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. 2022 Jan 13;480(5):918–928. doi: 10.1097/CORR.0000000000002111

What Is the Maximum Tibial Tunnel Angle for Transtibial PCL Reconstruction? A Comparison Based on Virtual Radiographs, CT Images, and 3D Knee Models

Yuanjun Teng 1,2, Lijun Da 3, Gengxin Jia 2, Jie Hu 4, Zhongcheng Liu 2, Shifeng Zhang 1,2, Hua Han 1,2, Yayi Xia 1,2,✉
PMCID: PMC9029965  PMID: 35023873

Abstract

Background

To minimize the killer turn caused by the sharp margin of the tibial tunnel exit in transtibial PCL reconstruction, surgeons tend to maximize the angle of the tibial tunnel in relation to the tibial plateau. However, to date, no consensus has been reached regarding the maximum angle for the PCL tibial tunnel.

Questions/purposes

In this study we sought (1) to determine the maximum tibial tunnel angle for the anteromedial and anterolateral approaches in transtibial PCL reconstruction; (2) to compare the differences in the maximum angle based on three measurement methods: virtual radiographs, CT images, and three-dimensional (3D) knee models; and (3) to conduct a correlation analysis to determine whether patient anthropomorphic factors (age, sex, height, and BMI) are associated with the maximum tibial tunnel angle.

Methods

Between January 2018 and December 2020, 625 patients who underwent CT scanning for knee injuries were retrospectively reviewed in our institution. Inclusion criteria were patients 18 to 60 years of age with a Kellgren-Lawrence grade of knee osteoarthritis less than 1 and CT images that clearly showed the PCL tibial attachment. Exclusion criteria were patients with a history of tibial plateau fracture, PCL injuries, tumor, and deformity around the knee. Finally, 104 patients (43 males and 61 females, median age: 38 [range 24 to 56] years, height: 165 ± 9 cm, median BMI: 23 kg/cm2 [range 17 to 31]) were included for analysis. CT data were used to create virtual 3D knee models, and virtual true lateral knee radiographs were obtained by rotating the 3D knee models. Virtual 3D knee models were used as an in vitro standard method to assess the true maximum tibial tunnel angle of anteromedial and anterolateral approaches in transtibial PCL reconstruction. The tibial tunnel’s entry was placed 1.5 cm anteromedial and anterolateral to the tibial tubercle for the two approaches. To obtain the maximum angle, a 10-mm- diameter tibial tunnel was simulated by making the tibial tunnel near the posterior tibial cortex. The maximum tibial tunnel angle, tibial tunnel lengths, and perpendicular distances of the tunnel’s entry point to the tibial plateau were measured on virtual radiographs, CT images, and virtual 3D knee models. One-way ANOVA was used to compare the differences in the maximum angle among groups, and correlation analysis was performed to identify the relationship of the maximum angle and anthropomorphic factors (age, sex, height, and BMI).

Results

The maximum angle of the PCL tibial tunnel relative to the tibial plateau was greater in the anteromedial group than the anterolateral group (58° ± 8° versus 50° ± 8°, mean difference 8° [95% CI 6° to 10°]; p < 0.001). The maximum angle of the PCL tibial tunnel was greater in the virtual radiograph group than the CT image (68° ± 6° versus 49° ± 5°, mean difference 19° [95% CI 17° to 21°]; p < 0.001), the anteromedial approach (68° ± 6° versus 58° ± 8°, mean difference 10° [95% CI 8° to 12°]; p < 0.001), and the anterolateral approach (68° ± 6° versus 50° ± 8°, mean difference 18° [95% CI 16° to 20°]; p < 0.001), but no difference was found between the CT image and the anterolateral groups (49° ± 5° versus 50° ± 8°, mean difference -1° [95% CI -4° to 1°]; p = 0.79). We found no patient anthropomorphic characteristics (age, sex, height, and BMI) that were associated with the maximum angle.

Conclusion

Surgeons should note that the mean maximum angle of the tibial tunnel relative to the tibial plateau was greater in the anteromedial than anterolateral approach in PCL reconstruction, and the maximum angle might be overestimated on virtual radiographs and underestimated on CT images.

Clinical Relevance

To perform PCL reconstruction more safely, the findings of this study suggest that the PCL drill system should be set differently for the anteromedial and anterolateral approaches, and the maximum angle measured by intraoperative fluoroscopy should be reduced 10° for the anteromedial approach and 18° for the anterolateral approach. Future clinical or cadaveric studies are needed to validate our findings.

Introduction

Arthroscopic PCL reconstruction has improved over recent decades, but postoperative clinical outcomes do not equal those achieved after ACL reconstruction [4, 14]. The transtibial technique has been used more frequently than other procedures for PCL reconstruction [15, 27]. However, this technique inevitably creates a sharp tunnel edge for graft bending at the proximal posterior tibia, which has been named the “killer turn” [20, 33]. Many studies have shown that the killer turn gradually causes abrasion, thinning, and permanent elongation of the PCL graft, which ultimately results in posterior laxity and graft failure [5, 9, 20, 33].

To reduce the killer turn in the transtibial technique, surgeons tend to try to make the angle of the tibial tunnel related to the tibial plateau as large as possible [30, 34]. However, recommendations for the drill guide angle have varied from 42° to 70° [1, 8, 26, 32]. To date, no consensus has been reached regarding the maximum angle for the tibial tunnel. Clinically, there is a limit to how much the tibial tunnel angle can be increased; an excessive tunnel angle would result in breakage of the posterior tibial cortex, which affects the aperture fixation and increases the risk of iatrogenic popliteal neurovascular injury [2, 7, 8, 16, 34]. Therefore, determining a maximum safe tibial tunnel angle is essential for PCL reconstruction.

During PCL reconstruction, intraoperative fluoroscopy is frequently used to obtain the maximum tibial tunnel angle. Nevertheless, multiple structures of the posterior tibial plateau, including the medial and lateral tibial condyles, the PCL fovea, and posterior cortex, overlap on a fluoroscopic image [17]. This leads to great difficulty in distinguishing the highest safe angle for the tibial tunnel via fluoroscopy. As an alternative to intraoperative fluoroscopy, a few authors have used CT images to preoperatively plan the maximum angle for the PCL tibial tunnel [18, 30]. However, the use of a 2-dimensional (2D) sagittal CT image has obvious limitations for preoperative planning because it is impossible to simulate the true surgical approaches of PCL reconstruction via a 2D image. Clinically, an anteromedial or anterolateral approach is usually used for PCL reconstruction, and an anterolateral approach has been shown to reduce stress at the graft-tunnel interface near the killer turn [9, 13]. However, there is little information available to surgeons regarding whether the maximum safe angle for these two approaches differs. Over the last decade, a virtual three-dimensional (3D) CT imaging technique has been widely recommended for the tunnel localization evaluation and surgical simulation in ACL and PCL reconstruction, since it provides an excellent 3D perspective of the bone morphology and has high accuracy and reliability in measurements [12, 19, 21, 25, 31]. We believe that 3D CT knee models can help create a computer simulation of PCL reconstruction, which may help us more accurately assess the maximum safe angle of the tibial tunnel.

The purposes of this study were (1) to determine the maximum tibial tunnel angle for the anteromedial and anterolateral approaches in transtibial PCL reconstruction, (2) to compare differences in the maximum angle based on three measurement methods (virtual radiographs, CT images, and 3D knee models), and (3) to conduct a correlation analysis to determine whether patient anthropomorphic factors (age, sex, height, and BMI) are associated with the maximum tibial tunnel angle.

Patients and Methods

Experimental Overview

This study is based on virtual radiographs, CT images, and virtual 3D models of the knee. First, CT data were obtained from eligible patients according to inclusion and exclusion criteria. Second, we created virtual 3D knee models using CT data, and virtual true lateral radiographs of the knee were obtained by rotating the 3D knee models. Third, the tibial tunnel with the maximum angle was simulated on the virtual radiographs, CT images, and virtual 3D knee models. Last, three endpoints were measured. To answer our first research question, we used the 3D knee models as an in vitro standard method to assess the true maximum tibial tunnel angle relative to the tibial plateau in different approaches of PCL reconstruction. To answer our second research question, we measured the maximum angle on virtual radiographs and CT images and compared the outcomes among groups. To answer our third research question, we conducted a correlation analysis to evaluate the relationship between patient anthropomorphic factors and the maximum tibial tunnel angle.

Patients

Between January 2018 and December 2020, we treated 625 patients who underwent CT scans for knee injuries. Of those, we considered patients who were 18 to 60 years of age, had a Kellgren-Lawrence grade of knee osteoarthritis less than 1, and had CT images that clearly showed the PCL tibial attachment as potentially eligible. Exclusion criteria were patients with a history of tibial plateau fracture, PCL injuries, tumor and deformity around the knee, and knee surgery. Based on this, 35% (218 of 625) were eligible; a further 12% (78 of 625) were excluded because patients had previous knee surgery, and another 6% (36 of 625) could not be analyzed because of unclear CT images. Finally, 104 patients (43 males and 61 females, median age: 38 years [24 to 56], height: 165 ± 9 cm, median BMI: 23 kg/cm2 [17 to 31]) were included in this study for analysis (Table 1).

Table 1.

Patient anthropomorphic data

Parameter Value (n = 104)
Side
 Right 39% (41)
 Left 61% (63)
Sex
 Male 41% (43)
 Female 59% (61)
Age in years 38 (24 to 56)
Weight in kg 62 (41 to 90)
Height in cm 165 ± 9
BMI in kg/m2 23 (17 to 31)
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Data presented as % (n), median (range), or mean ± SD.

Image Collection and 3D Knee Model Reconstruction

All included patients underwent routine knee CT scanning on a 64-multidetector-row CT scanner (Siemens AG) in the supine position. Scanning parameters included a gantry rotation speed of 1.00 s/rotation, 0.625 mm collimation width × 12 detectors, a CT pitch factor of 0.90, and a field of view of 25 to 30 cm. A 3D bone model of the knee was reconstructed using CT data via Mimics software (Version 21.0, Materialise).

Measurements and Outcome Measures

Measurements on 2D CT Images

To accurately determine the center of the PCL attachment, the following processes were performed, as described in previous studies [18, 30]. First, a sagittal CT image with the clearest and widest PCL tibial attachment site was selected. Second, the CT image’s grayscale value was manually adjusted to display the clearest and widest PCL attachment. Then, the sagittal width of the PCL attachment (proximal-distal) (13 ± 2 mm [range 8 to 16] ) and the length of the tibial facet (27 ± 3 mm [range 20 to 34]) were measured (Fig. 1A). The PCL’s center was defined as the exit center of the PCL tibial tunnel (Fig. 1A). The relative value for the width of the tibial attachment to the length of the tibial facet was 47% ± 6% (range 28% to 64%).

Fig. 1.

Fig. 1

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A-B Measurements that were taken on CT images. (A) The center of the PCL attachment (Point A) was determined on a sagittal-plane image of the knee. L1 is the sagittal width of the PCL attachment; L2 is the length of the tibial facet. The PCL tibial tunnel was simulated using a 10-mm rectangle (red line); the maximum angle was defined as the angle between the tibial plateau line and the tibial tunnel center line; LTT = length of the PCL tibial tunnel; PTT = perpendicular distance of the tunnel’s entry point to the tibial plateau. A color image accompanies the online version of this article.

The PCL tibial tunnel was simulated on 2D CT images using the following steps (Fig. 1B). First, a rectangle was built to simulate the tibial tunnel based on the tibial tunnel’s exit. The width of the rectangle was 10 mm, given that the diameter of the PCL tibial tunnel is usually 10 mm in PCL reconstruction [30]. Second, with a fixed exit center of the tibial tunnel, we adjusted the simulated tibial tunnel so it was near the posterior tibial cortex. Third, the center line of the tibial tunnel was defined as a line connecting two center points of the rectangle. The tibial tunnel’s entry point was defined as the intersection point of the center line of the tibial tunnel and the anterior tibial cortex. The tibial plateau line was defined as a tangential line passing through the anterior and posterior edge of the tibial plateau.

Lastly, we measured the following endpoints: the maximum angle of the PCL tibial tunnel, the length of the PCL tibial tunnel (LTT), and the perpendicular distance of the tunnel’s entry point to the tibial plateau (PTT). The maximum angle of the PCL tibial tunnel was defined as the maximum angle between the tibial plateau line and the center line of the tibial tunnel. The LTT was defined as a line connecting the tibial tunnel’s entry point and the PCL attachment center. The PTT was defined as the perpendicular distance of the tibial tunnel’s entry point to the tibial plateau line (Fig. 1B).

Measurements on 3D Virtual Knee Models

We used the 3D knee models as an in vitro standard method to simulate the PCL reconstruction and assess the true maximum angle of tibial tunnel. After the reconstruction of the 3D knee model, a virtual computer simulation of PCL reconstruction was performed using the Mimics software. Fully simulating the surgical procedures required three key steps, including determining the exit point of the tibial tunnel, creating tibial tunnels of the anteromedial and anterolateral approaches, and making the reference plane for the PCL drill guide. First, to determine the exit point of the tibial tunnel where the guide tip is located, we used the same center point of PCL attachment as determined on the 2D images. This point was automatically presented on the 3D knee models with the Mimics software. Second, to simulate the anteromedial and anterolateral approaches on the 3D knee models, the tibial tunnel’s entrance was placed either 1.5 cm medial or 1.5 cm lateral to the tibial tubercle, respectively. Then, a cylinder with 5-mm radius was created to obtain a 10-mm PCL tibial tunnel. To obtain a maximum tibial tunnel angle, successive 2D CT images were used to assess the tibial tunnel position, which allowed us to make the posterior wall of the tunnel as close as possible (approximately within 1 mm) to the tibial cortex (Fig. 2A). The 3D knee model was also monitored to ensure that no fracture occurred at the posterior tibial cortex (Fig. 2 B-C). Third, to make the reference plane for the PCL drill guide, we selected the medial tibial plateau as the reference plane for outcome measurements. This is in accordance with PCL reconstruction, since the medial tibial plateau was more easily visualized than the lateral tibial plateau during PCL reconstruction. We created the plane of the medial tibial plateau using three points: the most medial point on the axial view of the knee and the peak anterior and posterior points, which passed through the center of the medial tibial plateau on an axial view (Fig. 3) [29].

Fig. 2.

Fig. 2

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A-C The tibial tunnel was simulated on a 3D knee model. (A) The outline of PCL tibial tunnel was monitored on continuous sagittal planes (a-d) to ensure no breakage in the tibial posterior cortex. (B) PCL tibial tunnel (red region) on a 3D knee model (anterior view), and (C) PCL tibial tunnel (red region) on a 3D knee model (posterior view). A color image accompanies the online version of this article.

Fig. 3.

Fig. 3

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A-B The plane of the medial tibial plateau was established. (A) Three points were created on the 3D knee model: (a) the most medial point of the tibial plateau on an axial view, the (b) peak anterior and (c) posterior points on a sagittal view of the knee passing, and (d) the center of the medial tibial plateau. (B) The plane of the medial tibial plateau was established by connecting the three points in Fig. 3A. A color image accompanies the online version of this article.

Lastly, we created a plane perpendicular to the plane of the medial tibial plateau that passed through the center line of the tibial tunnel, and the intersecting line of these two planes was defined as the reference line (Fig. 4A). The maximum angle was measured between the center line of the tibial tunnel and the reference line (Fig. 4B). LTT was measured from the tibial tunnel’s entry point to the PCL attachment center. PTT was defined as the perpendicular distance from the tibial tunnel’s entry point to the reference line.

Fig. 4.

Fig. 4

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A-B Outcome measurements were made on the 3D knee models. (A) A plane perpendicular to the medial tibial plateau plane was created. Two planes were intersected on the reference line (green line). (B) The maximum angle was the angle between the reference line and the center line of the tibial tunnel; LTT = length of the PCL tibial tunnel; PTT = perpendicular distance of the tunnel’s entry point to the tibial plateau. A color image accompanies the online version of this article.

Measurements on 2D Virtual Radiographs

Based on 3D knee models, we created a virtual true lateral knee radiograph (sufficient superimposition of the distal medial and lateral femoral condyles) by rotating the 3D knee, as described by Ishikawa et al. [10]. We modified the transparency of the 3D knee model to 50% to better observe the position of the tibial tunnel (Fig. 2A). The tibial tunnel exit point was the PCL center point, which was determined on 3D knee models. With the fixed tunnel exit, a 10-mm tibial tunnel was adjusted so it was near the posterior cortex, which was aligned with the tibia. Lastly, the maximum angle, LTT, and PTT were measured on 2D virtual radiographs of the knee (Fig. 5). The maximum angle was the angle between the tibial plateau line and the tibial tunnel center line. LTT was the length between the tibial tunnel’s entry point and the PCL center point. PTT was the perpendicular distance of the tibial tunnel’s entry point to the tibial plateau line (Fig. 5).

Fig. 5.

Fig. 5

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Outcome measurements were made on virtual radiographs. The 3D knee model was rotated to create a true knee radiograph. The maximum angle of the tibial tunnel was the angle between the tibial plateau line and the center line of the tibial tunnel; LTT = length of the PCL tibial tunnel; PTT = perpendicular distance of the tunnel’s entry point to the tibial plateau. A color image accompanies the online version of this article.

The measurement reliability was evaluated using intraclass correlation coefficients (ICCs): an ICC less than 0.40 suggested poor reliability, an ICC between 0.40 and 0.75 suggested fair to good reliability, and an ICC greater than 0.75 suggested excellent reliability. One orthopaedic surgeon (YT) performed measurements twice with an interval of 4 weeks to assess the intraobserver reproducibility of the measurements, and another examiner (GJ) also performed measurements to access the interobserver reproducibility. Regarding the maximum tibial tunnel angle, the mean ICC values were 0.89, 0.95, 0.91, and 0.92 for intraobserver reliability, and 0.87, 0.90, 0.93, and 0.96 for interobserver reliability of measurements on virtual radiographs, CT images, and anteromedial and anterolateral knee models, respectively. Regarding LTT and PTT, the mean ICC value ranged from 0.89 to 0.94, indicating satisfactory intraobserver and interobserver reliabilities.

Ethical Approval

This study was reviewed and approved by our institutional review board.

Statistical Analysis

All data are expressed as means ± SDs, with the exception of patient age, weight, and BMI (expressed as median and range). The sample size was calculated using G*Power 3.1.9 (Heinrich Heine University) based on preliminary data (the maximum angle of the PCL tibial tunnel) collected for this study. Eighty-eight knees were needed in this study to obtain a power of 0.90 (effect size = 0.42; α = 0.05). Two board-certified orthopaedic surgeons (YT, GJ) were trained to perform standard measurements of outcomes. One-way ANOVA was used to compare differences among groups. The mean difference with 95% confidence intervals for continuous variables were calculated. A correlation analysis was performed to analyze the correlation between the outcomes and anthropomorphic characteristics (age, height, and BMI). For all bivariate correlation analyses, a Pearson correlation analysis was used to evaluate the associations between two normally distributed variables, while a Spearman correlation analysis was used for nonnormally distributed variables (nonnormally distributed variables: age, weight, and BMI). A correlation coefficient of 0.1 to 0.3 was considered to represent a weak correlation, 0.3 to 0.7 was considered to represent a moderate correlation, and 0.7 to 1.0 was considered to indicate a strong correlation. A subgroup analysis was conducted to compare the differences of outcomes between males and females. The statistical analysis was performed using SPSS software (version 22.0, SPSS Inc). A p value less than 0.05 was considered statistically significant.

Results

Maximum Angle, LTT, and PTT in the Anteromedial and Anterolateral Groups

The maximum angle of the PCL tibial tunnel relative to the tibial plateau was greater in the anteromedial group than the anterolateral group (58° ± 8° versus 50° ± 8°, mean difference 8° [95% CI 6° to 10°]; p < 0.001). The anteromedial group showed longer LTT (67 ± 9 mm versus 60 ± 8 mm, mean difference 7 [95% CI 5 to 10); p < 0.001) and PTT (60 ± 9 mm versus 51 ± 10 mm, mean difference 9 [95% CI 7 to 12); p < 0.001) than the anterolateral group (Table 2).

Table 2.

Measurements in the groups with virtual radiographs, CT images, 3D-AM, and 3D-AL

Parameter Maximum angle in degrees LTT in mm PTT in mm
Radiograph group 68 ± 6 87 ± 10 80 ± 11
CT image group 49 ± 5 66 ± 10 62 ± 9
AM approach group 58 ± 8 67 ± 9 60 ± 9
AL approach group 50 ± 8 60 ± 8 51 ± 10
Comparison between groups Mean difference (95% CI), maximum angle in degrees p value Mean difference (95% CI), LTT in mm p value Mean difference (95% CI), PTT in mm p value
Radiograph versus CT image 19 (17 to 21) < 0.001 21 (18 to 23) < 0.001 18 (16 to 21) < 0.001
Radiograph versus AM approach 10 (8 to 12) < 0.001 20 (17 to 22) < 0.001 20 (17 to 22) < 0.001
Radiograph versus AL approach 18 (16 to 20) < 0.001 27 (24 to 29) < 0.001 29 (26 to 31) < 0.001
CT image versus AM approach -9 (-11 to -6) < 0.001 -1 (-2 to 3) 0.54 2 (-1 to 4) 0.31
CT image versus AL approach -1 (-4 to 1) 0.79 6 (4 to 9) < 0.001 11 (8 to 14) < 0.001
AM approach versus AL approach 8 (6 to 10) < 0.001 7 (5 to 10) < 0.001 9 (7 to 12) < 0.001
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Data presented as mean ± SD and mean difference (95% CI); AM = anteromedial; AL = anterolateral; LTT = length of the PCL tibial tunnel; PTT = perpendicular distance of the tunnel entry point to the tibial plateau.

Differences in Measurement Methods for the Maximum Angle

The maximum angle of the PCL tibial tunnel was greater on virtual radiographs than CT images (68° ± 6° versus 49° ± 5°, mean difference 19° [95% CI 17° to 21°]; p < 0.001), the anteromedial approach (68° ± 6° versus 58° ± 8°, mean difference 10° [95% CI 8° to 12°]; p < 0.001), and the anterolateral approach (68° ± 6° versus 50° ± 8°, mean difference 18° [95% CI 16° to 20°]; p < 0.001) in 3D knee models, but we found no difference between the CT image and the anterolateral approach (49° ± 5° versus 50° ± 8°, mean difference -1° [95% CI -4° to 1°]; p = 0.79) (Table 2).

The mean values of LTT and PTT were larger on virtual radiographs than CT images (LTT: 87 ± 10 mm versus 66 ± 10 mm, mean difference 21 [95% CI 18 to 23]; p < 0.001; PTT: 80 ± 11 mm versus 62 ± 9 mm, mean difference 18 [95% CI 16 to 21]; p < 0.001), the anteromedial approach (LTT: 87 ± 10 mm versus 67 ± 9 mm, mean difference 20 [95% CI 17 to 22]; p < 0.001; PTT: 80 ± 11 mm versus 60 ± 9 mm, mean difference 20 [95% CI 17 to 22]; p < 0.001), and the anterolateral approach (LTT: 87 ± 10 mm versus 60 ± 8 mm, mean difference 27 [95% CI 24 to 29]; p < 0.001; PTT: 80 ± 11 mm versus 51 ± 10 mm, mean difference 29 [95% CI 26 to 31]; p < 0.001) in 3D knee models, but we found no difference between the CT image and the anteromedial approach in LTT (66 ± 10 mm versus 67 ± 9 mm, mean difference -1 [95% CI -2 to 3]; p = 0.54) and PTT (62 ± 9 mm versus 60 ± 9 mm, mean difference 2 [95% CI -1 to 4]; p = 0.31) (Table 2).

Anthropomorphic Factors

Correlation analysis (Table 3) and subgroup analysis (Table 4) showed that the maximum angle of the tibial tunnel had no associations with age, sex, height, or BMI. However, a taller patient had a modest correlation with longer LTT and PTT in virtual radiographs (LTT: correlation coefficient = 0.36; p < 0.001; PTT: correlation coefficient = 0.32; p < 0.001), CT images (LTT: correlation coefficient 0.33; p < 0.001; PTT: correlation coefficient 0.35; p < 0.001), the anteromedial approach (LTT: correlation coefficient 0.39; p < 0.001; PTT: correlation coefficient 0.34; p < 0.001), and anterolateral approach (LTT: correlation coefficient 0.31; p < 0.001; PTT: correlation coefficient 0.37; p < 0.001) in 3D knee models. Subgroup analysis showed that the mean LTT and PTT were longer in males than females in the anteromedial approach (LTT: 70 ± 8 mm versus 65 ± 9 mm, mean difference 6 [95% CI 2 to 9]; p < 0.001; PTT: 63 ± 9 mm versus 59 ± 8 mm, mean difference 4 [95% CI 1 to 8]; p = 0.02) and the anterolateral approach (LTT: 62 ± 9 mm versus 58 ± 7 mm, mean difference 4 [95% CI 1 to 7]; p = 0.02; PTT: 55 ± 10 mm versus 48 ± 9 mm, mean difference 6 [95% CI 3 to 10]; p < 0.001).

Table 3.

Correlation analysis between outcomes and patient characteristics

Parameter Age Height BMI
r valuea p value r valuea p value r valuea p value
Radiograph: maximum angle -0.01 0.93 -0.01 0.96 -0.12 0.23
Radiograph: LTT -0.04 0.72 0.36 < 0.001 0.04 0.72
Radiograph: PTT -0.03 0.74 0.32 < 0.001 -0.01 0.92
CT image: maximum angle 0.08 0.40 0.02 0.85 -0.04 0.66
CT image: LTT -0.14 0.17 0.33 < 0.001 -0.09 0.34
CT image: PTT -0.18 0.07 0.35 < 0.001 -0.14 0.17
AM approach: maximum angle -0.06 0.53 0.09 0.37 -0.15 0.14
AM approach: LTT -0.15 0.12 0.39 < 0.001 -0.12 0.24
AM approach: PTT -0.16 0.10 0.34 < 0.001 -0.14 0.15
AL approach: Maximum angle -0.12 0.25 0.16 0.10 -0.17 0.08
AL approach: LTT -0.09 0.37 0.31 < 0.001 0.16 0.10
AL approach: PTT -0.12 0.24 0.37 < 0.001 -0.18 0.07
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a

r value = Pearson or Spearman correlation coefficients; AM = anteromedial; AL = anterolateral; LTT = length of the PCL tibial tunnel; PTT = perpendicular distance of the tunnel entry point to the tibial plateau.

Table 4.

Subgroups based on sex

Parameter Male (n = 41) Female (n = 63) Mean difference (95% CI) p valuea
Radiograph: maximum angle 68 ± 6 69 ± 7 -1 (-3 to 2) 0.78
Radiograph: LTT 89 ± 9 85 ± 10 5 (1 to 9) 0.01
Radiograph: PTT 82 ± 10 78 ± 11 4 (0 to 8) 0.07
CT image: maximum angle 49 ± 5 50 ± 6 -1 (-3 to 1) 0.49
CT image: LTT 68 ± 9 65 ± 10 3 (-1 to 7) 0.11
CT image: PTT 64 ± 9 60 ± 10 3 (0 to 7) 0.08
AM approach: maximum angle 58 ± 9 58 ± 7 0 (-3 to 3) 0.90
AM approach: LTT 70 ± 8 65 ± 9 6 (2 to 9) < 0.001
AM approach: PTT 63 ± 9 59 ± 8 4 (1 to 8) 0.02
AL approach: maximum angle 51 ± 8 50 ± 8 1 (-2 to 5) 0.34
AL approach: LTT 62 ± 9 58 ± 7 4 (1 to 7) 0.02
AL approach: PTT 55 ± 10 48 ± 9 6 (3 to 10) < 0.001
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Data presented as the mean ± SD.

a

p value: male versus female; AM = anteromedial; AL = anterolateral; LTT = length of the PCL tibial tunnel; PTT = perpendicular distance of the tunnel entry point to the tibial plateau.

Discussion

Studies have demonstrated that residual posterior laxity and graft failure after PCL reconstruction is associated with the killer turn effect, which is caused by the sharp edge of the tibial tunnel exit [5, 9, 20, 33]. In the transtibial PCL technique, surgeons have made efforts to relieve the killer turn by maximizing the tibial tunnel angle [30, 34]. However, little is known about the maximum tibial tunnel angle for transtibial PCL reconstruction. To our knowledge, this is the first study to use a 3D knee model to simulate the guide pin angle in PCL reconstruction. The principal finding of our study was that the mean maximum angle of PCL tibial tunnel relative to the tibial plateau is greater in the anteromedial than anterolateral approach in PCL reconstruction. We believe that this is an important finding for surgeons, especially for those who use the anterolateral approach, to note this difference to select a maximum but safe angle during surgery. In addition, the maximum tibial tunnel angle was overestimated on virtual radiographs and underestimated on CT images. Surgeons should reduce the maximum angle planned by intraoperative fluoroscopy to prevent the tibial tunnel fracture.

Limitations

Several limitations to this study should be addressed. First, many patients with PCL injuries do not have CT scans; it would be beneficial to use MRIs for preoperative planning. However, MRIs were unavailable to produce accurate 3D knee models. We used CT images to create virtual radiographs, which do not present the shadow of soft tissue surrounding the knee, making them somewhat different from actual plain radiographs. However, virtual radiographs have been confirmed to be a valid alternative to plain radiographs [3, 10]. Second, this was a single-center study, and the included population is of Chinese Han descent, which may not be representative of other populations globally. Consequently, this may limit the reference value of our findings for patients of other races. Third, the width of the PCL attachment was measured on CT images, which might raise concern regarding the accuracy of this technique. To accurately determine the PCL attachment’s center, the grayscale value of the CT image was manually adjusted. It has been reported that the tibial attachment width in MRI or cadaveric studies varied from 10 to 15 mm, and the relative value of the tibial attachment width to the tibial facet length varied from 43% to 49% [22, 24, 28]. In our study, the mean value of PCL tibial attachment width was 13 mm and the relative value was 47%, which are consistent with these published studies. Fourth, we excluded patients who had PCL injuries, which might be a potential limitation of our study because there is a concern that patients prone to PCL tears are associated with a decreased posterior tibial slope [6]. Thus, it may have been more representative to include patients with PCL injuries in our study. Lastly, our findings were based on radiographs, CT images, and 3D knee models, but we did not consider soft tissues in clinical practice. Future clinical or cadaveric studies are needed to validate our findings. In addition, further biomechanical studies are also needed to verify whether PCL tibial tunnels with the maximum angle have superior cyclic and load-to-failure properties to those with a smaller angle.

Maximum Angle in the Anteromedial and Anterolateral Groups

The maximum angle of the tibial tunnel was about 8° larger in the anteromedial than the anterolateral approach. This finding is potentially important to surgeons because it adds to our existing knowledge that the PCL drill system should be set differently when PCL reconstruction is performed using different approaches. We also noticed a happy coincidence during data processing. We named this as “three sixties” and “three fifties” for PCL reconstruction. The term three sixties means that if surgeons use the anteromedial approach, they can set the PCL tibial guide to about 60°, limit the tunnel length to a conservative but safe length of about 60 mm, and the starting point of the guide pin should be about 60 mm below the joint line. A similar explanation can be used to describe the three fifties when using the anterolateral approach. We believe this information could potentially be memorable and practical to surgeons. However, several points should be noted while using this information. Firstly, three sixties and three fifties are only the approximate mean values of these outcomes, whereas each patient evaluated had their individual values for the maximum angle, LTT, and PTT. Secondly, these two terms are based on an assumption that the guide pin starts 1.5 cm medial or lateral to the tubercle; the tibial tunnel exit is at the center of PCL attachment, and the PCL guide arm is parallel to the medial tibial plateau plane. In clinical practice, the PCL drill guides are sometimes difficult to match with the tibial plateau because of the limitations from the ACL, the interposed soft tissue of the portals, and the fat pad, which might affect the accuracy of the maximum angle. Therefore, more clinical studies are needed to validate our findings. Currently, there is no consensus regarding the most appropriate approach for PCL reconstruction. Previous studies used the anterolateral approach to reduce the killer turn on the coronal plane [8, 10]. In our study, we increased the tunnel angle to reduce the killer turn on the sagittal plane. However, a few surgeons realized that the mean maximum angle of PCL tibial tunnel was less in the anterolateral (50°) than anteromedial (58°) approach. Based on our findings, it seems that by increasing the tibial tunnel angle, the anteromedial approach may have less killer turn effect than the anterolateral approach. However, this hypothesis has not been verified, and future biomechanical studies may focus on this topic.

Differences in Measurement Methods for the Maximum Angle

The mean maximum angle of the tibial tunnel was nearly 20° greater when measured on radiographs than on CT images. We believe radiographs overestimate and CT images underestimate the actual angle because 3D CT knee models provide a standard simulation and measurement for the maximum angle of tibial tunnel in vitro [11, 19, 31]. During transtibial PCL reconstruction, intraoperative fluoroscopy is frequently used to obtain the maximum tibial tunnel angle [18, 30]. However, it is difficult to recognize the accurate tibial tunnel position on 2D fluoroscopic images because the posterior aspects of the medial and lateral tibial condyles overlap [18]. We found that the maximum angle of intraoperative fluoroscopy should be reduced approximately 10° for the anteromedial approach and 18° for the anterolateral approach. Surgeons can use this information to perform this surgery more safely. Some surgeons have also used CT images to evaluate the maximum tibial tunnel angle preoperatively. Lee et al. [18] performed a cadaveric study using 10 fresh tibias to assess the maximum angle of the PCL tibial tunnel, and they found that the mean maximum angle was 52° on CT. In a large-population study including 408 adult knees, the permissible maximum angle was measured, and the mean maximum angle was 48° when a 10-mm-diameter tibial tunnel was simulated using a 2D CT image [30]. Our study showed the maximum angle was 49° on CT images, which is similar to the results of the above studies [18, 30]. However, a 2D CT scanning image has its limitation for preoperative planning of the PCL tibial tunnel. That is, the tunnel entrance cannot be adjusted from medial to lateral via a 2D image. If the PCL tibial tunnel is planned on a 2D sagittal CT image, the tunnel entrance is estimated to be approximately on the tibial crest. This is not in accordance with clinical practice because few surgeons use the tibial crest approach for PCL reconstruction. Our findings showed that the maximum angle planned on CT images was smaller than the anteromedial approach, but it was similar to the anterolateral approach. This might be due to the special anatomical shape of the proximal tibia [1, 23]. Therefore, surgeons should be aware that a tibial tunnel angle planned on CT images may not represent the maximum angle used for the anteromedial approach in PCL reconstruction.

Anthropomorphic Factors

We did not find many important associations between anthropomorphic factors and measurements, other than—as expected—the LTT and the PTT being longer in males than females and in taller people. This information is nonetheless important because the LTT and the PTT were important reference data for placing the tibial tunnel in PCL reconstruction. The LTT contributed to limiting the depth of the reamer during surgery. Our study indicates that the mean maximum depth of the reamer should be limited to 67 mm for the anteromedial approach and 60 mm for the anterolateral approach. This is particularly important given that an excessive reamer depth might cause iatrogenic injury to popliteal neurovascular bundles [2, 13]. The PTT is a useful reference value for determining the accurate position of the tunnel’s entry point. Based on our results, surgeons should be aware that the safe entry point of the anteromedial approach was lower than that of the anterolateral approach. We also found no differences regarding the outcomes of the LTT and the PTT between CT images and the anteromedial approach. This did not conflict with the results of a greater maximum angle in the anteromedial approach because the proximal tibia viewed in a cross plane is triangular [23]. If the PCL tibial tunnel is planned on CT images, the tunnel starting point is approximately on the tibial crest, which would prolong the tunnel lengths. In our study, the LTT and the PTT should be interpreted with caution, as they were based on the 3D bone models and did not consider soft tissues in clinical practice. Therefore, further clinical studies are needed to validate our findings.

Conclusion

Surgeons should note that, in this computer simulation of PCL reconstruction using 3D knee models, the maximum angle of the tibial tunnel relative to the tibial plateau was greater in the anteromedial than anterolateral approach in PCL reconstruction, and the maximum tibial tunnel angle might be overestimated on radiographs and underestimated on CT images. Our findings suggest that to perform PCL reconstruction more safely, the PCL drill system should be set differently for the anteromedial and anterolateral approaches, and the maximum angle measured by intraoperative fluoroscopy should be reduced 10° for the anteromedial approach and 18° for the anterolateral approach. Further clinical or cadaveric studies are needed to validate our findings and to verify whether PCL tibial tunnels with the maximum angle are superior to those with a smaller angle.

Acknowledgment

We thank Dr. Meng Wu for his role in gathering clinical information and assisting with drafting the manuscript revision.

Footnotes

The institution of one or more of the authors (YT, LD, GJ, JH, ZL, SZ, HH, YX) have received, during the study period, funding from the National Natural Science Foundation of China (82060413), the Fundamental Research Funds for the Central Universities (lzujbky-2021-kb30), the Science and Technology Project of Chengguan District/Lanzhou City (2020JSCX0071), the Project of TCM Inheritance and Innovation Platform Construction (TCM-IPC-2020-04), and the Cuiying Science and Technology Innovation Project of Lanzhou University Second Hospital (CY2019-QN02).

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.

Ethical approval for this study was obtained from the Lanzhou University Second Hospital, Lanzhou, People’s Republic of China (number D2020-29).

Contributor Information

Yuanjun Teng, Email: tengyj06@126.com.

Lijun Da, Email: 15117258336@126.com.

Gengxin Jia, Email: 1198367383@qq.com.

Jie Hu, Email: dada111087@126.com.

Zhongcheng Liu, Email: liuzhch_14@163.com.

Shifeng Zhang, Email: zsf199171@163.com.

Hua Han, Email: 674125998@qq.com.

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