The symptoms of knee osteoarthritis (OA) are mechanical; that is, they occur with physical activity. However, patients with the same degree of structural damage experience widely different levels of pain, a phenomenon that is poorly understood. Differences in joint forces and joint stress during functional activities may assist in explaining the dissociation between radiographic structural findings and pain.
The human knee is a complex joint, and considerable forces are exerted on the articular surfaces during weight bearing. The knee joint is located between the 2 longest lever arms of the body. Both femoral length and tibial length are important factors contributing to the torques or moments around the knee. The longer the legs, the more torque will be exerted at the fulcrum of this lever arm, the knee.
The etiology of knee OA is multifactorial. The female knee is particularly vulnerable to the development of OA. Several risk factors that differ between men and women may explain the female predisposition in this condition. These factors include differences in quadriceps strength (1), a higher fat mass and lower muscle mass in women (2), sex differences that affect joint loading, including pelvic dimensions, knee morphology, quadriceps angle (Q angle), and neuromuscular strength (3), and differences in knee stability/stiffness as a result of decreased neuromuscular strength and increased ligamentous laxity (3–5). In an analysis of risk factors for incident knee OA in the Chingford Study, investigators reported the incidental finding that tall height was a risk factor (6). The rationale for this finding was not explained and heretofore has remained unexplored.
Loss of height occurs frequently in the elderly. Knee height is independent of age among adults and does not appear to decrease over time (7). Our goal was to explore the association between the length of the lower leg, usually assessed as knee height, and knee OA. We first determined whether there is an association between knee height and both prevalent radiographic OA and prevalent symptomatic OA. If the amount of torque corresponds to the likelihood of knee pain, then one would expect increased symptoms in persons with longer legs. We then investigated whether symptoms in patients with knee OA are associated with knee height, independently of radiographic OA.
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- PATIENTS AND METHODS
The characteristics of participants in the Beijing OA Study are presented in Table 1. The mean knee height was 49.6 cm in men and 45.7 cm in women. The prevalence of radiographic tibiofemoral OA was 21.9% in men and 41.8% in women, and the prevalence of radiographic patellofemoral OA was 25.9% in men and 35.7% in women. Symptomatic OA was determined to be present in 9.7% of men and 20.3% of women. In the assessment of knee symptom severity, 82.9% of men reported having no knee pain, compared with 69.2% of women.
Table 1. Characteristics of the study population*
|Characteristic||Men (n = 1,006)||Women (n = 1,500)|
|Age, mean ± SD years (range)||68.4 ± 6.4 (60–95)||67.5 ± 6.1 (60–93)|
|Height, mean ± SD cm||166.5 ± 6.0||153.8 ± 5.5|
|Weight, mean ± SD kg||70.1 ± 10.7||61.6 ± 10.5|
|Body mass index, mean ± SD kg/m2||25.3 ± 3.4||26.0 ± 4.0|
|Quadriceps strength, mean ± SD kg||39.7 ± 10||26.9 ± 7.7|
|Physical Activity Index, mean ± SD score||32.2 ± 3.6||33.0 ± 3.5|
|Knee height, mean ± SD cm||49.6 ± 2.2||45.7 ± 1.9|
|Knee height as a proportion of total body height, mean ± SD %||29.8 ± 0.8||29.7 ± 0.8|
|% with patellofemoral radiographic OA||25.9||35.7|
|% with tibiofemoral radiographic OA||21.9||41.8|
|% with radiographic OA||34.0||52.3|
| Knee height, mean ± SD cm||49.9 ± 2.2||45.9 ± 2.0|
|% with symptomatic radiographic OA||9.7||20.3|
| Knee height, mean ± SD cm||50.0 ± 2.3||46.2 ± 2.0|
|% with knee pain without radiographic OA||7.3||9.9|
|Knee symptom severity, %|| || |
| No pain||82.9||69.2|
| Usually bearable||14.2||23.8|
| Sometime unbearable||2.8||6.4|
| Mostly or always unbearable||0.1||0.6|
Results of the analysis of the association between knee height and radiographic OA are shown in Table 2. Higher knee height was associated with an increasing prevalence of radiographic OA, especially among women. Knee height affected both patellofemoral OA and tibiofemoral OA.
Table 2. Prevalence and adjusted ORs for the risk of radiographic OA in men and women, according to knee height*
|Group||Knee height, quartile||P for trend|
|1 (shortest)||2||3||4 (highest)|
|Men|| || || || || |
| Mean knee height, cm||46.9||48.9||50.3||52.5|| |
| Patellofemoral OA|| || || || || |
| No. (%) of subjects||86 (17.3)||88 (17.6)||111 (22.2)||118 (23.8)||0.002|
| OR (95% CI)||1.0||1.0 (0.8–1.3)||1.2 (0.9–1.5)||1.4 (1.1–1.8)|| |
| Tibiofemoral OA|| || || || || |
| No. (%) of subjects||75 (14.9)||84 (16.7)||100 (19.9)||92 (18.3)||0.031|
| OR (95% CI)||1.0||1.1 (0.8–1.4)||1.2 (0.9–1.6)||1.3 (1.0–1.6)|| |
|Women|| || || || || |
| Mean knee height, cm||43.2||45.0||46.3||48.2|| |
| Patellofemoral OA|| || || || || |
| No. (%) of subjects||169 (22.5)||189 (25.6)||229 (30.5)||264 (36.3)||<0.0001|
| OR (95% CI)||1.0||1.2 (1.0–1.4)||1.4 (1.2–1.6)||1.7 (1.5–2.0)|| |
| Tibiofemoral OA|| || || || || |
| No. (%) of subjects||224 (29.8)||237 (31.9)||284 (37.5)||331 (44.9)||<0.0001|
| OR (95% CI)||1.0||1.3 (1.1–1.5)||1.4 (1.3–1.6)||1.7 (1.5–2.0)|| |
Results of the analysis of the association between knee height and symptomatic OA are shown in Table 3. Higher knee height was associated with an increasing prevalence of symptomatic OA among women.
Table 3. Prevalence and adjusted odds ratios (95% confidence intervals) for the risk of symptomatic osteoarthritis in men and women, according to knee height*
|Knee height, quartile||Men||Women|
|2||0.7 (0.4–1.2)||1.1 (0.9–1.5)|
|3||1.5 (1.0–2.3)||1.5 (1.2–2.0)|
|4 (highest)||1.3 (0.8–2.0)||2.2 (1.7–2.7)|
|P for trend||0.07||<0.001|
Analyses in which knee height as a proportion of body height was evaluated yielded similar results in both men and women. We also assessed the relationship using total height and leg length and found associations in both men and women that were similar to those observed when knee height was used. Given that knee height in older adults is stable and highly correlated with stature (7), we chose this measure, because stature or total height can be influenced by other factors, including vertebral crush fractures, kyphosis, mobility problems, and hip abnormalities (7, 11, 14).
We also explored whether different definitions of patellofemoral OA influenced the observed associations. For this analysis, we used a definition of patellofemoral joint OA as the presence of grade ≥2 osteophytes. No differences in the odds ratios (ORs) were noted when this definition was used.
In the final part of our analysis, we explored the association between knee height and the severity of knee symptoms independently of structural change. Results of the assessment of knee pain severity in women are presented in Figure 2. No significant relationship was detected in men. Among women, increasing knee height was strongly associated with more severe knee pain (P = 0.0004 for the highest quartile versus the lowest quartile). Knee height was also associated with severity of knee pain in women without OA (OR 1.8 for the highest quartile versus the lowest quartile of knee height [95% CI 1.3–2.5]). This result was the same when the analysis was restricted to women with radiographic OA. Although the K/L grade was associated positively with knee pain (OR 1.6 [95% CI 1.4–1.8]), the ORs from the ordinal logistic model showed that the K/L grade did not have much confounding effect on the association between knee height and knee pain (data not shown).
Figure 2. Association of severity of knee pain with knee height in women. Logistic regression analysis was performed, using severity of pain (0–3-point scale) as the dependent variable and adjusting for age, physical activity, quadriceps strength, radiographic severity (Kellgren/Lawrence grade), and body mass index. ∗ = P = 0.04 versus lowest quartile. ∗∗ = P = 0.004 versus lowest quartile.
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- PATIENTS AND METHODS
The results of our study suggest that higher knee height is associated with prevalent radiographic and symptomatic knee OA, especially in women. Moreover, in women, knee height appears to be an important factor associated with the severity of knee symptoms independently of radiographic changes.
We hypothesize that the likely mechanism for this effect involves the role knee height plays in determining knee joint loads. The external knee adduction moment is related to the distribution of forces between the medial and lateral compartments of the knee joint. It is defined as the torque that tends to adduct the knee during gait. Increased external knee adduction moments are indicative of increased loads on the medial compartment relative to the lateral compartment (15). The adduction moment is known to be an important determinant of dynamic loading of the medial tibiofemoral compartment (16). In addition to the coronal plane (adduction and abduction) knee joint moments, flexion/extension moments are abnormal in persons with knee OA (17). Both adduction moments and flexion/extension moments are higher in legs that are longer. Any force that crosses a joint can contribute to the total torque or moment at that joint. This contribution to the total joint moment is the product of that force and the length from that force's line of action to the center of rotation of the joint (i.e., the moment arm length).
Of great interest is the observation that this association appears to be important not only in increasing the risk of radiographic and symptomatic OA, but also in mediating symptoms, independently of radiographic change. The pain of OA is usually referred to as mechanical or activity-related. By the same principles of lever mechanics, the load imparted through a longer leg appears to play a significant role in producing mechanical pain and determining its severity in women.
Knee height provides an estimate of moment arm length. In addition, it provides advantages in estimating length over what can be obtained from body height (7). Stature decreases with aging, and this height loss is greater in women than in men. Stature estimated from knee height is more accurate than measured stature in persons with kyphosis (18).
What are the differences between men and women that may determine this sex-specific finding? OA is more prevalent in women than in men, and women may be more vulnerable to knee OA for several reasons. Quadriceps strength in men is greater than that in women, and this difference may play an important role in reducing postural sway and improving joint stability (1). The higher fat mass and lower muscle mass in women may explain some of the sex difference in OA susceptibility, although this conjecture needs to be formally tested (2). Other sex differences that impact joint loading include pelvic dimensions, knee morphology, Q angle, and neuromuscular strength (3). For instance, disproportionate loading of the lateral compartment in women likely arises from differences in knee stability/stiffness, which is reduced in women as a result of decreased neuromuscular strength and increased ligamentous laxity (3–5). Thus, laxer passive restraints (such as the anterior cruciate ligament) that help to determine knee stability during loading may have a deleterious effect if the lever arm is longer. As a result of greater mechanical instability, the female knee joint may be more vulnerable to the development of knee OA and thus more susceptible to a proportionally longer lever arm.
This research has a number of limitations that must be mentioned. The assessment of knee pain severity was accomplished by using an instrument that was developed for this study and is not knee specific. The standard responses on the verbal rating scale (no, mild, moderate, severe, and extreme) were thought to be unclear in translation, and thus the word “bearable” was used to qualify this. The measurements of knee height were made on the right leg only. This is unlikely to be a source of asymmetry, because leg length discrepancy is very infrequently attributable to structures below the knee (14). The associations found between knee height and OA were, in general, modest, although the association observed for the highest quartile of women was more marked.
In summary, we observed that knee height is associated with prevalent radiographic and symptomatic OA. It also appears to play an important role in knee symptoms independently of radiographic changes in OA. The findings were stronger in women than in men, which is consistent with the female predisposition for OA and the concept of the vulnerable female knee joint. This study highlights the importance of mechanical forces in the assessment of OA and knee symptoms.