The institutional review board of the authors’ institution approved this study, and all patients who participated provided written informed consent.
From February 2016 to October 2019, 226 knees underwent TKA using Journey II BCS at the authors’ institution. In this study, the inclusion criteria were the following: (1) knees with varus deformity, (2) the use of the image-free navigation system (Precision N; Stryker Orthopedics, Mahwah, NJ, USA), and (3) availability of complete data (patient characteristics, preoperative variables, and intraoperative measurements). One hundred fifty-eight knees met the inclusion criteria and thus were included in this study. Their characteristics and preoperative variables were shown in Table 1. All the procedures were performed using the same surgical technique by five knee surgeons. A senior surgeon (HI) participated in all procedures as either the chief surgeon or first assistant.
Table 1
Patient characteristics and preoperative variables
Age (years)
|
72.7 ± 8.6
|
Gender (female/male)
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21/137
|
BMI (kg/m2)
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26.7 ± 4.1
|
Preoperative range of motion
Maximum extension (degrees)
Maximum flexion (degrees)
|
−9.5 ± 7.2
118.3 ± 15.4
|
Preoperative HKA angle (degrees)
|
169.4 ± 5.7
|
Data are expressed as mean ± standard deviation |
BMI; body mass index, HKA; hip–knee–ankle |
Surgical procedure
All patients underwent TKA using a paramedian approach, and the patella was not everted. The medial soft tissues were minimally released for bone resection. The balancing techniques focused on medial compartment stability [12, 13]. The distal femur and proximal tibia were osteotomized through the navigation system. Femoral alignment was aimed at a placement of 90° to the mechanical axis in the frontal plane and 4° of flexion in the sagittal plane. For the tibia, alignment was aimed at 90° to the mechanical axis in the frontal plane and 3° of posterior slope in the sagittal plane. The extension and flexion gaps were measured using a ligament tensioner, and the amount of posterior femur resection was adjusted to make the extension and flexion gaps of the medial compartment equal to acquire medial joint stability. Femoral rotation was determined as being parallel to the surgical epicondylar axis, allowing residual lateral ligamentous laxity [12, 13]. Tibial rotational alignment was determined using the range of motion technique in which the knee was put through a full range of flexion and extension, allowing the tibial trial to orient itself to the best position relative to the femoral component and reducing component rotational mismatch [14].
Intraoperative gap measurement
After these procedures, the extension and flexion gaps between the osteotomized surfaces were measured twice by the chief surgeon using the same ligament tensioner with a distraction force of 80 N for each compartment, and the averages were used. The mean (± standard deviation [SD]) joint gaps at extension and flexion were 22.1 ± 1.7 mm and 22.5 ± 1.9 mm, respectively, in the medial compartment and 24.2 ± 2.2 mm and 23.8 ± 2.4 mm, respectively, in the lateral compartment.
After evaluating the soft tissue balance between the osteotomized surfaces, the tensor device was put on the osteotomized surface of the tibia by placing the femoral trial component and reducing the patellofemoral joint. The tensor device consisted of three parts: upper compartment-specific plates, a lower platform plate, and an extra-articular main body [12]. The upper plates had identical shapes to that of the medial and lateral compartments of the polyethylene trial surface of the Journey II BCS system. This device was designed to allow surgeons to measure every millimeter of the joint component gaps of medial and lateral compartments. Using this tensor device, the component gaps of medial and lateral compartments were assessed at maximum extension and 30°, 60°, and 90° flexion with a joint distraction force of 80 N for each compartment. The medial or lateral joint laxities, which were defined as the value of component gap minus the selected thickness of the tibial component, were evaluated.
Intraoperative tibial rotational angle evaluation
The tibial rotational angles after implantation at maximum extension and 30°, 60°, 90°, and maximum flexion were obtained for each patient using the navigation kinematic data during the motion cycles from maximum extension to maximum flexion. Among these rotational kinematic data, the following two parameters were evaluated because previous studies have considered that the amount of tibial internal rotation from midflexion to deep flexion influenced postoperative clinical outcomes [7, 8, 10]: (1) the rotational angular difference between 30° flexion and maximum flexion (RAD 30) and (2) the rotational angular difference between 60° flexion and maximum flexion (RAD 60). The tibial internal rotation relative to the femur was defined as a positive value.
Statistical analyses
A previous study has shown that intraoperative joint laxities in BCS TKA were nearly constant at 2.8 ± 1.6 mm in the medial compartment and 3.3 ± 2.3 mm in the lateral compartment [15]; therefore, the patients enrolled in this study were divided into two groups based on their intraoperative joint laxities in the medial and lateral compartments at maximum extension and 30°, 60°, and 90° flexion, respectively. The groups according to the medial compartment were as follows: Group M-stable (medial joint laxity of ≤ 2 mm) and Group M-loose (medial joint laxity of ≥ 3 mm). The groups according to the lateral compartment were as follows: Group L-stable (lateral joint laxity of ≤ 3 mm) and Group L-loose (lateral joint laxity of ≥ 4 mm). Among the 158 knees in this study, the numbers at maximum extension and 30°, 60°, 90° flexion between Group M-stable and Group M-loose and between Group L-stable and Group L-loose were shown in Tables 2 and 3.
Table 2
The numbers at each angle between Group M-stable and Group M-loose
|
Group M-stable
|
Group M-loose
|
Maximum extension
|
156 knees
|
2 knees
|
30° flexion
|
93 knees
|
65 knees
|
60° flexion
|
88 knees
|
70 knees
|
90° flexion
|
73 knees
|
85 knees
|
Table 3
The numbers at each angle between Group L-stable and Group L-loose
|
Group L-stable
|
Group L-loose
|
Maximum extension
|
131 knees
|
27 knees
|
30° flexion
|
51 knees
|
107 knees
|
60° flexion
|
68 knees
|
90 knees
|
90° flexion
|
62 knees
|
96 knees
|
Statistical analyses were performed using the statistical software EZR (version 1.31; Saitama Medical Center, Jichi Medical University, Saitama, Japan) [16]. The unpaired Student’s two-tailed t-test was used to compare RAD 30 and RAD 60 between the two groups in the medial or lateral compartments, respectively. The estimated sample size was 132 (1 − β = 0.80, α = 0.05) according to the statistical power analysis using G*Power (version 3.1.9.4, Heinrich Heine University, Düsseldorf, Germany) [17] Post hoc power analyses were adequate (> 0.80) except for the comparisons of medial or lateral joint laxity at maximum extension; therefore, the authors investigated the comparisons between RAD 30 or RAD 60 and medial or lateral joint laxities at 30°, 60°, and 90° flexion, respectively. The level of significance was set at p < 0.05. The data were shown as the mean ± SD.