**3. Discussion**

### **3.1 Intrinsic factors**

#### *3.1.1 Anatomical factors*

The anatomic factors have been studied thoroughly these days as these factors could be picked up easily by investigations pertaining to the diagnosis of ACL injuries like knee radiographs and MRI. Establishing correlation with these anatomic factors could help us in establishing a reliable tool in screening individuals at risk of ACL injury. Variations in these anatomical features are well documented in individuals especially between men and women which may explain why women are maybe at more risk of ACL injury than men. Various anatomic factors commonly studied include those related to knee geometry, alignment of the lower extremity, knee laxity, and body mass index [12, 13]. Factors related to knee geometry can be divided into a) tibial parameters like posterior tibial slope, depth of medial tibial plateau b) femoral/ notch parameters like notch width, bicondylar width, notch width index.

#### *3.1.1.1 Notch parameters*

Notch parameters have been one of the widely researched risk factors related to an ACL injury but there are wide differences in the way these parameters are measured. The femoral attachment of ACL is in the medial aspect of lateral femoral condyle over lateral intercondylar ridge [14, 15]. It is observed that impinging of ACL at various knee positions are well documented especially in those with narrow notch width. But controversy exists whether it's due to geometry and size of notch itself, or due to volume of ACL or amalgamation of these characteristics [16]. It's proven that women are known to have smaller notches when compared to males and that the people with smaller notches tend to have thinner and weaker ACL compared to those with wider notch [17–19]. The stenotic notches cause ACL impingement over the lateral femoral condyle and if the knee is subjected to any anterior shear force or tibial rotational forces causing rupture of ACL ligament [20–23].

LaPrade et all demonstrated that there is an increased risk of ACL injury in people with a narrow intercondylar notch [24, 25]. Souryal et all in their study on

#### *Risk Factors of ACL Injury DOI: http://dx.doi.org/10.5772/intechopen.99952*

bilateral ACL injuries found narrow notch width to be associated but they used only the plain radiographs to calculate the Notch width. The major drawback of these studies was that they were done on plain radiographs where errors due to magnification were common. A notch width index was thus used to overcome these issues. Notch width index (NWI) can be measured using tunnel view radiographs or Coronal section MRI. The NWI was identified as the ratio of the width of the intercondylar notch to the width of the distal femur at the level of the popliteal groove. Souryal et al. used a tunnel view of radiographs and compared patients with unilateral and bilateral ACL injuries and found that patient with bilateral ACL ruptures have significantly smaller notch and NWI when compared to those with unilateral ACL ruptures. Moreover, they found no difference in NWI of unilateral ACL ruptures when compared to those with normal knees. Furthermore, they conducted a cohort study and reported that men athlete had higher NWI compared to female athletes who suffered ACL injuries when compared to those uninjured ones [26, 27]. S.M. Fahim et all in their randomised control study used MRI for the calculation for NWI, reported that there were no differences in NWI in either sex and the NWI was significantly lower in those with ACL injury [28]. Ashwini et al., in their MRI based comparative study, demonstrated that people with ACL injury had NWI of 0.29 ± 0.02 or lesser compared to those without an ACL injury, whereas Bhasukala et al., postulated the cutoff of NWI to be 0.28 ± 0.06.21. S.M. Fahim Et all reported cut off value of 0.29 with a sensitivity of 90% and specificity of 86.7%. However, Gormeli CA et al. In their study on bilateral knees with ACL injury demonstrated cut-off values of NWI of 0.22 ± 0.008 in bilateral injured knees and 0.24 ± 0.01 in the unilateral injured knee [29–31]. So the cut off varies depending on the ethnicity and the exact cutoff values are still controversial but most authors have concluded the critical value NWI ranging from 0.19 to 0.26.

The shape of the intercondylar notch is another parameter frequently studied with regards to the notch shape that is most associated with an ACL injury [32–34]. We could use a semi-quantitative approach to classify the various intercondylar notch shapes. It is classified into three shapes i.e. A, Inverse U, and Omega (U). Two parameters are calculated to ascertain the notch shape viz. notch width at the level of the popliteal groove(NWP) and notch width at the joint line(NWJ). If NWP equal to or near equal to NWJ, then the notch is of "U" shape. If NWP < NWJ then the notch is of "A" shape.If the NWP > NWJ then the notch shape is of Omega shape. 'A' shaped femoral notch is commonly associated with ACL injury whereas an inverted U shaped notch is more favourable to prevent ACL injury [29, 30, 35].

Overall, most authors found a positive relationship with an ACL injury and narrow notch and smaller NWI increased the risk of ACL injury I.e as intercondylar notch width decreases, an increase in ACL injury risk is observed.

### *3.1.1.2 Posterior tibial slope*

Several studies related to proven multiple times that the Posterior tibial slope could be the major factor contributing to the stability of the knee joint [35, 36]. Cadaveric studies have shown that an increased posterior tibial slope has resulted in an increased anterior translation of tibia over the femur, thus increasing stresses on the ACL ligament especially during active gait [37, 38]. The posterior tibial slope is traditionally measured on lateral knee radiograph, the tibial axis is to be drawn connecting midpoints of lines connecting anterior and posterior cortex 4-5 cm apart as caudally as possible from the joint line. The angle between tangent connecting anterior and posterior cortex at joint and line perpendicular to the tibial axis corresponds posterior tibial slope in lateral knee radiograph. But again controversies regarding errors due to magnification or lack of true lateral radiographs and

inability to measure lateral posterior tibial slope(LPTS) and medial Posterior tibial slope(MPTS) have resulted in calculating these parameters in an MRI. To measure the medial and lateral posterior tibial slope on MRI first is to determine the tibial axis. For this, a sagittal section at the centre of the knee which contains intercondylar eminence and PCL attachment is selected. Two lines joining anterior and posterior cortices was drawn approximately 4-5 cm apart as caudally as possible from the joint line, the line joining the midpoint of the above two lines corresponds to the tibial axis. (**Figure 1**) Now the sagittal section of the centre of the lateral tibial plateau is identified, a tangent connecting the anterosuperior cortex to the posteroinferior cortex is drawn. The angle between the above tangent and the line perpendicular to the tibial axis corresponds to the medial posterior tibial slope. A similar technique is to be used to measure Lateral posterior tibial slope by drawing tangent at the centre of the lateral tibial plateau (**Figures 2** and **3**). A detailed view of MRI have found lateral posterior tibial slope(LPTS) to be more involved with ACL injury than medial Posterior tibial slope(MPTS) [39] but a meta-analysis has shown that ACL injury was associated with

**Figure 1.** *MRI picture depicting calculation of Tibial Axis.*

## *Risk Factors of ACL Injury DOI: http://dx.doi.org/10.5772/intechopen.99952*

medial and lateral PTS [40]. Hashemi et all [41], in their study of 104 patients, both MTPS and LTPS were increased, injured group. Though females had higher LTPS than males they concluded that increased lateral posterior-directed tibial plateau slope is one of the major risk factor irrespective of gender. Whereas MPTS was significantly increased only in men. Stijak et all, in their utilised both knee radiographs and the MRI, to determine the correlation between PTS and lateral plateau and concluded that increased lateral PTS was seen among the ACL injury cohort compared to the ACL intact cohort but they found increased MPTS in the uninjured group when compared to the injured cohort. They suggested that an increase in LTPS would influence the rotation of the knee joint causing pivoting of the knee. In our study, Both MPTS and LTPS had a positive correlation with ACL injury they were significantly increased in the ACL tear group when compared to uninjured group irrespective of the gender. Females had higher MPTS and LTPS when compared to males in both groups but the difference was statistically significant in either sex.

**Figure 2.** *MRI picture depicting calculation of medial posterior Tibial slope(MPTS).*

**Figure 3.** *MRI picture depicting calculation of lateral posterior Tibial slope(MPTS).*
