Early clinical outcome and learning curve following unilateral primary total knee arthroplasty after introduction of a novel total knee arthroplasty system

Table 1 Baseline characteristics

	Control group	Introduction group	P value (Control vs Introduction)	Follow-up group	P value (Control vs Follow-up)	Test
Subjects count, n (%)	75 (100)	75 (100)		62 (100)
Age, mean (range)	65.6 (49-84)	61.1 (39-81)	0.009	65.1 (45-86)	0.90	Mann-Whitney U test
Gender, male/female (%)	31/44 (m = 41%, f = 59%)	38/37 (m = 51%, f = 49%)	0.25	24/38 (63.2)	0.76	χ2
BMI, median (range)	29.4 (17.3-45.1)	29.4 (19.4-46.9)	1.0	28.8 (20.3-51.9)	1.0	Mann-Whitney U test
ASA Score, median	2	2	0.23	2	0.80	χ2
ASA Score I, n (%)	11 (14.7)	14 (18.7)		7 (11.3)
ASA Score II, n (%)	46 (61.3%)	49 (65.3%)		38 (61.3)
ASA Score III, n (%)	18 (24%)	10 (13.3%)		17 (27.4)
Pre-operative TFA, mean (95%CI)	1.2 valgus (0.5 varus–2.4 valgus)	0.1 varus (1.2 varus–1.0 valgus)	0.12	0.4 varus (1.7 varus–0.9 valgus)	0.076	Unpaired T-test
Varus Outliers, n (%)	47 (62.7)	55 (73.3)		50 (80.6)
Valgus Outliers, n (%)	5 (6.7)	2 (2.7)		2 (3.3)

For normally distributed data, in accordance with Shapiro Wilk-test, kurtosis, skewness and histogram inspection mean values are presented and p-value calculated using the unpaired T-test. Otherwise Mann-Whitney U test was applied. P values for categorical data is calculated using Pearsons Chi-squared (χ²) test or Fisher’s exact t-test. Fischer’s exact test was applied when chi-squared (χ²) conditions may be violated due to expected cell number count below 5 and/or total cell count below 40. Pre-operative TFA outliers found based on the principles of neutral knee alignment[13]. Pre-operative TFA is measured in degrees. 95%CI: 95% Confidence interval; TFA: Tibio-femoral angle; BMI: Body mass index; ASA: American Society of Anaesthesiologists.

Table 2 Alignment of the total joint arthroplasty including the tibial and femoral components

	Control group	Introduction group	P value (Control vs Introduction)	Follow-up group	P value (Control vs Follow-up)	Test
Tibial AP-alignment, mean (95%CI)	88.5 (88.1-88.9)	88.5 (88.1-88.9)	0.68	88.5 (88.1-88.8)	1.0	Mann-Whitney U test
Outliers, n (%)	11 (14.7)	10 (13.3)	0.81	10 (16.1)	0.81	χ2
Femoral AP-alignment, mean (95%CI)	96.0 (95.5-96.5)	95.4 (95.0-95.8)	0.06	95.6 (95.2-96.0)	0.22	Unpaired T-test
Outliers, n (%)	21 (28)	14 (18.7)	0.18	11 (17.7)	0.16	χ2
Post-operative TFA, mean (95%CI)	4.4 valgus (3.8-5.0)	3.8 valgus (3.3-4.4)	0.16	4.2 valgus (3.6-4.8)	0.62	Unpaired T-test
Outliers, n (%)	25 (33.3)	26 (34.7)	0.86	23 (37.1)	0.65	χ2
Medial Tibial Overhang, n (%)	5 (6.7)	8 (10.7)	0.38	6 (9.7)	0.52	χ2
Lateral Tibial Overhang, n (%)	6 (8.0)	7 (9.3)	0.77	4 (6.5)	0.73	χ2
Tibial Slope, mean (95%CI)	88.8 (88.2-89.4)	85.1 (84.4-85.8)	< 0.0001	84.9 (84.2-85.5)	< 0.0001	Unpaired T-test
Outliers, n (%)	21 (28.0)	13 (17.3)	0.12	12 (19.4)	0.24	χ2
Femoral Flexion (FF), mean (95%CI)	86.2 (85.3-87.0)	88.2 (87.8-88.7)	< 0.0001	88.2 (87.8-88.6)	< 0.0001	Unpaired T-test
Outliers, n (%)	44 (58.7)	28 (37.3)	0.009	16 (25.8)	< 0.0001	χ2

For AP alignment femoral and tibial components, < 90 degrees correspond to varus placement and > 90 degrees correspond to valgus placement. Outliers were found based on intervals for optimal alignment of the components[13]. For normally distributed data, in accordance with Shapiro Wilk-test, kurtosis, skewness and histogram inspection mean values are presented and p-value calculated using the unpaired T-test. Otherwise Mann-Whitney U test was applied. P values for categorical data is calculated using Pearsons Chi-squared (χ²) test. 95%CI: 95 % Confidence interval; TFA: Tibio-femoral angle.

Table 3 Indicators of inter- and early postoperative outcome for patients

	Control group	Introduction group	P value (Control vs Introduction)	Follow-up group	P value (Control vs Follow-up)	Test
Length of operation, mean (95%CI)	50 min (48-52)	64 min (62-67)	< 0.0001	59 min (57-61)	< 0.0001	Unpaired T test
Length of stay, median (range)	2 d (0-5)	1 d (0-3)	0.24	1 d (0-9)	0.032	Mann-Whitney U test
Intra-operative blood loss, median (range)	200 mL (0-500)	250 mL (20-1200)	0.002	200 mL (50-1150)	0.84	Mann-Whitney U test
Intra-operative complications, n (%)	0 (0)	1 (1.3)	1.0	0 (0)	1.0	Fisher’s exact t-test
Readmissions within 90 d, n (%)	9 (12)	12 (16)	0.56	7 (11.3)	0.83	χ2
Deaths within 90 d, n (%)	0 (0)	0 (0)	1.0	0 (0)	1.0	Fisher’s exact t-test

For normally distributed data, in accordance with Shapiro Wilk-test, kurtosis, skewness and histogram inspection mean values are presented and P value calculated using the unpaired T-test. Otherwise Mann-Whitney U test was applied. P values for categorical data is calculated using Pearsons Chi-squared (χ²) test or Fisher’s exact t-test. Fischer’s exact test is applied when chi-squared (χ²) conditions may be violated due to expected cell number count below 5 and/or total cell count below 40. 95%CI: 95% Confidence interval.

Table 4 Utilization frequency of novel system following introduction

	Introduction group	Follow-up group
Orthopedic Surgeon 1, n (%)	25 out of 36 (69.4)	25 out of 35 (71.4)
Orthopedic Surgeon 2, n (%)	25 out of 47 (53.2)	25 out of 46 (54.3)
Orthopedic Surgeon 3, n (%)	25 out of 55 (45.4)	12 out of 16 (75)

Frequencies shown as the fraction of total knee alloplastic surgeries where the newly introduced prosthesis was utilized for the three orthopedic surgeons. Percentages are shown in parenthesis.