This is one of the most common conditions found in growing individuals, but can also be observed in adults. In children, mostly girls are affected whilst in adults there is no gender prevalence. The patella or kneecap is positioned in the front part of the knee joint. It has a wedge shaped rear surface (facets) which is positioned in the V-shaped trough (trochlea) created by the femur (thighbone), with which it directly communicates during knee flexion. Only in full knee extension is it free floating and not in contact with the femur. The thigh muscle (quadriceps) consists of three distinct muscles acting on the patella and play an important part in its positioning during knee movement. Additional ligaments positioned on the inside and outside of the patella work as static stabilisers preventing displacemnt in the coronal plane. In some individuals, the patella may not be positioned in the centre of the trochlea either because of muscle imbalance, ligament tightness, deformity of the articulating surfaces or patella mal-positioning (too high or too low).

Classically patients suffering from a wrongly aligned patella (mal-tracking) present to the clinician with discomfort or a dull ache situated around the front of the knee. This is generally described as anterior knee pain, although patella mal-tracking can express itself through a variety of different symptoms. It is recognized that the condition has a strong female preponderance occurring most commonly in the late teens. However, the condition may also be seen in individuals who expose their knees to certain stresses (patello-femoral loading activities) such as squatting, kneeling, descending stairs, or sporting activities. The majority of patients suffer from a so called lateral patello-femoral overload syndrome, which implies that the patella is pushed towards the outside usually causing discomfort around the outer border of the patella.

MRI scan showing the patello-femoral joint in full knee extension. The wedge shape of the retro-patellar surface is clearly demonstrated. The bright signal represents the surface cartilage, which measures between 3 to 5mm in thickness.

It is important to distinguish between patients with normal tracking and those with tracking abnormalities as both groups may present with similar symptoms. In the group where patella tracking is within normal limits, the underlying cause for symptoms is thought to be due to general patella overload (excessive sporting activities, occupational hazards such as prolonged kneeling or squatting).

Dynamic CT-scan sequence showing the patello-femoral joint of both knees in full extension. Images, which are usually obtained in 0, 15 and 30 degrees of knee flexion allow for precise angle measurements.

Through a thorough clinical assessment augmented by specific investigations, the clinician is able to distinguish between the aforementioned abnormalities and to instigate the best treatment approach. This process includes measurements of the tubercle-sulcus angle and the Q-angle, both of which determine the direction of force during leg extension and hence provide important information on the maldistribution of force.

The TUBERCLE-SULCUS ANGLE measure (left) is performed with the patients leg bend to 90°. Measurements above 0° are considered abnormal. The Q-ANGLE measure is performed with the leg being straight. Angles beyond 25° are generally considered abnormal.

Obtaining a proper history is of paramount importance and often leads to the diagnosis. Sometimes it is necessary to obtain an MRI scan to establish the problem. In cases where simple conservative measures have failed to improve the patient it is occasionally favourable to quantify the degree of mal-tracking or mal-positioning. This is usually possible through a dynamic CT-scan. This investigation allows measurements to be applied to the patello-femoral articulation in varying degrees of knee flexion with and without quadriceps contraction. It also allows for quantification of the distance between the centre of the trochlea (TG) and the attachment of the patella tendon onto the tibial tuberosity (TT). Distance values beyond 20 mm are considered pathologic and likely to require a tibial tuberosity transfer (see surgery for mal-alignment).

Arthroscopic view before and after lateral retinacular release. Patella tilt and lateralised tracking are significantly improved after release.

The general treatment approach for mild to moderate mal-tracking is based on physiotherapy and muscle strengthening activities. Measures include stretching of the lateral patello-femoral ligament, strengthening of the vastus medialis obliquus (VMO), tape application (McConnell) and the use of a so-called in-care brace (DonJoy True-Pull). If these measures fail to improve an arthroscopic release of the lateral structures (lateral patello-femoral ligament), should be considered (see figure 1). It is important to continue with patella mobilisation for at least 6 weeks post arthroscopy as the released tissue might otherwise scar together.

Dynamic CT-scan showing significant lateral patella overhang and tilt indicative for patella mal-tracking. This patient required complete re-alignment of the patello-femoral mechanism as simple lateral release proved unsuccessful.