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=B/A. This ratio is normally r = 1. A ratio r < 0.8 suggests a high-riding patella (patella alta), or patellar tendon rupture.
The third important x-ray projection is the 30º tangential view, which is obtainable in 45° knee flexion. If a longitudinal, or osteochondral fracture, is suspected, the 30º 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 also 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.
Bipartite patella is an anatomical variant that results from developmental lack of assimilation of the bone during growth. Located on the proximal lateral quadrant of the patella, the condition is without clinical relevance, is usually bilateral and has a characteristic x-ray feature with rounded, sclerotic lines rather than the sharp edges of a fracture.
There are various techniques for patellar salvage:
- Patellar tendon repair and augmentation
- Partial patellectomy
- Total patellectomy
- Patellar reconstruction for central articular impaction injuries
- Repair of sleeve fractures of the patella (click here for a detailed description of this technique)
Partial patellectomy is preferred to total patellectomy, whenever possible, as it maintains the extensor lever arm. A comminuted upper, or lower pole, and even a comminuted zone in the middle of the patella, can be managed best by taking out all small bone fragments. If the damaged zone is in the middle of the patella, a proximal and distal osteotomy with reduction of the main fragments, as in a transverse fracture, can be performed. If the comminuted area is marginal, the bone fragments should be removed in order to prevent osteophyte formation. To avoid tilting the patellar fragment and increasing patellofemoral contact forces, the patellar tendon should be attached near the posterior aspect of the remaining patellar fragment. In a suboptimal fixation and/or poor quality bone, a temporary patellotibial cerclage may be needed to protect the primary transosseous suture repair.
If patellar salvage fails, patellofemoral osteoarthritis may well occur. Late salvage after patellar fractures may ultimately require a specialist knee arthroplasty, such as some form of semi-constrained knee prosthesis.
Augmentation of patellar tendon repair
After the patellar tendon has been repaired, or if fixation of the patellar tendon origin is inadequate, because of a multiple, small fragments, the transosseous sutures must be protected with a patellotibial cerclage wire between patella and the tibial tuberosity.
2 Reduction techniques and tools top
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.
3 Patellar tendon repair and augmentation topenlarge
Patellar tendon repair
If the patellar tendon has been torn from the distal pole of the patella the extensor mechanism must be repaired, in order to regain active knee extension.
Suture the patellar tendon to the patella with 5 nonabsorbable transosseous sutures.
This repair needs augmentation with a patello-tibial wire, because it is never strong enough to sustain knee flexion extension forces.
Cerclage wire around the patella
After the patellar tendon has been repaired, or if fixation of the patellar tendon origin is inadequate, because of a multiple, small fragments, the transosseous sutures must be protected with a cerclage wire between patella and tibial tuberosity.
The anchoring at the tuberosity can be achieved with a simple figure-of-zero cerclage wire through a transverse drill hole just below the tibial tuberosity.
When tightening the loop, it is necessary to ensure that the knee is able to flex at least to 90°. This means that in full extension there will be some laxity of the cerclage wire.
Pearl: usage in small A1-type fractures
The same technique can also be used in very small A1-type fractures.
Alternative: figure-of-eight and/or cannulated screw
As an alternative in osteoporotic bone, the anchorage at the tuberosity can be achieved around a 3.5 mm cortex screw, or through the hole of a cannulated screw.
It is the surgeon’s decision whether a figure-of-zero, or figure-of-eight, loop is made, based on personal experience and preference.
Alternative: cerclage wire through the patella
Use a patellotibial cerclage wire between the patella and tibial tuberosity to augment the repair. The anchoring at the tuberosity can be achieved around a 3.5 mm cortex screw, or through the hole of a cannulated screw. When tightening this, ensure that the knee is able to flex to 90°. This means that in full extension there will be some laxity of the cerclage wire.
Pearl: correct patellofemoral biomechanics
One way to ensure that the patellofemoral biomechanics are restored to more functional normality is to position the cerclage wire in a more anatomical position. It is preferable to pass the cerclage wire transversely through the middle, or lower, third of the patella. If the cerclage wire is passed over the proximal pole of the patella this tends to cause increased patellofemoral flexion load, which may lead to anterior knee pain.
If the cerclage wire is tensioned too tightly it will not be possible to flex the knee and a patella baja will occur.
This complication must be avoided as it can cause severe limitation of knee flexion and anterior knee pain. If there is need for a cerclage wire to protect the patellar tendon, a patella baja can be produced by misjudging the exact length of the patellar tendon. The opposite knee will indicate the correct position of the patella, as revealed by x-ray evaluation.
4 Partial patellectomy topenlarge
It is most often the distal pole of the patella that is fractured. If this is not reparable, remove these fragments. Suture the most proximal part of the remaining patellar tendon to the remaining patella, using transosseous sutures, or suture anchors.
Approximate the patellar tendon firmly to the remaining patella with the sutures. Then repair the lateral and medial parapatellar retinacula.
The biomechanics of the remaining patellofemoral joint will be abnormal, but will give a better clinical outcome than a total patellectomy.
The addition of a patellotibial cerclage wire should be used to protect the sutures.
5 Total patellectomy topenlarge
Total patellectomy is reserved for severe multifragmentary fractures of the patella, which may be combined with significant osteochondral damage to the patellofemoral joint. In situations where repair is not possible, the entire patella can be excised.
To perform this procedure remove the entire patella by sharp dissection, as close to the bone as possible. Suture the proximal extensor mechanism, with a nonabsorbable suture, to the distal extensor mechanism. The lateral and medial patellar retinacula – if torn – should also be repaired.
After total patellectomy early functional results are surprisingly good, but deteriorate with time.
Pearl: “shelling out” the bony portions of the patella
When “shelling out” the bony portions of the patella from the extensor mechanism, use a series of new, sharp scalpel blades (they blunt quickly) and cut into the “axilla” of the fiber attachments to the bone. This keeps the dissections as close to the bone as possible, ensuring maximal preservation of the extensor tendinous tissue. This is especially important over the front of the patella.
6 Patellar reconstruction for central articular impaction injuries topenlarge
Severe central comminution of the patella is a rare, but very difficult injury to treat.
As a salvage procedure, the injured segment may be removed by osteotomy.
The osteotomy may be secured with two 3.5 mm partially threaded, cortical, cannulated screws inserted as lag screws. Ensure that there is maximally possible alignment of the articular surfaces of the residual 2 portions of the patella: this may result in an anterior step at the osteotomy, which is of no consequence.
This patellar construct may well require augmentation of the fixation with a cerclage wire. Click here for further details on cerclage wire application.