Page 341 - Canine Lameness

19.2 ­ormal Anatomy 313

the tibia, which is the main reason why the medial meniscus is injured more frequently than the
lateral meniscus when CCL deficiency is present.
The patella is the articular sesamoid bone of the quadriceps complex. It is held in place by con-
gruent positioning within the trochlear groove and by its centralized attachment of the patellar
ligament on the tibial tuberosity and soft tissue support. Tension from the quadriceps group, patel-
lofemoral ligaments, joint capsule, and the alar fibrocartilages which glide over the trochlear ridges
through ROM is also a critical stabilizer of the patella. Derangement or injury of any of these
structures can predispose to patellar luxation.

19.2.2 Muscles of the Stifle Joint
The primary muscle stabilizers of the stifle joint are the quadriceps group, which perform stifle
extension, and the stifle flexors, the gastrocnemius and hamstring group (Hayes et al. 2013). The
quadriceps muscle is innervated by the femoral nerve and consists of the vastus medialis, vastus
intermedius, vastus lateralis, and the rectus femoris. The vastus lateralis and intermedius originate
on the vastus ridge of the proximal femur, while the vastus medialis originates on the craniomedial
aspect of the proximal femur on the cranial intertrochanteric crest. The rectus femoris is the only
member of the group to cross the hip joint, with its origin on the tuberosity of the rectus femoris
and the iliopectineal eminence located on the ilium. These four muscles form the tendon of inser-
tion of the quadriceps muscle that includes the patella itself and the portion from the patella to the
tibial tuberosity, the patellar ligament (sometimes referred to as the patellar tendon). The patellar STIFLE REGION
ligament is the terminal insertion of the quadriceps mechanism and therefore contributes to stifle
extension. The dynamic tension of the quadriceps muscle helps to keep the patella tracking within
the trochlear groove.
The main muscles of the hamstring group are the semimembranosus, semitendinosus, and
biceps femoris muscles. The hamstrings originate from the ischial tuberosity and are innervated by
the sciatic nerve. The semitendinosus muscle branches, and part of it attaches to the proximal
medial tibial fascia as well as courses distally with the tendon of the gracilis muscle as part of the
common calcanean tendon. The semimembranosus inserts as a distinct but short tendon near the
origin of the gastrocnemius and just beneath the MCL on the medial tibial condyle. The biceps
femoris attaches via an aponeurosis of the fascia lata to the tibial tuberosity laterally, continuing
distally to become a part of the common calcanean tendon. The hamstring group works reflexively
via the cruciate ligament mechanoreceptors to limit cranial tibial translation, hyperextension, and
excessive internal rotation, hence protecting the CCL from damage during movements of the tibia
relative to the femur (Hayes et al. 2013).
The gastrocnemius has a medial and a lateral head, which originate from the caudal femur at the
supracondylar tuberosities and attach on the tuber calcanei. Each muscle head contains a sesa-
moid bone (fabella) just below its origin. The gastrocnemius is innervated by the tibial branch of
the sciatic nerve and its action is to flex the stifle and extend the hock. In contrast to the action of
the hamstrings, the pull of the gastrocnemius works to pull the femur caudal relative to the tibia
(Hayes et al. 2013).
The long digital extensor muscle originates intra-articularly in the extensor fossa of the lateral
femoral condyle and courses through the stifle joint and into the extensor groove of the tibia. The
tendon is located deep into the tibialis cranialis muscle and just cranial to a palpable prominence
on the lateral aspect of the tibia, called the Tubercle of Gerdy in humans. In most dogs, this protu-
berance is large enough to palpate. The muscle inserts on the dorsal surface of the distal phalanx
of digits II–V.