Copyright 2009
Definition
There are two types of dislocations of the knee: patellofemoral dislocations and tibia-femur dislocations. A “knee dislocation” usually refers to the latter. Knee dislocations are due to either high energy or low energy traumatic events. High energy events are things such as motor vehicle collisions while lower energy events are things such as sporting injuries. With the higher energy injuries, there is a greater risk of damage to nerves and vessels and there will be more soft tissue damage as well.
Knee dislocations are injuries which need to be taken extremely seriously since accompanying damage to the popliteal artery can occur in up to 20% of the cases. This will lead to possible amputation if missed due to an avascular limb. They are characterized by 5 types of dislocations: anterior (most common), posterior (2nd most common), lateral, medial, rotary. These types are categorized based on the tibias position in relation to the femur.
Knee dislocations require significant energy to cause dislocation of the tibia on the femur. Depending on the type of load, and the direction of the load, the tibia will dislocate in a particular direction. For instance, a posterior load directed on the tibia as might happen in a dashboard injury would cause a posterior dislocation. Tearing of at least one, and usually both of the cruciate ligaments occurs, and collateral ligaments are almost always involved as well.
As stated above, vascular and nerve injury are very common, and it is critical to assess vascularity in a timely manner or there could be dire consequences for the patients. The poplliteal artery can be torn, or there can also be an intimal tear which may not be easy to detect with routine pulse checks. Peroneal nerve injury can occur in up to 40% of dislocations due to the nerves course about the posterolateral knee.
Diagnosis is fairly obvious on clinical exam, but plain x-rays should always be obtained as well. Exam should always include an assesment of a pateints distal pulses and ankle brachial indexes (ABI) should always be obtained. A low threshold for obtaining angiograms should be practiced, and check muscles innervated by the peroneal nerve should always be checked. Since many of these patients are trauma patients, the ABCs should be the first priority in caring for these patients. A careful exam of the ligaments (ACL, PCL, PLC, MCL, LCL) must be performed carefully but thoroughly.
Imaging includes the use of plain x-rays in the acute setting. MRI is not needed acutely, but after the patient has been stabilized, MRIs should be obtained to assess ligamentous and meniscal injury.
Treatment includes reduction of the knee right away by manual traction. This can be done with local anesthesia if the patient is awake, or without if the trauma patient is unconscious. The knee should then be stabilized and vascular and nerve status should then be addressed. MRI should be obtained and the ligaments will then have to be reconstructed on a acute or delayed manner depending on the injury. Patients will almost always need surgery to reconstruct the torn ligaments of the knee.
References
http://www.wheelessonline.com/ortho/traumatic_dislocations_of_the_knee