In knee osteoarthritis, what is the knee deformity threshold after which the ankle realigns beyond the neutral zone?

Table 1 Basic demographics of the included patients and their knee and ankle alignment distribution

Variable			Value
Number of patients			523
Number of limbs			845
Gender		Female	418 (79.9)
		Male	105 (20.1)
Age (year)			56.8 ± 8.1 (32-81)
KL grade		1	133 (15.7)
		2	202 (23.9)
		3	234 (27.7)
		4	276 (32.7)
Alignment	Direction	Knee	Ankle
	Varus	632 (74.8)	459 (54.3)
	Neutral	171 (20.2)	310 (36.7)
	Valgus	42 (5.0)	76 (9.0)

Data were presented as n (%) or mean ± SD (range). KL: Kellgren-Lawrence.

Table 2 The association between ankle alignment and each knee deformity category

Knee alignment	Neutral ankle	Valgus ankle	Varus ankle	P value1	Effect size2
Varus (n = 632)	203 (32.1)	43 (6.8)	386 (61.1)	< 0.001a	0.19
Neutral (n = 171)	89 (52.0)	21 (12.3)	61 (35.7)
Valgus (n = 42)	18 (42.8)	12 (28.6)	12 (28.6)

¹Associations between knee and ankle alignment categories were analyzed using the Pearson χ² test.

²Effect size was quantified using Cramér’s V, interpreted as negligible (< 0.10), small (0.10-0.19), moderate (0.20-0.39), or strong (≥ 0.40).

^aP < 0.05 indicates statistical significance.

Data were presented as n (%).

Table 3 Sex based differences in the demographics and alignment

Variable		Females	Males	P value	Effect size4
Age (year)		56.8 ± 7.9	56.7 ± 8.7	0.8381	NA
HKAA (°)		175.0 (172.0-178.0)	174.0 (172.0-177.0)	0.0182,a	NA
TTA (°)		85.0 (81.0-90.0)	85.0 (81.0-90.0)	0.6922	NA
Knee alignment	Varus	501 (74.2)	131 (77.5)	0.3843	0.05
	Neutral	142 (21.0)	28 (16.6)
	Valgus	32 (4.7)	10 (5.9)
Ankle alignment	Varus	365 (54.1)	94 (55.6)	0.6263	0.03
	Neutral	246 (36.4)	63 (37.3)
	Valgus	64 (9.5)	12 (7.1)

¹Continuous variables were compared using the independent-samples t-test.

²Continuous variables were compared using the Mann-Whitney U test.

³Categorical variables were compared using the Pearson χ² test.

⁴Effect size quantified using Cramér’s V, interpreted as negligible (< 0.10), small (0.10-0.19), moderate (0.20-0.39), or strong (≥ 0.40).

^aP < 0.05 indicates statistical significance.

Data were presented as n (%) or mean ± SD or median (interquartile range). HKAA: Hip to knee to ankle angle; TTA: Tibiotalar angle; NA: Not applicable.

Table 4 Knee osteoarthritis severity-based differences in demographics and alignment

KL grade		1	2	3	4	P value	Effect size3
Age (year)		44.39 ± 9.65	52.72 ± 10.03	56.36 ± 8.16	60.19 ± 6.98	< 0.0011,a	NA
HKAA (°)		176.0 (173.0-178.0)	176.0 (173.0-179.0)	175.0 (172.0-178.0)	173.0 (169.0-177.0)	< 0.0011,a	NA
TTA (°)		87.0 (84.0-90.0)	86.0 (83.0-90.0)	85.0 (81.0-89.0)	83.0 (79.0-88.0)	< 0.0011,a	NA
Knee alignment	Varus	100 (11.8)	154 (18.2)	191 (22.6)	239 (28.3)	< 0.0012,a	0.12
	Neutral	31 (3.7)	36 (4.3)	30 (3.6)	22 (2.6)
	Valgus	2 (0.2)	12 (1.4)	13 (1.5)	15 (1.8)
Ankle alignment	Varus	53 (6.3)	91 (10.8)	127 (15)	188 (22.3)	< 0.0012,a	0.16
	Neutral	66 (7.8)	92 (10.9)	88 (10.4)	64 (7.6)
	Valgus	14 (1.7)	19 (2.3)	19 (2.3)	24 (2.8)

¹Continuous variables were compared using the Kruskal-Wallis.

²Categorical variables were compared using the Pearson χ² test.

³Effect size quantified using Cramér’s V, interpreted as negligible (< 0.10), small (0.10-0.19), moderate (0.20-0.39), or strong (≥ 0.40).

^aP < 0.05 indicates statistical significance.

Data were presented as n (%) or mean ± SD or median (interquartile range). KL: Kellgren-Lawrence; HKAA: Hip to knee to ankle angle; TTA: Tibiotalar angle; NA: Not applicable.

Table 5 Multivariable regression analyses of ankle alignment

Predictor	β (SE) or OR	95%CI	P value
Multivariable linear regression (outcome: TTA)
Intercept	-0.35 (11.22)1	-22.42 to 21.71	0.97
HKAA (per 1° increase)	0.49 (0.06)1	0.36 to 0.61	< 0.001a
Age (per year)	0.04 (0.03)1	-0.03 to 0.10	0.27
Sex (male vs female)	0.96 (0.69)1	-0.41 to 2.32	0.17
KL grade (per grade)	-0.66 (0.32)1	-1.29 to -0.02	0.04a
Multivariable logistic regression (outcome: Varus ankle malalignment)
Intercept	-	-	-
HKAA (per 1° increase)	0.792	0.71 to 0.87	< 0.001a
KL grade (per grade)	1.122	0.82 to 1.54	0.48
Age (per year)	0.982	0.95 to 1.02	0.34
Sex (male vs female)	0.322	0.17 to 0.62	< 0.001a

¹Data were showed as β (SE).

²Data were showed as OR.

^aP < 0.05 indicates statistical significance.

Linear regression coefficients (β) represent the mean change in tibiotalar angle (TTA; °) per unit increase in the predictor. OR represent the change in odds of varus ankle malalignment (TTA < 85.9°). All models were adjusted for age, sex, and Kellgren-Lawrence grade. KL: Kellgren-Lawrence; HKAA: Hip to knee to ankle angle; TTA: Tibiotalar angle.

Table 6 Knee deformity thresholds for ankle malalignment probability (°)

Direction	Outcome	50% probability if HKAA	70% probability if HKAA	80% probability if HKAA
Varus knee leading to varus ankle alignment	TTA < 85.9	≤ 175.7	≤ 170.8	≤ 167.6
Valgus knee leading to valgus ankle alignment	TTA > 91.9	≥ 189.2	≥ 192.5	≥ 194.6

Knee deformity thresholds were derived from direction-specific logistic regression models. Varus knees were modeled for varus ankle malalignment [tibiotalar angle (TTA) < 85.9°], and valgus knees were modeled for valgus ankle malalignment (TTA > 91.9°). Thresholds correspond to hip to knee to ankle angle values associated with 50%, 70%, and 80% predicted probabilities of ankle malalignment. HKAA: Hip to knee to ankle angle; TTA: Tibiotalar angle.