Normal patellofemoral stability is guaranteed by the complex interaction of skeletal geometry, soft tissues, and neuromuscular control.¹ The shape of the femoral trochlea and its relationship to the patella dictate the patellofemoral gliding mechanism.²,³

The normal cartilaginous surface of the trochlea consists of the lateral and medial facets of the femoral sulcus and is defined by different criteria in the proximal-distal, mediolateral, and anteroposterior direction.⁴ The normal trochlea deepens from proximal to distal and is longest laterally and shortest on the medial side in the proximal-distal direction²,⁴,⁵ (Figure 21.1). The deepened trochlear groove separates the lateral facet from the medial part. In the anteroposterior measurements, the most anterior aspect of the lateral condyle is normally higher than the medial condyle, and the deepest point is represented by the center of the trochlear groove.⁶

Trochlear dysplasia is an abnormality of shape and depth of the trochlear groove, mainly in its proximal extent.⁷,⁸ It is a strong risk factor for patellar instability.⁹,¹⁰ Femoral trochlear dysplasia is present in 85% of patients with recurrent patellar dislocation and in 96% with objective patellar dislocation.⁷

Different forms of trochlear dysplasia are described, such as decreased depth, decreased inclination of the lateral facet, flat trochlea, trochlear bump (anterior translation of the trochlear floor), and hypoplasia of the medial trochlea.⁷,⁸ These well-defined forms of trochlear variations are located proximally and cause decreased bony stability in the trochlear groove. The patella is insufficiently guided at the entrance into the trochlea at the beginning of knee flexion, and lateral instability may occur.

A less described form of trochlear dysplasia is when the articular trochlea is too short in its proximal-lateral extension¹,²,¹¹ (Figure 21.2). This entity is not known very well. If surgery is required in patients with such dysplasia, then surgical treatment should be tailored to correct this specific type of dysplastic trochlea.¹,²,¹¹

Loss of patellotrochlear engagement could be because of high-riding patella or too short trochlea. The current surgical technique is meant to correct a short trochlea (in the presence of normal patellar height). In cases with patella alta, other surgical interventions, such as distalization of tibial tubercle (see Chapter 16)
or patellar tendon shortening (see Chapter 17), may be required.

The lateral-facet lengthening trochleoplasty (described later) to address the short lateral facet of trochlea is different from the lateral-facet elevating trochleoplasty (see Chapter 20), which is meant to address a less inclined or flat lateral facet of trochlea.

Table 21.1 lists indications and contraindications for lateral-facet lengthening trochleoplasty.

The patients with a short lateral trochlear facet suffer from dynamic lateral patellar instability.

The patella is well centered in the trochlea under relaxed conditions (Figure 21.3A). With the knee in extension, muscular contraction of the extensor mechanism would lead to proximalization and lateralization of the patella, resulting in “dynamic superolateral patellar subluxation” (Figure 21.3B). The lateral subluxation of the patella is caused by the absence of osteochondral opposing force of the lateral trochlear facet. The discrepancy between a well-centered patella under relaxed conditions and the dynamic superolateral instability caused by quadriceps contraction confirms the proximal-lateral patellar instability.