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Reduced muscle strength is suggested as a risk factor for knee osteoarthritis (OA) (1–4). Meniscectomy patients constitute one subgroup at high risk of developing knee OA (5, 6), with approximately one-half of the patients showing radiographic signs of knee OA 10–15 years after undergoing a meniscectomy (6–8). Meniscus tears, referred to as symptomatic degenerative tears, have been suggested to be associated with incipient OA and/or represent early-stage OA in the middle-aged population (9, 10). Therefore, patients undergoing surgery for symptomatic degenerative tears constitute a model to study the state prior to knee OA.
In meniscectomy patients, conflicting results have been reported on the rehabilitation of muscle strength and functional performance in the operated leg compared with the nonoperated leg (11–17). In one study, where a control group was included, a reduction in bilateral quadriceps maximal voluntary contraction (MVC) and voluntary activation was observed in meniscectomy patients (18), but no differences were seen between the operated and nonoperated leg. This finding suggests bilateral muscular deficits and emphasizes the need for a control group if the magnitude and nature of lower extremity muscle strength deficiencies in these patients are to be identified and examined.
Testing maximal isometric muscle strength might not adequately describe all aspects of lower extremity muscle function and changes in functional performance compared with healthy controls. For instance, reduced levels of eccentric thigh muscle strength might reduce the ability to absorb impacts and thereby increase knee joint loading (19). Furthermore, impaired capacity for rapid force exertion (i.e., a right shift in the torque time curve indicating a reduced rate of force development [RFD]) is thought to be important for functional performance (20). Reduced rapid force capacity would decrease the percentage of force produced in the early phase of muscle contraction and thereby reduce the ability to react on sudden perturbations, potentially representing a functional deficiency in postural control and other types of reactive motor tasks. Understanding in detail these muscle strength deficiencies and their relationship to self-reported function in patients at high risk of knee OA would be important in clinical decision making and in developing future knee OA prevention regimens.
The aim of the present study was to identify reductions in various aspects of muscle function for the knee extensor and flexor muscles (i.e., maximal concentric, eccentric, and isometric muscle strength, as well as rapid force capacity) together with deficiencies in objectively measured and self-reported physical function in middle-aged meniscectomy patients compared with population-based controls. It was hypothesized that meniscectomy patients would show a bilateral deficiency in muscle strength and objectively measured function compared with controls.
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- PATIENTS AND METHODS
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A detailed overview of the recruitment flow is shown in Figure 1. Thirty-one patients (10 women) and 31 controls (12 women) were ultimately examined. Due to a slow recruitment process of patients in the beginning of the study, the first 7 controls were included before patients were included, which is the reason for the discrepancy between the number of women, 10 and 12, respectively, in the patient and control groups. Therefore, the study is matched on a group level instead of on a case level.
Characteristics of patients and controls are shown in Table 1. No differences were observed between patients and controls except for physical activity at work, where patients were more active than controls. At the first test session, 5 patients reported knee injuries (which had not been reported previously) in the contralateral knee, 3 patients underwent a meniscectomy, 1 patient had a deficient anterior cruciate ligament (ACL), and 1 patient had an ACL reconstruction and meniscectomy. These patients were not excluded since our a priori hypothesis was a bilateral strength deficit between patients and controls. Furthermore, their results in the strength and functional tests were within 2 SDs of the mean of the nonoperated leg. All other patients had a healthy control leg.
Table 1. Characteristics of subjects*
| ||Patients (n = 31)||Controls (n = 31)|
|Subjects, no. male/female||21/10||19/12|
|Time since surgery, months||20.6 ± 6.1||N/A|
|Age, years||46.0 ± 5.5||45.9 ± 5.8|
|Height, cm||174.5 ± 6.9||175.0 ± 9.1|
|Weight, kg||77.7 ± 14.0||78.5 ± 16.3|
|Body mass index, kg/m2||25.5 ± 3.8||25.5 ± 4.3|
|Fat free mass, kg||57.8 ± 9.4||58.5 ± 12.0|
|VO2max, ml O2 × kg−1||35.8 ± 8.9†||37.0 ± 11.1|
|Hand grip strength, kg||39.1 ± 9.1||38.1 ± 8.8|
|Vigorous and moderate physical activity level, hours/week‡|| || |
| Work||2.3 (0, 16)†||0.0 (0, 1)§|
| Leisure||0.9 (0, 2.5)†||1.3 (0, 3)|
|Self-reported comorbidities¶|| || |
| 1–2 musculoskeletal, no.||13||11|
| 1–2 general, no.||6||10|
Thigh muscle strength and objectively measured functional capacity.
Overall, no differences were observed between the operated and nonoperated legs of the patients or when compared with controls in any of the strength variables (Table 2). No differences were observed in rapid force capacity (i.e., no right shift in the torque time curve and therefore no reduction in RFD) during maximal isometric knee extension (P = 0.18) (Figure 2) and knee flexion (P = 0.71) (data not shown). Furthermore, no significant differences were observed in the one-leg hop test or the maximum number of knee bends in 30 seconds test (Table 2).
Table 2. Isokinetic muscle strength and functional performance tests*
| ||Patients|| || |
|Operated leg (n = 31)||Nonoperated leg (n = 31)||Controls (n = 31; 62 legs)||P|
|Isokinetic muscle strength†|| || || || |
| Knee extension, Nm × kg−1|| || || || |
| Concentric peak torque||2.66 ± 0.08||2.73 ± 0.08||2.55 ± 0.07||0.20|
| Eccentric peak torque||3.37 ± 0.12||3.48 ± 0.12||3.27 ± 0.11||0.29|
| Isometric MVC||2.80 ± 0.10||2.88 ± 0.10||2.70 ± 0.09||0.26|
| Knee flexion, Nm × kg−1|| || || || |
| Concentric peak torque||1.37 ± 0.05||1.39 ± 0.05||1.39 ± 0.04||0.84|
| Eccentric peak torque||1.76 ± 0.07||1.79 ± 0.06||1.87 ± 0.06||0.46|
| Isometric MVC||1.20 ± 0.05||1.26 ± 0.05||1.20 ± 0.04||0.23|
|Functional performance|| || || || |
| Knee bends/30 seconds, no.||25.8 ± 1.8||26.1 ± 1.8||28.6 ± 1.8||0.45|
| One-leg hop, cm||82.4 ± 4.5||84.5 ± 4.5||91.1 ± 4.3||0.27|
Figure 2. Mean torque time curve during knee extension for the operated leg (solid line), nonoperated leg (broken line), and control legs (dotted line). No difference in rapid force capacity using the time points 0–30, 50, 100, and 200 msec among any of the legs (mixed linear model).
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Self-reported function and other outcomes.
Patients reported worse knee function than controls in the KOOS subscales for ADL (P ≤ 0.001) and for Sport/Rec (P ≤ 0.001), as well as worse general physical function scores for the SF-36 subscale (P ≤ 0.001). Additionally, patients reported more knee pain, bodily pain, other knee symptoms, and worse knee-related quality of life scores than controls (Figures 3 and 4).
Figure 3. Knee Injury and Osteoarthritis Outcome Score (KOOS) results for the meniscectomy group (solid circles) and the control group (open circles). Scores are means with 95% confidence intervals presented as an outcome profile of the 5 dimensions of the KOOS scale, where a score of 100 represents no knee problems and a score of 0 represents extreme problems. There was a significant difference in all subscales (P ≤ 0.001) between meniscectomy patients and controls (Mann-Whitney test). ADL = activities of daily living; Sport/Rec = sports and recreation; QOL = quality of life.
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Figure 4. Short Form 36 (SF-36) health survey results for the meniscectomy group (solid circles) and the control group (open circles). Scores are means with 95% confidence intervals presented as an outcome profile, where a score of 100 represents no problems and a score of 0 represents extreme problems. There was a significant difference (Mann-Whitney test) between meniscectomy patients and controls in PF (P ≤ 0.001) and BP (P ≤ 0.01). PF = physical function; RP = role physical; BP = bodily pain; GH = general health; VT = vitality; SF = social function; RE = role emotional; MH = mental health.
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Relationships between self-reported function and objectively measured function and maximal muscle strength.
Weak and nonsignificant correlations were observed between Sport/Rec and the various muscle strength variables, i.e., concentric and eccentric peak torque and isometric MVC (rs = 0.07–0.23, P ≥ 0.22).
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- PATIENTS AND METHODS
- AUTHOR CONTRIBUTIONS
To our knowledge, the present study is the first to examine detailed aspects of muscle strength together with objectively measured and self-reported functional capacity in meniscectomy patients compared with age- and sex-matched controls. Despite that, patients in accordance with previous studies reported impaired function, elevated pain, and more symptoms than controls (17, 36, 37). The hypothesized bilateral reductions in various muscle strength variables and functional tests between patients and controls were not observed. Likewise, no differences were observed between the operated and nonoperated leg in the patients, even though this finding could be masked by the patients with injuries to the contralateral knee.
Previous knee injury and reduced muscle strength are considered risk factors for knee OA (1, 2, 38). This study investigated different aspects of muscle strength and functional capacity in meniscectomy patients who were considered to represent a “pre-OA” state (10, 18). Some previous studies report quadriceps strength deficiencies (11, 13), whereas others observe no difference in muscle function (12, 14, 15) when comparing the operated with the nonoperated leg in meniscectomy patients. In 2 recent studies on strength deficiencies 4 years after meniscectomy in patients similar to those in the present study, impaired knee extensor strength and functional capacity were observed in the operated versus the nonoperated leg (17) and bilaterally compared with controls (18). The present study was conducted to combine the strength of these 2 studies by elucidating detailed aspects of muscle strength and functional performance impairments in meniscectomy patients compared with age- and sex-matched controls, and relating these aspects to self-reported knee function. Unexpectedly, however, the initial hypothesis of bilateral strength deficiencies between patients and controls could not be verified. Furthermore, no differences were detected between the operated and nonoperated legs of the meniscectomy patients in maximal muscle strength and functional capacity tests (Table 2 and Figure 2).
The discrepancies between the muscle strength data obtained in the present study and previous studies (17, 18) might be due to differences in patient selection, selection of controls, and/or methodologic differences. In fact, patient recruitment per se could be suspected to be a major reason for the divergent results on muscle strength reported in meniscectomy patients. The recruitment process is often sparsely reported in previous studies. Patient recruitment from a clinical setting or advertising for patients may introduce selection bias toward patients with more symptoms. In the present study, great care was taken during the recruitment process to ensure that patients were representative of a population who had undergone a meniscectomy for a degenerative tear. Furthermore, the meniscectomy patients in the present study were very similar to controls except for the self-reported knee problems (Table 1 and Figure 3). In the study by Becker et al (18), none of the patients participated in sports activities and no information on participation in sports was given for the controls. Therefore, the reported difference between the patients and controls (18) might be influenced by a higher level of physical activity in the control group. In the current study, patients self-reported more moderate and vigorous physical activity at work than controls, whereas physical activity during leisure time was equal between the patient and control groups (Table 1). However, general muscle strength (grip strength) and general fitness (VO2max) were equal in patients and controls supporting similar physical activity levels (Table 1). In the present study, controls were not excluded if they had minor self-reported comorbidities that were similar to those of the patients, since this was to be expected in a population of middle-aged individuals. No information was given on comorbid conditions in the study by Becker et al (18); therefore, it is not possible to know if different levels of comorbidities between the studies could help explain the inconsistent results.
In the current study, patients were examined ∼21 months after their meniscectomy, whereas in the studies by Becker et al (18) and Ericsson et al (17), patients were examined ∼48 months postmeniscectomy. Therefore, it is possible that the patients in our study showed no reduction in muscle strength, since the potential development of knee OA may not have progressed as dramatically due to the shorter postsurgery time interval. However, the ages of the participants in the present study and the 2 previous studies are similar, with a mean age range of 42.8–46.0 years (17, 18).
Changed muscle activation patterns and increased agonist–antagonist coactivation have been observed in knee OA patients compared with healthy age-matched controls (39–42). Hence, impaired neuromuscular function might precede knee OA and also precede muscle weakness. It seems from the present results that when patients performed isolated unilateral single-joint motor tasks in an isokinetic dynamometer, a test with very few degrees of freedom, no difference in performance was observed compared with controls. However, in functional tests that rely more on sensory input and postural (sensory motor) control, there was a borderline clinically relevant difference of 10% between patients and controls. The study was, however, not statistically powered to detect this difference, since sample size estimation was done on quadriceps MVC. Consequently, future studies should examine whether meniscectomy patients have altered neuromuscular activity when performing functional tasks/daily activities or whether sensory input is impaired. Another implication of these findings is that it might be useful if the training of symptomatic patients at risk of OA included training to increase neuromuscular control instead of only muscle strengthening.
A weak and nonsignificant relationship was observed between Sport/Rec and muscle strength variables, indicating the tests were measuring different constructs. Factors other than maximal isolated leg muscle strength could potentially also affect self-reported knee function. Factors like pain, mental health, general fitness (VO2max), and physical activity level are likely candidates to influence self-reported Sport/Rec function. Furthermore, self-reported knee function has been shown to be related to general subject characteristics like age, sex, and body mass index (BMI) (43, 44). A backward stepwise multiple regression analysis with these variables was conducted to explore other potential determinants of self-reported Sport/Rec function. Meniscectomy, poor mental health, and a higher BMI were found to be associated with worse KOOS Sport/Rec function (multiple regression coefficient r2 = 0.52, P < 0.001). Having had a meniscectomy was the strongest predictor (r2 = 0.35) of a worse Sport/Rec function score in the model. Other factors like fear of pain and re-injury or self-efficacy could also affect self-reported function, but we have not evaluated these aspects.
There are some limitations to the present study. The results cannot be generalized to all meniscectomy patients since we followed strict inclusion/exclusion criteria to include a majority of patients with degenerative meniscal tears. Therefore, the results may only apply to middle-aged patients with degenerative tears and not to younger patients who have experienced traumatic tears. Furthermore, only approximately one-third of the patients who received an invitation participated in the study. We have no information on patients that did not participate in the study and direction of any bias due to this can only be speculative. However, the patients self-reported the same amount of pain, symptoms, and functional limitations as previously seen in other studies with similar patients (17, 37). Controls from the same geographic region as the patients were recruited through the Danish Civil Registration System. Nevertheless, one cannot be sure that the controls are representative of the general population from which they were recruited, even though care was taken during the recruitment process to exclude only controls with knee problems related to the research question and to include controls with minor comorbidities common in the general population to this age group.
Study groups (i.e., patients and controls) were not exactly matched on sex (i.e., 10 and 12 women in the patient and control groups, respectively), which could potentially have influenced the results. However, all variables concerning strength and functional capacity had been adjusted for age and sex. Five patients had an injury to the contralateral knee. These patients were not excluded since our a priori hypothesis was a bilateral strength deficit between patients and controls. Furthermore, their results in the strength and functional tests were within 2 SDs of the mean of the nonoperated leg. However, it cannot be ruled out that these patients could mask differences between the operated and nonoperated leg. Excluding these 5 patients from the analysis resulted in a difference range of 6–8% in isometric quadriceps and hamstrings strength between the patients' operated and nonoperated legs, respectively. Differences less than 10% are often not considered clinically relevant (see supplementary Table 1, available in the online version of this article at http://www3.interscience.wiley.com/journal/77005015/home).
In conclusion, thigh muscle strength is not impaired in middle-aged adults 2 years after resection of a degenerative meniscal tear. Our findings indicate that factors other than muscle strength are responsible for the perceived functional limitations and suggest that training to improve strength alone may not be sufficient to improve self-reported function in patients at high risk of knee OA.