knee pain or knee OA. Physical therapists can offer education and strategies that may offset this risk for patients in these professions. Protective equipment like knee pads, proper lifting techniques, or assistive devices like mechanical lifts may offer protection against OA development, although these interventions have yet to be studied in this context. When treating patients with or without OA, it is important to keep risk factors for OA in mind. Being overweight or obese is one of the most common and modifiable risk factors for OA. Previous joint injury and female sex are two common Biomechanical pathogenesis Knee OA is somewhat unique in that there is a biomechanical pathway of disease initiation and progression in addition to the risk factors previously discussed. The magnitude, rate, and location of loading within the knee joint is thought to play an important role in whether or not a person develops knee OA. Understanding how biomechanics can influence OA risk is important because physical therapy interventions, such as gait retraining or bracing, may help to offset the pain and cartilage degeneration associated with OA progression. Knee adduction moment During weight-bearing activities, such as walking, a reaction force acts upwards from the ground and pushes back into our feet according to Newton’s Third Law of Motion. These reaction forces create joint moments, which can be conceptualized as the product of the distance between the reaction force and the center of the weight-bearing joint; this is commonly described as the force × 3 distance. A common application of this is the relationship between the ground reaction force and the knee when standing and squatting. When standing, the ground reaction force acts in an upward direction with a trajectory that passes close to the knee joint center when viewed from the sagittal plane. As a result, the distance between the knee joint and the ground reaction force is essentially zero, and thus the joint moment in the sagittal plane is zero. During a squat, the knee joint moves in an anterior direction, which increases the distance between the joint and the force. As a result, the force passes behind the knee and the joint moment that would act to flex the knee increases. Muscle force must be generated from the quadriceps to help overcome the moment caused by the ground reaction force to prevent collapsing into the ground. The adduction moment is similar but occurs in the frontal plane (Figure 4). The adduction moment has been shown to be related to OA severity and OA progression. Larger adduction moments at baseline predict more OA progression (Miyazaki et al., 2002) and more severe knee OA is associated with larger adduction moments (Zeni & Higginson, 2009). This is likely attributed to the fact that larger adduction moments are associated with greater contact forces in the medial side of the tibiofemoral joint and larger forces may degrade the cartilage more quickly. The adduction moment can increase as the ground reaction force becomes oriented in a medial direction, or it can be larger in patients who have a more genu varus alignment (see Figure 4). Because the adduction moment is related to OA progression and is a potentially modifiable biomechanical parameter, reducing this moment has been the target of many interventions. Interventions designed to reduce the adduction moment include gait retraining, unloader bracing, variable stiffness shoes, and contralateral cane use. The benefits and considerations of these interventions will be discussed in subsequent sections of this course.
nonmodifiable factors. Biomechanical factors also predict OA incidence and progression, and these will be discussed in the next section. In summary, the following is a list of factors that are generally accepted to increase the risk of OA development or progression: ● BMI >25. ● Previous joint injury, such as ligament tear or meniscus injury. ● Genetic predisposition. ● Female sex.
Figure 4: Ground Reaction Force During Gait
The external adduction moment can be conceptualized as the distance between the ground reaction force vector and the center of rotation of the knee joint in the frontal plane. When the ground reaction force vector is close to the knee joint, the adduction moment is relatively low, as shown in (A). A change in ground reaction force direction that moves the force vector medially will increase the external adduction moment by increasing the distance between the axis of rotation and the force, as shown in (B). This change in vector position can occur when the individual pushes laterally into the ground when walking. A greater distance between the vector and the knee joint also may occur when the patient presents with greater knee varus, which shifts the knee laterally in space relative to the foot-ground contact. This could increase the adduction moment even if the direction of the ground reaction vector does not change. Note. From Western Schools, 2018. Joint compression Although larger adduction moments may contribute to greater compression forces in the medial tibiofemoral compartment, other patient characteristics can change the overall magnitude of forces in the knee joint. Obesity, which has been discussed in this course as a strong predictor of knee OA, increases the compressive forces within the knee joint. Data from mathematical models of human walking have found that for every 1 pound of weight gained or lost, there is a fourfold change in joint compression forces (Messier, Gutekunst, Davis, & DeVita, 2005). Findings from recent experimental and simulated weight gain have also revealed that joint compression exceeds the amount of weight gained (Knarr, Higginson, & Zeni, 2016). Diet and exercise interventions that reduce body mass also reduce knee joint loads (Messier et al., 2013). In a landmark study by Felson et al., women who lost weight demonstrated a substantial reduction in the risk of developing knee OA in the future (Felson, Zhang, Anthony, Naimark, & Anderson, 1992). It appears that excessive body weight increases the risk of knee OA, at least in part, by increasing the forces experienced by the joint.
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Book Code: PTNC1023
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