The Patellofemoral Joint
Robert J. Nascimento
Anthony A. Schepsis
The patella is the largest sesamoid bone in the body. Its blood supply arises mainly from the peripatellar plexus. The patella articulates with the femoral sulcus, otherwise known as the trochlea. It is asymmetrically oval in shape with the apex distal. It is enveloped by fibers of the quadriceps tendon and blends with the patellar tendon distally. The patella serves as a fulcrum for the quadriceps muscles. The main biomechanical function of the patella is to increase the moment arm of the quadriceps mechanism.
The patellar surface is divided into two large facets — medial and lateral, which are separated by a central ridge. The facets are covered by the thickest hyaline cartilage in the body that may measure up to 6.5 mm. The superior three-fourths of the patella are articular, and the inferior one-fourth is non-articular. The contact area between the patella and femur varies with knee position. At 10-20 degrees of knee flexion, the distal pole of the patella contacts the femoral trochlea. As flexion increases, the contact of the patella moves proximally and medially with the most extensive contact being made at approximately 45 degrees (2,6,12,23). Contact stresses on the patellofemoral joint are higher than any other major weight-bearing joint in the body. The contact area and load across the joint increase with knee flexion. Compressive forces on the patella can range from 3.3 times body weight with stair climbing up to 7.6 times body weight with squatting (20).
The patellar medial facet varies anatomically. It may be divided into a medial facet proper and a small odd facet. The odd facet may develop as a response to functional loads and does not contact the medial femoral condyle except in extreme flexion.
Several ossification centers contribute to the patella. Failure of fusion can lead to bipartite patella that can be classified into three types: type I — inferior; type II — lateral; and type III — superolateral (most common). Bipartite patella is most often discovered accidentally during radiographic examination of the knee for another disorder. The rate of bipartite patellae is approximately 1%, and of these cases, more than 55% are unilateral. A bipartite patella may mimic a patellar sleeve fracture in children or an osteochondral fracture after trauma on plain radiographs.
Within the medial retinaculum is the medial patellofemoral ligament and the medial patellotibial ligament. The medial patellofemoral ligament originates from the adductor tubercle and inserts on the medial border of the patella. This ligament plays a major role in preventing lateral displacement of the patella (4) and is the most commonly injured structure in acute patellar dislocations. The lateral retinaculum is composed of a superficial and deep layer and runs from lateral margin of the patella and patellar tendon to the anterior aspect of the iliotibial band.
The patellar tendon varies in length from 4.0-6.9 cm (average, 4.6 cm) (18,24,27,32). It connects the apex of the patella to the tibial tuberosity and is slightly wider proximally than distally. The patella is commonly associated with either alta or baja, meaning it is high or low, respectively, in reference to standardized controls. Patients with patella alta may have instability (32), whereas patients with patella baja may display limited range of motion; this is commonly a consequence of surgery or trauma. The thickness of the patellar tendon is 4-7 cm, with the mean being 5.5 cm, proximally and 5-7 cm, with a mean of 5.4 cm, in its midsubstance (18,24,27).
The single most common cause of knee pain involves pathology related to the patella (9,29,35). Patellar pain or discomfort may be the result of direct trauma, repetitive direct pressure, constant repetitive movements with the knee in a flexed position, instability, malalignment, or a combination of these factors. It can arise from bony, soft tissue structures or even can be referred pain from the hip or spine.
Symptoms of patellar pain that begin during relatively normal activities/sports should alert the physician that the knee may not have been normal in the first place or that there is a chronic overuse entity. If the knee swells significantly within the first 12-24 hours after traumatic knee injury, this signifies that there is blood or a hemarthrosis within the joint. The most common cause of an acute hemarthrosis after a sports-related knee injury is a tear of the anterior cruciate ligament, with the second most common cause being a traumatic patellar dislocation/subluxation with bleeding as a result of either soft tissue tearing, osteochondral fracture, or both.
PHYSICAL EXAMINATION (12)
The physical examination should begin with general inspection for skin abrasions, contusions, lacerations, and prior surgical incisions.
It is important to document the overall alignment of the leg while the patient is standing, seated, and supine. Start with the patient in the standing position and observe the amount of standing varus or valgus alignment. Patellofemoral crepitus is best elicited by having the patient stand and then squat down with the examiner’s hands over the patella noting at what arc of motion the crepitus occurs. Retropatellar crepitus that is painful and occurs in either early flexion or terminal extension indicates disease on the distal part of the patella. A painful arc with crepitus in greater degrees of flexion indicates disease on the more proximal portion of the patella. Also observe for excessive pes planus or other abnormal foot alignments while standing. This is usually best accomplished by standing behind the patient.
With the patient in the seated position, observe patellofemoral tracking with the patient seated over the edge of the table while slowly flexing and extending the knee from 0-90 degrees. Observe for high or lateral patellar positioning (grasshopper eyes), small patella, patella alta (the patella faces the ceiling rather than straight ahead), and vastus medialis oblique (VMO) dysplasia. Also, the examiner should observe for signs of J tracking (the patella deviates laterally in terminal extension as the patient extends the knee from a flexed position, thus mimicking a J). Thigh circumference should be measured at a consistent level above the knee to assess for quadriceps atrophy. Palpate for crepitation, suggesting possible chondral injury, noting at what degree of knee flexion that crepitus is present to delineate between proximal and distal lesions.
With the patient supine, patellar alignment and Q angle (angle between anterior superior iliac spine, patella, and tibial tubercle) are measured. A Q angle of less than 15 degrees is generally considered normal. The Q angle should be assessed with the knee flexed at 30 degrees when the patella should be centered in the trochlear groove. An angle greater than 25 degrees with this method is indicative of an abnormally lateralized patellar vector. If the patella is subluxated, the Q angle can yield a falsely low measurement (21). One-finger palpation is important to localize tenderness, whether it is in the retinaculum or the medial and lateral patellar facets. These are best palpated by placing one finger under the respective facet and pushing the patella over to that side with the other hand. The quadriceps tendon and patellar tendon are also best palpated in a resting position with the knee extended. Point tenderness at the inferior pole of the patella at the attachment of the patellar tendon is typically consistent with patellar tendonitis (24
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