CASE 41 knee injuries

A 43-year-old male presents to the emergency department after falling approximately 15 ft while trimming tree branches. Per the report of emergency medical service personnel at the scene, the patient complained of excruciating right knee pain and was found to have a deformity of the right knee that improved when it “was pulled on” as the patient felt a “clunk.” On examination, the patient complains of persistent pain about the right knee. The skin is intact and there appears to be no gross deformity. There is a palpable pulse over the dorsalis pedis artery; however, you are unable to palpate a pulse over the posterior tibial artery.

What is your next step in evaluating the injured extremity?

1. Perform a ligamentous knee examination

2. Obtain a computerized tomogram with intravenous contrast of the right leg

3. Obtain orthogonal radiographs of the right knee

4. Measure the ankle brachial index (ABI) of the right leg

 

Discussion

The correct answer is (D). The vignette describes a circumstance that should raise one’s suspicion for a knee dislocation, and ABIs must be determined as approximately 40% of knee dislocations are associated with injuries to the popliteal artery. Of these injuries, anterior dislocations are most common and result in anterior translation of the tibia relative to the femur, stretching the popliteal vessels and causing intimal tears. An ABI of <0.9 has been shown to have 100% sensitivity, specificity, and positive predictive value for significant arterial injury. An abnormal

ABI should prompt vascular surgery consultation.

Based upon your examination, you are also concerned about the possibility of a concomitant nerve injury.

Which nerve is most likely to be injured?

1. Peroneal nerve

2. Tibial nerve

3. Saphenous nerve

4. Sural nerve

 

Discussion

The correct answer is (A). The incidence of neurologic injury after knee dislocation has been documented to range from 16% to 40%. Of these injuries, the peroneal nerve is most commonly injured, occurring in approximately one-third of all knee dislocations. It is vulnerable to injury especially in lateral and posterolateral knee dislocations as it winds around the proximal fibula and courses into the anterior compartment of the leg.

Upon testing of the ligamentous stability of the involved knee, several ligaments are suspected to be injured. Figure 6–44 demonstrates an MRI typical of this injury.

 

 

 

Figure 6–44

 

Which of the following structure(s) is most likely to be injured if increased

posterolateral rotation is noted on physical examination?

1. Anterior cruciate ligament (ACL)

2. Posterior cruciate ligament (PCL)

3. Popliteus and fibular collateral ligament

4. Semimembranosus and medial collateral ligament (MCL)

 

Discussion

The correct answer is (C). The finding of increased posterolateral rotation is suggestive of an injury to the posterolateral corner (PLC). The main static stabilizers of the PLC include the popliteus, fibular collateral ligament, and the popliteofibular ligament. Together, these structures restrain varus, external rotation, and coupled posterior translation and external rotation of the tibia on the femur.

 

Objectives: Did you learn...?

 

The incidence of vascular injury and the evaluation in the setting of knee dislocation?

 

The incidence of nerve injury in the setting of knee dislocation?

 

The ligamentous anatomy about the knee and the description of multiligamentous knee injuries in the setting of dislocation?