Risk Factors for Jumper’s Knee

Background: The prevalence of jumper’s knee is high in sports characterized by high demands on leg extensor speed and power, such as volleyball, basketball, football and athletics. A prevalence up to 50% has been reported among male, elite volleyball players. The complex process from a healthy tendon to jumper’s knee is not fully understood. Jumper’s knee is usually described as an overuse injury, although previous studies on risk factors are not conclusive. Previous cross-sectional studies among volleyball players have found that athletes with jumper’s knee have higher body mass compared to asymptomatic controls, suggesting a relationship between body composition and jumper’s knee. The “jumper’s knee paradox”, where symptomatic athletes appear to perform better in a counter movement jump (CMJ) compared to asymptomatic controls, is not fully understood. Tendons adapt to mechanical loading, albeit not always adequately. Tendon structural changes from childhood to adulthood are not well investigated, and there is a need for prospective studies examining the response to intensive training during adolescence. In many athletes with symptoms of jumper’s knee, ultrasound or MRI imaging of the painful tendons will reveal morphological abnormalities, typically as localized tendon thickening with hypoechoic areas and increased vascularity. However, it is not clear whether the presence of ultrasound changes in asymptomatic tendons precede (and predict) future tendon problems. Aims: First, we wanted to study whether training load and competition load as well as body composition or change of body composition represented a risk factor for developing jumper’s knee (Paper I). Furthermore, we wanted to investigate jumping ability and change of jumping ability as potential risk factors for developing jumper’s knee (Paper II). We also wanted to investigate the relationship between the development of ultrasound changes in the patellar and quadriceps tendons and symptoms of jumper’s knee, as well as the medium-term effects of intensive training on tendon thickness among adolescent athletes (Paper III). Methods: Papers I-III were based on a prospective cohort study with a total data collection period of 5 years. All participants were recruited from the same cohort, players entering the Toppvolley Norway (TVN) program, but the time and duration of inclusion differed between participants. TVN is located in Sand, Norway and combines an elite volleyball training program with a three-year senior high boarding school program. All students at TVN were eligible for this study. In Paper I & II only athletes without jumper’s knee at baseline were included, while Paper III also included athletes with pre-existing jumper’s knee. The diagnosis of jumper’s knee was based on a clinical examination alone. All athletes were examined twice a year. Training volume and match exposure were recorded prospectively on a weekly basis. Body composition was assessed by measuring height and weight, waist circumference and through skin fold caliper tests. Jumping ability was tested on a portable force plate and included two different jumps (CMJ and standing jump (SJ)). An ultrasound examination was done twice a year as long as students remained at TVN using greyscale and color Doppler. Main results: Altogether, 192 students were registered in the school record at TVN during the 5-year study period. Nearly one in three boys developed jumper’s knee during their time at TVN (32%) compared to only 8% of the girls (Paper I). Paper I: Athletes developing jumper’s knee had higher total training volume and tendon load compared to those who remained asymptomatic. A multivariate regression analysis showed that match exposure was the strongest sports-related predictor for developing jumper’s knee with an OR of 3.88 (95% CI 1.80 to 8.40) for every extra set played per week. Volleyball training had an OR 1.72, (95% CI 1.18 to 2.53) when combined with match exposure and gender. We did not detect any significant differences in body composition at the time of inclusion or change of body composition during the study period between players who developed jumper's knee and those who did not developed jumper’s knee. Males had three to four times higher risk of developing jumper’s knee compared to females, independent on training and match exposure in the multivariate regression analysis. Paper II: At the time of inclusion, male athletes who went on to develop jumper’s knee had significantly better results in CMJ (38.0±5.8 cm) compared to asymptomatic males (34.6±5.5 cm, p=0.03), while no difference was detected in SJ. In a multivariate logistic regression analysis corrected for gender and previous volleyball training, the OR was 2.09 (1.03 to 4.25) per cm difference in CMJ at the time of inclusion. Our results did not reveal any significant differences in the change in jumping ability between the groups, although both groups improved their jumping performance. Paper III: About half of the asymptomatic athletes who went on to develop jumper’s knee (17 of 35 tendons) had hypoechoic areas when they started at TVN, while 10% of the tendons that remained asymptomatic (24 of 238 tendons) had the same changes. Neovascularisation was found in 4% (9 of 238 tendons) of asymptomatic tendons and in 48% (15 of 35 tendons) of those who later developed jumper’s knee. In a multivariate logistic regression analysis, a baseline finding of a hypoechoic tendon area (OR 3.3, 95% CI 1.1 to 9.2) and neovascularisation (OR 2.7, 95% CI 1.1 to 6.5) increased the risk of developing jumper’s knee. Patellar tendon thickness among healthy athletes did not change (Wilk’s lambda, p=0.07) while quadriceps tendon thickness increased (p=0.001). The athletes reported a VISA score of 72 (SD 23) when first diagnosed, and knee function did not change during subsequent examinations and was 73 (SD 21) when they left TVN. Conclusion: A high volume of volleyball training and match exposure were important risk factors for developing jumper’s knee. Body composition or change in body composition were not associated with injury risk. Volleyball players with a natural ability for jumping high have an increased risk of developing jumper’s knee, while change in jumping ability was not identified as a risk factor. Hypoechoic areas and neovascularisation at baseline were risk factors for developing jumper’s knee. Quadriceps tendon thickness increased 7-11% among healthy athletes, while there was no change in patellar tendon thickness