Chapter 24
Anterior Knee Pain

with Jenny McConnell and Jill Cook

Patellofemoral syndrome 

As this is the single most common condition seen by most sports medicine practitioners we devote substantial attention to it in this book. We discuss nomenclature, etiology of the pain, functional anatomy and predisposing factors, before outlining clinical assessment and treatment in detail.

Nomenclature and origin of the pain

Patellofemoral syndrome is the term used to describe pain in and around the patella. From the 1930s until the 1970s, the term 'chondromalacia patellae' was synonymous with patellofemoral pain, because softening was noted on the undersurface of the patella. This term is now out of vogue and has been replaced by nonspecific terms such as patellofemoral syndrome (which we use throughout this book), patellofemoral pain syndrome, patellofemoral joint syndrome, anterior knee pain, extensor mechanism disorder.

There are two schools of thought as to the aetiology of patellofemoral pain. The longer standing theory invokes malalignment -- patellar malalignment relative to the femoral trochlea causes abnormalities within articular cartilage, which may or may not be visible to the naked eye. Correction of the malalignment should therefore decrease the patient's symptoms and even out loading of the articular cartilage. There is, however, poor correlation between articular cartilage lesions and pain, so patellar malalignment only explain a proportion of patellofemoral problems.

A more recent theory suggests that patellofemoral problems are due to 'a supra physiological mechanical loading and chemical irritation of the nerve endings denoting loss of tissue homeostasis', which causes an inflammatory cascade to occur and a consequent peripatellar synovitis. The peripatellar synovium is richly innervated and has been shown to be extremely sensitive to light touch. Once the synovium is inflamed, it will be continually aggravated by activities of daily living, resulting in prolonged symptoms.

Unfortunately, the issue of what has caused the pain and which structure is involved is largely still unanswered. Many patients with marked patellar malalignment never experience pain, whilst others, with apparently no malalignment, or changes to their activities, experience problems. 

Functional anatomy

At full extension, the patella sits lateral to the trochlea. During flexion, the patella moves medial and comes to lie within the intercondylar notch until 130o when it starts to move laterally again. The patella's excursion is controlled by the quadriceps muscles, particularly the vastus medialis obliquus and vastus lateralis components. With increasing knee flexion, a greater area of patellar articular surface comes into contact with the femur, thus, offsetting the increased load that occurs with flexion. Loaded knee flexion activities subject the patellofemoral joint to loads many times the body-weight (Table 24.3). Anatomically, the lateral structures of the patellofemoral joint are much stronger than the medial, so any imbalance in the forces will cause the patella to drift laterally.

Table 24.3 Forces transmitted through the patellofemoral joint with different activities

Predisposing factors (c)

A number of factors predispose to patellofemoral syndrome (Table 24.4) and their role must be assessed as some of them can be corrected (e.g., pronation, soft tissue hip joint stiffness).

Table 24.4 Factors that predispose to patellofemoral syndrome

Many individuals with patellofemoral pain present with internally rotated femurs, which may have a bony or soft tissue origin; or may be a secondary compensation for problems elsewhere. Bony internal femoral rotation (femoral anteversion) is often referred to as 'squinting patellae' (Fig. 24.3) and is associated with an increased lateral force on the patella due to an increased in the valgus force that occurs during knee extension. Internal femoral torsion, in association with internal tibial rotation, can be secondary to excessive subtalar pronation. Internal femoral rotation can also occur in conjunction with external tibial rotation.

Fig 24.3
'Patellar squinting' due to femoral anteversion

Hip internal rotation can be limited by soft tissues restriction such as a tight anterior hip joint capsule, and or short adductors, tensor fascia lata, iliopsoas and rectus femoris muscles. All of these restrict movement in the knee and hip and thus, predispose to patellofemoral pain. Tightness in the iliotibial band results in overactivity in the tensor fascia lata and diminished activity in the posterior fibers of gluteus medius. The muscle imbalance persists because the muscles acting from a shortened position (that usually cross two joints) are readily recruited and are strong, whereas muscles acting from an elongated position (usually postural muscles) are difficult to recruit and are weak. A subject with a short ITB demonstrates excessive medial rotation of the hip during the stance phase of gait, which means that the pelvis on the opposite side drops (Trendelenberg sign). This hip movement will increase the dynamic Q angle and hence increase the potential for patellofemoral pain.

Soft tissue tightness is particularly prevalent during the adolescent growth spurt. The resultant inflexibility alters stress through the patellofemoral joint and also compromises muscle control, as the muscles attempt to control a much longer lever. Lateral structures are frequently tight, with the superficial structures (vastus lateralis and iliotibial band) restricting medial glide and deep structures (lateral retinaculum) restricting medial tilt. Hamstrings and gastrocnemius tightness cause a lateral tracking of the patella, by increasing the dynamic Q angle. When an individual with tight hamstrings runs, knee flexion increases with foot strike. Because the knee cannot straighten easily, ankle dorsiflexion must increase for the body to pass over the planted foot. If the talocrural joint is already maximally dorsiflexed, the foot will pronate, particularly at the subtalar joint. This increases the valgus vector force at the knee and hence, increase the dynamic Q angle. Thus, altered foot biomechanics, as a secondary, as well as a primary problem, can alter tibial rotation and affect patellofemoral joint mechanics. Chapter 5 provides a detailed explanation of foot biomechanics.