The Validity of a New Low-Dose Stereoradiography System to Perform 2D and 3D Knee Prosthetic Alignment Measurements

Abstract

IntroductionThe EOS stereoradiography system has shown to provide reliable varus/valgus (VV) measurements of the lower limb in 2D (VV2D) and 3D (VV3D) after total knee arthroplasty (TKA). Validity of these measurements has not been investigated yet, therefore the purpose of this study was to determine validity of EOS VV2D and VV3D.MethodsEOS images were made of a lower limb phantom containing a knee prosthesis, while varying VV angle from 15° varus to 15° valgus and flexion angle from 0° to 20°, and changing rotation from 20° internal to 20° external rotation. Differences between the actual VV position of the lower limb phantom and its position as measured on EOS 2D and 3D images were investigated.ResultsRotation, flexion or VV angle alone had no major impact on VV2D or VV3D. Combination of VV angle and rotation with full extension did not show major differences in VV2D measurements either. Combination of flexion and rotation with a neutral VV angle showed variation of up to 7.4° for VV2D; maximum variation for VV3D was only 1.5°. A combination of the three variables showed an even greater distortion of VV2D, while VV3D stayed relatively constant. Maximum measurement difference between preset VV angle and VV2D was 9.8°, while the difference with VV3D was only 1.9°. The largest differences between the preset VV angle and VV2D were found when installing the leg in extreme angles, for example 15° valgus, 20° flexion and 20° internal rotation.ConclusionsAfter TKA, EOS VV3D were more valid than VV2D, indicating that 3D measurements compensate for malpositioning during acquisition. Caution is warranted when measuring VV angle on a conventional radiograph of a knee with a flexion contracture, varus or valgus angle and/or rotation of the knee joint during acquisition.

Do CAS measurements correlate with EOS 3D alignment measurements in primary TKA?

“…LLRs were made, and 2D measurements and 3D reconstructions were performed for these different positions. It was concluded that 2D alignment measurements differed considerably from the preset angle of the artificial leg, while the 3D reconstructions showed small deviation [ 30 ]. Besides validity, excellent intra- and interobserver reliability has been shown in the same study when performing knee prosthesis alignment measurements using EOS 3D reconstructions [ 29 ].…”

“…In this study, postoperative LLRs were taken 6 weeks postoperatively, at which point patients are generally able to apply full weight on their operated leg and can extend the knee. Moreover, the EOS system corrects malpositioning during acquisition, including flexion of the knee [ 30 ]; therefore, the moment of acquisition is not expected to influence our results.…”

“…The EOS 2D/3D system [ 13 , 22 ] is a new model-based technique that can be used to perform pre- and postoperative alignment measurements. Advantages of EOS are that it uses 3D software, by which the system mathematically corrects for malpositioning during acquisition; thus, measurements might potentially be more valid [ 30 , 41 ]. Because the system scans the lower limb using a C-arm, there is no divergence in the vertical plane.…”

“…Performing coronal alignment measurements both pre- and postoperatively with EOS 3D has been proven to be valid and reliable [ 16 , 29 ]. With the EOS 3D system, these measurement errors due to malpositioning are eliminated [ 30 , 41 ]. Also, validity of the images may be improved since divergence in the vertical plane is diminished.…”

Effect of limb rotation on radiographic alignment measurement in mal-aligned knees

“…Studies have reported that limb rotation or foot rotation could significantly affect the outcome [6] [9][11] [15]. The measurement error of hip-knee-ankle (HKA) angle could reach 4-10 degrees or even greater [14] [15]. While the recommended HKA after TKA ranges from -3°to 3°, which represents a proper knee alignment and prosthesis position.…”