Patella fractures refer to fractures of kneecap, a sesamoid bone on the anterior aspect of the knee. It accounts for about 1% of all skeletal injuries.
The patella is the largest sesamoid bone in the body.
It is contained within the extensor mechanism which consists of the quadriceps tendon proximally and the patellar ligament distally. Extensor retinaculum fans out on both sides of the patella to reinforce this mechanism.
Any fracture that results in displacement of the fragments in the longitudinal axis represents a disruption of the quadriceps mechanism.
Loss of quadriceps function means loss of active extension of the knee and loss of the ability to lock the knee in extension.
Anatomy of Patella
The patella is triangular in shape with a superior base and inferior apex. The posterior surface is smooth, composed of articular cartilage, and is divided into medial and lateral facets articulate with the medial and lateral condyles of the femur. The anterior surface is rough, for the attachment of tendons and ligaments.
The patella serves for the attachment of the quadriceps tendon superiorly and the patellar ligament inferiorly. The patellar ligament [some call it tendon] to the tibial tubercle, further inferiorly. Actually, the quadriceps tendon and patellar ligament are really the same structure bearing the patella as a sesamoid bone.
The medial and lateral patellar retinaculum are condensations of fascia attaching the patella margins to surrounding fascia. The medial patellar retinaculum attaches to the vastus medialis / sartorius fascia and the lateral patellar retinaculum is attached to the fascia of vastus lateralis and iliotibial band.
The medial patellofemoral ligament originates near adductor tubercle of the femur and inserts into the superomedial aspect of the patella. It helps to prevent lateral patellar dislocation during knee extension.
Biomechanics and Types of Patella Fractures
Being subcutaneously located, the patella is prone to injury.
Fractures of the patella may be caused by compressive force [direct blow] or a sudden tensile force (as occurs with hyperflexion of the knee), or a combination of these.
Following types of fractures of patella are seen.
A direct blow to the patella most often results in a stellate fracture. The compressive forces applied to the patella result in a comminuted pattern.
About 65% of these fractures do not involve the extensor retinaculum in which case the fracture may be treated nonoperatively.
These fractures occur due to sudden, violent contraction of the quadriceps muscle. An example is when an individual stumbles and tries to stop the fall. A displaced fracture may disrupt the quadriceps mechanism.
The sudden contraction may result in either tendon avulsion at either end or transverse fracture of patella together with a tear into the retinacular expansion
This type of fracture, with less than 2 mm of intra-articular step off, can be treated in a non-operative manner. More displaced fractures require surgery.
Multifragmentary Displaced Patella Fractures
These are usually the result of a high-energy force, frequently seen in victims of motor vehicle accidents, and are often associated with fractures of the femur.
Sleeve fractures are seen exclusively in the children population. They are seen as small bony avulsion fractures.
These are the result of patellar dislocation involving varying portions of the medial facet and subjacent bone. This fracture may be visualized in the skyline view only and often CT/MRI is required.
Quadriceps mechanism is not generally affected. Surgery is required to remove the intra-articular loose body and augment the quadriceps mechanism to prevent recurrences of the dislocation.
Clinical Presentation of Patellar Fractures
Pain and swelling in the affected knee after a blow to the knee or a fall is the usual complaint.
Abrasions, contusion over the anterior aspect of the knee, or both may be present
Examination reveals effusion of knee in a closed fracture. A communicating wound is observed in open fractures
If the fracture is displaced, a defect is palpable at the fracture site.
Active extension of the knee is performed for evaluation of the extensor mechanism. If a patient is able to extend his knee, the extensor mechanism is intact.
But it may be affected by pain associated with the injury and hemarthrosis.
The examination in such cases may be performed after instillation of local anesthetic.
If the patient is unable to extend the knee against gravity, it implies that a tear is present in the medial and lateral quadriceps expansion.
Standard views are anteroposterior, lateral, and axial ( skyline/sunrise) radiographs of the knee
Anteroposterior view helps in assessing the fracture pattern and the direction of displacement, whereas the lateral and skyline views assist in evaluating the amount and location of comminution.
CT may be indicated for assessment of osteochondral fractures and intra-articular fragments.
Bone scanning may be helpful in skyline fractures.
Treatment of Patella Fractures
Fractures which are not displaced and the extensor mechanism is intact, are treated by immobilization in cylinder cast for 4-6 weeks. The patient is allowed to bear weight on the extremity.
After that, with the presence of clinic-radiological signs of healing a removable brace is advised. The range of motion and strengthening exercises are initiated. As the patient gets better and is able to perform straight leg raising on her own and has flexion of 90 degrees, the brace is discontinued.
Patients who have open fractures are not treated conservatively. So is any kind of fracture with extensor mechanism disruption.
However, patients with fixed flexion contractures of the knee, in spite of the fracture pattern are candidates for conservative treatment.
Surgical treatment of Fracture Patella
Indications for operative treatment of a patella fracture include the following:
- Disruption of the extensor mechanism
- > 2 mm of stepoff [articular incongruity]
- > 3 mm displacement of fracture fragments
Surgical treatment aims to align articular incongruities, repair extensor mechanisms and allow early motion.
Surgical Approaches and Biomechanical Considerations
The patella may be exposed through a transverse or a longitudinal incision made over the middle of the patella. A longitudinal incision is preferred.
Fractures of the patella are treated mostly with tension band fixation with wire wherever there are fragments available for tension band wiring.
Compression can also be achieved with lag screws, especially distal pole fractures.
Screws can be used if multiple fragments can be converted to two main fragments for tension band wiring.
In comminuted fractures where tension band wiring is not feasible, the patella is reduced and held together with a cerclage wire passed circumferentially. This is especially helpful in stellate fractures.
A 1-mm cerclage wire may be placed around the periphery of the patella. If this measure is combined with tension band fixation, the circumferential cerclage should be placed first.
In case of tension band wiring, immediate postoperative mobilization is begun after pain could be tolerated. In the case of cerclage, 4-6 weeks of immobilization is done.
In the avulsion fractures of the proximal or distal pole, adequate fixation may not be achievable with hardware and, the nonabsorbable heavy suture material can be used for the repair.
Rarely, in fractures where none of the repairs can work [severely comminuted] partial of complete removal of patella [patellectomy] must be performed.
Before wound closure retinaculum repair is warranted.
Complications of Surgery for Fracture Patella
- Hardware prominence
- Loss of fixation or reduction
Prognosis of Patella Fractures
Articular reduction and chondral damage at the time of injury determines the prognosis. Intra-articular incongruities lead to posttraumatic arthritis. Anatomic restoration of the joint minimizes this.
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