Quadriceps weakness and its relationship to tibiofemoral and patellofemoral knee osteoarthritis in Chinese: The Beijing osteoarthritis study
Weakness has been documented as a feature of tibiofemoral knee osteoarthritis (OA) and may cause disease in this compartment by shock absorption during impulse loading at heel strike, when the patellofemoral joint is not engaged. Our objective was to determine the association of muscle weakness with compartment-specific knee OA, to evaluate sex-specific differences in this relationship, and to determine, by evaluating asymptomatic individuals with OA, whether symptoms may produce the weakness seen in OA.
This cross-sectional study involved 2,472 subjects (1,475 women and 997 men) ages 60 years or older from 4 central districts of Beijing, China. For all subjects, a skyline view of each knee and an anteroposterior (AP) or posteroanterior (PA) radiograph of both knees were obtained during weight bearing. Radiographs were read by one reader for Kellgren/Lawrence (K/L) grade, joint space narrowing (JSN), and osteophytes. We defined a subject as having tibiofemoral OA when the K/L grade was ≥2 on AP/PA view, patellofemoral OA on skyline view when the osteophyte score was ≥2 (or when the JSN score was ≥2 and the osteophyte score was ≥1), and mixed OA when the knee had both patellofemoral and tibiofemoral radiographic OA. Strength was measured isometrically for each leg separately, and knee pain was evaluated by questionnaire.
In women, quadriceps weakness was associated with tibiofemoral OA (odds ratio [OR] 0.7, 95% confidence interval [95% CI] 0.4–1.0), patellofemoral OA (OR 0.6, 95% CI 0.4–0.9), and mixed OA (OR 0.4, 95% CI 0.3–0.6). In men, weakness was associated with mixed OA (OR 0.5, 95% CI 0.3–0.8), and the ORs suggesting an association of patellofemoral OA with weakness were the same as those in women, although in men this trend did not reach statistical significance (P = 0.12). In men, isolated tibiofemoral disease was not associated with weakness; however, the sample size in this analysis was limited. When subjects with knee symptoms were excluded, the relationship of quadriceps weakness to OA was attenuated, with only the relationship between muscle weakness and mixed OA remaining significant.
There is a relationship between quadriceps weakness and knee OA in all compartments, with the strongest association in mixed disease. Pain may contribute to some of this weakness.
Lower extremity muscle weakness may play an important role in knee osteoarthritis (OA). It has been well established in cross-sectional studies that individuals with symptomatic knee OA have weaker quadriceps than do age-matched subjects without knee OA (1–3). It is not clear whether the weakness develops before or after the onset of radiographic disease. The weakness associated with knee OA is largely thought to be the result of disuse atrophy secondary to joint pain. Slemenda et al suggested otherwise; they presented cross-sectional data showing that quadriceps weakness occurs in knee OA when pain is absent, and longitudinal data showing that weakness was present in women prior to the onset of radiographic knee OA (2, 4). Results of other studies suggest that quadriceps weakness increases the risk of disability in persons with knee OA (1, 5). Because muscle strength is modifiable, it is important to understand its relationship to knee OA.
Currently, all studies investigating muscle weakness in knee OA have defined OA by disease in the tibiofemoral compartment. However, the disease is not limited to this compartment and may occur in the patellofemoral compartment, separately or in conjunction with the tibiofemoral compartment. In patients with patellofemoral OA and tibiofemoral OA (mixed OA) in the same knee, no correlation has been found between OA severity in the patellofemoral and tibiofemoral compartments (6). In addition, there is some evidence that the risk factors may be different depending on which compartment is affected (7, 8).
The quadriceps muscle acts differently on the patellofemoral joint than on the tibiofemoral joint. Quadriceps contraction occurs before heel strike and thereby absorbs impulse loads transmitted through the tibiofemoral joint; the patellofemoral joint is not engaged at this point in the gait cycle. The joint comes into play from ∼20° flexion to 90° flexion, during which time the contraction of the quadriceps muscles constitutes a main source of patellofemoral loading; this contraction pulls the patella into close contact with the underlying femoral trochlea (9). These differences suggest the importance of evaluating the relationship between quadriceps weakness and knee OA by compartment.
The multicompartmental nature of knee OA has been overlooked in studies of muscle strength/weakness. Indeed, there are no data on the relationship of muscle weakness to patellofemoral OA. Addressing the relationships of quadriceps weakness with compartment-specific OA is difficult. Most persons with radiographic knee OA have either mixed tibiofemoral/patellofemoral disease or have isolated disease in the tibiofemoral compartment. Isolated patellofemoral disease occurs less frequently (10). Thus, a large-scale study of knee OA would be necessary to evaluate patellofemoral involvement independent of involvement of other compartments. Furthermore, imaging of the patellofemoral joint is challenging, with the optimal radiographic method involving weight-bearing skyline views (11, 12). Until now, skyline views have not been available in large-scale studies of OA. We took advantage of a large-scale population-based study of OA in elderly persons living in Beijing that included both weight-bearing tibiofemoral and patellofemoral radiographs (skyline) to address whether quadriceps weakness was related to OA in either compartment. The population consisted of persons with and persons without radiographic knee OA (either symptomatic or asymptomatic). This presented a unique opportunity to study the effect of muscle weakness on compartment-specific disease, to evaluate sex-specific differences in this relationship, and to determine the difference in this relationship between symptomatic and asymptomatic individuals.
PATIENTS AND METHODS
The Beijing OA Study consisted of men and women ages 60 years or older from 4 central districts of Beijing, China, for whom radiographs were obtained and who completed quadriceps strength testing at Peking Union Medical College Hospital between 1998 and 2000 (13). Our recruitment plan drew from 4 central districts, each of which includes 9–24 health sections. We randomly chose one health section from each of the districts and then randomly selected a neighborhood from that health section. All streets in the selected neighborhoods were listed in random order. Interviewers went door-to-door, beginning with the first street on the list, to enumerate and interview all men and women ages ≥60 years who were self-described residents of Beijing. Recruitment continued until a quota was met for each sex and age stratum (60–69, 70–79, and ≥80 years) based on the 1995 National Intercensus data. Subjects were interviewed in their homes by trained health professional interviewers with a standardized questionnaire on joint symptoms and risk factors for OA, such as squatting. Strength testing was performed and radiographs were obtained on the same day; interviews occurred roughly 2 weeks before this testing.
Height and weight.
Height and weight were measured twice for each subject. Height was measured using a wall-mounted Harpenden stadiometer (Holtain, Crosswell, Wales, UK). If the difference between 2 measurements was ≥4 mm, examiners obtained 2 more measurements. The average of the measurements was then used in the analysis. Weight was assessed using a balance beam scale with precision to 0.1 kg. External standards were used on a regular basis to calibrate the scale and stadiometer.
Bone mineral density (BMD).
Bone mineral density was measured at the femoral neck with either a Lunar DPX-L or a Lunar Prodigy DEXA (dual-energy x-ray absorptiometry) bone densitometer (Lunar, Madison, WI). A cross-calibration study (n = 50 subjects) was done to estimate Prodigy-equivalent BMD values from the DPX-L measurements (r2 = 0.95).
Strength was tested isometrically on each leg separately using a specially designed chair fitted with a spring-loaded strain gauge (14, 15). The height of the chair allowed the subject's legs to hang freely. Subjects sat in the chair with the hip and knee at a 90° angle. The strain gauge was attached at one end by a cable to the back leg of the chair, and the other end was strapped to the subject's shin, 10 cm above the ankle joint (lateral malleolus), using a 2-inch–wide Velcro strap. The angle of pull on the strain gauge was perpendicular to the lower leg. Subjects pushed against the strap with maximal force in extension, and the greatest force was recorded in kilograms. The best of 3 trials was taken as the measurement. Test–retest reliability was assessed in 30 participants who were tested twice on the same day. Intraclass correlations were 0.95 for left peak force and 0.93 for right peak force.
For all subjects, skyline views of each knee were obtained during weight bearing, using the protocol described by Buckland-Wright (16). The radiograph was obtained with the x-ray tube placed in front of the subject's face and the beam angled straight down; the subject's knees were bent at a 30° angle. The bucky was placed underneath the knee, parallel to the floor, resting on a 20.3-cm step stool. For the first 1,800 subjects, AP radiographs of both knees in the fully extended position were obtained using the Framingham OA Study protocol; radiographs of the knees in this view were obtained to compare the prevalence of radiographic OA in this study with that in the Framingham Study. For the last 700 subjects, we switched to a method in which posteroanterior (PA) views of the semiflexed knee were used, as described by Buckland-Wright et al (17). For 70 subjects, both AP and PA views were available, and we found no difference in the prevalence of OA between these views. Therefore, data from subjects for whom either PA or AP views were available were used to assess tibiofemoral OA; when both views were available, the PA view was used.
Radiographs were read by one reader, an academically based bone and joint radiologist (PA). The presence of osteophytes and joint space narrowing (JSN) in the tibiofemoral and patellofemoral joints was graded on a 4-point scale (range 0–3) using an atlas (18). The AP/PA views of each knee were assessed for the overall Kellgren/Lawrence (K/L) grade. The weighted kappa on K/L grade for reliability readings in all batches was 0.83 (95% confidence interval [95% CI] 0.77–0.90) for interreader reliability and 0.79 (95% CI 0.73–0.84) for intrareader reliability. The small number of disagreements did not occur in any particular direction, suggesting that there was no likelihood of bias in the estimates. The weighted kappa on patellofemoral disease definition for intrareader reliability readings was 0.82 (95% CI 0.74–0.90).
Interview of subjects.
A subject was classified as having knee pain based on a positive response to 1 of 2 questions, as follows: 1) “Have you ever had knee pain lasting a month?” If the respondent answered “yes,” questions about the last time the pain occurred were asked, and if the answers revealed that the subject had experienced pain within the past year, he or she was characterized as being positive for knee pain; 2) “Have you had knee pain lasting a month in the past year?” If the respondent answered “yes,” he or she was considered to be positive for knee pain.
A questionnaire about squatting assessed the average number of minutes per day during which the subject squatted at age 25 years. Knee injury was assessed with the following question: “Have you ever injured your knee badly enough to limit your ability to walk for at least a week?” (Squatting and knee injury were assessed because they may be risk factors for disease and may affect muscle strength. Therefore, they were controlled for in the analysis). The reliability of the questionnaire was evaluated by administering it to 40 subjects, each of whom was asked the questions 2 times (once at the home interview and once at the clinic interview). The weighted kappa on knee pain lasting a month in the past year was 0.93 and for current squatting ≥30 minutes/day was 0.80.
Definition of compartment-specific OA.
Because the K/L grade was defined using only the AP/PA view, we adopted the validated approach described by Felson et al to define patellofemoral OA (16). This approach characterizes the patellofemoral compartment as positive for OA on radiographs when the patient has either grade 2 osteophytes (range 0–3) or grade 1 osteophytes and JSN of grade 2 or higher (range 0–3).
Tibiofemoral OA only.
Tibiofemoral OA only was defined by a K/L grade of ≥2 on an AP/PA radiographic view and the presence of both grade <2 osteophytes (or grade <2 JSN and grade <1 osteophytes) on skyline radiographic views (the latter to rule out coexistent patellofemoral disease).
Patellofemoral OA only.
Patellofemoral OA only was defined as the presence of grade ≥2 osteophytes (or grade ≥2 JSN and grade ≥1 osteophytes) on skyline radiographic views and a K/L score of <2 on AP/PA views. Individuals meeting these criteria for patellofemoral OA were further categorized as having either lateral or medial patellofemoral OA. Medial patellofemoral OA was defined as grade ≥2 JSN in the medial patellofemoral joint, and lateral patellofemoral OA was defined as grade ≥2 JSN in the lateral patellofemoral joint.
Mixed OA was defined by a K/L score of ≥2 on AP/PA radiographic views and the presence of grade ≥2 osteophytes (or grade ≥2 JSN and grade ≥1 osteophytes) on skyline radiographs.
We performed a knee-specific analysis in order to investigate separately the different compartments of the knee joint. We found that strength was highly correlated with sex and age; if we used strength/weight as our measure of strength, we found that strength was correlated with weight. Because of these large potential confounders, we first created sex, age (10-year), and weight (10-kg) strata within each stratum, and ranked subjects (by quartiles) according to their quadriceps strength. Because the quadriceps strength measure was now ranked within strata of weight, and because weight was separately correlated with the strength/weight measure, we decided to evaluate strength per se, as opposed to strength/weight. We performed analyses in which the dependent variable was the type of OA, and the independent variables for multivariate analyses were strength (in quartiles as described above), age, body mass index (BMI), history of knee injury, bone mineral density, and squatting (minutes/day) at age 25 years.
Generalized estimating equations (GEE) were used to determine the association of strength by quartile to patellofemoral, tibiofemoral, and mixed OA. GEE permitted us to perform knee-based analyses, adjusting for the correlation between knees in a person. GEE were also used to determine the association of strength by quartile to isolated medial or lateral patellofemoral disease. A subset analysis of those knees without pain was done to eliminate the effect pain may have in causing quadriceps weakness. A formal trend analysis was performed by using GEE after creating dummy variables for each of the quartiles of strength and using this P value as the trend.
A total of 2,472 subjects were studied. Characteristics of the population studied are presented in Table 1. In general, women were more likely than men to report knee pain (31% versus 17.1%) and were also more likely to squat for longer periods of time at age 25 years. Table 2 presents the prevalence of knee OA by compartment. Isolated patellofemoral OA was more common than isolated tibiofemoral OA in men, but in women the prevalence of OA by compartment was similar.
Table 1. Characteristics of the elderly Chinese study subjects*
|Age, mean ± SD years||68.9 ± 6.4||67.9 ± 6.1|
|BMI, mean ± SD kg/m2||25.3 ± 3.4||26.0 ± 4.0|
|BMD, mean ± SD gm/cm2||0.93 ± 0.14||0.81 ± 0.13|
|% with history of knee injury||4.8||5.8|
|% with knee pain||17.1||31.0|
|Squatting time at age 25 years, % of patients|| || |
| 0–29 minutes/day||28.8||15.9|
| 30–59 minutes/day||30.0||17.8|
| 60–119 minutes/day||28.7||37.9|
| ≥120 minutes/day||12.5||28.4|
Table 2. Prevalence of compartment-specific knee OA*
|No OA of the knee||1,473 (74)||1,648 (56)|
|Tibiofemoral OA||98 (5)||304 (10)|
|Patellofemoral OA||171 (9)||230 (8)|
|Mixed OA of the knee||244 (12)||751 (26)|
Table 3 presents the association between compartment-specific knee OA and quadriceps weakness. In terms of compartment-specific OA, quadriceps weakness was associated with OA occurring in the tibiofemoral and patellofemoral compartments in women. Men had less OA compared with women, and the odds ratios (ORs) suggesting an association of patellofemoral OA with weakness were the same in men and women, although in men this trend did not reach statistical significance (P = 0.12). The number of men with isolated tibiofemoral disease was low; even so, the association of weakness with tibiofemoral disease in men appeared weaker (e.g., OR 0.9 in the strongest quartile) than the association with patellofemoral OA.
Table 3. Association of quadriceps strength with compartment-specific radiographic OA in Chinese subjects*
|Patellofemoral OA only|| || || || || || |
| 0 (reference)||51/387 (13)||1.0||1.0||62/419 (15)||1.0||1.0|
| 1||40/407 (10)||0.7 (0.4–1.2)||0.7 (0.4–1.1)||66/466 (14)||1.1 (0.6–1.5)||1.1 (0.6–1.5)|
| 2||45/427 (11)||0.8 (0.5–1.3)||0.8 (0.5–1.2)||55/479 (12)||0.8 (0.5–1.2)||0.8 (0.5–1.2)|
| 3 (strongest)||35/423 (8)||0.6 (0.4–1.1)||0.6 (0.4–1.0)||47/514 (9)||0.6 (0.4–0.9)||0.6 (0.4–0.9)|
| P†|| || ||0.12|| || ||0.008|
|Tibiofemoral OA only|| || || || || || |
| 0 (reference)||26/362 (7)||1.0||1.0||73/430 (17)||1.0||1.0|
| 1||22/389 (6)||0.8 (0.4–1.5)||0.7 (0.4–1.4)||101/501 (20)||1.2 (0.9–1.8)||1.2 (0.8–1.7)|
| 2||25/407 (6)||0.9 (0.5–1.7)||0.7 (0.4–1.4)||63/487 (13)||0.7 (0.5–1.1)||0.7 (0.5–1.1)|
| 3 (strongest)||25/413 (6)||0.9 (0.5–1.8)||0.8 (0.4–1.6)||67/534 (13)||0.7 (0.5–1.1)||0.7 (0.4–1.0)|
| P†|| || ||0.51|| || ||0.013|
|Mixed OA|| || || || || || |
| 0 (reference)||80/416 (19)||1.0||1.0||251/608 (41)||1.0||1.0|
| 1||68/435 (16)||0.8 (0.5–1.2)||0.7 (0.4–1.0)||197/597 (33)||0.7 (0.5–0.9)||0.7 (0.5–0.9)|
| 2||49/431 (11)||0.6 (0.4–0.9)||0.5 (0.3–0.8)||166/590 (28)||0.6 (0.4–0.8)||0.5 (0.4–0.7)|
| 3 (strongest)||47/435 (11)||0.5 (0.3–0.9)||0.5 (0.3–0.8)||137/604 (23)||0.4 (0.3–0.6)||0.4 (0.3–0.6)|
| P†|| || ||0.0016|| || ||<0.0001|
In the analysis of isolated medial and lateral patellofemoral disease, there was a relationship between lateral patellofemoral disease and quadriceps weakness in both men and women (see Table 4). Nine subjects had both medial and lateral patellofemoral disease and were excluded from the analysis.
Table 4. Association of quadriceps strength with compartment-specific radiographic patellofemoral OA*
|Men|| || || || || || || || || |
| 0 (reference)||13/349 (4)||1.0||1.0||33/369 (9)||1.0||1.0||8/344 (2)||1.0||1.0|
| 1||12/379 (3)||1.3 (0.5–3.4)||1.2 (0.4–3.3)||21/388 (5)||0.7 (0.3–1.3)||0.7 (0.4–1.2)||8/375 (2)||0.9 (0.3–2.8)||0.8 (0.3–2.5)|
| 2||11/393 (3)||1.2 (0.4–3.3)||1.1 (0.4–3.3)||28/410 (7)||0.9 (0.5–1.6)||0.8 (0.5–1.6)||7/389 (2)||0.8 (0.3–2.3)||0.7 (0.2–2.1)|
| 3 (strongest)||9/397 (2)||1.1 (0.4–3.1)||1.1 (0.4–3.1)||14/402 (4)||0.5 (0.2–1.0)||0.5 (0.2–0.9)||11/399 (3)||1.2 (0.4–3.5)||1.1 (0.4–3.2)|
| P‡|| || ||0.94|| || ||0.07|| || ||0.93|
|Women|| || || || || || || || || |
| 0 (reference)||4/361 (1)||1.0||1.0||42/399 (11)||1.0||1.0||15/372 (4)||1.0||1.0|
| 1||10/410 (2)||2.3 (0.6–8.1)||2.0 (0.6–7.2)||45/445 (10)||1.0 (0.6–1.6)||1.0 (0.6–1.6)||9/409 (2)||0.5 (0.2–1.2)||0.5 (0.2–1.2)|
| 2||10/434 (2)||2.1 (0.6–7.1)||1.9 (0.5–7.0)||30/454 (7)||0.6 (0.4–1.0)||0.6 (0.4–1.0)||15/439 (3)||0.8 (0.4–1.7)||0.8 (0.3–1.7)|
| 3 (strongest)||7/474 (2)||1.4 (0.4–5.1)||1.3 (0.3–5.1)||29/496 (6)||0.5 (0.3–0.9)||0.6 (0.3–0.9)||12/479 (3)||0.6 (0.3–1.2)||0.5 (0.2–1.1)|
| P‡|| || ||0.8644|| || ||0.008|| || ||0.18|
When the analysis was performed for subjects without symptoms, the relationship of quadriceps weakness to disease in both compartments or only one compartment was attenuated (see Table 5). The only associations that remained statistically significant were those evaluating quadriceps weakness and mixed OA. The attenuation of these associations was seen also in OR measures, in that subjects in the strongest quadriceps quartiles did not have a substantially lower risk (i.e., OR 0.8–0.9) of compartment-specific OA.
Table 5. Association of quadriceps strength with nonpainful knee OA*
|PF OA only|| || || || || || |
| 0 (reference)||35/330 (11)||1.0||1.0||41/327 (13)||1.0||1.0|
| 1||31/367 (8)||0.8 (0.5–1.4)||0.8 (0.4–1.3)||51/376 (14)||1.1 (0.7–1.8)||1.1 (0.7–1.8)|
| 2||37/387 (10)||0.9 (0.5–1.6)||0.9 (0.5–1.5)||43/402 (11)||0.9 (0.5–1.4)||0.8 (0.5–1.4)|
| 3 (strongest)||32/395 (8)||0.8 (0.5–1.4)||0.8 (0.5–1.4)||45/454 (10)||0.8 (0.5–1.3)||0.8 (0.5–1.3)|
| P†|| || ||0.61|| || ||0.16|
|TF OA only|| || || || || || |
| 0 (reference)||20/315 (6)||1.0||1.0||47/333 (14)||1.0||1.0|
| 1||18/354 (5)||0.8 (0.4–1.7)||0.7 (0.4–1.5)||73/398 (18)||1.4 (0.9–2.1)||1.3 (0.8–2.0)|
| 2||16/366 (4)||0.7 (0.3–1.6)||0.6 (0.3–1.3)||43/402 (11)||0.7 (0.5–1.2)||0.7 (0.4–1.2)|
| 3 (strongest)||23/386 (6)||1.0 (0.5–2.1)||0.9 (0.4–1.9)||52/461 (11)||0.8 (0.5–1.3)||0.8 (0.5–1.3)|
| P†|| || ||0.71|| || ||0.08|
|Mixed OA|| || || || || || |
| 0 (reference)||50/345 (15)||1.0||1.0||115/401 (29)||1.0||1.0|
| 1||43/379 (11)||0.8 (0.5–1.2)||0.6 (0.4–1.0)||123/448 (28)||1.0 (0.7–1.3)||0.9 (0.6–1.3)|
| 2||28/378 (7)||0.5 (0.3–0.9)||0.4 (0.2–0.7)||95/454 (21)||0.7 (0.5–1.0)||0.6 (0.4–0.9)|
| 3 (strongest)||38/401 (9)||0.7 (0.4–1.2)||0.6 (0.3–1.0)||88/497 (18)||0.6 (0.4–0.8)||0.5 (0.3–0.8)|
| P†|| || ||0.03|| || ||0.0003|
We found a strong relationship between muscle weakness and combined patellofemoral and tibiofemoral OA (mixed disease) in both sexes, and a more tenuous relationship between weakness and isolated patellofemoral or tibiofemoral disease in women. There was also a trend in men associating weakness with isolated patellofemoral disease, but the small numbers of men with isolated patellofemoral or tibiofemoral disease may have limited our ability to discern relationships in these compartments. In a separate analysis of isolated patellofemoral disease, lateral (but not medial) patellofemoral disease was associated with muscle weakness. Thus, our analysis of all radiographic OA suggests that weakness is associated with almost all types of knee OA, except perhaps medial patellofemoral OA.
Our findings are similar to what has been observed in other cross-sectional studies of tibiofemoral disease (i.e., less of an association with weakness in men), but as yet no studies have included large numbers of men with tibiofemoral disease (2). This is the first report to describe an association between muscle weakness and patellofemoral disease in both men and women.
When, in an attempt to remove the effect of pain on causing weakness, we performed a similar analysis among asymptomatic subjects, the association with muscle weakness was attenuated in all compartments and remained significant only in persons with mixed disease. This finding contradicts what Slemenda et al reported regarding symptomatic and asymptomatic radiographic knee OA (2). The attenuation of the effect of muscle weakness in persons without symptoms was not likely to be secondary to a lack of statistical power, and many asymptomatic knees had fairly severe structural OA. Although it is difficult to make etiologic inferences from cross-sectional associations, our data suggest that pain, in large part, is responsible for decreased strength. This could be reflected in 2 ways, as follows: painful knees cannot elicit a maximal contraction due to pain during testing, or painful knee OA comes first and causes disuse, muscle atrophy, and muscle weakness.
Even in subjects without symptoms, the relationship between weakness and mixed OA remained statistically significant. Mixed disease may represent more severe disease with greater structural damage that alters input from articular mechanoreceptors in the joint. These receptors provide information for motor control and proprioceptive acuity in the muscles. Abnormal discharge from these receptors can result in muscle inhibition, causing weakness in the muscles crossing that particular joint (19). This would suggest that radiographic damage comes first and weakness follows. It may also be that individuals with mixed disease have had the disease longer, and that more weakness has developed over time. Once muscle weakness is present, it can set up a vicious cycle of decreased joint stability and altered proprioception that, in turn, alters the biomechanics of the joint, causing further deterioration of the diseased joint.
Unlike our findings in mixed disease, we were unable to detect any association of weakness with isolated asymptomatic structural OA in a single knee compartment. Asymptomatic knees with OA in isolated compartments could represent disease that is milder than multicompartment disease or could represent earlier disease. If asymptomatic, isolated OA represents milder disease, this suggests that weakness propels disease severity. If it represents earlier disease, weakness could be hypothesized to contribute to disease progression. Either way, our findings could be interpreted as suggesting that weakness has less effect on disease incidence than on disease progression and/or severity. However, a study by Brandt et al did not demonstrate a statistically significant difference in loss of muscle strength over 2.5 years between women with progressive knee OA relative to those with stable knee OA (20). It remains unclear what role muscle strength plays in the onset and progression of knee OA.
Based on these cross-sectional data, the effect of quadriceps weakness appears to be most marked for the lateral patellofemoral joint. With increasing knee flexion, activation of the quadriceps mechanism assists in controlling tracking of the patella and preventing excessive lateral displacement (21). The overall pull of the quadriceps mechanism is thought to be slightly lateral (22). The lateral pull of the patella is resisted, in part, by the vastus medialis, one of the 4 quadriceps muscles. Weakness in the vastus medialis may increase loading across the lateral patellofemoral compartment. Ineffectual quadriceps activation, in particular poor vastus medialis control, will allow excessive lateral displacement of the patella during knee flexion, leading to excessive compressive forces on the lateral patellofemoral joint. Interestingly, weakness was not associated with medial patellofemoral disease; this suggests that quadriceps weakness alters the balance of quadriceps contraction, leading to excessive lateral loading.
There are some limitations to the study. Although this is the largest sample of subjects with patellofemoral OA to be studied with skyline radiographs, the small numbers of men with isolated patellofemoral or tibiofemoral OA limit the interpretation of the data. In addition, the Chinese population studied has some distinct differences compared with the Western population. There is more symptomatic knee OA among women in the Beijing study than has been reported in the US (13). Women in Beijing are more active and have lower body weights, and squatting is much more common. Although the squatting questionnaire is subject to recall bias by asking subjects to estimate squatting time years ago, we found associations of squatting with OA in those without knee pain, a group of subjects in whom recall bias would be unlikely. Most important, it is difficult to theorize about etiology in this study, because of the cross-sectional design. However, the results in individuals without symptoms suggest that pain may contribute to the weakness, and that the association of OA and weakness may reflect their mutual association with knee pain. In light of this, the limitations in the knee-pain question must be kept in mind.
In conclusion, we observed a relationship between quadriceps weakness and both isolated patellofemoral and tibiofemoral OA and combined disease. For patellofemoral disease, this association was more marked for lateral disease than for medial disease. When we removed knees with pain from the analysis, only knees with disease in both the tibiofemoral and patellofemoral compartments were weak, not knees with disease in an isolated compartment. This suggests that pain may contribute to this weakness.