Knee Joint Instability and Mechanical Stiffness in Knee OA
Self-reported joint instability is a common complaint in individuals with knee osteoarthritis (OA). Knee joint instability can be defined as "the sudden loss of postural support across the knee at a time of weight bearing". Of import, sensations of instability can compromise an individual's capacity to perform their daily activities and are associated with poorer physical function.
Intuitively, sensations of knee joint instability (i.e. feelings of shifting, buckling or giving way of the knee) in patients with knee OA may be partly related to the mechanical stability of the joint. The provision of adequate resistance to motion (i.e. mechanical stiffness), is an important component of mechanical joint stability and is contingent upon both passive (e.g. ligaments) and active (i.e. muscle-tendon units) structures. Whilst mechanical stability in all three planes of motion is likely to influence sensations of joint stability, evidence suggests that frontal plane mechanics may be particularly important in those with medial knee OA. Some studies, but not all, have shown that patients with OA of the medial tibiofemoral joint demonstrate excessive varus-valgus passive laxity; there is also emerging evidence that passive mechanical stiffness is reduced with medial tibiofemoral OA. While passive laxity, stiffness, and joint instability are not synonymous, it is possible that a lax or low-stiffness knee, when exposed to high frontal plane moments during locomotor activities (that are associated with the medial OA knee), may experience joint instability. In contrast, patients with stiffer joint structures may report less instability given greater resistance against external perturbations. However, it is not known if knee joint passive mechanical stiffness is related to the symptomatic stability of the medial OA knee. An understanding of the relationship between self-reported knee instability and stiffness/instability may help guide the development of approaches to minimise symptomatic instability in this population.
Measurements related to frontal plane knee laxity (knee varus-valgus range-of-motion or medial joint opening on varus-valgus stress xray) indicate that such laxity does not differ between self-reported stable and unstable OA knees. Similarly, frontal plane knee laxity (as indicated by medial joint opening on varus-valgus stress xray) is not related to the severity of symptomatic instability. These previous investigations however, did not separate laxity measurements into varus and valgus, but rather measured knee laxity through the entire varus-valgus range-of-motion. Conceivably, angular laxity under varus and valgus loading may hold a different relationship with symptomatic knee instability, and this is worthy of investigation. An additional consideration is that passive knee laxity (i.e. range-of-motion), is not necessarily indicative of passive mechanical stiffness (i.e. resistance to motion). Early work in this field clearly demonstrates that the passive stiffness of the knee is dependent upon the portion of the moment-angle curve that is evaluated: typically passive stiffness is higher toward the end of range than in the mid-range. Thus, stiffness within a given range of the moment-angle curve is not synonymous with range-of-motion across the entire moment-angle curve. Moreover, when compared with healthy knees, those with medial tibiofemoral OA demonstrate lower varus-valgus mechanical stiffness in the mid-range, but not at the end of range. Thus, it may be important to evaluate the relationship between joint instability and passive stiffness in the mid- and end-range independently, as well as the relationship between instability and maximum range-of-motion (i.e. laxity). Conceivably, the mechanical behaviour of the knee close to its usual, relatively small, varus-valgus 'operating range' (i.e. mid-range stiffness), may be particularly important for joint function and may reveal associations with joint instability.
The aim of this study therefore, was to evaluate the relationship between self-reported instability, and varus-valgus angular laxity and passive end- and mid-range stiffness, in individuals with medial knee OA. We hypothesised that greater instability would be associated with greater angular laxity and less passive stiffness.
Background
Self-reported joint instability is a common complaint in individuals with knee osteoarthritis (OA). Knee joint instability can be defined as "the sudden loss of postural support across the knee at a time of weight bearing". Of import, sensations of instability can compromise an individual's capacity to perform their daily activities and are associated with poorer physical function.
Intuitively, sensations of knee joint instability (i.e. feelings of shifting, buckling or giving way of the knee) in patients with knee OA may be partly related to the mechanical stability of the joint. The provision of adequate resistance to motion (i.e. mechanical stiffness), is an important component of mechanical joint stability and is contingent upon both passive (e.g. ligaments) and active (i.e. muscle-tendon units) structures. Whilst mechanical stability in all three planes of motion is likely to influence sensations of joint stability, evidence suggests that frontal plane mechanics may be particularly important in those with medial knee OA. Some studies, but not all, have shown that patients with OA of the medial tibiofemoral joint demonstrate excessive varus-valgus passive laxity; there is also emerging evidence that passive mechanical stiffness is reduced with medial tibiofemoral OA. While passive laxity, stiffness, and joint instability are not synonymous, it is possible that a lax or low-stiffness knee, when exposed to high frontal plane moments during locomotor activities (that are associated with the medial OA knee), may experience joint instability. In contrast, patients with stiffer joint structures may report less instability given greater resistance against external perturbations. However, it is not known if knee joint passive mechanical stiffness is related to the symptomatic stability of the medial OA knee. An understanding of the relationship between self-reported knee instability and stiffness/instability may help guide the development of approaches to minimise symptomatic instability in this population.
Measurements related to frontal plane knee laxity (knee varus-valgus range-of-motion or medial joint opening on varus-valgus stress xray) indicate that such laxity does not differ between self-reported stable and unstable OA knees. Similarly, frontal plane knee laxity (as indicated by medial joint opening on varus-valgus stress xray) is not related to the severity of symptomatic instability. These previous investigations however, did not separate laxity measurements into varus and valgus, but rather measured knee laxity through the entire varus-valgus range-of-motion. Conceivably, angular laxity under varus and valgus loading may hold a different relationship with symptomatic knee instability, and this is worthy of investigation. An additional consideration is that passive knee laxity (i.e. range-of-motion), is not necessarily indicative of passive mechanical stiffness (i.e. resistance to motion). Early work in this field clearly demonstrates that the passive stiffness of the knee is dependent upon the portion of the moment-angle curve that is evaluated: typically passive stiffness is higher toward the end of range than in the mid-range. Thus, stiffness within a given range of the moment-angle curve is not synonymous with range-of-motion across the entire moment-angle curve. Moreover, when compared with healthy knees, those with medial tibiofemoral OA demonstrate lower varus-valgus mechanical stiffness in the mid-range, but not at the end of range. Thus, it may be important to evaluate the relationship between joint instability and passive stiffness in the mid- and end-range independently, as well as the relationship between instability and maximum range-of-motion (i.e. laxity). Conceivably, the mechanical behaviour of the knee close to its usual, relatively small, varus-valgus 'operating range' (i.e. mid-range stiffness), may be particularly important for joint function and may reveal associations with joint instability.
The aim of this study therefore, was to evaluate the relationship between self-reported instability, and varus-valgus angular laxity and passive end- and mid-range stiffness, in individuals with medial knee OA. We hypothesised that greater instability would be associated with greater angular laxity and less passive stiffness.
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