Lab instructions:

Dissection teams are to dissect, in parallel, an equine and ruminant pelvic limb, as provided on the day of the lab. Divide into teams of 3 and 2 to accomplish the dissection.  Be sure to review the dissection and anatomy of both limbs as a team.  Supplement dissection learning with liberal study and review of the limb models available in lab, wet and dry. The limbs can be tagged and stored at the conclusion of the lab for subsequent review. In addition, external anatomy of the hoof capsule (see Lab 7) may be reviewed on these limbs.

This lab will start with important anatomy at the ungulate stifle joint and this content is to be learned on lab prosections and models.  Following stifle anatomy, the lab guide is split into a horse section and a ruminant section. Please refer to those sections for specific dissection instructions for the limb you are working on. It is ok to start with the limb dissections and return to the stifle anatomy as specimens become available for review.

Stifle (genual) joint – horse and ruminant

The stifle joint consists of the femur, tibia, and patella, three joint compartments, and various ligaments and menisci.  Attachments of muscles are also incorporated. The fibula does not contribute directly to the stifle joint.

Recall that the patella is attached to the tibial tuberosity by a single patellar ligament in the carnivore. The ox and horse have three (3) patellar ligaments, the pig and small ruminants have one (1). The three ligaments in the ox and horse are termed the medial, intermediate (or middle) and lateral patellar ligament.

In addition the patella is attached to the femur via medial and lateral femoropatellar ligaments.  The stifle joint is stabilized by very strong medial and lateral collateral ligaments of the stifle, and by cranial and caudal cruciate ligaments.  A medial meniscus and lateral meniscus are slotted between the femoral condyles and tibial plateau, to increase joint congruence.  The lateral meniscus is bound to the femur by the caudally located meniscofemoral ligament.

Patella locking mechanism – horse

Extending from the medial aspect of the patella is the hook-shaped parapatellar fibrocartilage that is continuous with the medial patellar ligament. During flexion and extension of the joint, the large gliding surfaces of the patella and trochlea are in contact. Narrow resting surfaces at the distal aspect of the patella and its fibrocartilage, and the proximal aspect of the medial ridge of the femoral trochlea, engage when the patella is pulled proximally and rotated medially to ‘lock’ over the medial ridge in a resting position. This is the normal patellar locking mechanism of the horse. You may have noticed that when horses are just standing around, taking it easy, they tend to stand with all their weight on one pelvic limb, with the other one relaxed and sort of half-cocked, resting on the toe of the hoof. When they do this, they lock the patella of the supporting limb over the medial ridge of the femoral trochlea (i.e., they “put it in park”). Fixation of the stifle joint by the patellar locking mechanism is the key to converting the pelvic limb into a ‘passive stay’ for support of the body weight (more of this passive stay apparatus is revealed as we continue distal dissection).

Structures of tarsus – horse

(NOTE: see further along for ruminant instructions)

At this point, dissection of the provided limb specimen is to be performed.  Learn the following equine limb anatomy from directed dissection activities and reference to prosections and models (wet, dry, plastinated).

Starting our tarsus dissection on the plantar aspect of the joint, the flexor canal of the tarsus is a space for tendons to pass through, heading to the digit.  It is bound dorsally by the sustentaculum tali of the calcaneus, laterally by the calcaneus, and plantaromedially, by the fibrous structure, the flexor retinaculum. A tarsal sheath envelops the tendons.  There are calcaneal bursae at the tarsus, cushioning tendons as they pass by or insert in the region.  The subtendinous calcaneal bursa lies between the SDF tendon and the calcaneal tuberosity, i.e. at the site the tendon forms a cap over the bony prominence.

The tarsus is supported and stabilized by strong, complex medial and lateral collateral ligaments.  The long plantar ligament binds the calcaneus to the distal tarsus and metatarsus, counteracting the proximal pull of the common calcaneal tendon.

Moving to the cranial crus and dorsal tarsus, the tendons of the cranial tibial m., fibularis tertius m., long digital extensor m. and lateral digital extensor m. all cross the tarsus.  The extensor muscle tendons continue to the digit and we will follow those shortly.  Recall from Lab 5 the tendons on the craniolateral crus are bound down to the limb by three (3) horizontal bands of thickened fascia, the extensor retinacula (pl.), as they cross over the tarsal region.

At the distal attachments of the cranial tibial and fibularis tertius mm. both of these muscles have two tendons of insertion that are closely related. The medial tendon of the cranial tibial m. is referred to as the cunean tendon and it passes over the cunean bursa before it inserts on the fused first and second tarsal bones.

Okay, before we dissect more distally, lets consider the tarsocrural joint in detail. This is the most proximal joint of the four joints that make up the tarsus. The tarsocrural joint in the horse is lined by one extensive synovial joint sac that is subdivided into four (4) pouches: the dorsomedial, dorsolateral, plantaromedial and plantarolateral tarsocrural joints pouches.  The pouches represent clinically accessible regions of the joint sac, between bones, tendons and ligaments. The pouches are quite obvious when there is synovial effusion present (increased volume of synovial fluid) causing excessive filling of the joint sac and bulging of the sac between bones, tendons and ligaments, creating the pouch locations.

Clinical relevance:

Tarsal hydrarthrosis/effusion (“bog spavin”), arthrocentesis, surgical access via arthroscopy to tarsocrural joint (making sure to avoid the cranial branch of the medial saphenous v. passing over the dorsomedial pouch).

 

Metatarsus – horse

The next step is to continue following the tendons extending distally through to the digit. Dorsally, the lateral digital extensor tendon combines with the long digital extensor tendon, usually in the proximal half of the metatarsus, such that only the long digital extensor tendon travels into the digit.  On the plantar aspect the flexor tendons pass into the digit as they do in the thoracic limb.  The suspensory ligament is also of a similar construct as was dissected in the thoracic limb.  In the pelvic limb, only an accessory (check) ligament of the DDF may be present, if at all.  A small band of fibrous tissue passing from the plantar proximal metatarsal region to the DDF tendon represents the accessory ligament of the DDF – it may be vestigial (small, underdeveloped) or absent.  There is no accessory (check) ligament of the SDF in the pelvic limb.

Digit – horse

The pelvic limb digit anatomy is essentially identical to that already studied in the thoracic limb and it will not be considered further in this section. However, if time allows and you wish to explore further, please feel free to dissect ligaments and tendons in the digit area. Refer back to this lab for assistance with identification.

Hoof (foot) anatomy will be studied in the next lab (Lab 7) and these limbs can also be used in that lab to identify external anatomy of the hoof capsule.

Passive Stay Apparatus – Horse

Horses spend much of their adult life standing, and have a mechanism to rest one of their hindlimbs while the other one is “locked” or “placed in park” and is carrying the bulk of the weight in the hind end. This mechanism is referred to as the Pelvic Limb (Passive) Stay Apparatus. The limb that is bearing most of the weight is locked anatomically to decrease the need for muscle activity (i.e. reduce energy expenditure) to keep the limb extended and the horse standing!

The horse has a set of fibrous support structures that can convert the limb into a passive, weight-bearing stilt (stay). These structures make up the stay apparatus. Being the “efficiency experts” that they are, horses only have to lock the patella over the medial ridge of the femoral trochlea, engaging their resting surfaces, and what do you know, they’ve “put the limb in park.” If the stifle can’t flex, neither can the tarsus (due to the reciprocal apparatus). The lower joints are passively supported by the fibrous SDF, the DDF with its fibrous accessory ligament (variably developed), and the fibro-osseous sling, known as the suspensory apparatus, formed by the 3rd interosseus muscle (suspensory ligament), the proximal sesamoid bones, and the distal sesamoid ligaments (straight, oblique, cruciate, short). But, aren’t we forgetting the highly movable hip joint? No problem. It’s not going anywhere, as long as the horse’s weight is balanced over and pushing straight down on the limb. In order for the head of the femur to move downward (as in hip flexion on a weight bearing limb), the medial trochlear ridge must be free to rotate upward. This is prevented by the patellar lock – the femur is hooked under the patella like a crowbar under the edge of a building. Unless the horse shifts backward or forward, the hip joint doesn’t need to be fixed in any way; it’s a balancing act.

Structures of tarsus – ruminant

At this point, dissection of the provided limb specimen is to be performed.  Learn the following ruminant limb anatomy from directed dissection activities and reference to prosections and models (wet, dry, plastinated).

Starting our tarsus dissection on the plantar aspect of the joint, the flexor canal of the tarsus is a space for tendons to pass through, heading to the digit.  It is bound dorsally by the sustentaculum tali of the calcaneus, laterally by the calcaneus, and plantaromedially, by the fibrous structure, the flexor retinaculum. A tarsal sheath envelops the tendons.  There are calcaneal bursae at the tarsus, cushioning tendons as they pass by or insert in the region.  The subtendinous calcaneal bursa lies between the SDF tendon and the calcaneal tuberosity, i.e. at the site the tendon forms a cap over the bony prominence.

Moving to the cranial crus and dorsal tarsus, the tendons of the cranial tibial m., fibularis tertius m., fibularis longus m., long digital extensor m. (with medial and lateral bellies in the ruminant), and lateral digital extensor m. all cross the tarsus.  The extensor muscle tendons continue to the digit and we will follow those shortly.  Recall from Lab 5 the tendons on the craniolateral crus are bound down to the limb by two (2) horizontal bands of thickened fascia, the extensor retinacula (pl.), as they cross over the tarsal region.

Ok, before we dissect more distally, lets consider the tarsocrural joint. As in the horse, the tarsocrural joint in the ruminant is lined by one extensive synovial joint sac.  When there is synovial effusion present (increased volume of synovial fluid), causing excessive filling of the joint sac, the extent of the tarsocrural joint space is more apparent.

Metatarsus – tendons – ruminant

The next step is to continue following the tendons extending distally through to the digit. Recall the metatarsus in the ruminant consists skeletally of the fused 3rd and 4th metatarsal bones.  Dorsally, the long digital extensor tendon will have a medial and lateral tendon extending from the respective muscle bellies.  The medial tendon attaches to the medial digit (digit 3). The lateral tendon bifurcates at the metatarsophalangeal joint to send a branch to the medial and lateral digits (digits 3 and 4).  The lateral digital extensor tendon passes down independently to attach to the lateral digit only.  Therefore each digit receives two extensor tendon attachments.  On the plantar aspect of the metatarsus the flexor tendons pass into the digit as they do in the thoracic limb.  There are no accessory ligaments of the flexor tendons present in the pelvic limb of the ruminant.  The suspensory ligament (fused 3rd and 4th interosseous mm.) is also of a similar construct as was dissected in the thoracic limb.

Digits – ruminants

The pelvic limb digit anatomy is essentially identical to that already briefly studied in the thoracic limb and it will not be considered further in this section. However, if time allows and you wish to explore further, please feel free to dissect ligaments and tendons in the digit area.