1 Introduction

Graft of the anterior cruciate ligament (ACL) is a method of reconstruction involving placement of the ligament in the original anatomical position. However, because the graft cannot be made to completely mimic the original morphology, this surgery comes with a risk of roof and wall impingement of the femur, even after the anatomic double-bundle ACL reconstruction.^1–3 Subsequent graft impingement can cause knee extension loss or graft rupture.⁴

The intercondylar roof line, viewed from lateral view by radiography, is one of the indicators used during ACL reconstruction to see the relation to the position of the tibial tunnel. Tanaka et al.⁵ reported that the chronicity of ACL deficiency had an effect on the tibiofemoral relationship of the sagittal plane in a supine position. The tibial tunnel can be made posteriorly in the anteriorly subluxated tibia. Although avoiding the roof impingement is important, posterior tibial tunnel placement also has a high risk of requiring revision surgery.⁶ During ACL reconstruction, the tibiofemoral relationship of the opposite or normal knee should be considered.

The purpose of this study was to examine the radiographic tibiofemoral relationship of the sagittal plane in a standing position in ACL deficient knees. We hypothesized that the radiological tibiofemoral relationship of an ACL deficient knee in the standing position is different from that of a normal knee.

2 Materials and methods

This study received the approval of the ethics committee of our institution (No. 2020016). From September 2017 to August 2018, a total of 75 patients underwent ACL reconstruction at our institution. Of these, 64 patients were evaluated for inclusion in this study. We excluded patients with (1) extension deficit of the injured knee in physical examination, (2) past history of contralateral knee injury, (3) reinjury after ACL reconstruction, (4) radiologically poor condition (distance of the posterior point of the medial and lateral condyles was more than 10 mm).

Lateral radiographs of the injured and uninjured knee were obtained preoperatively in a standing position. The knee was fully extended with the opposite foot on a step, asking the patients to bear weight fully on one leg (Fig. 1).

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Fig. 1 Positioning for radiography. The knee was fully extended with the opposite foot on a step.

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The radiographs were evaluated using previously described techniques as follows (Fig. 2).1)Anterior tibial subluxation (ATS).^5,7–9 First, a line was drawn on the tibial plateau (P line). Lines were drawn perpendicular to this line at the most posterior aspect of the medial and lateral plateau. The shortest distance from these lines to the most posterior extent of the medial and lateral femoral condyle were measured and averaged. A positive value indicates that the perpendicular line is anterior to the posterior margin of the femoral condyle.2)Space for the ACL (sACL).⁵ Measured as the distance between the most inferior portion of the intercondylar roof and the tip of the tibial eminence.3)Roof-plateau intersection ratio.¹⁰ The points of intersection of the P line and the intercondylar roof line were measured using the Amis and Jacob line⁴ that passes parallel to the P line through the posterior corner of the shelf. The roof-plateau intersection ratio was calculated as a percentage of anteroposterior length. The most anterior point indicates 0% and the most posterior point indicates 100%.4)Roof-plateau angle.¹⁰ Roof-plateau angle was defined by the P line and the intercondylar roof line.

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Fig. 2 Measurements of radiographs. (A) Anterior tibial subluxation (ATS). (A-1) A line was drawn on the tibial plateau (P line). Lines were drawn perpendicular to this line at the most posterior aspect of the medial and lateral plateau. (A-2) The shortest distance from these lines to the most posterior extent of the medial and lateral femoral condyle were measured and averaged. A positive value indicates that the perpendicular line is anterior to the posterior margin of the femoral condyle. (B) Space for the ACL (sACL). Measured as the distance between the most inferior portion of the intercondylar roof and the tip of the tibial eminence. (C) Roof-plateau intersection ratio. The points of intersection of the P line and the intercondylar roof line were measured using the Amis and Jacob line that passes parallel to the P line through the posterior corner of the shelf (the dotted line). The roof-plateau intersection ratio was calculated as a percentage of the anteroposterior length. The most anterior point indicates 0% and the most posterior point indicates 100%. (D) Roof-plateau angle. Roof-plateau angle was defined by the P line and the intercondylar roof line.

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Statistical analyses were performed using GraphPad Prism 6 software (GraphPad Software, Inc, San Diego, CA, USA). The results were analyzed using Mann-Whitney U test to compare two groups. Statistical significance was set at p < 0.05.

3 Results

The patients comprised 36 men and 28 women aged 14–61 (mean, 31.3) years. The average period from injury to evaluation was 33.4 months (5 days–11 years). The results are summarized in Table 1. The mean value of ATS was 1.2 mm in the injured side and −1.6 mm in the uninjured side. The maximum difference was 10.6 mm. The tibia was located in a significantly anterior position in the injured knee (p < 0.0001). The mean distance of the sACL was 9.7 mm in the injured side and 10.7 mm in the uninjured side. The sACL was significantly wider in the injured knee than in the uninjured knee (p < 0.01). The roof-plateau intersection ratio was 28.5% in the injured side and 29.1% in the uninjured side. There was no significant difference between the sides. The roof-plateau angle averaged 63.6° in the injured side and 67.4° in the uninjured side (p < 0.001).

4 Discussion

The main function of the ACL is to restrain the anterior translation of the tibia. The AP translation of the tibia is maximal at approximately 30° of flexion in the normal knee. The results of this study indicate that the tibia of the ACL injured knee was located anteriorly in the standing position with extended knee, and the maximum difference of the sides was 10.7 mm. During ACL reconstruction, the tibiofemoral relationship of the normal knee should be considered not to have made the tibial tunnel posteriorly. Tensho et al.¹¹ reported in a computed tomographic study that there are bony landmarks corresponding to the ACL tibial footprint. In an operation room, however, the tunnel was placed using only X-ray photography or fluoroscopy as a reference12. In the remnant preservation technique,¹³ it is difficult to determine the tunnel placement without radiography. In such cases, the risk of roof impingement after anatomic ACL reconstruction still remains. Therefore, consideration of the tibiofemoral relationship of the normal knee is important for the tibial tunnel placement.

Previous radiographic studies were performed in the supine position,^5,9 where the knee was hyperextended and the tibia was passively forced to the anterior position. Therefore, a standing position seemed to demonstrate the tibiofemoral relationship more physiologically than a supine position. Previous studies reported that ACL deficient knee had larger ATS and sACL values in the supine position,^5,9 which was compatible with our result that was performed in the standing potion.

The incidence of further meniscus and cartilage injury could increase if the torn ACL was not treated surgically. These lesions usually happen after performing sports activities. McDonald et al.¹⁴ reported that chondral lesions and meniscus tears have implications for anterior tibial subluxation. Considering our results that the tibiofemoral relationship of the ACL deficient knee was different from that of a normal knee in the physiological position, the ACL deficient knee might experience secondary lesion while the patient performs activities associated with daily life.

In this study, the roof-plateau angle was measured to reflect the knee extension angle because it is difficult to accurately draw lines along the axis of the femur and tibia. Our results showed that the roof-plateau angle of the injured side was lower than that of the uninjured side. This result suggests that the limitation of extension remains before the surgery even if the knee was deemed to be fully extended in the physical examination. Buzzi et al.¹⁰ reported that the larger the roof-plateau angle, the more posterior the roof-plateau intersection ratio. In our result, a smaller roof-plateau angle might cause no significant difference in the roof-plateau intersection ratio.

There are some limitations to this study. First, intraobserver and interobserver variability were not evaluated; the methods of measurement of this study have been reported in several studies where intraobserver and interobserver variability were performed. The reliability of these measurements was evaluated.^5,9,10 Second, we included patients in whom the distance of the posterior point of the medial and lateral condyles was less than 10 mm. Because radiographic measurements may be confounded by rotation of the knee, the distance is shortest when viewed from a true lateral view. We evaluated patients with distance less than 5 mm and greater than 5 mm, the results were the same as all patients. Haasper et al.¹⁵ reported that 20° of rotation showed only a difference in tibial point of 3.3% compared to the optimal rotational alignment. Therefore, the minor rotation with a distance within 10 mm should only minimally affect the results of this study.

5 Conclusion

The tibiofemoral relationship of the ACL deficient knee was different from that of normal knee in the standing position. The relationship of the normal knee should be considered during ACL reconstruction and the risk of secondary lesions in the ACL deficient knee in activities of daily life should be considered.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.