The Effect of Functional Pelvic Tilt on the Three-Dimensional Acetabular Cup Orientation in Total Hip Arthroplasty Dislocations.

BACKGROUND: Anterior and posterior pelvic tilt appears to play a role in total hip arthroplasty (THA) stability. When changing from the standing to the sitting position, the pelvis typically rotates posteriorly while the hips flex and this affects the femoro-acetabular positions. This case-control study compares changes in 3-D acetabular cup orientation during functional pelvic tilt between posterior THA dislocations vs stable THAs. METHODS: Standing and sitting 3-D cup orientation was compared between fifteen posterior dislocations vs 233 prospectively followed stable THAs. 3-D cup orientation was calculated using previously validated trigonometric algorithms on biplanar radiographs. Those algorithms combine the angles in the three anatomical planes (coronal inclination, transverse version, and sagittal ante-inclination) in the standing position with the change in sagittal pelvic tilt from standing to sitting to calculate the 3-D orientation in the sitting position. RESULTS: The standing cup orientation of the dislocated THAs was only characterized by a lower coronal inclination (P = .039). Compared with the controls, from standing to sitting, they showed less posterior pelvic tilt (P < .001). This led to a significant lower coronal inclination (P < .001) and sagittal ante-inclination (P < .001) in the sitting position but similar transverse version (P = .366). CONCLUSIONS: Comparing posterior THA dislocations to stable THAs, there is a lower increase of all three orientation angles from standing to sitting. This leads to a decreased sitting coronal inclination and sagittal ante-inclination which may lead to an increased risk of impingement ensued by THA instability. By contrast, the transverse version was not significantly different in both positions. This confirms the importance of biplanar data on functional cup orientation. LEVEL OF EVIDENCE: Diagnostic, Level III.

Quantifying the Effects of Hip Surgery on the Sphericity of the Femoral Head in Patients with Mucopolysaccharidosis Type I.

BACKGROUND: The introduction of stem cell transplantation has improved life expectancy and cognitive outcome in patients with mucopolysaccharidosis I, but this condition remains associated with substantial residual disease in several parts of the body. Many patients have hip dysplasia with progressive medial flattening of the femoral head. Quantitative evidence on the effect of surgery on remodeling to sphericity of flattened femoral heads is lacking. In the present study, we used statistical shape modeling to quantify the effect of hip surgery on the sphericity of the femoral head in patients with mucopolysaccharidosis I. METHODS: We performed a retrospective case control study involving a series of 23 patients with hip dysplasia due to mucopolysaccharidosis I. Surgery was not offered to the first 11 children (control group). Following a change in treatment protocol, the next 12 children underwent bilateral proximal femoral varus derotation osteotomy and Pemberton osteotomy for the treatment of acetabular dysplasia with progressive femoral head flattening (surgery group). The surgery and control groups were compared with a reference group of patients with normal hips. Statistical shape modeling was used to quantify the shape of the femoral head (i.e., flattening and/or roundness of the epiphysis). RESULTS: The mean age at the time of stem cell transplantation in the surgery and control groups was comparable (1.2 years). The mean age at the time of surgical intervention was 5.5 years, and mean duration of postoperative follow-up was 3.3 years. Statistical shape modeling showed variations within the total group in terms of medial indentation, width, height, and sphericity of the femoral heads. In contrast to the progressive femoral head flattening in the control group, the surgery group showed improvement of the sphericity of the femoral head after surgery. The overall shape characteristics of the femoral head in the surgery group were similar to those of the reference group of patients with normal hips. CONCLUSIONS: To our knowledge, this is the first study in patients with mucopolysaccharidosis I that has shown quantitative remodeling of the dysplastic, flattened femoral head to normal sphericity after increasing containment of the femoral head. LEVEL OF EVIDENCE: Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.

Non-equivalent Results from Different Anteversion Measurements Methods for the Evaluation of the Acetabular Cup Orientation in Total Hip Arthroplasty.

OBJECTIVE: To determine the comparability among 10 radiographic anteversion methods for acetabular cup orientation in total hip arthroplasty (THA) found in the literature and the "gold" standard of assessing the anteversion with CT. METHODS: This is a retrospective study that blindly compares 10 different conventional radiographic anteversion measurements with the "gold" standard, the measurement of anteversion on the transverse plane of the 3-D images made with CT. The patient archiving and communications system (PACS) was systematically searched for subjects that had undergone a CT angiogram of the abdomen and lower extremities, including the pelvis, had at least one THA in situ and had undergone anterior-posterior (AP) and cross-lateral pelvic radiography between January 2013 and August 2016 in the Diakonessenhuis Hospital Utrecht/Zeist, a non-academic institution. CT scans of patients (n = 16) were systematically collected. Three observers independently measured cup anteversion from radiographs, using a total of 10 different methods, and measured the "gold" standard on CT images. The outcomes of the 10 radiographic anteversion were compared in terms of linear correlation with the "gold" standard on CT images. RESULTS: The correlations of the radiographic measured anteversions with the "gold" standard measured on CT images were 0.528 for the method of Liaw, 0.556 for Wan, 0.562 for the cross-lateral method, 0.586 for Hassan, 0.594 for Dorr, 0.602 for Lewinnek, 0.624 for Widmer, 0.671 for the lateral CT, 0.747 for Ackland, and 0.771 for the method of Riten Pradham. CONCLUSION: Anteversion measurement methods represent different projectional angles of the acetabular cup in different planes around different axes. Therefore, they differ from the "gold" standard and are not interchangeable, as is shown by this study. We consider the anatomical anteversion in the transverse plane rotating around the longitudinal axis as the "gold" standard and recommend avoiding using the term anteversion for other projectional angles in different planes.