Acetabular dysplasia: multiple pathologies and myriad solutions

Does pelvic tilt change with a peri-acetabular osteotomy?

Change in pelvic tilt (PT) during and after peri-acetabular osteotomy (PAO) is important for surgical planning. The aims of this study were to (i) determine how PT varies throughout the course of treatment in patients undergoing PAO, (ii) test what factors influence the change in PT and (iii) assess whether changes in PT influenced achieved correction. This is an retrospective, single-centre, consecutive case series of 111 patients treated with PAO for global (n = 79), posterior (n = 49) or anterior dysplasia (n = 6) (mean age: 27.3 ± 7.7 years; 85% females). PT was determined on supine, anteroposterior pelvic radiographs pre-, intra-, 1 day, 6 weeks and 1 year post-operatively, using the sacro-femoral-pubic (SFP) angle, a validated, surrogate marker of PT. An optimal acetabular correction was based on the lateral centre-edge angle (25°-40°), acetabular index (-5° to 10°) and cross-over ratio (<20%). There was a significant difference across pre- (70.1° ± 4.8°), 1-day (71.7° ± 4.3°; P < 0.001) and early post-operative SFP (70.6° ± 4.7°; P = 0.004). The difference in SPF between pre-operative and 1-year post-operative was -0.5° ± 3.1° (P = 0.043), with 9% of cases having a difference of >5°. The difference in SFP did not correlate with age, sex, body mass index, type of dysplasia or achievement of optimal acetabular correction (P = 0.1-0.9). In the early post-operative period, PT is reduced, leading to a relative appearance of acetabular retroversion, which gradually corrects and is restored by annual follow-up. The degree of change in PT during PAO did not adversely affect fragment orientation. PT does not significantly change in most patients undergoing PAO and therefore does not appear to be a compensatory mechanism.

What is the pelvic tilt in acetabular dysplasia and does it change following peri-acetabular osteotomy?

To quantify the pelvic tilt (PT) in patients with symptomatic acetabular dysplasia and determine if it represents a compensatory mechanism to improve femoral head coverage, we studied a cohort of 16 patients undergoing 32 bilateral staged PAOs for acetabular dysplasia and compared this to a matched cohort of 32 patients undergoing PAO for unilateral acetabular dysplasia all with >1 year follow-up. The change in PT was determined with two validated methods, namely, the sacro-femoral-pubic (SFP) angle and the pubic symphysis to sacroiliac index (PS-SI). Despite an improvement in the lateral centre-edge and Tönnis angles to within normal limits following PAO, patients with unilateral and bilateral acetabular dysplasia have similar PT pre-operatively (8° ± 5°) and post-operatively (9° ± 5°). A change of >5° was observed in only six patients (13%) using the SFP angle, and five patients (10%) using the PS-SI, all increased (posterior rotation of the pelvis). No patients were observed to have a change in PT >10°. The observed PT in our study group is equivalent to that found in the normal population and in patient with symptomatic acetabular retroversion. These findings all suggest that PT is morphological rather than a result of a compensatory mechanism, and even if it was compensatory, it does not appear to reverse significantly following PAO. The target for acetabular reorientation, therefore, remains constant.

Is the Femoro-Epiphyseal Acetabular Roof (FEAR) index on MRI a relevant predictive factor of instability in a borderline dysplastic hip?

AIMS

A borderline dysplastic hip can behave as either stable or unstable and this makes surgical decision making challenging. While an unstable hip may be best treated by acetabular reorientation, stable hips can be treated arthroscopically. Several imaging parameters can help to identify the appropriate treatment, including the Femoro-Epiphyseal Acetabular Roof (FEAR) index, measured on plain radiographs. The aim of this study was to assess the reliability and the sensitivity of FEAR index on MRI compared with its radiological measurement.

PATIENTS AND METHODS

The technique of measuring the FEAR index on MRI was defined and its reliability validated. A retrospective study assessed three groups of 20 patients: an unstable group of 'borderline dysplastic hips' with lateral centre edge angle (LCEA) less than 25° treated successfully by periacetabular osteotomy; a stable group of 'borderline dysplastic hips' with LCEA less than 25° treated successfully by impingement surgery; and an asymptomatic control group with LCEA between 25° and 35°. The following measurements were performed on both standardized radiographs and on MRI: LCEA, acetabular index, femoral anteversion, and FEAR index.

RESULTS

The FEAR index showed excellent intraobserver and interobserver reliability on both MRI and radiographs. The FEAR index was more reliable on radiographs than on MRI. The FEAR index on MRI was lower in the stable borderline group (mean -4.2° (sd 9.1°)) compared with the unstable borderline group (mean 7.9° (sd 6.8°)). With a FEAR index cut-off value of 2°, 90% of patients were correctly identified as stable or unstable using the radiological FEAR index, compared with 82.5% using the FEAR index on MRI. The FEAR index was a better predictor of instability on plain radiographs than on MRI.

CONCLUSION

The FEAR index measured on MRI is less reliable and less sensitive than the FEAR index measured on radiographs. The cut-off value of 2° for radiological FEAR index predicted hip stability with 90% probability. Cite this article: Bone Joint J 2019;101-B:1578-1584.