The stifle joint (often simply stifle) is a complex joint in the

hind limb A hindlimb or back limb is one of the paired articulated appendages (limbs) attached on the caudal ( posterior) end of a terrestrial tetrapod vertebrate's torso.http://www.merriam-webster.com/medical/hind%20limb, Merriam Webster Dictionary-Hindli ...

s of

quadruped Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...

s such as the

sheep Sheep or domestic sheep (''Ovis aries'') are domesticated, ruminant mammals typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus '' Ovis'', in everyday usage it almost always refers to domesticate ...

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...

or dog. It is the equivalent of the human

knee In humans and other primates, the knee joins the thigh with the leg and consists of two joints: one between the femur and tibia (tibiofemoral joint), and one between the femur and patella (patellofemoral joint). It is the largest joint in the ...

and is often the largest

synovial joint A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articul ...

in the animal's body. The stifle joint joins three bones: the

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates ...

patella The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as ...

, and

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it conn ...

. The joint consists of three smaller ones: the femoropatellar joint, medial joint, and lateral femorotibial joint. The stifle joint consists of the femorotibial articulation ( femoral and tibial

condyles A condyle (;Entry "condyle"
in
trochlea and the

), and the proximal articulation. The joint is stabilized by paired collateral ligaments which act to prevent abduction/adduction at the joint, as well as paired

cruciate ligaments Cruciate ligaments (also cruciform ligaments) are pairs of ligaments arranged like a letter X. They occur in several joints of the body, such as the knee joint and the atlanto-axial joint. In a fashion similar to the cords in a toy Jacob's lad ...

. The cranial cruciate ligament and the caudal cruciate ligament restrict cranial and caudal translation (respectively) of the

on the

. The cranial cruciate also resists over-extension and inward rotation, and is the most commonly damaged stifle ligament in dogs. "Cushioning" of the joint is provided by two C-shaped pieces of cartilage called menisci which sit between the medial and lateral condyles of the distal femur and the tibial plateau. The main biomechanical function of the menisci is probably to divide the joint into two functional units—the "femoromeniscal joint" for flexion/extension movements and the "meniscotibial joint" for rotation—a function analogous to that of the disc dividing the temporomandibular (jaw) joint. The menisci also contain nerve endings which are used to assist in

proprioception Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...

. The menisci are attached via a variety of ligaments: two ligaments for each meniscus, the from the lateral meniscus to the femur, the from the medial meniscus to the medial collateral ligament, and the transverse ligament (or ) which runs between the two menisci. There are between one and four

sesamoid In anatomy, a sesamoid bone () is a bone embedded within a tendon or a muscle. Its name is derived from the Arabic word for 'sesame seed', indicating the small size of most sesamoids. Often, these bones form in response to strain, or can be prese ...

bones associated with the stifle joint in different species. These sesamoids assist with the smooth movement of tendon/muscle over the joint. The most well-known sesamoid bone is the patella, more commonly known as the "knee cap". It is located cranially to the joint and sits in the trochlear groove of the femur. It guides the patellar ligament of the

quadriceps The quadriceps femoris muscle (, also called the quadriceps extensor, quadriceps or quads) is a large muscle group that includes the four prevailing muscles on the front of the thigh. It is the sole extensor muscle of the knee, forming a large ...

over the knee joint to its point of insertion on the tibia. Caudal to the joint, in the dog for example, are the two

fabella The fabella is a small sesamoid bone found in some mammals embedded in the tendon of the lateral head of the gastrocnemius muscle behind the lateral condyle of the femur. It is an accessory bone, an anatomical variation present in 39% of huma ...

e, which lie in the two tendons of origin of

gastrocnemius The gastrocnemius muscle (plural ''gastrocnemii'') is a superficial two-headed muscle that is in the back part of the lower leg of humans. It runs from its two heads just above the knee to the heel, a three joint muscle (knee, ankle and subtalar ...

. Fourth, there is often a small sesamoid bone in the tendon of origin of popliteus in many species. Humans possess only the patella. In horses and oxen, the distal part of the tendon of insertion of quadriceps ("below" the patella) is divided into three parts. An elaborate twisting movement of the patella allows the stifle to "lock" in extension when the medial portion of the tendon is "hooked" over the bulbous medial trochlear ridge of the distal femur. This locking mechanism enables these animals to sleep while standing up.

External links

{{Wiktionary, stifle
A stifling problem
Animal anatomy Dog anatomy Horse anatomy Joints Skeletal system