The patella is the largest sesamoid bone in the human body and is located anterior and proximal to the trochlea in full extension.⁷ It is enveloped by the quadriceps tendon anteriorly, with the patellar tendon emerging distally to insert into the tibia.⁸ The patella possesses the thickest articular cartilage of all the joints in the human body, approximately 5 to 9 mm at its thickest, which reflects the large forces it has to accommodate over its relatively small surface area.⁹,¹⁰,¹¹ This cartilage lines the articular surface of the proximal two-thirds of the underlying surface of the patella and is intra-articular; the distal patellar pole attaches to the patellar tendon and is extra-articular.¹² The cartilage acts as a bearing surface but is not sensate.¹³

The median cartilaginous ridge of the patella separates its medial and lateral facets. Its location has wide intersubject variability.⁵ It is reported to overlie the bony ridge in only 15% of cases and is positioned lateral to the ridge in 60% of subjects and medially in 25%; in some it is barely visible, whereas in others it is very pronounced.⁷ There are three medial and three lateral facets on the patella, which are congruent with those of the trochlear surfaces lying beneath during progressive degrees of knee flexion.¹² A seventh odd facet is positioned on the medial border, forming an articulation against the medial femoral condyle in deep knee flexion when the patella bridges across the intercondylar notch.¹⁴

The patella has a rich blood supply arising from six arteries, which form a vascular anastomotic ring positioned anterior to the patella.¹⁵ It is supplied by the descending genicular artery, the medial inferior genicular artery, the medial superior genicular artery, the lateral inferior genicular artery (together with the anterior tibial recurrent artery), and the lateral superior genicular artery.¹⁵ The inferior patella is known to have a rich vascular supply.¹⁶ However, the margins of the patella have a poor blood supply.¹⁷

Innervation to the patella is by branches of the infrapatellar branch of the saphenous nerve and medial, intermediate, and lateral cutaneous nerves of the thigh. These nerves all contribute to the patellar plexus of cutaneous nerves at the anterior thigh and patella.¹⁸ Nerves have also been identified in the periosteum and loose connective tissue around the patella, deriving from a medially based neurovascular bundle.¹⁹ A full understanding of the function of these has still to be established.

A classification system has been proposed to subcategorize patellae on the basis of the position of the median ridge and the size and shape of their medial and lateral facets when viewed along the patella to give a “skyline” transverse profile.¹¹ The least common configuration (˜10%), type I, occurs when the median ridge is
in the center of the patella and the concave medial and lateral facets are almost equal in size. Type II describes the ridge to be present slightly toward the medial border of the patella and a flattened or slightly convex medial facet, which is smaller in size than the lateral facet. It is the most common type (65%). Finally, type III patellae (25%) have the ridge displaced medially to such a degree that there is hardly any room left for the medial facet. They have smaller medial facets and larger lateral facets, both of which are convex.¹¹,¹² Underdeveloped medial facets and Wiberg type III patellae are commonly identified in patients with patellar dislocation.²⁰

The distal femur splits into two asymmetrical femoral condyles, which are connected by a sulcus, forming a groove referred to as the femoral trochlea on the anterodistal aspect (Figure 2.1). This happens in utero, with the trochlear shape evident on examination of fetuses.³ The femoral diaphysis is vertical in the newborn baby and then, as weight-bearing begins in early childhood years, a femoral obliquity angle develops, inducing a valgus force on the extensor mechanism of the knee.²¹

The trochlea provides a surface for the patella to glide over during knee flexion and extension.²¹ Continuing posteriorly and distally, as the knee flexes, the trochlea ends in the intercondylar notch, a nonarticulating groove (Figure 2.1). Similar to the patella, and reflecting its need to provide load distribution, the articular cartilage has been reported to be thicker centrally over the sulcus compared with the outer trochlea.²² Sulcus angle is a common measure of trochlear geometry and can be examined with magnetic resonance imaging (MRI) or axial radiographic images taken in 30° knee flexion. It is measured as the angle between the tangential line to the medial and lateral trochlear slopes (normal range: 135°-145°)²³,²⁴ (Figure 2.2). Trochlear depth is measured from MRI images, with the maximum anterior-posterior (A-P) distance of the medial and lateral femoral condyles and the minimum A-P distance of the deepest point of the trochlear groove from the line parallel to the posterior outline of the femoral condyles²⁵ (Figure 2.2). Trochlear depth is calculated with the formula ([(a + b)/2] − c). Mean trochlear depth distances from 5 to 6 mm have been reported in populations of individuals with normal trochlear anatomy.²⁵,²⁶

The geometry and size of the femoral condyles and the trochlea demonstrate wide intersubject variability. On axial view, generally the lateral condyle is larger, with the lateral part of the trochlear groove typically wider and more anterior compared to its medial counterpart.²⁷ When the knee is flexed, the medial facet becomes more prominent because the skyline moves distally and posteriorly. 27 In extension, the patella lies proximal to the trochlear groove, before it engages with the groove at approximately 30° flexion.²⁸ The shape of the lateral facet in extension therefore helps to guide the patella into the trochlear groove as the knee flexes. Relatively uniform patellar contact pressures are reported to be present with large lateral facets as a result of the laterally acting vectors on the patella.²⁹ A measurement of the lateral femoral condyle is the lateral trochlear inclination (LTI) taken from MRI scans. It is calculated as the angle between the line tangential to the cartilage surface of the lateral trochlear facet and the line tangential to the posterior condyles of the femur³⁰ (Figure 2.2). The mean LTI value in subjects with no history of patellofemoral joint pathology has been reported as 17°.³⁰

The most frequent pathology identified in populations of recurrent patellar dislocators is trochlear dysplasia, where the femoral trochlea loses its concave shape and patellar congruence and is instead flattened or convex. Tecklenburg et al⁵ described a sulcus angle of greater than 150° and an LTI of less than 11° in this population. Trochlear depths ranging from −0.6 to 2.7 mm have been reported in populations with trochlear dysplasia. 25 Dysplasia is reported to be present in up to 96% of patients suffering patellar dislocation.³¹

Two axes pass through the proximal-distal length of the femur³² (Figure 2.3). The anatomic axis refers to the line passing along the center of the shaft of the femur to the center of the knee joint, when viewed in the coronal plane. The second axis, referred to as the mechanical axis, relates to the lower limb in a weight-bearing position. It runs from the center of the femoral head to the center of the knee joint and top of the intercondylar notch.³² The angle formed by these axes has been reported to range from 5° to 7°.³³

Limb alignment in the coronal plane is routinely assessed, as discussed earlier. However, despite advances in medical technology, including the advent of computed tomography (CT) and MRI, transverse plane alignment is not commonly considered. Proximal femoral geometry is known to be closely related to trochlear morphology.⁴³ A CT study identified significant correlations between the presence of femoral anteversion and trochlear depth and between sulcus angle and the LTI.⁴⁴ Cadaveric testing has demonstrated that increasing femoral anteversion by 30° resulted in a 30% increase in pressure under the lateral patellar facet, with the opposite effect identified when the external torsion of the femur was increased.⁴⁵ Failure to address femoral anteversion has been linked with poorer outcomes in tibial tuberosity transfer surgery.⁴⁶ Thus, accurate measurement of rotational alignment seems critical to ensuring good outcomes in this patient population.

The subcutaneous soft tissues medial to the patella, the medial retinaculum, are made up of three layers that provide stability against lateral motion of the patella⁴⁷ (Figure 2.5). The superficial fascial layer has a close relationship with the vastus medialis obliquus fascia but contributes very little to patellar stability.⁴⁸ It is joined to layer 2 near the medial patellar border proximally and distally. The second layer is comprised of fibers from the superficial medial collateral ligament aligned proximal-distal, the medial patellofemoral ligament (MPFL) lying horizontally (ie, in a transverse plane), and the medial patellotibial ligament (MPTL), which is conjoined to layer 1. Beneath this, layer 3 constitutes the deep medial collateral ligament, the medial patellomeniscal ligament
(MPML), and the joint capsule. The retinaculum is tightly attached to the medial side of the patella and can thus impose a medial restraining force on the patella to enhance patellar stability, particularly during early knee flexion prior to the patella engaging into the trochlea.

In addition to the patellar tendon, the patella is connected to the tibia distally via the much smaller MPTL and MPML; both ligaments are thinner than the MPFL.⁴⁹ The MPTL inserts into the inferior and medial margin of the patella and proximal patellar tendon and crosses the anteromedial joint line to attach to the rim of the tibia. The MPML extends from the inferior and medial patella, attaching to the anterior rim of the medial meniscus.⁵⁰

The lateral side of the knee has a multilayered retinaculum, which consists of thin ligaments that provide passive restraint to the patella. However, the ligaments are less distinct than on the medial side. The lateral retinaculum comprises three layers: superficial, intermediate, and deep and, as its name implies, is positioned on the lateral side of the knee. The sequence and exact arrangement of the layers as well as the dimensions and attachments of the bands have been contentious and traditionally difficult to delineate given its converging, multilayer structure.⁵¹ The superficial layer is directly subcutaneous and constitutes the deep fascia, which does not attach to the patella but thickens laterally, forming the iliotibial band attaching to the proximal part of the lateral condyle and blending with the quadriceps aponeurosis.⁵² The superficial layer separates easily from the patella given its lack of direct attachment; however, it does provide a bracing resistance to lateral patellar motion. The most substantial layer, the intermediate layer, constitutes derivatives of the iliotibial band and quadriceps aponeuroses. Iliotibial band fibers predominantly insert into Gerdy tubercle distally, with some of the anterior fibers blending completely with the quadriceps aponeurosis, adhering to the lateral aspect of the patella and distal patellar tendon.⁴⁹ These fibers notably lack direct femoral attachment; however, their orientation and position suggest they have a function in providing lateral patellofemoral restraint. Beneath this layer lies the deep layer, primarily constituting the joint capsule. The lateral patellofemoral ligament has been inconsistently reported to form a strand of the deep joint capsule; however, in contrast to its medial counterpart, it is not thought to be the primary restraint to medial patellar motion. Instead, the iliotibial band, patella fibers present as a consistent short segment despite their lack of fixed femoral attachment, are more substantial, and are aligned in the transverse plane. A further attachment from the inferolateral patella to the lateral meniscus has also been reported.⁵² The main role of the lateral retinaculum is as the primary restraint to medial displacement of the patella in relation to the femur. It contributes to patellofemoral tracking and helps with the distribution of medial and lateral compressive loads acting on the patella.⁵³

The infrapatellar fat pad (IFP) is situated intracapsularly, between the femoral condyles, tibial plateau, and patellar tendon, with its posterior boundary the synovial lining of the knee joint⁵⁴ (Figure 2.6). A recent anatomy study has highlighted its complex anatomic attachments.⁵⁵ In 36 specimens, the IFP consistently attached to the inferior patellar pole, femoral intercondylar notch, proximal
patellar tendon, intermeniscal ligament, and both menisci and the anterior tibia via the meniscofemoral ligaments. In 30 specimens, the IFP attached to the anterior fibers of anterior cruciate ligament via the ligamentum mucosum. In 29 specimens, the IFP attached directly to the central anterior tibia, and in the other 7 specimens it was free from direct tibial attachment. Proximal IFP extensions were identified alongside the patella in all specimens medially and 83% of specimens laterally.