Abstract
Objectives :
Mucoid degeneration of the anterior cruciate ligament (ACL) is an under recognized cause of knee pain and loss of motion. Although its pathogenesis remains unclear, morphological factors of the knee may predispose certain individuals to this degenerative process. Our objective was to investigate the association between specific knee morphological parameters and the presence of mucoid degeneration of the ACL.
Methods :
The study was conducted using our institutional database to identify patients diagnosed with mucoid degeneration of the ACL on MRI between 2021 and 2025. Imaging of these patients was reviewed to assess the following morphological parameters: medial and lateral tibial slope, percentage of intercondylar notch occupied by the ACL, maximum ACL width in sagittal and coronal planes, notch width, notch width index, notch depth, notch shape, tibial tubercle - trochlear groove (TT-TG) distance, Dejour classification of trochlear dysplasia, presence of anvil osteophytes and Caton-Deschamps index. The data on continuous variables was summarized using mean and standard deviation when normally distributed, or median and interquartile range (IQR) when the distribution was skewed. Categorical variables were summarized as frequencies and percentages. We compared our findings with the normal findings described in the literature.
Results :
A total of 50 knees from 48 patients diagnosed with ACL mucoid degeneration were included, with 2 patients presenting bilateral involvement. The median age of the cohort was 67.3 years (IQR: 57 - 72.2) and 58% were male. Compared to normative data from existing literature, patients in this cohort demonstrated decreased medial and lateral tibial slopes and not the increase that would add load to the ACL. Contrary to existing reports which suggest A-shaped notches are most associated with mucoid ACL, U-shaped notches were most prevalent in our study (58%), followed by A-shaped notches (32%). The median percentage of the notch occupied by the ACL was 100%. The mean maximum ACL width was 16.2 ± 2.76 mm in the sagittal plane and 13.1 ± 3.42 mm in the coronal plane, parameters not described in prior literature. Notch shape analysis revealed decreased Sagittal Notch Angle (SNA) and Transverse Notch Angle (TNA), consistent with steeper and more stenotic notches. Notch Width Index (NWI) was also reduced, aligning with the presence of narrower intercondylar notches. The mean trochlear groove to tibial tubercle (TG–TT) distance was 9.64 ± 3.67 mm, within the normal range. Using the Dejour MRI classification, 82% of patients had no trochlear dysplasia, while 18% had high-grade (Type 3) dysplasia. Anvil osteophytes were observed in 28% of cases. The Caton–Deschamps Index (CDI) was within normal limits (mean 1.04 ± 0.158).
Conclusions :
This study highlights several distinct morphological features on MRI associated with mucoid degeneration of the ACL. Whilst increased tibial slope was not seen and the TG-TT offset and Caton Deschamps Index were within normal limits, there were several new findings. These include complete notch occupancy by the ACL in all cases and increased ACL width in both sagittal and coronal planes. Narrower, steeper, and stenotic notches characterized by reduced SNA, TNA, and NWI were commonly observed. These anatomical variations may contribute to altered knee biomechanics and potential impingement, thereby predisposing to mucoid changes. Recognition of these factors on MRI may aid early diagnosis.