Candidate:
Free body diagrams are a useful tool in orthopaedics to visualize the forces acting on a body segment and determine the resultant force and moment about a fulcrum. By isolating the body part and ensuring a state of static equilibrium, we can calculate the forces acting on the body segment.
Candidate:
There are several assumptions that are made when using free body diagrams, including:
Bones are rigid rods.
Joints are frictionless hinges.
No antagonistic muscle action.
Weight of body is concentrated through the center of mass.
Force acts in the direction of the muscle belly.
Muscles act only in tension.
Internal forces cancel each other out.
Joint reaction force (JRF) is presumed to be compressive only.
Candidate:
JRF is the force generated within a joint in response to external forces from muscle contractions and body weight. It is calculated by taking the vector sum of all the forces acting on the joint. In the case of the hip joint, for example, the JRF is equal to the body weight plus the total muscle forces acting on the joint.
Candidate:
Several factors can influence the JRF at the hip joint, such as coxa vara, valgus osteotomy, and Trendelenburg gait. Coxa vara causes an increase in the distance A, which reduces F(Ab) and the JRF. Valgus osteotomy causes a shortening of distance A and therefore increases F(Ab). Trendelenburg gait involves shifting the patient’s body weight to reduce distance B and the resulting moment arm. Ways to reduce the hip JRF include losing weight to reduce F(BW), holding a walking stick in the contralateral hand, or carrying a weight on the ipsilateral side.
Candidate:
The knee joint acts as a class 3 lever, where the effort is at the tibial tuberosity where the patella tendon inserts. The knee joint itself serves as the pivot, with the femur and tibia acting as either the fixed or mobile component depending on the case. In orthopaedics, this is significant because there is an increased JRF with the knee in flexion due to the increased pull of the quadriceps muscles. Additionally, patellectomy reduces the quadriceps moment arm and increases the quadriceps force.
Candidate:
The ankle joint acts as a class 1 lever at midstance and a class 2 lever at both heel strike and toe off. During the gait cycle, the heel, ankle, and 1st MTPJ all serve as fulcrums, with the foot typically fixed and the tibia mobile. The ground reaction force is equal and opposite to the force generated by the ankle joint, which is transmitted up the leg. This has implications for conditions such as ankle equinus, which can cause altered forces and moments at the ankle joint and lead to foot deformities.