The most important finding of this study was that the lateral height of the tibial spine was greater in ACL-injured patients than in ACL-intact controls. In patients with ACL injury, a significant difference existed in the tibial spine height between males and females. In males, the LTSH/TPW was a risk factor for ACL injury: 1% increase in LTSH/TPW was associated with a 1.7-fold increase in the risk of ACL injury.
ACL injury is multifactorial and the anatomic morphology of proximal tibia is of great concern. Numerous studies revealed that larger PTS, shallower medial plateau depth, and smaller medial tibial spine volume played important roles in ACL injury [11–17]. Sturnick et al. [15] reported that the decrease of medial tibial spine volume was associated with an increased risk of ACL injury and such correlation was only found in men. The anatomic morphology of the tibial spine to some extent assists the forward shift of the tibia relative to the femur, increasing the tension loaded on ACL until ACL ruptures [19].
Previous studies have shown that men tend to have larger bony structures than women [26]. In our study, TPW was greater in males than in females in patients with ACL injury and was smaller in females with ACL injury than females with intact ACL. Un-normalized parameters could result in unreliable results due to not excluding the negative influence of individual differences. As a result, TPW was used to normalize these length parameters in this study. We reasoned that these ratios could better and more reliably reflect the morphological characteristics of the tibial spine when the individual differences were significant.
In a study by Iriuchishima et al. [22], the TSW was 12.5 ± 1.9 mm in the study group and 13.9 ± 2.1 mm in the control group, while the TSW in our study was 10.6 ± 1.7 mm and 12.7 ± 2.2 mm, respectively, which may be attributed to the individual differences. Our study, however, obtained the same result—the TSW was smaller in patients with ACL injury than in healthy individuals—as Iriuchishima et al. reported. Besides, this study also found that the TSW/TPW in ACL tear group was significantly reduced and considered as a risk factor for ACL injury: 1% increase in TSW/TPW was associated with a 1.5-fold increase in the risk of ACL injury.
In accordance with the results of previous studies [14, 15, 22], the LTSH and MTSH were found to have no significant difference between patients with ACL injury and patients with intact ACL. However, after normalized by TPW in this study, the LTSH was significantly higher in patients with ACL injury than in normal subjects, which may be caused by the influence of individual body size. This study also found that a 1% increase in LTSH /TPW was associated with a 1.3-fold increase in the risk of ACL injury, indicating that LTSH could contribute to ACL injury. Previous research ignored the individual difference between groups, resulting in not considering the LTSH as a risk factor.
Previous studies have shown that the risk of initial ACL injury varies from male to female [14, 15, 19, 27–29]. Decreased width of femoral intercondylar notch, decreased height of media posterior meniscus [14], increased posterior slope of the articular cartilage in the middle portion of the lateral plateau, and decreased height of the posterior angle of the lateral meniscus were only associated with an increased risk of ACL injury in females [27, 28]. On the other hand, Sturnick et al. [15] suggested that decreased medial tibial spine volume was only associated with ACL injury in males and concluded that the greater variation between individuals was one of the possible reasons for this difference.
In our study, the differences between men and women in terms of tibial spine morphologies were found, making them at different risks for ACL injury. The results of this study suggested that the LTSH/TPW and MTSH/TPW were greater in males than in females in the study group. Furthermore, the two ratios were greater in males with ACL injury than in males with intact ACL and were considered risk factors for male ACL injury: a 1% increase in LTSH /TPW and MTSH/TPW was associated with a 1.7-fold and 1.5-fold increase in the risk of ACL injury, respectively. While such a relationship was not statistically significant in females. Although the females in the study group had a slightly smaller MTSH/TPW, compared with the females in the control group, this may be due to the inclusion of the patients. Likewise, the results of ROC analysis found that the LTSH /TPW had a fair diagnostic capacity for male ACL injury (AUC = 0.78), while it was not corelated with ACL injury in females.
From the kinematic point of view, the tibial spine is closely related to femoral intercondylar notch during knee flexion and extension [13]. Li et al. [30] applied the computer model based on double orthogonal fluorescence and magnetic resonance imaging to evaluate the kinematics of knee joints and found that during knee flexion, the contact points between the femoral condyle and tibia were located near the tibial spine. Histologically, ACL is attached closed to the intercondylar spine of the tibia [20, 21]. We reasoned that increased LTSH could contribute to an impingement of the ACL and grafts with tibial spine before and after ACL reconstruction, accelerating ACL injury and grafts failure, respectively. A portion of the lateral tibial spine should be removed in some cases to alleviate such impingement during arthroscopy surgery. The influence of the increased lateral tibial spine height in patients with ACL, male patients in particular, on the impingement between the grafts and lateral tibial spine warrants further investigation.
Identifying risk factors for ACL injury is to reduce its incidence, promote improvements in surgical techniques, and help patients with individualized rehabilitation programs [14, 31]. This study provides the characteristics of the tibial spine morphologies by introducing the normalized height and width of tibial spine, which are easy to get by a simple measurement on radiographs clinically. Besides, the significant correlation between the normalized height of tibial spine and male ACL injury suggests that orthopedic surgeons should pay more attention to the height of tibial spine in males regarding ACL injury, especially during arthroscopy surgery.
The clinical relevance of this study lies in findings that the greater lateral height of the tibial spine in patients with ACL injuries was found, especially in male subjects. This to some extent resolves the controversy in the literature regarding the height of the tibial spine in patients with ACL injury, and found the differences between men and women. These results also remind the orthopedic surgeon of the concerns during the reconstruction of ACL: more attention should be paid to the height of lateral tibial spine, male patients in particular, to avoid the impingement between the grafts and lateral tibial spine. It warrants further investigation.
Our study had some limitations. First, all measurements were only performed on radiographs and whether data by computerized tomography images or magnetic resonance imaging could draw the same conclusion warranted further investigation. Second, Men's exercise intensity is higher than that of women, which is also one of the important reasons for a high risk of ACL injury. This study, however, did not collect and analyze relevant data. Third, the subjects in the control group of this study were not completely healthy individuals, but, they did not have ACL injury. Forth, femoral condyle morphology and other tibial anatomical parameters were not included in this study.