Individuals are often satisfied with their immediate postoperative pain relief compared with their severe limitations before THA, and most patients report an overall improvement in health-related quality of life during the first year after surgery [17, 34, 39]. However, self-reported quality of life after THA may begin to decline as early as 18 months after THA  and functional mobility deficits persist several years after surgery [3, 29, 43, 53]. Therefore, the full recovery of functional mobility for individuals after THA may be a challenge in rehabilitation. The persistent functional deficits present after THA may have substantial consequences , including increased fall risk . Furthermore, mobility disability (eg, the inability to independently walk and climb stairs)  places a burden on our healthcare system as a result of increased use  and is a risk factor for decreased life expectancy in older adults .
Although several studies regarding THA have been completed [30, 33, 34, 39], few have included performance-based measures, which represent common activities of daily living. Historically, the success of THA has been based on surgical outcomes and prosthesis survival  or has relied on patient self-reports with questionnaires to evaluate functional outcomes after THA [6, 17, 53]. Because self-report measures of function do not correlate well to performance measures of physical function  and may overestimate patients’ true functional abilities , a more comprehensive evaluative strategy is needed. Moreover, although some recent investigations have examined periodic recovery after THA [8, 13], other investigations have been cross-sectional or have examined only two time points after THA. Therefore, the ability to identify the time course of functional recovery for comparison to self-reported outcomes is limited [9, 41, 53]. Furthermore, few have examined both self-reported and performance-based outcomes as early as 1 month after THA to characterize outcomes during early recovery . Information regarding early strength and function decline after THA can guide decision-making not only for surgeons prescribing rehabilitation early after surgery, but also to therapists to design effective intervention. Finally, no previous study has compared the combination of self-reported and performance-based strength and mobility outcomes with an identically tested cohort of healthy peers.
Considering the limitations of previous investigations, there exists a need to objectively and comprehensively evaluate functional and strength deficits after THA provide insights into modifiable targets for postoperative rehabilitation. We therefore sought to (1) compare measures of postoperative hip and knee strength, functional performance, and quality of life measured over the first year after THA to preoperative levels; and (2) compare the outcomes 1 year post-THA with adults with no hip pathology. We hypothesized that adults undergoing THA will have deficits in postoperative hip and knee strength and demonstrate poorer mobility such that performance on each measure of function would be worse 1 month after THA (primary end point) when compared with preoperative levels. Furthermore, we hypothesized that patients would report worse outcomes on self-reported measures of quality of life and be less physically active 1 month after THA. Additionally, we hypothesized that 1 year after THA, deficits in surgical knee and hip muscle strength, functional performance, and quality of life will persist when compared with healthy peers.
Patients and Methods
This investigation was a prospective, longitudinal study. We enrolled patients undergoing THA between June 2010 and August 2011 and evaluated all outcomes before and 1, 3, 6, and 12 months after THA. We also examined a cross-sectional cohort of healthy older adults for further comparison.
Twenty-six patients undergoing THA, performed through a posterior approach, were recruited from four community hospitals by physician referral or advertisement at preoperative educational sessions. Nineteen healthy adults from the community were recruited by email advertisement. All participants were considered eligible if they were between the ages of 45 and 80 years and had no history of uncontrolled hypertension or diabetes, body mass index > 40 kg/m2, additional orthopaedic pathology, or neurologic disorders that impaired daily function. Healthy adults had no known history of knee/hip osteoarthritis or joint arthroplasty. Each participant provided written, informed consent and the study was approved by the Colorado Multiple Institutional Review Board.
Sample size estimates were based on previous work examining lower extremity muscle strength before and 5 weeks after THA . Estimates from the nonsurgical leg were used to represent healthy control subjects. For calculations, differences in hip abductor strength between surgical and nonsurgical legs 5 weeks after THA were (mean ± SD) 23 ± 9 kg (surgical) and 37 ± 13 kg (nonsurgical). This represented a large effect size (1.25 SD). Presently, there is no evidence to determine the minimum clinically meaningful difference. From these numbers, a sample size of 15 subjects per group would provide 90% power to detect differences between patients with THA and healthy control subjects 1 month after surgery. Therefore, we estimated that we should enroll 20 participants with THA to anticipate a 20% dropout rate through 1 year and at least 15 healthy control subjects. Although our focus and sample size estimates centered on the 1-month time point after THA, we chose to additionally follow the trajectory of recovery over 1 year to better characterize recovery.
There were no differences between the THA group and healthy adult group for age, sex, or body mass index (Table 1). During the 1-year follow up, we had eight subjects without complete testing. Two received contralateral THA before the 1-year follow up, one had a dislocation after 6 months, one an intraoperative fracture, and four were unable to be reached at the 1-year time point.
All patients receiving THA had osteoarthritis. All operative procedures were performed using a posterior approach and cementless THA implants. After surgery, patients were directed by their surgeon to adhere to postsurgical movement precautions, including no hip flexion above 90°, hip internal rotation, or hip adduction. All patients received inpatient physical therapy, including education, activities of daily living, and mobility training during a 2- to 3-day postoperative hospital stay. All patients were then discharged to home and received anywhere from zero to eight home and outpatient physical therapy (PT) visits (mean ± SD, 4.0 ± 3.0 home PT; 2.0 ± 3.0 outpatient PT visits). Anecdotally, communication with therapists, chart reviews, and patients’ reports indicated that treatment focused on mobility training, ROM exercises, stretching, and functional activity. The combination of the limited number of PT sessions as well as the low volume and intensity of exercise suggests PT may not have substantially improved the trajectory of recovery.
Functional Performance Measures
Measures of functional performance included the stair climb, five times sit-to-stand, timed-up-and-go, 6-minute walk tests, and single-limb stance time. The stair climb test determines the time to ascend and descend 12 stairs and assesses performance on a relatively demanding functional task. This test has excellent reliability (intraclass correlation coefficient [ICC] = 0.90) . Participants were instructed to climb a flight of stairs and turn and descend the same flight as quickly and safely as possible. They were allowed to use the handrail but were encouraged to refrain from bearing weight through the handrail. The five times sit-to-stand test is a test of dynamic balance  and measures the time it takes to stand from and sit in a chair five times . This test has high test-retest reliability (0.81)  and has shown to be correlated with other tests of dynamic balance (r = 0.64) . Each participant was seated in a standard chair (height 46 cm) and instructed to transfer to a standing position and return to a sitting position as quickly as possible five times. Participants were instructed not to use the arms of the chair unless they were unable to stand without upper extremity support. The timed-up-and-go test, which assesses walking and dynamic balance, measures the time to rise from a chair, walk 3 m, turn around, and return to sitting without physical assistance . This test is a reliable (ICC = 0.75)  and valid test and provides assessment of fall risk. The 6-minute walk test  assesses how far a person can walk in 6 minutes. This test has been used to measure endurance and has been validated as a measure of functional mobility after joint arthroplasty and has excellent reliability (ICC = 0.94) [23, 35]. Each participant performed this test in a 30.5-m hallway and the total distance covered, in meters, was recorded. In the single-limb stance test, a measure of static balance, participants were asked to stand unsupported on their surgical limb. Time, up to 30 seconds, was recorded. The test has excellent reliability in older adults (ICC = 0.86) .
Strength Testing Procedures
Surgical limb hip flexor, extensor, and abductor strength, and knee extensor and flexor strength were measured at each testing session using an electromechanical dynamometer (HUMAC NORM; CSMI Solutions, Stoughton, MA, USA). Positions chosen for testing were based on previous literature and considered patient safety for adherence to postoperative precautions after THA. Maximal voluntary isometric strength of the hip flexors and extensors was performed while participants were supine with the hip flexed to 40° using a strap around the waist to stabilize the pelvis . Strength testing of the hip abductors was performed while participants were sidelying positioned in 0° of abduction/adduction and flexion/extension with a strap to stabilize the pelvis [26, 36]. Maximal strength testing of the knee extensors and flexors was performed while patients were seated and stabilized with a shoulder harness and waist strap in 85° of hip flexion and 60° of knee flexion as previously described [31, 47]. Data were sampled using a BiopacData Acquisition System at a sampling frequency of 2000 Hz (MP 150; Biodex Medical Systems, Inc, Shirley, NY, USA) and analyzed using AcqKnowledge software, Version 3.8.2 (Biodex Medical Systems, Inc). Strength measurements were expressed in units of torque (Nm). Each set of maximal isometric contractions was preceded by two submaximal warm-up contractions. All patients were given visual targets from the dynamometer's output and strong verbal encouragement during each trial. Maximal voluntary isometric contractions for all muscle groups were performed twice; however, if maximal torque during the first two trials differed by more than 5%, a third trial was performed, as previously described [18, 46-48]. The trial with the highest torque was normalized to body mass (kg) and used for analysis .
Patient Perception of Quality of Life/Physical Activity
All participants completed the Medical Outcome Study SF-36 and patients competed the Hip Dysfunction and Osteoarthritis Outcome Score (HOOS) at each visit. The HOOS assesses pain, joint stiffness, physical, social, and emotional function of a person with hip osteoarthritis to determine the overall level of disability. The HOOS is a valid, reliable, and responsive self-administered instrument with ICC values ranging from 0.78 to 0.91 depending on the subscale . The SF-36 is a reliable self-report survey (ICC = 0.75-0.91)  for assessing health-related quality of life [5, 7, 55]. All participants reported their physical activity level using the UCLA Activity Scale . This scale consists of 10 activity levels ranging from wholly inactive (level 1) to regular participation in impact sports (level 10) and has been used to monitor physical activity after total joint arthroplasty .
Study data were collected and managed using REDCap (Research Electronic Data Capture) electronic data capture tools hosted at the University of Colorado, Anschutz Medical Campus . REDCap is a secure, web-based application designed to support data capture for research studies.
To address the first purpose of our investigation, comparing patient outcomes at various time points with preoperative values, we used a mixed-effects repeated-measures model. We designated the 1-month time point as our primary end point for statistical analysis, but we were interested in the full 1-year time course of recovery. Therefore, the mixed-effects model used data from each measure at each time point to infer the differences in these outcomes over the 1-year follow up period, including the 1-month time point. This mixed-effects model approach is similar to performing a repeated-measures analysis of variance with the benefit of retaining case data if missing values were present at any time point. To address the second purpose of our investigation, comparing patient outcomes with measures in healthy control subjects, we used two-group, two-tailed t-tests to assess differences between healthy control subjects and patients at the 1-year time point. To additionally characterize this patient population, we calculated percent changes from the preoperative time point to the 1-month time point (primary end point) and provided estimates of the percent differences between THA outcomes at 1 year and healthy adults.
As was expected, 1 month after THA, patients had 15% less hip flexor torque (p = 0.03), 15% less hip extensor torque (p = 0.08), 26% less abductor torque (p < 0.01), and 14% less knee extensor (p < 0.001) and knee flexor torque (p < 0.01) compared with preoperative levels (Table 2). Additionally, patients with THA performed more poorly on the stair climb test (p < 0.001), timed up and go test (p = 0.02), single-limb stance (p = 0.03), and 6-minute walk test (p = 0.03) than they did before THA (Table 2). However, performance on the five times sit-to-stand was similar preoperatively and 1 month after THA (p = 0.49; Table 2). Despite poorer strength and functional performance 1 month after THA, patients had significantly improved HOOS scores in all domains (p < 0.01) except sports and recreation (p = 0.08; Fig. 1). Furthermore, with patient data available, there was no difference in the Physical Component Score (PCS) of the SF-36 (p = 0.08) one month after THA. Finally, UCLA scores indicated a drop in physical activity 1 month after THA (p < 0.01) compared with before surgery and improvement in physical activity levels by the 1-year time point (p = 0.02; Fig. 1C).
Compared with the healthy adults, patients had 17% less knee extensor (p = 0.01) and 23% less knee flexor torque (p < 0.01; Table 2) after 1 year of recovery after THA. Furthermore, patients were 15% slower on the stair climb test (p = 0.53), 9% slower on the five times sit-to-stand test (p = 0.35), 11% slower on the timed up and go test (p = 0.48), and walked 8% less distance over 6 minutes (p = 0.24) (Figs. 2, 3). Furthermore, SF-36 PCS scores improved from preoperative levels (p < 0.001) but were worse than healthy adults 1 year after THA (p < 0.01; Fig. 1B). Physical activity levels were also lower than healthy adults 1 year after THA (p = 0.14; Fig. 1C).
Patients’ quality of life after THA may decline as early as 18 months after THA  and strength and functional deficits persist several years after THA [3, 29, 43, 53]. Although patients report reduction in pain after surgery, functional deficits that persist (and may worsen with age) suggest postoperative outcomes could be improved. The greatest change in strength and function may occur early after surgery [21, 22]; however, data quantifying acute postoperative changes are lacking. Furthermore, few studies have measured these outcomes at regular intervals during recovery [8, 13, 21]. Because rehabilitation is most likely to be recommended in this timeframe, information on the deficits present early after surgery is required to make informed decisions regarding rehabilitative intervention. Using a comparison group of healthy older adults, this study identified the deficits in strength and outcomes scores that persist 1 year after THA suggesting a possible need for improvements in postoperative care.
We acknowledge the following limitations to our study. First, postoperative rehabilitation was not standardized. We intended to capture the general course of recovery after THA, including typical patterns of rehabilitation use after surgery from several practices. Although this may introduce variability in our results, we believe this approach makes our results more generalizable. Second, our sample size estimates may not be adequately powered to see differences in all outcomes at the 1-year time point as a result of the fact that these calculations were powered to infer differences in strength 1 month after surgery. As a result of variability in recovery, we may not be adequately powered to see differences 1 year after surgery. However, documenting the trajectory of recovery over the year provides important information. Finally, our inclusion criteria may underestimate deficits present in the general THA population. By limiting contralateral disease and comorbidities, we limited our population to include a higher functioning cohort than the broader THA population. However, by excluding confounding conditions, our results were not influenced by compromised function for other reasons.
We found that individuals experienced muscle strength loss, functional performance deficits, and decreases in physical activity 1 month after THA. Interestingly, strength loss in the surgical limb was not isolated to the hip musculature. Although the hip abductors experienced the greatest percent strength loss of all the musculature evaluated, acute strength loss was more global. Previously, Reardon et al.  indicated the presence of quadriceps weakness in this population 5 months after THA. The present study suggests that quadriceps weakness is not only present, but is worse 1 month after THA compared with preoperative values, indicating THA negatively impacts quadriceps strength. Furthermore, Bertocci et al.  and Sicard-Rosenbaum et al.  demonstrated decreased torque in the hip flexors, extensors, and abductors several months to several years after surgery. Although these studies confirm the presence of prolonged hip muscle weakness after THA, our study provides direct evidence of muscle strength losses early after surgery at the time that rehabilitation could have the biggest impact on improving long-term outcomes. The presence of early strength loss supports the need for early rehabilitation intervention to remediate strength losses to optimize recovery beyond levels seen preoperatively. This may require increasing the frequency and intensity of current rehabilitation practices or require more consistent use of rehabilitation after surgery. Similarly, functional performance after THA was diminished 1 month after surgery. Previous investigations have indicated diminished functional capacity using patient self-report [34, 53] and performance tests [38, 44] several months to years after THA. However, no previous performance-based studies have evaluated these functional outcomes as early as 1 month after THA. The presence of long-standing deficits in functional performance in previous studies, combined with our findings of acute functional performance deficits, suggests the current approach to postoperative rehabilitation may not optimize recovery. Despite the strength and functional performance deficits 1 month after surgery, patients reported improvements in their self-reported function. This is likely the result of improvements in hip pain after surgery and further supports a growing body of literature indicating that self-reported measures may not correlate well with patients’ true ability after joint arthroplasty [32, 49, 51].
We also found differences in lower extremity muscle strength and functional performance in patients 1 year after THA compared with their healthy counterparts. In contrast with our initial projections, strength deficits were seen primarily in the knee extensors and flexors of the surgical lower extremity rather than in the hip musculature. A previous investigation by Reardon et al.  demonstrated quadriceps muscle weakness persists several months after surgery despite decreased hip pain, improvement in function, and participation in rehabilitation. Similarly, our study suggests that quadriceps weakness persists beyond the time point previously evaluated, to at least 1 year after THA. This finding is significant because of the role of quadriceps strength in daily function. Quadriceps weakness negatively effects mobility [14, 15, 28], which may help explain why other researchers have found difficulties with functional performance after THA. Despite the fact that significant functional differences were not seen in our data, possibly related to small sample size, several studies have confirmed the presence of functional deficits and difficulty walking after THA. Specifically, Trudelle-Jackson et al.  demonstrated that patients had impaired self-reported function and postural control, whereas Vissers et al.  indicated that by 8 months after THA, patients functionally recover to only 80% of that of healthy adults. Still, other investigators indicate that gait mechanics never fully recover after THA when compared with healthy adults  and mechanics while climbing stairs are also impaired compared with healthy adults . Although our study has characterized strength and functional performance-based outcomes during the first year after THA, we were unable to quantify the quality of movement, which may be crucial to understanding the difficulties present in these previous investigations. Taken together, there is evidence that, although patients do experience recovery and improvement in strength and functional performance after THA, mobility difficulties and functional deficits remain. The present study not only characterizes the time course of recovery during the first year after THA, but quantifies early postoperative deficits after THA. These measures are needed to plan effective rehabilitation programs. During the first few weeks after THA, patients experience hip and knee strength loss and decreased functional capacity, which improve initially, then plateau from 6 months to 1 year. However, some measures of strength remain less than the level of healthy adults, particularly quadriceps and hamstrings strength, suggesting rehabilitation strategies should be further optimized to include focused strengthening of the knee extensors and flexors in addition to those muscles around the hip.
We thank Jessica Shenk PT, DPT, for assisting with data collection and entry and Andrew Kittelson PT, DPT, for assisting with data entry. We also thank our patients for their time and participation.
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