of the effect of age on the difference.


  Conclusion Top


AI measured by these two methods has a significant difference. The reproducibility of measurements by both the techniques was satisfactory. Physicians should clearly indicate which of the two medial reference points in the medical records was used to reduce errors. All subsequent assessment should be consistently carried out by the same method.

Financial support and sponsorship

None of the authors received financial support for this study.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

1.
Weinstein SL. Natural history of congenital hip dislocation (CDH) and hip dysplasia. Clin Orthop Relat Res 1987;(225):62-76.  Back to cited text no. 1
    
2.
Tonnis D. Normal values of the hip joint for the evaluation of X-rays in children and adults. Clin Orthop Relat Res 1976;(119):39-47.  Back to cited text no. 2
    
3.
Salter RB. Role of innominate osteotomy in the treatment of congenital dislocation and subluxation of the hip in the older child. J Bone Joint Surg Am 1966;48:1413-39.  Back to cited text no. 3
    
4.
Brougham DI, Broughton NS, Cole WG, Menelaus MB. The predictability of acetabular development after closed reduction for congenital dislocation of the hip. J Bone Joint Surg Br 1988;70:733-6.  Back to cited text no. 4
    
5.
Boniforti FG, Fujii G, Angliss RD, Benson MK. The reliability of measurements of pelvic radiographs in infants. J Bone Joint Surg Br 1997;79:570-5.  Back to cited text no. 5
    
6.
Broughton NS, Brougham DI, Cole WG, Menelaus MB. Reliability of radiological measurements in the assessment of the child’s hip. J Bone Joint Surg Br 1989;71:6-8.  Back to cited text no. 6
    
7.
Kay RM, Watts HG, Dorey FJ. Variability in the assessment of acetabular index. J Pediatr Orthop 1997;17:170-3.  Back to cited text no. 7
    
8.
Skaggs DL, Kaminsky C, Tolo VT, Kay RM, Reynolds RA. Variability in measurement of acetabular index in normal and dysplastic hips, before and after reduction. J Pediatr Orthop 1998;18:799-801.  Back to cited text no. 8
    
9.
Kim HT, Kim JI, Yoo CI. Diagnosing childhood acetabular dysplasia using the lateral margin of the sourcil. J Pediatr Orthop 2000;20:709-17.  Back to cited text no. 9
    
10.
Pirpiris M, Payman KR, Otsuka NY. The assessment of acetabular index: Is there still a place for plain radiography? J Pediatr Orthop 2006;26:310-5.  Back to cited text no. 10
    
11.
Halanski MA, Noonan KJ, Hebert M, Nemeth BA, Mann DC, Leverson G. Manual versus digital radiographic measurements in acetabular dysplasia. Orthopedics 2006;29:724-6.  Back to cited text no. 11
    
12.
Race C, Herring JA. Congenital dislocation of the hip: An evaluation of closed reduction. J Pediatr Orthop 1983;3:166-72.  Back to cited text no. 12
    
13.
van der Bom MJ, Groote ME, Vincken KL, Beek FJ, Bartels LW. Pelvic rotation and tilt can cause misinterpretation of the acetabular index measured on radiographs. Clin Orthop Relat Res 2011;469:1743-9.  Back to cited text no. 13
    
14.
Upasani VV, Bomar JD, Parikh G, Hosalkar H. Reliability of plain radiographic parameters for developmental dysplasia of the hip in children. J Child Orthop 2012;6:173-6.  Back to cited text no. 14
    
15.
Spatz DK, Reiger M, Klaumann M, Miller F, Stanton RP, Lipton GE. Measurement of acetabular index intraobserver and interobserver variation. J Pediatr Orthop 1997;17:174-5.  Back to cited text no. 15
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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