1 Principles topenlarge
The patella is the largest sesamoid bone in the human body. It is located within the extensor apparatus of the knee. Anatomical features include the proximal articular body, with an extraarticular anterior surface and a posterior articular surface, and the extraarticular distal pole. The rectus femoris and vastus intermedius muscles insert at the superior pole of the body and the vastus medialis and vastus lateralis muscles on either side. The patellar tendon originates from the inferior pole and inserts into the tibial tuberosity. The articular surface has the thickest layer of cartilage in the body, up to 5 mm, reflecting the very high resultant loads across the patello-femoral joint, rendering it susceptible to chondromalacia and degenerative joint disease.
History and examination
Patellar fractures comprise about 1% of all fractures and are mostly caused by direct trauma to the front of the knee, for example, a direct fall, or a blow onto the flexed knee.
Bony avulsions of the adjacent tendons, or pure ruptures of the quadriceps and patellar tendons, are caused by indirect forces.
Typical signs are swelling, tenderness and limited, or lost, function of the extensor mechanism.
Preservation of active knee extension does not rule out a patellar fracture if the auxiliary extensors of the knee - the medial and lateral parapatellar retinacula - are intact.
If displacement is significant, it is possible to palpate a defect between the fragments, if present. The hemarthrosis is usually obvious. The examination must include assessment of the soft tissues, so as not to confuse with an injury to the prepatellar bursa, or to omit grading the injury if the fracture is open.
In addition to the standard x-rays of the knee in two planes, a tangential ("skyline") view of the patella is useful. In the AP view, the patella normally projects into the midline of the femoral sulcus. Its lower pole is located just above a line drawn across the distal profile of the femoral condyles. In the lateral view the proximal tibia must be visible to exclude a bone avulsion of the patellar tendon from the tibial tuberosity. A rupture of the patellar tendon, or an abnormal position of the patella like patella alta (high-riding patella), or patella baja (shortening of the tendon), can be recognized with the help of the Insall-Salvati method. This is the relationship between the length of the patella (B) and of the patellar tendon (A) on the lateral x-ray, r = A/B. This ratio is normally r = 1+/-0.2, i.e. 0.8-1.2. A ratio r > 1.2 suggests a high-riding patella (patella alta), or patellar tendon rupture.
The third important x-ray projection is the 30° tangential view, which is obtainable in 45° knee flexion. If a longitudinal, or osteochondral fracture, is suspected, the 30° tangential view is a helpful diagnostic adjunct.
Special imaging is helpful in certain cases, such as stress fractures, in elderly patients with osteopenia and hemarthrosis, and in cases of patellar nonunion, or malunion.
Computed tomography is recommended only for the evaluation of articular incongruity in cases of nonunion, malunion and patello-femoral alignment disorders.
Scintigraphic examination (or MRI) can be helpful in the diagnosis of stress fractures; a leukocyte scan can reveal signs of osteomyelitis.
MRI can be helpful to diagnose cartilage defects and lesions.
Tendon ruptures and patellar dislocation must be ruled out. Isolated rupture of the quadriceps, or patellar, tendon must be excluded by clinical evaluation (palpation) and ultrasound scan (or MRI). Dislocation, most commonly occurring to the lateral side, may result in osteochondral shear fractures with lesions of the medial margin of the patella, and occasionally impaction fractures of the lateral lip of the patellar groove of the femur.
X-ray by courtesy of Spital Davos, Switzerland, Dr C Ryf and Dr A Leumann.
2 Reduction and fixation top
Reduction techniques and tools
The knee joint and fracture lines must be irrigated and cleared of blood clots and small debris to allow exact reconstruction.
An image intensifier should always be available so that the final result can be checked in the AP and lateral planes.
Patellar tendon avulsion
Occasionally, the patellar tendon is avulsed from the lower pole of the patella, together with a thin shell, or “sleeve”, of bone as illustrated.
This may be very subtle on x-rays as the sleeve remaining within the avulsed tendon is thin and is often not apparent on over-penetrated images, whilst at the same time, the lower pole of the patella may have a virtually normal profile. A clue may be found by detection of a patella alta (as appreciated by the Insall-Salvati index).
Fixation of avulsion
Insert a partially threaded cannulated screw, with a washer, as a lag screw.
Neutralization of bending forces
As implant pull-out, or failure, is virtually inevitable, the bending distraction forces must be neutralized by additional patellotibial cerclage wiring.
Repair any tears of the lateral and medial patellar retinacula with sutures.