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Chapter 5. EQUINE PELVIC LIMB

 Identify the following features in skeleton and assess position or palpate them in live animal.

5.1 SKELETON:

Os coxae:

tuber coxae, tuber sacrale, tuber ischii, acetabulum: acetabular notch and fossa, subpubic groove (for accessory lig. of hip joint, a lig. which other spp. do not possess)

 

Femur:

head (articular), fovea capitis (for ligament of head of femur) trochanter major: cranial part on whose lateral surface lies trochanteric (gluteal) bursa, trochanter tertius, trochanter minor, trochlea (articular): large medial lip condyles (articular): intercondylar fossa, supracondylar fossa, and extensor fossa

 

Patella:

base, apex, medial angle prolonged by fibrocartilage

 

Fibula:

proximal extremity (head) and short shaft distal extremity (lateral malleolus)

 

Tibia:

tuberosity (on which patellar ligaments converge); crest; condyles (articular) lateral malleolus (distal extremity of fibula); medial malleolus (easily palpable, not to be confused with one of the tarsal bones).

 

Pes: (tarsus, metatarsus, phalanges, sesamoid bones )

Tarsus: calcaneus tuber calcanei (over which passes superficial digital flexor and on to which attaches common calcanean tendon); talus tuberosity on medial aspect (distinguish this from medial malleolus in live animal); central tarsal:- note prominent dorsomedial border. TIII:- note prominent dorsomedial border; TI & II - in most animals these fuse; note position (cunean tendon inserts here so 'cunean tenectomy' has to be performed proximal to this level). Pay particular attention to the bones most frequently affected in 'bone spavin' - central, TIII, TI & TII, i.e., on dorsomedial aspect of tarsus.

Metatarsal bones: note metatarsal tuberosity on MtIII, and proximal extremities of MtII and IV and distinguish them from other bones of tarsus. They are additional landmarks when palpating live animal for evidence of bone spavin. Other bones of pes as for manus.

 

Weight bearing:

Observe pelvic limb of live horse in standing position when foot is bearing weight:

a) as in thoracic limb, the structures which fix the distal interphalangeal, proximal interphalangeal and metatarsophalangeal joints are the ligaments and tendons on the plantar aspect of the pes. These are (in addition to ligaments specific to these joints): the interosseus sesamoidean ligaments, ligaments of distal sesamoid; deep digital flexor tendon (assisted by its check ligament) and superficial digital flexor tendon.

b) Fixing of tarsus in standing is effected by the almost wholly mechanical action of the common calcanean tendon but assisted also by the muscular tension in the deep digital flexor and gastrocnemius.

But fixing of tarsal joint and fixing of stifle is interdependent. When the stifle is fixed (see later) in standing, two "mechanical muscles" counterbalance each other, namely the peroneus tertius in front and the superficial digital flexor behind, assisted by femorocalcanean tendon. In the same way, during motion, stifle and tarsal movements are synchronized by the same "mechanical muscles" - When the stifle flexes, the tarsus must flex, and when the stifle extends, the tarsus must extend.

c) Fixing of stifle is actively effected by the tonus of quadriceps group (which inserts into the base of patella and, by the patellar ligaments, into the tuberosity of tibia): assisted by bracing effect of biceps laterally, and semitendinosus medially. The patellar fixation device can fix this joint "mechanically."

d) The hip joint is fixed in standing by the tonus of those powerful muscles which extend the hip joint, mainly those which play such a big role in rearing, kicking, and propulsion - middle gluteal, biceps and semitendinosus.

 e) Sacroiliac joint is a fixed joint.

With the aid of the diagrams, the skeleton and the dissected limbs, identify all the above structures and note their attachments; then examine the live horse.

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5.2 SYNOVIAL BURSAE AND SHEATHS

Bursae:

Trochanteric "gluteal" bursa ( false hip lameness)

Calcanean bursa (under superficial digital flexor as it passes over tuber calcanei, extending up and down for a couple of inches.)

Subcutaneous calcanean bursa (between skin and tuber calcanei: not always present).

Bursa of distal sesamoid( navicular bursa)

Other bursae: cunean bursa (between cunean tendon and medial ligament of tarsus); one under long extensor tendon at metatarsophalangeal joint; one on either side of PI under the prolongations of interosseus.

 

Sheaths:

Synovial sheaths surround those tendons whose destinations are beyond the tarsal joint except superficial digital flexor which has a bursa instead.

The tarsal synovial sheath surrounds the main part of the tendon of the deep digital flexor as it passes over the sustentaculum tali of the calcaneus: it extends about two inches above and below hock, distention of this sheath (thoroughpin) can be confused with distension of the tarsal joint capsule ( bog spavin )

The digital synovial sheath surrounds the tendons of both superficial and deep digital flexors as they pass over the metatarsophalangeal joint: it extends about two inches above fetlock. (Distention of this sheath "tendinous windgalls" can be confused with distention of joint capsule of the metatarsophalangeal joint (articular windgalls). The interosseus intervenes between these two synovial structures and can be used to distinguish one distention from the other.

 

Tarsal joint

All the ginglymus movement takes place at the tibiotarsal articulation. The movements between all the other bones are minimal gliding movements. The chief ligaments are:- Collaterals (lateral and medial); Plantar ligament (it is attached to plantar aspect of calcaneus, fourth tarsal and proximal end of MtIV. If excessive force is exerted upon the tuber calc. straining the plantar ligament and giving rise to a convexity seen on profile (curb).

The joint capsule. The synovial sac of the tibiotarsal articulation is the one which is chiefly involved in distention (bog spavin) by excess fluid. Distention may occur in two directions proximally and dorsomedially. Pressure of one may cause a detectable fluctuation in the other. The proximal distention is into the pouch which extends up behind the distal end of the tibia for about two inches, where it can be confused with distention of the tarsal synovial sheath (thoroughpin). The dorsomedial distention occurs over the medial ridge of the trochlea of the talus, where the joint capsule is not bound down by the tendons passing over the joint.

The tarsus is the focus for some of the major movements of the hind limb. The forces being exerted upon the tuber calcanei which is the point of attachment of the common calcanean tendon. It is not surprising, therefore, that the tarsal joint is the site of many clinical disorders.

 

Stifle Joint

The movement between patella and trochlea of femur is gliding movement. That between the condyles of femur and tibia is ginglymus, this movement being facilitated by the fibrocartilaginous discs and the menisci. During flexion the condyles of the femur and the menisci glide backward on the tibia: the cranial cruciate ligament is said to be tensed in flexion (and the caudal in extension). Rotation of the limb is a limited action in the horse; when outward rotation of the body and femur occurs while the foot is bearing weight, it causes the cruciate ligaments to be twisted against each other and, as in the dog, in which much more rotation is possible, this may be the cause of cruciate rupture - usually cranial cruciate. Much less frequent in the horse: more common in bovine species. The collateral ligaments (lateral and medial) are tensed in extension.

Note how, in full extension, the patellar fibrocartilage is hooked over the large medial lip of the trochlea which is then protruding between the medial and middle patellar ligaments. This is the patellar fixation device, and is peculiar to the horse. Manipulate the live horse stifle and palpate this. Locking in this position" upward fixation; upward luxation" can occur. In the normal animal, when the patella is fixed on the upper part of the trochlea, flexing of the hock or stifle cannot occur. Release of the patellar fixation device would appear to involve slight contraction of the quadriceps to ease the patella off the trochlea, followed immediately by flexion of the joints - as is the case in normal locomotion. The patellar fixation device is a factor in relieving muscle fatigue in standing. The large medial lip of the trochlea, together with the greater strength of the lateral patellar ligament predispose against medial luxation of the patella.

The extensor action of the quadriceps femoris upon the tibia via the patellar ligaments, assists in rendering the limb firm and rigid when the foot is on the ground. Only moderate contraction of the quadriceps is necessary to maintain fixation of the joint.

One route of access to the joint capsule is low down in the space between medial and middle patellar ligaments: another route is immediately in front of (or behind) the lateral collateral ligament, this reaches lateral femorotibial sac which sometimes does not communicate with the other two - medial femorotibial and the femoropatellar).

 

Hip Joint

Ligaments: Ligament of head of femur, accessory ligament (present only in horse): from subpubic groove it is directed outward, backward and upward to the fovea capitis behind the attachment of the ligament of head of femur. Thus, it largely restricts inward rotation, but also abduction.

5.3 Vessels

 

5.3.1 Arterial

Follow the main arterial trunk: femoral, popliteal, cranial tibial, dorsal pedal, dorsal metatarsal (on lateral aspect), and digital arteries.

 5.3.2Venous

From the liberal plexuses of valveless veins in the corium covering the distal phalanx, the blood is drained on either side by the digital veins. These run up in front of the corresponding arteries and anastomose above the fetlock, forming a venous arch. The largest vein draining this arch (it is in fact the direct continuation of the medial digital vein) is the dorsal metatarsal vein (medially placed, thus not satellite to dorsal metatarsal artery). It ascends on the medial border of the interosseus; then as it approaches the tarsus, it inclines forward on the large metatarsal bone (MtIII) to the dorso-medial aspect of the tarsal joint capsule and is continued as the saphenous vein: this ascends subcutaneously on the medial surface of the tibia and joins femoral or external pudendal vein.

 

5.3.3 Lymph Nodes

Two groups of lymph nodes are fairly easily palpable: The deep inguinal lymph nodes are situated high on the medial thigh region, in the proximal part of the femoral canal covering the femoral vessels. They receive nearly all of the lymph vessels of the limb below them. The subiliac lymph nodes are situated above the fold of the flank, halfway between tuber coxae and patella. They receive superficial lymph vessels from hip, thigh, and flank.

 

 

 

5.3.4 Cunean Tenectomy

Occasionally performed in relief of 'bone spavin'. The 'cunean' tendon is the medial tendon of insertion of m. tibialis cranialis.

 

Consider what might be the rationale behind the operation.

 

5.3.5 Lateral digital extensor tenectomy:

Occasionally performed in relief of 'stringhalt'. The tendon is easily felt on dorsolateral aspect just below tarsus, where it runs forward to join tendon of long extensor.

 

Consider what might be the rationale behind the operation.

 

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5.4 LUMBOSACRAL PLEXUS

Nerves Whose Innervation Reach the Digit:

The nerve of the lumbosacral plexus whose innervation ultimately reaches the digit is the sciatic n. It emerges from greater sciatic foramen, runs back along sacrosciatic ligament, turns down behind hip joint and runs down between biceps and semitendinosus. It gives off the peroneal N. and continues as the tibial N. that travels down the craniomedial aspect of the common calcanean tendon. It gives off a branch to the tarsal joint and divides into two plantar Nn. The plantar Nn. pass through the tarsal groove in company with the deep digital flexor tendon, then separate and come to lie one on either side of the flexor tendons in the metatarsal region like the palmar Nn. of thoracic limb and are distributed in like manner. The medial plantar N. sends an anastomotic branch over the tendons to the lateral plantar N., reaching it at, or just above, the level of the distal extremity of MtIV. Near the metatarsophalangeal joint the plantar Nn. become the digital Nn. Each digital N. gives off a dorsal branch and continues as the plantar digital N. which supplies most of the plantar aspect of the digit.

The peroneal N. divides at the origin of the lateral digital extensor into superficial and deep branches. The superficial branch runs down in the groove between long and lateral digital extensors and becomes cutaneous on dorsolateral aspect of tarsus and metatarsus. The deep branch runs down under the long extensor, gives a branch to the tarsal joint, and becomes cutaneous on the dorsal and lateral aspects of metatarsus to the level of the metatarsophalangeal joint.

5.5 Nerve Block Sites

5.5.1 Peroneal nerve block

About 2" below head of fibula, in groove between long and lateral digital extensors. The usual purpose of this is to partially desensitize the tarsal joint.

 

5.5.2 Tibial nerve block

A handsbreadth above tuber calcanei and at craniomedial aspect of common calcanean tendon. The nerve is situated under strong fascia and embedded in some fat: small vessels lie alongside it. This will partially desensitize the tarsus and completely desensitize the digit.

Low plantar nerve block (or neurectomy) - above metatarsophalangeal joint but below distal extremity of MtII and/or IV. Palpate groove between interosseus and flexor tendons V.A.N. in that order. N. almost directly on border of deep flexor tendon.

Low plantar nerve block (or neurectomy) will desensitize that aspect of the digit on which it is performed. It may be performed on both sides (double low plantar nerve block): if so, this will desensitize the whole digit.

Digital nerve block (or neurectomy) - midway between metatarsophalangeal joint and coronary border of hoof and on either side of flexor tendon. V.A.N. in that order. If neurectomy is performed, note that fascia is thick here; the N. is yellowish-white compared with glistening white ligament of ergot which crosses the nerve obliquely downward and forward, but could easily be confused with the N. At this site, it is the plantar digital N. which is being dealt with. Digital nerve block (or neurectomy) will desensitize the plantar part of the side on which it is performed.

Double digital nerve block (or neurectomy) will desensitize the plantar part of both sides.

 

5.5.3 Precis of Motor Innervation

Gluteal nerves to extensors and abductors of hip.

Femoral nerve to extensors of stifle.

Obturator nerve to adductors of thigh.

Sciatic nerve: In addition to supplying some powerful extensors of hip and tarsus, the sciatic divides in upper part of thigh into tibial and peroneal nerves.

Tibial division of sciatic to digital flexors and tarsalextensors.

Peroneal division of sciatic to digital extensors and tarsal

flexors.

 

synovial bursa

large subcutaneous vein, pelvic limb, equine

Cutaneous innervation - equine

 

left hip and stifle-lateral aspect