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Is joint stability a matter of ligaments only?

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Article
This investigation sought to identify neuromechanical mechanisms by which a subject with an anterior cruciate ligament (ACL)-deficient knee might cope with the potentially destabilizing joint stresses during both level and downhill walking. Kinematic, kinetic and electromyographic data were collected from 21 subjects with arthroscopically verified ACL-deficient knees and Lysholm scores of 55–100 ( ), as well as from 12 healthy control subjects. Electromyographic data were recorded from the skin surface overlying rectus femoris, biceps femoris and gastrocnemius muscles. A dismountable slope of 6 m length and a gradient of 19% was constructed for downhill walking. Sagittal plane net joint moments and muscle mechanical power at the knee joint were calculated from force platform and videographic records using the inverse dynamics approach. During level walking there were no kinematic nor kinetic differences seen between ACL-deficient subjects and normals. The typical profile of muscle power at the knee contained three peaks during stance: an eccentric peak during early stance (K1); a concentric peak at mid stance (K2); and a second smaller eccentric peak (K3) during late stance. During downhill walking ACL-deficient subjects displayed a significantly smaller K1 compared to normals and their K1:K3 ratio was significantly less than that of normal subjects. Whereas normal subjects showed no hamstring activity during stance in level walking there was continuous activity throughout the stance phase displayed by the ACL-deficient and normal subjects. During downhill walking both the ACL-deficient and normal subjects showed continuous hamstring activity. However, the ACL-deficient subjects showed a significant delay in peak hamstring activity during late swing. Both groups on average displayed gastrocnemius peak activity just on heel strike during downhill walking but the linear envelopes of the ACL-deficient subjects were much more tightly time-locked to this critical event.
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Accelerated rehabilitation after anterior cruciate ligament (ACL) reconstruction has become increasingly popular. Methods employed include immediate extension of the knee and immediate full weight bearing despite the risks presented by a graft pull-out fixation strength of 200-500 N. The purpose of this study was to calculate the tibiofemoral shear forces and the dynamic stabilising factors at the knee joint for the reasonably demanding task of downhill walking, in order to determine whether or not this task presented a postoperative risk to the patient. Kinematic and kinetic data were collected on six male and six female healthy subjects during downhill walking on a ramp with a 19% gradient. Planar net joint moments and mechanical power at the knee joint were calculated for the sagittal view using a force platform and videographic records together with standard inverse dynamics procedures. A two-dimensional knee joint model was then utilised to calculate the tibiofemoral shear and compressive forces, based on the predictions of joint reaction force and net moment at the knee. Linear envelopes of the electromyographic (EMG) activity recorded from the rectus femoris, gastrocnemius and biceps femoris muscles were also obtained. The maximum tibiofemoral shear force occurred at 20% of stance phase and was, on average, 1.2 times body weight (BW) for male subjects and 1.7 times BW for female subjects. The tibiofemoral compressive force was 7 times BW for males and 8.5 times BW for females during downhill walking. The hamstring muscle showed almost continuous activity throughout the whole of the stance phase.(ABSTRACT TRUNCATED AT 250 WORDS)
Article
In this paper we introduce the concept of the functional (or equivalent) geometry of the knee, which is an attempt to reduce the natural knee with its complex geometry, frictional resistance and deformable cartilage into a two-dimensional joint comprising rigid femur and tibia in frictionless contact. An apparatus and method are described to measure the slope of the tangent to the surfaces of the 'equivalent' bones at their 'point' of contact. An antero posterior force of +/-300-500 N and axial compressive load of twice body weight were applied on cadaveric knee joints. The corresponding displacement of the tibia in the saggital plane was measured firstly with both cruciates intact and then when each was severed in turn. From the data obtained both the slope of the tangent mentioned above and the tensions developed along the cruciates under the influence of the forces applied were calculated. The results showed that the functional geometry of the knee in the saggital plane can be represented by a convex femur and a concave tibia. The tensions along the cruciates calculated on the basis of the experimental measurements were nearly always lower than the antero posterior force applied, and although this corroborated the trend demonstrated in a previous theoretical analysis, they were lower still. The reason for this may be the deformation of the cartilage under load, thus modifying the geometry of contact resulting in a more concave tibia of the 'equivalent' knee joint, than that of the rigid model used in the theoretical analysis.