Flexion/extension, biomechanics

The primary sources of control for body-powered devices are biomechanical in nature. Movement, or force, from a body joint or multiple joints is used to change position, or develop a force/ pressure that can be transduced by a harness and Bowden cable and/or mechanical switches. Typically, inputs such as chin and head force/movement, glenohumeral flexion/extension or abduction/ adduction, biscapular and scapular abduction, shoulder elevation and depression, chest expansion, and elbow or wrist movements are used. However, direct force/motion from muscle(s) has also been used by way of surgical procedures such as muscle tunnel cinepiasty (Sauerbruch, 1916) and the Krukenberg cinepiasty (Krukenberg, 1917). 

The primary motion of the knee joint is flexion-extension rotation around an axis passing through the medial and lateral femoral condyles. The three-dimensional motions of the knee other than flexion-extension rotation are constrained by ligaments, menisci, and articular surface configuration. The biomechanical functions of the ACL are mainly to resist anterior tibial translation, and secondly to resist internal and valgus tibial rotation, or combined motions. 

The second study question was whether the clinically available transtibial procedure for anatomic DB reconstruction can really obtain significantly better knee stability in comparison with the conventional SB reconstruction procedure. The anterior translation laxity in response to a 90-N anterior drawer force was significantly less after the anatomic DB reconstmction than after the SB reconstruction from 0 to 75° of knee flexion. Previous biomechanical studies have shown that the PL bundle of the intact ACL carries one-half to two-thirds of the total force in the ACL near full extension of the knee, when the knee is subjected to an anterior tibial load [8, 26, 27]. As the conventional SB reconstmction reproduces only the AM bundle, loss of the function of the natural PL bundle is considered to result in the insufficient function in the conventional SB reconstmction in the range between 0 and 75° of knee flexion. On the other hand, Yamamoto et al. [22] and Yasuda et al. [28] reported that the reconstmcted PL bundle cannot restrain anterior tibial translation at flexion angles of the knee. This fact explains the similarity concerning the knee laxity between the two reconstmctions namely, only the reconstmcted AM bundle stabilizes the knee near flexion position in response to anterior tibial load. 

FIGURE 12.4 Four motion patterns of the UHMWPE Core in the CHARITfi total disc replacement observed during in vitro biomechanical testing. A With Motion Pattern 1, relative angular motion occurred predominantly between the superior endplate and the core, with little or no core translation. B In Motion Pattern 2, lift-off occurred by either the superior endplate from the core, or by the core from the inferior endplate. C In Motion Pattern 3, the UHMWPE Core locked in plane, resulting entrapment, over a portion of the flexion-extension range. D In Motion Pattern 4, angular motion occurred between the core and both the superior and interior endplates. Reproduced from [82] with permission. 

Studies of the normal biomechanics of the proximal wrist joint have determined that the scaphoid and lunate bones have separate, distinct areas of contact on the distal radius/triangular fibrocartil-age complex surface [Viegas et al., 1987] so that the contact areas were localized and accounted for a relatively small fraction of the joint surface, regardless of wrist position (average of 20.6%). The contact areas shift from a more volar location to a more dorsal location as the wrist moves from flexion to extension. Overall, the scaphoid contact area is 1.47 times greater than that of the lunate. The... 

The key limitations in the development of the discriminate functions for classification purposes, are the data-driven nature of the algorithms and the lack of theoretical orientation in the process of development and validation of these models. It is suggested that the mathematical simulation of flexion or extension trunk movement may identify an objective basis for the evaluation and assessment of trunk kinematic performance. A catalog of movement patterns that are optimal with respect to physical and biomechanical quantities may contribute to the emergence of a more theoretically based computational paradigm for the evaluation of kinematic performance of normal subjects and patients. It must be emphasized that in this paradigm one has no intention to claim that the central nervous system actually optimizes any single or composite cost function. 

It is critical to understand functional anatomy and biomechanics of the AM and PL bundles of the ACL in order to understand the theory of the anatomic double-bimdle ACL reconstruction. It has been well known that the mid-substance fibers of the AM and PL bundles have different functions The AM bundle mid-substance is stretched in the full extension position, relaxed at 20-60 ° of knee flexion, and again stretched in a flexion position of more than 90° [9]. The PL bundle mid-substance is stretched in the full extension position, whereas it becomes slack in a flexion position [9]. In response to an anterior tibial load, the magnitude of the in situ force in the PL bundle mid-substance was larger than that in the AM bundle mid-substance at knee flexion angles between 0 ° and 45 ° [10]. Under a combined rotatory load, the PL bundle mid-substance is as important as the AM bundle mid-substance, especially when the knee is in the near extension position [11]. 

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