10.21.96

PASSIVE STAY APPARATUS:

this week we'll talk about the hindlimb stay apparatus. The function of 
this is to bear weight in a very efficient manner, most economical 
method. animal can lock one limb and keep the other one relaxed. so one 
leg is relaxed - you see this in the horse when one limb is flexed and 
the tip of the hoof is the only part touching the ground, and the horse 
has weight on other leg.

components:

PATELLA LOCKING MECHANISM

the patellar ridges of the femoral trochlea - in the horse, the medial 
ridge is noticably larger than the lateral ridge. the patella for the 
most part sits over the groove between the ridges, or slightly to the 
lateral side. it is held in place by the intermediate patellar ligament, 
the lateral patellar ligament, and the medial patellar ligament. but, 
there's a huge medial ridge, so the horse has evolved the parapatellar 
fibrocartilage, extending the patella medially, such that the medial 
ligament can extend in more of a straigt line from the tibia to the 
fibrocartilage.

there is a gliding surface of the patellofemoral joint,  and there is a 
resting surface. when the animal is walking normally, the patella for the 
most part is gliding- the smooth surface on its caudal end just glides 
over the trochlea's gliding surface.

there is a narrow flattened area on the distal portion of the patella, 
and there is a flattened area on the craniodorsal surface of the 
trochlea. when the joint is extended, the patella rises up above the 
trochlea itself. by shifting weight, horse causes medial patellar 
ligament to shift, and the whole patella shifts such that it rests on the 
medial ridge and is locked into place

cut quadriceps and femoropatellar ligaments and play with your stay 
apparatus (if you have a horse, i guess.).

some young horses get upward fixation of the patella. then, the horse can 
lock the stifle but can't unlock it. this causes the leg to be fixed with 
the stifle extended. they fix that by cutting the medial patellar 
ligament and letting it heal naturally, so the whole joint is looser.

rich will be showing us a demo of this soon.

realize: when horse is bearing wt, it is centered over the proximal femur 
such that it pushes the stifle into flexion, the hock into flexion, and a 
hyperextension of the tarsus. the cannon bone would be pushed to the 
ground, if you can imagine that. the stay apparatus prevents this 
possibility - there is also a

RECIPROCAL APPARATUS:
tendinous muscles: peroneus tertius cranially, and superficial digital 
flexor caudally. these muscles for the most part work with the locking 
mechanism such that when the horse wants to flex its stifle, it also ends 
up flexing its hock. horse can not flex stifle w/o flexing hock due to 
reciprocal apparatus.

the superficial digital flexor passs over the calcaneus, but isn't really 
attached, per se, just held down over it by a lot of retinaculum and then 
it continues on

know the components of the common calcanean tendon which inserts on the 
calcaneal tuberosity.

so, similarly, when you lock the stifle in extended position, the hock is 
locked as well. there's a bit more- the deep digital flexor is underneath 
the superficialdigital flexor, and you may see coming off the metatarsal 
or more commonly off the metacarpal a small ligament from the metatarsal 
or metacarpal to the deep digital flexor tendon, called the "check" or 
"accessory" ligament of the deep digital flexor tendon. this helps to 
relieve some of the strain caused by having the distal joints constantly 
hyperextended.

may also notice a ligament even deeper to deep dig flexor, and that's 
called the interosseus or suspensory ligament, which dives in deep to 
deep dig flexor, bifurcates and attaches to both sesamoids, then 
continues on.

one last thing you might see, on cranial surface of hindlimb are the 
tendinous peroneus tertius and right deep to it the cranial tibial. and 
they're heading down to the tarsus and metatarsus, and they do an 
interesting thing....peroneus tertius actually divides its tendon of 
insertion into a lateral branch which insertson the lateral side of T4 
and the calcanean tuber, and the dorsal branch, which stays on the dorsal 
surface and inserts on the tarsal bone. underneath the bifurcation is the 
tendon of the cranial tibial muscle, which also stays on the dorsal side, 
passing over onto the proximal craniodorsal surface of the metatarsal, 
and branches as well to the medial side of the Tarsal bone- medial branch 
of cranial tibial, aka cunean tendon, inserting on T1 and T2. look at the 
bursas underneath these tendons.