Patella Fracture
Written by: John Lin, MD; Edited by: Timothy Khowong, MD, MSEd
Anatomy:
The knee encompassess bones, muscles and tendons. Proximally, the distal femur connects the quadricep tendon to the patella, which then connects to the tibial tuberosity on the proximal tibia through the patellar tendon. Various other ligaments (Figure 1) provide additional support at the knee joint, allowing for flexion and stability. The primary function of the patella is to protect the knee joint and to act as a sesamoid bone to allow the knee to extend more efficiently.
There may be variations of the normal patella; the most common is a bipartite patella (Figure 2). In 80% of cases the patella develops and forms as a single bone while in 20% of cases there may be two, or even three areas of ossification, forming multiple pieces of the patella bone. As depicted above, there may be a secondary part of the patella in the inferior (Type I), lateral or vertical (Type II), or in the superolateral aspect (Type III). These findings are rare (2-3%) and may be confused with a patella fracture (3). In 50% cases, both knees are affected (3). The distinction between bipartite patella vs pathological fractures lies in location and radiographic findings: locations are noted above while on X-Ray, the two separate bones would have little separation and each bone would have well-defined, well-corticated borders.
Figure 1 (left): Normal Knee Anatomy
Figure 2 (Above): Saupe classification of bipartite patella
Presentation of Injury:
There is a higher prevalence of 2:1 in men compared to women (1). Patella fractures can result from either a direct or an indirect mechanism. Direct blunt trauma, such as fall onto a flexed knee or during a motor vehicle accident where the knee hits the dashboard can cause a patella fracture. Indirect mechanisms can result from forceful contracture of the quadriceps muscle where the force of the quadricep muscle exceeds the tension on the patella. This may be from jumping and landing or from sudden stopping during a sprint (1,2).
After either direct or indirect trauma, patients often present with focal knee pain with surrounding swelling, bruising, and joint effusion. There may be an overlying laceration/open fracture or notable separation of the patella from the normal anatomic position. The injury may lead to loss of the extensor mechanism.
Assessment of Patella Fracture:
Physical Exam
Inspect for bruising, edema, abrasions, lacerations
Evaluate patella for deviation from normal anatomic position
Evaluate for ability to ambulate and for weakness in affected lower extremity
Evaluate for loss of extensor mechanism (e.g. cannot perform straight leg raise against gravity)
Imaging
Plain Radiographs:
Figure 3: Normal radiograph views of patella. Lateral (A), AP (B), and Axial/sunrise (C)
To assess for patella fractures, obtain plain radiographs to include the anteroposterior (AP), lateral, and axial (sunrise) views (Figure 3). Fracture patterns can be characterized as nondisplaced, displaced, transverse, vertical, comminuted, etc. The lateral view is best for transverse fractures while the axial view is best for vertical fractures (2). Radiographs can also be used to assess for quadricep and patellar tendon disruption with findings including patella alta and patella baja (Figure 5). On X-rays, the Insall-Salvati Index is used to determine patella height and assess for deviations. The ratio is measured by comparing the patellar tendon length to the patella length. The normal ratio is 0.8-1.2. Patella alta is greater than 1.2, while patella baja is less than 0.8 (4). (Figure 6).
Figure 4 (top left): Various patterns of patella fractures
Figure 5 (bottom left): Patella alta (right) and baja (left)
Figure 6 (top right): Insall-Salvati Ratio. Red line (patella tendon length) over yellow line (Patella Length).
CT/MRI and Ultrasound:
CT and MRI are uncommonly used to assess for patella fracture but may be used in few instances. CT is useful to assess for comminuted fractures on the distal pole of the patella while MRI can help assess for tendon injury.
Ultrasound is often readily available in the Emergency Room and can be performed at bedside to assess for patella bony and tendinous injuries. A high frequency, linear probe is often preferred and used in the long-axis configuration to assess the patella tendon, patella and quadricep tendon; may also show periarticular joint effusions. The image below demonstrates normal bone and tendon anatomy on ultrasound (Figure 7). The patella is shown to have a hyperechoic cortex while the tendon has varying layers of linear strands. Disruptions to the cortex or tendon (and joint effusions) may show as disruption of the normal hyperechoic lines of these structures (5)
Figure 7: Normal patella bone and tendon anatomy on ultrasound
Management:
During initial assessment, pain control is important to obtain adequate examination (e.g. straight leg raise) and radiographic views. In addition to oral medications, intra-articular aspiration of hemarthrosis may be considered to decrease pain and swelling. The treatment of patellar fractures can be determined as nonoperative vs operative (i.e. consult orthopedics).
Nonoperative
Intact extensor mechanism
Minimal or non-displacement
Vertical fractures
Old age/morbidities
Place in Knee Immobilizer, RICE therapy, discharge with Ortho and PT follow up.
Operative
Prserve patella if possible
Extensor mechanism failure (i.e. cannot perform straight leg raise)
Open Fractures
Articular step off > 2-3 mm
Displacement > 3 mm
Osteochondral
Consult orthopedic surgery