anatomical compatibility
The roto-translational dynamics is confirmedby the anatomy of the joint profiles, by the shape of cruciate ligaments and in the first phase of flexion of the
leg on the thigh, from the back-positioning of the meniscuses simultaneously decommit intercondylar eminence.
leg on the thigh, from the back-positioning of the meniscuses simultaneously decommit intercondylar eminence.
The conformationof the profilesarticular The femoral condyles have a rounded surface with a curvature radius somewhat reduced, but not uniform so that their profile is very similar to those of a spiral, while the tibial plateau are defined plateau because their curvate radius is much wider than those of the femoral condyles. The development in length of the femoral condyles is almost two times greater than the length of the tibial articular surface. This prevents the possibility of a pure rolling between the two articular heads, which is limited for the first 25-30 degrees of flexion. In subsequent degrees, the rolling is combined with a sliding phase which becomes more and more progressive. |
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The shapeof the cruciate ligaments In the flexion-extension, the cruciate ligaments retain the articular heads ensuring contact and antero-posterior stability of the knee during the whole movement. The cruciate ligaments have a helical shape and they are a reinforcing structure of the joint capsule; the descriptions of the tibial ads, in fact, we know that the fibers of the cruciate ligaments have not all the same length and direction; during the movements, therefore, fibers are not placed in tension simultaneously. Roud in 1913 had already proposed a thesis claiming that during the flexion-extension some fibers of the cruciate were always in tension due to their different lengths. This allows to counterbalances LCA \ LCP tension at any phase of flexion-extension avoiding movements drawer. In addition to this, starting from the mechanical model of Strasser (1917) it was established that the articular profile of the rear part of the condyles exactly represents the curve that wraps the different positions of the tibial plateau between the complete flexion and extension, confirming that not the anterior cruciate ligament, neither the rear cruciate ligament changes the length while the condyl profile remains tangent to the tibial plateau. From this it follows also that the shape of the condyles is geometrically determined by the length of the cruciate ligaments, from their position and arrangement of their ads. |
The movementof the meniscuses During flexion of the leg on the thigh the suggests joint mechanics the physiological slip back of the meniscus during flexion. The internal and the external meniscus in fact, in their rear part and in particular with their rear horns, are connected (via an fibrous expansion) respectively to the semimembranosus muscle and the popliteal muscle, which, however, are the motors muscles of flexion: their shortening determines the simultaneous of the flexion motor action and backwards recall of the meniscus. The roto-translational mechanical, faithfully interprets this movement, in fact the first phase of flexion, rotation of the femoral condyles provides a backward rollingon of them on the tibial plateau , that is proportional to the angle reached, at the same time with the rearward displacement of meniscus that are dragged back by the same motor muscles used in flexion. |
Legend: R - kneecap LLI - internal collateral ligament LLE - external collateral ligament Mi - internal meniscus Me - external meniscus Pop - popliteus muscle 4 - anterior horn external meniscus 5 - rear horn external meniscus 6 - anterior horn internal meniscus 7 - rear horn internal meniscus 8 - transverse ligament 9 - meniscus-kneecap wing 10 - popliteal muscle fiber expansion 11 - semimembranosus muscle fiber expansion |
Intercondylar eminence: the axial rotation along the longitudinal axis of the leg that can be carried out only with the bent leg after the 30 degrees (Kapandji 1977). The same author, in fact, says that only after the initial roll-back of the femoral condyles on the tibial plateau, there is disengagement of tibial condylar eminence that can rotate
in the femoral intercondylar fosse where it is stuck with the extended knee (as blocking cause of the axial rotation in extension -Kapandji 1977).
in the femoral intercondylar fosse where it is stuck with the extended knee (as blocking cause of the axial rotation in extension -Kapandji 1977).