Morphological analysis of the posterior elements of associated both-column acetabular fractures

Abstract

Background

It is still in dispute as to whether a posterior approach is applied to reduce the posterior fractures of associated both-column acetabular fractures (ABC-AFs).

Purpose

To analyze the morphological changes in the posterior elements including the posterior column (PC) and/or associated posterior wall (PW) fragments to provide a reference for surgical planning.

Material and Methods

The 3D computed tomography (CT) data of 100 cases of ABC-AFs were retrospectively analyzed using Mimics and 3-Matic software. The line distribution of the retroacetabular surface (RAS), the acetabular fossa, and the size of the PW fragments were analyzed.

Results

Fracture lines (n = 89) on RAS appeared in three patterns: transverse lines (n = 41) on the cephalic (65.8%) and caudal (29.3%) thirds; oblique lines (n = 34) on the mid-caudal thirds; and multifragmentary lines (n = 14). The lines of the displaced PW fragment (n = 61) were widely distributed in intra- and extra-articular regions. The mean radian of the PW fragments was >90° in 67.2% of cases and involved anteriorly to the vertex in 90.2% of cases. The average fracture span on the RAS was 0.60–1.00 in 63.9% of cases. The mean length of the spike of PW fragments was >20 mm in 80.3% of cases.

Discussion

For PC fractures, transverse lines on the cephalic third of RAS indicated a possibility of the anterior approach, while transverse lines on the distal third or oblique and multifragmentary lines suggest the posterior approach. A displaced PW fragment was involved more extensively both intra-and extra-articularly and may be optimally treated via a posterior approach.

Keywords

Associated both-column acetabular fractures fracture lines fracture mapping posterior column posterior wall

Introduction

Associated both-column acetabular fracture (ABC-AF) is the second most common type of acetabular injury, accounting for 22% of all acetabular fractures (1). This type of fracture is caused by impaction of the femoral head on the articular surface of the acetabulum, which is divided into the anterior column, quadrilateral plate (QP), and/or posterior wall fragments (2). According to the OTA/AO classification, this fracture is classified as type C and further divided into C1, C2, and C3 subtypes (3).

The posterior elements of ABC-AFs are composed of the posterior column (PC) and posterior wall (PW) fractures. A PW fragment is an isolated fracture, while the PC fragment is considered to be the continuation of the QP fragment. Most displaced ABC-AFs should be treated operatively. An appropriate surgery approach is crucial in obtaining an accurate reduction with the least incidence of complications. However, the choice of approach, especially when dealing with the posterior elements, remains in dispute (4 –6). A single anterior approach to reduce the PC fractures was reported (6,7), while a staged or simultaneous combined Kocher–Langenbeck (KL) approach is applied to reduce and fix the posterior elements of this complex pattern (5,8). It remains unclear when a posterior approach should be chosen.

The morphological change of the fractures is one of the key points in the decision-making process for the surgical treatment of these fractures. Fracture mapping techniques may show these changes more clearly (9). Previous studies have reported the morphological changes of the QP fragment and acetabular fossa in ABC-Afs (10,11); however, no studies have reported comprehensive morphology in the PC and PW yet. Thus, the aim of the present study was to analyze the morphological changes in the PC and associated PW fragments of ABC-AFs to provide a reference for decision-making in planning the surgical approach.

Material and Methods

Patients

In this retrospective study, the data of patients who had surgery at our institute for ABC-AFs were collected in the picture archiving and communication system (PACS) and medical record system from August 2016 until August 2019.

The inclusion criteria were as follows: (i) patients aged >18 years at time of injury; (ii) ABC-AFs identified according to the Letournel–Judet criteria and OTA/AO classification; and (iii) availability of pelvic X-ray images with anteroposterior and Judet views and the data of whole pelvic three-dimensional (3D) computed tomography (CT) with a slice thickness of 1 mm. The exclusion criteria were open or pathological fractures.

The classification was confirmed by two senior authors who both have >15 years of experience in treating these fractures. A third author was consulted if any disagreements occurred.

The present study was conducted in accordance with the Declaration of Helsinki. Ethical approval was granted from the institutional review board of our hospital, and informed consent was waived by our ethics committee because the academic use of patient's information, including the CT data, during their treatments was obtained from the patients by the hospital at the time of their hospitalization. There was no identifiable information of the participants included in the manuscript.

Fracture mapping

The fracture mapping was made by our first author who was trained in 3D modelling software and had sufficient knowledge in the surgical treatment of acetabular fractures. The 3D fracture map was generated as previously described by Xie et al. (9). The 3D models of all fractures were reconstructed by importing Digital Imaging and Communications in Medicine (DICOM) data into the Mimics version 17.0 software (Materialise, Leuven, Belgium) for segmentation. The segmented images of all fragments were then imported into the 3-matic version 10.2 software (Materialise, Leuven, Belgium). A simulated surgical reduction was performed by moving and rotating each fragment to restore the original anatomical structure. Subsequently, adjustments to the restored acetabulum were made to best match a pre-created standard 3D acetabulum model. The fracture model was mirrored if it was not on the same side as the template. The template surface was then marked and divided into small independent surfaces, the borders of which coincided with the fracture lines of the acetabular fragments. All the templates were subsequently superimposed to build a frequency diagram, and the borderlines of the rendered templates indicated the frequency of the fractures (Fig. 1).

In the present study, the posterior edge of the PC and corresponding retroacetabular surface (RAS), starting from the greater sciatic notch cephalically to the lesser sciatic notch caudally, was artificially divided into the three equal segments: the cephalic, middle, and caudal thirds (Fig. 2). The distribution of the fracture lines on the RAS was analyzed.

Quantitative evaluation of the PW

In the study, the size of the extra-articular part of the PW fragment, presented as the radian of the rim, fracture span on the RAS, and spike length on the lateral ilium, were measured.

The radian of the PW fragment's rim indirectly reflects the scope of the fragment involved in the articular surface of the acetabulum. This parameter was determined by using a modified method introduced by Olson et al. (12). Briefly, a clock-face position was applied by creating a best-fit circle in the plane of the acetabulum on the “reduced” 3D model. The vertex (a) of the acetabular rim was a point where a vertical line drawn from the midpoint (b) of the horizontal line connecting the insertion sites of the transverse ligament was intersected to the superior rim of the acetabulum. Then, the central point of the acetabulum was identified as the midpoint of the line connecting points a and b. The fracture site on the most superior rim was defined as point c and the most inferior as point d. The angle between c and d was defined as a radian of the PW fragment. The radian consisted of two parts: angle α is the angle between point a and point c; a minus was given when point c was located posteriorly to point a, and vice versa, a plus. Angle β was the angle between point a and point d (Fig. 3a).

The fracture span represents the “width or depth” of the fragment extended posteriorly on the RAS. This parameter was calculated according to the method described by Cho et al. (13). On the contralateral 3D model of each case, the widest RAS was identified. The fracture span was defined as the proportion of horizontal distance (Y) of the PW fragment (between the fracture beak and acetabular rim) to the whole width (X) of the RAS, which is from the edge to the rim (Fig. 3b).

The spike length of the PW fragment indicates its “height” that extended into the lateral ilium. To quantitatively assess this parameter, a clock-face position of the acetabulum was first determined on a reconstructed 3D model. A horizontal line passing through the vertex of the acetabulum was made. The model was then rotated in the coronal plane and the longest distance between the horizontal line and the tip of the spike of the fragment was measured (Fig. 3c).

In addition, a complete fracture was defined as a fracture displaced by >2 mm; otherwise, fractures were considered non-displaced. Incomplete, non-displaced, and small marginal fragments of the rim that could be left untreated without affecting joint stability were considered “non-fracture.”

Statistical analysis

SPSS version 20.0 software (IBM Corp. , Armonk, NY, USA) was used to perform all statistical analyses. Demographic data and fracture characteristics were analyzed using descriptive statistics. Data related to PC and PW fracture are presented as mean and standard deviation. The correlation analysis was performed using the Pearson chi-square test and Fisher’s exact test. A P value <0.05 was considered statistically significant.

Results

Demographic characteristics

From August 2016 to August 2019, 490 cases of acetabular fractures were surgically treated and 100 cases (81 men, 19 women; mean age = 52 years) met the inclusion criteria. The left side was involved in 49 cases and the right side in 51 cases.

According to the AO/OTA classification system, 75 cases were of type C1, nine of type C2, and 16 of type C3. The isolated acetabular fractures without concomitant injuries were presented in 32 cases.

Fracture lines on RAS

There were two origins of fracture lines identified on the RAS: those from the QP fragments and those from the PW fragments. Fracture lines from the two separate origins intersected longitudinally on the central part of the RAS (Fig. 4).

Distribution of fracture lines on the RAS

The fracture lines derived from the QPs distributed mainly on the RAS, ranged from the greater to lesser sciatic notch. Displaced PC fractures were confirmed in 89 cases. Three patterns of PC fracture lines were identified in this group as follows: (i) the transverse lines concentrated on the cephalic third (close to the greater sciatic notch) in 27 (30.3%) cases, middle third in 2 (2.2%) cases, and caudal third (around the sciatic spine) in 12 (13.5%) cases (Fig. 5). The ratio of traversed lines closer to the greater and lesser sciatic notches were 65.9% and 29.3%, respectively; (ii) the oblique lines distributed on the mid-caudal thirds of the RAS in 34 (38.2%) cases, including inverse lines from the posterosuperior edge of the PC to the anteroinferior rim in 23 cases, and reverse lines from the posteroinferior edge of the PC to the anterosuperior rim in 11 cases (Fig. 6); and (iii) multifragmentary fracture lines involving both the cephalic and caudal thirds of the RAS were found in 14 (15.7%) cases (Fig. 7). Non-fracture of the PC was identified in 11 cases (Table 1).

Table 1.

Distribution of posterior column fracture lines.

Fracture pattern	No. of fractures
Transverse lines
Cephalic third	27
Middle third	2
Caudal third	12
Oblique lines
Posterosuperior–anteroinferior	23
Posteroinferior–anterosuperior	11
Multifragmentary	14
Non fracture	11
Total	100

PW fracture

A complete PW fracture was identified in 61 (61%) cases, while no or incomplete PW fracture was found in 39 (39%) cases.

The fracture lines of PW fragments were widely distributed on the RAS posteriorly and the lines extended into the lateral ilium. A dense line area was found on the middle of the RAS (Fig. 8a, b). While in the fossa, the lines were concentrated on the area starting from the superior rim of the acetabulum (acetabular dome), passed inferoposteriorly through the junction between the lunate surface and the fossa, and terminated in the posteroinferior rim of the acetabulum (Fig. 8c). A few lines were found on the superoposterior rim of the acetabulum.

The mean radian of the PW fragments was 103.8° (range = 36.2°–175.3°). The radian was >90° in 41 (67.2%) cases, 40°–90° in 19 (31.1%) cases, and <40° in one case (Table 2).

Table 2.

Quantitative data of the posterior wall fragment.

Parameters
Radian on the rim
>90°	41 (67.2)
40°–90°	19 (31.1)
<40°	1 (1.7)
Fracture span on the RAS
0.60–1.00	39 (63.9)
0.30–0.59	21 (34.4)
<0.30	1 (1.7)
Spike length on the lateral ilium (mm)
<21	12 (19.7)
21–40	22 (36.1)
41–60	22 (36.1)
>60	5 (8.1)

Values are given as n (%).

RAS, retroacetabular surface.

Angle α was negative in two cases (−10.2° and −9.4°) and 0° in four cases. Angle α was positive in 55 (90.2%) cases, with a mean value of 24.2° (range = 3.2°–62.8°). Angle β was 79.7° on average (range = 35.8°–119°).

The average fracture span on the RAS was 0.67 ± 0.16 (range = 0.28–1.00). The fracture span measuring 0.60–1.00 of the RAS was confirmed in 39 (63.9%) cases, 0.30–0.59 of the RAS in 21 (34.4%) cases, and <0.30 in one case.

The mean spike length of PW fragments on the lateral ilium was 34.7 mm (range = 0–74 mm). The length was found to be <21 mm in 12 (19.7%) cases, 21–40 mm in 22 (36.1%) cases, 41–60 mm in 22 (36.1%) cases, and >60 mm in 5 (8.1%) cases. A length >40 mm was confirmed in 27 (44.3%) cases. The ratio of the length >20 mm was 80.3%.

Discussion

Controversy exists regarding whether a posterior approach should be selected for the surgical treatment of PC and/or associated PW fractures in ABC-AFs. A staged (5) or simultaneously (4,8) combined anterior with KL approach is recommended, although some surgeons have indirectly reduced and fixed these fractures through a single anterior approach (6,14). It is still unclear when a KL approach should be applied in PC and/or associated PW fractures for the surgical treatment of ABC-AFs.

In this morphological study, the fracture lines of the PC and PW fragments in ABC-AFs were analyzed using a fracture mapping technique. The authors found that there were two derivations of fracture lines presented on the RAS: those from QP and those from the PW fragments. The two lines intersected longitudinally in the middle part of the RAS, leading to a dense fracture line area. These outcomes indicated that the middle part of the RAS was the area most involved, as the fracture lines from the PC and PW intersected here. More attention may need to be paid to this area during surgical decision-making for the repair of these fractures.

In a case report, 22 cases of acetabular fractures, including 11 cases of ABC-AFs, were reduced through a supra-ilioinguinal approach; however, no indication was declared (14). Pierannunzii et al. (15) suggested that the closer the fracture line of the PC to the greater sciatic notch, the higher the fracture site and the higher the possibility of reduction and fixation through the anterior approach. In our study, three fracture patterns were identified on the PCs: the transverse lines located mostly (two-thirds) on the cephalic third and less (one-third) on the caudal third of the RAS, respectively; the oblique lines concentrated on the mid-caudal part of the RAS and the multifragmental lines involving the entire RAS. In ABC-AFs, PC fractures are the continuation of the QP fragment. A previous study reported that the fracture lines of the QP could be divided into anterior oblique (AO), superior posterior oblique (SPO), and mid-posterior oblique (MPO) lines (11). The closer the AO line to the arcuate line, the easier it was to reduce the QP fragment through an anterior approach. According to our study, the PC lines are an extension of the SPO line on the RAS. The reduction of the PC fracture is closely related to the reduction of the QP fragment, which is most commonly performed with an anterior approach, such as a modified ilioinguinal (16), Stoppa (17), or pararectal approach (18). This study found that in cases of transverse lines, more than two-thirds were located on the cephalic third of the RAS, indicating a short fracture line both on the QP and PC, providing a possibility to reduce the fractures using an anterior approach (Fig. 9).

However, when the posterior part of the QP fragment presented with a caudal third transverse or oblique (reverse or inverse) line on the RAS, a long line may exist on the inferoposterior QP (a long SPO line) or on the PC. In such cases, it is difficult to anatomically reduce PC fractures and to rigidly fix the fracture using long screws applied through an anterior approach. A simultaneously combined approach, such as a pararectal associated KL approach, is recommended (Fig. 10). The onset of mutifragmentary fractures on the PC is another indication to open reduction through the KL approach (Fig. 11).

In this series, PW fractures were present in 61% of the cases, which is significantly higher than that previously reported (34%–40%) (6,7,19). The present study found that the extra-articular fracture lines of the PW fragment were distributed extensively on the whole RAS posteriorly and on the lateral ilium cephalically. The intra-articular fracture lines originated from the superior rim of the acetabulum, passed inferoposteriorly through the junction of the lunate surface and fossa, and finally ended in the inferoposterior aspect of the joint. Our results showed that the distribution of the lines was more extensive than that of the isolated wall fragments reported by Cho et al. (13). Using a quantitative measurement, we identified that the mean fracture span on the RAS and radian on the rim of the fragments was 0.67 and 103.8°, respectively. These outcomes were similar to the outcomes of the isolated PW fragment reported by Cho et al. (13).

However, by calculating the angle between the vertex and most superior fracture site of the PW fragments, the authors identified that 90% of fractures with a superior fracture site were located anteriorly to the vertex of the rim (mean = 24.2°), which was greater than that of the isolated PW fragment involving the superoposterior rim (mean = 16.2°) reported by Cho et al. (13). In addition, by measuring the length of the cephalic spike of the fragments, we confirmed the mean spike was 34.7 mm, and the length was found to be >20 mm in 80.3% of all PW fracture cases. Our results demonstrated that the size of PW fragments associated with ABC-AFs was larger than that of the isolated PW fractures. They were involved more anteriorly on the rim, more cephalically on the lateral ilium, and, more importantly, more widely in the articular surfaces. Our results were in accordance with previous studies, which implied that PW fragments in ABC-AFs were “larger” than those in other types of acetabular fractures (isolated PW fracture, posterior column, T shape, and transverse fractures) (20 –22). These findings implied that displaced PW fractures in ABC-AFs were more unstable. A KL approach, and occasionally an axillary incision on the iliac wing, was suggested over open reduction and internal fixation of the PW fragment with long spikes extending into the lateral ilium (Figs. 10 and 11).

The present study has some limitations. First, due to the anatomy of the hemipelvis, the measurement of the length of the spike of PW fragment may be inaccurate. However, considering that there was no optimal method for calculating the PW involvement, we developed a method for this problem. The fracture lines that were hand-drawn on the template were based on the understanding of the acetabular fracture in 3D. Meanwhile, anatomical differences of pelvis lead to difficulties in fracture mapping. Some bias and mistakes may exist.

In conclusion, using a fracture mapping technique, we identified three fracture patterns on the RAS in ABC-AFs: the transverse line involved in the cephalic and caudal thirds of the RAS; the oblique line distributed on the mid-caudal third of the RAS; and the multifragmentary lines. A transverse PC fracture line that was located on the cephalic third of the RAS may be reduced and fixed through an anterior approach, while a PC fracture with transverse lines positioned on the caudal third of the RAS, or oblique or multifragmentary lines, suggests a KL approach. The PW fragments of ABC-AFs were larger, were positioned more anteriorly on the superior acetabular rim, more cephalically on the lateral ilium, and more of the intra-articular surface than that of isolated PW fragments. A displaced PW fragment may be optimally reduced and fixed though the KL approach.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Kai Ye

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