Effects of Lower Limb Rotation on the Measurement Accuracy of Coronal Alignment in Long-Leg Radiographs After Total Knee Arthroplasty
Total knee arthroplasty (TKA) is a widely performed surgical procedure aimed at restoring proper lower limb alignment, particularly in the coronal plane. Accurate alignment is crucial for reducing stress at the bone cement–implant interface, minimizing polyethylene wear, and preventing implant loosening. Weight-bearing long-leg radiographs (LLRs) are the standard imaging modality used to assess the mechanical axis in the coronal plane, identify anatomical variations in the femur and tibia, and plan the degree of bone resection preoperatively. However, the accuracy of these measurements can be significantly affected by lower limb rotation or flexion during imaging. This study investigates the impact of rotational errors on the measurement of coronal alignment parameters in LLRs after TKA and explores the relationship between lower limb rotation and alignment accuracy.
Background and Rationale
Restoring proper lower limb alignment is a critical goal of TKA. Misalignment can lead to increased stress on the implant, accelerated wear, and eventual failure of the prosthesis. LLRs are commonly used to evaluate the mechanical axis of the lower limb, which is defined by the hip–knee–ankle (HKA) angle. The HKA angle is formed by the intersection of the femoral and tibial mechanical axes and is a key parameter in assessing alignment. However, rotational errors during imaging can distort the measurement of this angle, leading to inaccurate assessments of alignment and component positioning.
Previous studies have shown that rotation-dependent linear changes can influence the measurement of alignment parameters. For instance, internal or external rotation of the lower limb can alter the apparent alignment of the femoral and tibial components, as well as the HKA angle. This study builds on these findings by analyzing the types and range of rotational errors encountered during LLRs and recalculating alignment parameters to determine the impact of these errors on clinical judgments.
Study Design and Methodology
This prospective study included patients with grade three or four end-stage osteoarthritis (classified by Kellgren and Lawrence) who underwent unilateral TKA at a single institution between December 2018 and January 2020. Patients with large bony defects, patellar subluxation, severe osteophytosis, or previous surgeries that prevented complete knee extension were excluded. The study protocol was approved by the hospital’s Research and Ethics Committee, and written consent was obtained from all participants.
All patients underwent the same surgical technique for TKA, with the distal femoral osteotomy performed perpendicular to the femoral mechanical axis as measured from preoperative LLRs. The goal was to achieve a neutral mechanical axis. Cemented posterior-stabilized total knee prostheses were implanted, and standardized postoperative care, including multimodal pain therapy, thromboembolism prophylaxis, and physiotherapy, was provided.
LLRs were obtained before surgery and three months postoperatively using the same imaging equipment and protocol. Patients were instructed to stand with their feet shoulder-width apart, with the patella positioned straight forward at maximum knee extension. The radiographic beam was centered on the knee joint, and imaging was performed at a distance of 180 cm from the radiography tube.
Two orthopedic surgeons trained in digital measurement techniques used ImageJ2x software to assess implant alignment and lower limb rotation. The HKA angle, femoral component alignment, and tibial component alignment were measured. Malalignment (outliers) was defined as a deviation of >2° from 90° for femoral or tibial component alignment or >3° from a neutral HKA angle (0°).
The degree of lower limb rotation was calculated using a formula based on fibular parameters visible in the LLRs. This formula incorporated the visible part of the fibula, the overlapped part of the fibular tip, and the distance between the fibular tip and the lateral fibular cortex. Multiple regression analysis showed a strong correlation between these fibular parameters and knee rotation.
Results
A total of 63 patients were initially enrolled, but 13 were excluded due to extra joint deformity, incomplete knee extension, or disagreement among observers regarding rotation degrees. The final analysis included 50 patients. The mean preoperative HKA angle was 4.07° ± 8.70°, ranging from 15° varus to 13.5° valgus. Twenty-two patients had grade three osteoarthritis, and 28 had grade four osteoarthritis.
The mean degree of rotation of the operated limb at three months postoperatively was 9.30° ± 8.37°, ranging from 26.70° internal rotation to 6.18° external rotation. The true values of the alignment angles were recalculated after correcting for rotational errors using regression analysis. The HKA angle before correction was 1.79° ± 3.47°, and after correction, it was 1.14° ± 3.53°. Before correction, 30 patients were classified as outliers, and after correction, this number decreased to 24. Fourteen patients changed from outliers to inliers, while eight changed from inliers to outliers.
For femoral component alignment, the mean value before correction was 89.96° ± 3.33°, and after correction, it was 90.73° ± 3.43°. Five measurements changed from outliers to inliers, and four changed from inliers to outliers. For tibial component alignment, five measurements changed from outliers to inliers, and two changed from inliers to outliers.
Discussion
The results of this study demonstrate that lower limb rotation significantly affects the measurement accuracy of coronal alignment parameters in LLRs after TKA. Despite instructions to position the patella forward during imaging, rotational errors ranging from 26.7° internal rotation to 8.4° external rotation were observed. These errors likely resulted from the anatomical position of the patella, which is slightly lateral, and the presence of osteophytes that may have obscured its true position.
The study highlights the importance of accounting for rotational errors when interpreting LLRs. The correction of these errors led to significant changes in the classification of alignment status, with 22 patients (44%) being reclassified as either malaligned or well-aligned. This finding raises questions about the reliability of LLRs as a standalone tool for preoperative planning and postoperative evaluation of alignment.
The formula used to calculate lower limb rotation was derived from a saw bone model, which may not fully reflect the complexities of human anatomy. Future research should incorporate three-dimensional weight-bearing imaging to provide more accurate assessments of alignment and component positioning. Additionally, the study’s small sample size and single-center design limit the generalizability of the findings. Larger, multicenter studies are needed to validate these results and establish standardized protocols for imaging and measurement.
Conclusion
Lower limb rotation has a significant impact on the measurement accuracy of coronal alignment parameters in LLRs after TKA. Surgeons should be aware of the potential for rotational errors and consider implementing correction values in clinical practice. While LLRs remain a valuable tool for assessing alignment, their limitations must be acknowledged, and alternative imaging modalities should be explored to improve the accuracy of preoperative planning and postoperative evaluation.
doi.org/10.1097/CM9.0000000000001982
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