Maximal facet tropism of 10.8° as a quantitative cutoff for predicting lumbar disk herniation in young elite athletes: a retrospective case-control study in Japan

Ethics statement

This retrospective, single-center, case–control study was approved by the ethics committee of Tokushima University Hospital (approval no., 3642) and was performed in accordance with the ethical standards of the 1964 Declaration of Helsinki and its later amendments. The requirement for written informed consent was waived owing to the retrospective observational nature of this study and the anonymity of the data analyzed.

Study design

This retrospective, single-center, case–control study was conducted following the Strengthening the Reporting of Observational Studies in Epidemiology guidelines.

Participants

The study included athletes who were treated at the study institution between November 2013 and May 2025. Cases included 43 athletes with symptomatic LDH in whom magnetic resonance imaging (MRI) findings were consistent with their clinical presentation. Eleven symptomatic nonherniated athletes with discogenic low back pain and evidence of Modic type 1 or 2 changes or a high-intensity zone on MRI but without disk herniation were selected as controls. Conversely, athletes who had spondylolysis, isthmic spondylolisthesis, significant scoliosis, and a history of lumbar surgery were excluded. Herniation status was adjudicated independently by two board-certified spine surgeons based on MRI and clinical records, and disagreements were resolved by consensus.

Data collection and measurements

Image analysis and measurement of FT

All participants underwent lumbar spine MRI using either a 1.5-T or 3.0-T scanner: Signa Explorer 1.5T (GE Healthcare, Chicago, IL, USA), Signa MR750 3.0T (GE Healthcare), or TRILLIUM OVAL 3.0T (Hitachi, Tokyo, Japan). Imaging data were visualized and analyzed using the Centricity Universal Viewer Zero Footprint Client system ver. 6.0 SP11.2.2 (GE Healthcare).

Two experienced spine surgeons conducted all imaging measurements, and consensus was reached for all parameters assessed. Axial T2-weighted images at the L3/4, L4/5, and L5/S1 levels were used for the assessment of FT. On each axial image, a sagittal midline was first established as a line passing through the centers of the vertebral body and the spinous process. The axis of each facet joint was then defined by a line connecting the midpoints of the inner and outer edges of the joint. The facet angle was measured as the angle between this sagittal midline and the facet joint axis for both sides. The FT at each level was calculated as the absolute difference between the left and right facet angles (Fig. 1).

Fig. 1

Method for measurement of the facet angle and tropism. On an axial T2-weighted magnetic resonance image, the facet angle is measured relative to the sagittal midline, defined by a line connecting the centers of the vertebral body and spinous process. The axis of each facet is determined by a line connecting the midpoints of its inner and outer edges. Facet tropism is calculated as the absolute difference between the left and right facet angles.

Statistical analysis

To ensure the reliability of the FT measurements, intrarater and interrater reliability was assessed for a randomly selected subset of 20 cases. Measurements were repeated by the first observer after a 4-week interval to determine intrarater reliability and by a second independent observer for interrater reliability. Reliability was quantified using the intraclass correlation coefficient.

All statistical analyses were performed using R ver. 4.4.0 (R Foundation for Statistical Computing, Vienna, Austria). Intergroup comparisons were performed using the Mann-Whitney U test for continuous variables and Fisher’s exact test for categorical variables. The association between the type of sport played and the presence of LDH was analyzed using a two-sided Fisher’s exact test because of the small number of cases in some cells. To assess the independent association between potential risk factors and the presence of LDH, a multivariable logistic regression model was constructed. The model incorporated maximal FT as the primary variable of interest, with adjustments for age and sex. A two-sided p-value of <0.05 was considered significant. A secondary subgroup analysis was also performed to explore potential differences based on the location of the disk herniation. The LDH group was stratified into patients with herniation at L4/5 and those with herniation at L5/S1. Maximal FT was then compared between these two subgroups using the Mann-Whitney U test.

Participant characteristics

The study population consisted of 43 athletes with LDH and 11 controls. Within the LDH group, disk herniations were most frequently observed at the L4/5 (n=25) and L5/S1 (n=22) levels, followed by the L3/4 level (n=1) (Supplement 1). Some patients presented with multilevel herniations. No significant difference was noted in the mean age (28.0±7.4 years in the LDH group vs. 32.1±12.7 years in the control group, p=0.17), sex distribution (83.7% male vs. 72.7% male, p=0.480), or athletic background, including the competition level or type of sport (Table 1). A breakdown of participant sports is presented in Supplement 2.

Table 1

Patient demographic and athletic data

Imaging findings

No significant difference was noted in the mean FT angle at each individual lumbar level; however, the maximal FT angle was significantly greater in the LDH group (11.8°±5.9°) than in the control group (7.6°±3.0°, p=0.028) (Table 2). The prevalence of severe FT (>10°) was analyzed to identify the anatomical level contributing most to this finding. This analysis demonstrated significantly higher prevalence of severe FT in the LDH group, exclusively at the L5/S1 level (34.9% vs. 0.0%, p=0.024), with no significant differences at other levels (Table 3).

Table 2

Comparison of facet tropism between study groups

Table 3

Prevalence of severe facet tropism (>10°)

Risk factors for LDH

In the multivariable logistic regression analysis adjusted for age and sex, maximal FT was the only factor identified as a significant independent predictor of LDH (odds ratio [OR], 1.234 per 1° increment; 95% confidence interval [CI], 1.008–1.511; p=0.041) (Table 4). Receiver-operating characteristic curve analysis for maximal FT was conducted to determine a clinically relevant threshold for this risk factor. The analysis yielded an area under the curve of 0.727, with an optimal cutoff value of 10.8°. This threshold demonstrated 55.8% sensitivity and 90.9% specificity for identifying LDH (Fig. 2).

Table 4

Multivariable logistic regression analysis of risk factors for lumbar disc herniation

Fig. 2

Results of receiver-operating characteristic curve analysis for maximal facet tropism as a predictor of lumbar disc herniation (LDH). The curve demonstrates the ability of maximal facet tropism to distinguish between athletes with and without LDH. The area under the curve was 0.727. The optimal cut-off point was identified to be 10.8°, which had a sensitivity of 55.8% and a specificity of 90.9% for identifying LDH.

Subgroup analysis based on the herniation level

A subgroup analysis of patients with single-level LDH was performed to determine whether the magnitude of a patient’s maximal FT was associated with the site of injury. The cohort included 15 patients with herniation exclusively at L4/5 and 16 with herniation only at L5/S1. A comparison of the patient-level maximal FT between these subgroups revealed no significant difference (mean 12.50°±5.60° for the L4/5 group vs. 10.47°±4.70° for the L5/S1 group, p=0.353) (Table 5, Fig. 3). Furthermore, when categorized by the 10.8° threshold, the proportion of patients with severe asymmetry was comparable between the two groups (53.3% in the L4/5 group vs. 43.8% in the L5/S1 group, p=0.724) (Table 5).

Table 5

Comparison of maximal facet tropism in patients with single-level herniation

Fig. 3

Box plot comparing maximal facet tropism in patients with single-level lumbar disc herniation. The distributions of maximal facet tropism are shown for the L4/5 group (n=15) and the L5/S1 group (n=16). The horizontal line within each box indicates the median, while the box itself represents the interquartile range. No significant difference was observed between the two groups (p=0.353, by Mann-Whitney U test).

Analysis of asymmetry and herniation characteristics

A subanalysis confirmed that the finding of a high maximal FT in the LDH group was robust and not dependent on the type of sport played, considering the lack of significant difference between athletes in rotational and nonrotational sports (p=0.125) (Supplement 3). Furthermore, in patients with unilateral herniation, no significant association was found between the side of the herniation and the side with the more sagittally oriented facet at either L4/5 (p=0.418) or L5/S1 (p=0.748) (Supplement 4).

Maximal FT as a holistic risk factor for the lower lumbar spine

This study’s key finding is that maximal FT across the lower lumbar spine is an independent predictor of LDH in athletes, with a proposed clinically relevant threshold of 10.8°. This shifts the clinical focus from tropism at a single, predetermined level to a more holistic view of the lower lumbar spine as a functional unit.

The present findings challenge the traditional view of tropism as a strictly local risk factor and suggest a mechanism of kinetic chain disruption. We hypothesize that severe asymmetry at the lumbosacral junction (L5/S1), where maximal FT was most prevalent, introduces rotational instability at the base of the lumbar spine [5,6]. A recent finite-element analysis demonstrated that structural compromise at L5/S1 (e.g., extensive foraminoplasty or resection) significantly altered lumbar biomechanics and increased stress distributions at adjacent levels [14]. Clinically, such altered lumbosacral kinematics showed significant association with symptomatic disk and facet degeneration at the cranial adjacent segment in young adults [15]. Therefore, L4/5 disk failure in these athletes may be the common sequela of chronic compensatory overload as a result of the underlying structural instability at the lumbosacral base. This model explains why maximal FT, representing the greatest point of instability in the kinetic chain, is a robust independent predictor of herniation, regardless of the specific level of failure.

Lumbosacral junction as a frequent origin of severe asymmetry

Despite the well-documented association between lumbar FT and disk herniation, the most significant spinal level for this relationship remains debated. For example, Degulmadi et al. [16] reported a significant association between tropism and herniation at the L4/5 level. In contrast, the present study demonstrated that severe FT was significantly more prevalent in the LDH group exclusively at L5/S1. This finding aligns with previous clinical data linking FT and LDH at this specific level [17], indicating that although a significant asymmetry at any lower lumbar level can increase risk, the lumbosacral junction is a particularly common site for such severe anomalies. The inherent biomechanical stresses at the transition from the mobile lumbar spine to the fixed pelvis, particularly the increased shear forces, likely contribute to L5/S1 being the most common source of the high maximal FT values observed in the LDH group [18,19]. Although the normally more coronal orientation of the L5/S1 facets helps resist this shear [20], severe tropism likely disrupts this protective architecture, leading to the development of a structural weak point.

Clinical implications for spine care in athletes

The clinical implications of identifying severe L5/S1 tropism are substantial, offering a pathway to a more proactive model of managing spinal health in athletes. Spine-related problems, particularly symptomatic LDH causing back and leg pains, are common in elite athletes and are often associated with high training volumes. The present findings can be integrated into this preventive framework, starting with screening. Quantitative assessment of FT during preparticipation imaging can identify athletes at higher risk for developing symptomatic LDH. For these asymptomatic cases, the focus can shift to primary prevention through education on safer movement patterns and targeted core muscle training to bolster stability [21,22]. For athletes already diagnosed with LDH, this finding serves as a critical prognostic marker. Concomitant severe FT is a critical prognostic marker, prompting clinicians to warn patients of a higher recurrence risk [23] and design individualized rehabilitation [22]. In surgical candidates, addressing the underlying instability warrants a discussion to improve long-term outcomes [17]. This stratified approach enables more efficient, personalized care to preserve spinal health in athletes.

Limitations and future directions

This study provides two key insights into the pathoanatomy of LDH in athletes: (1) maximal FT across the lower lumbar spine is an independent risk factor, and (2) severe asymmetry >10° is most often observed at L5/S1 in the LDH group. However, this study has several limitations. First, the most significant limitation is the small sample size of the control group (n=11). This factor limits the statistical power of our analyses and increases the risk of bias. However, detecting a significant difference (p=0.028) and high specificity (90.9%) despite this low power may suggest a substantial underlying effect size, and this finding requires urgent validation in larger cohorts. Second, the control group comprised symptomatic nonherniated athletes rather than truly asymptomatic individuals, reflecting real-world referral patterns in which asymptomatic athletes rarely undergo MRI. Although this design isolates herniation-specific morphological factors, it limits generalizability to asymptomatic populations. Third, our retrospective design limited our ability to adjust for all potential confounding factors. Specifically, detailed data on height, body mass index, race, and specific training loads or volumes were not consistently available. Notably, these unmeasured confounders may influence the LDH risk; therefore, the proposed 10.8° cutoff should be considered preliminary and requires validation in larger, more diverse athletic cohorts. Fourth, although no statistical difference was found between rotational and nonrotational sports, our classification was broad. Future studies with larger cohorts should analyze more granular, sport-specific biomechanical loading patterns to determine whether certain athletic activities amplify the risk posed by FT. Finally, this study was limited to radiologic parameters and did not evaluate clinical outcomes, such as the need for surgical intervention, recurrence rates, or return-to-play duration. Future prospective studies are warranted to establish the true clinical utility and prognostic value of this maximal FT threshold in guiding patient management.