Page 735 - Adams and Stashak's Lameness in Horses, 7th Edition

Lameness of the Proximal Limb 701

TIBIA AND CRUS

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INTRODUCTION IMAGING THE TIBIA/CRUS

The tibia is one of the major weight‐bearing bones of In a foal or young horse, the tibia can be radiographed
the hindlimb. The tibia extends obliquely downward easily. However, in an adult, it is better to focus on a
and backward from the stifle to the hock. It is a tubular particular region of the tibia and center the X‐ray beam
bone with a triangular‐shaped cross section proximally appropriately. Large plates should be utilized to obtain
changing to an oval shape as it courses distally. The tibia radiographs of the entire length of the tibia. Orthogonal
has three chief centers of ossification and two views should be acquired, and specific oblique projections
supplementary ones for the tibial tuberosity and the lat- can be focused at specific areas of interest depending on
eral malleolus. The proximal articular surface of the the tentative diagnosis. The proximal fibula is notorious
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tibia is roughly triangular in shape with the intercondy- for simulating fractures. The tibial tuberosity forms from
lar eminence protruding from the center of the triangle a separate ossification center and varies greatly with age.
providing attachment for the patellar ligaments. The The appearance of the tibial tuberosity varies substan-
proximal articular surface of the tibia has a flat table tially with different projection angles. The parallel ridges
surface with a centrally located intercondylar eminence. located on the caudal and caudolateral surfaces of the
The tibial tuberosity contains a deep extensor groove/ tibia (termed muscular lines) mark the attachment of the
muscular sulcus, which is filled by the cranial tibial (CT) deep digital flexor muscle and must not be mistaken for
and the long digital extensor tendon (LoDET) muscles. a so‐called bone reaction. The opposite tibial region can
The tibia is covered by muscle and tendon on the cra- be used as a normal comparison.
nial, lateral, and caudal aspects. The medial surface is Ultrasonographic examination can be very helpful in
without muscle covering and easily palpated under the evaluating the soft tissues of the crus, particularly the
skin. Proximally the medial surface is broad where it caudal and cranial muscle masses (gastrocnemius and
furnishes insertion to the medial collateral ligament of peroneus tertius/CT). The patellar ligaments can be eval-
the stifle and the sartorius and gracilis muscles. The uated quite effectively with diagnostic ultrasound.
caudal surface is flattened and provides attachment for Puncture wounds and lacerations of the crus can be
the popliteus muscle proximally and for the deep flexor carefully investigated to document the extent of soft tis-
muscle of the digit via a series of roughened lines that sue involvement with ultrasound. Clinical conditions
fade distally. The nutrient foramen is situated among that ultrasound may be helpful to define include gas-
these lines and can be quite prominent. The distal articu- trocnemius injury, peroneus tertius/CT tendon damage,
lar surface contains two deep sulci oriented in caudome- thoroughpin (or “false” thoroughpin), calcaneal bursi-
dial to craniolateral direction, which articulate with the tis, and tarsal sheath tenosynovitis.
trochlear ridges of the talus. Nuclear scintigraphy should be considered when the
The primary axis of tension strain in the tibia occurs on lameness cannot be blocked out or cannot be confirmed/
the cranial surface, while the caudal surface experiences defined to a specific region with radiographic and/or
compressive strains. These strains are particularly ultrasound examination. Any young racehorse with an
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large in the proximal metaphyseal and mid‐diaphyseal acute hindlimb lameness potentially due to a stress frac-
region. Distally the tibia experiences large torsional ture of the tibia is a candidate for scintigraphy. Lameness
forces. Casting the hindlimb does not alter the strains conditions suspected to originate more proximally on
experienced by the tibia. Casts may be effective to the limb or to involve multiple sites within the limb are
immobilize the tarsus and distal limb of the horse, but also candidates for scintigraphy. In older horses scintig-
they do not protect the tibia from weight‐bearing loads. raphy may permit detection of subtle bony changes
For more information on the muscular anatomy and undetected by other diagnostic methods.
innervation of the tibia, refer to Chapter 1.

CLINICAL CONDITIONS
DIAGNOSTIC ANALGESIA OF THE CRUS
Conditions that affect the tibia/crus can affect the
There is no practical method to use diagnostic anal- large muscle mass surrounding the tibia, the shaft of the
gesia to differentiate conditions of the tibia and crus. tibia, and/or the articulations of the tibia (stifle joint
The distal portion of the crus can be desensitized with a proximally and TC joint distally). Traumatic events are
peroneal and tibial nerve block. Pain from injuries the most common cause of injuries to the tibial region.
involving the distal tibia and caudal soft tissues of the Injuries in this area can manifest a wide variety of clini-
crus may also be abolished with this block, but the cal signs apparently dependent on the structures
results are unreliable. Lameness of the crus becomes involved. In the young racehorse soft tissue injuries are
more likely in horses in which perineural and intra‐ not as common as injuries to the bone. In the mature
articular techniques for the rest of the limb have been sport horse, husbandry practices can increase the inci-
exhausted. dence of soft tissue injuries, particularly lacerations and