The information presented here is for educational purposes only.

Introduction

This installment of The Med Cell looks at the knee. The knee is an important target in close combat. Injury can lead to loss of mobility and possibly even loss of the leg itself through disruption to blood supply. The amount of force required to cause ligamentous injury is minimal, and in fact can be brought about without external force such as in twisting the knee. The amount of external force that must be applied to dislocate the knee (a very serious injury) is probably around 80 – 100 pounds per square inch. Knowledge of knee anatomy can help predict which injuries are likely to result when a force such as a foot strike is directed at the knee from various angles.

In this installment diagnosis and treatment will not be examined due to limitations of space. In fact, knee injuries can be difficult to diagnose without proper imaging techniques e.g. MRI, and physical examination is of limited use in the acute phase due to pain and swelling, so it is inappropriate to discuss here.

Also, a future article on firearms related injuries will examine gunshot wounds to the knee including ‘knee capping’, but is outside the scope of the present article.

Epidemiology

More than 1 million patients are seen annually in U.S. emergency departments with complaints of acute knee injuries. Knee injury is the most common cause of disability in relation to sporting injuries.

More specifically, knee dislocations account for about 1 in 300 parachuting related injuries in military jumpers. The figure is higher for civilian jumpers.

Anatomy

The knee is the largest joint in the body and is also the most complicated. It is a modified hinge joint, and contains synovial fluid.

The knee joint connects the two longest mechanical levers in the human body, the thigh and the lower leg. As a result, large forces act in and around the joint, contributing to its instability. Stability is mainly provided by the ligamentous structures, and these are detailed below.

Three bones articulate at the knee joint: the femur, tibia and patella (commonly referred to as the knee-cap). See X-rays 1 and 2 below. As such, the tibiofemoral and patellofemoral joints make up the larger complex knee joint

X-ray 1: View of the right knee front-on showing the femur, tibia and fibula

X-ray 2: Lateral view of the right knee showing the femur, tibia and patella

Note that in proper terms the fibula is NOT included in the knee joint as it forms a separate articulation with the tibia called the tibiofibular joint.

Individual Bones:

Femur (see diagram 1 below)
The lower end of the femur consists of a lateral and medial condyle (condyle means the rounded projection of bone forming an articulating surface. It comes from the Greek word for knuckle. The condyles are separated by the intercondylar fossa behind and are joined in front by a trochlear (or pulley-like) surface for the patella to glide over. The lateral condyle projects further forward than the medial condyle, which helps to stabilize the patella (otherwise the patella would be pulled side-ways by the vastus lateralis muscle). The fossa provides attachment for the cruciate ligaments.

Importantly, the condyles give rise to small projections called epicondyles, which serve as sites of origin for the collateral ligaments.

Diagram 1: Bony aspects of the right femur.

Tibia (see diagram 2 below)
The tibia has a large upper end, flaring out to accommodate the large condyles of the femur. The tibia itself has a medial and a lateral condyle. The superior surface of the tibia is called the tibial plateau.

Diagram 2: Bony features of the tibia.

The medial surface of the plateau is larger and more oval while the lateral surface is smaller and more circular. There is a grooved ridge that runs from front to back to form the lateral and medial intercondylar tubercles. These serve as landmarks for the attachments of the menisci and the cruciate ligaments. The tibia narrows below to form the shaft. At the upper end of the shaft is the tibial tuberosity to which is attached the quadriceps tendon via the patellar ligament.

Patella (see diagram 3 below)
A sesamoid bone is a bone which is embedded within a tendon. The patella is the largest sesamoid bone in the body. It is embedded within the quadriceps tendon which then continues on as the patellar ligament to be inserted into the tibial tuberosity. This is what doctors hit with a tendon hammer to elicit the knee jerk. The patella itself is roughly triangular in shape, easiest to picture as an upside down triangle, with the base being the uppermost edge, tapering to a pointed apex at the lower margin. It has two surfaces: the rough anterior surface which is covered by the fibres of the quadriceps tendon, and the posterior or articulating surface which is smooth and has a vertical ridge dividing the surface into medial and lateral facets. In extension of the knee the medial facet does not make contact with the medial femoral condyle (it only does so in flexion), whereas the lateral facet of the patella is always in contact with the lateral femoral condyle.

Diagram 3: Bony features of the patella.

The patella is mobile from side to side. In the human, the femur is situated obliquely, like so:

This means that when the quadriceps contracts, it has a tendency to pull the patella laterally. This is prevented in 3 ways, the last being the most important.

1. The bony forward projection of the lateral femoral condyle
2. Fibrous extensions of the quadriceps called retinacula which connect the sides of the patella with the tibial condyles, and
3. Most importantly, the vastus medialis muscle, the lower fibres of which counteract the pull of the quadriceps muscle.

Functions:

1. Increases the leverage of the quadriceps by increasing the angle at which the muscle acts. (Twice as much torque is needed to extend the knee the final 15