Abstract
Background: Numerous radiographic indices have been described to help define the degree of acetabular deficiency in adult patients with developmental dysplasia. The lateral centre-edge angle (LCE) of Wiberg and the anterior centre-edge (ACE) angle of Lequesne are two of the commonly measured indices that are reported in the evaluation of procedures that are used to correct acetabular deformity. Unfortunately much of the reported literature that tries to define abnormal indices has been extrapolated from the evaluation of “normal” pelvi or those with osteoarthritis. The purpose of this study was to evaluate the application and limitations of the LCE and ACE angles in a group of patients with developmental dysplasia treated with periacetabular osteotomy.
Methods: 50 cases were randomly selected from a larger cohort of over 500 patients with acetabular dysplasia treated at our institution with periacetabular osteotomy. The preoperative and post osteotomy false profile and anteroposterior plain radiographs were reviewed. Cases were first grouped into one of three categories based on a general estimation of the location of acetabular deficiency as determined from the AP radiograph.
A number of radiographic indices were measured and compared including the VCE angle, LCE angle, and acetabular angles of Tönnis and Sharp. Variation in both the VCE and LCE angle was evaluated by measuring the index using two different reference points. This included (1) the traditional mark of the furthest point along the sclerotic density of the weight bearing zone and (2) an alternate point representing the furthest extent of lateral or anterior bony coverage. Additional sources of measurement error were determined. The relationships between the centre-edge angles and other radiographic indices were determined. An evaluation of the indices and correction on post osteotomy radiographs was also performed.
Results: Fifty cases in 45 patients were reviewed. There were 31 female and 19 males. The mean age was 30 years (range, 17–45). A general review of all preoperative AP radiographs revealed that all hips displayed some degree of lateral deficiency.
Nineteen of these cases displayed a “classic” lateral and anterior deficiency. However, 19 cases displayed a more uniform deficiency and 12 cases were in fact retroverted.
Evaluation of the radiographic indices revealed:
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A mean VCE angle of 2.3 degrees (SD±12.7) and LCE angle of 3.4 degrees (SD±9.3). These were corrected to 25.8 degrees (SD±11.6) and 28.6 degrees (SD±8.7) following osteotomy.
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The VCE and LCE angles did not appear to be correlated (r=0.35). This is contrary to previous studies evaluating non-dysplastic pelvi (Chosa et al., 1997). The LCE angle showed no significant correlation to other lateral coverage indices (Tönnis, Sharp).
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No correlation was seen either in the post osteotomy values, or in the absolute degree of correction.
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The alternate VCE (aVCE), using the most anterior aspect of the acetabular margin as the reference point was consistently larger (p< 0.001) with a mean difference of 27.1 degrees (SD±10.0). There was however a positive correlation between these two methods of measurements (r=0.77).
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A similar variation was seen when comparing the LCE angle and the alternate LCE (aLCE). The mean difference between measurements was 7.3 degrees (SD±8.7)(p< 0.001).
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The mean VCE in hips with primarily anterior and lateral deficiency (−6.7°±12.5) was significantly lower (p< 0.01) than those with uniform deficiency (5.1°±8.3) or those with retroverted acetabuli (8.9°±13.3)
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Dysplastic hips with a decreased LCE angle but relatively normal Tönnis angle should be treated carefully as osteotomy may result in excessive angular correction in the coronal plane, thus creating a negative Tönnis angle. This can ultimately lead to problems with lateral and/or anterolateral impingement.
Potential sources of error in measurement that were identified include:
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Deformity of the acetabulum and occasional abnormalities of the femoral head limit the ability to identify the center of the rotation necessary to measure the centre-edge angles. Subluxation of the femoral head also creates a degree of error. These difficulties were observed in over 20% of cases.
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Alteration in pelvic tilt and rotation theoretically decreases the accuracy of measurement. Practically over 30% of radiographs were seen as less than ideal.
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The absolute reference point for VCE and LCE angles as the end of the sclerotic line in the weight bearing area can be (1) difficult to define (2) does not always represent the most anterior or lateral extent of the acetabular margin. This discrepancy appears to increases in dysplastic hips. This has been suggested previously (Fabeck et al.,1999) and is now supported by our findings.
Conclusion: Centre-edge indices can be useful parameters in defining acetabular morphology. However, these parameters should not be used in isolation and the absolute values do little to define the overall location and degree of deficiency in hips with acetabular dysplasia. A number of significant sources of measurement error limit their accuracy especially in patients with hip dysplasia. Currently, it is our feeling that no single radiographic parameter fully defines the specific morphology in each individual case nor reflects the success of correction when treating patients with periacetabular osteotomy.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
