Comparison of radiographs and CT features between posterior Pilon fracture and posterior malleolus fracture: a retrospective cohort study

Abstract

Objectives:

This study explored the morphological differences between posterior Pilon fracture and posterior malleolus fracture from radiographs and CT to provide detail for diagnosis and treatment of them.

Methods:

Radiographs and CT imaging data of 174 patients with distal posterior tibial fractures who were treated from January 2013 to January 2019 were retrospectively analyzed. Based on the operation and imaging examination, the fractures were classified into posterior Pilon fractures and posterior malleolus fractures. Radiographic parameters including the width, height, depth, α angle, β angle, γ angle, fragment area ratio 1 (FAR1), δ angle and fragment area ratio 2 (FAR2) of ankle mortise were measured.

Results:

There were 96 posterior Pilon fractures (Type I: 30, Type II: 22 and Type III: 44) and 78 posterior malleolus fractures (Type I: 40 and Type II: 38). The ankle depth, α angle, γ angle, FAR1 and FAR2 of posterior Pilon fractures were larger than these of posterior malleolus fractures (p < 0.05). In addition, FAR1 and FAR2 of Type II and Type III posterior Pilon fractures were significantly larger than these of Type I (p < 0.05). FAR1 and FAR2 of Type I posterior malleolus fractures were significantly smaller than these of Type II (p < 0.05).

Conclusion:

Radiographs combined with CT analysis is an effective method to accurately distinguish morphological features between posterior Pilon fracture and posterior malleolus fracture.

Advances in knowledge:

Radiographs combined with CT distinguished the fracture of posterior malleolus and posterior Pilon rapidly and accurately, instead of operation.

Background

Posterior Pilon fracture was originally put forward by Hansen, ¹ which refers to the posterior intra-articular fracture of the distal tibia caused by vertical compression of high-energy force combined with or without torsional force. This fracture involves impaction of articular surface and proximal displacement of talus and posterior malleolus fracture fragments. ² The characteristics of posterior Pilon fracture determine the difference of surgical approach, fixation method and clinical prognosis compared with traditional ankle fracture and Pilon fracture. Due to the lack of awareness of posterior Pilon fracture, relatively low incidence, little of case reports, lack of relevant statistical data and understanding of posterior Pilon fracture by clinicians, it’s often confused with classical posterior malleolus fracture. Nevertheless, the treatment of the two fractures was markedly different. The operative indication of posterior malleolus fracture was 25% articular surface damage, ³ while Amorosa et al ⁴ suggested that the posterior Pilon fracture more than 10% of the damaged joint surface should be treated surgically. In recent years, some researches ^3–8 reported that all posterior Pilon fractures were treated with the open reduction and internal fixation (ORIF). And accurate diagnosis, patient position, surgical approach, reduction strategy and fixation methods also affect the surgical effect. The misdiagnosis and inappropriate treatment can easily lead to ankle dysfunction and traumatic arthritis. Therefore, the purpose of this study was to retrospectively compare the morphological differences of posterior Pilon fracture and posterior malleolus fracture and to measure some imaging parameters of radiographs and CT images, so as to help clinicians correctly and efficiently distinguish the characteristics of the two kinds of fractures and provide guidance for the necessity of surgical treatment.

Methods

Classification criteria for posterior Pilon fracture and posterior malleolus fracture

Posterior Pilon fracture was judged by surgery and met the following criteria ⁶ : (1) the fracture was an intra-articular fracture of the distal posterior tibia caused by vertical compression of the ankle joint combined with rotational violence. (2) There were some signs of impaction and compression on the articular surface of the fracture fragment and the talus. (3) The fracture fragment was large, involving in the posterior ankle joint surface and displaced proximally. (4) The fracture extended to 1/3 or 1/2 of the posterior or medial malleolus and subluxation of the talus is common. The fracture classification was determined by two surgeons together, and any differences between the two surgeons were sent to a third independent chief physician for discussion. Georg Klammer et al ⁶ classified posterior Pilon fracture into three types ( Figure 1) according to Weber/AO classification, the relationship between fracture line and medial malleolus and different surgical strategies. Type I: the fracture line extended to the medial malleolus, and the posteromedial fragment was single. The fragment could be completely exposed through the posterolateral incision. Type II: the fracture fragment was divided into two fragments, in which the posteromedial fragment was comminuted. An additional medial incision or a limited posteromedial incision was required to open and fix the posteromedial fragment or separate the medial malleolus fragment. Type III: the fracture line extended to the anterior colliculus of the medial malleolus, and there was an independent anteromedial fracture fragment. In most cases, an additional medial incision was made to fix the anteromedial fracture fragment.

Figure 1.

A represents Type I of posterior Pilon fracture: there is only one complete fracture fragment in the posterior malleolus. B represents Type II of posterior Pilon fracture: The fracture line of the posterior malleolus extends to the posterior colliculus of the medial malleolus. C represents Type III of posterior Pilon fracture: The fracture line of the posterior malleolus extends to the anterior colliculus of the medial malleolus. D represents Type I of posterior malleolus fracture: A single, smaller posterolateral fractur of the posterior malleolus. E represents Type II of posterior malleolus fracture: Small avulsion fracture of posterior malleolus.

The fracture of the posterior malleolus was diagnosed by operation and met the following criteria ⁹ : (1) no impact and compression marks were found on the articular surface of the fracture fragment and talus. (2) Fracture fragments had no effect on the weight-bearing articular surface of the posterior ankle and were not directly related to the medial malleolus fracture line. (3) Fracture fragments were generally small, without displacement or small displacement (easy to move backwards and outwards). Some researches ^10–15 had divided posterior malleolus fractures into three types based on their appearance on axial CT by Haraguchi classification. However, Hansen ¹ and Klammer et al ⁶ had named medial extension of posterior malleolar fracture (Haraguchi Type II) as posterior Pilon fracture. On the basis of this situation, we redivided posterior malleolus fracture into two types ( Figure 1): anterolateral oblique line (Type I) and small avulsion (Type II).

General information

This study was a case series study in a large-scale general hospital, integrated medical service, education and research, with retrospective in design which been reported in line with the PROCESS criteria. ¹⁶ 174 patients who were diagnosed as posterior Pilon fracture or posterior malleolus fracture by surgery between January 2013 and January 2019 were retrospectively analyzed (Table 1). Inclusion criteria: (1) all patients were diagnosed as posterior Pilon fracture or posterior malleolus fracture by surgery; (2) ≥18 years old; (3) radiographs and CT examinations were performed before the operation; (4) complete relevant information included signing the patient's informed choice and authorization letter and filling in the registration form of admission information for traumatic patients. Exclusion criteria: (1) congenital deformity of the ankle joint; (2) history of ankle surgery; (3) fracture line did not involve the posterior tibial joint; (4) refusing surgery or surgical contraindications. Among the patients who met the inclusion criteria, there were 96 cases of posterior Pilon fractures (36 cases on the left and 60 cases on the right), including 48 males and 48 females, and their ages ranged from 23 to 79 years (51.26 ± 13.64). In addition, there also included 78 cases of posterior malleolus fractures (36 cases on the left and 42 cases on the right), which were comprised 40 males and 38 females, and their ages ranged from 21 to 89 years (49.05 ± 15.97). The patients of both two fracture types had no significant difference in terms of gender, age and affected side (p > 0.05).

Table 1.

Comparison of general imformation between posterior pilon fractures and posterior malleolus fractures

	Posterior pilon fractures (96)	Posterior malleolus fractures (78)	F	p
Age	51.26 ± 13.64	49.05 ± 15.97	t = −0.968	0.335
Gender	Male: 48 Female: 48	Male: 40 Female: 38	X2 = 0.028	0.866
Affected side	Left: 36 Right: 60	Left: 36 Right: 42	X2 = 1.329	0.249
Ankle dislocation	50 cases	5 cases	X2 = 41.527	0.000a
Fracture classification	Type I: 30 Type II: 22 Type III: 44	Type I: 40 Type II: 38	-	-

^aMeans that there is statistical difference between the two groups.

Methods

Our emergency center would do the initial management for patients before imaging measurement. When all patients came to our emergency center for treatment of ankle fracture, we would firstly ask the patient to limit the movement of the affected limb and avoid the landing of the affected limb. In addition, patients were required to sit in wheelchairs or lie on carts to avoid secondary injuries of the injured area. When the patient was examined on the anterior and lateral radiographs of the tibia, our imaging doctors would keep the tibial axis of the affected limb perpendicular to the direction in which the ray passed through the tibia, which helped to obtain the most original imaging data of ankle fracture. After preliminary assessment and necessary imaging examinations, we carried out next step treatment for patients.

All of the radiographs and CT imaging measurements were done with two clinicians. Both of them have studied medical imaging course, with more than 5 years clinical experience in ankle fracture treatment, and the title of deputy director of orthopedics or above. And the measurement method adopted in this study was based on reports by Haraguchi et al, ¹⁰ Zhou et al ¹⁷ and Xie et al. ¹⁸ Furthermore, the CT transverse slice with the largest measured value was selected as the research slice. We choose the research slice on CT, which is the furthest distance between the medial malleolus and the lateral malleolus and the largest measured area of the fracture fragment. Any disagreement between the two authors was sent and discussed with a third independent author for consensus agreement. Posterior Pilon fracture and posterior malleolus fracture were classified according to the morphology of the fracture line in the transverse section. The radiographic parameters were measured ^17,18 :

• The height of the ankle mortise: a vertical line was drawn from the tip of the lateral malleolus on the anteroposterior radiographs to the medial axis line of the tibia. The height of the ankle mortise was the distance between the foot of the vertical line and the intersection where the tibial axis and tibial articular surface met (Figure 2a).

• The width of the ankle mortise: the width of the ankle mortise was the length of the line, which crossed the two talar domes and met the medial and lateral malleolus on the anteroposterior radiographs (Figure 2a).

• α angle: the angle between the medial axis line of the tibia and the perpendicular line to the line that crossed the two domes of the ankle mortise on the anteroposterior radiographs (Figure 2a).

• The depth of the ankle mortise: the distance between the anterior ankle edge and the posterior ankle edge on the ankle joint lateral radiographs (Figure 2b).

• β angle: the angle between the medial axis line of the tibia and the line that crossed the anterior and posterior ankle edges on the ankle joint lateral radiographs (Figure 2b).

• γ angle：on the transverse section of CT, the angle between the major fracture line and the line connecting the bimalleolar axis—the line connecting the bimalleolar was the axis connecting the maximum tibia and fibula incisures (Figure 3a).

• δ angle：the angle between the tibial axis and the line that crosses the anterior and the posterior ankle edges on the sagittal CT view (Figure 3b).

• The ratio of the area of the fracture fragment to the total area of the distal tibia on the transverse CT view (fragment area ratio 1, FAR1) = s1/(s1 +S1) (Figure 3a).

  • On the sagittal plane of CT, the ratio of the fracture fragment area to the total tibial area below the horizontal line of the fracture line apex (FAR2) = s2/(s2 +S2) (Figure 3b).

Figure 2.

(a) The height of the ankle mortise: on the anteroposterior radiographs, line CG is the tibial axis, point E is the intersection point of the tibial axis and the tibial joint surface, and point G is the intersection point of the vertical line from lateral malleolus to the tibial axis. Then the length of line EG is the height of the ankle mortise. (b) The width of the ankle mortise: on the anteroposterior radiographs, making the line AB spanning the two domes of talus from lateral malleolus to medial malleolus. Then line AB is the width of the ankle mortise. (c) α angle: on the anteroposterior radiographs, line AB is the width of the ankle mortise，line CG is the tibial axis, and the point F is the intersection point of line CG and AB. Scribe the vertical line DF of line AB from point F, then α is the coronal degree of ankle mortise. (d) The depth of the ankle mortise: on the lateral radiographs, scribe line CD from the edge of anterior malleolus to the edge of posterior malleolus. Then line CD is the depth of the ankle mortise. (e) β angle: on the lateral radiographs, line CD is the depth of the ankle mortise, line AE is the tibial axis, and the point E is the intersection point of line AE and CD. Scribe the vertical line BE of line CD from point E. Then β is the sagittal degree of ankle mortise.

Figure 3.

(a) γ angle: on the transverse CT view, the angle between the fracture line and the line connecting the two ankles is taken, and the axis connecting the maximum notch of the tibiofibula is taken. (b) FAR1 = s1/(s1 +S1): on the transverse CT view, the ratio of the area of the transverse fracture fragment (s1) to the total area of the distal tibia (s1 +S1). (c) δ angle: on the sagittal CT view, the angle between the sagittal fracture line and the horizontal line. (d) FAR2 = s2 / (s2 +S2): On the sagittal CT view, the ratio of the area of the sagittal fracture fragment (s2) to the total area of the tibia below the level of the fracture line apex (s2 +S2). FAR, fragment area ratio.

All the above measurements were completed by Philips DICOM Viewer R3.0-SP13 software.

Statistical analysis

SPSS 22.0 statistical software (SPSS Inc., Chicago, IL) was used to analyze the all data. T-test of two independent samples and one-way anova data were used, and p < 0.05 indicated that the difference was statistically significant.

Results

Patient characteristics of posterior Pilon fractures

A total of 96 adult patients of posterior Pilon fractures were assessed in this study (Table 1, Figure 1). Among them, 82 cases were combined with oblique fractures of the lateral malleolus from the posterior superior to the anterior inferior, 13 cases were complicated with fractures of the entire medial malleolus and 1 case was a simple posterior Pilon fracture. Furthermore, there are 50 cases of ankle dislocation in posterior Pilon fracture, including 48 cases of subluxation and 2 cases of total dislocation. Because total dislocation, obvious displacement or complete dissociation of fracture fragments would affect the measurement results, the measurement results were not included in the statistical analysis (Table 1).

Patient characteristics of posterior malleolus fractures

A total of 78 adult patients of posterior malleolus fractures were assessed in this study (Table 1, Figure 1). Furthermore, lateral subluxation of the ankle joint occurred in five cases (6.41%) of posterior malleolus fractures, but no total dislocation was found in all included cases of posterior malleolus fractures (Table 1).

Comparison of radiographic features between posterior Pilon fractures and posterior malleolus fractures

The results of image measurement showed that there was no significant difference (p > 0.05, Table 2, Figure 4a) in the height and width of the ankle points between posterior Pilon fractures and posterior malleolus fractures. This indicated that the two types of fractures had no effect on the anterior anatomical structure of the ankle joint. In addition, the β angle of posterior Pilon fractures and posterior malleolus fractures was variable, but no statistically significant difference was found between the two (p > 0.05, Table 2, Figure 4b). When the ankle depth and α angle were compared between posterior Pilon fractures and posterior malleolus fractures, the differences in the depth and α angle (coronal angle of ankle points) of the ankle point were statistically significant (p < 0.05, Table 2, Figures 4 and 5), indicating that the changes of ankle point depth and α angle (coronal angle of ankle points) in the two ankle fracture types were correlated with the ankle function of the affected side.

Table 2.

Radiographs comparison of height, width, depth, α angle and β angle between posterior pilon fractures and posterior malleolus fractures (‾X ± S)

Type of distal posterior tibial fracture	Posterior pilon fractures (96)	Posterior malleolus fractures(78)	t	p
Height (mm)	23.33 ± 6.87	24.57 ± 4.36	1.447	0.150
Width (mm)	38.11 ± 4.72	37.59 ± 4.82	−0.721	0.472
Depth (mm)	40.87 ± 6.25	35.44 ± 4.96	−6.223	0.000a
α angle (°)	8.36 ± 6.72	4.14 ± 3.38	−5.375	0.000a
β angle (°)	5.97 ± 4.50	5.59 ± 2.77	−0.685	0.494

^aMeans that there is statistical difference between the two groups.

Figure 4.

(a) Radiographic comparison of height, width and depth between posterior Pilon fractures and posterior malleolus fractures. (b) Radiographic comparison of α angle and β angle between posterior Pilon fractures and posterior malleolus fractures. *** means that there is statistical difference (p < 0.001) between the two groups.

Figure 5.

A represents the depth of the posterior Pilon fracture and B represents α of the posterior Pilon fracture. C, D represent the depth and α of posterior malleolus fracture, respectively.

Comparison of CT features between posterior Pilon fractures and posterior malleolus fractures

The γ angle, FAR1 and FAR2 of posterior Pilon fractures were significantly larger than these of posterior malleolus fractures, and the differences were statistically significant (p＜0.05, Table 3, Figures 6 and 7). Nevertheless, the results showed that the difference of the δ angle between posterior Pilon fractures and posterior malleolus fractures was not statistically significant (p＞0.05, Table 3, Figure 6a).

Table 3.

CT morphology comparison of γ angle, δ angle, FAR1 and FAR2 between posterior pilon fractures and posterior malleolus fractures (‾X ± S)

Type of distal posterior tibial fracture	Posterior pilon fractures(96)	Posterior malleolus fractures(78)	t	p
γ angle (°)	46.49 ± 11.29	39.76 ± 12.89	−3.671	0.000a
δ angle (°)	75.03 ± 8.73	72.03 ± 12.51	−1.795	0.075
FAR1 (%)	24.34 ± 12.19	12.87 ± 8.00	−7.451	0.000a
FAR2 (%)	32.04 ± 8.06	20.76 ± 6.98	−9.746	0.000a

FAR, fragment area ratio.

^aMeans that there is statistical difference between the two groups.

Figure 6.

(a) CT morphology comparison of γ angle and δ angle between posterior Pilon fractures and posterior malleolus fractures. (b) CT morphology comparison of FAR1 and FAR2 between posterior Pilon fractures and posterior malleolus fractures. *** means that there is statistical difference (p < 0.001) between the two groups. FAR, fragment area ratio.

Figure 7.

A, C represent γ and FAR1 of posterior Pilon fracture, respectively. B, D represent FAR2 of posterior malleolus fracture, respectively. FAR1 = s1 / (s1 +S1). FAR2 = s2 / (s2 +S2). FAR, fragment area ratio.

CT morphology comparison of different types of posterior Pilon fractures’ FAR1 and FAR2

There were significant differences (p＜0.05, Table 4) in FAR1 among Type I, Type II and Type III of posterior Pilon fractures. In addition, FAR1 of Type II and Type III were significantly larger than that of Type I (p＜0.05, Table 5), but no significant difference was found between Type II and Type III (p = 0.677＞0.05). Furthermore, significant difference could be observed (p＜0.05, Table 4) in FAR2 among Type I, Type II and Type III of posterior Pilon fractures. Furthermore, FAR2 of Type II and Type III was significantly larger than that of Type I, the difference being statistically significant (p＜0.05, Table 5), but no significant difference was found between Type II and Type III (p = 0.700＞0.05).

Table 4.

CT morphology comparison of different types of posterior plion fractures’ FAR1 and FAR2 (‾X ± S)

Type of posterior pilon fracture	Type I (30)	Type II (22)	Type III (44)	F	p
FAR1 (%)	12.65 ± 5.94	28.98 ± 6.16	30.00 ± 12.19	34.172	0.000a
FAR2 (%)	26.28 ± 7.00	34.17 ± 3.72	34.89 ± 8.37	14.339	0.000a

FAR, fragment area ratio.

^aMeans that there is statistical difference among the three groups.

Table 5.

CT morphology multiple comparison of different types of posterior plion fractures’ FAR1 and FAR2

Dependent variable	FAR1		p	FAR2		p
Type	I (30)	II (22)	0.000a	I (30)	II (22)	0.000a
		III (44)	0.000a		III (44)	0.000a
Type	II (22)	I (30)	0.000a	II (22)	I (30)	0.000a
		III (44)	0.677		III (44)	0.700
Type	III (44)	I (30)	0.000a	III (44)	I (30)	0.000a
		II (22)	0.677		II (22)	0.700

FAR, fragment area ratio.

^aMeans that there is statistical difference between the two groups.

CT morphology comparison of different types of posterior malleolus fractures’ FAR1 and FAR2

Significant differences were noted (p＜0.05, Table 6) regarding FAR1 and FAR2 of Type I and Type II of posterior malleolus fractures. In addition, FAR1 and FAR2 of Type II were both significantly larger than these of Type I (p＜0.05, Table 6).

Table 6.

CT morphology comparison of different types of posterior malleolus fractures’ FAR1 and FAR2 (X ± S)

Type of posterior malleolus fracture	Type I (40)	Type II(38)	t	p
FAR1 (%)	7.80 ± 2.70	18.22 ± 8.27	−7.403	0.000a
FAR2 (%)	18.64 ± 5.30	22.98 ± 7.86	−2.846	0.006a

FAR, fragment area ratio.

^aMeans that there is statistical difference between the two groups.

Discussion

Most of the literature evaluated the surgical approach and treatment of posterior Pilon fracture, but there was no definite definition of the specific size of the fracture fragments and the angle between the fracture lines of posterior Pilon fracture in the current imaging researches. ^9,19,20 Clinically, posterior Pilon fracture is often regarded as posterior malleolus fracture, which affects the determination of treatment plan. The operative indication of posterior malleolus fracture was 25% articular surface damage, ³ and many literatures ^3–8 reported that all posterior Pilon fractures were treated with ORIF. However, the indication for surgery and fixations in posterior malleolus fracture are debatable and should not be based on fragment size alone. Surgical indications also include open fracture, joint dislocation, articular surface collapse, and obvious displacement of fracture fragments. ^21,22 Thus, it’s necessary to strengthen the criteria or morphological study of posterior Pilon fracture and posterior malleolus fracture.

Radiography, a basically imageology examination, is cheap, rapid and low-radiation, which is the first choice for initial diagnosis after the patient is injured. If the diagnosis of radiography is soft tissue contusion and no ankle fracture, we don’t need CT examination, which can save the cost of patients. If the ankle fracture is found by radiography, we can make a preliminary judgment on the type of ankle fracture through the relevant measurement data of the radiographs, which is conducive to the preliminary formulation of the treatment plan in the early stage. In the morphological features’ comparison of radiographic images between posterior Pilon fracture and posterior malleolus fracture, no significant difference was found in the height and width of the ankle mortise. The reason may be that the involved anatomical structures of two kinds of ankle fractures mainly appear in the posterior part of the ankle joint, which less likely to involve the anterior anatomical structure of the ankle joint. Furthermore, the analysis showed p > 0.05 for the β angle of posterior Pilon fractures and posterior malleolus fractures. This is mainly because the posterior fracture fragment of ankle joint is variable, and there is no corresponding tissue nearby to fix it, which is easy to move posterior and superior. The degree of displacement of the fracture fragment is related to the magnitude of the external force.

Moreover, the radiographic results revealed the significant differences between the posterior Pilon fracture and the posterior malleolus fracture in terms of the depth and α angle of the ankle mortise. Because the fracture fragments in the back of ankle joint are changeable, different vertical forces will cause the displacement of the free fracture fragments to varying degrees, which lead to the variation of the depth of the posterior ankle. The main factors affecting the size of α angle are the magnitude of external force in the lateral and medial directions. The rotational violence mainly leads to posterior malleolus fracture and the posterior Pilon fracture is mainly caused by vertical violence, which may be accompanied by rotational violence. Therefore, the depth and α angle of the ankle mortise can be used to distinguish the posterior Pilon fracture from the posterior ankle fracture.

Comparing the CT image morphological features between posterior Pilon fractures and posterior malleolus fractures, we found that the γ angle, FAR1 and FAR2 of posterior Pilon fractures degrees were significantly larger than these of posterior malleolus fractures (p＜0.05). Rotational or vertical violence may lead to different γ angle in the posterior joint of the distal tibia. Posterior Pilon fracture is mainly caused by vertical violence or double violence of verticality and rotation. And it often involves the articular surface, resulting in larger fracture fragments and a larger range of fracture effects. However, posterior malleolus fracture is only caused by rotational violence, which results in the difference of measurement results. It is further proved that the violence direction of the posterior Pilon fracture is different from that of the posterior malleolus fracture, which helps us to distinguish the two fractures.

In addition, we found that 45.8% of posterior Pilon fractures FAR1 ≥25%, 79.2% of posterior Pilon fractures FAR2 ≥25%, 94.9% of posterior malleolus fractures FAR1 <25%, and 79.5% of posterior malleolus fractures FAR2 were <25%. Some scholars believed that the operative indication of posterior malleolus fracture was 25% articular surface damage. ^23–25 And many literatures ^3–8 reported that all posterior Pilon fractures were treated with the ORIF. Therefore, FAR1 and FAR2 of the ankle mortise can be used to distinguish between two types of fractures and determine whether ORIF is needed.

Forberger et al ²⁶ reported that 73% of posterior talus subluxation occurred in posterior Pilon fractures. In a retrospective study by De Vries et al, ²⁷ it was found that patients with posterior subluxation of talus had relatively large posterior malleolus fragments and had significantly worse functional prognosis. According to CT transverse images of this study, 50 cases (52.08%）of posterior Pilon fractures developed posterior subluxation of talus (2 cases of total dislocation). Five cases (6.41%）of lateral subluxation of ankle occurred in posterior malleolus fractures, but no complete dislocation was found in all posterior malleolus fractures. Therefore, this can be used as one of the differential diagnosis criteria for differentiating posterior Pilon fracture from posterior malleolus fracture.

The δ angle of posterior Pilon fractures and posterior malleolus fractures were variable and over 70 degrees. Nevertheless, there was no significant statistically difference of the δ angle between the two types fracture. This may be directly related to the direction of violence on the posterior intra-articular fracture of the distal tibia. The sagittal fracture line of the posterior intra-articular fracture of the distal tibia, whether caused by rotational or vertical violence, is basically vertical to the ground and parallel to the longitudinal axis of the tibia. The results of this study are basically consistent with those of Yao et al. ²⁸

The results of this study showed that FAR1 and FAR2 of Types II and III posterior Pilon fracture were significantly larger than these in Type I, but there was no significant difference between Type II and Type III. The reason may be related to the different proportion of Type I, Type II and Type III affected by vertical and rotational violence. Type I tends to rotational violence, while the latter two tends to vertical violence. Furthermore, FAR1 and FAR2 of Type II posterior malleolus fractures were significantly larger than these of Type I. Causes of differences are analyzed: Type I is mainly caused by the avulsion of the posterior tibiofibular ligament and Type II is mainly caused by the avulsion of the transverse tibiofibular ligament. However, the rotational violence of the posterior tibiofibular ligament is less than that of the transverse tibiofibular ligament in the process of ankle joint torsion.

All in all, anatomical reconstruction of articular surface is particularly important for the prognosis of posterior Pilon fracture. ²⁶ And ORIF should be performed regardless of the extent of articular surface involved and the size of the fragment. ⁶ But the operative indication of posterior malleolus fracture was 25% articular surface damage. ³ This study found that the ankle depth, α angle, γ angle, FAR1 and FAR2 of posterior Pilon fractures were larger than these of posterior malleolus fractures, which helps surgeons accurately distinguish between the two fractures and determine whether surgery is needed.

There are still the following limitations in this study: (1) we can't determine whether the patients suffer secondary injuries during transportation, which will further aggravate the fracture and may lead posterior malleolus fracture shows imaging features of posterior Pilon fracture. (2) The sample size of this study is small, which may also affect the measurement results. Therefore, more high-quality and large-sample studies are need for further analysis about it.

Despite the shortcomings, this study still has many advantages: (1) this is the first comprehensive analysis of radiographs and CT to systematically and comprehensively identify the posterior Pilon fracture and the posterior malleolus fracture. (2) The literature provides specific data on the differences between posterior Pilon fracture and posterior malleolus fracture, with clear data indicators, which is helpful for doctors to diagnose quickly and accurately so that patients can receive timely and effective individualized treatment. (3) This article classifies the posterior Pilon fracture and the posterior malleolus fracture, and measures the imaging features of different types of fractures. It is helpful for clinicians to further understand the severity of the fracture by analyzing the differences between different types of fractures. At the same time, we look forward to more high quality randomized controlled trials to provide high quality evidence.

Conclusion

In conclusion, radiographic diagnosis combined with CT analysis is a feasible method to distinguish posterior Pilon fracture from posterior malleolus fracture and fully assesses the morphological difference of the two kinds of fractures.