Valgus cut angle and the factors affecting distal femoral cut in total knee arthroplasty in the Turkish population

Abstract

Background

This study aims to comprehensively examine the distal femoral valgus cut angle (VCA) utilized during total knee arthroplasty (TKA) in the Turkish population and identify the radiological and demographic factors influencing this critical surgical parameter.

Methods

A retrospective analysis was performed on 193 lower extremity orthoradiographs from 120 patients diagnosed with end-stage knee osteoarthritis between January 2020 and December 2023. Key radiological variables assessed included VCA, mechanical femorotibial angle (MTFA), neck-shaft angle (NSA), medial offset (MO), and lateral distal femoral angle (LDFA).

Results

The mean VCA was 6.48 ± 0.83 degrees, with no statistically significant differences observed between genders (p = 0.755). A moderate negative correlation was identified between VCA and NSA (r = − 0.423, p < 0.001), while weak to moderate positive correlations were observed between VCA and both MO (r = 0.337, p < 0.001) and LDFA (r = 0.307, p = 0.002). No significant associations were found between VCA and the other evaluated parameters.

Conclusions

The results highlight the necessity of personalized assessment of VCA during TKA to optimize radiological outcomes, as opposed to a standardized approach. In the Turkish population, VCA is primarily influenced by NSA, MO, and LDFA, with coronal plane deformities showing no significant effect. This study emphasizes the importance of considering patient-specific anatomical variations during preoperative planning.

Introduction

Total knee arthroplasty (TKA) restores the mechanical alignment of the lower extremity and provides functional, effective results in treating end-stage osteoarthritis of the knee. The success of TKA is related to the precise alignment of the components. Malalignment causes implant wear and loosening, resulting in instability and revision surgeries [1–6]. Accurate bone cuts and balancing soft tissues are the key factors to achieve excellent mechanical alignment in TKA [1, 7].

The difference between the mechanical and anatomical axis of the femur, 3° valgus of the femoral, and 3° varus of the tibial joint surfaces form the mechanical femorotibial angle (MTFA) used to determine the coronal alignment of the limb. In conventional TKA, surgeons try to obtain a postoperative MTFA within ± 3°of neutral [4, 7, 8]. However, MTFA is found to vary from 2° to 9°9. Distal femoral valgus cut angle (VCA) is the difference between the anatomical and mechanical axis of the femur. Preoperative measurement and accordingly cutting the distal femur ensure perpendicular placement of components [4, 8–10]. Most surgeons prefer fixed VCA of 4–7° for all patients [8, 11]. Recent studies have recommended using variable VCA to improve coronal alignment and functional results of TKA [1, 4, 12].

Although the current trend is towards robotic navigation surgery, conventional surgery is performed frequently due to cost and accessibility. Besides, implant designs and surgical procedures are mostly based on the Western population. Due to ethnic differences in distal femur morphology, the VCA can differ in the Turkish population [9, 13]. We hypothesized that the valgus cut angle shows inter-individual variation within the Turkish population and that femoral radiographic parameters are the primary determinants of this variability. This study aims to determine the average VCA value and the radiological features of the femur that affect VCA in the Turkish population.

Methods

Patients with end-stage knee osteoarthritis who had undergone radiological evaluation with a standing lower extremity orthoscanogram between January 2020 and December 2023 enrolled in this retrospective study. Patients with congenital anomalies in the lower extremity, previous tibia or femur fracture, previous lower extremity surgeries, and radiographs taken in lower extremity rotation were excluded from the study. The Institutional Review Board of Gulhane Education and Research Hospital approved this study on 28.06.2024 with approval number 2024 − 353, and informed consent was obtained from all participants. The ethical standards in the 1964 Declaration of Helsinki were followed.

The patient’s age, sex, height, and weight were obtained from the records, and the body mass index (BMI) was calculated. Full-length lower extremity radiographs (orthoscanograms) were obtained using a standardized positioning protocol. Patients stood upright with both feet in a neutral rotational position (0°) and the patella directed anteriorly, while the X-ray tube was placed approximately 150 cm from the detector. The images had to provide sufficient clarity to identify the anatomical reference points required for precise alignment measurements. Short-leg knee radiographs were intentionally not used, as limiting visualization to the knee alone may result in erroneous assessment of both the mechanical femorotibial axis and the anatomical axis. Radiological evaluations include VCA, MTFA, femoral neck-shaft angle (NSA), hip medial offset (MO), femoral length, lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), and lateral femoral bowing angle (FBA). VCA is the difference between the anatomical and mechanical axis of the femur. MTFA is the angle between the mechanical axis of the femur and the tibia. FBA was defined as the angle formed by the line connecting the center points of the femur at the level of the lesser trochanter and 5 cm distally and proximally, and the line connecting the center points of the femur at the level of the femoral condyle and 5 cm proximally and distally. (Fig. 1)

Fig. 1.

A. The Neck-shaft angle (NSA) was the angle made by the line connecting the midpoint of the neck cortices and the center of the femoral head, with the mid-diaphyseal line (A-angle). The Valgus cut angle (VCA) is the angle between the anatomical and the mechanical axis. (B-angle). B Femoral length (FL) was measured from the tip of the greater trochanter to the intercondylar notch. C Hip medial offset (MO) is the perpendicular distance of the femoral head center from the mid-diaphyseal line. D Femoral bowing angle (FBA) was defined as the angle formed by the line connecting the center points of the femur at the level of the lesser trochanter and 5 cm distal proximally and the line connecting the center points of the femur at the level of the femoral condyle and 5 cm proximal distally. E The Lateral distal femoral angle (LDFA) was the lateral angle formed between the mechanical axis line of the femur and the knee joint line of the femur. The medial proximal tibial angle (MPTA) was defined as the medial angle formed between the tibial mechanical axis and the knee joint line of the proximal tibia. F Mechanical femorotibial angle (MTFA) is the angle between the mechanical axis of the femur and the tibia

Statistical analysis

Analyses were performed with IBM SPSS version 22.0 (IBM Corporation, Armonk, NY, USA), and statistical significance was set at p < 0.05. Continuous variables were expressed as mean ± standard deviation, and categorical data were expressed as numbers and percentages. Normality analyses of continuous variables were performed with the Kolmogorov-Smirnov Goodness-of-Fit test. The linear relationship between continuous variables was determined using the Spearman Correlation test. Additionally, a post-hoc power analysis indicated that, with 193 limbs, the study had ≥ 80% power (α = 0.05, two-tailed) to detect correlations of at least |r| = 0.20, suggesting adequate power for the observed associations between VCA and NSA, MO, and LDFA.

Results

We measured 193 orthoscanograms from 120 patients (25 male, 95 female) to determine the VCA in the Turkish population and the factors that affect it. The average VCA was 6.48±0.83, and there was no significant difference between genders (p = 0.755). Women had a higher BMI (mostly shorter in height) than men (p = 0.005). Radiological evaluations revealed that women had shorter medial offset (p < 0.001) and less bowing angle (p = 0.013) than men. The average age, MTFA, NSA, LDFA, and MPTA were similar between the genders (Table 1). In patients with bilateral measurements, the only parameter that differed between sides was VCA (p = 0.001).

Table 1.

Distribution of data between males and females. (BMI, body mass index; VCA, valgus cut angle; NSA, Neck-shaft angle; MTFA, mechanical femorotibial angle; LDFA, lateral distal femoral angle; MPTA, medial proximal tibial angle; FBA, femoral bowing angle)

	Age	BMI	VCA	NSA	Femoral length	Hip medial offset	MTFA	LDFA	MPTA	FBA
Male Mean± SD	70.6 ± 10.9	26.5 ± 3.5	6.5 ± 0.5	125.5 ± 6.2	48.7 ± 3.5	6.2 ± 0.8	171.2 ± 7.2	91.1 ± 2.7	84.5 ± 4.5	5.3 ± 3.7
Female Mean± SD	68.7 ± 7.3	28.6 ± 4.2	6.4 ± 0.8	126.2 ± 5.7	43.5 ± 2.8	5.4 ± 0.6	169.4 ± 6.4	91.7 ± 3.7	83.9 ± 4.0	3.9 ± 3.0
p-value	0.543	< 0.001	0.755	0.535	< 0.001	< 0.001	0.131	0.343	0.414	0.013

Bold values indicate statistically significant correlations (p < 0.01)

When we compared the alignment of the extremities, there were 157 varus deformities and 13 valgus deformities. Twenty-three extremities had neutral alignment. No significant differences were found in VCA, NSA, and FBA, while the other radiological parameters differ according to the extremity alignment (p < 0.001).

There was a negative, moderate correlation between VCA and femoral neck-shaft angle (r= − 0.423, p < 0.001). A weak to moderate positive correlation was found between VCA and medial offset (r= − 0.337, p < 0.001). A weak positive correlation was found between VCA and LDFA (r = 0.307, p = 0.002). No significant correlations were detected between VCA and femoral length, MTFA, MPTA, and bowing angle (p > 0.05). (Table 2)

Table 2.

Correlation analysis between VCA and femoral alignment parameters. (VCA, valgus cut angle; NSA, neck-shaft angle; MO, hip medial offset; MTFA, mechanical femorotibial angle; LDFA, lateral distal femoral angle; MPTA, medial proximal tibial angle; FBA, femoral bowing angle)

		VCA
NSA	r	− 0,423**
	p	0,000
Femoral length	r	− 0,07
	p	0,329
MO	r	0,330**
	p	0,000
MTFA	r	− 0,083
	p	0,250
LDFA	r	0,225**
	p	0,002
MPTA	r	0,02
	p	0,787
FBA	r	0,039
	p	0,585

* Correlation is significant at the 0.01 level (2-tailed) spearman correlation test

Bold values indicate statistically significant correlations (p < 0.01)

Discussion

Ensuring lower extremity alignment is one of the most important goals for achieving ideal TKA results and providing long-term survival of the implants. It is recommended that the alignment deviates from neutral by 3 degrees [4, 7–9, 14–16]. Previous studies demonstrated that using variable VCA is related to better coronal alignment of the components in TKA and better outcomes [4, 9, 12, 14, 15]. Since ethnic differences can be seen in distal femur morphology, it is necessary to determine the VCA value for the Turkish population [9, 13]. The average VCA was 6.48±0.83 (range 4.4–9.3) in patients with knee osteoarthritis in the Turkish population. The factors that affect VCA were NSA, LDFA, and MO in this study.

The correlations between NSA and MO can be explained according to a geometrical point of view. MO, the anatomical axis of the femur, and the mechanical axis of the femur form a triangle, and the changes in NSA or MO are expected to affect VCA. Marya et al. demonstrated a negative correlation between VCA and NSA in their recent study in 302 limbs of 160 patients with knee osteoarthritis in the Indian population. Also, they revealed that medial offset positively correlates with the VCA [9]. Previous studies demonstrated the relations between these parameters [17, 18]. The results of this study showed that there is a similar relationship in the Turkish society. Geometrically, the patients’ femoral length and height are expected to correlate with VCA. While Drexler et al. found a correlation between VCA and femur length (or height), many studies claim the opposite [9, 17–19]. In this study, patient height or femur length did not affect the VCA. Related to the geometrical equations, LDFA can considered to correlate with VCA, but Drexler et al. found no correlation between the VCA and LDFA [17, 18]. We found a weak positive correlation between LDFA and VCA attributable to ethnic differences.

One of the factors thought to affect VCA was gender in this study, but no significant differences were found. Similarly, previous studies investigating the effect of gender on VCA could not show there was a difference [17, 20, 21]. Bardakos et al. used three different measurement methods and found gender differences by two methods [18]. All changes in the femoral anatomy can affect the VCA from a geometrical point of view. Females have higher hip anteversion, which causes a decrease in MO and VCA [17, 20, 22, 23]. On the other hand, males have higher femoral length than females, which decreases VCA [17, 20]. These factors may cause similar VCA angles in men and women. In this study, gender differences in femoral length and MO resulted in similar VCA measurements.

The knee osteoarthritis mostly causes a varus deformity in the knee and the alignment of the lower extremity. Alignment of the lower extremity causes changes in almost every radiological parameter, but there is no consensus about the effect of alignment on VCA. In previous studies evaluating the relationship between lower extremity alignment and VCA, different results were obtained in other ethnic groups. Deakin et al. found higher VCA is related to higher varus deformity [4, 24]. Similar to this result, studies in the Chinese population demonstrated the correlation between alignment and VCA [4, 18, 19, 25]. Two different studies from India found opposite results. Mullaji et al. found that coronal plane deformity affected VCA, while a recent study by Marya et al. expressed opposing results [9, 26]. This study mainly included varus-aligned extremities (81%), and there were no significant differences in VCA according to the coronal plane alignment. Marya et al. advocated excluding patients with excessive femoral bowing, which could be the reason for not detecting such a relation. However, in this study, femoral bowing was not an exclusion criterion, and no correlation was found between VCA and FBA [9].

Almost every previous study has reported that VCA has a wide distribution, and individualized measurement of VCA has been recommended [1, 4, 9, 14, 17]. Although most radiological measurements have been investigated for similarities or differences between the right and left sides, because of the variability of the VCA, none of the previous studies have compared VCA side-to-side [27, 28]. Studies evaluating knee morphology in the Turkish population found no difference between the sides [13, 29]. In this study, VCA measurements ranged from 4.4° to 9.3°. Unlike previous studies, we compared VCA between the sides in patients with bilateral measurements and found a statistically significant difference. This result reveals the necessity of performing variable-angle distal femoral cutting rather than fixed-angle cutting in total knee arthroplasty.

This study has some limitations. Retrospective analysis of the orthoscanogram can lead to overlooking rotational errors in patient positioning. We excluded noticeable rotational errors from the study because they could cause inaccurate measurements, but as a result, the sample size was relatively small. Besides, flexion contractures in the knee can affect the measurements in our radiological evaluation. Additionally, computed tomography or computer navigation systems could make more accurate measurements. We chose an orthoscanogram over computed tomography to avoid radiation exposure. Also, computer navigation systems are not widespread and accessible in every hospital. Secondly, this study did not evaluate the clinical implications of variable VCA in TKA. This was a single-center study and may not fully represent the entire Turkish population. However, our center is a high-volume tertiary referral hospital that accepts patients from across Turkey, resulting in a relatively heterogeneous study population. This study lays the groundwork for further research, as it is the first to assess the factors affecting VCA in the Turkish population.

In conclusion, given the wide variability of VCA from side to side, it is appropriate to plan TKA by evaluating each extremity individually. The factors that affect VCA found in this study were NSA, MO, and LDFA in the Turkish population. Coronal deformity in the lower extremity did not affect VCA.

Abbreviations

BMI

Body mass index

FBA

Femoral bowing angle

FL

Femoral length

LDFA

Lateral distal femoral angle

MO

Medial offset (hip medial offset)

MTFA

Mechanical femorotibial angle

MPTA

Medial proximal tibial angle

NSA

Neck–shaft angle

TKA

Total knee arthroplasty

VCA

Valgus cut angle

Author contributions

Conception and design: U.Y., Ö.E., C.Y.; Collection and assembly of data: E.K., E.Y., A.A.; writing: U.Y., Ö.E.; review: E.Y., H.Y.T.; Analysis and interpretation of the data, drafting of the article: H.Y.T., E.K., A.A., supervision: C.Y. All authors read and approved the final manuscript.

Funding

No funding was received to conduct this study.

Data availability

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

This study was approved by the Ethical Committee of Gulhane Education and Research Hospital, Health Sciences University (Date: 28.06.2024; No: 2024/353). All procedures involving human participants were conducted in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. Written informed consent for participation was waived by the ethics committee, as the study was retrospective and all data were anonymized in accordance with national regulations.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.