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Physiological Joint Models
The forces inside our joints cannot be measured directly in a living person. These include contact forces between cartilage surfaces, tension in ligaments, and forces in individual muscles. To estimate them, we build musculoskeletal models whose joints follow real anatomy: the knee with its articular contact and coupled motions, the foot with multiple segments, and the spine with individual vertebrae and ribs. We drive these models with measured motion and ground reaction forces and solve for muscle forces and joint contact forces. We validate them against in vivo data such as forces from instrumented knee implants. Our knee contact model won the Sixth Grand Challenge Competition to Predict In Vivo Knee Loads in 2016.
How we use it
Knee contact models. We embedded contact elements on the joint surface of a full-body model to predict medial and lateral knee contact forces during walking, and validated the predictions against an instrumented knee implant. Using the same approach, we estimate knee contact force and ground reaction force together from motion and foot pressure data.
Knee kinematic coupling. From our biplanar fluoroscopy measurements, we modeled how the tibia translates and rotates with knee flexion during walking. This makes knee motion in simulations closer to real knee motion.
Multi-segment foot model. Our five-segment foot model calculates muscle forces, passive moments from connective tissue, and contact forces in the ankle, Chopart's, Lisfranc and metatarsophalangeal joints during walking. These forces cannot be obtained from net joint moments alone.
Reliability of simulation tools. We compared OpenSim and AnyBody Modeling System using the same subjects and motions to understand how the choice of model and solver affects calculated muscle activity.
Joint Loading in Injury and Deformity
Injuries and deformities change how the body moves, and they also change how forces are shared among the structures inside a joint. A torn ligament carries less load and lets bones shift, a collapsed foot arch moves contact toward one side of the ankle, and stiffened soft tissue cushions the foot less. These changes in load distribution can lead to secondary injury and joint degeneration. We modify our musculoskeletal and finite element models to represent a specific injury or deformity, run them with measured motion data, and calculate how loads on ligaments, cartilage and soft tissue change.
How we use it
Ligament injury in the knee. Using a knee model with 14 ligaments, we simulated ACL injuries of different severity by reducing ACL stiffness or increasing its initial length. We found the level of injury at which the tibia begins to slide forward significantly during walking, and showed that ACL force changes nonlinearly as the injury progresses.
Flatfoot and the ankle joint. We represented flexible flatfoot by reducing the stiffness of the spring ligament in a five-segment foot model. Patients with flatfoot had a greater lateral contact force in the tibiotalar joint, a more posterior center of pressure, and larger posterior tibialis muscle forces than people with normal feet.
Plantar soft tissue in diabetes. We combined indentation tests with CT-based, subject-specific finite element models of the heel to estimate the nonlinear properties of skin and fat. The plantar fat of diabetic patients was markedly stiffer than that of healthy adults, which may increase the risk of foot ulcers.
Knee loading and cartilage degeneration. In earlier work at Stanford, we studied how joint loading and contact geometry relate to cartilage thickness. Using a finite element model, we also showed how gait changes after partial meniscectomy alter strain in the menisci.
Virtual Surgery and Treatment Planning
Surgeons often choose among several techniques, such as where to place a ligament graft or which joint to fuse or realign. They usually cannot know in advance how each choice will change joint motion and loading during daily activities. We perform these surgeries virtually. We change the anatomy of our musculoskeletal models to reflect a surgical procedure, then simulate walking with each person's measured motion to compare how different options affect joint stability, ligament forces and joint contact forces. We also use 3D medical images to plan surgical details, such as tunnel positions for ligament reconstruction and the shape of patient-specific grafts.
How we use it
ACL graft placement. Using a knee model with 14 ligaments, we replaced the ACL with grafts attached at different femoral locations. We then compared knee kinematics and graft forces during walking to find attachment positions that restore natural knee motion.
Subtalar fusion. By fusing the subtalar joint in a foot model, we showed how loss of subtalar motion increases contact forces in the neighboring foot joints during walking. This offers a mechanical explanation for arthritis that can develop after fusion.
Tunnel positions for ligament reconstruction. With orthopedic surgeons, we used 3D CT of cadaveric knees at different flexion angles to calculate how the distance between candidate femoral and tibial tunnels changes. This helps identify isometric tunnel positions for ACL, PCL and posterolateral corner reconstruction.
Patient-specific grafts. We reconstructed cartilage defects from MRI and fabricated custom-shaped grafts using computer-aided design and rapid-prototyped molds.
Osteotomy simulation. We are extending virtual surgery to realignment procedures: osteotomies for flexible flatfoot, and knee osteotomies including high tibial osteotomy. The goal is to predict which correction best restores normal gait and joint loading.
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