Knee trauma

Mechanism-based PatternApproach to Classificationof Complex Injuries of the Knee Depicted at MR ImagingDr Wafik Ebrahim AliLecturer of radiodiagnosis faculty of medicie Alazhar university

Complex injuries of the knee are common, resulting from accidents or sports mishaps at all levels of competition. These injuries often occur as a result of multiple forces applied to the knee.

It is widely known that certain forces predictably produce specific individual or combined patterns of injury.

A comprehensive classification system based on mechanisms of injury would be useful because: (a) an understanding of the causativemechanism in a given case may improve detection of the complete constellation of injuries.(b) appreciation of the mechanism of injurymay help predict both immediate and delayed instability and need for surgery.

Acquisition of a precise history of the mechanism may be difficult, however, as is performance of an accurate physical examination in the setting of acute injury.Magnetic resonance (MR) imaging is widely used to assess knee injuries more completely. Radiologists are accurate at detecting individual injuries and combinations of injuries. However, little attention has been paid to the use of MR imaging to classify knee injuries into mechanismbased categories.

Functional AnatomyThe bones of the knee contribute little to the stability of the joint. Both the static and dynamic stability of the knee are dependent on its supporting soft tissues. Menisci, ligaments, tendons, muscles, and fascia all make contributions to knee stability.

Dynamically, the supporting structures can be divided by location: anterior, medial, lateral, posterior, and central.

Figure 1. Diagram of functional anatomy of the knee joint

Two areas in the knee are critical for stability: the posteromedial and posterolateral corners.Both corners are major resistors of rotational and translational stresses, particularly in extension.Traumatic loss of one corner may allow unstable rotation of the knee joint, with a pivoting out around the other corner. Injury at the posterolateral corner, in particular, may lead to severe disability.Therefore, recognition of injuries to these structures is critical in any MR imaging evaluation of the injured knee.

Impaction versus Avulsion Injury PatternsIn general, forces acting on the knee produce an impaction injury at the entry site of the force and a distraction injury, or avulsion, at the opposite, or exit, site of the force.Bone injuries caused by impaction tend to be broad, compared with smaller, more focal areas of bone marrow edema associated with ligament avulsions on the distraction side.

Illustration of impaction bone bruise pattern and avulsion bone bruise pattern. valgus force

Impactions may be further divided into: Contiguous,kissing, injuries or noncontiguous injuries, which are produced by the abrupt translation of two bones that occurs after ligamentous rupture.

The direction and type of force may be inferred from the patterns of bone marrow edema and soft tissue injury. Direct blows result in bone contusions and soft-tissue injury at the impaction site only.

Mechanism-based Classification System of Knee InjuriesOn the basis of information from the injury patterns to primary and secondary stabilizing structures, plus bone injury patterns, complex knee injuries are classified into 10 categories, according to the knee position (flexion, extension), direction of force, and presence or absence of rotation

Diagram illustrates the direction of injury-producing forces acting on the knee.

Forces Responsible for Knee Injuries and Supporting Structures Responsible for Primary and Secondary Resistance to Those Forces

ForcePrimary resistanceSecondary resistanceAnterior translation (displaces tibia anteriorly)ACLMCL, LCLPosterior translation (displaces tibia posteriorly)PCLNoneVarus (medial to lateral)LCLACL, PCLValgus (lateral to medial)MCLACL, PCLInternal rotation (femur fixed)LCLACL, capsuleExternal rotation (femur fixed)MCLPCL, capsuleHyperextensionPCLACL, posterior capsule

Hyperextension injuryHyperextension injuries, by virtue of the greater forces exerted on the extended or locked knee, produce more pronounced bone injury patterns, often with frank fractures. Severe distraction injuries on the posterior, exit side of the joint are common with this pattern. These injuries are particularly serious in that they involve the critical posteromedial and/or posterolateral corners of the knee.

Hyperextension

Hyperextension

Pure hyperextension

Hyperextension with varus

Hyperextension with varus

Hyperextension with valgus

Hyperextension with valgus

Flexion injury

Flexion injuries, unless combined with significant axial load, tend to show few contiguous impaction bone injuries but have a greater tendency to produce injury due to internal or external rotation. Noncontiguous impaction bone bruises are usually found, as well as smaller avulsion bone bruises. We have observed that flexion injuries with rotation show a greater association with meniscal tears than do hyperextension injuries.

Flexion with valgus and internal rotation

Flexion with valgus and internal rotation

Flexion with varus and internal rotation injury

Hyperflexion injury with posterior tibial translation

Hyperflexion injury with posterior tibial translation

Patellar dislocationDislocation of the patella is typically produced by combined flexion, valgus, and internal rotation of the femur on a fixed tibia. In theory, this is very similar to flexion, valgus, and external rotation (tibia on fixed femur),

Flexion with internal feroral rotation on fixed tibia

Transient patellar dislocation

Lateral patellar dislocation

Valgus and varus forcePure valgus and the rare pure varus categories are characterized by a simple coup-contrecoup pattern of impaction bone injuries and oppositesided distraction ligament injuries.

Pure valgus injury

Pure varus injury

Direct traumaDirect trauma is characterized by broad bone contusions located beneath the site of impaction. Typically, there are no injuries on the opposite side of the knee. Direct anterior trauma, which causes patellar and trochlear groove contusions, and lateral or medial blows are most common.

Direct trauma

Thank You

Wafik Ebrahim