THE ANATOMY OF THE PATELLA AND THE
EXTENSOR MECHANISM
RONALD P. GRELSAMER, MD
Knee Hip Pain
Mt SINAI HOSPITAL
5 East 98th Street
Box 1188
New York, NY 10029
Phone: 212-241 2914
Fax: 212 - 534-6202




Get Information on Dr. Grelsamer's Books Here!
The following has been modified from:
The Patella - A Team Approach by R Grelsamer and J McConnell, Aspen Publishers 1998, distributed by Pro-Ed Publishers (see www.proedinc.com)

The word patella means "little plate" in Latin and, indeed, as bones go it is rather small and rather round. Ossification is complete in girls by age ten and in boys by age thirteen to sixteen. In some patients there can be more than one center. When one of those centers is supero-lateral and when it fails to fuse with the rest of the patella the patient is said to have a bipartite patella, which can be painful.

Although just about every portion of the human anatomy from the pelvis on down (including hips and feet) has an effect on the extensor mechanism, the extensor mechanism proper begins above the hip joint and terminates at the tibial tuberosity. The extensor mechanism includes:

  • the four muscles of the quadriceps
  • the patella, the patellar tendon
  • all the other soft tissues attaching to the patella
  • the tibial tuberosity.

The Skin and Subcutaneous Layers

The skin is quite mobile over the knee as it is over the back of the hand. This mobility allows the knee to flex more than 120 degrees. Some fibrous connections do exist within the loose tissue between the kneecap and the skin, and these connections are critical to the success of patellar taping.

Between the skin and the patella are sacks called bursae. These are akin to collapsed balloons which make their presence felt when they become inflamed and painful (“Bursitis”!!!).

Muscles

The quadriceps muscles (“quads”) consist of the rectus femoris, the vastus intermedius, the vastus lateralis and the vastus medialis. The vastus medialis itself can be subdivided into the vastus medialis proper (or longus) and the vastus medialis obliquus (VMO). Taken as a group the four muscles act as extensors of the knee when the foot is off the ground (as when a person kicks a ball), but (more commonly) act as decelerators when the foot is on the ground, keeping the knee from collapsing when the foot strikes the ground.

  • Rectus femoris is the only one of the four muscles to cross the hip joint. However this suffices to give the hip joint considerable importance with respect to the extensor mechanism. Extension of the hip is an integral part of quadriceps stretching.
  • Vastus medialis longus originates from the medial aspect of the upper femur and inserts anteriorly into the quadriceps tendon. It has a line of action of approximately 18° off the long axis of the femur in the frontal plane. The pennation angle has been estimated to be about 5 degrees.
  • Vastus medialis obliquus (VMO) is distinct from the vastus medialis longus. It originates from the distal medial femur and adductor tubercle and inserts into the medial retinaculum and supero-medial portion of the patella. Normally, the VMO comes as far distal as the upper third or half of the patella and its lowermost fibers can be nearly horizontal. Its tendinous portion is short, broad, and blends in with the medial retinaculum. […] In some patients a thin layer of fat can be seen separating the two. It has a line of action of about 50-65 degrees off the long axis of the femur in the frontal plane - quite a bit less vertical than the bulk of the quadriceps.

See Anatomic Abnormalities.

The Quadriceps Tendon

This represents the confluence of all four muscles tendon units and it inserts at the anterior (top) aspect of the patella. It articulates with the trochlea as the knee flexes past 90 degrees and absorbs some of the stresses associated with deep knee flexion. It is prone to degeneration and rupture in patients in their fifties and sixties.

The Patellar Tendon

Semantic controversy exists among purists with respect to the naming of this structure. Is it a tendon or a ligament? Because it connects two bones (patella and tibia) it is a ligament, but because it represents the attachment of the quadriceps unit to the tibia it is a tendon. We will call it the patellar tendon. It originates at the inferior (non-articulating) pole of the patella and inserts onto the tibial tuberosity. It is approximately 5-6cm long and 3cm wide.

This tendon rarely ruptures, but when it does it tends to be in people in their teens and twenties.

The Q Angle

The muscles and tendons of the quadriceps don't form a straight line when viewed from the front (see red line on illustration to the right).

They point one way above the kneecap, and another way below the kneecap.

Therefore when the muscles contract, the kneecap has a tendency to be pulled off to the side. Fortunately, the trochlear groove is higher on the outside, and this keeps the kneecap from slipping out of its groove.

The Q angle is greater in some people than in others. When greater than average, the kneecap has more of tendency to slip out of the groove or to press excessively hard on the outer wall of the groove.

The Fat Pad

Behind and on either side of the patellar tendon lies the fat pad. It is usually both visible and palpable. It is richly innervated and sensitive to pain.

The Lateral Retinaculum

This thick structure lies along the lateral border of the patella and represents the confluence of many structures. See Anatomic Abnormalities.

The Iliotibial Band (ITB) (a.k.a. iliotibial tract and iliopatellar band)

This is a remarkable structure. It spans two joints, influences three articulations, and is quite often forgotten in the orthopaedic physical examination! It begins as a wide fascia covering the upper lateral pelvis and thigh in continuity with the fascia lata, narrows down to a band along the side of the thigh, remains attached posteriorly to the lateral intermuscular septum, and fans out distally to the patella, the lateral retinaculum, and the tibia (via its attachment to a prominence -Gerdy’s tubercle).

It has a number of functions. It bends the hip. It can bend or straighten the knee depending on the initial position of the knee. Because of its attachments to the lateral retinaculum and to the patella it has a significant effect on patellar position, patellar tracking, and patellar pain - especially when it is excessively tight.

The Medial Retinaculum and Medial Patellofemoral Ligament

The medial retinaculum is far thinner than its lateral counterpart and is not felt to be as significant with respect to patella position and tracking. Deep to the medial retinaculum are patello-femoral, patello-meniscal and patello-tibial ligaments. The medial patello-femoral ligaments is strong enough to influence patellar tracking, and in fact is the major (static) medial restraint.

Compartments

There are three compartments in the knee, akin to an apartment with three connecting rooms. The inner compartment (left side of right knee and vice versa) is the medial compartment. The outer compartment (right side of right knee) is the lateral compartment. The kneecap and the underlying trochlear groove form the patello-femoral compartment (a.k.a. patello-femoral joint).

Plicae

A plica is the name given to any fibrous band of tissue which spans part of the knee joint without clear functional role. A plica can become irritated and painful. The presence of a plica is often omitted from MRI reports.

The Tibial Tuberosity

The prominence onto which the patellar tendon attaches is the tibial tuberosity. It usually lies just lateral to the mid-sagittal plane (midline) of the tibia. In childhood and adolescence it is separated from the tibial shaft by a growth plate. Because the growth plate does not contribute to the overall length of the bone, it is called an apophyseal rather than an epiphyseal plate.

The Sensory Innervation

There is considerable variation with respect to the sensory territory covered by each nerve. In some patients the sensation over the lateral portion of the knee is provided by the medial nerves. In these patients, a medial or midline incision will often render numb the lateral half of the knee. By the same token, some of the pain relief following patella operations on the medial side might in part be due to the simple interruption of sensory nerves (See Surgery).

The Vascular Supply about the Patella

The vascular supply to the patella is quite rich. A major vessel enters the patella at the corner of each quadrant in the form of the superior and inferior medial and lateral geniculate arteries (with corresponding veins).

It is important to note that the vessels are wrapped in a mesh of fine nerves. Thus any condition affecting the nerves about the knee (RSD, smoking, etc…) will affect the blood supply - and therefore the color and temperature of the overlying skin.

Hamstring Muscles

These muscles are not part of the extensor mechanism either since they flex the knee. However, tightness of the hamstrings has been associated with patellar pain.

In patients with malalignment the anatomy will be abnormal in one or more ways.