What are the mechanisms behind the stay apparatus of the equine hindlimb?
1. Patellar lock
2. Reciprocal apparatus
3. Check ligament
4. Suspensory apparatus
What are the ligaments involved in the reciprocal apparatus?
Peroneus tertius (cranial)
Tendon of the superficial digital flexor muscle (caudal)
How does the reciprocal apparatus work in the equine hindlimb?
The reciprocal mechanism is comprised of two tendinous cords, the peroneus tertius muscle and the superficial digital flexor muscle (both of which are mostly tendinous).
This mechanism allows both the stifle and hock joint (tarsal joint) to move in unison, so when one flexes, the other flexes. This aides in both movement and the stay apparatus.
When the stifle joint is locked, the weight on the limb presses the hock joint (tarsal joint) to flex. However, tension in the superficial digital flexor resists this pressure, and the hock does not flex. The peroneus tertius is not involved in the stay apparatus, but is involved in flexing the hock joint when the the stifle joint flexes during movement.
Why doesn't the hindlimb have two check ligaments - superior and inferior - the way the forelimb does?
The hindlimb only has an inferior check ligament that connects the palmar aspect of the tarsus to the deep digital flexor tendon. There is no superior check connecting the tarsus to the superficial digital flexor tendon.
In the hindimb, the common calcaneal tendon, especially the tendon of the superficial digital flexor muscle, is largely tendinous with its proximal part a fibrous band that is attached to the femur and the other end "capping" the calcanean tuberosity. The SDFT’s attachment to the calcaneus eliminates the need for an accessory ligament.
There is a distal check ligament, like the forelimb, from the middle of the metatarsal 3 to the deep digital flexor tendon.
What are the muscles of the crus in the hindlimb? Ie., the extensors of the digits and the flexors of the tarsus?
FLEXORS OF THE TARSUS:
The flexors of the hock lie on the craniolateral surface of the crus. They originate from the distal femur and proximal tibia and insert on the tarsus and proximal metatarsus. They are:
Tibialis cranialis m. (flexes hock)
Peroneus tertius m. (flexes hock)
Long digital extensor m. (extends digits, flexes hock)
Which nerve innervates the flexors of the tarsus?
What are the main muscles that work as extensors of the tarsus, but flexor of the digits?
Gastrocnemius m (lateral & medial heads)
Superficial digital flexor
Deep digital flexor
Where does the gastrocnemius originate and insert? What is its innervation?
It originates in the distal femur and inserts on the calcanean tuberosity as part of the calcanean tendon.
It flexes the stifle and extends the tarsal joint.
It is innervated by the tibial nerve.
What is analogous to the common digital extensor of the forelimb in the hindlimb?
The long digital extensor.
What is the clinical significance of the cunean tendon?
The cunean tendon is the common name for the horse's medial insertion of the cranial tibial muscle (cranialis tibialis). It passes over the "seat of spavin" - the medial aspect of the tarsal joint near the meeting of tarsal 3, central tarsal and the cannon bone.
This area is susceptible to "bone spavin" - osteoarthritis or osteitis in the joint (specifically joints of the tarsal limb). When the cranial tibial muscle contract, the cuneon tendon presses against this area, which is why some vets perform tenectomies on the cunean tendon.