The ability to achieve a 90° squat appears to have important functional implications for patients rehabilitating from total knee arthroplasty. However, detecting impairment may require more rigorous methods than conventional sit-to-stand testing.
By Mark D. Rossi, PhD, PT, CSCS, and Denis Brunt, EdD, PT
By 2030 the number of knee replacements performed each year in the US is expected to grow to almost 3.5 million.1 Even though total knee replacement (TKR) has become a viable and successful intervention when conservative treatments have failed, some patients report being dissatisfied with surgery.2 Additionally, some TKR patients have difficulty with activities such as entering a car, descending stairs, bending to the floor, and getting on a toilet or chair.3-5 These activities require that an individual flex the knee and the hip in a controlled descent while the foot is in contact with the ground. In other words, a partial squat must be performed in order to complete the motion. People who have had TKR report that squatting is an important task for them to be able to complete after surgery.6 For this reason, and because squatting is a motion required for many functional activities, the ability to squat could be an important task to evaluate after knee replacement.
Why the squat?
After unilateral knee replacement, some individuals present with an interlimb weight-bearing asymmetry when completing functional tasks such as sit-to stand, walking, and squatting.7-11 Performing a squat maneuver successfully requires optimum eccentric control of the quadriceps and hip extensor muscles as well as maintenance of balance over both limbs. Farquhar et al’s study of individuals with unilateral knee replacements found greater interlimb mechanical asymmetry during sit-to-stand when performed in a constrained position (with both tibias perpendicular to the floor) compared with a self-selected position.12 Thus, challenging an individual after TKR may present true asymmetry by placing greater demand on the individual compared with less demanding activities such as level walking and relaxed standing. Since squatting requires optimum strength and balance and is a component motion in many activities, the squat test could be a useful tool with which to challenge individuals after TKR to load their involved limb. Thus, future research should examine the use of a squat test as clinical tool.
In our recent study we noted that, on the first outpatient visit, individuals who had undergone unilateral TKR placed more body weight over the nonoperated limb than the operated limb during relaxed stand and 30° and 60° squat tests.9 We also noted that though loading of the operated limb improved by the time patients had been discharged from outpatient services (approximately two months after surgery), they continued to bear more weight on the nonoperated limb.9
An important feature of the study was that we created an asymmetry index (% body weight of operated limb/% body weight of nonoperated limb) and used a receiver operator characteristic (ROC) curve to determine the probability of identifying individuals who reported at least moderate difficulty with functional tasks based on the Western Ontario and McMaster Universities Arthritis Index (WOMAC) questionnaire. We evaluated this relationship approximately one week after surgery at the first outpatient visit. We found that the areas under the ROC curves for the 30° and 60° squat tests were .81 and .89, respectively. Not surprisingly, the asymmetry index score in relaxed standing was not good (.64) at discriminating between those who had difficulty with functional tasks and those who did not. Clearly, the relaxed stand position does not challenge the individual and is a low-demand activity compared with squatting.
Even though interlimb weight-bearing asymmetry improves after surgery, individuals who have had unilateral TKR continue to rely on the nonoperated side. A recent study found that in individuals who had undergone unilateral TKR, weight-bearing asymmetry during sit-to-stand at three months after surgery remained at preoperative levels.10 Detecting weight-bearing asymmetries during a squat test may help identify some of these potential problem areas during recovery after TKR.
Our goal in another recent pilot study was to determine if individuals awaiting TKR who could squat to 90° (90-sq) perceived their overall well-being and self-reported function as better than those who could not squat to 90°(< 90-sq).13 Further, we set out to determine if individuals who could squat to 90° would have better clinical orthopedic scores, greater quadriceps strength, and better mobility than those who could not squat to 90°. Sixty-nine individuals (49 women; average age, 73 years) who were awaiting primary unilateral TKR secondary to osteoarthritis were tested two weeks before surgery. From a standing position, each subject squatted to 30° of knee flexion, measured on the operated side using a hand-held goniometer. This test was repeated for a 90° squat. Individuals were then categorized in two groups: the 90-sq group included those who could squat to 90°, and the < 90-sq group included those who could not. There was no difference in age or comorbidities (assessed through the Charlson index) between the 90-sq and < 90-sq groups. Our dependent measures included the WOMAC physical function and total scores, the Hospital for Special Surgery Knee Society Score (HSSKS), the Knee Society Function Score (KSFS), and the Timed Up and Go (TUG) test.
The study participants had no difficulty completing the 30° squat. Twenty-two people could squat to 90°. Compared with the 47 individuals in the < 90-sq group, these 22 individuals perceived their physical function as better in terms of the WOMAC physical function score (p = .02) and WOMAC total score (p = .02). Those who were able to squat to 90° also had significantly better clinical scores for the HSSKS (p = .03) and KSFS (p = .03) than those who could not. With regard to performance, individuals in the 90-sq group had significantly faster TUG test mobility times than those in the < 90-sq group (p = .006, Figure 1).
Our results showed that, for those people awaiting primary total knee replacement secondary to osteoarthritis, the ability to squat with high knee flexion was associated with better perceived function scores, clinical knee scores, and performance-based scores compared with those individuals who could not squat with high knee flexion. Although research is needed to examine if the ability to squat to higher flexion angles before TKR translates into better function after surgery, the results of this study may help identify individuals awaiting knee replacement who may not be at optimal function before surgery. It may be that individuals who cannot squat to 90° before surgery could benefit from intervention before surgery, such as prehabilitation,14 or more intensive rehabilitation after TKR.15 Further research is also needed to determine if other angles of squatting (e.g., squatting to 60°) yield similar results.
In a separate study we evaluated 17 women and 16 men who underwent primary total knee replacement secondary to knee joint osteoarthritis.8 Primary unilateral TKR was completed by the same surgeon and the same prosthesis and fixation type (cemented) was used in all cases. All participants were evaluated at the initial outpatient physical therapy visit approximately one week after surgery and at physical therapy discharge, approximately two months later. Percentage of body weight placed over the involved and uninvolved limbs was assessed using a dual-force platform during standing, a 30° squat, and a 60° squat. An asymmetry index (previously described) was calculated to document and describe weight-bearing asymmetry between limbs during each condition. The WOMAC index was used to document self-reported pain and physical function in both groups.
At both time points we found that men placed more body weight over the surgical limb for the 30° squat (p = .004) and 60° squat (p = .021) compared with women. Therefore, at both time points, men had significantly higher asymmetry index scores (indicating more symmetry) than women for both levels of knee flexion (.74 ± .033 vs .59 ± .032 for 30° squat, p = .005; .77 ± .032 vs .65 ± .011 for 60° squat, p = .021). Interestingly, men and women had similar improvements from the initial visit to discharge from physical therapy for all variables. There was no difference between genders for the WOMAC pain (p = .22) and physical function dimensions (p = .23) at initial visit. At discharge, men perceived less pain (WOMAC pain, p = .003) and reported less difficulty in functional tasks (WOMAC physical function, p = .016). Stair negotiation was a highly ranked, painful, and difficult task to complete for both genders at initial visit and discharge.
A separate investigation showed women produced less knee extensor force than men for both involved and uninvolved limbs before knee replacement surgery.16 Moreover, the involved limb generated less knee extensor force (normalized to body mass index) in women than in men.16 Thus, our finding that women bear less weight than men over the involved limb than the uninvolved limb during a 30° and 60° squat may be due in part to gender differences in muscle strength and associated muscle weakness secondary to osteoarthritis.
With regard to the change in outcome measures from initial visit to discharge, Vincent et al reported that men had higher Functional Independence Measure (FIM) scores than women at admission and at discharge.17 The authors also reported equivalent change scores for men and women between initial visit and discharge. These results were similar to our results in that we found no difference in change scores (initial to discharge) between women and men. For both women and men, descending stairs was a difficult task to complete at initial visit and discharge. Women perceive greater pain and difficulty during functional tasks at discharge, yet change scores are similar to their male counterparts. During a free-standing squat, women are less inclined than men to bear weight on the operated limb. Women do not “match” the outcomes of men following knee replacement surgery, even though the magnitude of recovery is the same.
In our three studies, we noted that before and within two months after unilateral TKR, squatting presents as a challenging task. Approximately one week after surgery and at the beginning of outpatient physical therapy, squatting to 30° and 60° can identify those people who report at least moderate functional difficulty. Moreover, individuals awaiting TKR secondary to osteoarthritis who can squat to 90° perform better on performance-based measures, clinical scores, and scores of self-reported function than their counterparts who cannot squat to 90°.
As stated previously, Farquhar et al noted more mechanical asymmetry between limbs during a sit-to-stand from a constrained position than when the patient completed the task in a self-selected position.12 Based on their results and our findings, compared with relaxed stand or sit-to-stand, tasks that are dynamic and require greater strength and balance appear better able to identify individuals who will have interlimb weight-bearing asymmetries and difficulty with activities of daily living such as squatting, bending to the floor, stair negotiation, and getting in and out of a car. Further research is necessary to examine the squat in particular as a challenging task that will identify true problematic asymmetries. Individuals who cannot squat to 90° before surgery may need remediation either before or after surgery to optimize recovery.
When we evaluate gender differences in the ability to perform a squat, men and women improve at the same rate over the course of a two-month rehabilitation program. The gender differences in weight-bearing asymmetry we observed may be due to the fact that women are generally weaker than men. If women and men improve at the same rate on perceived and performance based measures over the course of physical therapy, but women perceive higher levels of pain and physical dysfunction approximately two months after surgery, further research is warranted to examine this disparity and possible implications for gender specific rehabilitation.
Mark D. Rossi, PhD, PT, CSCS, is an associate professor and Denis Brunt, EdD, PT, is professor and chair of the Department of Physical Therapy, College of Nursing and Health Sciences, at Florida International University in Miami.
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