Few risk factors for knee osteoarthritis (OA) are appreciated, and the discordance between symptoms and the severity of structural disease has not been explained. Knee height contributes to moments around the knee. The longer the leg, the more torque is present. Although this would suggest that having long legs would be related to the occurrence of knee OA and pain, this issue has not been studied. Our aim was to explore the association between knee height, knee pain, and knee OA.
We recruited a random sample of Beijing residents ages 60 years and older. Subjects answered questions about joint symptoms, and radiographs of their knees were obtained. A knee joint with a Kellgren/Lawrence grade of ≥2 was defined as having radiographic OA. Patellofemoral OA was defined as being present when grade ≥1 osteophytes or grade ≥1 joint space narrowing was observed on skyline views of the patella or anterior femur. Subjects were considered to have symptomatic OA when both radiographic OA and self-reported pain were present in the same joint. Knee height was measured on the right leg using a sliding broad-blade caliper; the subject was seated, and the subject's feet were bare. We used logistic regression analyses to assess whether knee height was associated with prevalent radiographic and symptomatic OA. We then assessed whether knee height was associated with knee symptoms independently of structural change.
A total of 1,006 men (mean ± SD age 68.4 ± 6.4 years) and 1,500 women (mean ± SD age 67.5 ± 6.1 years) participated in this study. Higher knee height was associated with an increasing prevalence of both radiographic and symptomatic OA, especially among women. For radiographic OA, the magnitude of association was similar for the patellofemoral and tibiofemoral compartments. Among women with knee pain, higher knee height was associated with more severe knee pain (P = 0.0004 for the highest quartile versus the lowest quartile of knee height) independently of the severity of radiographic OA.
Knee height is associated with prevalent radiographic and symptomatic knee OA. It may also play an important role in knee symptoms. This study highlights the importance of mechanical forces in the determination of OA and knee symptoms.
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.
PATIENTS AND METHODS
The Beijing OA Study
The study design has been described in detail previously (8). Briefly, we studied a random sample of men and women ages 60 years and older from 3 central districts of Beijing, China. Subjects were interviewed at their homes. Trained health professional interviewers administered a standardized questionnaire that focused on joint symptoms and possible risk factors for OA. At the end of the interview, subjects were invited to the central examination site at Peking Union Medical College Hospital for a clinical examination, laboratory testing, and radiography. Transportation to the hospital was provided.
Radiographic evaluations consisted of weight-bearing anteroposterior, 14 × 17-inch, fully extended knee radiographs and weight-bearing skyline view films. Radiographs were read for the overall severity of OA using the Kellgren/Lawrence (K/L) grade (9), and for joint space narrowing (JSN) and osteophytes in both the medial and lateral compartments using a 0–3-point scale based on the atlas of the Osteoarthritis Association Research Society International (10). Radiographs were read by one reader, an academically based bone and joint radiologist (PA). Reproducibility of the intrareader assessments was high (for OA versus no OA, κ = 0.79).
Definition of risk factors and outcomes.
Radiographic OA was defined as being present when the K/L grade was ≥2. Patellofemoral OA was defined as being present when the osteophyte score was ≥1 or the JSN score was ≥1 on skyline views of the patella or anterior femur.
Knee pain was assessed by asking, “In the past 12 months, have you had pain, aching, or stiffness lasting at least a month in your knee?” Symptomatic knee OA was defined as being present if the subject answered “yes” to this question and had radiographic changes in the tibiofemoral joint and/or the patellofemoral joint of that knee. Knee pain severity was ascertained by asking, “How severe is your knee pain usually?” The responses on this Likert scale were (a) usually bearable, (b) sometimes unbearable, (c) mostly or always unbearable, and (d) don't know.
Knee height was measured on the right leg, using a sliding broad-blade caliper, with the subject in the seated position (see Figure 1) (11). The patient's shoes and socks were removed, and pants were rolled up past the knee. The subject's heel rested on the caliper blade, and sandbags placed under the foot ensured that the foot remained level with the heel. The knee was bent to a 90° angle, and the distance from the undersurface of the heel along the calf to the anterior surface of the thigh over the femoral condyles (just proximal to the kneecap) was measured. Measurements to the nearest 0.1 cm were obtained and then repeated. If the measurements differed by ≥0.5 cm, 2 more measurements were obtained, and the second set was used. The mean of the 2 measurements was used in the analysis.
Knee height as a proportion of body height was defined as knee height/body height. Standing height was measured in millimeters with a wall-mounted Harpenden stadiometer (Holtain, Crosswell, UK). Height was measured twice. If a difference of ≥4 mm between the 2 measurements was found, height was remeasured, and the second set of measurements was entered into the data system. The mean of the 2 measurements was used. Weight was measured in kilograms, using a standard balance-beam scale, and was recorded to the nearest 0.1 kg.
Knee extension strength was tested in both legs using a spring gauge attached to a fixation point behind a specially constructed chair (12). The primary measure of physical activity was derived from the Framingham Physical Activity Index, a measure of metabolic work during a typical day (13).
First, we examined whether higher knee height was associated with prevalent radiographic and symptomatic OA. Considering that the risk profiles between men and women may be different, we conducted separate analyses for each sex. We created sex-specific quartiles of knee height and then calculated the prevalence of compartment-specific radiographic OA. We obtained the prevalence ratios and 95% confidence intervals (95% CIs), with subjects whose knee height was in the lowest quartile serving as the reference group, using the generalized estimating equation. We used the same approach to analyze the relationship between knee height and the prevalence of symptomatic OA. In the multiple regression model, we adjusted for physical activity, quadriceps strength, age, and body mass index.
We then assessed whether knee height was associated with severity of knee symptoms independently of structural change. We performed an ordinal logistic regression analysis to examine the association between knee height (predictor variable) and the severity of knee pain (dependent variable). Severity of knee pain was scored as 0 (no pain), 1 (usually bearable), 2 (sometimes unbearable), or 3 (mostly or always unbearable) while adjusting for age, physical activity, quadriceps strength, radiographic severity (K/L grade), and body mass index. In this analysis, we used the radiographic severity score from the more severely affected knee and the quadriceps strength measurement from the same side. To test whether the relationship between knee height and the prevalence of knee pain is modified by structural changes of the knee, we performed additional analyses by either adding or removing the K/L grade from the regression model and then restricting the analysis to subjects without radiographic OA (K/L grade <2) to determine whether knee height was still associated with the severity of knee pain.
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.
Knee height as a proportion of total body height, mean ± SD %
29.8 ± 0.8
29.7 ± 0.8
% with patellofemoral radiographic OA
% with tibiofemoral radiographic OA
% with radiographic OA
Knee height, mean ± SD cm
49.9 ± 2.2
45.9 ± 2.0
% with symptomatic radiographic OA
Knee height, mean ± SD cm
50.0 ± 2.3
46.2 ± 2.0
% with knee pain without radiographic OA
Knee symptom severity, %
Mostly or always unbearable
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*
Knee height, quartile
P for trend
Odds ratios (ORs) were adjusted for age, body mass index, physical activity, and quadriceps strength. OA = osteoarthritis; 95% CI = 95% confidence interval.
Mean knee height, cm
No. (%) of subjects
OR (95% CI)
No. (%) of subjects
OR (95% CI)
Mean knee height, cm
No. (%) of subjects
OR (95% CI)
No. (%) of subjects
OR (95% CI)
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
Values were adjusted for age, body mass index, physical activity, and quadriceps strength.
P for trend
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).
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.
We would like to thank the participants and staff of the Beijing Osteoarthritis Study.