Patella Fractures and Extensor Mechanism Injuries

Anatomy

The extensor mechanism of the lower leg consists of the following:

• 1.

  The four muscles of the quadriceps :

  Muscle	Origin
  Rectus femoris	Anterior inferior iliac spine/groove
  Vastus intermedius	Anterior/lateral femur
  Vastus lateralis	Greater trochanter/lateral lip of linea aspera of femur
  Vastus medialis (longus and obliquus)	Intertrochanteric line/medial lip of linea aspera of femur

• 2.

  The quadriceps tendon is formed by the convergence of the quadriceps muscle.

• 3.

  The patella is the largest sesamoid bone of the human body and lies embedded between the quadriceps tendon and patellar tendon. A vertical ridge divides the retropatellar articular surface into a larger lateral facet (approximately two-thirds of the area) and a smaller medial facet (one-third of the area; Fig. 60.1 ). The anterior surface of the patella is covered with an extraosseous arterial ring derived mainly from branches of the geniculate arteries ( Fig. 60.2 ). The intraosseous blood supply of the patella is provided by two systems of vessels, both derived from this extraosseous vascular ring: the mid-patellar vessels, which penetrate the middle third of the anterior surface of the patella, and the polar vessels, which enter the patella at its apex.

  
  

• 4.

  The patellar tendon originates at the inferior pole of the patella and inserts onto the tibial tuberosity. It receives its blood supply from two sources. The infrapatellar fat pad supplies the deep surface of the patellar tendon with contributions from the inferior medial and inferior lateral geniculate arteries. The anterior or superficial surface of the tendon is supplied by the retinaculum, which receives its supply from the inferior medial geniculate artery and the recurrent tibial artery (see Fig. 60.2 ).

• 5.

  Soft tissue attaching to the patella. The patella is stabilized by the medial and lateral retinaculum, which arise from aponeurotic fibers of the quadriceps muscle and serve as the so-called “auxiliary extensors of the knee” (see Fig. 60.1 ). The medial patellofemoral ligament (MPFL) is the primary passive restraint to lateral patellar dislocation ( Fig. 60.3 ).

  

• 6.

  The tibial tuberosity is a tibial prominence onto which the patellar tendon attaches.

Biomechanics

The patellofemoral joint reaction force is a measure of the compression of the patella against the femur. The magnitude of the patellofemoral joint reaction force depends on the angle of knee flexion and the tension of the quadriceps/patella tendon. It increases with knee flexion due to the angle between the quadriceps tendon and patellar tendon becoming more acute and the increasing quadriceps force produced during knee flexion.

Patellofemoral joint reaction forces are as follows :

• Level walking: half of body weight

• Ascending/descending stairs: three to four times body weight

• Squatting: seven to eight times body weight

The contact area at the patella increases with knee flexion and decreases with knee extension. The patella femoral contact stress is determined by dividing the patellofemoral joint reaction force by the patellofemoral contact area. The contact stress increases from 0 to 90 degrees of knee flexion.

Patient's History and Physical Examination

The history usually describes a fall from a height; a near fall; a direct blow to the patella, such as, for example, in cases of dashboard injury; or a combination of these mechanisms. Especially in cases of dashboard injury and high-velocity trauma, concomitant injuries such as fractures of the proximal tibia and distal femur, ruptures of the posterior cruciate ligament, knee dislocations, and acetabular fractures should be suspected. Because the contact area of the patellofemoral joint changes between flexion and extension, higher flexion angles during impact result in more proximal patellar pole fractures and lower flexion angles cause more distal fractures. At 90 degrees of knee flexion, a more central, transverse fracture occurs.

The physical examination should include an evaluation of the skin to look for contusions, abrasions, blisters (if treatment has been delayed), and the presence of an open fracture or an open-joint injury. In patients with a displaced patella fracture, the physical examination will reveal a visible or palpable defect between the fragments. Significant hemarthrosis usually develops secondary to the fracture. If a palpable bony defect is present with little or no effusion, a large retinacular tear should be expected. Knee extension is then evaluated. A tense hemarthrosis will make this part of the examination extremely painful for the patient. Arthrocentesis with aspiration of the hemarthrosis and subsequent injection of lidocaine or bupivacaine into the joint can be helpful. The patient's ability to extend the knee does not rule out a patella fracture and may simply mean that the patellar retinaculum is intact. Inability to extend the knee, however, suggests a discontinuity in the extensor mechanism. With a patellar fracture, such inability implies a tear of both the medial and the lateral quadriceps expansion. Occasionally, a proximal laceration may be noted in proximity to a patella fracture. It may represent an open fracture or an open-joint injury. It is imperative to diagnose these injuries early. A simple means of evaluation is the saline load test. A large-bore needle (18 gauge or higher) and a 50-mL syringe are used to perform joint aspiration. A significant amount of joint effusion may be removed, usually resulting in the relief of pain.

The needle is left in place while the syringe is removed and filled with saline solution, which is then injected into the knee joint. Any communication between the fracture or joint and the outside environment will become obvious if the saline solution exits the wound.

Radiographic Evaluation

Radiographic evaluation of the patella includes the standard radiographs (anterior-posterior and lateral view) plus a tangential view of the patella, usually in 60 degrees of knee flexion.

In the lateral view with the knee flexed 90 degrees, the proximal patellar pole normally lies posterior to the anterior surface of the femur.

Injuries of the patellar tendon, or an abnormal position of the patella, such as patella alta or patella baja, can be diagnosed with the help of radiologic indices. The Insall-Salvati method involves the determination of the ratio of the greatest vertical patella length to patellar tendon length ( Fig. 60.4A ). In a normal subject, this ratio is 1.0. Most authors deem a ratio between 0.8 and 1.2 as normal; otherwise, a ratio of 0.95 would have to be regarded as patella alta . A ratio less than 0.8 suggests a high-riding patella (patella alta), which may indicate a rupture of the patellar tendon. A ratio larger than 1.2 is called patella baja and can be an indicator for a quadriceps tendon rupture. Up to 20% variance is normal.

In individuals with transfer of the tuberosity, a large inferior patellar pole, following Sinding-Larsen-Johansson syndrome or after healed patella fractures, the Insall-Salvati index may be misleading and cannot be used in the same manner. Therefore a modified Insall index or the Caton-Deschamps (see Fig. 60.4B ) and Blackburne-Peel indices (see Fig. 60.4C ) have been introduced as alternate measurements that can likewise be used on lateral radiographs, computed tomography (CT) scans, and magnetic resonance imaging (MRI). The length of the patella surface is compared with the length of the patellar tendon or the anterior border of the tibial plateau surface.

Long-leg standing radiographs are important to determine potential risk factors for patella maltracking, such as valgus- or varus-malalignment or a pathologic q-angle.

CT can be helpful to quantify articular steps and provide more detailed information about the fracture pattern for the preoperative planning. CT can also be used to detect malrotation or trochlea dysplasia leading to patella maltracking and/or dislocation.

MRI is indicated in case of suspected severe soft tissue injury and patellar dislocation.

Ultrasound plays a role in the diagnostic of tendinous/muscular injuries and in the follow-up of patients.

Radiographic evaluation of the uninvolved knee is only rarely recommended as a tool for preoperative planning in selective cases.

Injuries

Patella Fractures

Patella fractures account for around 1% of all fractures. Fractures of the patella can be transverse, vertical, or oblique, with various degrees of displacement and step formation in the articular surface ( Fig. 60.5 ). Fractures without relevant displacement and articular steps smaller than 2 mm can be treated conservatively. Fractures of the inferior pole can also often be treated conservatively because the distal part of the patella is extraarticular and does not engage with the articular surface of the trochlea. Vertical fractures often occur with intact retinacula and preservation of competence of knee extension.

Treatment

The knee is splinted in a position of comfort (usually slight flexion), cooled, and elevated. There exists no evidence that partial weight bearing is beneficial for the conservative treatment of patella fractures. As a common practice, conservative treatment is performed by partial weight bearing of 15 kg on crutches for 3 weeks, then with a pain-adapted increase of weight bearing of half the body weight for another 3 weeks. Flexion should be limited to 60 degrees for the first 3 weeks and 90 degrees for another 3 weeks. A hinged knee brace can be helpful for the limitation of flexion in patients with poor coordinative skills, such as elderly patients.

Key Points: Surgical Indications

• Open fractures

• Intraarticular steps of 2 mm or more

• Inability of the patient to extend the knee actively

The main goals of operative treatment are to preserve extensor function and vascularization and restore articular congruency. As in all articular fractures, anatomic reduction with step-free reconstruction of the articular surface and stable fixation are the major principles. In open fractures, a thorough débridement and washout of the wound must be performed before osteosynthesis as usual. The patient is placed in a supine position. A tourniquet can be applied optionally. It has to be positioned as proximal as possible and should be inflated with the knee flexed to avoid entrapment of the quadriceps femoris muscle, which could otherwise disturb reduction. An anterior longitudinal midline incision of the skin with a medial parapatellar approach to the knee joint is recommended. Some authors instead suggest a transverse, a pure median, or a lateral parapatellar approach. In our opinion, these are not helpful and hamper potential revision surgery (e.g., total knee arthroplasty).

Key Points: Osteosynthesis

• Modified tension-band wiring

• Lag screw fixation

• Cerclage

• Cannulated lag screw with tension band

• Plate osteosynthesis

• Partial patellectomy

• Total patellectomy

Tension-band wiring is the most widely used technique to fix patellar fractures. After reduction, the fracture is fixed with two parallel, 2.0-mm Kirschner wires (K-wires) placed perpendicular to the fracture. An 18-gauge wire is passed behind proximally and distally. In the modified tension-band–wiring procedure, the wire converts anterior distractive forces to compressive forces at the articular surface. One or two (for more symmetric tensioning) twists are placed on opposite sides of the wire and tightened simultaneously to achieve symmetric tension. A repair of any retinacular tear has to be performed as well. A recently suggested transosseous suturing technique for transverse or comminuted fractures of the patella as an alternative to the standard tension-band wiring was proposed to be more safe and effective, with a significantly lower complication rate than the tension-band–wiring technique.

Lag screw fixation is indicated for stabilization of comminuted fragments in conjunction with tension-band wiring. It may also be used as an alternative to tension-band wiring for transverse or vertical fractures. It is contraindicated for extensive comminution and osteopenic bone. Small secondary fractures may be stabilized with 2.7- or 3.5-mm cortical screws. Transverse or vertical fractures require 3.5- or 4.5-mm cortical screws. Retrograde insertion of screws may be technically easier. A stellate fracture pattern may be fixed with additional cerclage wiring .

As a combination, osteosynthesis can be performed by cannulated lag screws with tension-band wiring . Partially threaded screws are placed with a lag technique. Wires are shuttled through the screws and across the anterior patella in a figure-of-eight tension band. This procedure achieves the most stable construct. The combination of screws and tension-band wiring eliminates both possible separations seen at the fracture site with modified tension band and screw failure due to excessive three-point bending.

Plate osteosynthesis is a further option to fix multifragmentary fractures of the patella. However, biomechanical studies providing data to support increased stability are lacking.

Osteochondral fragments can be refixed with bioabsorbable pins or small fragment screws (e.g., 2.5 or 2.0 mm) with submergence of the screw head underneath the chondral surface ( Figs. 60.6, 60.7, and 60.8 ). Significantly better results in terms of less early postoperative pain, better mobility angles of the injured knee, higher functional score of the injured knee, and decreased incidence of complications were shown for the novel cable pin system as a minimally invasive technique compared with the conventional open K-wire tension-band method in transverse patella fractures. Application of extraarticular arthroscopy with hanger-lifting procedure could potentially offer new options for surgical treatment.

In comminuted fractures, anatomic reduction of all fragments often is not achievable. In this case, a partial patellectomy with the maintenance of the main articular fragment and resection of all smaller, nonrefixable fragments is indicated. The periosteum and the retinacula have to be kept intact. After proximal partial patellectomy, the quadriceps tendon is reinserted with either transosseous sutures or suture anchors as an alternative. The same applies to distal partial patellectomy and the reinsertion of the patellar tendon. However, in a recent human cadaveric study, MRI demonstrated that the largest arterial contribution to the patella is entering at the inferior pole inferomedially. The authors, therefore, recommended that distal-pole patellectomy should be avoided to retain vascularized bone at the reduced fracture site.

For displaced, comminuted fractures not amenable to reconstruction, total patellectomy sometimes remains as the last salvage procedure. The bone fragments are sharply dissected. The defect may be repaired through a variety of techniques. The most common technique is the quadriceps turndown by inverted V-plasty of Shorbe and Dobson. Total patellectomy usually results in extensor lag and loss of strength ( Figs. 60.9 and 60.10 ).

Postoperative Management

Surgery is usually followed by mobilization with a knee brace. As with most injuries, there exists no evidence that partial weight bearing is beneficial for the postoperative treatment of patella fractures. However, most commonly and according to conservative treatment, partial weight bearing of 15 kg on crutches is performed for 3 weeks, then with pain-adapted increase of weight bearing of half the body weight for another 3 weeks. Range of motion is based on intraoperative assessment of repair. Active flexion with passive extension is commonly recommended. In most cases, flexion should be limited to 60 degrees for the first 3 weeks and 90 degrees for another 3 weeks ( Table 60.1 ).

Table 60.1

Postoperative Physiotherapy Regime After Patella Fractures

	Partial Weight Bearing	Range of Motion
1–3 Weeks	15 kg	0–60°
4–6 Weeks	1/2 body weight	0–90°
>6 Weeks	Full	Free

Quadriceps strengthening is begun when there is radiographic evidence of healing, usually around 6 weeks after surgery.

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