Rotation angle between the femoral and tibial components in varus/valgus stress X-rays following total knee arthroplasty

Abstract

Background

The impact of rotational angle between the femoral and tibial components is often overlooked in the 2D evaluation of varus/valgus stability after TKA with anterior-posterior knee X-rays. The rotation angle between the femoral and tibial components may influence the measured angle and distance between these components in 2D stress X-rays following TKA.

Objective

The purpose of this study was to assess the impact of the rotational angle between the femoral and tibial components on the evaluation of varus/valgus stability using stress X-rays following total knee arthroplasty (TKA).

Methods

This prospective study analyzed 48 consecutive rTKAs (three males, aged 68 ± 6.4 years; 45 females, aged 75 ± 5.9 years). Postoperative varus/valgus stress X-rays were taken at maximum manual stress during knee extension under anesthesia, and were analyzed three-dimensionally using a 2D-3D image matching technique with 3D bone and component models. The rotation angles of the components (CR angles) were assessed under conditions of no stress, valgus stress, and varus stress. Additionally, the varus/valgus angle (VV angle) between components was evaluated under the same conditions. Medial joint opening (MJO) and lateral joint opening (LJO) were also measured in both stressed and non-stressed states.

Results

The CR angles under no stress, valgus stress, and varus stress were 9.9 ± 5.5°, 10.1 ± 6.2°, and 10.8 ± 5.1°, respectively. The VV angles under no stress, valgus stress, and varus stress were 3.6 ± 1.1°, 1.1 ± 1.4°, and 7.1 ± 1.9°, respectively. The MJO in the non-stress condition and under valgus stress were 0.0 ± 0.4 mm and 1.3 ± 1.0 mm, respectively. The LJO in the non-stress condition and under varus stress were 0.9 ± 0.9 mm and 2.9 ± 2.7 mm, respectively.

Conclusions

This prospective study revealed two key findings: (1) the CR angle in varus stress was significantly more externally rotated compared to the CR angle in the non-stress condition, and (2) no significant correlations were found between the rotational angle of the components and the VV angle, MJO, or LJO.

Keywords

robotic total knee arthroplasty stress X ray 2D-3D image matching technique rotation angle between components

Introduction

Robotic-assisted total knee arthroplasty (rTKA) has become increasingly widespread, with recent reports highlighting favorable clinical outcomes.^1,2 These positive results are attributed to improved postoperative varus/valgus stability, which is achieved through precise prosthesis positioning that is tailored to each patient's unique anatomy and soft tissue properties. Stability following TKA is crucial to the procedure's overall clinical success.³ In our previous study, we demonstrated that rTKAs can achieve excellent postoperative varus/valgus stability, as evidenced by varus/valgus stress X-rays.⁴

Stress X-rays have been established as a standard method for assessing postoperative varus/valgus stability. However, this assessment is typically conducted using a two-dimensional (2D) approach, rather than a three-dimensional (3D) one.^5,6 The limitation of the 2D evaluation of anterior-posterior knee X-rays after TKA is that it does not account for the rotational angle between the femoral and tibial components.⁷ Consequently, the impact of this rotational angle on the assessment of varus/valgus stability is often overlooked. It is possible that the rotation angle between the femoral and tibial components may influence the measured angle and distance between these components in 2D stress X-rays following TKA.

Our group has previously conducted 3D static alignment measurements and knee motion analyses using the 3D-2D image registration technique.^8–19 In this study, we aim to evaluate the rotational angle between the femoral and tibial components three-dimensionally using the same 3D-2D image registration technique in varus/valgus stress X-rays following rTKA. The primary objective of this study is to determine the impact of the rotational angle between the femoral and tibial components on the evaluation of varus/valgus stability in stress X-rays after TKA.

The hypothesis is that the rotational angle between the tibial and femoral components affects the assessment of varus/valgus stability in stress X-rays after rTKA.

Materials and methods

This study was approved by the ethical review board of Niigata University (IRB number: 2020-0448). This prospective observational study included a consecutive series of rTKAs performed on subjects over 60 years old with advanced varus knee osteoarthritis (OA) classified as grades 3–4 according to the Kellgren–Lawrence (K–L) classification.²⁰ Out of 55 knees (50 patients) treated between January 2021 and March 2023, 48 knees (45 patients) were enrolled, excluding cases with incomplete data. Of the 48 knees, 29 were left-sided, and 19 were right-sided. The cohort included three males (three knees) and 42 females (45 knees), with mean ages of 68 ± 5.2 years and 74 ± 5.1 years, respectively (Table 1).

Table 1.

The demographic data.

Total	48	Knees
Male/Female	3/45
Left/Right	29/19
	Mean ± SD	95%CI
Age, years	74 ± 6	73–76
Body height, cm	153.2 ± 6.0	151.5–155.0
Body weight, kg	59.8 ± 9.2	57.1–62.5
Body mass index (BMI), kg/m²	25.8 ± 3.6	25.0–27.0
Pre operative FTA, °	183.8 ± 7.6	181.6–186.0
Post operative FTA, °	176.6 ± 3.4	175.6–177.6
Pre operative KOOS
Pain, %	60.6 ± 24.3	53.6–67.7
Symptoms, %	68.6 ± 21.6	62.4–74.9
ADL, %	69.2 ± 25.4	61.9–76.6
Sports and recreation, %	33.9 ± 25.7	26.4–41.3
QOL, %	37.9 ± 23.7	31.0–44.8
Stiffness, %	59.9 ± 27.0	52.0–67.7

FTA = femorotibial angle; KOOS = knee injury and osteoarthritis outcome score; ADL = activities of daily living; QOL = quality of life.

All procedures utilized the Navio^® and CORI^® surgical systems (Smith & Nephew, Memphis, TN, USA), which are handheld, imageless, and semi-active robotic systems.²¹ The CORI system, compared to the NAVIO system, offers improved workflow efficiency due to its higher-speed camera technology. Both systems provide image-free mapping of bone geometry, intraoperative planning, gap assessment, and confirmation of alignment and knee balance. All cases in this study used the bi-cruciate substituting (BCS)-TKA (Journey II BCS^®; Smith & Nephew, Memphis, TN, USA).

Preoperative planning was conducted using 3D preoperative planning software (JIGEN^®; LEXI, Inc, Tokyo, Japan) to determine the size and default positioning of the femoral and tibial components based on the anatomical coordinate systems. The preoperative plan included component size selection and condylar twist angle between the posterior condyle axis (PCA) and surgical epicondylar axis (SEA).¹⁶ The default femoral component position was set to 0° relative to the mechanical axis (MA) in the coronal plane, 3° flexion to the MA in the sagittal plane, and 0° to the SEA in rotational alignment. The default tibial component position was set to 0° relative to the MA in the coronal plane, 3° posterior inclination to the MA in the sagittal plane, and 0° to the Akagi line.²²

Intraoperative adjustments were made to the component positions and total lower limb alignment, taking into account soft tissue balance. Fine-tuning was performed only on the femur, with the following ranges of adjustment: 0 to 3° varus alignment to the MA (coronal plane), 0 to 6° flexion alignment to the MA (sagittal plane), and 0 ± 3° external rotation to the SEA (rotational alignment). The tibial component positions were fixed at 0° to the MA in the coronal plane and 3° to the MA in the sagittal plane. Tibial rotational alignment was determined at 0° to the Akagi line, based on preoperative planning or range of motion techniques. The final lower limb alignment was set within the range of 0 to 3° varus alignment to the MA in the coronal plane. Intraoperative tuning was performed after resection of the anterior cruciate ligament, posterior cruciate ligament, and all osteophytes, except for the posterior femoral condyles where feasible. The actual intraoperative procedures included a medial parapatellar approach, creation of intraoperative 3D images, determination of bony reference points, temporary setting of default component positions, soft tissue balance assessment through manual maximum stress applied by surgeons, fine-tuning of component positions based on soft tissue balance, and bone cutting using a handheld end-cutting burr. Finally, the components were set using cement.

Postoperative evaluation involved applying varus/valgus stress X-rays in knee extension under anesthesia at maximum manual stress following surgery. First, 3D models of the femur and tibia were reconstructed from CT data using 3D visualization and modeling software (ZedView^®; LEXI Inc., Tokyo, Japan). The postoperative component positions relative to bone were calculated from postoperative CT using a 3D-3D image matching technique (JIGEN^®; LEXI, Inc, Tokyo, Japan). The relative 3D positions between femoral and tibial components were determined using the 3D-2D image matching technique (Zed Motion^®; LEXI, Inc, Tokyo, Japan) with the 2D stress X-rays and the 3D complex in component and bone models obtained from the 3D-3D image matching technique (Figure 1). The anatomical and component coordinate systems were established according to previous studies.^8,9 The spatial relationship between the anatomic coordinate systems and the component coordinate systems automatically calculated the relative positions of the femur and tibia (Figure 2).

Figure 1.

Image matching in the 3D complex and stress X-rays with a 3D-2D image matching technique.

Figure 2.

Analysis of femoral component position relative to the tibial component in the coordinate system of the tibial component.

As an evaluation parameter, the components’ rotation angle (CR angle) was defined as the angle between the x-axis of the femoral component coordinate system and the x-axis of the tibial component coordinate system (Figure 3). CR angles were measured under non-stress (non-stress CR angle), valgus stress (valgus CR angle), and varus stress (varus CR angle) conditions. The varus/valgus angle between components (VV angle) was defined as the angle between the line connecting the medial and lateral most distal points of the femoral component and the x-axis of the tibial component coordinate system (Figure 4). The femoral component's distal surface inherently includes a 3° valgus angle; thus, the actual varus/valgus angle between components is the VV angle minus 3°. However, for simplicity, this 3° angle is not subtracted in this study, and the measured varus/valgus angles are presented as the VV angle under non-stress (non-stress VV angle), valgus stress (valgus VV angle), and varus stress (varus VV angle) conditions. The joint gap between the medial and lateral most distal points of the femoral component and the upper surface of the insert, which represents the thickest part of the medial and lateral insert, was defined as the medial joint opening (MJO) and lateral joint opening (LJO), respectively (Figure 5). MJO and LJO were measured under valgus and varus stress conditions, respectively.

Figure 3.

The component rotation angle: CR angle.

Figure 4.

Varus/valgus angle: VV angle.

Figure 5.

The separation distance between femoral and tibial components: the medial joint opening (MJO) and lateral joint opening (LJO).

Statistical analysis

The Shapiro-Wilk test was used to assess the normality of all data. Paired t-tests were conducted to analyze differences in CR angles under non-stress, valgus stress, and varus stress conditions. Correlations were evaluated using Pearson's product-moment correlation coefficient for normally distributed data, or Spearman's rank correlation coefficient for non-normally distributed data. Statistical significance was set at p < 0.05. All statistical analyses were performed using SPSS software (version 27; SPSS Inc., Chicago, IL, USA).

Results

The demographic data for this study are presented in Table 1. The preoperative and postoperative femorotibial angles (FTA) were 183.8° and 176.6°, respectively.

The non-stress CR angle, valgus CR angle, varus CR angle, non-stress VV angle, valgus VV angle, varus VV angle, MJO, and LJO were 9.9 ± 5.5°, 10.1 ± 6.2°, 10.8 ± 5.1°, 3.6 ± 1.1°, 1.1 ± 1.4°, 7.1 ± 1.9°, 0.0 ± 0.4 mm, 1.3 ± 1.0 mm, 0.9 ± 0.9 mm, and 2.9 ± 2.7 mm, respectively (Table 2). The varus CR angle was significantly more externally rotated compared to the non-stress CR angle, whereas the valgus CR angle did not differ significantly from the non-stress CR angle (Table 3). No significant correlations were found between CR angles and any other parameters (Table 4).

Table 2.

Evaluation parameters.

	Mean ± SD	95%CI
CR angle in no stress, °	9.9 ± 5.5	8.3–11.5
CR angle in valgus stress, °	10.1 ± 6.2	8.3–11.9
CR angle in varus stress, °	10.8 ± 5.1	9.3–12.2
VV angle in no stress, °	3.6 ± 1.1	3.3–3.9
VV angle in valgus stress, °	1.1 ± 1.4	0.7–1.5
VV angle in varus stress, °	7.1 ± 1.9	6.5–7.7
MJO in no stress, mm	0.0 ± 0.4	−0.2–0.1
MJO in valgus stress, mm	1.3 ± 1.0	1.1–1.6
LJO in no stress, mm	0.9 ± 0.9	0.7–1.2
LJO in varus stress, mm	2.9 ± 2.7	2.1–3.7

CR angle = components rotation angle; VV angle = varus/valgus angle; MJO = medial joint opening in valgus stress; LJO = lateral joint opening in varus stress.

Table 3.

Statistical significance with paired t-tests.

	CR angle variation	t	p value
No stress to valgus stress	0.2°externaly	0.462	0.646
No stress to varus stress	0.9°externaly	2.343	0.023

CR angle = components rotation angle.

Table 4.

Correlation coefficient to CR angle variation.

	VV angle		MJO or LJO
	r	p value	r	p value
No stress to valgus stress	0.074	n.s.	0.204	n.s.
No stress to varus stress	0.259	n.s.	−0.005	n.s.

CR angle = components rotation angle; VV angle = varus/valgus angle; MJO = medial joint opening; LJO = lateral joint opening.

Discussion

The most important findings of this study were that (1) the varus CR angle was significantly more externally rotated compared to the non-stress CR angle, and (2) CR angles were not correlated with VV angles, MJO, or LJO.

Stress X-rays are commonly used to evaluate varus/valgus stability after TKA; however, applying only varus/valgus stress with precision using manual force is challenging. Consequently, varus/valgus stress might influence not only varus/valgus alignment but also the rotational relationship between the femur and tibia. This study is the first to report on the effect of varus/valgus stress on tibiofemoral rotation after TKA.

The current study demonstrated that the varus CR angle was significantly more externally rotated compared to the non-stress CR angle. One possible explanation for this result is the direct internal rotational force applied to the tibia by the hand holding the lower limb, which may cause relative external rotation of the femur during varus stress. When the affected side is on the left, the examiner's right hand holds the lower leg of the affected side while applying varus stress. Therefore, the tibia's tendency to rotate internally could be attributed to the pronation of the examiner's forearm. According to this assumption, when the affected side is on the left, the tibia should rotate externally due to the pronation of the examiner's left hand while applying valgus stress, leading to internal rotation of the femur relative to the tibia. However, this result was not observed. Thus, the possibility that a direct internal rotational force applied to the tibia by the hand holding the lower limb causes external rotation of the femur relative to the tibia during varus stress was ruled out. Excluding direct rotational force, this result may be attributed to the external rotation of the femur relative to the tibia during knee flexion, known as “medial pivot motion”.^23–28 Medial pivot motion is a movement observed in the natural knee, and Inui et al. reported similar movement after TKA with BCS.²⁹ Although varus/valgus stress was applied in full knee extension, the knee tends to flex slightly during varus/valgus stress, which may cause the femur to rotate externally rather than internally due to medial pivot motion. However, there was no significant difference between the non-stress CR angle and the valgus CR angle, in contrast to the varus CR angle. Medial pivot motion suggests that the rotational axis of the knee is placed on the medial side, making external rotation more likely to occur with the effect of a medial fulcrum during varus stress, rather than with a lateral fulcrum during valgus stress (Figure 6). Kinoshita et al. reported that medial stability and moderate lateral laxity could induce external rotation of the femur after TKA.³⁰ Therefore, varus stress may create a soft tissue balance that induces external rotation of the femur.

Figure 6.

The difference of relationship about rotational axis of the knee and fulcrum of valgus and varus stress.

Most reports on varus/valgus stability after TKA, as assessed by stress X-rays, have used a 2D method.^5,6 However, Ueyama et al. reported that the agreement between 2D and 3D measurements in identifying outliers in TKA was poor.³¹ Additionally, evaluating the rotational angle in the axial plane of AP knee X-rays using the 2D method is challenging.⁷ While the rotational angle in the axial plane could have influenced the varus/valgus angle and joint gap in 2D evaluations, this study's results suggest otherwise, and our hypothesis was rejected. Therefore, the validity of 2D evaluation of stress X-rays may be supported.

This study had several limitations. First, the results were obtained by a single researcher with a single evaluation. Second, the sample size of this study is relatively small. Third, the study focused only on measuring varus/valgus stress X-rays immediately after TKA, without a sufficient postoperative duration or long-term clinical outcomes. Further studies are needed to address these issues.

Conclusions

This prospective study demonstrated that (1) the CR angle under varus stress was significantly more externally rotated compared to the CR angle under non-stress conditions, and (2) no significant correlations were found between the rotational angle of each component and the postoperative assessment of varus/valgus stability (VV angle, MJO, or LJO) in rTKA using the 3D-2D image registration technique.

Footnotes

Acknowledgments

The authors would like to thank the entire staff of Niigata Rehabilitation Hospital for their technical support and cooperation.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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