Classification

Classification of orthopedic injuries can be useful for both effective communication between physicians and development of appropriate treatment strategies. Multiligament-injured knees can be classified based on the following parameters: (1) joint position, (2) injured structures, (3) energy level, and (4) chronicity. Classification of knee dislocations is summarized in Table 56-1.

Historically, knee dislocations like other joint dislocations, have been classified based on the position of the distal bone (tibia) in relation to the proximal bone (femur). Kennedy2 noted five main types of dislocations: anterior (40%), posterior (33%), lateral (18%), medial (4%), and rotational (5%). This classification system is limited because the majority of dislocations present following spontaneous reduction.

Although the least common type, the posterolateral dislocation, is well described and is one of the few useful aspects of the positional classification system. The hallmark of this type of dislocation is its frequent irreducibility secondary to "buttonholing" of the medial femoral condyle through the medial capsule and invagination of the MCL into the medial joint space6 (Fig. 56-1). The posterolateral dislocation often produces a dimple sign on the medial aspect of the knee (Fig. 56-2).

The anatomic system classifies multiligament-injured knees based on functional evaluation of injured structures,7 which is determined by a thorough clinical examination. The anatomic system clearly defines which structures are injured and can be used to guide surgical planning.

Table 56-1

Classification of Knee Dislocations

Anatomic Classification

KDI

Cruciate intact knee dislocation

KDII

Both cruciates torn, collateral intact

KDIII KDIIIM KDIIIL

Both cruciates torn, one collateral torn MCL torn LCL torn

KDIV

All four ligaments torn

KDV

Periarticular fracture-dislocation

Joint Position

Anterior

40%

Posterior

33%

Lateral

18%

Rotational

5%

Medial

4%

Energy Level

High

Fractures and neurovascular injury common

Low

Chronicity

Acute

<3wk

Subacute

3-12wk

Chronic

>3mo

Data obtained from references 2, 4, 6, 9, 11.

Figure 56-1 Coronal magnetic resonance image of a multiligament-injured knee showing displacement of the medial collateral ligament and medial capsule into joint space.

Figure 56-2 Clinical photograph of a right posterolateral knee dislocation showing dimple sign. Orientation is with the patient's head toward the top. This photograph corresponds to the magnetic resonance image shown in Figure 56-1.

In addition to the anatomic classification, knee dislocations can be classified based on the energy level or chronicity of the injury. Higher energy level injuries are often associated with additional musculoskeletal or systemic injuries. Low-energy dislocations have a lower incidence of vascular, neurologic, and meniscal injuries.8 In high-energy dislocations, the rate of popliteal artery injury ranges from 14% to 65%. Fifty percent to 60% of patients will have fractures, and 41% will have multiple fractures. Patients with these high-energy injuries often have other significant injuries to their head or chest, which precludes early aggressive treatment of their knee ligaments.9

The chronicity of the injury is also important with regard to associated injuries and surgical planning. Surgical treatment gives improved results in acute versus chronic cases.10 Some structures may be repaired if treated acutely, whereas late treatment may require a reconstruction. A knee dislocation is acute if seen within 3 weeks, subacute if seen between 3 to 12 weeks, and chronic if seen after 3 months.11

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