Anatomy

The crus is located between the stifle and hock joints. Anatomy of the hock and stifle are discussed in Chapters 44 and 46. The medial aspect of the tibia lacks muscle covering and is easily palpated, but muscles and tendons cover the cranial, lateral, and caudal aspects. The tibia has proximal and distal physes and a separate center of ossification for the tibial tuberosity.

The fibula is lateral, does not share axial load, and fuses with the tibia to form the lateral malleolus. The proximal aspect of the fibula may develop from two or more separate centers of ossification, and fibrous union may persist throughout life. The result of this union is transverse radiolucent lines that are evident radiologically and should not be mistaken for fractures (see discussion of rare fibular fractures later).^1,2

Muscles and tendons of the crus are important in locomotion and support of the hindlimb. The long digital extensor muscle originates from a common tendon with the fibularis tertius from the extensor fossa of the femur and is located cranially and laterally in the crus. The fibularis tertius courses distally to divide into dorsal and dorsolateral tendons of attachment. Avulsion injury of the tendon of origin of the fibularis tertius and long digital extensor causes a classic disruption of the hindlimb reciprocal apparatus. The tibialis cranialis muscle is deep to the long digital extensor muscle and distally splits into two parts, the medial of which is called the cunean tendon (jack tendon) and may play a minor role in distal hock joint pain. The lateral digital extensor muscle originates proximally, and its tendon courses laterally over the tarsus and joins the long digital extensor muscle in the proximal metatarsal region. Myotenectomy of the lateral digital extensor tendon and muscle is often performed for management of stringhalt.

The paired gastrocnemius muscles originate from the distal, caudal aspect of the femur and share a single, strong tendon that courses distally and inserts on the calcaneus. The superficial digital flexor and deep digital flexor muscles and tendons arise caudally. The superficial digital flexor tendon (SDFT) begins medial to the gastrocnemius tendon and courses from medial to caudal to attach to the calcaneus before continuing distally. The deep digital flexor tendon (DDFT) is deep to both the SDFT and the gastrocnemius tendons and courses distally over the plantaromedial aspect of the calcaneus, the sustentaculum tali. The combined SDFT and gastrocnemius tendons form the common calcaneal tendon, the major extensor of the hock, and injuries can cause partial or complete loss of hock support. The tarsal sheath begins in the distal, caudal aspect of the crus and surrounds the DDFT.

The fibular (peroneal) nerve originates from the sciatic nerve and branches in the proximal aspect of the crus to become the superficial and deep fibular nerves that run between the long digital extensor and lateral digital extensor muscles and tendons. The tibial nerve is palpable in the distal, caudal aspect of the crus, cranial to the common calcaneal tendon.

Clinical Characteristics and Diagnosis of Lameness of the Crus

Degree of lameness can vary from a subtle, high-speed lameness seen in horses with early stress-related bone injury to an acute, non–weight-bearing lameness, swelling, and deformity seen in horses with complete tibial fractures. Horses with tearing of the gastrocnemius tendon, musculotendonous junction, origin, and insertion have varying degrees of hyperflexion of the tarsocrural joint and partial loss of the reciprocal apparatus. Because the hock drops during weight bearing, the degree of pelvic hike is less than expected and the degree of lameness may be underestimated. Foals with fibularis tertius and long digital extensor avulsion injury lose integrity of the reciprocal apparatus and have swelling on the lateral, proximal aspect of the crus. Rarely, foals can injure the fibularis tertius distally near the tarsus. Fibularis tertius injury in adult horses may cause swelling of the distal, cranial aspect of the crus near the tarsocrural joint, but lesions in the midcrus are not associated with swelling. In calcinosis circumscripta, round-to-oblong, nonpainful mineralized masses are attached to the distal aspect of the lateral patellar ligament and lateral femorotibial joint capsule or collateral ligament.

No gait characteristics are pathognomonic for pain associated with the crus. Lameness of the crus is similar to that seen in horses with pain originating anywhere from the tarsus to the hind foot. Horses with tibial stress fractures have a shortened cranial phase of the stride and, most often, a stabbing type of hindlimb gait when viewed from behind. Injuries of the crus should be suspected when lameness is pronounced, but sites in the rest of the limb are eliminated by using diagnostic analgesia. In Thoroughbreds (TBs), in which the risk of tibial stress fractures is high, a tentative diagnosis of stress fracture is made when lameness is pronounced and recurrent after work.

Palpation of the crus should be done with the limb in both the standing and flexed positions, but this often yields no information. The medial side of the tibia is most easily felt, and occasionally in horses with tibial stress fractures, mild swelling is present and pain is elicited by deep compression (see Figure 6-27). Unfortunately, most tibial stress fractures involve the caudolateral cortex, and it is difficult to compress this area during palpation. Digital tibial percussion sometimes elicits a painful response in horses with tibial stress fractures, but there are many false-positive and false-negative responses. Forced tibial torsion with the limb in a flexed position may elicit pain in horses with tibial stress fractures (see Chapter 107), but I have found the results of this test to be inconsistent.

Specific Conditions of the Crus

Tibial Stress Fractures

Tibial stress fractures are the most common lameness condition of the crus and occur most commonly in TB racehorses. In my experience, tibial stress fractures are rare in other sports horses, including the Standardbred (STB) racehorse. In an 8-year period, of 1020 STBs in which scintigraphic examination was performed, only three horses (two of which were trotters) had tibial stress fractures.³ Thirteen STB racehorses, 11 pacers, and two trotters with tibial stress fractures were reported in one study,⁴ but based on my experience this is a highly unusual clustering of horses. In that study pacers were overrepresented, and factors such as breeding, track size, training methods, and referral bias may have played a role. In my experience and practice area tibial stress fractures occur more commonly in trotters than in pacers. The caudal tibial cortex appears prone to stress-related bone injury because it is under compressive forces when loaded. The highest compressive forces were recorded in the middiaphysis at the walk, but loading at the gallop, pace, and trot was not determined.^4-6

Tibial stress fractures usually occur in 2- and 3-year-old TB racehorses. In one study, tibial stress fractures occurred most commonly in 2-year-olds or unraced horses.⁷

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