Lab instructions:

Part A: An instructor will provide an overview of this lab and highlight a few examples of comparative bone features across the species being studied.

Part B: Use ‘bone boxes’ (containing carnivore skeletons), bone models, articulated skeletons and images, to study the regions, bones and joints of the pelvic limb.  Refer to the terms tables for focused studying. Be able to identify pelvic limb regions, the bones and their features, and joints and structures related to joints, in any species (i.e. dog, cat, horse, ruminant and pig), as applicable to that species.  It is understood that not all material will be covered in lab and it is anticipated that continued learning and review of this content will continue as self-study effort.

Pelvic Limb Regions

Os Coxae

The pelvic girdle, or pelvis, consists of two hip bones, which are united at the symphysis pelvis midventrally and join the sacrum dorsally. The pelvis shape and angles vary between species.  Each hip bone, or os coxae, is composed of four distinct bones developmentally. These are the ilium, ischium, pubis, and acetabular bone. They fuse during the twelfth postnatal week, and form the socket, i.e. the acetabulum, that receives the head of the femur in creation of the ball and socket hip joint. The small acetabular bone, which helps form the acetabulum, is incorporated with the ilium, ischium, and pubis when they fuse so it is not identified in the mature animal.  The articular surface of the acetabulum is indented by the acetabular fossa  and the ligament of the head of the femur attaches to this fossa.  The circumference of the articular surface is broken at the caudomedial part by the acetabular notch. The two sides of the notch are connected by the transverse acetabular ligament. The largest and most cranial of the os coxae bones is the ilium, which articulates with the sacrum (forming the sacroiliac joint). The ischium is the most caudal, whereas the pubis is located ventromedial to the ilium and cranial to the large obturator foramen.  The obturator foramen is closed in life by the obturator membrane and the external and internal obturator muscles that the membrane separates.  Vessels and nerves pass through the foramen.

Femur and Patella

The proximal end of the femur consists of a head, neck and two processes, the greater trochanter and lesser trochanter.  The horse greater trochanter is subdivided into cranial and caudal parts.  A very prominent third trochanter is present in the horse.  The fovea capitis is a circular pit on the medial part of the head. The fovea capitis serves for the attachment of the ligament of the head of the femur (ligamentum capitis ossis femoris) (and the accessory ligament in the horse, see arthrology section). Distally the femur consists of a trochlea with medial and lateral ridges and a trochlear groove, and medial and lateral condyles and epicondyles.  The extensor fossa of the femur is an important muscle attachment site for cranial crus muscles.  The patella articulates with the articular surface of the trochlea of the femur. This is the sesamoid bone associated with the tendon of the m. quadriceps femoris.  Additional features for the femur and patella are noted in the terms list.

Tibia & Fibula

The tibia, the shin or leg bone, has a proximal surface that articulates with the femur and that is formed largely by two relatively flat condyles (medial and lateral). An intercondylar eminence separates the two condyles.  The fibula articulates with the lateral condyle, and cranial tibial and fibularlis longus mm. arise from here.  A sesamoid bone in the tendon of origin of the popliteus (seen in radiographs) articulates with the caudolateral condyle of the tibia.  Two biconcave fibrocartilages, the menisci, fill part of the space between the apposed condyles of the femur and tibia, making the joint congruent (see stifle joint below). The cruciate ligaments of the stifle are named according to their cranial or caudal attachment to the tibial intercondylar areas (see stifle joint below).  The tibial tuberosity is the large quadrangular process on the proximal cranial surface of the tibia. The quadriceps femoris, the biceps femoris, and the sartorius attach to this tuberosity by means of the patella and patellar ligament.  Distally, the tibia articulates with bones of the tarsus.  The prominent medial projection of the tibia is the medial malleolus.  Centrally, the distal tibial articular surface (the cochlea) has an intermediate ridge that locks into the trochlear groove of the talus. In equine clinical jargon, this ridge is referred to as the distal intermediate ridge of the tibia (DIRT). It is an important bone feature because it is a very common location for osteochondritis dessicans (OCD, a developmental bone disease) to develop and DIRT lesions are a thing :).

The fibula construct varies considerably between species, as shown in the images.  It is a full length bone in the carnivore and pig and is missing much of its body in the ox and horse.  A proximal extremity (head of fibula) is consistent and articulates with the lateral condyle of the tibia.  The distal extremity, the lateral malleolus, articulates with the lateral distal tibia in the carnivore, swine, and ruminant.  In the ruminant the lateral malleolus of the fibula is a solitary articulating bone, with no body attached to it (this is one key feature to keep in mind when looking at radiographs of the ruminant tarsus).  The horse has a vestigial fibular body of varying length and distally the lateral malleolus of the fibula has fused completely to become a part of the tibia (recall this is a similar set up to the horse ulna and radius story, where the lateral styloid process of the ulna has fused to the radius).

Tarsus

The tarsus, between the metatarsals and the crus (leg), is composed of six to seven tarsal bones and the related soft tissues. It is also called the hock. The bones are arranged in three irregular rows with variation between species in fusion of the bones with each other.  The proximal row is composed of a long, laterally located calcaneus and a shorter, medially located talus. The talus has a trochlea on its proximal end in the carnivore and horse. In the artiodactyla there is a proximal and distal trochlea, a defining feature of animals in this order.  The trochlea has medial and lateral trochlear ridges separated by a groove for articulation with the tibial cochlea.  The tuber calcanei is a traction process of the calcaneus that projects proximally and caudally. The extensor muscles of the tarsus insert on this process via the common calcanean tendon. On the medial side of the calcaneus is a bony process, the sustentaculum tali. A tendon of the deep digital flexor glides over the plantar surface of this process.

There is a central tarsal bone occupying the ‘middle row’. The distal tarsal row consists of up to four separate bones or a combination of these bones fused to each other.  Species variation is notable in this part of the tarsus as the images show.  The large fourth tarsal bone, which completes the distal row laterally, articulates with the calcaneus proximally. The fourth tarsal is as long as the combined lengths of the third and central tarsal bones against which it lies.

Metatarsals & Phalanges

The metatarsal bones of the carnivore resemble the metacarpal bones except for the first metatarsal, which may be divided, rudimentary, or absent.  Similarly, the metatarsal bones of the ungulates resemble their metacarpal counterparts.  However in the ruminant, there is a metatarsal sesamoid bone in place of a 5th metatarsal bone.  The phalanges and sesamoids form the skeleton of the digit. Those of the pes, are similar to those of the manus.

Joints & Ligaments

Pelvic ligaments and coxofemoral (coxal) joint and ligaments

The ischium and pubis of the right and left sides are joined on the median plane at the symphysis pelvis.

The sacroiliac joint is an articulation of stability rather than mobility. The right and left wings of the ilia articulate with the right and left wings of the sacrum. In the adult most of the apposed articular surfaces are united by fibrocartilage that surrounds a small area of articulating hyaline cartilage with synovial fluid. Dorsal and ventral sacroiliac ligaments reinforce the joint.  Sacroiliac joint injections are performed in equine practice, particularly in sports medicine cases with lameness located to this site.

The sacrotuberous ligament in the carnivore runs from the transverse processes of the last sacral and first caudal vertebrae to the ischiatic tuberosity. It serves as an origin for several muscles that have been dissected. In the ungulates this ligament has expanded into a sheet and is referred to as the sacrosciatic (or broad sacrotuberous) ligament.

As mentioned, the hip joint  is a ball-and-socket joint whose main movements are flexion and extension, although any direction is possible. to a degree. The joint capsule passes from the neck of the femur to a line peripheral to the acetabular lip. Also already mentioned, the ligament of the femoral head is a thick band of collagenous tissue that extends from the acetabular fossa to the fovea capitis, anchoring the femoral head into the acetabulum. See the horse addition below.

The transverse acetabular ligament is a small band that extends from one side of the acetabular notch to the opposite side. It is located at the ventrocaudal aspect of the acetabulum and continues as the acetabular lip, which deepens the acetabulum by forming a fibrocartilaginous border around it.

Coxal (hip) joint – horse specific feature.

Besides the standard ligament of the femoral head, uniquely, the horse has an additional accessory ligament of the femoral head passing from the femoral fovea capitis through the acetabular notch to the pectin pubis where it inserts in the prepubic tendon. This additional powerful ligament in the horse may be one explanation for the lower incidence of hip luxation in this species.

Stifle joint and ligaments

The stifle is a complex, condylar joint, with motion possible in 3 planes. Primarily it acts as a hinge joint (extension and flexion) in the craniocaudal plane, supported by extra- and intra-articular ligaments.  The femur, patella (‘kneecap’ in human), and tibia contribute to the joint surfaces of the stifle, forming femoropatellar and femorotibial joints.  Collateral, femoropatellar, and patellar ligaments are extra-articular ligaments and the cranial and caudal cruciate ligaments are intra-articular. The cruciate ligaments are named for their attachment on the tibia, e.g. the cranial cruciate ligament attaches to the cranial tibial plateau. There is a single patellar ligament in the carnivore, swine, and small ruminant and three patellar ligaments in the ox and horse, namely the lateral, intermediate and medial patellar ligament.  Fat accumulates between the patellar ligament(s) and the deeper stifle joint capsule and is noted on radiographs as the infrapatellar fat body.  Medial and lateral, C-shaped, cartilaginous menisci, lie between the femoral condyles and the tibial plateau, increasing joint congruency between these two bones. These menisci are attached by ligaments to each other and to the tibia and stifle.  The meniscofemoral ligament connects the caudal part of the lateral meniscus to the femur.

In the carnivore, the medial and lateral femoropatellar ligaments are thin fascial bands extending between the patella and the sesamoids of the gastrocnemius muscle bellies. The gastrocnemius sesamoids are more commonly called the fabellae and form an important anchor for suture in stifle surgeries (e.g. stabilization after cruciate ligament rupture).

A little more on the all important cruciate ligaments. The cruciate ligaments limit craniocaudal motion of the femur and tibia. The cranial cruciate ligament  keeps the tibia from sliding cranially beneath the femur when the limb bears weight. It also limits medial rotation of the tibia when the stifle is flexed.  The caudal cruciate ligament prevents caudal movement of the tibia beneath the femur when the limb bears weight.  See clinical relevance box below.

Lastly, the equine stifle has been designed to allow for a patellar locking mechanism that results in holding the stifle in extension with minimal muscle activity required (this helps the horse stand on the ‘locked’ leg for a bit, while resting the other pelvic limb).  The mechanism involves an anatomical loop of structures being ‘hooked’ over the very large medial trochlear ridge of the stifle. The loop of structures includes the intermediate patellar ligament, the patella, the parapatellar fibrocartilage, and the medial patellar ligament.  The parapatellar fibrocartilage is a medial extension of the patella made of, yes, fibrocartilage.  This locking mechanism will be revisited in detail in a later lab.

Stifle joint synovial sacs

The stifle consists of femoropatellar and medial and lateral femorotibial synovial sacs.  These sacs are named for the bones they are between.  In the carnivore and pig these 3 sacs communicate freely as one large joint sac, meaning synovial fluid and anything injected into one of the sac spaces will readily move between all three sacs. The ruminant typically has communication between the 3 synovial sacs.  In the horse the medial femorotibial and femoropatellar sacs communicate most of the time and the lateral femorotibial sac is more commonly a closed space, not connected to the other sacs.  Knowledge of the communications between synovial sacs has important bearing on joint injection sites for therapy and diagnostic analgesia during lameness exams (with particular reference to the horse lameness exam).

The lateral femorotibial sac continues distally through the extensor groove of the tibia as the tendon sheath for the tendon of origin of the long digital extensor (and fibularis tertius).

Tarsal joint and ligaments

The tarsus is a composite of several articulations, supporting ligaments, and intervening joint sacs. Proximal to distal the joints are the tarsocrural, proximal intertarsal, distal intertarsal and tarsometatarsal joints.  Greatest flexion and extension movement in the tarsus occurs at the tarsocrural joint.   The remaining tarsal joints are considered low motion joints, i.e. they move very little, however they do handle the very high loads of weight bearing.  Powerful and complex medial and lateral collateral ligaments cross the tarsus from the tibia (and fibula, as present per species) to the metatarsal bones, providing stability on each side of the tarsus.  Numerous ligaments join the individual tarsal bones to each other.  The long plantar ligament extends from the calcaneus to the 4th tarsal and 4th metatarsal bones, acting to anchor the calcaneus distally to counter the pull of the common calcanean tendon proximally. This ligament will be revisited in a later ungulate lab.

The flexor canal of the tarsus is the equivalent of the thoracic limb carpal canal.  This space provides passage for tendons of the deep digital flexor to pass through, enroute to the digit.  The canal is bound by the calcaneus laterally, the sustentaculum tali of the calcaneus dorsally, and the flexor retinaculum plantaromedially.  A tarsal sheath wraps around the tendons to lubricate their path through the canal.

Tarsal joint synovial sacs

Along with the four major articulations of the tarsus there are four joint sacs.  The tarsocrural joint sac is the largest and incorporates the articulations of the distal tibia (and fibula as present for the species) with that of the talus and calcaneus.  Synovial fluid sampling is relatively easy from the spacious tarsocrural joint sac.  Notable in the horse, the tarsocrural joint sac projects outward (superficially) in spaces between tendons, ligaments and bones, to form 4 pouches.  These pouches are so named for their location on the tarsus – dorsomedial, dorsolateral, plantaromedial, and plantarolateral pouch.  The dorsomedial pouch is invariably the largest and most accessible for joint access.  The pouches become very apparent with excessive fluid distension of the tarsocrural joint sac.  The other 3 joint sacs include the proximal and distal intertarsal sacs and the tarsometatarsal sac.  With reference to clinically relevant anatomy, the tarsocrural and proximal intertarsal joint sacs communicate 100% of the time in the horse and ox (but not in the pig).  The distal intertarsal and tarsometatarsal joint sacs have variable communication in the horse. Knowledge of these communications is important to planning joint therapies and diagnostic analgesia strategies during lameness exams.