2.1. Study design and statement of ethics
This prospective cohort study was approved by the Institutional Research Ethics Board of the authors’ institution. Informed consent, which was publicly announced on the institution’s website, had been obtained from all participants.
2.2. Patient selection
At our hospital, unilateral TKA is performed in a single operation, and bilateral TKA is performed in a separate operation. The target subjects of this study were patients who underwent TKA in one knee between October 2018 and October 2020. Among them, patients were selected with total CDL stage 3 in the evaluation before TKA who consented to participate in the evaluations both before and three months after TKA. In addition, patients who could not walk without support in evaluations before TKA due to musculoskeletal disorders (hip, knee and spinal disorders), neurological disorders, or dementia were excluded from this study.
There were 70 patients with complete data sets of outcome measures: 0 patients in total CDL stage 1, 11 patients in stage 2, and 59 patients in stage 3. Therefore, the subjects for the analysis of this study were 59 patients with total CDL stage 3 (Table 1). There were 16 subjects who received bilateral TKA after receiving unilateral TKA during this period. These subjects had knee joint diseases including osteoarthritis, osteonecrosis, trauma, and rheumatoid arthritis.
The typical length of hospital stay for TKA was 18 days. The number of rehabilitation days was 15 days, excluding days of hospitalization, surgery, and discharge. Post-TKA rehabilitation started the day after TKA when all subjects were allowed to support their full body weight. All subjects underwent rehabilitation for three months after TKA with the aims of reducing pain, increasing range of motion, and improving activities of daily life by strengthening the muscles and motor functions.
2.3. Outcome measures
2.3.1. Primary outcome measures
In this study, LS improvement was evaluated by using the LS risk tests introduced by JOA: the stand-up test, the two-step test, and the GLFS-25 [11]. LS was conducted before and three months after TKA. The results of the CDL for each test and the total CDL were classified as stage 0, 1, 2, or 3.
- The stand-up test is to assess leg strength. The stools of four different heights 40, 30, 20, and 10 cm in accordance with JOA guidelines were prepared. The subjects were tested by standing up from a sitting position, first with both legs and then with one leg, at each height starting from 40 cm. This test is to quantify leg strength [9]. A nine performance scoring system was adopted [11]: 0 (inability to stand); 1, 2, 3, or 4 (stand using both legs from a height of 40, 30, 20, and 10 cm, respectively); and 5, 6, 7, and 8 (stand using one leg from a height of 40, 30, 20, and 10 cm, respectively). Scores < 2, < 3, and < 5 were classified as CDL stages 3, 2, and 1, respectively.
- The two-step test is for measuring stride length. Furthermore, the results of this test can also be used to assess walking ability, including leg strength, balance, and flexibility of the patients. The subjects were tested by taking two steps with the longest possible stride, and then the stride lengths for the two steps were measured. The test score was calculated using the total length of the two steps divided by the subject’s height. Scores <0.9, ≥1.1 to <1.3, and ≥0.9 to <1.1 were classified as CDL stages 3, 2, and 1, respectively.
- GLFS-25, a self-assess questionnaire of 25 questions, was used for assessing the physical status and living circumstances. This test measures the physical pain and activities of daily life over the past month prior to the test. Scores ≥24 points, ≥16 to <24 points, and ≥7 to <16 points were classified as CDL stages 3, 2, and 1, respectively.
- The total CDL is determined based on the results of the tests mentioned above, namely the stand-up test, two-step test, and GLFS-25. The stage from each of the tests that showed the mobility function has decreased the most was used to classify the patient’s final total CDL outcome for analysis.
2.3.2. Secondary outcome measures
Electronic equipment was used for measuring quiet standing posture and walking movement: optical motion capture system (Vicon Nexus 2.10, Vicon Motion Systems, London, UK) with 13 infrared cameras (MX T20-S and Vantage 8, Vicon Motion Systems) and six force plates (OR6-5 and BP400600, Advanced Mechanical Technology, Inc., Watertown, MA, USA). The sampling frequency was set at 100 Hz, and all the equipment took the measurements synchronously. The subjects were required to wear skin-tight clothes (Under Armour, Baltimore, MD, USA). Thirty-five, 14-mm-diameter, reflective markers were attached to the subjects according to the Plug-in-Gait model protocol [14,15]. All outcome measures were taken both before and three months after TKA.
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Mean weight-bearing ratio of the quiet standing posture:
Quiet standing posture was measured for five seconds using two force plates. The mean weight-bearing ratio (the mean load on the operated side divided by the mean load on the unoperated side) was calculated from the data for the measured quiet standing posture using analysis software (Vicon Nexus 2.10).
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Temporo-spatial parameters in walking:
The patients were asked to practice walking barefoot freely on a floor force plate. Measurements were taken three times when patients could walk without feeling uncomfortable. The data from the movement which the subjects were most satisfied with were adopted. The analysis software (Vicon Nexus 2.10) was used to calculate the cadence, stride time, step time, single support time, double support time, stride length, step length, and walking speed on the operated side of the knees. In addition, presentation software (Polygon 4.3) was used to standardize one gait cycle as 100%.
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3m-Timed Up & Go test (3m-TUG):
The subjects were asked to stand up from a sitting position on a chair, walk to a marked spot of 3 meters away, return to the chair, and sit down. The time taken for these actions was measured twice and the shorter one was adopted for analysis [16].
2.4. Statistical Analysis
All data were expressed as means ± standard deviation (SD). The distribution of each CDL stage ratio for the three LS tests and the total CDL stage was performed using Wilcoxon’s signed-rank test. The paired t-test was applied to examine the changes in various LS test scores and each motor functional parameter between before TKA and three-month-after TKA groups. The unpaired t-test was applied to examine the differences in age, body mass index (BMI), preoperative various LS test score, each of the preoperative motor functional parameter between the two groups; improvement and non-improvement groups, which were divided based on the improvement of the total CDL stage before TKA to three months after TKA. Stepwise multiple logistic regression and decision tree analyses were performed to determine the most important preoperative factors of improvement in total CDL stage. All statistical analyses were performed with a significance threshold set at p < .05 using IBM SPSS 27.0 (IBM Corp., Released 2020. Armonk, NY, USA).