American Society of Biomechanics Clinical Biomechanics Award 2017: Non-anatomic graft geometry is linked with asymmetric tibiofemoral kinematics and cartilage contact following anterior cruciate ligament reconstruction

Abstract

Background: Abnormal knee mechanics may contribute to early cartilage degeneration following anterior cruciate ligament reconstruction. Anterior cruciate ligament graft geometry has previously been linked to abnormal tibiofemoral kinematics, suggesting this parameter may be important in restoring normative cartilage loading. However, the relationship between graft geometry and cartilage contact is unknown.

Methods: Static MR images were collected and segmented for eighteen subjects to obtain bone, cartilage, and anterior cruciate ligament geometries for their reconstructed and contralateral knees. The footprint locations and orientation of the anterior cruciate ligament were calculated. Volumetric, dynamic MR imaging was also performed to measure tibiofemoral kinematics, cartilage contact location, and contact sliding velocity while subjects performed loaded knee flexion-extension. Multiple linear regression was used to determine the relationship between non-anatomic graft geometry and asymmetric knee mechanics.

Findings: Non-anatomic graft geometry was related to asymmetric knee mechanics, with the sagittal plane graft angle being the best predictor of asymmetry. A more vertical sagittal graft angle was associated with greater anterior tibial translation (β = 0.11mmdeg, P = 0.049, R² = 0.22), internal tibial rotation (β = 0.27degdeg, P = 0.042, R² = 0.23), and adduction angle (β = 0.15degdeg, P = 0.013, R² = 0.44) at peak knee flexion. A non-anatomic sagittal graft orientation was also linked to asymmetries in tibial contact location and sliding velocity on the medial (β = -4.2mmsdeg, P = 0.002, R² = 0.58) and lateral tibial plateaus (β = 5.7mmsdeg, P = 0.006, R² = 0.54).

Interpretation: This study provides evidence that non-anatomic graft geometry is linked to asymmetric knee mechanics, suggesting that restoring native anterior cruciate ligament geometry may be important to mitigate the risk of early cartilage degeneration in these patients.

Keywords: Anterior cruciate ligament graft; Anterior cruciate ligament reconstruction; Cartilage; Knee; MRI; Osteoarthritis.

Fig. 1. Workflow for MR Image Analysis

(Top row) Subject-specific bone, cartilage, and ACL geometries were created by segmenting IDEAL-SPGR and 3D FSE Cube MR images of both knees of each subject. (Bottom row) Subjects performed an active flexion-extension motion against an inertial load while volumetric, dynamic MR images were collected with an SPGR-VIPR sequence. The bones were tracked in the dynamic images using the bone geometries. Tibiofemoral kinematics were then computed using the position and orientation of the tibia relative to the femur (Grood and Suntay, 1983). Cartilage contact was computed by prescribing the tibiofemoral kinematics to the femoral and tibial cartilage geometries and measuring the overlap of the cartilage surfaces.

Fig. 2. ACL Geometry, Kinematics, and Cartilage Contact Metrics

(A) The orientation of the ACL relative to the tibial plateau in the sagittal and frontal planes, the location of the tibial footprint in the axial plane, and the location of the femoral footprint in the sagittal plane were computed for both knees of each subject. (B) Representative plot showing internal tibial rotation throughout the flexion-extension motion for both knees of one subject. The extension phase of the motion is denoted with an arrow. Similar plots were created for the other five degrees of freedom of the tibiofemoral joint. Kinematics metrics were then computed as the kinematics at peak knee flexion and the range in kinematics during knee extension. (C) Top row shows the proximity of the tibial cartilage to the femoral cartilage at peak knee flexion for the contralateral knee of a representative subject. Red is indicative of cartilage contact. Similar maps were generated for both knees of each subject and used to compute the center of contact location on the tibial plateaus. Bottom row shows the cartilage surface sliding velocity on the medial and lateral tibial plateaus during knee extension for the contralateral knee of a representative subject. The mean absolute sliding velocity during extension and the sliding velocity at peak knee flexion were computed for both knees of each subject.

Fig. 3. Relationship between Kinematics and ACL Sagittal Plane Angle

Scatter plots show the relationship between asymmetric kinematics and non-anatomic graft sagittal plane angles for anterior tibial translation, internal tibial rotation, and adduction angle at peak knee flexion. The coefficient of the linear regression model (β) is shown for each relationship.

Fig. 4. Relationship between Cartilage Contact and ACL Sagittal Plane Angle

Scatter plots show the relationships between asymmetric cartilage contact and non-anatomic graft sagittal plane angles for (A) the center of contact location and (B) the mean absolute contact sliding velocity for the medial and lateral tibial plateaus. The coefficient of the linear regression model (β) is shown for each relationship.

Fig. 5. Representative subject with a vertical ACL graft

Graphic shows the anterior tibial translation, internal tibial rotation, and center of contact location at peak knee flexion and the side-to-side difference in contact sliding velocity during knee extension for a subject with a more vertical ACL graft in the sagittal plane, relative to the native ACL. The asymmetries in kinematics and cartilage contact measured in this subject are representative of the relationship between asymmetric knee mechanics and non-anatomic ACL graft sagittal plane orientation observed across all subjects.