Total Knee Arthroplasty Assessments Should Include Strength and Performance-Based Functional Tests to Complement Range-of-Motion and Patient-Reported Outcome Measures Free

Abstract

Range of motion (ROM) and pain often define successful recovery after total knee arthroplasty (TKA), but these routine clinical outcomes correlate poorly or not at all to functional capacity after TKA. The purpose of this Perspective is to underscore the importance of muscle strength and performance-based functional tests in addition to knee ROM and patient-reported outcome (PRO) measures to evaluate outcomes after TKA. Specifically: (1) muscle strength is the rate-limiting step for recovery of function after TKA; (2) progressive rehabilitation targeting early quadriceps muscle strengthening improves outcomes and does not compromise ROM after TKA; (3) ROM and PROs fail to fully capture functional limitations after TKA; and (4) performance-based functional tests are critical to evaluate function objectively after TKA. This Perspective also addresses studies that question the need for or benefit of physical therapy after TKA because their conclusions focus only on ROM and PRO measures. Future research is needed to determine the optimal timing, delivery, intensity, and content of physical therapy.

Introduction

Patients after total knee arthroplasty (TKA) rarely meet age-matched norms for strength, function, or activity levels.^1–4 Muscle weakness, ubiquitous prior to and further amplified following TKA,^3,5–8 is linked to poor function.^8–11 Individuals with knee osteoarthritis (OA) who have poorer strength and function are more likely to exhibit lower activity levels, which are associated with poor general health,^12–14 including higher rates of metabolic syndrome, obesity, and cardiovascular disease.¹⁵ Individuals after joint replacement are also at substantially higher risk for future OA progression and joint replacement,¹⁶ which may be due in part to muscle weakness, poor function, and the associated altered movement and activity patterns. Objectively assessing outcomes after TKA is critical to determine prognosis and to evaluate the efficacy and effectiveness of treatments including rehabilitation.

Outcome measures include range of motion (ROM), pain, patient-reported outcomes (PROs), strength, and performance-based functional tests. The most common outcome measures used in orthopedic research studies evaluating TKA are ROM, PROs, and/or pain¹⁷; these measures take little time and few resources to assess. Although these measures are important metrics of recovery, they capture only part of the clinical assessment of the individual and thus should not be used in isolation. Knee ROM correlates only weakly to function acutely after TKA; ROM does not correlate to and seldom limits function in Western culture more than 1 month after TKA.^9,18–20 PROs including pain are highly correlated to patient satisfaction^21–24 and reimbursement^25–27 and may be conducted without an in-person follow-up visit (ie, over the phone, by mail, or via email/internet). PROs, however, do not accurately reflect strength or functional limitations following TKA surgery or recovery thereafter.^4,9,18,19

Strength and performance-based functional tests, the most critical patient outcomes to assess after TKA for prognosis and to evaluate treatment efficacy and effectiveness, are routinely overlooked in orthopedic research¹⁷ and may also be underused in rehabilitation practice and research.²⁸ The Osteoarthritis Research Society International recommends performance-based tests to assess physical function²⁹ in OA and arthroplasty. The most recent American Physical Therapy Association Clinical Practice Guideline for TKA rehabilitation also recommends using performance-based functional tests yet does not specifically recommend quantifying strength.³⁰ Although ROM and pain are the most common metrics, quadriceps strength is actually the rate-limiting step for recovery after TKA. Persistent muscle weakness remains years after TKA and negatively affects function.^5,31

The purpose of this perspective is to underscore the importance of evaluating muscle strength and function (using performance-based functional tests) in addition to knee ROM and perceived function and pain (ie, PROs) to evaluate outcomes after TKA (Figure). Specifically, we explain that (1) muscle strength is the rate-limiting step for recovery of function after TKA; (2) progressive rehabilitation targeting early quadriceps strengthening improves outcomes and does not compromise ROM after TKA; (3) ROM and PROs fail to fully capture functional limitations after TKA; and (4) performance-based functional tests are critical to evaluate function objectively after TKA. We also address studies that question the need for or benefit of physical therapy after TKA and highlight future directions in TKA rehabilitation research.

Role of the Funding Source

The funders played no role in the design, conduct, or reporting of this study.

Muscle Strength Is the Rate-limiting Step for Recovery of Function After TKA

The early postoperative period following TKA often includes a period of substantially reduced activity. Some postoperative protocols promote significant rest and carry strong warnings against too much activity during the first 2 weeks following TKA.³² Yet during this early postoperative period, profound strength loss occurs throughout the lower extremities and especially in the quadriceps. Strength loss approaching or exceeding 50% occurs in both the quadriceps and hamstrings approximately 1 month after TKA.^{6–8,33–35} Other lower extremity muscle groups, such as the ankle plantar flexors and dorsiflexors, are affected less dramatically but still lose nearly 20% strength 1 month after TKA.³⁴ Although recovery of strength occurs over the course of rehabilitation and beyond,^10,33 muscle weakness in the involved limb remains prevalent years after TKA.^5,31,36 For example, 1 year after surgery, Yoshida et al³⁶ found a 13% quadriceps strength deficit in the surgical limb relative to the contralateral limb, and Skoffer et al⁵ found a 15% surgical to contralateral limb deficit in the control group but only a 7% deficit among an experimental group that received progressive strengthening. Silva et al found quadriceps strength deficits of approximately 19% to 31% at 2 years after TKA in the surgical limb of patients after TKA relative to limbs of healthy controls.³¹ Individuals after TKA have bilateral weakness and seldom achieve age-matched norms from healthy controls.^1,3,31,36,37

Muscle weakness, especially in the quadriceps, substantially influences functional recovery after TKA. Mizner et al found that preoperative quadriceps strength predicts functional performance assessed via the stair climb and Timed “Up & Go” Tests (TUG) 1 year after TKA but does not predict PROs; in contrast, neither preoperative knee ROM nor pain predicted any functional outcome measure.⁹ In a study of 105 adults scheduled for TKA, Zeni et al identified that preoperative quadriceps strength predicts stair-climbing ability 2 years after TKA; in contrast, neither knee flexion nor extension ROM predicted stair climbing ability.¹¹ Mizner et al found that quadriceps strength in the involved limb was strongly correlated to function preoperatively and at 1, 2, 3, and 6 months after TKA.⁶ Another study by Zeni et al also found that early postoperative function was the best predictor of function on the same task 1 and 2 years after TKA.¹⁰ In an analysis of 195 patients approximately 1 month after TKA, Suh et al found quadriceps strength in the surgical limb and the nonsurgical limb as well as pain were associated with both gait speed and gait endurance (ie, 6-Minute Walk Test [6MWT]); neither knee flexion nor extension ROM predicted gait speed or gait endurance.³⁸

Quadriceps strength in the contralateral (ie, non-operated) limb also influences function after TKA. Even after accounting for other potential predictors, including functional performance, ROM, and demographics, quadriceps strength in the contralateral limb is a significant predictor of functional performance (TUG and stair climb tests) 1 and 2 years after TKA.¹⁰ Suh et al similarly found that quadriceps strength in the non-operative limb is the most strongly correlated predictor variable for both gait speed and gait endurance.³⁸ These studies^10,38 indicate that strength in the contralateral limb is also critical to function and should not be neglected during and beyond postoperative rehabilitation. Weakness in and/or learned disuse of the surgical limb may also place greater demand and loading on the contralateral knee, leading to rapid deterioration and progression of OA, resulting in subsequent joint replacement.

Given the importance of developing muscular strength prior to and after TKA, patients should be referred to physical therapy to address strength deficits before and after TKA. We recommend testing quadriceps and hamstring strength in both the involved (surgical) and contralateral limbs preoperatively and regularly throughout the postoperative recovery period and beyond. Quadriceps strength and activation may be evaluated early after surgery during the performance of isometric quadriceps contractions (assess for superior glide of the patella) and straight leg raises (assess for the presence of an extension lag). After patients can perform a straight leg raise without a lag, more formal isometric strength testing should begin, typically around 3 weeks postoperatively, although “tolerance criteria” and individual healing timeframes should also be considered. Strength testing should be conducted at least every few weeks or when there is a change in patient status during the rehabilitation period and at follow-up visits after formal rehabilitation. Strength testing should occur using an electromechanical dynamometer ideally but a secured, hand-held dynamometer and 1-repetition maximum on a knee extension machine are viable alternatives.³⁹ After TKA, the surgical (involved) limb strength should at a minimum achieve both the preoperative involved limb strength and concurrent contralateral limb strength values, although the contralateral limb may not be a valid comparison, particularly if it has OA. Lower extremity strengthening, especially quadriceps strengthening, should continue throughout the rehabilitation period and beyond. Orthopedic surgeons and other health care providers should encourage patients to participate in rehabilitation that includes progressive resistance training to increase muscular strength, particularly of the quadriceps. Further research is needed to determine the optimal timing, frequency, content, and dosage of supervised outpatient, postoperative physical therapy, and how rehabilitation may be tailored to an individual patient after TKA.

Progressive Rehabilitation Targeting Early Quadriceps Strengthening Improves Outcomes and Does Not Compromise ROM After TKA

Given the relationship between strength and functional performance after TKA,^6,9–11,38 improving strength prior to surgery and minimizing strength loss early after surgery may be critical to long-term functional outcomes. Although relative rest immediately following surgery is necessary for biological healing to occur, prolonged inactivity leads to rapid declines in a host of biological and physiological factors, including muscle strength,^40,41 body composition,^40–42 bone health,⁴¹ cardiorespiratory fitness,^41,42 and function.⁴⁰ Exercise training counteracts the deleterious effects of inactivity.^41,43,44 Postoperative rehabilitation, including early ambulation, motor training (ie, balance, walking, and movement symmetry), resistance training for muscle strength, and neuromuscular electrical stimulation, is important for optimizing outcomes after TKA.^30,45 A comprehensive, progressive rehabilitation program should include pain and swelling management, ROM and flexibility exercises, targeted and progressive strengthening, functional activities, balance activities, neuromuscular control exercises, cardiovascular exercise, patient education, and a home exercise program (Tab. 1). Physical therapists should carefully monitor “tolerance criteria,” including swelling, ROM, pain, medication use, and changes in functional status throughout rehabilitation, especially during within the first few weeks after surgery and following changes in activity levels (eg, after adding weight-bearing exercises or when the patient returns to work).

Some individuals fear that early progressive rehabilitation including targeted strengthening may compromise ROM because of increased lower extremity swelling. However, these concerns are unfounded provided appropriate rehabilitation criteria (eg, swelling, ROM milestones, pain) are followed.^1,5,33,46,47

Bade et al⁴⁷ evaluated the safety and efficacy of an early high-intensity rehabilitation protocol versus a low-intensity rehabilitation protocol after TKA. Both rehabilitation protocols were initiated at a mean 4 days after surgery and occurred at outpatient rehabilitation facilities. The high-intensity protocol emphasized progressive resistance exercises targeting all major lower extremity muscle groups, and the low-intensity protocol first focused on isometric and active ROM exercises; both groups progressed to weight-bearing exercises and functional activities. One year post-TKA, both groups exceeded their preoperative (baseline) level of performance on several performance-based functional tests, including the stair climb test, 6MWT, and TUG; quadriceps and hamstring strength also improved beyond baseline performance. Active knee flexion ROM at 12 months was, on average, 129 degrees in the high-intensity group and 128 degrees in the low-intensity group, and both groups achieved 2 degrees of knee hyperextension ROM. Although the high-intensity group outperformed the low-intensity group only marginally on a few outcomes during the early postoperative period, participants in both the high- and low-intensity groups achieved outstanding outcome measures that were superior to previous studies despite comparable inclusion/exclusion criteria. The lack of differences between groups may have due to several factors, including (1) the overall volume of rehabilitation (26 sessions for each group), (2) arthrogenic muscle inhibition that limited intensity early after TKA (neither group received neuromuscular electrical stimulation [NMES]), and (3) the progressive training load coupled with outside activities performed by the low-intensity group that were comparable with the overall training intensity performed by the high-intensity study. This study⁴⁷ demonstrates, however, that progressive rehabilitation is safe and effective and does not compromise ROM.

Stevens-Lapsley et al³³ implemented a randomized control trial in which participants received progressive physical therapy including 10-rep maximum strengthening with and without NMES. Both groups achieved favorable knee flexion and extension ROM, suggesting that progressive strengthening with and without NMES does not compromise ROM.³³ Furthermore, progressive strengthening and NMES (greater benefits at higher intensities⁴⁸) resulted in superior quadriceps and hamstring strength, functional performance (6MWT, stair-climbing test, TUG), active knee extension ROM, and global rating scale of perceived function 3.5 weeks after TKA (primary endpoint) compared with rehabilitation alone.³³ Progressive strengthening combined with NMES also resulted in superior quadriceps strength, hamstring strength, functional performance, and PROs 1 year after TKA.³³

Pozzi et al¹ compared strength, ROM, functional performance, and PROs among (1) participants after TKA who underwent progressive strengthening, (2) participants after TKA who received standard of care physical therapy, and (3) age-matched healthy controls. Participants after TKA who received progressive strengthening (compared with standard of care) had fewer physical therapy visits and still had better outcomes in the stair climb test and quadriceps strength; they also tended to perform better on the TUG.¹ Moreover, participants after TKA who received progressive strengthening were more likely than participants in the standard of care group to achieve the lower bound cutoff for active knee extension ROM among healthy controls, and both TKA groups achieved an active knee flexion ROM of approximately 120 degrees.¹

Several other studies promote the benefit of postoperative rehabilitation for individuals after TKA.^30,45,49,50 Given the relationship between peak swelling, quadriceps weakness, and poorer functional performance,⁵¹ rehabilitation specialists should carefully monitor swelling as well as other indicators of too aggressive interventions or activities such as increased pain, medication use, and joint soreness. Supervised rehabilitation protocols including strengthening and functional exercises progressed using clinical milestones may provide the best short- and long-term outcomes after TKA.^30,49,50

Although traditional physical therapy is often prescribed 2 to 3 times per week for 6 to 8 weeks, progressive rehabilitation does not necessarily equal high volume rehabilitation. The progressive strengthening group reported by Pozzi et al received an average of 6 fewer physical therapy visits than the standard of care group.¹ Future work should evaluate the frequency and number of rehabilitation sessions throughout the course of recovery, including several months after TKA when deficits often persist and formal rehabilitation is typically complete. Many new and emerging technologies⁵²—such as telerehabilitation, online applications (“apps”), instrumented insoles for biofeedback, and a “people-like-me” approach (ie, using patient reference charts to identify similar patients as a way to guide prognosis, rehabilitation, and recovery)⁵³—may improve rehabilitation by providing more individually tailored care while decreasing overall treatment volume. Future research is needed to determine which individuals may benefit from different types of early rehabilitation (eg, more or less progressive interventions, more or fewer supervised visits, and alternative pathways for individuals at high risk for complications) and to evaluate rigorously other new and emerging trends, including telemedicine and group therapy.

ROM and PROs Fail to Fully Capture Functional Limitations After TKA

Although achieving knee ROM milestones is an important marker of progress, knee flexion ROM seldom restricts long-term function except in arthrofibrosis, a complication of TKA. An early, tailored, comprehensive rehabilitation program should be the first line of defense against arthrofibrosis in the rare case that individuals are not meeting ROM milestones.²⁰ Only 110 degrees of knee flexion ROM, however, is needed for most activities in which healthy, older adults participate in Western culture,⁵⁴ which is why knee flexion ROM rarely limits functional activities. Notably, there are some activities (eg, deep squat sitting, prayer positions, and yoga) that are more common in other cultures that require significantly more knee flexion ROM. In contrast, a knee flexion contracture of only a few degrees (ie, limited knee extension ROM) may significantly hinder basic functional activities such as walking.⁵⁴ Yet neither knee flexion nor knee extension ROM meaningfully predict performance-based functional outcomes at medium- or long-term follow-up after TKA.^10,11,38,55

As with knee ROM, PROs also do not fully capture functional limitations after TKA. Individuals report marked functional recovery and satisfaction after TKA.^56–58 Patient perception of function, however, does not accurately capture functional deficits after TKA.^4,18,19 Mizner et al¹⁸ found substantial decreases in function from preoperatively to 1 month postoperatively as assessed by the TUG, stair climb, and 6MWT tests; additionally, the involved quadriceps strength decreased nearly 50%, knee flexion ROM decreased approximately 18 degrees, and swelling increased compared with preoperative values.¹⁸ During the same time frame, however, the Global Rating of Perceived Function did not decline and the Knee Outcome Survey – Activities of Daily Living Scale significantly improved.¹⁸ Simply put, individuals perceive no change or even improved function despite profound impairments in strength, ROM, and function 1 month after TKA.^18,19 Perception of function is strongly correlated to pain,^4,18,19 which likely influences PROs and satisfaction after surgery. Among those individuals who are dissatisfied with their outcomes after TKA, persistent pain and poor functional recovery are the 2 most common reasons.⁵⁹ Similarly, objectively measured activity levels 6 months to 1 year after TKA are similar to or only marginally higher than preoperative levels^2,4,60,61 despite significantly improved self-reported physical activity.^2,37 Taken together, these findings highlight the need to collect objective, performance-based functional measures in addition to PROs to more adequately capture and address functional deficits.

Performance-Based Functional Tests Are Critical to Evaluate Function Objectively After TKA

Preoperative and early postoperative (ie, approximately 1 month after TKA) functional performance strongly predict function 6 months to 2 years after TKA. Zeni et al found that early postoperative functional performance on a specific functional task is the strongest predictor of performance on the same task 1 and 2 years after TKA; knee ROM, in contrast, did not predict functional performance.¹⁰ Preoperative performance on a stair climb test and muscle strength predict the ability to ascend and descend stairs without a handrail among individuals 2 years after TKA; active knee flexion ROM did not.¹¹ Similarly, poor preoperative performance on the 6MWT, stair climb, and TUG predict poor performance in the same measures 6 months after TKA; neither knee flexion nor extension ROM predicted functional performance.⁵⁵ A recent systematic review supports the association of preoperative functional performance and quadriceps muscle strength with function 6 months after TKA.⁶² ROM and PROs, in contrast, seldom predict functional performance.^9–11,38,55 Performance-based functional tests are superior to ROM and PROs at predicting function after TKA.

Health care providers should use objective, functional performance-based tests as opposed to ROM and/or PROs alone to evaluate function before and after TKA. Several valid and reliable functional tests⁶³—such as the TUG, 30s sit to stand chair test, and 4-Meter Walk Test—require few resources (eg, chair and/or hallway), little time (ie, <5 minutes), and no special equipment or training. Integrating these performance-based functional tests within the electronic medical record could facilitate collection in routine clinical practice (Tab. 2). Research studies evaluating the efficacy or effectiveness of interventions for individuals with TKA must include strength and performance-based tests in addition to ROM and PROs to accurately capture impairments and functional limitations prevalent after TKA (Figure).

Limitations of Studies Suggesting No Need for or Benefit of Physical Therapy

Studies that use ROM (especially knee flexion ROM) and/or PROs alone cannot capture the spectrum of strength and functional performance deficits that are prevalent early after TKA and often persist for years. Yet, many studies^64–67 suggesting no benefit of supervised rehabilitation rely exclusively on these outcomes, which might compromise the conclusions of these studies. For example, an award-winning paper⁶⁶ concluded non-inferiority of unsupervised home exercise relative to supervised outpatient physical therapy based on knee flexion ROM (primary outcome), PROs (the Knee Injury and Osteoarthritis Outcome Score [KOOS]), time for return to activities of daily living, time until discontinuation of opiate pain medications, and complications. Strength and performance-based functional tests were not assessed.⁶⁶ Similarly, Wang et al⁶⁵ compared 2 weeks of formal outpatient physical therapy versus a home exercise program on ROM, subsequent procedures (eg, manipulation under anesthesia), and 2 PROs (ie, Short Form 12 and Knee Society Scores). The authors concluded no difference between formal rehabilitation and a home exercise program even though no strength or performance-based functional measures were assessed and over one-third of the participants in the home exercise group required subsequent physical therapy following the 2-week home exercise program.⁶⁵ Klement and colleagues⁶⁷ conducted a retrospective chart review of a cohort of patients after TKA who participated in only a web-based “self-directed physical therapy” program for 2 weeks following hospital discharge. The outcome measures included knee ROM and PROs (ie, Short Form 12, KOOS, and KOOS Joint Replacement); strength and functional performance were not assessed.⁶⁷ Given the lack of objective measures of strength and functional performance in these^65–67 and other studies,^64,68 no conclusion regarding the effect of supervised, outpatient physical therapy on strength or functional performance can be made.

The available evidence, however, suggests that supervised physical therapy may be beneficial, especially for those who initially perform at lower levels.⁶⁹ In the current American Physical Therapy Association Clinical Practice Guideline for TKA rehabilitation,³⁰ several of the most strongly recommended interventions (ie, motor function training [eg, balance, movement symmetry], NMES, and high-intensity strength training) are more readily conducted within a supervised program. The Clinical Practice Guideline also recommends that physical therapists teach individuals preoperatively and that postoperative physical therapist management start within 24 hours of surgery.³⁰ Brennan et al found that fewer days to initiate outpatient physical therapy was a strong predictor of better function and lower pain scores at the completion of outpatient rehabilitation.⁷⁰ Falvey et al found that Medicare beneficiaries receiving more visits of home health physical therapy after TKA made greater functional improvement than those who received fewer visits, even after adjusting for potential confounders.⁷¹ Although these studies do not directly answer the question of whether supervised physical therapy is superior to group- or home-based exercise programs, they suggest that direct interaction of patients after TKA with a physical therapist improves outcomes.

Another problem with studies suggesting no benefit of physical therapy is a lack of comparison with favorable outcomes. For example, 2 studies^64,72 that concluded no benefit from outpatient relative to home-based physical therapy reported average knee flexion ROM < 100 degrees in both treatment groups 6 weeks⁷² and 1 year⁶⁴ after TKA; thus, neither treatment regimen in either study^64,72 was effective. In contrast, 12 months after TKA, individuals who received progressive TKA rehabilitation can expect to have knee flexion ROM values of approximately 120 degrees⁷³ to 129 degrees⁴⁷ and full knee extension as well as high strength and functional outcomes.^1,33,47,73 Although the study by Rajan and colleagues⁶⁴ did not have any objective strength or performance-based functional outcomes, Han et al⁷² had 1, the 50-Foot walk time for maximal walking speed. The average maximal walking speed among the participants (mean age 65 years old, ranging from 45 to 75 years) in Han et al⁷² was 1.3 m/s, which is slightly slower than or comparable with the average comfortable walking speed among healthy men and women ages 60 to 69 years⁷⁴ (Tab. 2). The maximal walking speeds among healthy men and women ages 60 to 69 years are 1.93 m/s and 1.77 m/s, respectively.⁷⁴

There are significant limitations in current research evidence evaluating the effectiveness of physical therapy after TKA, particularly relative to alternative approaches such as home-based or group-based programs. Several studies evaluate PROs, ROM, and/or pain, but few evaluate strength and functional performance. Reproducibility and quality of rehabilitation protocols is another concern, because important details regarding interventions, exercise prescription and progression (including intensity), criteria guiding rehabilitation, and even the frequency and duration of physical therapy are often missing. Evaluating outcomes relative to established norms or acceptable thresholds is needed so that individuals do not have to settle for comparable, but poor, outcomes. Future studies should thoroughly describe rehabilitation programs according to established reporting guidelines (ie, the Template for Intervention Description and Replication),⁷⁵ use objective strength and performance-based outcomes measures (in addition to ROM and PROs), and compare outcomes with normative thresholds.

We have highlighted that (1) muscle strength is the rate-limiting step for recovery of function after TKA, (2) progressive rehabilitation targeting early quadriceps strengthening is vital to function and does not compromise range-of-motion after TKA, (3) ROM and PROs fail to fully capture functional limitations after TKA, and (4) performance-based functional tests are critical to fully evaluate function objectively after TKA. Clinicians must therefore assess strength and function using performance-based functional tests in addition to ROM, pain, and patient-perceived function (ie, PROs) to evaluate outcomes comprehensively after TKA. Studies evaluating the efficacy and effectiveness of rehabilitation after TKA must also include objective measures of strength and performance-based function. By evaluating strength and performance-based function in addition to ROM and PROs, we will gain a comprehensive understanding of outcomes after TKA and the role rehabilitation plays in achieving better outcomes.

Author Contributions

Concept/idea/research design: J.J. Capin, M.J. Bade, J.M. Jennings, L. Snyder-Mackler, J.E. Stevens-Lapsley

Writing: J.J. Capin

Fund procurement: J.J. Capin

Consultation (including review of manuscript before submitting): M.J. Bade, J.M. Jennings, L. Snyder-Mackler, J.E. Stevens-Lapsley

Funding

This work was supported in part by an Advanced Geriatrics Fellowship (J.J.C.) from the United States Department of Veterans Affairs Geriatric Research Education and Clinical Center, by the National Institute on Aging of the National Institutes of Health (F32-AG066274), and by the Academy of Orthopaedic Physical Therapy Career Development Award (J.J.C.). The work is solely the responsibility of the authors and does not necessarily represent the official views of the funding sources.

Disclosures

The authors completed the ICMJE Form for Disclosure of Potential Conflicts of Interest. J. Jennings has received royalties from Total Joint Orthopedics; consults for Total Joint Orthopedics, Xenex, and DePuy; receives research support from Porter Adventist Hospital and DePuy; and has stock options from Xenex. L. Snyder-Mackler has served as a consultant to Flexion Therapeutics. M. J. Bade is a member of PTJ's Editorial Board. The other authors (J.J.C., J.S.L.) have nothing to disclose.

References