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. 2023 Sep 18:23:101206.
doi: 10.1016/j.artd.2023.101206. eCollection 2023 Oct.

Three-Dimensional CT-Based Limb Length Evaluation Is Highly Dependent on Anatomical Landmark Selection and Pelvic Asymmetry

William B O'Callaghan  1   2 , Matt Thompson  3 , Thies Wuestemann  3 , Sarah L Whitehouse  1   4 , Ross W Crawford  1   4
Affiliations

Three-Dimensional CT-Based Limb Length Evaluation Is Highly Dependent on Anatomical Landmark Selection and Pelvic Asymmetry

William B O'Callaghan et al. Arthroplast Today. .

Abstract

Background: Pelvic skeletal asymmetry can result in rotational differences and morphologic bony prominence variance between the left and right hemipelvis. When selecting bony reference points for modern computed tomography-based robotic total hip arthroplasty planning, it is unclear which bony landmarks are the most reliable and accurate, especially in the presence of significant pelvic asymmetry.

Methods: A retrospective study was conducted utilizing a database of computed tomography scans. Multiple bony landmarks in the pelvis and femur were selected for comparison, with the aim of measuring pelvic asymmetry. Specifically, the study measured the average difference in lateral offset between the left and right hemipelvis caused by pelvic asymmetry. Landmarks were also compared to determine the impact of pelvic asymmetry on hip length, femur length, and limb length discrepancies. Furthermore, a scenario was simulated in the software whereby a total hip replacement was inserted, potentially changing the hip length. The impact of pelvic reference point selection on the measurement of this simulated change in hip length was examined.

Results: This study population showed widespread pelvic asymmetry. The anatomical landmarks of the opposite side cannot be relied upon for predicting the anatomy of the affected side. The center of rotation axis is more reliable than the inferior obturator foramen axis for hip length discrepancy due to pelvic asymmetry (P < .05).

Conclusions: Current computer-assisted surgery THR software reports measurements of global offset and hip length that do not consider pelvic asymmetry. Surgeons are not given confidence ranges to represent the potential impact of asymmetry on the global offset and hip length values. Surgeons following these numbers to guide implant position may incur implant placement error should significant pelvic asymmetry be present in a given patient.

Keywords: Digital templating; Global offset; Hip length; Pelvic asymmetry; Robotics; Total hip arthroplasty.

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Figures

Figure 1
Figure 1
General landmark and measurement selection of the ASIS, COR, and iOF axes for hip length measurement on the corresponding model (top left) and how they are mapped on a specific individual’s bones (top right). The 3 different measures of hip length (LT-to-ASIS, LT-to-iOF, and the control hip length measure, LT-to-COR) (bottom).
Figure 2
Figure 2
Measurements on the correspondence model used to calculate and compare hip length, femur length, and limb length discrepancies (left). The same measurements were taken on the 106 patients in the subgroup in which the pelvis and corresponding femoral and tibial models were available (example shown in middle). Visual representation of hip length (LT-to-COR), femur length (MidE-to-COR), and limb length (MM-to-MidE + MidE-to-COR) (right).
Figure 3
Figure 3
Scatterplot of femur and limb length discrepancies.
Figure 4
Figure 4
Scatterplot of hip and limb length discrepancies.
Figure 5
Figure 5
The difference in hip length discrepancy relative to the CORs when using the ASIS or inferior obturator foramen axes as the pelvic reference.
See this image and copyright information in PMC

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