Relationship Between Acetabular Hounsfield Unit Values and Periprosthetic Fractures in Cementless Total Hip Arthroplasty: A Matched Case-Control Study

Abstract

Background: The association between regional bone status around the acetabulum and the incidence of intraoperative acetabulum fractures has not been extensively studied. We investigated the association of Hounsfield unit (HU) values on computed tomography in the regions of the acetabulum with periprosthetic fractures.

Methods: We retrospectively reviewed records of 301 consecutive patients who underwent cementless total hip arthroplasty between October 2016 and December 2020. Using preoperative computed tomography taken in the 4 weeks preceding total hip arthroplasty, we measured HU values in 4 different acetabulum regions (anterior, medial, posterior, and superior). After identifying fracture cases, we identified a control group-matched in terms of sex, age, and preoperative diagnosis-selected in a 1:3 ratio among nonfracture patients treated in the same inclusive period. As the average HU values differed by region, we used the standardized value to compare fracture-site HUs. We ranked the standardized HU values for each acetabular site and compared the fracture site rank between the groups.

Results: Intraoperative acetabular fractures were observed in 10 hips (3.2%), occurring most frequently in the superior region (40%). The standardized HU values of the fracture site were statistically lower in the fracture group (P = .039). We compared the ranks of the standardized HUs of the fractured parts with those of the corresponding parts in the control group; the fracture site had a significantly lower standardized HU rank, indicating that fractures tended to occur in the relatively "weaker-than-expected" parts.

Conclusions: Periprosthetic fractures tended to occur at relatively weak parts of the acetabulum.

Keywords: Acetabulum; Hounsfield unit; Occult fracture; Periprosthetic fracture; Total hip arthroplasty.

Figure 1

Postoperative coronal computed tomography image of the hip joint. The axial image was sliced at the center of the head.

Figure 2

Preoperative axial computed tomography image of the hip joint. Image is at the same level as the postoperative image and sliced at the center of the head. Anterior, posterior, and medial walls were measured with the maximum elliptic mean value and did not include the cortex. The measurement of the medial wall was taken at the bisector of the anterior-posterior diameter of the acetabulum without a double floor.

Figure 3

Maximum elliptic mean. In the superior wall, the coronal image at the center of the medial wall was used to measure the maximum elliptic mean without the cortical bone.

Figure 4

Scatter plot of reliability at each region of the acetabulum. The plot shows inter-rater reliability assessed at the (a) superior, (b) anterior, (c) posterior, and (d) medial walls.

Supplemental File 1

Bland-Altman plot for inter-rater reliability analysis of each acetabulum location. The limits of agreement are shown as dotted, black lines with 95% confidence intervals (green and red areas), bias (as dotted black line) with 95% confidence intervals (blue area), and regression fit of the differences of the means (as solid black line). (a) Medial wall. (b) Posterior wall. (c) Superior wall. (d) Anterior wall.