Anterior Knee Pain

'Anterior Knee Pain' is a generic term from pain from the extensor apparatus that connects the quadriceps muscles to the tibia. It is characterised by diffuse, poorly localised pain at the front of the knee, particularly exacerbated by activities that put a lot of load through the extensor apparatus. It is most commonly cause by maltracking of the kneecap, which is controlled by the overall position of the leg during walking, stairclimbing and running, referred to as 'biomechanics'.

The patella actually sits within the tendon of the quadriceps muscle, and runs around it to attach to the tubercle of the tibia as the patellar tendon. This tendon has a sheath around it (paratenon) Behind it is a surface of smooth articular cartilage, which slides against the groove in the lower femur. This is a very 'congruent' articulation, so minor maltracking of the patella in the groove causes very high contact pressures which can irritate the surface, causing the cartilage to swell. this swelling is uneven and can often cause a crunching or popping sound.

Below the patella and behind the patellar tendon is the 'fat pad' which can also get inflamed.

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The bony patella has a constant ratio of height to width, this can be expressed as the “Patellar Index” (Width x 100 / length). This is always > 100 (with minor racial variation). The patellar thickness varies  2 to 3 cm  (Av. 25mm), not including the articular cartilage.

The lower (distal) 25% is non-articular; the upper (proximal) 75% is covered by hyaline cartilage with a thickness 3 to 5 mm (the thickest in the body) It has very little reparative capacity.

The deep surface is divided into medial and lateral facets separated by a median ridge, as well as an 'odd facet'. medially.

The lateral facet articulates with lateral condyle in flexion. It is always concave, and is divided into upper, middle, lower thirds, each of which contacts the  femur at different degrees of knee flexion, initially with the lower facet. It may translate medially during flexion and rotate.

The medial facet has a great variation of shape, most commonly convex, but may be flat. The shape of the articular surface does not correspond to x-ray appearance
the medial facet may not actually articulate until late flexion, and is separated by ridge from the 'odd facet'.

The groove in the distal femur (trochlear groove) extends more proximally and more anteriorly. It has an articular surface of 2-3mm, and is of variable shape.

The patella articulates with lateral facet at greater than 15 degrees of flexion
There is no contact pressure when in extension, unless resisted extension in terminal 15 degrees.

The patella is stabilised by active and passive restraints.

The patellar tendon runs slightly obliquely from the lower pole of the patella to the tibial tubercle.
The lateral retinaculum is reinforced by the superficial oblique retinaculum (from the ITB to patella) and the deep transverse retinaculum
This deep layer is divided into three bands, namely the epicondyo-patellar band (AKA the lateral patellofemoral ligament), a central band, and a lateral meniscopatellar ligament.

The medial retinaculum is mainly thickened capsule, and is not as strong as the lateral restraints. It is formed of the medial patellofemoral ligament, the medial meniscopatellar ligament
Centrally, the quadriceps extension is a wide band that also acts as passive stabiliser.

 

Active stabilisers include the quadriceps which inserts in 3 layers. 

Rectus femoris most superficially attaches to the upper pole and the anterior third of upper surface, and is continuous with patellar tendon inferiorly. 
Joint aponeurosis of vastus medialis and vastus lateralis continue down medial and lateral borders; the medial extends more distal.
Vastus Intermedius tendon lies deepest.
The muscle of Vastus Medialis Obliquus (VMO) is separate from medial fibres of medialis, and has a more distal insertion.
Vastus Lateralis Obliquus is anatomically separate from vastus lateralis by a thin layer of fat, and originates from the intermuscular septum. It has a direct lateral pull, and provides static and dynamic stability.

The functions of the patella are to increase the lever arm of extension, to provide a layer of hyaline cartilage since tendons not good with shear or compression. It also controls the line of pull of the tendon. The shape of patella resists dislocation, and the divergent pull of quads is nullified. Finally it provides mechanical protection of the front of the knee.

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The patella has equal balanced forces at rest, from which the pressure in the patellofemoral joint can be calculated. It increases dramatically with knee flexion, but as the knee flexes the surface area of contact of the patella reduces. The patella also rotates laterally with flexion, which combines to make a slightly 'S-shaped' curve

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During prolonged stance, the knee is locked into extension. The patella sits superolateral to trochlea, and the quads are relaxed, so there is no patellofemoral pressure. 
At 'quads setting' the patella is pulled proximally about 1cm, limited by the patellar tendon, and moves slightly laterally.

At the start of flexion, the medial femoral condyle rotates on the tibia, like a 'ball in a saucer'. The lateral femoral condyle rolls back on the tibia, and this movement posteriorly “unlocks” knee. The line of contact rotates externally, which results in the tibia rotating internally, and the femur rotating externally. This tends to reduce the Q-angle in the first 20 degrees of flexion.
 

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Q-angle is the angle between the pull of the quads, and the patellar tendon.

At 20-30 degrees flexion, the patella becomes more prominent as it rides up the lateral femoral condyle. The ITB moves posteriorly, and pulls the patella into external rotation. The ITB also works with medial retinaculum to pull patella into groove.   
The centre of gravity of the body moves laterally, such that it is over the knee in single leg prolonged stance. There is much less shift when running, but this depends on control around the hip and gluteal muscle tone. 

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The pressures through the patellofemoral joint vary markedly with activity. They are also increased by small changes in PFJ articulation; as little as 5mm lateral translation and 5 degrees of rotation can dramatically increase pressures.

An overly tight ITB can cause this, and other problems such as shortening of the patella tendon reduces the contact area significantly
Muscle imbalance has a dynamic effect; and is different during gait / running / stair climbing .

The orientation of the trochlear groove is a function of the position of the femur; what is often conceptualised as external rotation of the patella is actually internal rotation of the leg, either due to the weakness around the hip, or foot weakness.

Hip abductors in prolonged stance are not needed as trunk / COG moves laterally. In hip flexion/ walking/ running more control is needed. Weak abductors cause adduction and lateral shift of the trunk. Poor core stability contributes to this, and weak gluteals allow internal rotation of the femur. These combine to create torsional force through PFJ, often identifiable in single limb squat.

 

Abnormalities of femoral rotational profile can also contribute to this; the hip is most comfortable at mid-rotation. The femoral condyles are anterior at mid-rotation, in persistent femoral anteversion the condyles are internally rotated, which causes increased contact pressure on the lateral facet, and can contribute to subluxation. There may be corresponding external tibial torsion.
Tibialis posterior muscle fatigue may flatten the medial arch of the foot, causing 'over pronation' during gait. The subtalar torque converter internally rotates the knee, and provides valgus thrust. These can also contribute to patellofemoral malalignment.

Assessing femoral anteversion

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Tibialis posterior insufficiency.

The differential diagnosis of anterior knee pain includes both intra- and extra-articular causes.

Intra-articular problems:

  • Chondromalacia Patellae (which can range from 'lateral over-pressure syndrome' to articular surface fibrillation, or full thickness articular surface defects)

  • Anterior horn meniscal tears (usually lateral meniscus)

  • Intra-articular loose bodies

  • Plica syndrome

  • Femoral condylar articular degeneration

​Extra-articular problems:

  • Fat pad impingement

  • Patellar tendon problems (rupture, tendinosis / tendinitis or shortening)

  • Subluxation / dislocation

  • ITB bursitis

  • Prepatellar bursitis

  • Osgood Schlatters

  • CRPS/ neuroma

  • Retinacular pain

  • Referred pain

  • Femoral nerve

  • Hip arthritis

  • Bone bruising (traumatic or spontaneous)

An accurate diagnosis should combine a careful clinical assessment, and possibly further investigations such as MRI scanning. A duplex doppler ultrasound may show increased blood flow in tendinitis, or scarring and inflammation of the fat pad.

Treatment of anterior knee pain is almost always physiotherapy in the first instance, with a lot of work on assessment of core stability, gluteal control, gait assessment, foot positioning and walking/ running gait. Neuromuscular control and appropriate muscle strengthening may take months of tedious exercise, and other adjuncts such as ultrasound guided injection may be necessary. Arthroscopy is only rarely needed unless due to an intra-articular injury. Occasionally recurrent dislocation may result in reconstruction of the medial patellofemoral ligament, or if there is an underlying bony abnormality osteotomies to realign the patellar tendon (tibial tubercle reconstruction) or femoral derotation or valgus osteotomy may be needed.
 

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Medial Patello-Femoral Ligament (MPFL) reconstruction

Elmsie-Trillat realignment

Operative treatment of chondromalacia patella is reserved for the worst cases, and the results are unpredictable at best. An articular surface defect can be treated with arthroscopic chondroplasty, which involves debridement of loose flap edges, and multiple holes are drilled into the base. This forms a blood clot that becomes fibrocartilage. It is not as effective in patella as femur.
With fibrillation/ fissuring, ‘smoothing’ of the articular surface can be performed. Other surgeries such as lateral retinacular release are of limited benefit, and may be merely denervation ie destroying pain nerves.

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'Fibrillation' (fronding and softening) of the articular surface smoothed off.

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Loose debris from a chondral lesion removed, then a chondral pick used to release fat into the knee and form a 'superclot'

For the most recalcitrant of cases of patellofemoral arthritis, joint replacement is possible. By far the most common replacement is total knee replacement, but replacement of just the patellofemoral joint is possible. The results however are variable, and the longevity of a partial patellofemoral joint arthroplasty is not nearly as good as a total knee replacement. I myself do not undertake knee replacement surgery, but can provide recommendations if it is indicated.

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Patellofemoral Joint Replacement