Hock & Distal Limb Muscles

What type of bones compose the tarsus?

The tarsal bones

Type - short bones

Describe the three rows of bones composing the tarsus.

Proximal row:

• Talus

• Calcaneus - palpable

Middle row:

• Central Tarsal Bone & 4th TBs

Distal row:

• 1, 2,3 & 4 TBs

• 4th TB bridges middle and distal rows (In middle and distal row)

Describe the differences between the equine and canine species considering the tarsal bones. 

Horse

• 1 & 2 fused

• 3 very large, as it articulates with the third metacarpal

Describe the 4 joints making up the tarsus and what articulating bones form these joints. 

What type of joints are the intratarsal joints?

1. Tibio-tarsal joint (TT):

• Talus - tibia & fibula

• Large range of movement

• Calcaneus - not articular

2. Proximal intertarsal joint (PIT):

• Talus & calcaneus -

• Central TB & 4 Tbs

3. Distal intertarsal joint (DIT):

• Central - 1, 2 & 3 TBs

• 4th articulates with this

4. Tarso-metatarsal joint (TMT):

• 1,2,3 & 4 TBs - metatarsal bones

• Fibrous Joint

Describe what type of movement occurs in the tarsus and which of these joints have the most-least mobility.

• Large range of flexion:

• Most movement tibio-tarsal joint

• Little movement at other joints

What is the significance of a oblique trochlea in the talus of the equine? What angle are these trochlea in the dog?

• Produces rotation as they create an oblique angle

• In dogs these trochlea are vertical, so they move more cranial with their limbs, restricting to extension and flexion only

  • In horses: the movement is a combination of flexion/extension and a slight outward rotation. 

  • This anatomical adaptation is a result of the horse's evolution for cursorial (adapted for running) locomotion, optimizing the hind limb for powerful propulsion and weight-bearing. 

What direction are the distal hindlimbs in flexion and protraction?

• Distal hindlimb directed lateral to forelimbs

• Prevents 'over-reach' injuries

What direction are the distal hindlimbs in extension and retraction?

• Distal hindlimb directed in axial plane, midline

• Aids in propulsion!

What type of joint is the tarsus?

• Features of typical synovial joint:

• Extensive joint capsule due to complexity of joint

• Poor communications between compartments in ALL species, most marked in the larger species

What aspects of the tibio-tarsal joint are palpable?

• What are the compartments?

Tibio-tarsal joint separate from the rest

• Dorsomedial aspect

• Plantarolateral aspect

• Plantaromedial aspect

Distal Regions

• Dorsomedial aspect

• Dorsolateral aspect

What provides the tarsal joint with the necessary stability?

• Stability:

1. Collateral ligaments (A series of them)

• Long: Tibia - metatarsal bone

• Short: Bridge bone - bone

Clinical significance: dressage horses

2. Fibrocartilagenous reinforcement of joint capsule

3. Retinaculum

4. Plantar ligament

•  'Curb' - inflammation or swelling, can cause mineralization, called curb in equines

Describe the centers of ossification found within the tarsal bones.

• Development:

• All single centre of ossification:

• Except calcaneus = 2

List the following for the cranial tibial muscle.

• Origin

• Insertion

• Function

• Nerve Supply

Muscle located in cranial aspect of tibial region.

1. Cranial tibial muscle

• O - proximal tibia

• I - metatarsal bones (all species)

• I - Additionally, on medial aspect hock (horse) = Cunean tendon

• Function - Hock flexion

• Nerve Supply: Peroneal / fibular Nerve (branch of Sciatic)

List the following for the peroneus muscle. For this muscle note the aspect present in the horse, and the dog and cat.

• Origin

• Insertion

• Function

• Nerve Supply

A. Peroneus longus / long fibular muscle - (Dog & cat (not horse)

• O = Lateral tibia & fibula

• I = Plantar aspect tarsus

B. Peroneus tertius / third fibular muscle

• Horse (not dog and cat)

• O - lateral femoral condyle (with long digital extensor muscle)

• I - 3rd MT (with cranial tibial muscle)

• I - lateral aspect tarsus

• Function: Hock flexion

• Nerve supply: Peroneal / fibular Nerve (branch of Sciatic)

List the following for the long digital extensor muscle.

• Origin

• Insertion

• Function

• Nerve Supply

• O - extensor fossa (femur)

• Tendon of origin incorporated into stifle joint capsule

• Provides lateral collateral support to stifle

• I - all digits

• distal phalanx - extensor process

• Function: Hock flexion, Digital extension

• Nerve supply: Peroneal / fibular Nerve (branch of Sciatic)

List the following muscles which are components of the common calcanean tendon. Also list the other following details:

• Insertion

• Function

• Nerve Supply

Components:

1. Biceps femoris

2. Semitendinosis

3. Gracilis

4. Gastrocnemius

5. Superficial digital flexor

• I - calcaneus

• Tuber calcis / calcanean tuberosity

  • Acts as lever for distal limb

  • Plantar ligament

• Function = hock extension

• Nerve supply - reflects contributors

List the following for the gastrocnemius muscle.

• Origin

• Insertion

• Function

• Nerve Supply

• O - femur, 2 tendons of origin (fabellae)

• I - calcaneus

• Most significant component of common calcanean tendon

• Function: Hock extensor, (Stifle flexor)

• Nerve supply: Tibial Nerve (branch of Sciatic)

List the following for the superficial digital flexor muscle.

• Origin

• Insertion

• Function

• Nerve Supply

• O- distal femur (with gastrocnemius)

• I - calcaneus (part of common calcanean tendon)

• I - branches to all digits (middle phalanx)

• Function: Hock extension, Digital flexion

• Support of distal limbs joints in extension

  • No accessory check ligament

• Nerve supply: Tibial Nerve (branch of sciatic)

List the following for the deep digital flexor muscle. 

• Origin

• Insertion

• Function

• Nerve Supply

O- tibia (Runs over tarsus)

• Not part of common calcanean tendon

• Passes through slit of SDFT

• I - branches to all digits (distal phalanx)

• Function: Hock extensor, Digital flexor

• Support of distal limbs joints in extension

  • Weak accessory check ligament

• Nerve supply: Tibial Nerve (branch of sciatic)

Summarize the distal hindlimb muscles.

What is the purpose of the hindlimb stay apparatus?

• Natural weightbearing position = extension

• Ability to lock weight bearing hindlimb in extension while other rested

• NB: Resting forelimb = lame

What systems are in place that makes the hindlimb stay apparatus possible?

• Hip / stifle / hock:

  • Prevention of collapse into flexion, keep in natural extension

• Hip - no passive system

• Stifle locked into place via the: Patellar locking mechanism

• Hock locked into place via the: Reciprocal apparatus

What is the reciprocal apparatus?

• How does it function?

Part of the equine, hindlimb stay apparatus, part of the hock. Formed by interaction of two structures:

• Peroneus tertius

• Superficial digital flexor

• Fibrous bands - acts like a pulley system, if stifle is extended, tarsus must also extend, if flexed tarus and stifle must both be flexed

• Proximal movement of origin = proximal movement of insertion

• Stifle and hock must move together

What prevents the MTP/PIP/DIP joints from hyperextending, thus contributing to the hindlimb stay apparatus?

• MTP joint:

• Suspensory apparatus:

• Suspensory ligament

• Proximal sesamoids

• Distal sesamoidean ligaments

• Long digital extensor

• MTP, PIP & DIP joints:

• SDFT (no accessory check lig)

• DDFT + accessory check lig

• Annular ligaments

What allows the horse to produce massive power during hindlimb extension?

• Combined force from huge muscle mass

  • Gluteal muscles → Very well developed

  • hip extensors / limb retractors (extra heads)

  • stifle extensors

  • hock extensors (reciprocal apparatus)

• Forces directed caudally in axial plane:

• Angled trochlea on talus - limb is extended, makes limb direct caudally

  • Great for jumping!

• Significance for you?

  • Horses kick straight back

  • Don't stand directly behind a horse!