The osseous structures which make up the patellofemoral joint are the patella and distal femur, specifically the trochlea.
The patella is divided by an eccentric longitudinal median ridge on the articular side (often miscalled the apex) and is thought of as having two major articulating facets: the medial facet and lateral facet (though a total of seven total facets exist).
The lateral facet is typically longer and less acutely sloped (from the horizontal) compared to the medial facet (Fig. 5.1). The position of the median ridge delineates four different types of patellar morphology [59] (Fig. 5.2). Importantly, the articular surface of the patella constitutes only the superior 2/3 of the patella, as the distal pole serves only as the patellar tendon insertion [49].
The trochlear groove (TG) is the articulating surface of the distal femur with the patella. The normal TG depth is about 5.2 mm [43]. The lateral wall of the trochlear groove (i.e., the lateral femoral condyle) serves as the primary osseous restraint to lateral patellar subluxation and dislocation.
V. Kalia (*) · D. N. Mintz
Hospital for Special Surgery, New York, NY, USA e-mail: [email protected]
Fig. 5.1 Axial radiograph demonstrating normal anatomy of the patellofemoral joint. The lateral patellar facet (black block arrow) is typically longer and less acutely sloped (from the horizontal) compared to the medial patellar facet (open block arrow). The position of the median ridge (solid thin arrow) delineates four different types of patellar
morphology. The trochlear sulcus is depicted by the dashed arrow
a
c d
b
Fig. 5.2 Four axial radiographs demonstrate the four categories of patellar morphology. In type I, the median ridge is near the midline of a measurement of the medial-to-lateral distance of the patella. In type II, the median ridge resides in the normal position slightly off midline, creating an elongated lateral patellar facet and a relatively shorter medial patellar facet. In type III, the median ridge is more medialized, resulting in a very short medial patellar facet and a longer lateral patellar facet. In type IV, the median ridge is further medialized, leaving a nearly flat laterally sloped
Soft Tissue Anatomy
The extensor mechanism of the knee is a vital dynamic/active stabilizer of the patel- lofemoral joint. The quadriceps tendon is a confluence of four individual muscle tendons, the rectus femoris, the vastus lateralis, the vastus intermedius, and the vas- tus medialis muscles. The patella is a sesamoid bone within the quadriceps tendon.
The portion of the patellar tendon distal to the patella is the patellar tendon. It runs from the inferior pole of the patella and inserts on the tibial tubercle. The average length of the patellar tendon is 4.6 cm (3.5–5.5 cm) [45].
The medial patellar stabilizers about the knee include the medial patellofemoral ligament (MPFL), medial retinaculum, medial patellotibial ligament, and the vas- tus medialis oblique (VMO), a portion of the vastus medialis muscle. The MPFL serves as the primary passive restraint to lateral patellar translation, particularly in early knee flexion. Laxity of the MPFL (whether congenital, traumatic, or iatro- genic) predisposes to patellar instability [1]. Together, the medial retinaculum and MPFL are the most important ligamentous stabilizers of the patellofemoral joint.
The VMO muscle is the primary dynamic muscular restraint to lateral patellar tracking [21].
The primary lateral patellar soft tissue stabilizer is the lateral retinaculum, which comprises multiple (superficial and deep) layers. Tightness of the lateral retinacu- lum may result in lateral patellar tilt and lateral patellofemoral compartment overload.
Biomechanics
Anatomic function of the patellofemoral compartment requires congruency and synergy of its osseous and soft tissue components. In full extension, the patella sits in a slightly lateralized position relative to the trochlea. Throughout the normal motion arc, different components play critical roles depending on the degree of flexion of the knee. From 0° (full extension) to 30° flexion, the MPFL plays the primary role in preventing lateral patellar maltracking and a slight medial patellar shift occurs as the patella begins to engage with the trochlear groove. At about 20–30° of flexion, the patella engages in the trochlea, providing an additional (bony) stabilizer against lateral patellar maltracking. Up to about 60° of flexion, the surface area of contact between the trochlea and patella increases, increasing stability. As flexion angle increases further, the force vectors conveyed by the quadriceps and patellar tendons converge into an axially oriented patellofemoral joint reaction force (PFJRF), further increasing patellofemoral joint stability [49].
Anatomic Variants Not Typically Associated with Pain
A bipartite patella (Fig. 5.3) results from failed fusion of a secondary ossification center with the body of the patella during development. It is usually asymptomatic and can be bilateral in up to 40% of cases. It is nine times more common in males than females. Of importance, the associated patellar articular cartilage in a bipartite patella remains intact. In the setting of trauma or overuse, the synchondrosis at the failed fusion site may be disrupted and allow for abnormal motion or friction, which may result in pain and associated imaging findings of bone marrow edema on MRI [27].
The dorsal defect of the patella or central patellar defect (Fig. 5.4) is an anatomic variant represented by a benign well-defined subchondral lesion in the superolateral aspect of the patella. The overlying cartilage is nearly always intact, and on MRI, the overlying cartilage may appear thickened to fill the subchondral defect. It may be part of the spectrum of a bipartite patella but further along in evolution to normal.
Anatomic Variants Commonly Associated with Pain and/or Instability
Patients with trochlear dysplasia have trochlear grooves which are significantly shal- lower than normal. Trochlear dysplasia alters the contact forces of the patellofemoral joint and increases the risk of osteoarthritis. There is loss of bony restraint of the patella within the trochlear groove. Measurements of trochlear depth are best made on midsagittal MR images about 3 cm above the femorotibial joint space [43]. The Dejour classification [11–13] of trochlear dysplasia [30] (Fig. 5.5) is commonly ref- erenced and is actually two slightly different classification schemes developed by two different French orthopedic surgeons Henri Dejour and his son David Henri Dejour.
Fig. 5.3 Axial radiograph and single AP radiograph of a patient with bilateral bipartite patellae
a
Fig. 5.3 (continued) b
a b
c d
Fig. 5.5 Dejour classification system for trochlear dysplasia, types A, B, C, and D (panels a–d, respectively)
Fig. 5.4 Axial radiograph of both knees demonstrates bilateral findings of rounded radiolucencies (black block arrows) with vague sclerotic margins in the lateral aspect of the patellae, consistent with bilateral dorsal defects of the patella, which are normal anatomic variants and represent “do not touch” lesions
Patella alta (Fig. 5.6) results from high positioning of the patella relative to the trochlea and an elongated patellar tendon. This anatomic alignment of the patella and femur requires higher flexion at the knee for the patella to become engaged in the trochlear groove, allowing for a larger arc along which patients are predisposed to abnormal patellar translation such as subluxation or dislocation [47]. Patella alta (and its opposite, patella baja, a low-riding patella [Fig. 5.7]) is defined by various measurements (detailed below). Patella alta increases the contact forces on the dis- tal patella and can contribute to patellofemoral joint pain.