A 26 year old man was admitted after a single vehicle road traffic accident. He was an unrestrained driver travelling at approximately 50 mph and was ejected from his vehicle. His initial hospital management followed standard Advanced Trauma and Life Support guidelines. He had no loss of consciousness and complained only of severe left thigh pain. On examination, there was gross angular deformity at the left mid-thigh level, consistent with a femoral shaft fracture. Left pedal pulses were palpable and there was no neurological deficit. When fully exposed, the patient was noted to have bruising on the posterior aspect of his right knee. Despite normal colour, the right leg below the knee was cool to palpation. Distal motor and sensory assessment was normal although passive and active right knee movements were resisted. The right femoral pulse was palpable. The right dorsalis pedis, posterior tibial, and popliteal pulses were absent.

Doppler ultrasound confirmed the presence of weak arterial signals distally. Plain radiographs identified no bony injury, joint dislocation or subluxation in the right lower limb. Percutaneous-transfemoral digital-subtraction angiography identified an intimal flap of the right popliteal artery (fig 1) and emergency exploration was performed. A large haematoma was noted over the posterior aspect of the right knee joint with rupture of the hamstring tendons, the posterior joint capsule, both collateral ligaments, and both cruciate ligaments. The involved arterial segment was isolated and excised, a segment of saphenous vein was interposed, and arterial perfusion was restored. The muscular compartments of the right leg were decompressed with medial and lateral fasciotomies. The left femoral shaft fracture was managed with a statically locked retrograde intramedullary nail.

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Figure 1

 Digital subtraction angiogram of the right femoropopliteal vessels, showing intimal disruption of the right popliteal artery, characterised by segmental loss of the normal smooth luminal contour.

Delayed reconstruction of the right knee ligamentous injuries was subsequently undertaken.

Traumatic knee dislocation is uncommon, typically anterior and in up to 50% of cases the knee joint will be in a reduced position at presentation. Tethered proximally at the adductor hiatus and distally as it passes deep to soleus, the popliteal artery is injured in 30% of knee dislocations.^1–3 The presence of normal arterial pulses or Doppler signals, although reassuring does not exclude an arterial injury. Kaufman et al in their series showed a low but definite (13%) frequency of nonocclusive arterial injury (spasm or intimal flap) after traumatic dislocation of the knee.³ Progression to complete occlusion, although rare, has been reported.⁴ Prompt recognition of the presence of an arterial injury and the restoration of blood flow are paramount for limb salvage.

The type of injury described above is an occult dislocation and although it is not uncommon in the high velocity trauma patient, more overt injuries may distract the attending doctor. It is evident from the case described that a high index of suspicion and careful secondary survey are necessary to prevent the possible complications of this injury.