Hock and Distal Limb Muscles (Week 3, Mod 7)

How many rows of bone are in the tarsus? List the names of the bones in each row, from LATERAL to MEDIAL

There are THREE rows of bone (not 2 like the carpus)

Proximal row:

• Talus

• Calcaneus - PALPABLE

Middle row:

• Central + 4th tarsal bone

Distal row:

• 1st, 2nd, 3rd, and 4th tarsal bone

• 4th TB bridges the middle and distal rows

How is the horse tarsus different from the canine tarsus? Think of 2 differences

1) 1st and 2nd TB fused

2) 3rd TB is very large

• Articulates with the 3rd metatarsal, which is weight bearing in this species

How many joints are within the hock? Name them (proximal to distal), and list which bones are involved in each.

1) Tibio-tarsal joint (TT joint):

• Talus + tibia & fibula

  • LARGE range of movement here (flexion and extension)

• Includes the calcaneus to some degree, but is not articular here

  • Main function is a lever, not for movement within the joint 

• Trochlea of the talus are here

2) Proximal Intertarsal joint (PIT joint):

• Talus & calcaneus + central + 4th TBs

3) Distal Intertarsal joint (DIT joint):

• Central + 1, 2, and 3rd TBs

4) Tarso-Metatarsal joint (TMT joint):

• 1, 2, 3, and 4, TBs + the metatarsal bones

*** REMEMBER: between rows, its a fibrous joint, so not much movement 

Describe the range of motion within the hock… How well can it flex and extend? What is unique about the horse hock that may increase its range of motion?

Has a large range of flexion at the tibio-tarsal joint… little movement at all of the other joints.

In the horse, the trochlea of the talus are NOT VERTICAL… more diagonal, allowing for rotation

• Essentially, this means that when the hock is flexed, instead of the distal limb folding in line with the rest of the limb, it will flare out laterally 

• Prevents “over-reach” injuries, meaning that the horse doesn’t kick its front limbs on accident while sprinting 

What kind of joint is the hock? What are 2 key features of this joint? (are similar to the stifle…)

The hock is a typical synovial joint…

• Has and EXTENSIVE joint capsule 

• Poor communication between compartments 

What areas of the horse hock are palpable? Think of 2 main regions, and where on those regions they can be palpated. Why is this important clinically?

Tibio-tarsal joint

• Is separate from the rest of the hock, so has different points that are palpable:

  • Dorso-medial aspect

  • Plantaro-lateral aspect

  • Plantaro-medial aspect (CALCANEUS BONE)

Distal regions of the joint:

• Dorsal-medial aspect

• Dorso-lateral aspect

Is helpful clinically in diagnosing where effusion may be coming from 

What 4 features contribute to the stability of the hock?

1) Collateral ligaments

• Long ligaments: Tibia → metatarsal bones

• Short ligaments: bridges bone → bone (in yellow)

2) Fibrocartilagenous plate reinforcement of joint capsule on the PLANTAR aspect

• Comes with an accessory check ligament

3) Retinaculum 

4) Plantar ligament 

• Attaches from tip of calcaneus bone to the metatarsals, much like the palmar ligament of the carpus

What is the clinical significance of damage at the tarsus joint in dressage horses?

Since they are encouraged to bear more weight on their hindlimbs, rather than their forelimbs, this can cause damage at the tarsus joint 

How many centers of ossification to the tarsal bones have? Who is the exception?

All have a SINGLE ossification center

• EXCEPT for the calcaneus bone → has 2

There are 3 muscles associated with the cranial region of the distal limb… what are their names?

1) Cranial tibial muscle

2) Peroneus muscle

• Dog and cat → peroneus longus (long fibular muscle)

• Horse → peroneus tertius (third fibular muscle)

3) Long digital extensor muscle

Describe the cranial tibial muscle… origin, insertions, function, and nerve supply.

Origin - proximal tibia 

Insertions - has TWO possible insertions… depend on species 

• Metatarsal bones (all species)

• Medial aspect of hock (via cunean tendon) in the HORSE, as well as inserting at the metatarsal bones

Function - hock FLEXION 

Nerve supply - Peroneal / fibular nerve (branch of SIATIC)

Describe the peroneus muscle… what are the two different kinds? Know what species they belong to, the origins, insertions, function, and nerve supply.

a) Peroneus longus

• DOG AND CAT

• Origin - lateral tibia & fibula

• Insertion - Plantar aspect of tarsus; wraps around from the front to back 

b) Peroneus tertius 

• HORSE ONLY

• Origin - Lateral femoral condyle 

• Insertion - Has TWO 

  • 3rd metatarsal (WITH cranial tibial muscle)

  • LATERAL aspect of the tarsus

  • Essentially inserts the opposite way from the cranial tibial muscle, forming a sort of girdle around the hock 

• Is more of a fibrous cord than an actual muscle belly 

FUNCTION - hock FLEXION

Nerve supply - Peroneal / fibular nerve (branch of SIATIC)

Describe the long digital extensor muscle… origin, insertion, function, and nerve supply.

Origin - extensor fossa (in the FEMUR)

• Tendon of origin is incorporated into the stifle joint capsule 

• Provides LATERAL collateral support to stifle 

Insertion - ALL digits  (the distal phalanx precisely)

• in the horse, the joints form little bursas as the tendon runs dorsally to the digit; done for cushioning and protection, same as the forelimb

Function - hock FLEXION, and digit EXTENSION 

Nerve supply - Peroneal nerve 

CAUDAL ASPECT OF LIMB

What exactly is the common calcanean tendon?? What 5 muscles is it made up of? Where does it insert, what is its function, and what nerve supplies it?

The calcanean tendon IS THE ACHILLES TENDON

Composed of:

• Biceps femoris 

• Semitendinosis

• Gracilis 

• Gastrocnemius 

• Superficial Digital Flexor 

Insertion - Calcaneus bone

• AKA calcanean tuberosity

  • Acts as a lever for the distal limb

  • Plantar ligament ALSO attaches here to provide support

Function - hock EXTENSION

Nerve supply - All nerves of the contributing muscles 

Describe the Gastrocnemius muscle… origin, insertion, function, and nerve supply.

Origin - Femur

• Has 2 tendons of origin

• Fabellae (sesamoids of the distal portion of the femur

Insertion - calcaneus bone

• is the MOST SIGNIFICANT contributor to the calcanean tendon 

Function - hock EXTENSOR, AND acts as a stifle FLEXOR (also crosses behind stifle)

Nerve supply - Tibial nerve (branch of SIATIC)

** Would be comparable to the calf muscle in humans

Describe the Superficial digital flexor muscle… origin, insertions, function, and nerve supply.

Origin - Distal femur (WITH gastrocnemius muscle)

Insertion - has MULTIPLE

• Calcaneus bone (part of calcanean tendon)

• Branches to ALL DIGITS (middle phalanx precisely)

Function - hock EXTENSION w/ digital FLEXION

• Also provides support of the distal limb joints in extension

• No accessory check ligament necessary (especially because of calcanean tendon)

Nerve supply - Tibial nerve

Describe the Deep digital flexor muscle… origin, insertion, function, and nerve supply

Origin - tibia

• Runs over tarsus to insert at the digits

• Is NOT a part of the calcanean tendon 

Insertion - ALL digits (distal phalanx)

Function - hock EXTENSOR and a digital FLEXOR 

• Also functions to support the distal limb joints in extension (has weak accessory check ligaments)

Nerve supply - Tibial nerve 

Describe the concept of the hindlimb stay apparatus… where in the hindlimb do horses have “stay apparatuses” in place?

Have them at the level of the stifle (patellar locking mechanism) and the hock (reciprocal apparatus)

Essentially keeps the limbs in the natural weight bearing position of extension

• Is able to keep one hindlimb in extension while the other rests, due to hindlimbs not being the weight bearing limbs

What is the reciprocal apparatus composed of? How does it work to keep the limb in extension?

Reciprocal apparatus -> peroneus tertius at cranial limb, superficial digital flexor at caudal limb

• Acts as a pulley system 

• Connects the stifle directly to the tarsus… if the stifle is extended, tarsus must also be extended; if its flexed, tarsus also must be flexed 

• CAN’T move independently 

If the reciprocal apparatus takes care of the proximal limb, then what stay mechanisms are at the distal aspect of the limb? What is their function?

Overall function: prevention of collapse into hyperextension

MTP joint:

Suspensory apparatus:

• Suspensory ligament

• Proximal sesamoids

• Distal sesamoidean ligaments

• Long digital extensor

• Purpose of suspensory apparatus; same as forelimb 

  • Also has a suspensory ligament that runs down the plantar aspect of the THIRD metatarsal, then wraps around dorsally to attach to long digital extensor muscle (like the common digital extensor of the forelimb)

At the proximal and distal interphalangeal joints:

• The SDFT and DDFT provide enough support; these are paired with annular ligaments (in blue) which gives extra stability to the tendons