Patella
Sesamoid Bone (the largest in the human body); part of the Appendicular Skeleton. · Located in the anterior compartment of the lower limb, embedded within the quadriceps tendon, anterior to the femoral condyles at the knee joint.
Also known as: Kneecap
Classification
Sesamoid Bone (the largest in the human body); part of the Appendicular Skeleton.
Location
Located in the anterior compartment of the lower limb, embedded within the quadriceps tendon, anterior to the femoral condyles at the knee joint.
Origin
Endochondral ossification from a single primary center (rarely multiple) that typically appears between ages 3 and 6 years. Fusion of accessory centers is usually complete by puberty.
Dimensions
Typically measures 5cm in diameter and 2-3cm in thickness. It is roughly 1/4 the size of the distal femur. The bone shown in these medical plates appears to be from a skeletally mature adult.
Description
The patella acts as a biological pulley. As the largest sesamoid bone, it is an essential component of the extensor mechanism of the knee, shielding the joint from trauma while significantly increasing the torque of the quadriceps muscle.
Key Features
Triangular shape, posterior vertical ridge, large articular facets, and a roughened anterior surface with vertical striations.
Anatomical Description
A flat, triangular or heart-shaped bone situated at the front of the knee joint. It features a superior base, an inferior apex, an anterior surface, and a posterior articular surface divided by a vertical ridge into medial and lateral facets.
Dimensions & Proportions
Typically measures 5cm in diameter and 2-3cm in thickness. It is roughly 1/4 the size of the distal femur. The bone shown in these medical plates appears to be from a skeletally mature adult.
Surface Features
The anterior surface is convex and perforated by small apertures for nutrient vessels, marked by longitudinal striae. The posterior surface is smooth and articular in its upper two-thirds, while the non-articular lower third (the apex) provides attachment for the patellar ligament.
Articulations
Articulates with the patellar surface (trochlear groove) of the femur via a synovial plane-type joint. It does not articulate directly with the tibia or fibula.
Muscle Attachments
The base (superior border) and sides provide insertion for the Quadriceps femoris tendon (vastus lateralis, medialis, intermedius, and rectus femoris). The apex serves as the origin for the Patellar ligament.
Blood Supply
Supplied by a vascular anastomosis known as the genicular plexus, primarily derived from the superior and inferior medial and lateral genicular arteries.
Developmental Origin
Endochondral ossification from a single primary center (rarely multiple) that typically appears between ages 3 and 6 years. Fusion of accessory centers is usually complete by puberty.
Clinical Significance
Prone to transverse or comminuted fractures from direct impact. Clinical conditions include patellar dislocation (usually lateral), patellofemoral pain syndrome, and 'Jumper's Knee' (patellar tendonitis).
Age & Sex Determination
Limited utility for sexing compared to the pelvis, though male patellae are generally larger and more robust. Age estimation is primarily useful in subadults based on the appearance of the ossification center.
Forensic Importance
The patella is often well-preserved in forensic contexts due to its dense cortical bone. It can be used for stature estimation if other long bones are absent, using specific regression formulae.
Comparative Anatomy
Absent in most marsupials and many reptiles; highly developed in cursorial (running) mammals to increase the leverage of the hind limb extensors.
Evolutionary History
Evolved as a biomechanical adaptation to protect the knee joint and increase the mechanical advantage of the quadriceps muscle group by increasing the angle of pull.
Imaging Characteristics
Easily visualized on lateral X-rays to assess height (Insall-Salvati ratio) and on 'Sunrise' or 'Merchant' views to evaluate patellofemoral alignment.
Pathological Conditions
Osgood-Schlatter disease (at the tibial tuberosity attachment), Patellar bipartita (a normal variant mimicking fracture), and Chondromalacia patellae (softening of the cartilage).
Surgical Relevance
Surgical access via medial parapatellar approach; fractures often require Open Reduction Internal Fixation (ORIF) with tension band wiring to counteract the pull of the quadriceps.
Similar Bones
In fragmentary form, it could be confused with a large carpal (like the scaphoid) or a large tarsal, but its unique heart shape and large articular facets are diagnostic.
Interesting Facts
You are actually born without a 'bony' kneecap; it exists as a cartilaginous structure until ossification begins in early childhood.
Photography Tips
Capture the posterior surface to show the facets. Use oblique lighting to highlight the vertical striae on the anterior surface. Include a scale to distinguish it from smaller sesamoids.