INTRODUCTION

The concept of a dislocated knee has changed markedly over the past few decades. In 1971, Meyers and Harvey¹⁰ predicted that most orthopaedists would not see more than one knee dislocation in their entire career. However, evidence indicates that knee dislocations are being seen with increasing frequency for a variety of reasons. Increasing trauma, changes in automotive design, and recognition of spontaneously reduced knee dislocations have all added to the increased incidence. The increasing number of knee dislocations has created the need for a classification system that will help guide the management of these complex injuries. Classification systems should be simple and reproducible; they should give information to the clinician that will be helpful in making treatment decisions and aid in the communication of like injuries as well as in developing prognoses for injuries. This chapter reviews the shortcomings of the older classification systems of knee dislocations and presents the anatomic classification system that is simple, accurate, reproducible, and useful.^18–20,28

DEFINITION OF KNEE DISLOCATION

Classically, a knee dislocation was defined as greater than 100% displacement of the tibiofemoral articulation. This displacement had to be present on either physical examination or radiographic imaging. Since the 1990s, several authors have recognized the existence of “reduced” knee dislocations. These are knees that present to the treating physician reduced but have multiple injured ligaments (usually including both cruciates) and gross instability on stress testing. Some of these injuries represent a spontaneous reduction of a knee dislocation, and others are dislocated knees that are reduced in the field before transport to a medical facility. The incidence of this type of reduced knee dislocation may be as high as 50% of all knee injuries classified as a dislocation.^3,28,32 A study from our institution showed that the incidence of vascular injury in the reduced dislocations was equal to that of knees that presented dislocated (Fig. 26-1).³² In general, more recent studies have shown that the risk of arterial injury is approximately 7% to 12% with a diagnosis of a dislocated knee.²⁶ Clinicians must be alert to the presence of reduced dislocations in order not to miss a limb-threatening arterial injury, as shown in Figure 26-1. A multiligament-injured knee with gross instability must be treated the same as a knee that presents dislocated.

FIGURE 26-1 Bilateral knee injuries, both classified as dislocation, presenting in the same patient. A, A lateral radiograph of the right knee demonstrates an anterior dislocation. On examination, the patient was noted to have sustained an anterior cruciate ligament/posterior cruciate ligament/medial collateral ligament (ACL/PCL/MCL) injury (knee dislocation [KD] III-M). B, An anteroposterior (AP) view of the left knee on initial presentation shows that the knee is reduced. On physical examination, the injury pattern involved the ACL/PCL/fibular collateral ligament [FCL] (KD III-L-C). C, An arteriogram of the right knee reveals good flow through the popliteal artery. D, An arteriogram of the reduced left knee demonstrates complete occlusion of the left popliteal artery and hence the knee required revascularization.

(A–D, Reprinted from Wascher, D. C.: High-energy knee dislocations. In Drez, D., Jr.; DeLee, J. C. [eds]: Operative Techniques in Sports Medicine, Vol. 11. Philadelphia: W. B. Saunders, 2003; p. 237.)

Although most knee dislocations involve tears of both cruciate ligaments (posterior cruciate ligament [PCL] and anterior cruciate ligament [ACL]), case reports of cruciate-intact knee dislocations do exist. A PCL-intact knee dislocation was first described in 1975.¹¹ Several authors^2,23 have reported on patients with a radiographically documented knee dislocation that, upon reduction or operative exploration, demonstrated a functioning PCL or ACL. Although some of these patients were noted to have suffered partial PCL tears, the presence of a functioning PCL provides increased stability and changes the management of these PCL-intact dislocations. In most PCL-intact knee dislocations, the tibia is perched anterior on the distal femur (Fig. 26-2). In addition to an ACL tear, there is often complete rupture of either the medial collateral ligament (MCL) or the posterolateral structures.

FIGURE 26-2 This patient presented with a PCL-intact knee dislocation. Note how the tibia is perched anterior on the distal femur. This type of injury would be classified as a KD I according to the anatomic classification system, to be discussed later.

Similarly, ACL-intact knee dislocations have also been reported. These are also extremely rare injuries. The tibia is perched posterior on the femur, and there is a complete tear of the PCL associated with a complete collateral ligament injury. Again, the presence of a functional ACL provides increased stability in the knee and changes the management of these injuries. A classification system for knee dislocations must take into account the presence of both reduced and cruciate-intact knee dislocations.

POSITION CLASSIFICATION SYSTEM

In 1963, Kennedy⁹ was the first to classify knee dislocations. He proposed a classification system based on the tibial position with respect to the femur. For example, an anterior knee dislocation implies that the tibia is positioned anterior to the femoral condyles. He identified five types of knee dislocation: anterior, posterior, lateral, medial, and rotatory. Rotatory dislocations were further subdivided into four groups: anteromedial, anterolateral, posteromedial, and posterolateral, with posterolateral being the most frequently described type of rotatory knee dislocation.^5,6,14,15 This classification system has been widely utilized in the literature.*

The position classification system can help the physician plan a reduction maneuver for the dislocated knee. However, most knee dislocations can be easily reduced with longitudinal traction. The position classification system can also be useful to alert the clinician to the possibility of coexisting injuries. For example, several studies have shown a higher incidence of popliteal artery injury in anterior dislocations.^5,8–10 However, vascular injury can occur with any position of knee dislocation; one must have a high index of suspicion for associated neurovascular injuries in any dislocated knee, regardless of position. Although knowing that an anterior dislocation carries an increased risk of vascular injury is helpful to the clinician, it does not give any information about what structures (ligamentous, neurovascular) have been injured and what treatment is required.

One example of the utility of the position classification system is the rare but important posterolateral dislocation (Fig. 26-3). The mechanism of injury is an abduction force to a flexed knee coupled with internal rotation of the tibia. This force causes the medial femoral condyle to “buttonhole” through the medial capsule and the MCL invaginates into the knee joint. The trapped condyle and MCL prevent closed reduction. The clinical hallmark of the posterolateral dislocation and irreducibility by closed means is a transverse furrow seen on the medial aspect of the knee.^6,15 Peroneal nerve palsy, theoretically resulting from a traction injury to the nerve over the lateral femoral condyle, is frequently associated with this type of dislocation. Skin necrosis secondary to pressure from the medially displaced femur has also been reported. For a posterolateral knee dislocation, the position classification system alerts the clinician to the injured structures and the means of treating the injury.

FIGURE 26-3 AP radiograph (A) and magnetic resonance imaging (MRI) study (B) of a posterolateral dislocation. The injury pattern involved the ACL/PCL/MCL and included damage to the peroneal nerve (KD III-M-N).

The major limitation of the position classification system is that up to half of knee dislocations are unable to be classified. Knees that present with a spontaneous reduction or that have been reduced in the field cannot be described within the structure of the position classification system. The clinician does not have any information about damaged ligaments nor of the possibility of coexisting vascular or nerve injury. Even more dangerous is the fact that by this injury not being included in the position classification system, the inexperienced physician may not recognize the reduced knee dislocation as a significant ligamentous injury and will not undertake the rigorous vascular examination required to detect a popliteal artery injury.

ENERGY OF INJURY CLASSIFICATION SYSTEM

Knee dislocations have also been classified by the degree of energy imparted to the knee joint. Most commonly, knee dislocations result from high-energy mechanisms such as motor vehicle wrecks, industrial accidents, and falls from a height (Fig. 26-4). The injury mechanisms in high-energy knee dislocations often cause significant associated head, chest, abdominal, and extremity injuries as well as soft tissue injury about the affected knee. Many of these injuries can be life-threatening, and their management takes precedence over the knee ligaments. Regardless of the energy of injury, the clinician must remain vigilant about the possibility of a popliteal artery injury in any knee dislocation. The orthopaedic surgeon must be alerted to the possibility of other life-threatening trauma in a patient with a knee dislocation. Conversely, trauma surgeons must include a gross assessment of knee stability in all patients involved in high-energy injury mechanisms. The risk of vascular and nerve injury is increased in high-energy knee dislocations.^9,13,22,30

FIGURE 26-4 This patient sustained a severe dashboard injury in a high-speed car accident. He presented with a severe open knee dislocation, with rupture of the ACL, PCL, FCL, and posterolateral capsule (KD III-L).

(Reprinted from Wascher, D. C.: High-velocity knee dislocation with vascular injury: treatment principles. In Miller, M. D.; Johnson, D. L. [eds]: Clinics in Sports Medicine: The Dislocated Knee, Vol. 19. Philadelphia: W. B. Saunders, 2000; p. 467.)

Knee dislocations can also occur from low-energy trauma such as that seen in sports activities or in minor falls. Sports in which knee dislocations are occasionally encountered include football, rugby, horseback riding, and skiing. Patients with dislocated knees from minor falls are often obese. Patients who sustain low-energy knee dislocations rarely have associated traumatic injuries. Although the risk of vascular injury is lower in low-energy mechanisms, the physician must still be vigilant in assessing the popliteal artery in all patients with a knee dislocation. In the past, knee dislocations have been described as low- or high-velocity injuries.²² It is our opinion that the term energy, rather than the term velocity, more accurately describes the injury mechanism.

The energy of injury system often dictates management of the dislocated knee. The associated injuries in high-energy dislocations frequently preclude early operative management of the knee ligaments. Initial management of the knee dislocation in a multiply injured patient commonly includes the use of external fixation spanning the knee joint to stabilize the knee in a reduced position and to facilitate patient transport and positioning. The use of an external fixator should be limited to 1 to 2 weeks so that an early protected range of motion program may be instituted to prevent arthrofibrosis. Delayed treatment of high-energy dislocations often requires collateral ligament reconstruction in addition to reconstruction of both cruciate ligaments. Conversely, low-energy dislocations can frequently undergo early operative intervention. The collateral ligaments can often be primarily repaired or reconstructed, yielding results superior to those obtained with delayed reconstruction.

One of the problems with the energy of injury classification system is that the categories are somewhat arbitrary. If a patient sustains a knee dislocation when hit by a slow-speed car in a parking lot, is this a high- or low-energy dislocation? What about a patient who falls off of a roof or a high tower? Although it is useful for the clinician to think in terms of the energy imparted to the knee, ultimately it is the pattern of ligament injury, the presence or absence of neurovascular injuries, and the overall clinical assessment that will dictate treatment of the knee dislocation. A morbidly obese patient with a major psychiatric illness who dislocates a knee slipping on a wet bathroom floor (low-energy dislocation) may be treated definitively with external fixation. Conversely, an athletic patient with an isolated knee dislocation from a motorcycle wreck (high-energy dislocation) might be best treated by early ligament repair and reconstruction. Finally, the energy of injury classification system does not alert the orthopaedic surgeon as to which knee structures need to be repaired or reconstructed.

ANATOMIC CLASSIFICATION SYSTEM

A classification system for knee dislocations has been described and is based on what ligamentous structures are torn.²⁸ The ligament anatomy of the knee is complex, with many injury patterns possible when a knee dislocation occurs.^18,20,32 Conceptually, the knee can be separated into four simply identifiable structures: the ACL, the PCL, the medial ligamentous structures, and the posterolateral corner. The medial structures include the superficial and deep MCL as well as the posteromedial capsule. The posterolateral corner is primarily composed of the fibular (lateral) collateral ligament, the popliteofibular ligament, the popliteus tendon, and the lateral joint capsule. This simplistic representation of knee anatomy can be useful to the clinician in planning treatment of a dislocated knee.

In order to classify a knee dislocation by the injured anatomy, it is important to obtain a thorough assessment of the injured knee. The clinician must perform a thorough physical examination of the knee on initial presentation to the emergency room or office. At a minimum, this should include a Lachman test, a posterior drawer test, varus and valgus stress testing at 0° and 30°, and a spin or dial test at 30° and 90°. A thorough neurovascular examination must also be performed. Any vascular abnormality warrants careful evaluation, which may include serial examinations, ankle brachial index evaluation, angiography, and/or popliteal artery exploration and repair/reverse saphenous vein graft. A complete neurologic examination may be difficult in a patient with head trauma, but in most cases, a gross assessment of tibial and peroneal nerve function can and should be obtained. As soon as the patient’s overall condition allows, we recommend obtaining a magnetic resonance imaging (MRI) scan of the knee to better assess the degree and location of ligament injuries. Several authors^3,17,19,31 have commented on the utility of the MRI in the evaluation of ligament injury in knee dislocations. Certain injuries (e.g., the presence of a PCL peel-off injury or a popliteus tendon avulsion) can be accurately detected preoperatively only by MRI. Knowing preoperatively which ligaments are torn and detecting the presence of avulsions can help the surgeon plan the number and types of allografts required and the fixation devices necessary to restore functional stability to the knee.^19,21,31 The final and most critical assessment of knee ligament function occurs with an examination under anesthesia (EUA) at the time of knee reconstruction. The EUA provides a final assessment of the degree of translation, the presence of an endpoint on ligament stress testing, and the overall functionality of the specific ligamentous structure. Whereas MRI can identify which ligaments are injured, the EUA gives an accurate assessment of the functional stability of the injured knee ligaments.^1,7,20,23,29 In the dislocated knee, partial ligament tears can often be treated nonoperatively. To summarize, the assessment of the dislocated knee consists of three parts: an initial examination in the trauma room or office, a preoperative MRI, and an EUA at the time of surgery.

The anatomic classification system of knee dislocation (KD) is a simple system based upon which ligaments are torn. Five major injury patterns can occur (Table 26-1). Injuries are classified by roman numerals; in general, higher-number injuries have sustained greater trauma than lower-number injuries. A KD I is a cruciate-intact knee dislocation and includes both PCL-intact and ACL-intact injuries (see Fig. 26-2). A KD II is a bicruciate injury with functionally intact collateral ligaments (Fig. 26-5). KD III, the most common injury pattern, involves tears of both cruciate ligaments and one of the collateral ligaments. These injuries are further subdivided into medial (KD III-M) (Fig. 26-6) or lateral (KD III-L) (Fig. 26-7) injuries. A KD IV involves complete disruption of all four major knee ligaments (Fig. 26-8). Finally, the KD V group is a knee dislocation with the presence of a major periarticular fracture and is described by other authors as a fracture-dislocation of the knee (Fig. 26-9).^12,21 Fracture-dislocations of the knee involve a ligamentous injury to the knee in association with a major fracture of the tibial plateau or femoral condyles that further destabilizes the knee joint. Although avulsion injuries are frequently seen in purely ligamentous knee dislocations (arcuate avulsion fractures, tibial spine avulsions, or segond fractures), they should be classified as a ligamentous injury in one of the first four categories, despite the small fracture present.^4,25

TABLE 26-1 Anatomic Classification of Knee Dislocations

ACL, anterior cruciate ligament; KD, knee dislocation; L, lateral; M, medial; PCL, posterior cruciate ligament.

FIGURE 26-5 Lateral radiograph demonstrates a bicruciate injury with functionally intact collateral ligaments (KD II).

FIGURE 26-6 AP radiograph (A) and MRI (B) of a bicruciate injury with involvement of the MCL (KD III-M).

FIGURE 26-7 Lateral radiograph (A) and MRI (B) of a bicruciate injury with involvement of the FCL (KD III-L).

FIGURE 26-8 AP radiograph (A) and MRI (B) demonstrate complete disruption of all four major knee ligaments (KD IV).

FIGURE 26-9 AP radiograph demonstrates a knee dislocation with the presence of a major periarticular fracture (KD V). In this case, the patient has a femoral-sided fracture-dislocation, which includes a fracture of the medial epicondyle.

(Courtesy of Robert C. Schenck, Jr.)

The final component of the anatomic classification system is the use of additional labels to identify associated neurovascular injuries. A subtype C indicates a significant arterial injury and alerts the clinician to the need for revascularization of the limb. A subtype N indicates a peripheral nerve injury, either the tibial nerve or, more commonly, the peroneal nerve. This N subtype is used for any neurologic deficit (neurapraxia, neurotmesis, or axonotmesis). For example, a dislocated knee with complete tears of the ACL, PCL, and posterolateral corner with an associated popliteal artery injury and a peroneal nerve palsy would be classified as a KD III-L-CN.

The anatomic classification system has significant advantages over older classification systems. Unlike the position classification system, all knee dislocations can adequately be described by the anatomic classification system. Even knee dislocations that present reduced can be classified after a physical examination and MRI have identified the torn ligaments and associated injuries. In contrast to the energy of injury classification system, no judgment is required for placing a dislocated knee within a specific group. If a ligament is injured and functionally incompetent, it is “torn” for the purposes of the anatomic classification system. Most important, however, is the ability of the anatomic classification system to help guide the clinician in treatment. The surgeon can readily identify the injured structures and can then proceed to plan how best to “fix what is torn.” The anatomic classification system rapidly communicates between clinicians about the extent of the injuries and allows for comparisons of like injuries in the wide spectrum of knee dislocations. At our institution, residents and attending physicians quickly describe the ACL/MCL/PCL knee injury as a KD III-M. The injury diagnosis and ligamentous involvement are easily communicated. Here are a few additional examples of the usefulness of the anatomic classification system: The C subtype quickly alerts the physician to the need for urgent vascular surgery to restore flow to the ischemic limb. A KD II injury has functionally intact collateral ligaments and can be treated just as successfully with either an early or a delayed reconstruction of the cruciate ligaments. A KD IV injury identifies a knee with severe instability that may require external fixation to maintain joint reduction prior to ligament reconstruction.

Finally, the anatomic classification system will help the clinician predict outcomes in the treatment of dislocated knees. On average, knees with a higher roman numeral classification will have inferior outcome scores. Likewise, it has been shown that KD III-L injuries fare worse than KD III-M injuries.^3,28 Knee dislocations with neurovascular injuries (subtypes C or N) are also more likely to have unsatisfactory outcome measurements. KD V injuries are associated with a high complication rate related to the increased incidence of arterial injury, inadequate fracture reduction, or difficulty in achieving a satisfactory ligament reconstruction.

CONCLUSION

Successful treatment of the dislocated knee requires a thorough understanding of the knee injury. Older classification systems are not able to categorize all knee dislocations, do not guide treatment, and are not predictive of outcome. The anatomic classification system that we have used for the past 15 years readily allows the physician to classify all knee dislocations on the basis of the injured structures identified by physical examination and imaging studies. This simple system helps the surgeon plan treatment of the dislocated knee and has been predictive of outcomes in published reports. Hopefully, more widespread use and reporting of this classification system in future series of knee dislocations will yield additional insight into the optimal treatment of these uncommon yet challenging injuries.