Method for quantitative assessment of acetabular bone defects

Abstract

The objective of the study was to suggest a novel quantitative assessment of acetabular bone defects based on a statistical shape model, validate the method, and present preliminary results. Two exemplary CT-data sets with acetabular bone defects were segmented to obtain a solid model of each defect pelvis. The pathological areas around the acetabulum were excluded and a statistical shape model was fitted to the remaining healthy bone structures. The excluded areas were extrapolated such that a solid model of the native pelvis per specimen resulted (i.e., each pelvis without defect). The validity of the reconstruction was tested by a leave-one-out study. Validation results showed median reconstruction errors of 3.0 mm for center of rotation, 1.7 mm for acetabulum diameter, 2.1° for inclination, 2.5° for anteversion, and 3.3 mm³ for bone volume around the acetabulum. By applying Boolean operations on the solid models of defect and native pelvis, bone loss and bone formation in four different sectors were assessed. For both analyzed specimens, bone loss and bone formation per sector were calculated and were consistent with the visual impression. In specimen_1 bone loss was predominant in the medial wall (10.8 ml; 79%), in specimen_2 in the posterior column (15.6 ml; 46%). This study showed the feasibility of a quantitative assessment of acetabular bone defects using a statistical shape model-based reconstruction method. Validation results showed acceptable reconstruction accuracy, also when less healthy bone remains. The method could potentially be used for implant development, pre-clinical testing, pre-operative planning, and intra-operative navigation. © 2018 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 9999:1-9, 2018.

Keywords: acetabular bone defects; quantification; statistical shape model; volume analysis.

Figure 1

Workflow of the quantitative assessment. Determination of a solid model of the pelvis with bone defect on the basis of a segmented CT‐data set. Masking of the pathological regions and reconstruction based on the remaining healthy bone. Definition of clinically relevant bone defect sectors on defect pelvis and native pelvis. Analysis of bone volume loss (light red), bone defect shape, and bone formation (light green).

Figure 2

Bone defect sectors. (A) Defect sectors are cranial roof (1), anterior column (2), posterior column (3), and medial wall (4). (B) Defect sectors are constructed on the acetabular plane and aligned to the patient coordinate system. (C) Defect sectors are scaled in relation to the acetabulum diameter using the parameters R ₁, R ₂, and R ₃ as well as angles α and β.

Figure 3

Calculation of bone loss and bone formation. (A) Subtracting defect pelvis from native pelvis results in a body that represents bone loss. (B) Subtracting native pelvis from defect pelvis results in a body that represents bone formation.

Figure 4

Validation of the statistical shape model. The validation comprises a Leave‐one‐out study representing four clinical scenarios (Scenario_0, Scenario_1, Scenario_2, Scenario_3) by four masks (Mask_0, Mask_1, Mask_2, Mask_3) and comparing six anatomical parameters. (A) Scenario_0 represents ground truth of the reconstruction. Scenario_1 represents a scenario in which an implant and/or bone defect on one side is masked and excluded before the reconstruction. Scenario_2 represents a scenario in which both acetabuli are excluded. Scenario_3 represents a scenario in which both acetabuli and the cranial areas of the os ilium are excluded due to an incomplete CT‐scan. (B) Parametrical masks were constructed for each scenario. (C) Parameters used to asses the quality of the reconstruction are root‐mean‐square of surface deviation (RMS), position of center of rotation (CoR), diameter of acetabulum (AcD), anatomical inclination (Inclination), anatomical anteversion (Anteversion), and bone volume within the left acetabulum mask (Volume).

Figure 5

Reconstruction errors of Mask_2 using SSMs with different numbers of shape variation modes. Shown are distance and differences between each individual pelvis and its reconstruction using the scaled mean shape (0), 10 shape variation modes (10), 20 shape variation modes (20), 30 shape variation modes (30), 40 shape variation modes (40), and 65 shape variation modes (all). (A) RMS of surface deviation. (B) CoR position. (C) Acetabulum diameter. (D) Inclination. (E) Anteversion. (F) Bone volume within acetabulum mask. The boxplots show median, 25% and 75% percentile, and extreme values including all outliers.

Figure 6

Reconstruction errors of the SSM with 20 shape variation modes using different masks. Shown are distance and differences between each individual pelvis and its reconstruction using Mask_0 (0), Mask_1 (1), Mask_2 (2), and Mask_3 (3). (A) RMS of surface deviation. (B) CoR position. (C) Acetabulum diameter. (D) Inclination. (E) Anteversion. (F) Bone volume within acetabulum mask.

Figure 7

Quantitative defect assessment. (A) Native pelvis, defect pelvis, bone loss and bone formation of two exemplary specimens. (B) Volume of native pelvis (brown bars), defect pelvis (gray bars), bone loss (red bars are absolute values, blue dots are relative values), as well as bone formation (green bars) for each sector.