Patellofemoral Anatomy and Biomechanics

Seth L. Sherman, MD∗dr.seth.sherman@gmail.com, Andreas C. Plackis, BS and Clayton W. Nuelle, MD,     Department of Orthopaedic Surgery, University of Missouri, 1100 Virginia Avenue, DC953.00, Columbia, MO 65212, USA

∗Corresponding author.

Patellofemoral disorders are common in the general population and result in a high frequency of patient visits to both primary care and orthopedic clinics. A thorough understanding of the basic anatomy and biomechanics of the patellofemoral joint is critical for any clinician who wishes to treat the broad spectrum of disorders that occur within the patellofemoral joint.

Keywords

Patella anatomy

Trochlea anatomy

Patella pathology

Trochlea pathology

Patellofemoral anatomy

Patellofemoral biomechanics

Key points

• Patellofemoral disorders encompass a large spectrum of disease including patellofemoral pain, instability, focal chondral disease, and arthritis.

• Most patellofemoral disorders are the result of aberrant anatomy (ie, soft tissue injury, bony malalignment) that predisposes the patient to biomechanical abnormalities (ie, patella maltracking).

• There are multiple bony and soft tissue stabilizers to the patella. Soft tissue stabilizers (ie, medial patellofemoral ligament [MPFL]) are critical from 0° to 20° of knee flexion, while the trochlear groove provides stability at greater than 20°.

• Abnormalities of dynamic muscle strength (ie, vastus medialis obliquus [VMO]), static soft tissue restraint (ie, MPFL, lateral retinaculum), patella height and tilt, trochlear morphology, and tibial tubercle position have profound effects on patellofemoral kinematics and may lead to clinical dysfunction.

Introduction

A thorough understanding of the basic anatomy and biomechanics of the patellofemoral joint is critical for any clinician who wishes to treat the broad spectrum of disorders that can occur.

Epidemiology

In orthopedic and musculoskeletal clinics, evaluation of patellofemoral pain encompasses up to 10% of all visits and has been reported as high as 30% in the 13- to 19 year-old age group.1,2 Patellofemoral disorders comprise nearly 25% of all knee injuries.3–5 They are more common in women than in men.6 The incidence of primary patellar dislocation is 5.8 cases per 100,000 population, and up to 29 cases per 100,000 population in patients aged 10 to 17 years.7 Chondral lesions have been reported in upwards of 60% of patients who underwent routine knee arthroscopies.8 Patellofemoral pathology has a significant impact on time lost from sport or work.9

Spectrum of disease

Patellofemoral disorders encompass a large spectrum of disease, including patellofemoral pain, instability, focal chondral disease, and arthritis. Dysfunction can be the direct result of trauma (ie, patella dislocation) or insidious in nature (ie, patellofemoral pain, arthritis). Most patellofemoral disorders are the result of aberrant anatomy (ie, bony malalignment) that predisposes the patient to biomechanical abnormalities (ie, patella maltracking). Successful treatment requires an understanding of the anatomy/biomechanics of the joint, in order to recognize and correct common patterns that lead to patellofemoral dysfunction. Anatomic and biomechanic abnormalities may be addressed nonoperatively (ie, dynamic strengthening/stability, bracing) or operatively (ie, tubercle osteotomy, MPFL repair/reconstruction, patellofemoral cartilage restoration or resurfacing, or trochleoplasty). A comprehensive treatment plan must address both the biology and the biomechanics for optimal results.

Anatomy

Osseous Anatomy of the Patella

The patella is the largest sesamoid bone in the body. It resides within the trochlear groove of the distal femur and links the extensor mechanism through connections to the quadriceps tendon at its superior pole and the patellar tendon at its inferior pole.

The patella is convex on its anterior surface, but is divided by a longitudinal median ridge on the articular side. The patella has 7 total facets, but is primarily divided into the 2 large medial and lateral facets.

The lateral facet is typically longer and more sloped to match the lateral femoral condyle, while the medial facet is smaller, with a shorter but consequently steeper slope.10 The Wiberg classification delineates 4 different types based on the location of the median ridge (Fig. 1).11 The primary blood supply to the patella occurs from a complex arterial plexus that forms an anastomotic ring surrounding the patella.12,13 Patellar articular cartilage is the thickest found in the body, measuring up to 7 mm.14

Fig. 1 Illustration demonstrating the Wiberg classification of patella anatomy.

Patella cartilage has much greater congruency in the axial plane compared with the sagittal plane, contributing to the gliding capability of the joint itself. Contour of the cartilage does not always follow that of its underlying subchondral bone.15

The articular surface is only present on the superior two-thirds of the patella, as the distal pole serves as the patellar tendon insertion and is extra-articular (Fig. 2).

Fig. 2 Illustration demonstrating the posterior aspect of the patella, which consists of cartilage over the proximal two-thirds and an extra-articular portion along the distal one-third of the patella.

Osseous Anatomy of the Trochlea

The trochlea is formed by the anterior aspect of the distal femur. It has a centralized trochlear groove (TG) with associated medial and lateral facets.

The lateral facet is larger and extends more proximally than the medial facet. The depth of a normal TG is 5.2 mm, with the lateral femoral condyle being 3.4 mm higher than the medial femoral condyle in the axial plane.10

The TG deepens as it extends distally and deviates lateral before it terminates at the femoral notch. The facets transition into the medial and lateral femoral condyles.16

The depth of the TG can be measured by the sulcus angle (Fig. 3).

Fig. 3 The sulcus angle (red lines) is the angle formed in the axial plane from the highest point on the lateral facet, to the trochlear groove, to the highest point on the medial facet. An angle of 138° represents normal anatomy, with an angle of 150° or greater representing an abnormally shallow groove. The congruence angle (green lines) is formed from a line drawn through the apex of the trochlear groove with a line through the lowest point on the articular ridge of the patella. A value of -6° represents normal anatomy, while a value greater than 16° represents an abnormal patellofemoral articulation. Patellar tilt (blue lines) is the angle formed by a line drawn parallel to the posterior femoral condyles and a line drawn through the transverse axis of the patella.

Trochlear dysplasia is characterized by a loss of the normal concave anatomy and depth of the TG, creating a flat trochlea with highly asymmetrical facets. This frequently predisposes to patellar dislocation during knee flexion secondary to the loss of restraint of the patella within the groove.

Dejour and colleagues17 quantified trochlear dysplasia radiographically and defined the trochlear bump, deemed pathologic when greater than 3 mm, and the trochlear depth, deemed pathologic at 4 mm or less (Figs. 4 and 5). The lateral condyle forms the lateral wall of the patellofemoral articulation and is the primary restraint to lateral patellar translation once the patella is deeply engaged in the groove.18 Hypoplasia of either the medial or lateral femoral condyle can also contribute to abnormal trochlear anatomy and subsequent patellofemoral articulation abnormality.

Fig. 4 Illustration depicting the Dejour classification of trochlear dysplasia with the corresponding lateral radiograph of each type.
Fig. 5 Illustration depicting the crossover sign (white arrow), as seen on a lateral radiograph in the setting of trochlear dysplasia.

Anatomy of patellofemoral soft tissue structures

Quadriceps Mechanism

The quadriceps mechanism is an important contributor to dynamic patellofemoral joint stability. It is formed by the convergence of 4 muscles: the rectus femoris, vastus medialis, vastus lateralis, and vastus intermedius.

The tendon results as a confluence of these individual muscle tendons 5 cm to...