The Knee Joint - Bones
What is the knee joint?
The knee joint is a hinge-type synovial joint, which mainly allows for flexion and extension (and a small degree of medial and lateral rotation). It is formed by articulations between the patella, femur and tibia.
What is the femur?
The femur is the only bone in the thigh and the longest bone in the body.
The shaft of the femur descends in a slight medial direction. This brings the knees closer to the body’s centre of gravity, increasing stability.
A cross section of the shaft in the middle is circular but flattened posteriorly at the proximal and distal aspects.
On the posterior surface of the femoral shaft, there are roughened ridges of bone, called the linea aspera (Latin for rough line). These split inferiorly to form the medial and lateral supracondylar lines. The flat popliteal surface lies between them.
Proximally, the medial border of the linea aspera becomes the pectineal line. The lateral border becomes the gluteal tuberosity, where the gluteus maximus attaches.
Distally, the linea aspera widens and forms the floor of the popliteal fossa, the medial and lateral borders form the medial and lateral supracondylar lines. The medial supracondylar line ends at the adductor tubercle, where the adductor magnus attaches.
The distal end of the femur is characterised by the presence of the medial and lateral condyles, which articulate with the tibia and patella to form the knee joint.
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The medial and lateral condyles are rounded areas at the end of the femur. The posterior and inferior surfaces articulate with the tibia and menisci of the knee, while the anterior surface articulates with the patella. The more prominent lateral condyle helps prevent the natural lateral movement of the patella; a flatter condyle is more likely to result in patellar dislocation.
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The medial and lateral epicondyles are bony elevations on the non-articular areas of the condyles. The medial epicondyle is the larger. (The medial and lateral collateral ligaments of the knee originate from their respective epicondyles)
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The intercondylar fossa is a deep notch on the posterior surface of the femur, between the two condyles. It contains two facets for attachment of intracapsular knee ligaments; the anterior cruciate ligament (ACL) attaches to the medial aspect of the lateral condyle and the posterior cruciate ligament (PCL) to the lateral aspect of the medial condyle.
(For the purpose of this article about the knee, we will not be discussing the anatomy of the proximal femur, as it will be covered in the article about the hip)
What is the tibia?
The tibia is the main bone of the lower leg, forming what is more commonly known as the shin.
It expands at its proximal and distal ends; articulating at the knee and ankle joints respectively. The tibia is the second largest bone in the body and it is a key weight-bearing structure.
The proximal tibia is widened by the medial and lateral condyles, which aid in weight-bearing.
The condyles form a flat surface, known as the tibial plateau. This structure articulates with the femoral condyles to form the key articulation of the knee joint.
Located between the condyles is a region called the intercondylar eminence, which projects upwards on either side as the medial and lateral intercondylar tubercles. This area is the main site of attachment for the ligaments and the menisci of the knee joint. The intercondylar tubercles of the tibia articulate with the intercondylar fossa of the femur.
The shaft of the tibia is prism-shaped, with three borders and three surfaces; anterior, posterior and lateral:
Anterior border – palpable subcutaneously down the anterior surface of the leg as the shin. The proximal aspect of the anterior border is marked by the tibial tuberosity; the attachment site for the patella ligament.
Posterior surface – marked by a ridge of bone known as soleal line. This line is the site of origin for part of the soleus muscle, and extends inferomedially, eventually blending with the medial border of the tibia. There is usually a nutrient artery proximal to the soleal line.
Lateral border – also known as the interosseous border. It gives attachment to the interosseous membrane that binds the tibia and the fibula together.
(For the purpose of this article about the knee, we will not be discussing the anatomy of the distal tibia, as it will be covered in the article about the ankle)
What is the fibula?
The fibula is a bone located within the lateral aspect of the leg. Its main function is to act as an attachment for muscles, and not as a weight-bearer.
At the proximal end, the fibula has an enlarged head, which contains a facet for articulation with the lateral condyle of the tibia. On the posterior and lateral surface of the fibular neck, the common fibular nerve can be found.
The fibular shaft has three surfaces – anterior, lateral and posterior. The leg is split into three compartments, and each surface faces its respective compartment (i.e. anterior surface faces the anterior compartment of the leg)
(For the purpose of this article about the knee, we will not be discussing the anatomy of the distal fibula, as it will be covered in the article about the ankle)
What is the patella?
The patella (knee-cap) is located at the front of the knee joint, within the patellofemoral groove of the femur.
It is classified as a sesamoid type bone due to its position within the quadriceps tendon, and is the largest sesamoid bone in the body.
The patella has a triangular shape, with anterior and posterior surfaces. The apex of the patella is situated inferiorly, and is connected to the tibial tuberosity by the patella ligament.
The base forms the superior aspect of the bone, and provides the attachment area for the quadriceps tendon.
The posterior surface of the patella articulates with the femur, and is marked by two facets:
Medial facet – articulates with the medial condyle of the femur.
Lateral facet – articulates with the lateral condyle of the femur.
The patella has two main functions:
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Enhances the leverage that the quadriceps tendon can exert on the femur, increasing the efficiency of the muscle. As the patella is both formed and resides within the quadriceps femoris tendon, it provides a fulcrum to increase the power of the knee extensor.
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Protects the anterior aspect of the knee joint from physical trauma, and serves as a stabilising structure that reduces frictional forces placed on femoral condyles.
Understanding the knee joint
The knee joint consists of two main articulations – tibiofemoral and patellofemoral. The joint surfaces are lined with hyaline cartilage, and are enclosed within a single joint cavity.
Tibiofemoral – medial and lateral condyles of the femur articulate with the tibial condyles. It is the weight-bearing component of the knee joint.
Patellofemoral – anterior aspect of the distal femur articulates with the patella. It allows the tendon of the quadriceps femoris (knee extensor) to be inserted directly over the knee – increasing the efficiency of the muscle.
The proximal tibiofibular joint is formed by an articulation between the head of the fibula and the lateral condyle of the tibia — it is a plane type synovial joint; where the bones to glide over one another to create movement.
What is a meniscus?
The medial and lateral menisci are fibrocartilage structures in the knee that serve two functions:
To deepen the articular surface of the tibia, thus increasing stability of the joint.
To act as shock absorbers by increasing surface area to further dissipate forces.
They are C shaped, and attached at both ends to the intercondylar area of the tibia.
In addition to the intercondylar attachment, the medial meniscus is fixed to the tibial collateral ligament and the joint capsule. Damage to the tibial collateral ligament usually results in a medial meniscal tear.
The lateral meniscus is smaller and does not have any extra attachments, rendering it fairly mobile.
What is a bursa?
A bursa is synovial fluid filled sac, found between moving structures in a joint – with the aim of reducing wear and tear on those structures.
There are four main bursae found in the knee joint:
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This is an extension of the synovial cavity of the knee, located between the quadriceps femoris and the femur.
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Found between the apex of the patella and the skin.
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Split into deep and superficial. The deep bursa lies between the tibia and the patella ligament. The superficial lies between the patella ligament and the skin
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Located posteriorly in the knee joint, between the semimembranosus muscle and the medial head of the gastrocnemius.
There are also a number of smaller bursae found around the knee joint.
Fibular collateral ligament bursa
Iliotibial tract bursa
Biceps femoris tendon bursa
Pes anserine (subartorial) bursa
Gastrocnemius muscle bursa (Medial and lateral heads)
