PATELLA

PATELLA (CHONDROMALACIA)


ANATOMY & BIOMECHANICS

The patello-femoral joint refers to a specific part of the knee joint. Medically, the kneecap is known as the patella and the thigh bone is called the femur. The knee joint is composed of three bones - the femur (or thigh bone), the tibia (or shin bone), and the patella (or kneecap). The parts of the knee joint are subdivided into the tibio-femoral joint which refers to the joint space between the tibia and the femur; and the patello-femoral joint which is the joint space between the patella and the femur. Both of these joints (patello-femoral and tibio-femoral) form the knee joint.

The patella is connected to the quadriceps tendon at the top of the patella. The quadriceps tendon attaches to the quadriceps muscle which attaches to the pelvis. The patellar tendon goes from the bottom of the patella to the front of the tibia known as the tibial tubercle. When the quadriceps muscle contracts (shortens), it pulls the patella which in turns pulls on the tibial tubercle, which causes the knee to straighten (go into extension). As the knee moves, the patella glides across the front of the knee joint in a shallow groove on the front of the femur which is known as the trochlear groove of the femur. (Click HERE for a computer animation of basic knee motion (mpg file) courtesy of Rob Kroeger.)

There are several basic types of abnormalities that may occur with the patella: it may dislocate (slip out of place), sublux (partially slip out of place), fracture, develop degenerative arthritis, or develop a tracking problem. A tracking problem refers to the fact that the patella stays in place in front of the knee, but it no longer remains centered in the front part of the femur known as the trochlear groove.

When tracking problems occur, the kneecap develops an abnormal set of biomechanics that results in abnormally increased pressure on the underside of the patella (patellar articular surface). The pain that results from this has a variety of different names, and while there are some technically medical differences between the various names listed below, for the sake of simplicity, we will assume that they all refer to the same type of patellar problem. A few of the most common diagnoses associated with tracking problems are: Lateral Facet Syndrome (of Ficat), Anterior Patello-femoral Pain Syndrome, Chondromalacia, Lateral Pressure Syndrome, Malalignment Syndrome, Maltracking Syndrome, and sometimes Patello-femoral Degenerative Arthritis.

Basically, all of the above diagnoses refer to a biomechanical abnormality of the joint space between the patella and the trochlear groove of the femur. Normally, the patella sits centered in the groove.

Provided by Nucleus Communications, Inc.

Figure 1: Cross Sectional View of Right Knee.

However, if it begins to move towards one side of the groove, the amount of pressure on the underside of the kneecap (patellar articular surface), changes. This results in the development of pain initially. If untreated, the end result is arthritis of the patella.

The centering of the patella in the trochlear groove is related to the strength of the vastus medialis obliqus (a part of the vastus medialis muscle) and the medial patello-femoral ligaments which pulls the patella towards the opposite knee while the vastus lateralis and lateral patello-femoral ligaments pull the knee cap towards the outside (lateral) aspect of the knee. When all of these forces are in proper alignment, the patella is centered in the trochlear groove of the femur.

Provided by Nucleus Communications, Inc.

Figure 2: Stabilizing Forces on the Patella

If an imbalance develops with weakness of the vastus medialis muscle and/or weakness of the medial patello-femoral ligaments and/or over-development of the vastus lateralis muscle and/or tightness of the lateral ligaments of the patello-femoral joint, then a force imbalance develops. When this happens, the patella begins to move laterally (towards the outside) within the trochlear groove. As the knee is flexes, the tension increases on the tight lateral structures. In turn, this causes pain with bent knee activities.

Figure 3: Lateral Tightness Pain

This results in abnormally increased contact between the femur and the patellar articular surface which may eventually result in arthritis. If the imbalance is overwhelming, then the patella may actually slip out of place (dislocate).

DIAGNOSIS OF ANTERIOR KNEE PAIN

Initially, a history is taken of the problem. This is the interview between the physician and the patient during which specific complaints are identified and the physician attempts to identify any contributing factors to the knee problem.

The next step is to perform a physical exam. During the physical exam a number of areas are checked including tracking of the patella, strength of the muscles around the knee cap, tightness of the tissue and ligaments around the kneecap, areas of tenderness in the soft tissue, areas of tenderness on the underside of the patella, the location of the patella, the presence of abnormal grinding of the patella as it moves, and the ability of the patella to be subluxed or dislocated.

During the exam, it is usually necessary to take x-rays of the knee. Usually multiple views are taken with the knee in different positions. This lets the physician examine the bones from different angles. Sometimes, it will be necessary to obtain more sophisticated imaging exams such as a bone scan, MRI, or CT scan. In some cases, if the knee is swollen, it will be necessary to drain (aspirate) the knee under local anesthesia in the office. This allows the physician to determine the type of fluid (inflammatory, blood, etc.) that may be within the joint.

X-RAY EXAM

Typically, when checking for problems at the patello-femoral joint, it is necessary to obtain special x-ray views of the patella. These are taken at in such a fashion that the relationship of the patella to the femur may be better identified to allow identification of tracking abnormalities.

Figure 4: Patello-femoral X-ray of Right & Left Patello-femoral Joints.

In figure 4, notice how the patella is symmetrically placed in the trochlear groove of the femur. The space between each side of the patella and the femur is approximately the same. This x-ray appearance is that of a normal relationship between the patella and the femur.

Figure 5: Patello-femoral X-ray of Right & Left Patello-femoral Joints - Abnormal.

In figure 5, notice how the patella are unevenly (asymetrically) located in the trochlear groove of the femur. The space between each side of the patella and the femur is no longer the same. Each patella is tilted abnormally away from the medial side of the knee and towards the lateral side of the joint. (Compare figure 5 to the normal appearing x-ray in figure 4.)

TREATMENT

Fortunately, most of the pain syndromes that result in pain around the front of the patella and the front of the knee usually resolve with non-surgical treatment. This treatment is directed at re-establishing the normal biomechanical relationship between the patella and the femur. Usually Physical Therapy is necessary and a home exercise program is necessary.

Unfortunately, in some cases, surgery may be indicated. However, the results are not always predictable or successful and in some cases, the surgery may have no effect on the patient's problem. One of the more common surgical procedures is an arthroscopic lateral release. (152 KB avi file depicting surgical release of the lateral muscles from the patella.) This procedure is aimed at releasing the tight lateral restraints, thereby decreasing the abnormal pressure on the underside of the patella.



THIS MATERIAL DOES NOT CONSTITUTE MEDICAL ADVICE. IT IS INTENDED FOR INFORMATIONAL PURPOSES ONLY. PLEASE CONSULT A PHYSICIAN FOR SPECIFIC TREATMENT RECOMMENDATIONS.

THE CENTER FOR ORTHOPAEDICS AND SPORTS MEDICINE
1211 JOHNSON FERRY RD.; MARIETTA, GA., 30068
770-565-0011




    Last Update: 04/03/2003
    Copyright (c)1999 by The Center for Orthopaedics & Sports Medicine - ALL RIGHTS RESERVED