The most common hind limb orthopedic/sports medicine conditions afflicting active dogs are iliopsoas strains, cranial cruciate
ligament (CCL) insuffiency and gracilis and semitendinosus contracture. Of the hind limb muscular injuries iliopsoas strain
is, by a considerable margin, seen more frequently than gracilis and semitendinosus contracture. Because contracture of the
gracilis and/or semitendinosus is less common, it is often undiagnosed until significant pathology is established. Diagnosis
and treatment in early stages of the disease process can significantly inhibit progression. For these reasons, it is included
in this presentation.
Acute, stretching-induced muscle injuries are estimated to account for over 30% of injuries seen in a typical human sports
medicine practice and have been reported to be the most common injury seen in human general practices. Acute muscle injuries
have rarely been reported in the small animal veterinary literature, and discussion of chronic muscle disorders in dogs is
limited to a handful of classical syndromes or inflammatory conditions. Given the similarities between the human and canine
musculoskeletal system, it seems unlikely that such common injuries in human athletes would not also be common in the canine
athlete. It is probable that the low reported prevalence of muscle injury in dogs is due to a failure to diagnose the condition.
The iliopsoas muscle represents the fusion of the psoas major and the iliacus muscles as seen inthe anatomy illustration.
The psoas major muscle arises from the transverse processes of the lumbar vertebrae of the lower spinal column at L2 and L3
and the bodies of L4-7, and the iliacus arises from the ventral or lower surface of the ilium. The two muscles combine and
have a common insertion on the lesser trochanter of the femur. The action of this muscle is to move the pelvic hind limb relative
to the trunk via hip flexion, primarily moving the pelvic hind limb forward.
Iliopsoas strains occur as the result of excessive force acting on this muscle, and are commonly associated with highly athletic
activities such as agility. These injuries often occur at or near the muscle-tendon junction, which is the weakest part
of the myotendinous unit. Eccentric contraction, in which the muscle is activated during stretch, is known to be an important
factor in the development of these acute strain injuries. Traumatic incidents that result in active eccentric muscle contraction,
such as slipping into a splay-legged position, jumping out of a vehicle, aggressive agility training, or roughhousing with
other dogs are often suspected in precipitating acute lameness. It is not uncommon to find dogs with iliopsoas strains that
have other concurrent orthopedic problems, or that have recently undergone surgical treatment for another orthopedic condition,
such as cranial cruciate ligament rupture.
Dogs with iliopsoas strains commonly present with a history ranging from a subtle intermittent offloading of the hind limb
to significant unilateral hind limb lameness that is exacerbated with activity. These dogs commonly demonstrate performance
issues such as knocking bars with the hind limbs and slowing in the weave poles.
On direct palpation, discomfort and spasm of the myotendinous unit may be noted. Pain and spasm will also be noted when stretching
the myotendinous unit by either placing the hip in extension with abduction, or by simultaneous extension of the hip with
internal rotation of that pelvic hind limb.
Radiographs are of little value in the early phase, but may reveal mineralization just cranial to the lesser trochanter in
chronic cases. The use of advanced imaging modalities to demonstrate lesions of the affected muscle and/or tendon can increase
confidence in the diagnosis. Ultrasonography is a relatively inexpensive noninvasive imaging modality for canine musculoskeletal
evaluation with the additional advantage that general anesthesia is not required. This imaging modality is particularly dependent
on the expertise of the operator, which may limit its practical application in some settings. Advanced diagnostics such as
CT (computerized tomography) scan and MRI (magnetic resonance imaging) may be used to identify iliopsoas strains and are both
widely used in diagnosing acute, stretch-induced muscle injury in human patients. Although CT is valuable for imaging soft
tissue lesions, the use of MRI has greatly increased the ability to detect submacroscopic lesions.