Femoral notch and proximal tibial morphology and their relationship with an anterior cruciate ligament injury in the Indian population: A magnetic resonance imaging-based study

INTRODUCTION

The anterior cruciate ligament (ACL) is a key stabilizer of the knee joint which opposes anterior tibial translation and internal rotation. ACL injuries are most common ligament injuries in the knee joint and cause significant disability in both the athletic and the general population. These injuries have also been known to predispose to early knee joint osteoarthritis in spite of reconstructive surgery.^[1-3] For this reason, considerable literature and research have focused on determining the several possible risk factors that predispose a patients toward an ACL injury.^[4-6]

Research has revealed that ACL injuries have numerous possible risk factors ranging from the nature of sports, limb alignment, hormonal variation, neuromuscular variations, and anatomic variations in knee joint morphology.^[7,8] The aim of such research is to identify high-risk patients to develop preventative strategies reducing the occurrence or recurrence of an ACL injury. Our study focuses on the measurements of the femoral and tibial bony morphology in ACL injured patients in the Indian population to identify any anatomic bony variables and its variations with published literature.

MATERIALS AND METHODS

With ethical committee clearance, we performed a prospective case–control study to assess the bony knee morphology of patients who sustained an ACL injury. The inclusion criteria of our study were male patients between the ages of 20 and 50 years old who sustained non-contact ACL injury with a body mass index under 25 kg/m² proven by magnetic resonance imaging (MRI). All subjects in the study were not sports persons and participated only in occasional recreational activities. We included patients who presented to our center between October 2015 and December 2016. The bony measurements of these patients were compared against 40 control MRI scans. Control MRI scans were those performed in patients with knee joint pathologies not associated with a ligamentous injury such as meniscal lesions, osteochondral lesions, or injuries which on MRI were not detected. Patients with contact injuries, mucoid degeneration of the ACL, or osteotomies around the knee were not excluded from the study.

Notch measurement

The notch morphological measurements were performed, as described by Stein et al.^[10] [Figure 1] illustrates the methodology we used for measuring the femoral notch morphology.

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Figure 1:

Illustrating the method used to measure (a) notch depth, (b) notch width, (c) notch.

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1. Notch depth was measured between the highest point of the notch apex to the posterior bicondylar line of the distal femur

2. Notch width was determined between the medial projection of lateral condyle and lateral projection of medial condyle at two-third of the notch depth

3. The notch angle was determined in the axial sequence by a line drawn from the superior most aspect of the notch to the inferior most aspect of the notch on the medial and lateral condyle.

Measurement of posterior tibial slopes

We developed our own technique for measuring the posterior tibial slope on MRI as determining the long axis of the limb was not possible in the regular MRI sequence window, which is possible in a computer tomography (CT) scan. To overcome this, we performed our measurements in the following sequence. In the axial view, the sequence demonstrating the largest femoral popliteal groove was utilized as the scout image, next the corresponding coronal MRI image was used to define the midpoint of the medial and lateral articular surface. This was measured from the medial most point of the medial femoral condyle to the medial most part of the medial tibial spine and similarly in the lateral compartment, the lateral most aspect of the lateral tibial plateau to the lateral most point of the lateral tibial spine. Next, the corresponding sagittal image of both the mid articulating points of the medial and lateral compartments was used to measure the posterior tibial slope. Next, the posterior tibial slope was determined by drawing a vertical line over the anterior most part of the tibial cortex and then a second line connecting the peak anterior and posterior points on the tibial plateau in both the medial and lateral compartments. The angle formed between the two lines was determined and recorded is the tibial slope. [Figure 2] illustrates the methodology we used for determining the posterior tibial slope.

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Figure 2:

Illustrating the method used to measure (a) posterior tibial slope in the lateral compartment (b) posterior tibial slope in the medial compartment.

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RESULTS

A total of 80 male patients (40 ACL injury and 40 nonACL injury) MRI scans were analyzed and the notch morphology and tibial slopes measured in each case. The mean age of ACL injured group was 32.98 (range 20–50) years and the non-ACL injured group was 34.23 (range 20–49) years [Table 1].

Notch shape

There were 35 Type A, 33 Type U, and 12 Type W notches in the total 80 studied patients [Figure 3]. Out of 40 patients in ACL injured group, 19 patients had a type A notch, 15 a type U, and 6 a Type W. There was a higher incidence of type U notches in the control group of patients when compared to the ACL injury group.

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Figure 3:

Graph demonstrating the number of each notch shape included in our study.

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DISCUSSION

As ACL injuries are common injuries causing considerable disability with a high socioeconomic burden in all parts of the population,^[11] it is well worth studying the risk factors that predispose patients to an ACL injury. Multiple risk factors have been reported, for example, female sex, ligament laxity, hormonal changes, and even specified sporting activities.^[7,12] Arthroscopy and ACL reconstructive techniques have improved significantly overtime with various implants and techniques being developed for an improved ligament reconstruction, however, due bone morphology some patients result in retears of the reconstructed graft.^[13] Therefore, it is important to note the bony morphologies that predispose to a primary ACL injury as well as more importantly identifying patients who are susceptible for another tear which would make our observations more clinically applicable.

In our study, we noted that the mean femoral notch width in the ACL injury group was smaller than that of the non-ACL injury group and the difference between the two groups was statistically significant. This indicates that narrower notch width could be the principal risk factor for an ACL injury. This is a well-studied parameter and our results are comparable with published literature.^[14-17] It is thought that a narrower notch results in greater degrees of impingement of the ACL along with a higher incidence of a mucoid hypertrophy. Both the notch width and depth did not correlate a higher incidence of ACL injury. Literature has reported that a notch angle <50° was a significant risk factor for ACL injury.^[18,19] We did not note similar findings and found that despite measuring a lower mean notch angle in the ACL injury group, it was not significantly reduced when compared to the non-injured group. Studies have classified femoral notches into different shapes. These include Type A, U and W of which Type A was found to be the most common type of femoral notch morphology. Reports suggested that Type A notches had a higher incidence of ACL injury, however, in our study results, we did not a statistically significant incidence of Type A notches in the ACL injury group.^[20] Notch morphology and shape have also been thought to affect surgical technique where visualization of the lateral aspect notch through a standard anteromedial arthroscopy portal may be difficult in tapering stenotic notches as seen in Type A.^[21]

Posterior tibial slope angles have also been considered a significant risk factor for ACL injury, especially during compressive axial loading, where it results in increased anterior tibial translation.^[22] More recently, studies have evaluated medial and lateral posterior tibial slopes to determine which of the two contributes to increase ACL injury predisposition. Our results suggested that there was no significant relationship between the medial and lateral posterior tibial slope and ACL injury. Our findings are in agreement with other literature^[16,23] but also contrary to a few studies that have shown significance in the medial posterior tibial slope and others in the lateral posterior tibial slope and ACL injury.^[24-26] Since the results of posterior tibial slope angle and ACL injury are not uniform, it is worth considering the variations in measurement techniques in each study. Defining the midpoint of the medial and lateral compartment is challenging as well as determining which MRI sequence to use. Standardization techniques using fixed bony landmarks to define image sequences or sections for subsequent measurements are used and therefore there is a vast amount of possible interobserver variability. This could be the reason for many different study outcomes. There is enough biomechanical evidence and sound theory in that an increased posterior tibial slope would allow for further anterior tibial translation adding more stress on the ACL.^[27,28]

The importance of these measurements in patients with an ACL injury is the increased risk factors predisposing the patient for a reinjury of the ACL. It also gives an indication to the surgeon and patient for the chance of a contralateral knee ACL injury in view of similar bony knee morphology. MRI is also the preferred radiographic investigation for knee pathologies therefore using MRI for measurement reduces the need for an additional investigation therefore more economical.

We do recognize that our study has limitations, and this may affect our findings. We only had one observer perform measurements and our sample size was limited. In addition, the use of CT scan would have allowed for the accurate determination of the long axis of the tibia, however. We wanted to avoid the additional cost and radiation to the patient.

CONCLUSION

Femoral notch width is the most significant risk factor for ACL injury in the Indian population. Though our results did not show increased proximal tibial slope angles to be a significant risk factor, we did note higher slope angles in the ACL injury group and an increased sample size may have resulted in a significant finding.