The Deep Medial Collateral Ligament, Part 1: Quantitative, Qualitative, and Radiographic Anatomy

BACKGROUND: The deep medial collateral ligament (dMCL) has historically been described as capsular thickening with meniscofemoral and meniscotibial components deep to the superficial medial collateral ligament (sMCL). However, recent investigations suggest that the dMCL includes capsular expansion beyond the traditionally described midthird ligament region. PURPOSE: To define the attachment geometry and radiographic relationships of the dMCL using 3-dimensional coordinate mapping and standardized radiographic correlation. STUDY DESIGN: Descriptive laboratory study. METHODS: A total of 10 fresh-frozen cadaveric knees (mean age, 70.7 ± 6.8 years) were dissected to isolate the dMCL. A 6 degrees of freedom robotic testing apparatus with motion capture established 3-dimensional specimen-specific coordinate systems. The periphery of each femoral and tibial attachment site was digitized to determine attachment site geometry and spatial relationships to key medial landmarks. Radiopaque markers enabled standardized anteroposterior and lateral radiographic measurements. RESULTS: Qualitatively, the dMCL demonstrated broad anteromedial capsular extension, with an anterior margin length of 27.0 ± 3.4 mm from the posterior aspect of the sMCL. Posteriorly, the dMCL blended with the anterior margin of the posterior oblique ligament. The femoral attachment was 10.7 ± 1.5 mm in anteroposterior width. The center of the femoral attachment of the dMCL was located 5.1 ± 2.0 mm posterior and 12.4 ± 3.5 mm distal to the medial epicondyle as well as 10.6 ± 3.2 mm distal to the center of the sMCL femoral attachment. The tibial attachment was 29.3 ± 3.7 mm in anteroposterior width and was located 2.2 ± 0.8 mm distal to the joint line. Radiographically, the center of the dMCL femoral attachment was 7.9 ± 2.9 mm from the center of the sMCL femoral attachment. The tibial attachment center was 15.0 ± 3.6 mm proximal to the center of the proximal sMCL tibial attachment and 41.1 ± 5.3 mm proximal to the center of the proximal edge of the distal sMCL tibial attachment. CONCLUSION: The dMCL demonstrated a broad and consistent anatomic footprint. Substantial anteromedial capsular expansion was consistently identified, extending anteriorly beyond the historically described narrow midthird capsular ligament deep to the sMCL. CLINICAL RELEVANCE: These findings provide an updated qualitative and quantitative anatomic framework for future biomechanical investigations and should assist the surgical identification and radiographic assessment of the dMCL during medial-sided knee reconstruction procedures.