Background
Optimizing knee function in older people is important. It is increasingly clear that causes of knee pain are multifactorial. Despite the evidence for a significant contribution of patellofemoral diseases to knee pain and disability [
1], a significant focus of work has been on the tibiofemoral compartment with little work at the patellofemoral compartment. Patellofemoral pathology tends to cause knee pain on performing functional activities involving knee bending which are not well captured in most commonly used instruments which examine knee pain.
Patellar tendinopathy is a clinical condition characterised by activity-related anterior knee pain [
2,
3]. It is commonly known as ‘jumper’s knee’ as it is traditionally recognised in athletes who are involved in sports that require jumping [
3,
4]. However, patellar tendinopathy has also been found in people who do not participate in sports involving jumping [
3‐
6]. The prevalence of patellar tendinopathy varied among athletes participating in different sports, with values ranging between 2.5 and 45 % [
6‐
8]. The diagnosis of patellar tendinopathy is primarily based on clinical examination. However patellar tendon abnormalities identified by imaging modalities such as magnetic resonance imaging (MRI) and ultrasonography have been shown to be corresponded to the characterised pathological features of patellar tendinopathy histopathologically [
5,
9,
10]. Although not all individuals with evidence of patellar tendinopathy on imaging have symptoms [
5,
11], there is evidence that abnormalities identified by imaging modality within the asymptomatic tendon predict the development of tendon-related symptoms and disability [
12,
13].
Currently there is little data on the prevalence and factors associated with patellar tendinopathy in community-based non-athletic populations, and it is unknown whether patellar tendinopathy contributes to knee symptoms in older people. Understanding these is important as patellar tendinopathy may contribute to activity-related knee pain which may impair the functional activities of the knee [
14]. We have recently reported the prevalence (28.3 %) of MRI defined patellar tendinopathy in community-based older individuals with past and current obesity being the major risk factor [
15]. However the relationship between patellar tendinopathy and knee pain is not fully investigated [
15]. Given the mechanical mechanism in the pathogenesis the patellar tendinopathy [
3], physical activity [
16] and quadriceps strength [
2] have been recognized as risk factors for patellar tendinopathy in athletes, although the evidence is not strong [
17]. It is not known whether physical activity and quadriceps strength are risk factor for patellar tendinopathy in community-based older populations. The cross-sectional area of a muscle has been validated as a measure of the force producing capability of that muscle [
18,
19] and could be used as a surrogate for muscle strength. Thus the aim of this study was to (1) determine whether MRI defined patellar tendinopathy was prevalent, (2) identify factors associated with this condition, and (3) examine whether patellar tendinopathy was associated with knee pain, in a cohort of community-based middle-aged women. We hypothesised that (1) MRI defined patellar tendinopathy will be common, (2) higher levels of physical activity and greater quadriceps muscle size will be associated with increased prevalence of patellar tendinopathy, and (3) patellar tendinopathy will be associated with knee pain in community-based women.
Results
The characteristics of the 176 women are summarized in Table
1. The women were largely asymptomatic, which was evidenced by the very low score of the WOMAC pain subscale at the time of knee MRI, with the median scores being 21 out of 500. Fifty-three (30.1 %) women had MRI defined patellar tendinopathy. There were no significant differences in terms of age, BMI, vastus medialis cross-sectional area, physical activity, and WOMAC pain score between those with and without patellar tendinopathy.
Table 1
Characteristics of study participants
Agea, years | 52.2 (7.1) | 52.5 (5.6) | 0.80 |
Body mass indexa, kg/m2 | 27.1 (5.4) | 27.1 (5.7) | 0.94 |
Vastus medialis cross-sectional areaa, mm2 | 1047 (180) | 1130 (293) | 0.06 |
Physical activity scoreb, n (%) | | | 0.07 |
Low (1–6) | 53 (44.2) | 16 (30.8) | |
Moderate (7–8) | 39 (32.5) | 15 (28.8) | |
High (9–12) | 28 (23.3) | 21 (40.4) | |
WOMAC pain scorec | 21 (0–240) | 21 (0–311) | 0.92 |
In univariate analysis, vastus medialis cross-sectional area and physical activity levels were positively associated with the prevalence of MRI defined patellar tendinopathy. After adjustment for age and BMI, greater vastus medialis cross-sectional area (OR per cm
2 = 1.22, 95 % CI 1.04–1.43,
p = 0.02) and higher levels of physical activity (OR per tertile = 1.65, 95 % CI 1.09–2.51,
p = 0.02) remained to be associated with increased prevalence of patellar tendinopathy (Table
2). When vastus medialis cross-sectional area and physical activity were included in the same regression model and adjusted for age and BMI, the association for vastus medialis cross-sectional area remained significant (OR 1.19, 95 % CI 1.01–1.40,
p = 0.04) but the association for physical activity was attenuated and no longer significant (OR 1.52, 95 % CI 0.99–2.32,
p = 0.06).
Table 2
Factors associated with the prevalence of MRI defined patellar tendinopathy
Age (years)a | 1.01 (0.96, 1.06) | 0.82 | 1.01 (0.96, 1.06) | 0.82 |
Body mass index (kg/m2)b | 1.00 (0.95, 1.06) | 0.94 | 1.00 (0.94, 1.06) | 0.95 |
Cross-sectional area of vastus medialis (cm2) c | 1.18 (1.02, 1.37) | 0.03 | 1.22 (1.04, 1.43) | 0.02 |
Physical activity (tertiles)c | 1.58 (1.05, 2.36) | 0.03 | 1.65 (1.09, 2.51) | 0.02 |
A total of 148 women had knee MRI at 2 year follow-up, of whom 46 women had patellar tendinopathy at baseline. Over 2 years, the persistence of patellar tendinopathy was observed in 20 out of 46 (43.5 %) participants. The incidence of knee pain was 3 out of 46 (6.5 %), and the worsening knee pain was 22 out of 148 (14.9 %). The association between MRI defined patellar tendinopathy and changes in knee pain over 2 years was examined (Table
3). Although there was no significant association between the prevalence of patellar tendinopathy and the incidence of knee pain (OR 6.46, 95 % CI 0.49–85.68,
p = 0.16), the persistence of patellar tendinopathy was associated with the worsening of knee pain (OR 10.65, 95 % CI 1.14–99.77,
p = 0.04).
Table 3
Association between MRI defined patellar tendinopathy and change in knee pain over 2 years
Prevalence of patellar tendinopathy | 5.82 (0.48, 70.62)a | 0.17 | 6.46 (0.49, 85.68)a | 0.16 |
Persistence of patellar tendinopathy | 10.71 (1.17, 98.24)b | 0.04 | 10.65 (1.14, 99.77)b | 0.04 |
Discussion
MRI defined patellar tendinopathy was common in community-based middle-aged women without clinical knee diseases, with a prevalence of 30.1 %. Both higher levels of physical activity and greater vastus medialis size were associated with greater prevalence of MRI defined patellar tendinopathy. The findings suggest that being physically active may be a risk factor for patellar tendinopathy, indicating a mechanical pathogenesis of patellar tendinopathy. Furthermore, persistent patellar tendinopathy predicted the worsening of knee pain over 2 years.
Imaging, in particular ultrasonography defined patellar tendinopathy has been described in asymptomatic young athletes [
12,
13]. It has been shown to be common and predict the development of knee symptoms and future tendon-related disability [
12,
13]. There is little literature characterising patellar tendinopathy in community-based populations. Recently we reported a prevalence of 28.3 % for MRI defined patellar tendinopathy in community-based older adults with obesity being the risk factor [
15]. In the current study of community-based in middle-aged women, we found that the prevalence of MRI defined patellar tendinopathy was 30.1 %. These findings suggest that imaging identified patellar tendinopathy is a common condition occurring in the general populations but not just a condition of athletes [
2,
6,
17]. Our study found an increased risk of patellar tendinopathy in relation to higher levels of physical activity and greater vastus medialis size. Previously it has been shown in athletes that the number of jumps performed correlated with an increased risk of patellar tendinopathy [
16] and that there was an association between quadriceps muscle strength and patellar tendinopathy [
17]. Our results in a community-based population support these findings. Moreover, we showed that the positive association between vastus medialis size and patellar tendinopathy was independent of the level of physical activity. In contrast to our previous study showing obesity being a risk factor for MRI defined patellar tendinopathy in community-based older adults [
15], we found no relationship between patellar tendinopathy and BMI in middle-aged women in the current study. Evidence is not consistent across the literature. In athletes an association has been identified between patellar tendinopathy and increased body weight, waist circumference, and BMI [
2,
6]. However, some argue that weight is not a risk factor for the development of patellar tendinopathy [
27,
28], which is in keeping with the findings of this study. Taken together, these results suggest that MRI defined patellar tendinopathy is prevalent in community-based women and is associated with both muscle size (a surrogate of quadriceps muscle strength) and levels of physical activities. Although in this study there are no measures of intermediate variables which are more representative of knee biomechanics, our findings indicate a mechanical rather than systemic metabolic pathogenesis of patellar tendinopathy. This is consistent with the conclusion from a systematic review of the risk factors for patellar tendinopathy that all nine identified risk factors affect the loading of the patellar tendon in some way, and could be explained within the framework of a mechanical pathophysiological theory [
17].
Although the clinical significance of ultrasonography identified patellar tendinopathy has been reported in asymptomatic young athletes [
12,
13], it is unknown for the clinical implications of the MRI defined patellar tendinopathy in community-based populations. We found that persistent patellar tendinopathy was associated with the worsening of knee pain. However the wide confidence interval for the significant association can be explained by the moderate sample size of the study. Our study did not show a significant association between the prevalence of patellar tendinopathy and the incidence of knee pain (
p = 0.16). Our study sample had a low incidence of knee pain (6.5 %) which limited the power of our study to show a significant result. Larger prospective cohort studies with longer follow-up will be needed to further explore the relationship between MRI defined patellar tendinopathy and knee symptoms.
There are several limitations in our study. The sample size of our study is modest. Further research of larger prospective cohort studies should be conducted to assess the natural history of patellar tendinopathy over time and its clinical significance in relation to knee symptoms. Although we collected WOMAC data at two time points, we did not specifically ask about knee pain on bending, which would be more specific for patellar tendinopathy. This study investigated the risk factors for MRI diagnosed patellar tendinopathy in community-based women without significant knee pain. Thus the findings may not be generalizable to men or people with clinical patellar tendinopathy. A strength of our study is that patellar tendinopathy has been measured using the non-invasive MRI technique based on both T1- and T2-weighted sequences which showed high reproducibility, and analysis and disease classification have been conducted using well validated methodologies.
Patellofemoral compartment pathology has been recognised as a source of pain and impaired function with ageing [
1]. It has been found that knee pain and functional limitations in activities such as knee bending, which have significant impact on knee function and quality of life, are common in older people. Understanding the modifiable risk factors for knee pain in community-based older populations will have the potential to inform preventive and therapeutic strategies for reducing knee pain and improving knee function. Symptomatic patellar tendinopathy has negative impacts on activity and quality of life [
14]. Most of the instruments currently used in epidemiological studies of older people focus on the tibiofemoral compartment and do not address symptoms that may be attributable to patellar tendinopathy, such as activity-related anterior knee pain associated with important functional activities such as knee bending. Our study suggests that patellar tendinopathy is common in community-based middle-aged women. Further work using specific tools will be needed to determine whether patellar tendinopathy contributes to some of the common functional impairment seen in older people but not captured by the commonly used knee instruments.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.
The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
FMC and YW were involved in conception and design of the study. JT, JF, SRD, RJB, and FH were involved in the acquisition of data. JT, YW, FMC, and JK were involved in statistical analysis and interpretation of the data. JT, YW, and FMC drafted the manuscript. All authors reviewed the manuscript with critical revision of the article for important intellectual content and approved the final manuscript. YW and FMC took the responsibility for the integrity of the work as a whole, from inception to finished article.