Does Compensatory Anterior Pelvic Tilt Decrease After Bilateral Periacetabular Osteotomy?

Abstract

Background The kinetic link among the lumbar spine, pelvic tilt, and the hip has been hypothesized, but this relationship requires further study in acetabular dysplasia. Anecdotal reports suggest that patients may compensate for acetabular dysplasia with an involuntary increase in anterior pelvic tilt; it is not known if this relationship is affected by acetabular reorientation. Questions/purposes (1) Does compensatory pelvic tilt decrease on preoperatively obtained standing AP pelvis radiographs compared with those obtained at a minimum of 6 months after bilateral periacetabular osteotomy (PAO)? (2) Does a modified surrogate measurement of pelvic tilt, the pubic symphysis to sacroiliac (PS-SI) index, correlate with a physical synthetic bones model in which pelvic tilt can be directly measured? (3) Can the PS-SI index demonstrate high interrater reliability? Methods We assessed the surgical records of one surgeon, who participates in the longitudinally maintained Academic Network of Conservational Hip Outcomes Research (AN-CHOR) registry, for patients who had undergone the second side of a staged bilateral PAO between 2007 and 2016; there were 113 such patients. Of those, 70 (62%) were lost to followup within 6 months of the second PAO or did not have adequate imaging studies, and another three (3%) were excluded for prespecified reasons, leaving 40 (35%) for evaluation in this retrospective study. Standing preoperative and most recent postoperative AP pelvis radiographs were used to measure the Tönnis angle, anterior wall index, posterior wall index, lateral center-edge angle, pubis symphysis-tosacrococcygeal junction distance, and the PS-SI index. The most recent radiographs were obtained at a mean of 16 6 6 months after the second PAO. We chose 6 months as the minimum because at this time point, the majority of patients have reached their maximum clinical improvement and are no longer limited by postoperative muscle dysfunction. Statistical analysis was performed using the intraclass correlation coefficient (ICC) for interrater reliability and paired t-tests for assessing change in measurements from pre-to postoperative. Additionally, a model was created using a physical synthetic bones model in which pelvic tilt could be directly measured. This model was secured through bilateral acetabuli on a mount and rotated through 5°increases in One of the authors certifies that he (IZ) has received or may receive payments or benefits, during the study period, an amount of less than USD 10,000 from OrthoPediatrics (Warsaw, IN, USA), outside this submitted work. Clinical Orthopaedics and Related Research® neither advocates nor endorses the use of any treatment, drug, or device. Readers are encouraged to always seek additional information, including FDA approval status, of any drug or device before clinical use. Each author certifies that his or her institution approved the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research.

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

“…In contrast, Daley et al . [ 18 ] reported a modest mean reduction in pelvic tilt following bilateral PAO in their series of 40 patients when using the PS-SI index to measure change in pelvic tilt on standing AP pelvic radiographs. A closer look at the distribution of change shows that 27 patients (67.5%) showed 0°–10° change, seven patients (17.5%) showed 10–20° change, and 6 patients (15%) showed >20° change in PT following PAO.…”

“…The Pubic Symphysis to Sacro-Iliac (PS-SI) Index [ 18 ]—the ratio of the length of a line drawn from the superior border of the centre of the pubic symphysis to its intersection with the sacroiliac line (a line drawn between the inferior aspect of the sacroiliac joints), and the length of the sacroiliac line, whereby a change of 1mm in the PS-SI equates to 3.6° change in pelvic tilt.…”

“…However, in patients with acetabular dysplasia, this remains inadequately characterized. Furthermore, whether the pelvic tilt changes following correction of the acetabular deformity is still debated [ 11 , 17 , 18 ].…”

“…It has been postulated that patients with acetabular dysplasia try to reduce their pelvic tilt in different functional positions, effectively retroverting their acetabulum to increase anterior–superior femoral head cover [ 18 , 19 ]. If this were the case, one would expect that after correction of the acetabular dysplasia, this compensatory mechanism would resolve.…”

Functional outcomes are preserved in adult acetabular dysplasia with radiographic evidence of lumbosacral spine anomalies: an investigation in hip-spine syndrome

“…To describe lumbar spine, the pubic symphysis to sacroiliac index (PS-SI) was used, an easily reproducible, validated method to characterize pelvic tilt, especially in PAO patients, which is represented by Fig. 6 [ 24 ]. The interpedicular distance at L4, L5, and S1 (Fig.…”