The nCommon Vein
Copyright 2010
Definition
The patella of the musculoskeletal system is characterized by being the largest sesamoid bone in the body. Colloquially it is known as the knee cap.
It is part of the knee joint of the lower leg. It consists of bone and cartilage tissue once mature.
Its unique structural features include its shape. It is flat and ovoid progressing to a rounded apex on the anterior inferior margin. The blood supply to the patella is both intraosseous and extraosseous. Extraosseous blood is supplied by the genicular anastomostic ring or circumpatellar anastomosis. The quadriceps tendon inserts on the superior pole. The patellar tendon origin is the inferior pole. The patella has 7 articular facets. The layer of articular cartilage can be up to 1 cm.
The lateral support structure for the patella is known as the lateral patellar retinculum. The lateral retinaculum is made up of two layers; the superficial oblique retinaculum and the deep transverse retinaculum. The superficial oblique retinaculum receives fibers from the patellar tendon, vastus lateralus, and iliotibial band. The deep transverse retinaculum consists of the epicondylopatellar band/lateral patellofemoral ligament, the midportion, and the patellotibial band.
The medial support structure for the patella is known as the medial retinaculum. There are three ligaments present; the medial patellofemoral ligament (MPFL), the medial patellomeniscal ligament (MPML), and the medial patellotibial ligament (MPTL). The MPFL along with the vastus medialis obliquus form the primary restrictive mechanism for lateral patella deviation.
The anterior surface of the patella consists of the upper third, middle third, and lower third. The upper third of the patella is the site of the quadriceps tendon attachment. The middle third has vascular components. The lower third is the origin of the patella ligament/tendon.
The posterior surface of the patella is the articulating surface. The articular surface is approximately 12 cm2. The upper three-fourths articulates with the femur and can be broken down further into the medial and lateral facets. The lower fourth has vasculature and is located near the infrapatellar fat pad (Hoffa’s fat pad).
There are many anatomic variants to the patella. Bipartite patellas are the result of a second ossification center. Tripartite and multipartite patellas also occur.
The patellar ossification center develops between ages 3-4. The patella bone as well as all other bones, muscles, and ligaments of the body are derived of mesodermal origin in embryo.
The function of the patella is to improve the biomechanics of the knee during extension. The patella protects the articulating surface of the femur from trauma. The patella also helps provide nourishment to the articulating portion of the femur.
Common diseases include arthritis, fracture, tendinitis, tumor, bursitis, Hoffa’s Disease, osteomyelitis, dislocation/subluxation, osteochondrosis, bipartite patella, patella alta, patella baja, chondromalacia patella, patellofemoral pain syndrome (PFPS), Sinding-Larsen Johanssen, plica synovialis, and stress fractures.
Fractures of the patella can occur in the following patterns; transverse, vertical, marginal, comminuted, osteochondral, and sleeve.
Bursitis of the knee typically occurs in the prepatallar or infrapatellar bursa when it becomes inflamed due to increased friction
Hoffa’s disease is inflammation of the infrapatellar fat pad.
Plica synovialis is a condition of folds of embryonic synovial membrane remnants that can lead to pain if they do not degenerate as they are supposed to in the developing fetus.
Bipartite patella is a variant of the patella when there is a second ossification center. It occurs more commonly in men. It is often mistaken for a fracture.
Patella baja and alta refer to the anatomic relationship of the patella to the femur. Alta is a high riding patella and baja is a low riding patella. These can be symptomatic or asymptomatic.
Patella dislocation/subluxation typically occurs laterally due to the stronger lateral restraints. Often the medial patellofemoral ligament is injured.
Patellofemoral pain syndrome (PFPS) or runner’s knee is a common cause of anterior knee pain where there is abnormal tracking of the patella leading to increased contact pressures of the patella on the femur.
Chondromalacia patella is a softening and degeneration of the cartilage under the kneecap.
Osteomyelitis is an infection of the bone usually due to bacteria.
Sinding-Larsen-Johanssen disease is a condition of pain at the inferior pole of the patella in preteen boys due to traction on the patella by the patellar ligament.
Commonly used diagnostic procedures include clinical history, physical exam, x-ray, MRI, bone scan, and CT scan.
It is usually treated with internal or external surgical fixation or non-operative approaches for fractures. Patellar (Jumper’s) or quadriceps tendintis, patellofemoral syndrome, chondromalacia patella, dislocation/subluxation, plica synovialis, Hoffa’s disesase and bursitis are treated with physical therapy, NSAIDs and steroid injections. Tumors can be treated with surgery, chemotherapy, and radiation. Stress fractures are treated with rest, physical therapy, and dietary modification. Arthritis is treated initially with physical therapy, NSAIDs, steroid injections, and braces. Arthritis can also be surgically treated with arthroscopic debridement or knee replacements. Patellofemoral replacements are becoming more commonly used to try to defer total knee replacements for later in life. Osteomyelitis requires surgical debebridement and antibiotics typically. Sinding-Larsen-Johanssen is treated non-operatively with quadriceps strengthening exercises.
References
Elstrom J, Virkus W, Pankovich (eds), Handbook of Fractures (3rd edition), McGraw Hill, New York, NY, 2006.
Koval K, Zuckerman J (eds), Handbook of Fractures (3rd edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.
Lieberman J (ed), AAOS Comprehensive Orthopaedic Review, American Academy of Orthopaedic Surgeons, 2008.
Moore K, Dalley A (eds), Clinically Oriented Anatomy (5th edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.
Wheeless’ Textbook of Orthopaedics: Tibia Fracture Menu (http://www.wheelessonline.com/ortho/menu_for_the_tibia_tibia_frx)
Waryasz GR, McDermott AY. Patellofemoral pain syndrome (PFPS): a systematic review of anatomy and potential risk factors. Dynamic Medicine 7:9, 2008. (http://www.dynamic-med.com/content/7/1/9)