The principal function of the extensor mechanism of the knee is to maintain the erect position. Ambulation, rising from a chair, and ascending or descending stairs are examples of this ability to overcome gravity. The biomechanical principles of these actions should be understood to treat extensor mechanism injuries successfully.
The following video is included with this chapter and may be viewed at expertconsult.inkling.com :
Judet quadricepsplasty for treatment of extension contractures of the knee.
The patella is the largest hypomochlion 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). 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” ( 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 midpatellar vessels, which penetrate the middle third of the anterior surface of the patella; and the polar vessels, which enter the patella at its apex. The patellar tendon 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 ).
In a recent human cadaveric study, magnetic resonance imaging (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.
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.
As 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 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, 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 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 of the patella includes the standard radiographs (anteroposterior and lateral view) plus a tangential view of the patella, usually in 60 degrees of flexion of the knee.
In the lateral view with the knee flexed 90 degrees, the proximal patellar pole normally lies posterior to the anterior surface of the femur.
The most reliable means of assessing patellar height is the method of Insall and coworkers, which involves determination of the ratio of the greatest vertical patella length to patellar tendon length. 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 index may be misleading and cannot be used in the same manner. Therefore, a modified Insall index or the Caton-Deschamps and Blackburn indices have been introduced as alternate measurements that can likewise be used on lateral radiographs, computed tomography (CT) scans, and 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.
CT can be helpful to quantify articular steps and provide more detailed information about the fracture pattern for the preoperative planning.
MRI is indicated in case of suspected severe soft tissue injury and patellar dislocation.
Radiographic evaluation of the uninvolved knee has been promoted for many years. Nowadays, such imaging is only rarely recommended as a tool for preoperative planning in selective cases.
At times mistaken for a patellar fracture, a bipartite or tripartite patella is a developmental residuum from a variation in which the patella arises from two or more ossification centers that fail to fuse. It is usually a bilateral finding. The most common type is a bipartite patella, in which a bony mass is located in the upper outer quadrant of the patella. It is separated from the main patellar mass by opposing smooth bony surfaces. The condition is generally asymptomatic and requires no treatment, but it can cause confusion when treating patients with a history of injury to the knee area.
Fractures of patella can be transverse, vertical, or oblique, with various degrees of displacement and step formation in the articular surface ( Fig. 60-3 ).
After the diagnosis has been made, the knee is splinted in a position of comfort (usually slight flexion), cooled, and elevated.
Patella fractures account for around 1% of all fractures.
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 as 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 a preservation of competence of knee extension.
As with most injuries, there exists no evidence that partial weight-bearing is beneficial for conservative treatment of patella fractures. As common practice, conservative treatment is performed by partial weight-bearing of 15 kg on crutches for 3 weeks, then with 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 like elderly patients.
Indications for surgery include open fractures, intraarticular steps of 2 mm or more, and the inability of the patient to extend his knee actively.
Biomechanics of Patellar Fracture Fixation
The main goals of operative treatment are to preserve extensor function, 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 prior to 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, such as total knee arthroplasty, and so forth.
Osteosynthesis options include modified tension-band wiring, lag screw fixation, cerclage, cannulated lag screw with tension band, partial patellectomy, and 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-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 to distractive forces to compressive forces at the articular surface. Two 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.
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. Fully threaded screws are placed with a lag technique. Wires are shuttled through the screws and across anterior patella in a figure-of-8 tension band. This procedure achieves the most stable construct. Screws and tension-band wire combination eliminates both possible separation seen at the fracture site with modified tension band and screw failure due to excessive three-point bending.
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. 60-4, 60-5, and 60-6 ).
Mao and colleagues performed a randomized controlled trial comparing the novel cable pin system as minimally invasive technique with the conventional open K-wire tension-band method in transverse patella fractures. They reported 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.
As recently reported in a case series of six patients, application of extraarticular arthroscopy with hanger-lifting procedure could 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 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 for distal partial patellectomy and the reinsertion of the patellar tendon. The drill holes should be near the articular surface to prevent tilting of the tendon and minimize articular step-off. A load-sharing wire passed through drill holes in the tibial tubercle and patella may be used to protect the repair and facilitate early range of motion. Special attention is required to avoid patellar tilting by reinsertion of tendons.
According to the recent findings of Lazaro and colleagues, who demonstrated by MRI that the largest arterial contribution to the patella is entering at the inferior pole inferomedially, it is 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. 60-7 and 60-8 ).