X-ray-based Schatzker classification (Schatzker Ⅰ-Ⅵ) is a traditional classification for TPF. Generally, the higher fracture classification corresponds to the greater injury, severer fracture displacement and more serious soft tissue injury. However, there are few studies on the relation between Schatzker classification and PAI, and its mechanism analysis. Wicky  and Macarini  et al found that using X-ray-based Schatzker classification alone may underestimate the severity of fractures, thus affecting the diagnosis and correct treatment of TPF, and also influencing the correct determination of degree of soft tissue injury and PAI. Therefore, CT-based three-column classification was introduced to evaluate the severity of TPF more comprehensively, so as to discover the association between classification for TPF and PAI.
In this study, in terms of the cause of TPF, there were 16 cases of traffic injury, 2 cases of falling injury and 4 cases of crushing injury, and all patients had high-violence injury. According to the analysis of injury mechanism, such patients are injured mostly under flexion or semi-flexion of knee joints, and fractures of the posterior coronal plane, namely posterior column fractures, can be caused when subjected to axial stress. At this moment, Schatzker type Ⅳ fractures can be caused when subjected to varus stress. In other words, the fracture line leans posteriorly, and the medial and posterior columns are further involved, so the popliteal artery is squeezed or crushed by posterior bone blocks, resulting in popliteal artery contusion or rupture. If the knee joint is subjected to axial stress and valgus stress under flexion or semi-flexion, the fractures will involve both lateral and posterior columns, and the popliteal artery may also be injured. In the case of extremely great axial stress, varus stress and valgus stress under flexion of knee joints, the lateral, medial and posterior columns will be affected simultaneously, causing Schatzker type VI fractures. The reason is that the medial articular surface is larger and its strength is greater than that of lateral articular surface. Therefore, ligament, vascular and nerve injuries are likely to occur in the case of medial plateau fractures often caused by great violence. Moore et al [10, 11] also argued that the risk of vascular and nerve injuries is the highest in Schatzker type IV fractures. In this study, Schatzker type VI fractures accounted for 50%(11/22), type V fractures for 4.54%(1/22), and type IV fractures for 45.45%(10/22). Based on the CT-based three-column classification, the posterior column was involved in all 22 cases, 2 columns in 15 cases, 3 columns in 6 cases, and only the posterior column in 1 case. Therefore, it is believed that PAI should be suspected when the posterior column is involved in complex TPF, and the risk of PAI is the highest when the medial and posterior columns are involved in TPF.
The results in this study also indicate that when the posterior column or tibial shaft is involved in complex TPF, the popliteus can pull the entire tibial plateau or fracture blocks of posterior column move backwards due to great violent injury. As a result, the popliteal artery is directly impacted, and stabbed, squeezed or crushed by fracture blocks, thus resulting in popliteal artery rupture or severe contusion and then extensive thrombosis.
In this study, in terms of the type and location of PAI, PAI was in the segment P1 in 3 cases, segment P2 in 6 cases and segment P3 in 12 cases. It can be seen that the possibility of PAI was the highest in segment P3 (57.14%), and the same conclusion was made regardless of Schatzker classification or CT-based three-column classification. The possible reason is that the popliteal artery at the same plane (segment P2 and P3 of popliteal artery) is directly squeezed or crushed by the posterior border of tibial plateau and fracture blocks of posterior column. However, during surgical exploration of PAI, it is recommended that an S-shaped incision be made at the posterior popliteal fossa, and the popliteal artery be explored from the distal end. This is because the residual end may retract to the distal and proximal ends after PAI, so exploring from far to near can effectively avoid omissions.
It was found via intraoperative exploration that the popliteal artery was completely ruptured in 11 cases, partially ruptured in 1 case, and severely contused with thrombosis in 10 cases. At the same time, popliteal vein rupture and popliteal vein contusion with thrombosis were found in 3 cases and 3 cases, respectively, so the popliteal vein was also repaired during operation. It is believed that in TPF accompanied by PAI, both popliteal artery rupture and severe popliteal artery contusion with extensive thrombosis may occur regardless of Schatzker type Ⅳ fractures or Schatzker type VI fractures, consistent with the research results of Drapanas et al . Moreover, the popliteal vein injury is more likely to occur in Schatzker type Ⅳ TPF accompanied by PAI, which was also confirmed by the CT-based three-column classification that the risk of popliteal vein injury was the highest when the medial and posterior columns were involved in fractures. In clinic, the pulse of dorsal pedal artery can still be felt in some TPF patients with PAI in the early stage. Meek  et al found that the weaker pulse than normal can be felt at the distal end in about 22% of patients with injury of major limb arteries, also consistent with the findings in this study that the weaker pulse of dorsal pedal artery than that at the contralateral side could be felt in 5 cases in the early stage. The possible reason is that the popliteal artery has not been completely ruptured but severely contused in some patients with PAI, causing damage to the artery intima and thrombosis, so some blood supply remains in the initial stage. Later, due to the worsening of thrombosis, the artery becomes completely occluded, the blood supply of popliteal artery completely disappears, and lower limb ischemia is gradually enhanced, leading to limb necrosis. Therefore, it is necessary to determine whether PAI is accompanied in TPF in the clinical examination, rather than only based on the presence or absence of pulse of dorsal pedal artery. Besides, bilateral comparison is needed, and CTA or DSA should be performed promptly for definite diagnosis if it is unable to determine accurately. If there are no conditions for CTA or DSA in a hospital, the patients should be transferred to the conditional hospital in time to avoid missed diagnosis.
In this study, the MESS of the 22 patients at admission was 6-10 points, with an average of 7.59 points. In the case of MESS ≥9 points, the possibility of amputation was greatly raised. There were 20 cases with the MESS ≥7 points, and the amputation rate was 15%, lower than the traditional rate in PAI (30-50%) . The amputation rate was 60% when the MESS ≥9 points. Therefore, PAI patients with the MESS at admission ≥9 points should be highly alerted, in which case the amputation rate is significantly increased. In addition, Mullenix et al  argued that incision decompression is needed for patients with the increased pressure of osteofascial compartment. Similarly, the results in this study demonstrate that therapeutic or prophylactic incision decompression is necessary when the pressure of osteofascial compartment rises in the leg. At the same time, the deep fascia and other osteofascial compartments should be completely separated during incision decompression. Otherwise, the decompression effect will be unsatisfactory, and the treatment of PAI will not be benefited.
It is reported by Wani et al  that the amputation rate is 42.1% in patients undergoing operation 6 h after PAI, the success rate of popliteal artery repair is 89% within 8 h, and the amputation rate is up to 86% after 8 h. Wager et al  also thought that the success rate of blood circulation reconstruction will dramatically decline with the prolongation of duration of limb ischemia. In this study, the amputation rate was 20% in patients undergoing operation after 6 h. It is believed that the duration of injury is not the absolute criterion for deciding whether amputation is needed. Limb salvage can still be considered if the patients have no severe infection or extensive leg muscle necrosis. This is because PAI patients are mostly adult men, and they have higher expectations and shoulder greater social pressure . Therefore, retaining the good function of knee is of great importance for the quality of life after operation.
Glass  and Percival  et al showed that whether fractures or vascular injury should be treated first during operation depends on the duration of ischemia. In the case of duration of injury <6 h, the fractured bones should be fixed first, followed by vascular repair. On the contrary, in the case of duration of injury >6 h, the blood vessels should be repaired first, followed by fixation of the fractured bones, so as to reduce the time of tissue ischemia and hypoxia. However, it is believed that except acute open PAI in which surgical repair is needed first, fracture reduction and external fixation should be considered first in other cases, so that the re-rupture of damaged artery can be avoided when the patient's posture is changed after artery repair.