The most important finding of this study was that the distribution of the lesions of spondylolysis on MRI differed between young soccer and baseball players, which suggested different pathomechanisms of spondylolysis in each sport activity. This was the first study, to the best of our knowledge, that investigated the differences in clinical factors and MRI findings between young soccer and baseball players with symptomatic spondylolysis. In fact, most previous studies have reported the results of patients with spondylolysis in individual sports [17-19], and only one study compared spondylolysis in cricketers and soccer players using single photon emission computerized tomography (SPECT) [20]. In the present study, soccer players with spondylolysis had more multiple and bilateral lesions compared with baseball players (p < 0.001). Baseball players with spondylolysis tended to have a single, unilateral lesion compared with soccer players, and pitching or batting with the dominant hand was likely to affect the contralateral side of the pars interarticularis. The symmetrical distribution of lesions of spondylolysis in soccer players was consistent with results previously described by Gregory et al, although the lesions were assessed by SPECT [20].
It was speculated that the greater numbers of multiple and symmetrical lesions of spondylolysis in soccer players were due to specific movements and tendencies required when playing soccer. Most soccer players mainly use the dominant foot, but often also use the non-dominant foot to kick a ball. Moreover, during training and matches, in general, they spend more than 95% of their time jogging and running quickly without kicking a ball. On the other hand, baseball players routinely generate high rotational and torsional forces on the lumbar spine during pitching and batting. Pitching particularly can lead to back stiffness and facet joint of discogenic pain, and swinging a bat generates high compressive loads on the spine [21]. They use only the dominant hand when pitching and batting, and they tend to repeat the same movements. These specific movements would lead to lesions of spondylolysis located contralateral to the dominant side of the hand for pitching or batting in baseball players. One of the strong points in this study was that information regarding the dominant leg in the sports field was collected prospectively in order to describe the location of the lesions relative to the dominant side of the leg in soccer and baseball. This was the first report, as far as we know, describing the correlation between the side of the lesions and the side of the dominant leg in soccer and baseball players with spondylolysis.
Lesions of spondylolysis were located at lower lumbar levels, 57.8% at L5, 31.9% at L4, and 10.3% at L3, with no lesions at L1 and L2 in the present study. Ladenhauf et al. reported that the levels of spondylolysis for 127 young athletes were L5 in 74% , L4 in 22.1%, and L3 in 1.6%, although they evaluated lesions of spondylolysis using various kinds of imaging modalities including MRI, SPECT, CT, and X-ray [22], and the present results showed a similar lesion distribution. The most affected lumbar level was L5 in both groups in the present study, 55.4% of soccer players and 60.0% of baseball players revealed high signal intensity lesions at L5 on STIR-MRI scans. Congeni et al. reported that 71% of spondylolysis was seen at L5 [23], and Gregory et al. reported 66.7% [20], and the present results were consistent with these results.
The risk of pediatric and adolescent sports injuries was high, and children 5-14 years accounted for nearly 40% of all sports-related injuries [24]. Spondylolysis is regarded as a main cause of LBP in young athletes [6]. Young athletes with spondylolysis are mainly treated conservatively with cessation of sports activity and rehabilitation with or without a thoracolumbar orthosis, with favorable clinical outcomes [25-27]. It was reported that the mean time of cessation of sports activity was 3.9 ± 0.8 months, with a mean time of 5.2 ± 2.1 months for complete return to soccer in 34 young soccer players with spondylolysis who were treated nonoperatively [28]. Lumbar flexion-extension movements, rotational forces of the trunk, and lumbar compression forces at landing have been reported as predisposing factors to spondylolysis [8,9]. Based on the results of the present study, the pathomechanism of spondylolysis would differ between young soccer and baseball players with spondylolysis, and in baseball players, the dominant side hand for pitching and batting would affect the contralateral side of the pars interarticularis. Therefore, clinicians should consider not only previously reported predisposing factors for spondylolysis, but also characteristic movements and training patterns in each sport and the side of the dominant leg in the sports field when determining the regimen of conservative treatments for young athletes with spondylolysis. For example, when we perform strengthening of core abdominal muscles, which is an important conservative treatment for spondylolysis that can stabilize pars defects [29, 30], for baseball players with spondylolysis whose dominant side hand for pitching is the right side, the left side of the core abdominal muscles should be strengthened more. For young soccer players with spondylolysis, we should consider symmetrical strengthening of abdominal muscles and resistance exercises of back extension and rotary torso. To clarify the efficacy of such individualized conservative treatments for athletes with spondylolysis, prospective studies will be needed in the future.
This study had several limitations. First, the sample size was small in each group. Second, all the young athletes included in this study were male; therefore, spondylolysis was not evaluated in young female soccer and baseball players. Third, young soccer and baseball players with spondylolysis who did not undergo MRI, including players with asymptomatic spondylolysis, were not included, which allows for selection bias. Fourth, CT and SPECT were not used for the assessment of the lesions of spondylolysis. Some authors reported that MRI was not as sensitive for detecting spondylolysis as a SPECT bone scan [31, 32], however it was confirmed that MRI has a high diagnostic performance for detecting a pars defect in young athletes [11, 12, 14, 16, 33]. MRI would be preferable for young athletes because of the lack of ionizing radiation. Fifth, young athletes with spondylolysis who participated in a sports activity other than soccer and baseball were not evaluated. Further case-controlled studies may be needed to clarify whether there are specific findings of spondylolysis depending on sports type. Finally, findings of physical examinations, such as tightness of the hamstrings and hip range of motion, were not assessed due to the retrospective study design. Despite these limitations, the present study provided important information for understanding the pathomechanism of spondylolysis in young soccer and baseball players.