What is the desired screw trajectory for fixation of noncontiguous posterior distal articular fractures associated with distal tibial shaft fractures?

Abstract

Introduction:

Posterior distal articular fractures (pDAFs) associated with tibial shaft fractures are often nondisplaced allowing for fixation with percutaneous screws. This study uses imaging of such injuries to determine trajectory for percutaneous screws such that the fixation is perpendicular to and engages the center of pDAFs.

Materials and Methods:

This retrospective review at a single academic level one trauma center included radiographic review of all patients with distal tibial shaft fractures and associated noncontiguous pDAFs between 2015 and 2020. The axial pDAF angle with reference to the trans-malleolar axis and sagittal pDAF angle with reference to the long axis of the tibial shaft were measured using computed tomography imaging. These measurements were used to determine the screw starting point and the trajectory angle in both axial and longitudinal axes that would ensure the screw crossed the fracture plane orthogonally and targeted the center of the posterior fracture fragment, defined for purposes of this study as the desired screw position.

Results:

DAFs were overall identified in 74/145 cases (51.0%), 56/74 (75.6%) of which were pDAFs. With the far end of the screw positioned in the center of the posterior fragment, the mean axial screw angle for perpendicular fixation of pDAFs was 31° ± 4°, corresponding to a starting point at 76% ± 9% of the total width of the tibial plafond from the most lateral extent. The mean sagittal screw angle for perpendicular fixation of pDAFs was 80° ± 5°. Screw placement placed at these angles would be accurate within 1 SD of the desired screw position in only 29/56 (52%) fractures in the axial plane and 44/56 (78%) fractures in the sagittal plane.

Conclusions:

This study demonstrates that a standardized screw angle and starting point would place a screw in only 52% of fractures in the axial plane and 78% of fractures in the sagittal plane within 1 SD of the desired screw position. Results of this study suggest that computed tomography scans are useful for preoperative templating of screw trajectory for pDAFs associated with tibial shaft fractures as there is no single percutaneous trajectory that targets the screw in the ideal manner (orthogonal to the fracture plane and centered in the fragment) for every one of these fractures.

1. Introduction

Distal tibia fractures are commonly associated with noncontiguous distal articular factures (DAFs). These most frequently are posterior DAFs (pDAFs), with rates up to 92% in spiral pattern tibial shaft fractures.^1–8 These fractures are often missed using plain radiography. This leads some to argue that preoperative computed tomography (CT) is indicated to identify these injuries when present,^3,4,7 whereas others argue that the use of preoperative CT scan for distal tibial shaft fractures to identify DAFs provides low yield with minimal effect on treatment strategy and outcomes and, therefore, should not be universally done.^9,10 Mitchell, et al¹¹ demonstrated that these fractures occur at a mean angle of 25° to the bi-malleolar axis, which may aid in intraoperative detection of these injuries without CT scan.

Regardless of how they are identified, management of noncontiguous pDAFs when nondisplaced may be treated with percutaneous placement of one or 2 screws from anterior to posterior before fixation of the tibial shaft fracture to prevent intra- or postoperative displacement.^12,13 Theoretically, one or both screws should be placed perpendicular to the fracture plane and central within the pDAF to provide optimal biomechanical fixation by minimizing shear and maximizing compression. To date, no studies have identified the trajectory and starting point for anterior to posterior screw placement across in the desired position based on CT analysis in a clinical series with variable pDAF patterns. In addition, it is unknown whether screw placement at this desired trajectory would provide accurate fixation for a large majority of the pDAFs. The purpose of this study is to identify the theoretical optimal location for placement of an anterior to posterior percutaneous screw for pDAFs in the setting of distal tibial shaft fractures and to determine what proportion of percutaneous screws would be placed in the desired position if they were all placed at this trajectory. Because this is a theoretical radiographic investigation, it is outside the scope of this study to report on clinical outcomes of preoperative templating for accurate screw placement.

2. Materials and methods

After institutional review board approval, a retrospective cohort study was performed at a single level one trauma center. No sources of funding were required for this study. Patients were identified for participation in this study using a billing database query by CPT code to include all patients 18 years of age and older who were treated operatively for tibial shaft fractures between January 1, 2015, and December 31, 2021. Patients were included in the study if they had a distal tibial shaft fracture occurring at or below the tibial isthmus, defined as the lower 40% of the total tibial length. Patients were excluded for fracture above the tibial isthmus, extension of the main tibial shaft fracture into the tibial plafond (OTA/AO 43B/C), ballistic injuries, and absence of a preoperative CT scan. Decisions to order a preoperative CT scan and to treat a DAF operatively or nonoperatively were left to the discretion of the managing surgeon.

Preoperative CT scans were evaluated in all patients for presence or absence of distal articular fracture. The patients with posterior malleolar fractures were included for angular analysis. Haraguchi classification of each fracture was determined.¹⁴ Other DAFs were not analyzed as these are not typically treated with percutaneous screw placement. Measurements were taken at a level 2 to 3 mm above the articular plafond on the axial CT image as demonstrated in Figure 1. The area of the pDAF was calculated relative to the area of the tibial plafond in the axial plane. The angle between the bimalleolar axis and the fracture line of the posterior malleolus (angle θ₁) was determined as previously described by Haraguchi, et al.¹⁴ The desired axial percutaneous screw trajectory was then determined to be at the angle perpendicular to the pDAF angle and at a starting point with reference to the distal tibio-fibular joint (angle θ₂). The desired screw starting point was determined based on the intersection of the screw angle and the most anterior aspect of the plafond (point Z). The location of the starting point was then defined as a percentage of the total width of the plafond (line a/line b). Sagittal measurements were taken at the point of the largest diameter of the pDAF as demonstrated in Figure 2. The pDAF fracture height was determined at its highest point from the articular surface to the metaphyseal exit point. The angle between the pDAF and the long axis of the tibial shaft (angle γ₁) was determined. The desired sagittal percutaneous screw trajectory was then determined to be at the angle perpendicular to the pDAF in reference to the long axis of the tibial shaft so that the trajectory would pass 2 mm below the physeal scar at the central axis point of the tibial shaft (angle γ₂). The average desired percutaneous screw trajectory and the percentage of fractures that would have screws placed in the desired position with this trajectory in both the axial and sagittal planes was determined. Pearson correlations between screw angles and fracture area in the axial plane and fracture height in the sagittal plane were determined. P < 0.05 were considered significant for all results.

Figure 1.

Computed tomography image of distal tibia 2 mm above plafond. This example demonstrates the measurement of the axial posterior distal articular fracture angle (θ₁) measured off the trans-malleolar axis (x). Screw trajectory angle is the same angle (θ₂) but is measured off the perpendicular to the transmalleolar axis (y). Start point location is marked by Z and is determined as the percentage of start point width (A) with respect to plafond width (B) measured from the most lateral aspect of the plafond.

Figure 2.

Computed tomography image of distal tibia at the maximum depth of the posterior distal articular fracture. This example demonstrates the measurement of the sagittal posterior distal articular fracture angle (γ₁) measured off the long axis of the tibial shaft as determined by a line through 2 separate center points indicated by the drawn circles. Screw trajectory angle (γ₂) is the angle perpendicular to γ₁ measured off the long axis of the tibial shaft passing 2 mm below the physeal scar at the central long axis of the tibial shaft.

3. Results

A total of 459 patients were treated operatively for tibial shaft fractures within the study period. Fifty patients were excluded for fracture location above the tibial isthmus, 68 patients were excluded for direct extension of the main tibial shaft fracture into the tibial plafond (OTA/AO 43B/C), 51 patients were excluded for ballistic injury mechanism, and 145 patients were excluded or absence of preoperative CT scan. A total of 145 patients were thus included for final analysis.

DAFs were identified in 74/145 cases (51%); 56/74 (76%) of these were pDAFs. pDAF morphology measurements are listed in Table 1. About 53/56 (95%) pDAFs were classified as Haraguchi type I posterolateral oblique fractures, and 3/56 (5%) pDAFs were classified as Haraguchi type II medial-extension type fractures. There were no Haraguchi type III small-shell type fractures identified. Only 22/56 (39%) pDAFs identified on CT scan were also identified on plain ankle radiographs in the emergency department preoperatively.

Table 1.

Morphology analysis of pDAF.

	Total (N = 56)
Haraguchi fracture classificationa
Type I	53 (95%)
Type II	3 (5%)
pDAF identified on plain radiographa	22 (39%)
pDAF area percentageb	22% ± 13%
pDAF heightb	24 ± 8 mm
Axial pDAF angle (θ1)b	31° ± 4°
Axial desired screw angle (θ1)b	31° ± 4°
Axial starting point percentageb	75% ± 9%
Axial screw placement accuracya	29 (52%)
Sagittal pDAF angle (γ1)b	10° ± 5°
Sagittal desired screw angle (γ2)b	80° ± 5°
Sagittal screw placement accuracya	44 (78%)

pDAF area was determined as a percentage of the total tibial plafond area. Axial pDAF and optimal screw angles are measured relative to the trans-malleolar axis. Axial starting point percentage is measured from the most lateral aspect of the plafond in reference to the total plafond width. Sagittal pDAF and optimal screw angles are measured relative to the long axis of the tibial shaft. Screw placement accuracy describes the percentage of fractures that would be accurately fixed with screw placed within 1 SD of the mean. Numbers reported as follows: a = N (%); b = mean ± SD.

The mean desired axial screw angle for fixation of pDAFs (Fig. 1, angle θ₂) was 31° ± 4° (range 15°–52°), corresponding to a starting point (Fig. 1 point Z) at 75% ± 9% (range 54%–99%) of the total width of the tibial plafond from the most lateral extent (Fig. 1 line a/line b). Screw placement at an axial angle of 30° with a start point at 75% of the plafond width would be placed perpendicular and central within 1 SD of the desired position in 29/56 of the identified pDAFs (52%). There was a low correlation between axial pDAF angle and pDAF area (r = −0.02; P = 0.9) or height (r = −0.03; P = 0.8).

The mean desired sagittal screw angle for fixation of pDAFs (Fig. 2, angle γ₂) was 80° ± 5° (range 59°–89°). Screw placement at an angle of 80° relative to the long axis of the tibial shaft passing 2 mm below the physeal scar at the central axis of the tibia would be placed perpendicular within 1 SD of the desired position in 44/56 of the identified pDAFs (78%). There was a low correlation between sagittal pDAF angle and pDAF area (r = 0.01; P = 0.9) or height (r = −0.1; P = 0.4).

4. Discussion

This cohort of infraisthmal tibial shaft fractures demonstrated a high rate of DAF, with most of these cases being pDAFs. Analysis of the pDAF morphology demonstrated a mean desired screw angle of approximately 30° off the distal tibio-fibular joint axis with a mean starting point approximately 75% from the lateral most aspect of the distal tibia on a mortise X-ray. The mean desired sagittal screw angle was 80° off the long axis of the tibial shaft. However, there was a large variation in the desired angle and starting point for pDAFs in both the axial and sagittal planes in this cohort. This corresponds to a broad range of possible screw angles and start points to best provide perpendicular fixation for these injuries, and placement of screws within 1 SD of the mean desired trajectory would provide perpendicular and central fixation for only 52% of fractures in the axial plane and 78% of fractures in the sagittal plane. CT scans may be beneficial in the presence of distal tibial shaft fractures to not only identify pDAFs but also appropriately preoperatively plan for screw placement as no single standard trajectory is sufficient to capture all pDAFs.

Rates of pDAFs associated with tibial shaft fractures have been reported to be as high as 92%.^1–8 Management of these fractures is dependent on multiple factors, including size, displacement, and associated injuries. Surgical fixation has been recommended for these injuries when fragment size is greater than 25% of the tibial plafond or if there is articular displacement greater than 2 mm.^15,16 However, many surgeons prefer to percutaneously fix even smaller and minimally displaced pDAFs as this has been shown to restore up to 70% of syndesmotic stability.^16–18 When associated with tibial shaft fractures that can be managed with intramedullary nailing, anterior to posterior percutaneous fixation before placement of the nail is favored for nondisplaced pDAFs to minimize risk of displacement with nail insertion and to allow simpler placement of the screws with the patient in the supine position.^12,13,19 To date, no studies have investigated the trajectory and position of percutaneous screws relative to the pDAF morphology.

Mitchell, et al¹¹ performed a similar analysis to the present study and determined that the average angle of the posterior malleolus was 24° with a range of 14° to 34°. This was slightly lower than the angles determined in the present study despite a similar percentage of Haraguchi type II fractures (7%, or 3/44). These authors used their morphology analysis to confirm the previously described technique for improved intraoperative detection of pDAFs by externally rotating the leg to place the fracture line parallel to the x-ray beam.^11,20,21 However, the wide range of angles reported both by Mitchell, et al, and in the present study indicate that there may not be one angle that can best detect and fix all fractures. They also comment that their findings may assist in screw placement at the appropriate angle, but they did not investigate the impact of screw starting point or sagittal screw angle.

Several studies have investigated the morphology of the pDAF in rotational ankle fracture patterns of injury. A retrospective review of 107 rotational ankle fractures demonstrated mean pDAF angles between 17.6° and 19.4° depending on the fracture pattern.²² These values are smaller than those described in the present study and by Mitchell, et al, suggesting that the mechanisms for pDAF in rotational ankle fractures and those associated with tibial shaft fractures may be different. There was a much lower rate of Haraguchi type II fractures in the present study compared with those associated with rotational ankle fractures, which ranged from 22% for supination-external rotation injuries up to 42% in pronation-external rotation injuries.²² Haraguchi type II patterns are often associated with impaction-type mechanisms that have lower pDAF angles relative to the transmalleolar axis, which can explain the differences between these and the present study.^22–25

Although no studies have investigated the sagittal angle of pDAFs in the setting of tibial shaft fractures, several have investigated this in the setting of rotational ankle fractures. Yi, et al²² demonstrated mean angles between 17.5 and 21.2 degrees depending on fracture pattern, which is higher than what was found in the present study. Steeper sagittal pDAF angles are also correlated with decreased articular contact area and decreased posterior ankle stability because of greater vertical shear forces.^22,26,27 The relatively steep pDAF angles (mean = 10°) associated with tibial shaft fractures identified in the present study may indicate risk of ankle instability if fracture displacement occurs. This highlights the importance of accurate screw placement perpendicular to the fracture line using preoperative templating to minimize risk of displacement because of shear forces.

The importance of accurate screw placement in fracture fixation has been well described in orthopaedic literature. Lag screw technique requires perpendicular screw placement to maximize interfragmentary compression and minimize shear.²⁸ However, little consideration has been given to optimizing this type of fixation for pDAFs of the tibia. Given that percutaneous screws are often the only fixation used for pDAFs, it may be important to place these implants in a central and orthogonal position to minimize displacement with axial loading.²⁹ In cases where a CT scan is not available before pDAF detection intraoperatively, this study demonstrates that an axial screw angle of approximately 30° and start point 75% across the width of the plafond along with a sagittal screw angle of approximately 80° relative to the long axis of the tibial shaft can be used maximize fixation accuracy for as many pDAFs as possible with the understanding that some fractures will be fixed outside of the desired screw trajectory without proper preoperative planning.

Although it can be challenging to accurately place screws in the appropriate trajectory based on CT scans when in an OR environment with only fluoroscopic imaging available, Sheth, et al²⁹ has described a technique for accurate screw placement in the axial plane for adolescent transitional ankle fractures that can be applied to adult pDAFs. The limb is positioned for a perfect mortise view, defined as minimum tibia-fibula overlap, with the c-arm oriented perfectly parallel to the incisura and directly perpendicular to the OR table. The start point for the screw is then obtained using a guide wire based on the distance measured from the incisura on CT imaging. The C-arm is then angled over the limb to the exact measured angle for screw trajectory on CT imaging, and the screw is placed parallel to this plane to achieve the appropriate trajectory.²⁹ A similar technique can be done in the sagittal plane by tilting the C-arm toward the head to the appropriate angle relative to the tibial shaft. Figure 3 demonstrates this technique in practice with start point and screw trajectory based on CT planning. Attention must be paid in the sagittal plane to the planned depth of nail seating so that the percutaneous screw does not block its insertion; this may limit the ability to place screws into the desired sagittal screw trajectory in certain instances of high sagittal angle pDAFs. It is important to note that the desired screw location and trajectory based on CT planning may increase the potential for anterior neurovascular or tendinous injury about the ankle. Providers must consider the location of these anatomic structures relative to the ideal screw location and may have to adjust screw insertion points to decrease risk of inadvertent injury.

Figure 3.

Demonstration of templating and screw placement in practice. A, This patient with a distal spiral tibial shaft fracture had a minimally displaced posterior malleolar fracture traced in white and indicated by the black arrow. The desired axial plane screw angle (θ₂) was 33° at an desired starting point Z was measured to be 55% of the width of the plafond (a = 2.5 cm, b = 4.5 cm); (B) fracture in sagittal plane traced in yellow and indicated by the black arrow. The desired sagittal plane screw angle (γ₂) was 4° from the axis of the tibia; (C) screw placement on intraoperative fluoroscopic mortise view with an approximate start point 52% of plafond width and trajectory medial to lateral ending in the posterolateral tibial plafond (noncalibrated image, a = 115 pixels, b = 220 pixels; (D) screw placement on intraoperative lateral view with screw angle 1° relative to long axis of the tibia.

Limitations of this study include the fact that not every patient with a distal tibial shaft fracture underwent CT imaging. This may lead to an inaccurate estimation of the overall rate of DAFs as patients with spiral tibia fractures, which are more commonly associated with DAFs, were more likely to get CT scans. However, rates of pDAFs from our study match well with those from prior literature.^1–7,11 Identification of pDAFs on plain radiographs was done using ankle images from the emergency department and thus are technologist dependent. This may have underestimated true frequency as these fractures may have been identified with intraoperative fluoroscopy at the correct radiographic angle.¹¹ As there were only a few cases with Haraguchi type II fractures, we did not stratify our results by Haraguchi Classification. Haraguchi type II fractures are thought to have higher angles of the posterior malleolus in the axial plane and would require further assessment of CT scans for proper screw trajectory. The desired sagittal screw angles were calculated to be placed below the level of the physeal scar, but consideration must be given to the planned depth of nail seating so that the percutaneous screw does not block its insertion; this may limit the ability to place screws into the desired sagittal screw trajectory. Finally, clinical and functional outcomes from fixation were not assessed. Because this study was completed retrospectively, it was not possible to determine the clinical benefit of accurate screw placement as it was unknown if surgeons preoperatively templated their screw trajectories. In addition, patients did not undergo postoperative CT scanning to reliably assess screw accuracy. This is, therefore, an investigation of theoretical ideal screw placement, and the clinical impact of this practice is outside of the scope of the present study. Despite a present lack of data regarding the clinical impact of this practice, it stands to reason that when surgeons deem percutaneous fixation to be appropriate an attempt should be made to optimize this fixation as much as possible. Future studies should aim to investigate how preoperative templating using CT scans with improved screw accuracy may affect clinical outcomes both in function and complications such as risk to anatomic structures with varied locations for percutaneous screw placement.

5. Conclusion

This study demonstrates the large variation in angles and starting points that are necessary to achieve perpendicular anterior-posterior percutaneous screw placement in the center of the pDAF associated with distal tibial shaft fractures. Only 52% of fractures in the axial plane and 78% of fractures in the sagittal plane would be fixed within 1 SD of the mean desired screw angle and starting point. Results of this study suggest that CT scans are useful for preoperative templating of screw trajectory for pDAFs associated with tibial shaft fractures as there is no single percutaneous trajectory that is appropriate for all of these fractures. Further research is indicated to identify how parameters of screw placement may affect clinical outcomes.

Data availability statement

The data that support the findings of this study are available on request from the corresponding author.