Knee Joint and Muscles: Anatomy, Movement, and Species Notes

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The knee joint is a compound synovial hinge that links the femur to the tibia through two articulations, the femorotibial joint and the femoropatellar joint, and it is moved by four major muscle groups: the quadriceps, hamstrings, gastrocnemius, and popliteus. In dogs this joint is called the stifle, and its most clinically important structure is the cranial cruciate ligament, which fails far more often in dogs than the anterior cruciate ligament fails in people.

This article walks through the bony geometry of the articulation of the knee, the menisci and ligaments that hold it together, and the muscles that drive it, with origin, insertion, action, and innervation for each group. It also explains why the canine stifle is not simply a smaller human knee, and what that difference means for injury and movement.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

The Two Articulations of the Knee

The knee is not one joint but a joint complex. Two separate articulations share a single fibrous capsule in most domestic species.

Femorotibial articulation

The femorotibial joint is the weight-bearing articulation of the knee. It forms between the two femoral condyles above and the tibial plateau below. The femoral condyles are convex and roll and glide over the relatively flat tibial plateau during flexion and extension. Because the surfaces do not interlock, stability depends heavily on soft tissue: the cruciate ligaments, the collateral ligaments, the menisci, and the joint capsule.

The tibial plateau is not level. It slopes caudally, meaning the back of the plateau sits lower than the front in the standing dog. This slope matters because it converts the downward load of body weight into a force that pushes the tibia forward relative to the femur, a phenomenon called cranial tibial thrust. The cranial cruciate ligament is the main structure that resists that thrust. A steeper plateau increases the thrust, which is one reason tibial plateau slope is measured before certain orthopedic procedures [1].

Femoropatellar articulation

The femoropatellar joint forms between the patella (the kneecap) and the trochlea, the groove on the cranial surface of the distal femur. The patella is a sesamoid bone embedded in the quadriceps tendon. It slides up and down within the trochlear groove as the knee flexes and extends, and it acts as a pulley that increases the leverage of the quadriceps.

In dogs the trochlear groove is shallower than the deep patellofemoral groove of humans. A shallow groove provides less bony containment of the patella, so the canine patella relies more on soft tissue restraints, especially the parapatellar fibrocartilage and the joint capsule. When those restraints fail or the limb alignment is abnormal, the patella luxates, meaning it slips out of the groove. Patellar luxation is a common cause of stifle disease in dogs and produces measurable damage to the articular cartilage and joint capsule [2][3].

Menisci

Two C-shaped fibrocartilage wedges sit between the femoral condyles and the tibial plateau. The medial meniscus and lateral meniscus deepen the articular surface, distribute load across a wider area, absorb shock, and improve joint lubrication. Each meniscus is anchored at its ends to the tibia by the cranial and caudal meniscotibial ligaments.

The menisci are not rigid. They deform and shift slightly during movement to keep the femoral condyles centered on the plateau. When the cranial cruciate ligament ruptures, the tibia slides forward abnormally, and the medial meniscus is often trapped and crushed between the femoral condyle and the tibial plateau. This produces a meniscal tear, which is a frequent companion injury to cruciate rupture. Magnetic resonance imaging of the canine stifle can resolve the meniscus and the cranial cruciate ligament in detail, and thin-slice techniques improve that structural visibility [4].

Joint capsule and synovium

The joint capsule is a layered sleeve around the whole complex. Its outer stratum fibrosum is fibrous and provides mechanical restraint. Deeper layers, the stratum subsynoviale and stratum synoviale, contain the synovial membrane that produces synovial fluid. That fluid lubricates the cartilage and nourishes it.

The capsule is not a passive wrapper. In dogs with patellar luxation or cranial cruciate ligament rupture, the capsule remodels. Dogs with these conditions show increased superficial synovial cell layers compared with controls, and chronic cases show reduced villous formation. In patellar luxation the stratum synoviale is frequently absent, while in cruciate rupture the capsule becomes thicker overall [3]. Collagen composition also shifts, with chronic joint disease associated with elevated collagen types V and VI in the stratum synoviale, and larger breed dogs showing less dense collagen networks than smaller breeds [5]. These changes help explain why chronic instability drives progressive joint degeneration.

The synovial membrane of the canine knee contains a defined set of glycosaminoglycans, including chondroitin sulfate A/C, dermatan sulfate, heparan sulfate, and hyaluronic acid, distributed in different concentrations across the intimal and subintimal layers [6]. These molecules bind water and give synovial fluid its viscosity, which is what keeps cartilage surfaces gliding rather than grinding.

Ligaments of the Knee

Four major ligaments govern the passive mechanics of the femorotibial joint.

The cranial cruciate ligament runs from the caudomedial aspect of the lateral femoral condyle to the craniomedial tibia. It prevents the tibia from sliding forward under the femur and limits internal rotation of the tibia. The caudal cruciate ligament runs in the opposite direction and prevents caudal tibial displacement. Together they form an X inside the joint, which is where the name cruciate comes from.

The medial and lateral collateral ligaments run along the sides of the joint and resist varus and valgus angulation, that is, bending sideways.

A less familiar structure is the ligamentum mucosum. In humans it is described as a ligamentous band from the femoral intercondylar notch to the infrapatellar fat pad. A dissection study found it bilaterally in 95.2% of cat hindlimbs and 83.0% of dog hindlimbs, appearing as an elastic, friable band of white-to-pink tissue tethering the infrapatellar fat pad to the intercondylar notch [7]. Because many veterinary anatomy texts omit it, students sometimes mistake the ligamentum mucosum for the cranial cruciate ligament during dissection [7].

The Four Major Muscle Groups

Movement at the knee comes from four functional groups. The quadriceps extends the joint. The hamstrings flex it. The gastrocnemius crosses the joint and contributes to flexion and to standing stability. The popliteus fine-tunes rotation and helps control the meniscus.

Quadriceps femoris

The quadriceps is the largest muscle mass on the cranial thigh. It has four heads: the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. All four converge on the patella and continue as the patellar ligament to the tibial tuberosity.

The rectus femoris arises from the ilium just cranial to the acetabulum. The three vastus muscles arise from the cranial and lateral surfaces of the femoral shaft. The common insertion is the tibial tuberosity via the patella and patellar ligament. The quadriceps extends the stifle and, through the rectus femoris, also flexes the hip. It is innervated by the femoral nerve.

Quadriceps function is measurable. Strain elastography of the quadriceps femoris muscle and the femorotibio-patellar joint shows higher stiffness in the patellar ligament and cranial cruciate ligament of dogs with cruciate rupture compared with healthy dogs, with strain ratios between the infrapatellar fat pad and the cranial cruciate ligament rising from roughly 1.1 in healthy dogs to roughly 2.6 to 2.7 in affected dogs [8]. That stiffness reflects the altered mechanical environment of the diseased joint.

Hamstrings

The hamstring group sits on the caudal thigh. In the dog it comprises the biceps femoris, semitendinosus, and semimembranosus.

The biceps femoris arises from the ischial tuberosity and the sacrotuberous ligament and inserts on the fascia of the crus, the patella, and the tibial crest. The semitendinosus arises from the ischial tuberosity and inserts on the medial tibia. The semimembranosus arises from the ischial tuberosity and inserts on the medial femoral condyle and medial tibia. All three flex the stifle and extend the hip. The biceps femoris also contributes to hock extension through its crural fascia attachment. The hamstrings are innervated by the sciatic nerve, with the semitendinosus and semimembranosus supplied by the tibial division and the biceps femoris by the common fibular division.

Gastrocnemius

The gastrocnemius is the large paired muscle of the caudal crus. Its two heads arise from the medial and lateral supracondylar tuberosities of the femur, just above the femoral condyles. It inserts on the calcaneus via the common calcaneal tendon.

Because it arises above the knee, the gastrocnemius crosses the stifle and acts as a stifle flexor as well as a hock extensor. It also contributes to the standing stability of the limb. It is innervated by the tibial division of the sciatic nerve.

Popliteus

The popliteus is a small muscle at the caudolateral corner of the knee. It arises from the lateral femoral condyle and inserts on the caudal surface of the tibia. Its classic description is that it unlocks the knee by rotating the tibia, and it also retracts the lateral meniscus during flexion.

A morphometric study of origin coordinates on the lateral femoral condyle proposed a broader functional role. Depending on the stifle angle during the stance phase, the popliteus may act as either an extensor or a flexor in addition to facilitating rotational movement [9]. The same study proposed that the long digital extensor, which also arises from the lateral condyle, functions as a stifle extensor and may prevent the lateral condyle from slipping caudally during extension, and may help prevent cranial tibial thrust on the lateral side of the tibial plateau [9]. The practical message from that work is that these small lateral muscles should be preserved as much as possible during stifle surgery [9].

The popliteus is innervated by the tibial nerve.

Muscle Table: Origin, Insertion, Action, Innervation, and Species Notes

How the Knee Moves

The knee is primarily a hinge. Flexion brings the caudal surfaces of the femur and tibia closer together. Extension straightens the limb. Rotation is limited but real, and it is coupled to flexion and extension because the femoral condyles are not perfectly symmetrical.

Movement at the stifle does not happen in isolation. A radiographic study of anesthetized Beagles found strong linear relationships between hip, stifle, and tarsal joint angles during passive hip movement. Hip-stifle and hip-tarsal relationships were strong in the unforced state and after force withdrawal, and stifle-tarsal relationships were strong across conditions, with adjusted R-squared values above 0.85 in several comparisons [10]. In plain terms, moving the hip changes the angle of the knee and hock in a predictable, nearly straight-line way. This is why rehabilitation protocols that mobilize the hip can affect the knee, and why a stiff hip can change how a dog loads its stifle.

Range of motion at the stifle is measurable with a goniometer, a protractor-like tool placed along the limb. A study of Anatolian Shepherd dogs found moderate to excellent inter-observer reliability for goniometry of the stifle, with no statistically significant difference between goniometric and radiographic measurements for that joint [11]. That supports goniometry as a practical clinical tool for tracking knee motion over time.

Range of motion also changes with disease, but not always in the way owners expect. In dogs with cranial cruciate ligament disease, goniometric range of motion and goniometric stifle extension correlated significantly with the severity of stifle osteoarthritis, while active range of motion measured during walking did not [12]. A dog can lose passive joint motion to arthritis while still swinging the limb through a near-normal arc during gait, because muscle activity and momentum compensate.

Dog Stifle Versus Human Knee

The canine stifle and the human knee share the same basic parts: femur, tibia, patella, two cruciate ligaments, two collateral ligaments, two menisci, and a quadriceps mechanism. The differences matter clinically.

The tibial plateau slope is more prominent in dogs. A steeper slope increases cranial tibial thrust, the forward push of the tibia under load. The cranial cruciate ligament must resist that thrust with every step, which is one reason cruciate disease in dogs is often a chronic, progressive problem rather than a single traumatic tear. In humans the anterior cruciate ligament more often fails in a single high-energy event during sport. Loading patterns and failure modes are therefore different between the species.

The trochlear groove is shallower in dogs than the deep patellofemoral groove of humans. Less bony containment means the canine patella depends more on soft tissue for stability, and patellar luxation is correspondingly more common in dogs than in people.

Anatomical variants around the canine stifle are also common and can be mistaken for disease. A pictorial review describes variations involving the femur, tibia, fibula, and sesamoid bones, most of which are asymptomatic and found incidentally on radiographs [13]. Growing dogs have age-specific radiographic appearances of the stifle that differ from adults [13]. Knowing these normal variants prevents unnecessary investigation.

Synovial fluid properties are broadly similar across species. Measurements of synovial fluid viscosity from pathologically changed canine joints and human knee joints differ by less than 4% at the shear rate tested [14]. Lubrication physics is therefore comparable, even though joint geometry is not.

Clinical Relevance, Limitations and Common Mistakes

The cranial cruciate ligament is the most clinically relevant structure in the canine stifle. Its rupture is one of the most common orthopedic conditions in dogs, and it drives osteoarthritis progression. Radiographic scoring across 14 anatomic locations in dogs with cruciate rupture shows that osteoarthritis is concentrated in the caudal aspects of the tibial plateau and in the lateral tibial and femoral condyles, consistent with the instability pattern of a cruciate-deficient joint [1]. Both tibial plateau leveling osteotomy and tibial tuberosity advancement redistribute stress effectively at most locations, though signs of incomplete restoration of tibial internal rotation can persist [1].

Patellar luxation is the other major stifle condition. It produces erosive cartilage lesions across multiple joint subdivisions, most frequently involving the distal patella and lateral trochlea, along with variable osteophytosis [2]. Higher-grade luxation produces greater chondrocyte loss and reduced proteoglycan content in cartilage [2]. Increased radiographic soft tissue opacity is common in luxated stifles even without overt cruciate pathology, found in 37 of 57 stifles in one case series, and weight and osteoarthritis presence were associated with that finding [15].

Common mistakes in reading the canine stifle include the following.

• Mistaking the ligamentum mucosum for the cranial cruciate ligament during dissection or arthroscopy. The ligamentum mucosum is present in most dogs and cats and tethers the infrapatellar fat pad to the intercondylar notch [7].
• Assuming a normal walking gait means a normal joint. Dogs with cruciate disease and osteoarthritis can lose passive range of motion while active range of motion stays near normal [12].
• Treating every radiographic irregularity around the stifle as pathology. Many femoral, tibial, fibular, and sesamoid variants are asymptomatic and incidental [13].
• Assuming the dog stifle is mechanically identical to the human knee. The steeper tibial plateau slope and shallower trochlear groove change both loading and patellar stability.

Imaging supports clinical examination. Ultrasound can assess joint anatomy, size, blood supply, and tissue elasticity, and it serves as a reliable alternative when CT or MRI is unavailable [16]. Magnetic resonance imaging with deep learning reconstruction improves structural visibility of the cranial cruciate ligament and meniscus at reduced slice thickness [4].

Individual cases need a veterinarian. Anatomy knowledge guides understanding, not diagnosis.

Frequently Asked Questions

What is the knee joint called in dogs?

The knee joint in dogs is called the stifle. It is the same joint complex as the human knee, comprising the femorotibial and femoropatellar articulations, but with different bony geometry.

Which muscles extend the knee?

The quadriceps femoris group extends the knee. It includes the rectus femoris and the three vastus muscles, all of which converge on the patella and insert on the tibial tuberosity through the patellar ligament.

Which muscles flex the knee?

The hamstrings and the gastrocnemius flex the knee. The hamstrings are the biceps femoris, semitendinosus, and semimembranosus, and the gastrocnemius crosses the joint from the femur to the calcaneus.

What is the most important ligament in the dog's knee?

The cranial cruciate ligament is the most clinically relevant structure in the canine stifle. It prevents forward sliding of the tibia and limits internal rotation, and its rupture is a leading cause of hindlimb lameness and osteoarthritis in dogs.

Why do dogs tear their cruciate ligament more often than people?

Dogs have a more prominent tibial plateau slope than humans, which increases cranial tibial thrust with every step. That chronic loading pattern makes cruciate failure in dogs more often degenerative and progressive than the single traumatic tears typical in human athletes.

Do dogs have a patellofemoral groove like humans?

Dogs have a trochlear groove, but it is shallower than the deep patellofemoral groove of humans. The reduced bony containment makes the canine patella more dependent on soft tissue restraints and more prone to luxation.

What are the menisci and what do they do?

The menisci are two C-shaped fibrocartilage wedges between the femur and tibia. They distribute load, absorb shock, and improve lubrication, and the medial meniscus is commonly injured when the cranial cruciate ligament ruptures.

What does the popliteus muscle do in dogs?

The popliteus rotates the tibia and retracts the lateral meniscus. A morphometric study proposed it may also act as an extensor or flexor depending on the stifle angle during stance [9].

Related Articles

• Equine Muscular System: Major Muscle Groups and Function
• Equine Hindlimb Anatomy: Bones, Joints, and Ligaments
• Bovine Musculoskeletal Anatomy: Axial Skeleton and Joints
• Veterinary Anatomy Notes: How to Create and Use Them for NAVLE
• Canine Stifle Joint Anatomy and Cranial Cruciate Ligament Rupture
• Muscle Protein Synthesis
• Shoulder Joint Anatomy: Articulation and Movement
• Condyloid Joint: Structure and Movement
• Ball and Socket Joints: Examples, Movement, and Anatomy

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10. An experimental radiographic study on the changes of stifle and tarsal joint angles related to passive hip joint movements in anesthetized Beagles.
11. Evaluation of joint range of motion in Anatolian shepherd dogs: inter-observer reliability and radiographic validation.
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