IN-DEPTH: HIND LEG FROM THE PELVIS DOWN


When the stifle joint is loaded, the cup-shaped menisci diminish part of the loading forces. Due to their broad contact surfaces to the condyles of the femur and tibia plateau, axial compression is turned into circumferential meniscal extension.18 The menisco-tibial ligaments take an important role in this force transmission.19 It was shown that forces occurring during different degrees of stifle move- ment lead to differential tensile loading of the indi- vidual ligamentous structures. In full stifle extension (160°) the tensile loading of the medial cranial meniscotibial ligament was four times higher, predisposing this structure and the medial meniscal anterior horn to structural tear.18 Since these structures incorporate a high density of mechanoreceptors, tear or distruction can lead to proprioceptive dysfunction20 and repetitive damage. While the stifle extends, the tibia slightly rotates externally and the center of joint contact moves cra- nially on both tibial condyles, with the lateral mov- ing a greater distance than the medial.21 A study revealed that the articular contact area in the me- dial femorotibial compartment decreases in exten- sion, as well as the area of highest joint contact on the medial tibial condyle, whereas both increase in the lateral compartment.21 To resist caudal translation of the tibia in exten-


sion, the cruciate ligaments restrict tibiofemoral transitional gliding. The caudal cruciate ligament is supported by the meniscofemoral ligament, a structure running from the lateral meniscus to- wards the medial femoral condyle. In horses the meniscofemoral ligament is a substantial structure related to the caudal tibial slope and size ratio be- tween the meniscofemoral and caudal cruciate liga- ment is proven to be greater in quadrupeds compared to human knees.22 In a resting position, the patella can be hooked


over the medial ridge of the femoral trochlea. The vastus medialis of the quadriceps femoris muscle creates a slight tensional load on the medial aspect of the patella, hence hooking-in a ligamentous loop construction built of heavy ligamentous structures in the front of the stifle together with the parapatel- lar cartilage. This leads to engagement of the framesaw construction of the reciprocal apparatus, thus stabilizing the hindleg in a stretched position.6 The required muscle activity for stabilization is con- sidered less than 2% compared to the amount of force that would be required without a patella mech- anism23 and the quadriceps femoris muscle is rela- tively relaxed in this position.2 In the mid-stance phase of movement, the rectus femoris part of the quadriceps muscle prevents stifle joint collapse. At the end of stance phase, the mus- cle activity is replaced by activation of the vastus lateralis portion, leading to lateral rotation of the patella around a longitudinal axis to prevent it from hooking on the medial ridge of the femoral trochlea in movement.16


To raise the distal limb in protraction, flexion


cycles of the stifle and tarsal joints are well coordi- nated during the swing phase by the reciprocal ap- paratus.14 Even synchronous movements between stifle, tarsal, and fetlock joints have been demon- strated.24 As soon as the calcaneus receives hori- zontal orientation in swing phase, the superficial digital flexor tendon is tensioned, leading to passive flexion of the digital fetlock and interphalangeal joints.1 From a biomechanical point of view, perfect syn- chronization of individual muscle activity is crucial for an energy-efficient stride and joint protective muscle stabilization, thereby preventing biome- chanical dysfunction, which may lead to long-term lameness.


From Stifle to Hip


The coxofemoral joint is built as a primary weight- bearing ball-and-socket joint, and is held in a slightly flexed position (115°) by the activity of the hamstrings and gluteal muscles. Stabilization re- quires only a minimum of energy, since it is mainly provided by heavy ligamentous structures. The firm femoral head ligaments, in particular the prom- inent accessory femoral head ligament, fixate the femur onto the acetabulum and give partial origin to the pectineus and gracilis muscle before blending into the prepubic tendon.2 Even though being built as a universal joint, rotation as well as ab- and adduction are rather limited by the preeminent surrounding muscular tissue and firm ligaments, and the primary move- ment occurring is flexion and extension in the sagittal plane.2 Flexion and extension around the hip joint as a


pivot point are responsible for protraction and re- traction of the entire limb. While maximum pro- traction occurs shortly before the end of the swing phase, maximum retraction occurs during break- over.14 The motion of the hind limb in the sagittal plane appears to be pendular around a rotation point in the acetabulum.24 Following complete limb extension at the end of propulsion, the non-weight-bearing swing phase is initiated. The moment the hoof breaks over toe- pivot point, the retraction of the hindlimb starts by joint flexion and shortening of the distance between hoof and tuber coxae, to facilitate lifting of the leg.1 Even though the shortening distance between hoof and stifle measures approximately 11 cm17 in the mid-swing phase of an average horse, overall the limb is only shortened about 6 cm in length, since the distance between the stifle and tuber coxae elon- gates8 through hip extension in protraction prior to ground contact in the landing phase.1 While hip flexion during the swing phase is


mainly provided by concentric contraction of the il- iopsoas muscle together with activation of the tensor fascia latae and rectus femoris muscle, flexion of the joint in stance phase is limited by eccentric contrac-


AAEP PROCEEDINGS  Vol. 65  2019 327


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