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Abstract

Background:

There are several known risk factors for ACL injury including knee valgus moment, smaller notch width index (NWI), female sex, increased tibial slope, and meniscal deficiency. Among these, the NWI has typically been determined using either coronal or axial magnetic resonance imaging (MRI) cuts; however, it is unknown whether these measurements differ in each plane.

Purpose:

To compare axial versus coronal NWI measurements in ACL-injured patients to establish a benchmark for calculation.

Study Design:

Cross-sectional study; Level of evidence, 3.

Methods:

Patients ≥14 years of age with a confirmed ACL injury on MRI were included for review. Two orthopaedic resident physicians measured the NWI using both axial and coronal cuts using previously described measurement criteria as well as a modification utilizing the MRI cuts with the widest bicondylar width. The NWI was computed and a t test was performed to compare the axial and coronal measurements. Interobserver reliability was assessed using 2-way mixed intraclass correlation coefficients (ICCs).

Results:

A total of 64 patients with ACL injuries were included. Modified axial and coronal NWIs for the cohort were a mean ± SD of 0.27 ± 0.02 and 0.23 ± 0.02, respectively. Modified axial NWIs were greater than coronal NWIs (P < .001). Standard axial and coronal NWIs for the cohort were 0.29 ± 0.03 and 0.25 ± 0.03, respectively. Standard axial NWIs were greater than coronal NWIs (P < .001). Modified axial and coronal NWIs for male and female patients were not different (P = .61 and P = .14, respectively). Axial notch width measurements were greater than coronal notch widths for both modified and standard measurements (P < .001). Axial intercondylar distances were not different from coronal intercondylar distances for both standard and modified measurements (P = .06 and P = .22, respectively). The ICCs for modified coronal and axial NWIs were 0.88 and 0.97, respectively. The ICCs for standard coronal and axial NWIs were 0.99 and 0.99, respectively.

Conclusion:

NWIs were greater when measured on an axial MRI compared with the coronal plane for both measurement methods. This was because notch widths were measured wider in this plane since intercondylar distances were not statistically different.

Keywords

notch width index notchplasty MRI ACL injury

Anterior cruciate ligament (ACL) tears are the most commonly reported ligamentous knee injuries.^{3,6,7,10,12,14,19} Several contributory extrinsic (modifiable) and intrinsic (nonmodifiable) risk factors have been identified, including a narrow intercondylar notch, poor tibiofemoral congruity, increased tibial slope, smaller ACL and tibial eminence size, and female sex.^{2,3,9
-11,14,15,19} Among these, intercondylar notch parameters have been studied to determine if certain populations are more predisposed to ACL injury than others. These parameters have been reported using a variety of measurements to characterize notch size. Calculation of one such measurement, the notch width index (NWI), is relevant for patients undergoing ACL reconstruction (ACLR) to guide operative technique.^2,18 Notch width may influence intraoperative decision making such as graft choice, double- versus single-bundle grafts, graft size, and whether to perform a notchplasty.¹⁸

The NWI is defined as a ratio between the widths of the intercondylar notch and distal femur (when measured at the level of the popliteal groove).^{3,6
,10,12,15} Though originally described for use on plain radiographs, it is now commonly measured on either coronal or axial magnetic resonance imaging (MRI) cuts.^3,4,12 Some studies have shown that both axial and coronal NWIs are statistically smaller in ACL-injured patients when compared with controls.^{2,3,6
,9
-12,14,15,19} However, no comparative assessment has examined whether possible differences exist when measurements are done in each plane. This lack of standardization in technique may be contributory to the discrepancies regarding NWI thresholds and risk of ACL injury seen throughout the literature. Establishment of standard thresholds and understanding of measurement discrepancies may assist surgeons with operative planning as well as patient counseling on risk factors for injury and reinjury.

In this study, we sought to examine the equivalence of axial versus coronal MRI measurements of NWI. Findings would aid surgeons in identifying risk factors for ACL injury with greater precision. We hypothesized that the NWI measured on an axial MRI would be greater than if determined using a coronal cut in ACL-injured patients.

Methods

Following institutional review board approval, we retrospectively reviewed the MRIs of patients with ACL injuries sustained from both contact and noncontact mechanisms. Patients ≥14 years of age with no age limit with a confirmed ACL injury on MRI were included. Patients without an ACL injury, previous surgery on the injured knee, and patients <14 years old were excluded. Patients with ACL injuries were chosen as the cohort because of the established association between NWI and ACL injury risk; thus, it was of particular interest to analyze differences in this patient population.

Two orthopaedic resident physicians (J.M. and R.D.) measured the NWI on T2-weighted MRI without contrast using both axial and coronal cuts. Calculations were done following the methodology reported by Raja et al¹² as well as a modification unique to this study. For simplicity, these measurements will be referred to as standard and modified. For the modified Raja et al measurement, the intercondylar line was measured at the greatest width from medial to lateral condyles parallel to the posterior condylar reference line on the axial cuts and parallel to the distal condylar reference line on the coronal cuts at the most distal portion of the notch on the coronal cuts and most posterior portion on the axial cuts. This was performed on the MRI cut with the greatest bicondylar width. The notch width was then measured as the line drawn along the largest intercondylar width at points of the notch parallel to the posterior condylar reference line on the axial cuts and parallel to the distal condylar reference line on the coronal cuts. These measurements were obtained on the same MRI slice in both coronal and axial MRI cuts and can be seen in Figures 1 and 2, respectively. The standard Raja et al measurement was performed by locating the MRI cut at the level of the popliteal hiatus and then measuring the bicondylar width from medial to lateral condyle parallel to the posterior condylar reference line on the axial cuts and parallel to the distal condylar reference line on the coronal cuts and at the most distal portion of the notch on the coronal cut and most posterior portion of the notch on the axial cut.¹² The notch width was then measured as the line along the intercondylar line at points in the notch parallel to the posterior condylar reference line on the axial cuts and parallel to the distal condylar reference line on the coronal cuts. These measurements were obtained on the same MRI slice in both coronal and axial MRI cuts and can be seen in Figures 3 and 4, respectively.

Figure 1.

A coronal magnetic resonance imaging cut demonstrating the measurements made for the modified notch width index calculation. The purple line represents the distal condylar reference line. The red line represents the intercondylar distance drawn parallel to the distal condylar reference line at the widest point of the femoral condyles and most inferior point of the intercondylar notch. The yellow line represents the intercondylar notch width at the most inferior point of the notch and collinear with the femoral intercondylar distance line.

Figure 2.

An axial magnetic resonance imaging cut demonstrating the measurements made for modified notch width index calculation. The purple line represents the posterior condylar reference line. The red line represents the intercondylar distance drawn parallel to the posterior condylar reference line at the widest point of the femoral condyles and most posterior point of the intercondylar notch. The yellow line represents the intercondylar notch width at the most posterior point of the notch and collinear with the femoral intercondylar distance line.

Figure 3.

A coronal magnetic resonance imaging cut demonstrating the measurements made for the standard notch width index calculation. The purple line represents the distal condylar reference line. The red line represents the intercondylar distance drawn parallel to the distal condylar reference line at the level of the popliteal hiatus and most inferior point of the intercondylar notch. The yellow line represents the intercondylar notch width at the most inferior point of the notch and collinear with the femoral intercondylar distance line.

Figure 4.

An axial magnetic resonance imaging cut demonstrating the measurements made for the standard notch width index calculation. The purple line represents the posterior condylar reference line. The red line represents the intercondylar distance drawn parallel to the posterior condylar reference line at the level of the popliteal hiatus and most posterior point of the intercondylar notch. The yellow line represents the intercondylar notch width at the most posterior point of the notch and collinear with the femoral intercondylar distance line.

The NWI was computed, and paired Student t test was performed to compare the axial and coronal measurements for each patient. Unpaired Student t tests were performed for comparison of sexes and contact versus noncontact injuries. A power analysis was completed before chart review and determined that 64 patients were needed for a power level of 0.80, a probability level of .05, and a medium effect size (Cohen d = 0.05). Interobserver reliability between the 2 orthopaedic surgery residents experienced in MRI interpretation was assessed using 2-way mixed intraclass correlation coefficients (ICCs).

Results

A total of 64 patients with an MRI-confirmed ACL injury were included in the study. There were 36 (56.25%) male patients. The mean ± SD age of the cohort was 26.54 ± 12.14 years. Race reported by the patients was 80% White, 14% Black, 2% mixed, 2% Asian, and 3% other. Ethnicity reported by the patients was 84% non-Hispanic, 14% Hispanic, and 2% other. When the cohort was analyzed by both race and ethnicity, the reported percentages were 72% White non-Hispanic, 11% Black non-Hispanic, 8% white Hispanic, 3% Black Hispanic, 3% other Hispanic, 2% Asian non-Hispanic, and 2% mixed other (Table 1).

Table 1

Patient Demographic Information

Patient Demographics
Age, y, mean ± SD	26.54 ± 12.14
Sex, male/female, n (%)	36 (56.25) / 28 (43.75)
Injury type, contact/ noncontact, n (%)	15 (23.44) / 49 (76.56)
Race, %, n	White (80), Black (14), mixed (2), Asian (2), Other (3)
Ethnicity, %, n	non-Hispanic (84), Hispanic (14), Other (2)

The modified coronal NWI for the entire cohort was 0.23 ± 0.02. The modified axial NWI for the entire cohort was 0.27 ± 0.02. The axial NWI was statistically greater than the coronal NWI (P < .001). The standard coronal NWI for the entire cohort was 0.25 ± 0.03. The standard axial NWI for the entire cohort was 0.29 ± 0.03. The axial NWI was statistically greater than the coronal NWI (P < .001).

The modified axial notch width measurement was 2.09 ± 0.26 cm and the modified coronal notch width measurement was 1.78 ± 0.23 cm. The modified axial notch width measurement was statistically greater than the coronal notch width measurement (P < .001). The standard axial notch width measurement was 2.03 ± 0.28 cm and the standard coronal notch width measurement was 1.83 ± 0.25 cm. The standard axial notch width measurement was statistically greater than the coronal notch width measurement (P < .001).

The modified axial intercondylar distance was mean ± SD 7.70 ± 0.66 cm and the modified coronal intercondylar distance was 7.72 ± 0.66 cm. The modified axial intercondylar distance was not statistically different from the coronal intercondylar distance (P = .22). The 2-way mixed ICC for the modified coronal NWI was 0.88 and the 2-way mixed ICC for the modified axial NWI was 0.97. The standard axial intercondylar distance was 7.13 ± 0.60 cm and the standard coronal intercondylar distance was 7.18 ± 0.60 cm. The standard axial intercondylar distance was not statistically different from the coronal intercondylar distance (P = .06). The 2-way mixed ICC for the standard coronal NWI was 0.99 and the 2-way mixed ICC for the standard axial NWI was 0.99. The NWI measurements are detailed in Table 2.

Table 2

Details on NWI Measurements ^a

NWI Measurements
	Standard	P	Modified	P
Axial bicondylar width, cm	7.13 ± 0.60	.06	7.70 ± 0.66	.22
Coronal bicondylar width, cm	7.18 ± 0.60	.06	7.72 ± 0.66	.22
Axial notch width, cm	2.03 ± 0.28	<.001	2.09 ± 0.26	<.001
Coronal notch width, cm	1.83 ± 0.25	<.001	1.78 ± 0.23	<.001
Axial NWI	0.29 ± 0.03	<.001	0.27 ± 0.02	<.001
Coronal NWI	0.25 ± 0.03	<.001	0.23 ± 0.02	<.001

Data are presented as mean ± SD. Bolded P values indicate signficance. NWI, notch width index.

The modified coronal NWI for male patients was mean ± SD 0.23 ± 0.02 and the modified coronal NWI for female patients was 0.23 ± 0.02. The modified coronal NWI was not statistically different between sexes (P = .14). The modified axial NWI for male patients was 0.27 ± 0.03 and the axial NWI for female patients was 0.27 ± 0.02. The modified axial NWI was not statistically different between sexes (P = .61).

There were 15 (23.44%) contact ACL injuries and 49 (76.56%) noncontact ACL injuries. The modified coronal NWI for contact injuries was 0.23 ± 0.02 and the modified coronal NWI for noncontact injuries was 0.23 ± 0.02. The modified coronal NWIs were not statistically different between contact and noncontact injuries (P = .93). The modified axial NWI for contact injuries was 0.27 ± 0.02 and the modified axial NWI for noncontact injuries was 0.27 ± 0.02. The modified axial NWIs were not statistically different between contact and noncontact injuries (P = .41).

Discussion

This study examined differences in NWI when measured on axial versus coronal MRI cuts. Axial calculations were significantly greater than their coronal counterparts due to increased axial notch widths for both the modified and the standard measurements. These findings align with the results of Raja et al¹² who, in their assessment of independent risk factors for ACL injuries, also observed higher mean axial NWIs. However, their study lacked statistical analysis, and it is unknown whether their findings were statistically significant.

Due to the paucity of literature, there are no clear thresholds for defining a “small” NWI. Previously reported critical NWI cutoffs of <0.18 to <0.29 have permitted a relative understanding of when to expect a narrow notch; however, future attempts to increase precise reporting warrant further clarity in these values.^{2,3,7,9,10,14,15,19} In their meta-analysis, Li et al⁹ explained that these threshold variations are likely due to differences in measurement methods, patient positions, and sample sizes. In addition, they observed smaller NWI values in male patients.⁹ Maheswari et al¹⁰ demonstrated the opposite with sex-based comparisons revealing smaller values in female patients of their ACL-injured cohort. However, their study was potentially skewed, with nearly 70% of the group being male.¹⁰ Other studies have corroborated our results in demonstrating no relationship between NWI and sex.^1,3,12,15 When taken altogether, it is likely that differences in study inclusion criteria are contributory to the literature discrepancies regarding male versus female notch measurements and the variations in critical NWI numerical cutoffs.

From an anatomic standpoint, it is well known that the notch becomes wider moving anterior to posterior⁵; and while both measurements (axial and coronal) should be measuring the same anterior to posterior position in the notch either at the largest bicondylar width or at the popliteal hiatus, the 2-dimensional MRI slice may not be capturing exact 3-dimensional anatomy. It is possible that measuring the most distal portion of the notch on the coronal cut is capturing a point consistently anterior compared with measuring the most posterior portion of the notch on the axial MRI cut, thus explaining the smaller NWI observed on coronal MRI. Although our study has identified that NWI differs depending on the plane of measurement, it does not establish which plane is more relevant nor does it address the threshold that defines a “small” NWI. Further studies are needed to address these unanswered questions.

With increased precision in NWI measurements, our findings question the implications, if any, that may occur regarding notchplasty rates in ACLR procedures. As an adjunct to ACLR, concerns of early-phase postoperative blood loss, distortion of knee biomechanics, and increased notch narrowing secondary to bony regrowth have already given way for judicious use of the technique.^8,13 Wolters et al. reported on intraoperative measurements taken during ACLRs and suggested that notch dimensions should assist in preoperative planning, intraoperative decision-making, and overall technique choice.^17,18 However, this recommendation has been subject to controversy. Wolf et al¹⁷ retrospectively reviewed 137 single and double ACLRs and found that notch dimensions were independent of the risk for graft failure. They concluded that notchplasty is not supported for cases involving smaller intercondylar notch dimensions, yet the overall study was underpowered for detecting significance within notch widths.¹⁷ It is likely that surgeon preference will remain the primary means for discerning notchplasty indications and the subsequent need for computing an NWI in the preoperative setting. In the probable event that an MRI is available, our findings present a way to standardize NWI measurement so that it can be used as an objective piece of information in the perioperative period.

This study provides a framework for future research in this area. First, establishing a standard measurement protocol will be needed for consistency of decision making. Second, establishing clear thresholds for risk or injury and reinjury from the new standard measurement protocol will be needed. Last, for this specific study, future research should be directed at establishing a noninjured control group to determine if the difference in notch width measurement holds true between the control and ACL-injured cohorts as well as to possibly establish better thresholds for NWI.

Limitations

This study had limitations warranting discussion. First, it did not control for notch morphology. Our findings had good to excellent interrater reliability, but NWI calculations may have been skewed, as “A-shaped” notches have been found to be narrower in all width dimensions.¹⁶ Furthermore, the presence of these notch shapes in conjunction with a low NWI has been found to significantly increase odds for ACL injury.² Patients with both phenotypes may have been outliers in our data set, and future studies should segregate these variables.

Second, inability to control the relationship of MRI cuts to the posterior condylar axis may have affected where the investigators made their measurements. Third, while the protocol for selecting the most optimal MRI cut for calculation of the NWI was established, there is some subjectivity in identifying landmarks based on what each MRI cut allows for visualization, and this was left up to the discretion of each researcher. While good intrarater reliability confirmed agreement between investigators, notch measurements may have been inaccurate if the most optimal slice was not selected by both examiners. Fourth, our findings likely require prospective challenges through clinical scenarios to determine their significance and applicability to clinicians.

Conclusion

Footnotes

Final revision submitted April 26, 2025; accepted May 20, 2025.

The authors declared that there are no conflicts of interest in the authorship and publication of this contribution. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Ethical approval for this study was obtained from St. Luke's University Health Network Institutional Review Board (IRB No. IRB-FY2024-290).

ORCID iD

Jonathan McKeeman

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A Comparison of Notch Width Index Differences When Measured on Axial and Coronal MRI