instability

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Related to patellofemoral instability: Jumper's knee

instability

1. tendency to variable or unpredictable behaviour
2. Physics a fast growing disturbance or wave in a plasma

instability

[‚in·stə′bil·əd·ē]
(control systems)
A condition of a control system in which excessive positive feedback causes persistent, unwanted oscillations in the output of the system.
(physics)
A property of the steady state of a system such that certain disturbances or perturbations introduced into the steady state will increase in magnitude, the maximum perturbation amplitude always remaining larger than the initial amplitude.

instability

In a structure, the sudden loss of stiffness that limits its load-carrying capability, and in some cases results in the structure’s failure.

instability

instabilityclick for a larger image
i. A state of the atmosphere in which a parcel of air will continue to rise, if for any reason, it is pushed up. Any instability in the atmosphere means turbulent conditions exist. The environmental lapse rate (ELR) in this case will be higher than both dry and saturated adiabatic lapse rates. Dry adiabatic lapse rate (DALR) is the rate of decrease of temperature with height of a parcel of dry air lifted adiabatically. Its value is 3°/1000 ft. Saturated adiabatic lapse rate (SALR) is the lapse rate of a parcel of saturated air and its value is 1.5°/1000 ft.
ii. A tendency of a body to continue moving away from its original state when disturbed by an external force and to continue after that force has been withdrawn.
iii. The failure of structures, such as the buckling of plates and panels under compression, and the failure of struts under end loads.
References in periodicals archive ?
Physical Assessment Common Abnormal Findings in Children Exam with Down Syndrome Measurements * BMI less than 5th percentile * BMI greater than 85th percentile Neck * Changes in size or consistency of the thyroid gland Atlantoaxial Instability * Weakness, neck pain, head tilt Heart * Mitral valve prolapse * Aortic regurgitation Musculoskeletal System * Hip instability * Patellofemoral instability * Deformities of the forefoot, pes planus, calcaneal valgus, pronation Note: BMI = body mass index.
1) Patients predisposed to patellofemoral instability may have an increased Q angle, dysplasia of the patella or trochlear groove, patella alta, or a dysplastic VMO.
1) Simmons and Cameron achieved a 0% recurrence rate in patellofemoral instability when the tibial tubercle was transferred distally without medialization in patients with instability and significant patella alta.
27,37,39,40,46,47) An additional limitation to research is the wide spectrum of patellofemoral instability predisposition and the difficulty in truly establishing similar cohorts.
Many proponents feel that this is the essential lesion in patellofemoral instability.