Abstract
Introduction
Revision total hip arthroplasty is often associated with acetabular bone defects. In most cases, assessment of such defects is still qualitative and biased by subjective interpretations. Three-dimensional imaging techniques and novel anatomical reconstructions using statistical shape models (SSM) allow a more impartial and quantitative assessment of acetabular bone defects [1]. The objectives of this study are to define five clinically relevant parameters and to assess 50 acetabular bone defects in a quantitative way.
Methods
Anonymized CT-data of 50 hemi-pelvises with acetabular bone defects were included in the study. The assessment was based on solid models of the defect pelvis (i.e. pelvis with bone defect) and its anatomical reconstruction (i.e. native pelvis without bone defect) (Fig.1A).
Five clinically relevant parameters were defined: (1) Bone loss, defined by subtracting defect pelvis from native pelvis. (2) Bone formation, defined by subtracting native pelvis from defect pelvis. Bone formation represents bone structures, which were not present in the native pelvis (e.g. caused by remodeling processes around a migrated implant). (3) Ovality, defined by the length to width ratio of an ellipse fitted in the defect acetabulum. A ratio of 1.0 would represent a circular acetabulum. (4) Lateral center-edge angle (LCE angle), defined by the angle between the most lateral edge of the cranial roof and the body Z-axis, and (5) implant migration, defined by the distance between center of rotation (CoR) of the existing implant and CoR of native pelvis (Fig. 1B).
Results
All data are presented as single values as well as median and [25th, 75th]- percentile (Fig.2). Bone loss was 53.6 [41.5, 76.7] ml with a minimum of 19.0 ml and maximum of 103.9 ml. Bone formation was 15.7 [10.5, 21.2] ml with a minimum of 3.5 ml and a maximum of 41.6 ml. Ovality was 1.3 [1.1, 1.4] with a minimum of 1.0 and a maximum of 2.0. LCE angle was 30.4° [21.5°, 40.1°] with a minimum of 11.6° and a maximum of 63.0°. Implant migration was 25.3 [15.1, 32.6] mm with a minimum of 5.4 mm and a maximum of 53.5 mm.
Discussion
Within this study, 50 hemi-pelvises with acetabular bone defects were successfully quantified using five clinically relevant parameters. Application of this method provides impartial and quantitative data of acetabular bone defects, which could be beneficial in clinical practice for pre-operative planning or comparison of surgical outcomes. Including a larger number of cases, this method could even serve as a basis for a novel classification system for acetabular bone defects.
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