Risk factors in sport and/or occupation
Retrieved studies
The results of the systematic literature search for the risk factors in sport and/or occupation are presented in figure 1. In total, ten articles met our inclusion criteria.
Risk factors
All ten retrieved studies were performed in a sports setting. We found no studies reporting on jumper's knee in an occupational setting (table 1). The case-control study of Lian et al. [7] compared 24 amateur volleyball players in the top division of the Norwegian competition with current jumper's knee with 23 of their counterparts without a history of jumper's knee. The cases significantly performed more hours of weight training per week (cases: 5 hours per week versus controls: 2 hours per week). The cases did not differ from the controls regarding the hours of jump training per week (cases: 0.4 hours per week versus controls: 0.6 hours per week) or the hours of volleyball training per week (cases: 8 hours per week versus controls: 7 hours per week). No association between exposure time and symptoms of jumper's knee was found.
Warden et al. [17] performed a case-control study including 30 cases with jumper's knee and 33 activity-matched controls without jumper's knee. The hours of sport activities per week, mainly volleyball, basketball and soccer, were not significantly different between the two groups: 4 hours per week versus 3 hours per week. This result is in agreement with the results of the case-control study of Kettunen et al. [18]. In this study, the 18 athletes with jumper's knee (mostly ball players and long-distance runners) and 14 control athletes without jumper's knee (also mostly ball players and long-distance runners) did not differ in the hours of physical activity per week. Notably, no mean hours of physical activity per week were reported. However, based on our calculations from table 1 in their study, the average number of hours was approximately ten. Taunton et al. [19] conducted a retrospective cohort study of two years among a subgroup of 96 runners with jumper's knee and found that the average number of hours per week of training (6) was not associated with an increased risk. Malliaras et al. [20] investigated, in a cross-sectional study, the association between years of volleyball playing and the weekly hours of training and playing over a period of 7 months, among male and female players in the Victorian State League competition in Australia. The mean number of years of volleyball playing was eight and the mean weekly hours of training and playing was five. The years of volleyball playing and the weekly hours of training had no relation with tendon abnormality and/or pain.
Crossley et al. [21] compared in their case-control study the number of sporting hours per week among three groups: 31 controls with no jumper's knee, 14 cases with jumper's knee in one leg and 13 cases with jumper's knee in two legs. They found mixed results regarding the reported number of sport hours per week. The number of sport hours in the cases with jumper's knee in both legs was significantly higher than in the cases with jumper's knee in one leg as compared to controls: 7 hours a week versus 4 hours and 3 hours per week, respectively (mean difference: 3.2 hours per week, 95% CI 0.6–5.8). The latter two groups did not differ in sport hours per week.
However, other studies did find an association between hours of training and jumper's knee. A cross-sectional study by Ferretti et al. [10] on elite-volleyball players (>14 hours per week) concluded that the number of playing and training sessions per week increased the prevalence of jumper's knee: 3% with two sessions a week, 15% with 3 sessions, 29% with 4 sessions and 42% with >4 sessions. Cook et al. [22] performed a prospective cohort study over a period of 16 months among 26 elite junior basketball players (number of playing and training hours is not mentioned; 14.5 hours of weight bearing activities per week). They found that 30% of the basketball players with hypoechoic tendons and 7% of the basketball players without hypoechoic tendons developed jumper's knee. Moreover, the significant increase in training volume for men was associated with a significant increase in tendon abnormalities. Cook et al. [23] performed a cross-sectional study among elite junior basketball players (n = 134) and state-level swimmers (n = 29). They documented 15 hours of exercise per week and 12 hours of weight-bearing exercise per week for the basketball players. For the swimmers, they reported 17 hours of exercise per week and 3 hours of weight-bearing exercise. At least 7% of the basketball players had jumper's knee but none of the swimmers had the condition. Gaida et al. [24] compared, in a case-control study, the number of training hours per week among elite female basketball players with no hypoechoic tendons (controls, n = 24), hypoechoic tendon in one leg (unilateral cases, n = 8) or in both legs (bilateral cases, n = 7). They found that in the preceding one to six months, both types of cases trained about 3 hours per week more than the controls. The cases trained on average 12 hours per week and the controls trained 9 hours per week on average.
Finally, Ferretti et al. [10] found a statistically significant association between a hard playing surface (cement versus parquet) and an increased prevalence of jumper's knee.