Patellofemoral Joint

Original Editor - Elvira Muhic

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Description

The patellofemoral joint is a unique and complex structure consisting of static elements (bones and ligaments) and dynamic elements (neuromuscular system).

Anatomy

Articulating Surfaces

The patella has a configuration of a triangle with its apex directed inferiorly. Superiorly, it articulates with the trochlea, the distal articulating surface of the femur, which are the main articulating surfaces of patellofemoral joint.

The articular cartilage of the patella is similar to that of other joints in that it contains a solid phase and a fluid phase that is mostly composed of collagen and glycosaminoglycans. The solid phase is somewhat permeable and when the articular surface is under load, the fluid gradually redistributes itself within the solid matrix. Therefore, the pressure within the fluid is strongly associated with the cushioning effect of the articular cartilage and the low friction coefficient of articular surfaces. Any damage to the articular surfaces causes a loss of pressure within the fluid phase, which subsequently results in higher stresses on the collagen fibers and more vulnerability leading to possible breakdown.

Ligaments & Joint Capsule

The Medial Patellofemoral Ligament (MPFL) - has an origin on the medial femur and a “sail-shaped” attachment on the patella and quadriceps tendon. Because of its wider attachment than its origin, several authors have promoted the technique of using a double-bundled graft to recreate the anatomy of this complex. Kang and colleagues described two components of MPFL fibers, using the term superior-oblique bundle and inferior-straight bundle. The clinical significance of this is not yet known, but the authors suggested the bundles may vary in their roles as dynamic versus static stabilizers. Furthermore, length differences between the two bundles or attachment sites, have been described. Mochizuki and colleagues showed the length of the MPFL fibers from the origin to the medial patella was 56.3+/-5.1 mm vs. 70.7+/-4.5 mm to the quadriceps tendon.

Muscles

Medial movement of the patella is controlled by the vastus medialis oblique (VMO) muscle. Lateral tracking is guided by both the vastus lateralis and the iliotibial band. Patellar motion is further constrained by the patellofemoral ligament, the patellotibial ligament, and the retinaculum.

Function

Compression at the patellofemoral joint:

Activity	Force	%Body Weight	Pounds of Force
Walking	850 N	1/2 x BW	100 Ibs
Bike	850 N	1/2 x BW	100 Ibs
Stair Ascend	1500 N	3.3 x BW	660 Ibs
Stair Descend	4000 N	5 x BW	1000 Ibs
Jogging	5000 N	7 x BW	1400 Ibs
Squatting	5000 N	7 x BW	1400 Ibs
Deep Squatting	15000	20 x BW	4000 Ibs

Arthrokinematics

The patella is engaged with the trochlea at 20-30 º of knee flexion. At 90 º, the patella contacts the lateral and medial femoral facets within the condylar fossa. At 130-135 º of knee flexion, the medial facets of the patella contact the articulating surface of the femoral condyles. In knee extension, the patella abuts the suprapatellar fat pad.

The patella lies within the quadriceps tendon and thereby increases the mechanical advantage of the quadriceps mechanism. Not only does the patella increase the force of knee extension by 50%, but it also provides stability to the patellar tendon and minimizes the forces placed on the femoral condyles. Tracking of the patella begins with the lower patellar border lying in contact with the suprapatellar fat pad when the knee is fully extended. With knee flexion, the patella moves proximally with a lateral shift, which is limited in excursion by the lateral retinaculum. As the knee continues to flex, the tibia internally rotates and the patella moves upward. The amount of force placed on the patellofemoral joint increases with increasing knee flexion. On the other hand, knee hyperflexion increases patellofemoral stress, as does extreme extension. The vector force placed on the patella may be affected by the Q-angle.[10] The Q-angle is a line created from the anterior superior iliac spine (ASIS) to the mid patella, which intersects with a line from the mid patella to the tibial tubercle when the knee is in full extension. An average Q-angle for a male is 14 º, whereas that for a female is 17 º. Q-angles larger than average can indicate abnormal patellar tracking.  Other factors that may affect the vector force on the patella include the following: 

Femoral anteversion
Tibial torsion
Hyperpronation of the foot
Atrophy of the VMO muscle
A tight lateral retinaculum
Patella position (patella alta/baja or subluxation)
Inflexibility of the quadriceps, hamstring, iliotibial, and calf muscle-tendon units
General ligamentous laxity

Pathology/Injury

Patellofemoral joint complaints are one of the most common musculoskeletal complaints in all age groups. Complaints vary from anterior knee pain to peripatellar knee pain to retropatellar knee pain. The etiology of patellofemoral joint syndrome is multifactorial and results from a combination of intrinsic and extrinsic factors. Treatment is often conservative in nature. Because of the variable nature of the complaints and an often lack of objective identifiable pathologic cause of patellofemoral joint complaints, this condition can be difficult to evaluate, diagnose, and treat, which may cause great frustration for the physician and patient alike. Patellofemoral joint syndrome may affect as many as 25% of all athletes.

The presenting symptom in patients with patellofemoral joint syndrome is knee pain. The quality of the knee pain varies from dull and achy to sharp and shooting; occasionally, there is a burning sensation. The location of the pain may also vary. The pain may be described as anterior knee pain, retropatellar knee pain, peripatellar knee pain, global knee pain, posterior knee pain, joint line pain, or a combination of these.
Patients may complain of painful or painless retropatellar crepitation. Symptoms may also include a painful catching sensation and a painful giving way of the knee. The pain may have an obvious etiology. Pain is often related to overuse or a change in exercise intensity.
Some activities that frequently trigger symptomatology are stair climbing, uphill running, hiking, deep knee bends, and squatting. The pain is often not noted until completion of the activity. The patient may also complain of pain with prolonged sitting in which the knees are in flexion.
Pain may be related to trauma, most frequently from falls onto the anterior knee or from the impact of the knees on the dashboard in motor vehicle accidents. Most commonly though, an inciting event cannot be determined. A family history of anterior knee pain may be positive. Children going through growth spurts may experience painful knees, which occurs as the bone grows and stimulates musculotendinous growth through traction. This results in a relative period of inflexibility, which translates to abnormal vector forces placed at the patella and patellar maltracking.

Genetics may predispose a person to develop patellofemoral joint syndrome. Genetic factors that are commonly associated with this condition include the following:
Hyperlaxity of the knee (genu recurvatum) or patellofemoral joint
Genu varus or genu valgus
Femoral anteversion or tibial torsion
Wide pelvic girdle
Pes planus or pronation of the foot
Muscle tightness, which itself may have a genetic component
Abnormal concentration of forces over a smaller articular surface of the patellofemoral joint.
This condition may either be caused or aggravated by overuse or a change in activity level. Repetitive knee flexion, especially on a weighted joint (eg, stair climbing, hiking, uphill running, kneeling, squatting, prolonged sitting with knee flexion) can cause symptomatology.
Trauma can be the underlying cause.
A forceful compression of the patellofemoral joint (eg, a fall onto the anterior knees, impact of the knees on a dashboard during a motor vehicle accident) may precipitate this condition.
Patellofemoral syndrome may occur after a patella subluxation or dislocation.
Patients may develop this condition after having ACL reconstruction with a bone-patellar tendon-bone technique. One year postoperatively, one third of these patients may have patellofemoral symptoms secondary to a weak quadriceps from patellar irritability or flexion contracture.
Osgood-Schlatter disease may be a predisposing factor to the development of patellofemoral pain later in life.
These patients continue to have the predisposing factors, such as muscle imbalance, that caused the Osgood-Schlatter disease.
Assess muscle strength, particularly VMO strength. Weakness is often associated with a decrease in muscle bulk. VMO weakness causes poor patellar tracking. The VMO displaces the patella medially during knee extension, thus guiding it through the trochlear groove during quadriceps contraction.

Techniques

The clinician should make a general observation for the presence of any predisposing factors, which may include the following:
Gait (walking and running) – Inversion/eversion of the hindfoot
Femoral anteversion or tibial torsion
Genu varus, genu valgus, or genu recurvatum
Foot with pes planus or pronation

Examination

Examination of the patellofemoral joint
Observe the position of the patella with the knee in 90° of flexion. Patella alta, patella baja, or patellar lateralization may be present.
Observe patellar tracking in terminal extension (30-0°). A J-curve may be present.
Assess the patellar glide. A tight lateral retinaculum can decrease the medial glide. A medial glide of less than 5 mm (1 quadrant) can indicate a tight retinaculum. If a positive apprehension sign (fear of the patella popping out of position) is elicited with assessment of the patellar glide, suspect a patella subluxation or dislocation.
Palpate for pain. Tenderness is often found on the patellar facets, the trochlea, and the peripatellar soft tissue. Tenderness to palpation at the superior or inferior poles of the patella usually indicates another pathology.
Assess the patella compression test. Compress and push the affected patella distally. Pain is a positive test associated with anterior knee pain. An active test, in which the patient contracts the quadriceps tendon against a compressed patella, has a high false-positive rate.
Assess the Q-angle. The Q-angle is the angle formed by a line created from the ASIS to the mid patella intersecting with a line created from the mid patella to the tibial tubercle with the knee in full extension. The average Q-angle for males is 14°, and the average for females is 17°. An increase in this angle can indicate abnormal patellar tracking.
Muscle force vectors may be unequal and cause patellar maltracking.
Assess hamstring flexibility. Tight hamstrings antagonize the quadriceps function and increase patellofemoral joint loading. Iliotibial band and rectus femoris flexibility should likewise be assessed.
In a study of 12 patients with patellofemoral pain syndrome, Hudson and Darthuy noted that these patients had a tighter iliotibial band.[11] However, it was unclear whether patellofemoral pain syndrome is caused by or results in a tighter iliotibial band.
Assess the muscle bulk of the VMO. The VMO controls medial movement of the patella.

References

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