Page 380 - Canine Lameness

352 20 Hip Region

have a common insertion on the lesser trochanter of the femur. The function of the iliopsoas is
primarily hip flexion, but it also contributes to external rotation of the femur as well as lumbar
flexion.
Hip extension, thigh adduction, stifle flexion, and tarsal extension are controlled by the following
group of muscles found superficially on the caudal portion of the medial thigh: the semitendino-
sus, semimembranosus, and gracilis muscles. The semitendinosus muscle originates on the
ischiatic tuberosity and inserts at the cranial border of the tibia. It has an aponeurosis that spreads
onto the crural fascia. A well-developed fibrous band extends from its caudal border and joins the
calcanean tendon, inserting on the tuber calcaneus giving it the ability to extend the tarsus in addi-
tion to flexing the stifle and extending the hip. The semimembranosus originates on the ischiatic
tuberosity and inserts on the distal mid-femur, as well as on the proximal end of the tibia. Its pri-
mary function is hip extension, with the ability to flex or extend the stifle depending upon the
position of the limb. Extension of the hip is also aided by the gracilis that originates on the pubic
symphysis and inserts on the cranial border of the tibia with the semitendinosus. In addition to hip
extension, it contributes to adduction of the pelvic limb as well as flexion of the stifle. The semiten-
dinosus and semimembranosus muscles are innervated by the sciatic nerve, while the gracilis is
innervated by the obturator nerve.
The sartorius is made up of a cranial and caudal head, with the cranial head originating on the
medial crest of the ilium and inserting on the patella with the quadriceps, and the caudal head
originating on the cranial ventral iliac spine, inserting on the cranial medial border of the tibia with
the gracilis muscle. While both heads contribute to flexion of the hip joint, the cranial belly extends
the stifle, and the caudal head flexes the stifle. The sartorius is innervated by the femoral nerve.
The last muscle to contribute to hip extension is the large biceps femoris. It originates on the
sacrotuberous ligament and ischiatic tuberosity, and inserts via the fascia lata to the patella, patel-
lar tendon, and cranial border of the tibia, and via the crural fascia to the tuber calcanei. In this
respect, it can contribute to stifle extension, and tarsal extension. Uniquely, the caudal portion of
the muscle will flex the stifle. The biceps femoris is innervated by the sciatic nerve.
HIP REGION functions to support the weight of the caudal portion of the body and to transfer propulsion forces
The sacroiliac (SI) joint is both a synovial and cartilaginous joint. Biomechanically, the SI joint

from the pelvic limbs to the spine. The apposed crescent-shaped surfaces of the medial wing of the
ilium and sacrum are covered by cartilage, with a thin joint capsule uniting their margins. This
allows for some rotation and translational movement of the ilium relative to the sacrum. Dorsal to
the auricular surfaces, the wing of the sacrum and the wing of the ilium are rough and possess
irregular projections and depressions that tend to interlock. Fibrocartilage fills this space between
the bony projections of both bones that unites the two wings. Through the medium of this fibro-
cartilage, the ilium and sacrum are firmly united in adulthood, forming the SI synchondrosis (syn-
chondrosis sacroiliaca). The SI synchondrosis is located craniodorsal to the synovial portion of the
joint.
The SI joint does have a small amount of normal motion, though it is very slight (Gregory et al.
1986). How much motion is not fully known, one study estimated ~7 degrees of rotation and likely
a small amount of craniocaudal and dorsoventral motion, while an in vivo CT imaging study found
only ~2 degrees of motion (Saunders et al. 2013). The SI joint is primarily stabilized by the dorsal
and ventral SI ligaments and the sacrotuberous ligament, and is innervated by S1–S3/4 with
sensory input from L1–S3.
The SI joint is connected to the wings of the ilium and sacrum via four soft tissue structures: a
craniodorsal synchondrosis component, a caudoventral synovial component, the dorsal SI
ligaments, and the ventral SI ligaments. The dorsal SI ligaments are divided into a short and a long