Participants were 90 female patients who were selected from among 527 patients with hip osteoarthritis who underwent THA at two hospitals in Japan between December 2015 and September 2017 (Fig. 1). Inclusion criteria were patients who (1) had primary unilateral cementless THA via an anterolateral minimally invasive approach; (2) were female; and (3) agreed with the purpose of this study. Since about 90% of our patients are female, we limited the subjects to females in order to minimize effects of gender differences in lifestyle. Exclusion criteria were (1) a history of lower limb or back surgery; (2) previously diagnosed painful orthopedic disease other than hip joint disease; (3) previously diagnosed hip osteoarthritis of the non-surgical side; (4) previously diagnosed mental disease or neuromuscular disease; and (5) postoperative complications such as fracture, dislocation, infection, or nerve paralysis. Four experienced physicians performed all surgeries. Surgery was performed with patients in the side lying position at a 60° angle to the floor. The incision site was limited to between the femoris fascia lamina muscle and anterior fiber of the gluteus medius muscle. No navigation system was used during surgery. For cementless stems, the SL-Plus stem (Smith & Nephew) was used in 74.4% of cases, Short Modular Femoral Hip System (Smith & Nephew) in 12.2%, and Global Tissue Sparing stem (Zimmer Biomet) in 7.8%. For acetabular components, the R3 Acetabular System (Smith & Nephew) was used in 57.8% of cases, Continuum Acetabular System (Zimmer Biomet) in 30.0%, and G7 Acetabular System (Zimmer Biomet) in 7.8%. The liner material was ceramic in 90.0% of cases, and cross-linked polyethylene in 10.0%. The head material was ceramic in 93.3% of cases, and Oxinium (Smith & Nephew) in 6.7%. These components were determined by the attending physician based on the age of the patient and X-ray picture of the hip joint.
This study was a six-month long prospective observational study. The study period was set at six months given the need to achieve sufficient recovery within the standard physical therapy period (i.e., 150 days after surgery), as defined in Japan. The JHEQ and JOA were measured before and six months after surgery. Physical functions were measured six months after surgery, and PA was measured five to six months after surgery. Postoperative rehabilitation was performed according to the clinical pathway at our institution. On the day after surgery, all patients were allowed to bear full weight and underwent inpatient rehabilitation. Rehabilitation consisted of gait exercises, passive ROM exercises, and muscle strengthening exercises. For gait exercises, patients used parallel bars in the beginning, and crutches or a walker from one week after surgery. By the time of discharge, all patients used a T cane during walking exercises. The length of hospital stay (LOS) according to the clinical pathway was three weeks. Most patients underwent outpatient rehabilitation roughly once a week after discharge, and performed activities such as getting up from the floor or ascending and descending stairs according to ability, in addition to inpatient rehabilitation.
We collected participant background information and surgical information, including age, height, body weight, body mass index (BMI), duration of disease (years), LOS (days), intraoperative blood loss (ml), operation time (minutes), and the size of the head from medical charts. The size of the head was included due to a report suggesting that it affects range of motion and ADL . Duration of disease was defined as the period from pain onset to surgery. We measured hip joint ROM (flexion, extension, abduction, abduction, external rotation, and internal rotation), lower extremity muscle strength on both sides (hip flexor, extensor, and abductor, and knee extensors and flexors), and maximum walking speed for physical functions, and PA, JOA score, and JHEQ score. All measurements were conducted by two physical therapists with a full understanding of measurement methods and practical experience.
Lower extremity maximal isometric strength was measured using a hand-held dynamometer (µTas F-1; Anima Corp., Tokyo, Japan). We referred to the measuring method proposed by Fukumoto et al . For the assessment of hip flexors, extensors, and abductors, patients were positioned on a platform in a supine position. For the assessment of knee extensors and flexors, patients were positioned on a platform in a sitting position at a 90° angle. After practice, muscle strength during 5 seconds of isometric contraction was measured twice, and the higher value shown on the hand-held dynamometer was used for analysis. In order to confirm the reliability of these measurement methods, we performed preliminary measurements and calculated the intra-class correlation coefficient (ICC) to assess intra-rater reliability. In all methods, ICC (1, 2) was over 0.95. The length of lever arm (m) was measured from the hip joint to the center of the sensor pad. Then, the torque to body weight ratio (Nm/kg) was calculated. ROM and lower extremity muscle strength were measured on both sides (surgical and non-surgical sides). JOA hip scores were measured according to the method prescribed by the JOA . The ADL category of the JOA hip score was adopted as an indicator of ADL ability. The maximum 10-m walking speed was measured as an indicator of walking ability. If patients requested the use a cane, we permitted cane use. The number of measurements was set to two, and the maximum value (m/s) of the two measurements was used for the analysis of walking speed. For the measurement of PA, we used a digital pedometer with 3-axis acceleration sensors (TH-400; YAMASA, Tokyo, Japan) to measure the number of steps and “fast walking time.” The validity and reliability of another device by the same manufacturer (EX-510; YAMAX, Tokyo, Japan), which uses the same algorithm, has been verified previously . Fast walking time was measured at a cadence of ≥ 120 (steps/minute) with this device. There is a strong correlation between walking cadence and exercise intensity , and faster walking time on this device indicates increased activity intensity. A cadence of ≥ 120 (steps/min) corresponds to moderate or higher activity intensity (≥ 3–4 METs) . Patients were instructed to wear the device on their body for 24 hours a day except when bathing or sleeping, for five to six months after surgery. After the measurement, we calculated the average number of steps (steps/day) and fast walking time (minutes/day) for the five-month period, excluding days when the device was not worn. Furthermore, the activity time per day was calculated from the number of steps (activity time per 1300 steps = 15 minutes for elderly people ), and the moderate intensity activity ratio per day (%) was calculated as the ratio of fast walking time to activity time per day.
JHEQ scores were determined according to the method described in previous studies [5–7]. The JHEQ consists of pain (28 points), movement (28 points), and mental (28 points) subscales, with a higher score indicating a better outcome. Patient dissatisfaction with their current condition and hip joint pain on each side were measured on a visual analog scale (VAS). The VAS for hip joint pain and questions were used to calculate subscale scores. The maximum total score for all subscales combined is 84 points.
The Mann-Whitney U test or t test was used to assess changes in total and subscale scores of the JHEQ from before to six months after surgery. In addition, one-way analysis of variance and a posteriori Bonferroni test were used to assess differences in each subscale score six months after surgery. For analyses, patients were divided into two subgroups (high-score and low-score groups) based on the median movement subscale score. Next, the chi-square test, t-test, or the Mann-Whitney U test was used to compare clinical background factors, surgical information, physical functions, and PA between the two groups. Finally, a stepwise logistic regression analysis was performed to identify factors that determine high and low movement subscale scores, with high and low movement subscale scores as the objective variable, and items with a significant between-group difference as explanatory variables. We used the Hosmer and Lemeshow test as an index of goodness-of-fit. P < 0.05 was considered statistically significant. R (ver. 3.4.1) was used for all analyses .