Association of lateral femoral condyle index, notch width index, and risk of anterior cruciate ligament injury – A retrospective study

INTRODUCTION

Anterior cruciate ligament (ACL) rupture is a common knee injury encountered in young and active individuals and is influenced by a combination of mechanical and anatomical factors. Apart from trauma, several structural aspects of the knee joint have been implicated in increasing vulnerability to ACL injury. These include femoral intercondylar notch morphology,^[1] tibial plateau geometry, and tibial slope.^[2]

The biological mechanics of the knee during pivoting activities depend on the interaction between the distal femur and proximal tibia. The medial tibial plateau is relatively concave, which allows greater rotational movement in the lateral compartment.^[3-5] Variations in the arc of the femoral condyle may alter this motion and lead to increased rotational instability and strain on the ACL.^[6]

Magnetic resonance imaging (MRI) enables accurate assessment of both ligament integrity and bony morphology.^[7]Several MRI-based parameters, such as medial and lateral tibial slope (LTS), lateral tibial height (LTH), medial tibial depth, and intercondylar notch width, have been investigated as potential risk factors for ACL rupture. More recently, the lateral femoral condyle index (LFCI) has been depicted as a measure of femoral condylar sphericity and may influence knee kinematics during flexion-extension movements.^[8]

While numerous studies have explored these morphological factors in various populations, data from the Indian population remains sparse. The present study was therefore undertaken to evaluate the association of lateral femoral condyle sphericity, measured using LFCI, and intercondylar notch dimensions, assessed using the notch width index (NWI), with primary ACL injury. Through this, we hope to contribute to a better understanding of anatomical factors influencing ACL injury risk in this specific population.

MATERIALS AND METHODS

This retrospective observational study was performed at a tertiary care center between August 2019 and December 2021 for a duration of 2.5 years. Ethical approval for the study was obtained from the institutional ethics committee (MMC/EC-72/2019). A total of 164 MRI scans fulfilling the inclusion criteria were analyzed.

• Group 1: 82 patients with primary ACL rupture

• Group 2: 82 patients with isolated meniscal tear/cartilage injuries or anterior knee pain without ACL rupture.

Exclusion criteria

1. Patellofemoral dysplasia

2. Multi-ligamentous injury (medial collateral ligament, lateral collateral ligament, posterior cruciate ligament)

3. Osteoarthritis >Grade 2 and malalignment

4. Malunited distal femur fracture

5. ACL avulsion injuries.

MRI examinations were performed using a 1.5-Tesla scanner. MRI examinations were performed using a uMR570 1.5 Tesla MRI machine (United Imaging Healthcare), uExceed/CS (united compressed sensing) software platform, maximum gradient amplitude: ~45 mTm (each axis)-Maximum slew rate: ~200 T/m/s with image slice thickness of 3 mm.

The LFCI was measured on the mid-sagittal image of the lateral femoral condyle corresponding to the level of the popliteal groove identified on coronal images. Two circles representing the anterior extension surface and posterior flexion surface of the condyle were fitted to the subchondral bone contour [Figures 1 and 2]. The LFCI was calculated as the ratio of the diameter of the posterior (flexion) circle to the anterior (extension) circle. Measurements were repeated on adjacent parasagittal slices to ensure consistency. The NWI was calculated on coronal images at the level of the popliteal groove as the ratio of the intercondylar notch width to the bicondylar width of the distal femur [Figure 3]. Using this ratio, differences in magnification that could arise from non-standard radiography techniques are eliminated.^[8]

$LFCI=\frac{\text{Diameter of Flexion circle (A)}}{\text{Diameter of Extension circle (B)}}$

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Figure 1: Two circles representing the anterior extension surface (green circle) and posterior flexion surface (yellow circle) were fitted to the subchondral bone contour. The level of popliteal groove in coronal image (vertical yellow line). Diameter of posterior yellow (flexion) circle (red line).

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Figure 2: The lateral femoral condyle index was calculated as the ratio of the diameter of the posterior (flexion -yellow) circle to the anterior (extension -green) circle. The level of popliteal groove in coronal image (vertical yellow line). Diameter of anterior green (extension) circle (red line).

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Figure 3: The notch width index was calculated on coronal images at the level of the popliteal groove as the ratio of the intercondylar notch width (line-b) to the bicondylar width of the distal femur (line-a).

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All measurements were independently performed by 2 blinded observers. To ensure consistency, measurements were repeated on adjacent parasagittal slices.

Interobserver reliability was assessed using the intraclass correlation coefficient (ICC) with a 2-way random effects model and absolute agreement.

RESULTS

A total of 164 patients who met the inclusion criteria were included in this study. The age range was 18–60 years, with a median age of 37.5 years. The mean LFCI in patients with ACL injuries was found to be 0.70 and in the control group was found to be 0.752. There was a significant difference in LFCI between the case group and the control group [Table 1].ROC AUC = 0.75. Cut off <0.745 predicted ACL rupture with 81% sensitivity and 65% specificity [Table 2].

A post hoc power analysis demonstrated that the sample size provided >80% power to detect a statistically significant difference in LFCI between groups at α = 0.05.

LFCI measurement demonstrated excellent interobserver reliability with an ICC of 0.93.

The mean NWI in patients with ACL injuries was found to be 0.237 and in control group was found to be 0.229. There was no significant difference in NWI between the knees with ACL ruptures and the control group [Table 1].

DISCUSSION

Distal femoral morphology influences knee kinematics and rotational stability. Reduced LFCI represents a prominent anterior condyle relative to posterior flexion radius, allowing increased translation of the lateral femoral condyle over the convex lateral tibial plateau, predisposing to ACL injury.

Fernandes et al. demonstrated that a longer flattened part of the lateral femoral condyle, as compared with the tibial plateau, is associated with ACL injury in conventional radiography.^[9] According to Pfeiffer et al., an increase in the lateral femoral posterior condyle produces a more elliptical and in-equidistant lateral femoral condyle, which may change tibiofemoral relative motion and alter knee kinematics and loading mechanics.^[10] This is one possible mechanism by which lateral femoral condyle morphology affects rotational stability of the knee.^[10]

In contrast, a decreased LFCI indicates a more prominent anterior condyle in relation to the flexion radius, whereas a greater LFCI characterizes a more congruent anterior radius in relation to the posterior flexion radius. The flattened anterior portion of the condyle may glide more excessively over the convex lateral tibial plateau as a result of this asymmetry, which could lead to a larger pivoting mechanism. Variable anatomic femoral ACL insertion sites, which are probably connected to the distal femur’s bone morphology, could also be a factor in a lower LFCI in patients with ACL re-ruptures or ruptures. Graft misplacement or greater graft tension could result from this. Individuals who have elevated LTH and LTS, along with a decreased LFCI, are more vulnerable to an ACL rupture or re-rupture. This lends credence to the theory that patients with ACL injuries pivot more frequently because of the lateral compartment’s tibial and femoral bone morphology. The most indicative of ACL damage was a combination of a lower LFCI and higher LTH and LTS. This highlights how crucial the lateral compartment’s bone morphology is to understanding the ACL injury mechanisms. This could make it easier to identify patients with significant rotatory knee laxity who might benefit from more precise risk assessment and evaluation of non-operative treatment, as well as from an additional extracapsular reconstruction.

According to Hodel et.al.’s findings, a lower LFCI is linked to ACL injuries. These findings could help surgeons identify patients who are at risk for ACL injuries and advise them about their increased risk of re-rupture following reconstruction.^[5] In 2021, Nowak et al. came to the conclusion that LFCI is not associated with an increased risk of primary non-contact ACL injury.^[6] From our study, we found that decreased LFCI is a significant risk factor for ACL injuries. Hence, surgeons should assess the pre-operative MRI and add additional procedures like lateral extra-articular tenodesis (LET) along with ACL reconstruction, and also advise the patients regarding possible re-rupture of the graft. Hence, decreased LFCI is a confirmatory finding in MRI to decide on adding the LET procedure.

Van Eck et al. categorized the intercondylar notch’s shape as Type A, Type U, and Type M. Type A was defined as an acute triangle-shaped, narrow notch. The apex of Type U was wider than Type A’s. In contrast to the other two varieties, Type M was wider and possessed two apices.^[1] According to research by Al-Saeed et al., having a Type A femoral notch increased the probability of an ACL injury, whereas having a small femoral NWI did not.^[7]

The NWI was developed by Souryal et.al., when they observed that athletes with bilateral ACL injuries had significantly narrower intercondylar notches than controls. The NWI is calculated as the ratio between the intercondylar notch width and the width of the distal femur. NWI values ≤0.20 were used to define critical notch stenosis. A notch width of ≤15 mm was identified as the critical absolute notch width.^[8] According to LaPrade and Burnett’s theory, a stenotic notch and a subsequently smaller ACL led to higher stress concentration and ACL-notch impingement, which in turn raised the risk of ACL rupture. “The notch width itself, or the shape of the notch, is not responsible for the ACL tear,” according to prospective research by Shelbourne et.al. “A narrow intercondylar notch simply reflects the smaller ACL it houses’’.^[11] Houseworth et.al after using computed tomography scan, came to the conclusion that a small posterior notch may put a person at risk for an ACL injury.^[12] Anterior outlet notch stenosis has been shown by Anderson et.al to raise the risk of ACL injury.^[13] Tanzer and Lenczner have also shown that, even in non-stenotic knees, any ligament graft with a diameter of more than or equal to 8 mm will impinge on the inferomedial border of the lateral femoral condyle.^[14] On the basis of our findings and the results of the studies noted above, we recommend that notchplasty should be considered during ACL reconstructions in athletes with stenotic notches to reduce the risk of notch impingement and potential deterioration of the ligament reconstruction.^[15,16] From our study, we found that there was no significant difference in NWI between the case group and the control group. The findings do not support NWI as an independent predictor of ACL injury; however, decisions regarding notch plasty should remain based on intraoperative assessment.

Injury to the ACL and the tibial slope have been linked in certain research; the results were conflicting and uneven. From a biomechanical perspective, a steeper tibial slope might produce more translational force, which might cause the tibia to move more anteriorly. It is well recognized that the ACL’s primary function is to limit anterior tibial mobility. Consequently, an elevated tibial slope may result in elevated ACL strain or possibly ACL rupture.^[17]

Patients with significant rotatory knee laxity who might benefit from an additional extracapsular procedure might be identified with the use of LFCI. Individuals who have certain risk factors related to bone morphology may be more vulnerable to non-operative therapy failure. More accurate risk stratification can support the treating surgeon in prophylactic measures, patient counseling, surgical technique modifications, and post-operative rehabilitative protocol adaptation.

The limitation of the study was that it was a retrospective observational study, so prone to bias, and also, we have not taken tibial parameters such as tibial slope and tibial depth into consideration. The control group consisted of symptomatic patients rather than completely asymptomatic individuals because obtaining MRI scans in normal volunteers is ethically and practically difficult. However,patients with ligament injuries, osteoarthritis, and malalignment were excluded to minimize confounding factors and create a near-normal comparison cohort.

As the measurement is based on MRI, it is difficult to use LFCI as a screening tool for possible ACL tears due to economic reasons. However, the excellent interobserver reliability observed in this study confirms that MRI-based measurement of LFCI is reproducible and can be reliably used in clinical assessment.

CONCLUSION

A decreased LFCI is associated with ACL injury and NWI was not an independent risk factor for ACL tear. These findings might aid clinicians in identifying patients at risk for an ACL injury using MRI, and at the same time, this also helps to formulate ACL reconstruction strategies and reduce the risk of re-rupture after ACL reconstruction. From the study, it can be concluded that a decreased LFCI was found to be significantly associated with ACL injury, whereas the NWI was not identified as an independent risk factor for ACL rupture. These findings suggest that the LFCI may serve as a useful radiological marker for identifying patients at higher risk of ACL injury. In addition, this information could be valuable in guiding clinical decision-making for ACL reconstruction and potentially reducing the risk of re-rupture following surgical intervention. Further studies are needed to validate these findings and explore the clinical implications in a larger cohort.