18 Fractures of the Patella

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18.3 Classification

18 Fractures of the Patella

J. Schatzker

18.1

Introduction

The patella is a sesamoid bone that develops within the tendon of the quadriceps muscle. Any fracture that results in displacement of the fragments in the longitudinal axis represents a disruption of the quadriceps mechanism.

18.2

Methods of Evaluation and Guides to Treatment

Just as one cannot expect a return of function in the hand without repair of a ruptured flexor tendon, one cannot expect a return of function of the quadriceps mechanism if the displaced fracture of the patella is left unreduced. Loss of quadriceps function means loss of active extension of the knee and loss of the ability to lock the knee in extension.

Individuals with such lesions can neither walk on the level without the knee being unstable and buck- ling, nor can they walk upstairs or downstairs or on inclined planes. The disability is profound. Thus, the indication for open reduction and internal fixation of the patella is any fracture that leaves the quadriceps mechanism disrupted. The patella is intimately bound to the quadriceps tendon proxi-

mally, to the infrapatellar tendon distally, and on either side to the retinacular expansions that are adherent to the capsule. We have come to differen- tiate three groups of fractures of the patella, each requiring different treatment.

18.3

Classification

18.3.1

Osteochondral Fractures

Osteochondral fractures of the patella (Fig. 18.1), which usually involve varying portions of the medial facet and subjacent bone, are the result of patellar dislocation. Surgery is required to remove the intra-articular loose body and repair the quadriceps mechanism to prevent recurrences of the dislocation. In this injury, the extensor mechanism as such is not interfered with. This fracture may only be visualized in the «skyline» radiographic projec- tion of the knee as on CT and therefore is frequently missed in routine radiographs of the knee. One must therefore remember that if a dislocation of the patella is suspected, one must obtain a skyline view of the patella in addition to the routine views of the knee.

18.3.2

Stellate Fractures

A stellate, undisplaced fracture of the patella (Fig. 18.2), or a vertical fracture, is usually the result of a direct blow to the patella. The continuity of the quadriceps mechanism is undisturbed and the retinacula are not torn. The fracture is stable and will not displace under the normal physiologic stresses of active motion. Surgery is unnecessary.

Fig. 18.1. Osteochondral fracture of the medial facet sus- tained in a lateral dislocation of the patella. This fracture is usually not visible on an anteroposterior or lateral radiograph of the knee, and must be looked for on a “skyline” view of the patella

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18.3 Classification

18.3.3

Transverse Fractures

A sudden, violent contraction of the quadriceps muscle, such as might occur when an individual stumbles and tries to stop the fall, may disrupt the quadriceps mechanism. Thus, there may be an avulsion of the quadriceps tendon, or of the infrapatellar tendon, or there may be a transverse fracture of the patella, together with a tear into the retinacular expansion (Fig. 18.3).

18.3.4

Multifragmentary Displaced Fractures

These are usually the result of a high-energy force, frequently seen in victims of motor vehicle accidents, and are not infrequently associated with fractures of the femur. They are the result of a combination of forces such as direct blows to the patella com- bined with displacing forces that rip the patella apart.

Because of the compressive force across the patello- femoral articulation, as in the complex olecranon fractures, we frequently find in the patella separate fragments of the articulation that are displaced and may be impacted into the underlying bone. These must be recognized and reduced in addition to the reduction and fixation of the major fragments. To leave them displaced is to accept an incongruous articular surface that will predispose the knee to posttraumatic arthritis.

18.4

Surgical Treatment

We shall concern ourselves here only with disrup- tions of the extensor mechanism.

18.4.1

Undisplaced Fractures

A transverse, undisplaced fracture of the patella is an avulsion fracture. The function of the quadriceps mechanism is not disrupted, and the patient is able to maintain the knee extended because the retinacula on either side of the patella are not torn. However, the fracture is potentially hazardous because, with further sudden strong contractions of the quadriceps, the retinacular expansions might tear. The fracture will displace and the quadriceps function will be disrupted. The knee requires simple protection. Excellent results have been achieved by allowing motion, but keeping the patient on crutches, and also by immobilizing the knee in a cylinder cast and allowing weight bearing.

18.4.2

Displaced Fractures

A transverse fracture, either simple or comminuted, has, as an integral part, an associated disruption of

Fig. 18.3. In a displaced transverse simple or comminuted fracture of the patella, the retinacular expansions are torn and the patient loses the ability to extend the knee against gravity

Torn retinacular expansion Femoral condyle (medial) Retinacular

expansion

Tibial tubercle

Fig. 18.2. In a vertical or stellate fracture of the patella, the retinacular expansions remain intact. The patient is able to execute straight leg raising against gravity

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18.4 Surgical Treatment

the quadriceps retinacula (see Fig. 18.3). Thus, the quadriceps function is lost and surgical repair is necessary.

18.4.2.1

Surgical Approaches

The patella may be exposed through a transverse or a longitudinal incision made over the middle of the patella. What is important is to make the incision in such a way that it can also be readily extended if greater exposure is needed to visualize the retinacular tears. These must be exposed so that the articular surface is visible as the patella is being reduced and fixed.

18.4.2.2

Biomechanical Considerations

Fractures of the patella and fractures of the olecranon are the two ideal indications for tension band fixation with wire (Müller et al. 1979). If the patella is reduced and held together with a cerclage wire passed cir- cumferentially, reduction is maintained as long as the knee is not flexed or the quadriceps muscle is not contracted. The moment the knee is flexed, the fracture gapes anteriorly, the contour of the patella is changed, and congruency is lost (Fig. 18.4). This will also happen as the result of a strong quadriceps contraction, even if the knee is immobilized in extension in a plaster cylinder. With tension band fixation, flexion of the knee results in an increase of compression across the fracture (Fig. 18.5), and contraction of

the quadriceps if the knee is extended will not cause the fragments to gape.

18.4.2.3

Techniques of Internal Fixation

The tension band wire is passed through the quadriceps and patellar tendons before it is tied over the anterior surface of the patella. The placement of the wires over the anterior surface of the patella is very important, and great care must be taken to ensure that they remain in this position as the tension band is tightened. We do not recommend the figure-of-eight configuration for the tension band wire. Crossing the wires removes their ability to control rotation of the fragments about the long axis of the patella. One wire is usually enough, although in large individuals two wires should be used to increase the strength of the fixation. The wire should be tied on both sides to ensure uniform compression of the fracture. We prefer a large-bore needle (gauge 12–14) to guide the wire through the tendons. This ensures its accurate placement and allows its passage through the sub- stance of the tendon close to the bone. The fracture must be reduced and the knee extended before the wire is tied under tension. We observe the fracture as the wire is tightened, and we tighten the wire until a slight overcorrection occurs and the articular fissure begins to gape. The knee is then flexed to beyond 90°. This maneuver places the whole fracture surface under compression, impacts the fragments, and closes the articular fissure without an anterior gap or distortion of the patella. Compression can

Fig. 18.4. Circumferential cerclage wiring of the patella is unable to neutralize the pull of the quadriceps and infrapatellar tendons. Under load, such as ﬂ exion of the knee, the fracture gapes anteriorly and stability of the fracture and con- gruity of the patella are lost

Cerclage wire

Fig. 18.5. A tension band wire applied to the anterior sur- face of the patella absorbs the distracting forces. In ﬂ exion, the patella is pulled against the intercondylar groove and the fracture closes with the fragments under axial compression.

This is an example of dynamic compression Tension band

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18.4 Surgical Treatment

also be achieved with lag screws, a technique that is particularly necessary with fractures of the distal pole, because if only tension band wiring is used, the small distal fragment displaces and tilts into the joint (Fig. 18.6). The tension band wire is applied once the lag screw is tightened. It serves to protect the lag screw fixation. Similarly, if there are major comminuted fragments, we like to reduce and fix them in place with lag screws (Fig. 18.7). If Kirschner wires are used to secure fixation of the fragments, they must not be crossed; otherwise impaction and compression of the fragments will not occur. If the tension band is placed around the Kirschner wires rather than through the

tendon, there is great danger of pulling the Kirschner wires forward, deforming them, and having the tension band wire slip off (Fig. 18.8). We use Kirschner wires particulary if the patella is grossly comminuted and the wires are necessary for stability. We feel that the patella should be preserved at all cost, and that a primary patellectomy should be performed only if the fracture is such that it is impossible to save even a portion of the patella. If a portion of the patella is retained, the suture of the tendon to bone is protected with a tension band wire (Fig. 18.9). The patient should be warned that such a wire will usually break somewhere between 8–14 weeks from the time of insertion, but by then it will already have served its purpose. Its breakage is usually signaled by pain.

Distally the wire can be passed either through a hole in bone or around a transversely placed screw. The latter is preferable because a wire passing through bone as the knee is moved often causes discomfort.

Since stainless steel wire tends to break with repetitive motion, we have often used a # 5 Merseline suture instead. It is invisible on x-ray and does not break with repetitive movement.

18.5

Postoperative Care

The joint is immobilized in 40°–60° flexion for 2–3 days, and the extremity is kept elevated on a Böhler–Braun splint. On the second or third day, if Tension

band Lag screw

Fig. 18.6. Small fragments of the patella, such as an avulsed inferior pole or a lateral fragment, are best ﬁ xed to the remain- der of the patella with a lag screw. This ﬁ xation must be further protected with a tension band wire

Lag screw Tension

band

Fig. 18.7. Major comminuted fragments of the patella can be lagged together. This is best for vertical or oblique fracture lines, because a tension band applied about the quadriceps and infrapatellar tendons will not compress a vertical fracture line. The remaining fracture lines are ﬁ xed with a tension band. Note the double twist

Tension band wire Tension

band K-wire

a b

Fig. 18.8a,b. Kirschner wires provide rotational and lateral stability. If used, they must be inserted parallel or they will block interfragmental compression of the tension band. The K-wires should be bent over on one side only to facilitate removal. Note the two twists in the tension band, which ensure uniform compression on both sides of the fracture

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18.5 Postoperative Care

we judge that the wound is healing satisfactorily, we instruct the patient to begin active flexion and extension exercises, but we allow only partial weight bearing for the first 6 weeks. We do not splint the knee in extension. It is wrong to immobilize the knee after open reduction and internal fixation of an articular fracture. Not only is motion required to enhance the healing of the articular cartilage, but also in a patellar fracture knee flexion is necessary to enhance the stabilizing effect of dynamic compression on the fracture interface, and this is exerted only when the knee is flexed. Thus, we do not use cylinder casts to protect the internal fixation. Usually, in 6 weeks or so the fracture will have consolidated and the patient

Tendon bone suture

Tension band wire wound around the screw

Tension band wire

Fig. 18.9. If the inferior half of the patella is sacriﬁ ced because of severe comminution, the infrapatellar tendon is sutured to the remaining bony fragment. The suture line is protected with a tension band wire, which allows early mobilization of the knee

will have regained a good range of movement. At this point, the patient can begin full weight bearing. Some protection is advisable until full quadriceps strength has been regained. We use continuous passive motion (CPM) only in patients who have difficulty regaining a range of motion.

Reference

Müller ME, Allgöwer M, Schneider R, Willenegger H (1979) Manual of internal fixation, 2nd edn. Springer, Berlin Hei- delberg New York, pp 42–47