1 Introduction

A fibrous nodule that occurs in the intercondylar notch after an anterior cruciate ligament injury or reconstruction is called a cyclops lesion, and when it is accompanied by limited knee extension, the condition is called cyclops syndrome. The incidence of cyclops lesions ranges between 14.5 and 35.4 % on arthroscopic viewing^1–6 and 33.0–46.8 % on magnetic resonance imaging (MRI) scans^7,8 after anterior cruciate ligament (ACL) reconstruction. The incidence of symptomatic cyclops syndrome (extension loss >5°) ranged from 0.9 to 3.6 % after double-bundle ACL reconstruction.^5,9 Multiple factors such as age, sex,^5,10 body mass index (BMI),⁸ and bone tunnel position¹¹ are thought to be related to cyclops lesions or syndrome; however, this remains an unsolved problem despite advances in ACL reconstruction techniques. Bone tunnel position is considered one factor in cyclops lesion formation; however, there are contradicting opinions.^2,5 Particularly, the relationships between the bone tunnel position, cyclops lesion, and extension loss in double-bundle ACL reconstruction remain unclear. Furthermore, although quadriceps stun has been implicated in cyclops syndrome,⁹ there have been few reports on the relationships between cyclops lesions/syndromes, knee extension, and flexion strength. Our hypothesis is that an anteriorly located tibial bone tunnel or a highly located femoral bone tunnel is more likely to cause a cyclops lesion and limitation of extension, and that knee extensor strength is more likely to be reduced in the presence of cyclops lesions. The aim of this study was to investigate background factors and bone tunnel location related to cyclops lesions and knee extension loss after anterior cruciate ligament double-bundle reconstruction and the relationship between cyclops lesions and postoperative muscle strength changes.

2 Materials and methods

3 Results

In total, this study included 744 patients who underwent ACL reconstruction, which was performed using the knee flexor tendon (semitendinosus tendon or semitendinosus and gracilis tendon). Of the 744 cases with a reconstructed knee flexor tendon, 11 had complex ligament injuries, 27 had meniscal locking, 5 had revision, 72 had bilateral injuries, 72 had previous fractures or surgery in the lower limb, 23 patients could not provide consent, 39 patients had difficulty coming to the hospital because of relocation, 5 patients had to change their schedule because of a change in the surgical technique, and 4 had mechanical trouble or other problems. In addition, 231 had no HHD measurements at 12 months, 27 had no postoperative CT imaging, 30 had single bundle reconstruction, and 6 had no MRI imaging or arthroscopy. Thus, 192 patients (101 male and 91 female patients, mean age of 27.9 years [range, 13–70 years]) who underwent MRI or second-look arthroscopy at least one year after reconstruction were included in this study. There were 30 patients with cyclops lesions and 162 patients without cyclops lesions, with an incidence of 18.5 %. Fourteen patients had cyclops syndrome with HHD >5 mm, with an incidence of 7.3 %. The two groups showed a significant age difference (no-cyclops lesion: 28.7 ± 11.8 years and cyclops lesion: 23.3 ± 10.2 years, p = 0.02) (Table 1). After one year, the HHD was significantly higher and extension strength was significantly lower in the cyclops group than in the no-cyclops group (p = 0.004).

The intraclass correlation coefficients (ICCs) of this measurement method were preliminarily verified; ICC (1,1) was 0.969 for %h, 0.940 for %d, 0.986 for %ap, and 0.916 for %ml, whereas ICC (2,1) was 0.877 for %h, 0.961 for %d, 0.986 for %ap, and 0.899 for %ml.

Logistic regression analysis for each bundle regarding the presence of a cyclops lesion as the dependent variable and the position of the bone tunnel and graft size as independent variables revealed significant differences in %d PL (no-cyclops lesion: 29.7 ± 5.4 % and cyclops lesion: 31.8 ± 5.2 %, p = 0.03) and PL graft size (no-cyclops lesion: 3.3 ± 0.2 % and cyclops lesion: 3.5 ± 0.3 %, p = 0.01). (Fig. 1, Table 2). Multiple linear regression analysis for each bundle with HHD as the dependent variable and bone tunnel position and graft size as independent variables within the cyclops group showed that %h and %ap AM (B = −0.15, p = 0.01; B = 0.16, p = 0.048) were significantly associated with HHD (Table 3). Within the no-cyclops group, %ap PL was significantly associated with HHD (B = −0.08, p = 0.01). The AM bundle tended to be located anteriorly; however, this was not significant (p = 0.073) (Table 4). The measured values of knee extension and flexion muscle strength and changes over time are shown in Table 5 and Figs. 2 and 3, respectively. Analysis using linear mixed models with repeated effects showed an interaction between time and cyclops lesions in knee extension muscle strength (p = 0.035), with a main effect of time only (p < 0.0001) but no main effect of cyclops lesions (p = 0.060).

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Fig. 1 Mapping of the bone tunnels.

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Fig. 2 Changes in knee extension strength after anterior cruciate ligament reconstruction with and without cyclops. CI, confidence interval.

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Fig. 3 Changes in knee flexion strength after anterior cruciate ligament reconstruction with and without cyclops. CI, confidence interval.

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When comparing the difference in knee extension strength between the cyclops and no-cyclops groups, there was no significant difference between the two groups preoperatively (p = 0.73), and the postoperative difference at four months (p = 0.042), 6 months (p = 0.0094), 9 months (p = 0.035), and 12 months (p = 0.0039) were significantly larger. In other words, postoperative muscle strength changes in knee extension differed depending on the presence or absence of a cyclops lesion. Postoperative strength loss was greater in the group with cyclops than in the group without cyclops.

For knee flexion strength, there was no interaction between time and cyclops lesions (p = 0.86), no main effect of cyclops lesions (p = 0.10), but only a main effect of time (p < 0.0001). Comparing the differences, we found no significant difference between the two groups preoperatively (p = 0.24), and this difference did not significantly change at 4 months (p = 0.55), six months (p = 0.98), 9 months (p = 0.85), and 12 months (p = 0.61). In other words, the postoperative change in knee flexion muscle strength was not affected by the cyclops lesions.

4 Discussion

The most important findings of the present study were the analysis of risk factors for cyclops lesions, which included a younger age, large graft size relative to height, and a shallow location of the PL bone tunnel; and that extension loss was more likely with a higher AM bundle bone tunnel position. Risk factors for cyclops lesions and syndromes reported to date include: being a young woman,^5,10 BMI,⁸ anterior placement of the tibial bone tunnel,¹¹ narrow intercondylar size,⁵ limited immediate postoperative active knee extension and quad muscle stunning,⁹ hamstring spasm,¹³ >4 weeks from injury to reconstruction,¹⁰ and remnant.^14,15 In our study, we were unable to examine all previously reported factors; however, among these, we found no differences based on sex. Moreover, cyclops lesion was predominant in younger patients than in older patients. Surgery-related factors, such as a large graft size relative to the height and the slightly shallow location of the PL bone tunnel, were associated with the development of cyclops lesions. Furthermore, in the presence of cyclops lesions, the higher the bone tunnel position of the AM bundle, the closer it was to the roof and the more likely it was to be restricted in extension.

The incidence of cyclops lesions is not related to the position of the AM bundle and is higher when the PL bundle is located at a shallow position along the Blumensatz line; the bigger the PL bundle, the higher the incidence. The PL bundle is positioned posterior to the AM bundle in extension; however, as the knee flexes, the PL bundle moves forward such that the femoral side wraps around the AM bundle.¹⁶ The normal PL bundle is at maximum tension during extension; however, as the knee flexes, the tension decreases and then increases above 90°. Previous reports have shown that when the femoral foramen is shallow, the graft tension tends to be higher during flexion.¹⁷ In other words, it is assumed that the femoral bone tunnel of the PL bundle is shallow, which causes the graft tension to begin to increase early during knee flexion, and that the graft tension is tighter during deep flexion than when it is made deep. Wall impingement of the PL bundle has also been reported to be particularly likely to occur during flexion.¹⁸ Thicker reconstructed ligaments may cause the PL bundle to impinge on the wall or anterior tissues, such as the Hoffa, during flexion, or the excessively tensioned PL bundle may push the AM bundle forward during full flexion, resulting in injury and tissue hyperplasia due to impingement. Based on the results of this study and the literature review, we cannot rule out the possibility that, contrary to our hypothesis, a mechanism related to flexion rather than extension is primarily responsible for the formation of cyclops lesions. However, this mechanism is speculative, and biomechanical investigations using cadavers are required.

A sub-analysis of the positional relationship of the bone tunnel leading to extension limitation in the presence of a cyclops lesion was also performed. In this case, the position of the bone tunnel of the AM bundle was found to play a significant role. The higher the position of the AM bundle's femoral bone tunnel, or the closer it was to the roof, the more likely it was to cause extension limitation. In other words, the closer it was to the so-called non-anatomic reconstruction position, the greater the likelihood of extension limitation. This may be related to the position of the graft, which makes it more prone to graft impingement.¹⁹ Normally, the AM bundle is considered to have lost tension at 0–30°. If the femoral bone tunnel is close to the intercondylar notch, the cyclops lesion may become trapped in the intercondylar notch during extension, limiting knee extension. The posterior location of the tibial bone tunnel in AM also contributes to the appearance of extension limitation, a result that contradicts a previous report¹¹ stating that the tibial bone tunnel is at a more anterior position in cyclops syndrome, where extension limitation is more probable. Historically, when non-anatomic reconstruction of the femoral bone tunnel, aiming for over-the-top, was performed, the tibial bone tunnel was moved posteriorly to avoid anterior impingement; however, the current mainstream anatomic reconstruction is aimed at the anatomical ligament position or the slightly anterior position. Shifting the tibial bone tunnel to the posterior location may create a space anterior to the reconstructed ligament, which may provide space for the cyclops lesion to grow. In contrast, in the present study, a sub-analysis of cases without cyclops lesions also showed a trend towards higher HHD when the tibial bone tunnel was significantly more anteriorly positioned at the PL bundle and, although not significantly different, when the AM bundle was more anteriorly positioned. Although the anterior bone tunnel position may contribute to generalized extension limitations, it appears to be different in cases of cyclops lesions and syndromes.

The degree of improvement in postoperative extension strength differed depending on the presence or absence of a cyclops lesion. However, the cyclops lesion group showed a significantly greater decline in knee extension strength in the first four months than the no-cyclops group. Because the present study did not involve MRI at an early postoperative time point, the causal relationship remains unclear. As previously reported, a decline in knee extension muscle strength, such as AMI, can cause cyclops,²⁰ and the results of the present study may support this. Although muscle strength was significantly reduced in the early postoperative period, the differences did not resolve, although they showed a similar trend of improvement thereafter. It will be interesting to see if the proposed treatment for AMI²¹ will change the incidence of cyclops lesions and syndromes, and future studies regarding this are warranted.

This study suggests that cyclops lesions and syndromes may be influenced by the non-anatomic bone tunnel location and are more likely to occur with prominent postoperative knee extension muscle weakness, which is associated with delayed muscle recovery at one year postoperatively. Efforts should be made to reduce the incidence of cyclops lesion syndrome because symptoms such as limited range of motion and poor muscle recovery are likely to present significant challenges after anterior cruciate ligament reconstruction and delay a safe return to activities like sports.

The present study has some limitations. As the study was performed up to approximately one year postoperatively, the long-term effects are unknown. However, cyclops lesions have been reported to form within the first 6 months⁸ and occur between 6 and 12 months,⁷ and in most cases, appear to form within 1 year. In the present study, the presence or absence of cyclops lesions was confirmed via imaging or arthroscopy at approximately one year postoperatively, which is considered a sufficient follow-up period for cyclops formation. Previous reports have shown that the removal of cyclops lesions improves knee extension limitation; however, the number of cases showing this is small.⁴ In the present study, we could not confirm whether the removal of the cyclops lesion improved knee extension strength or even resulted in easing restriction.

5 Conclusions

Cyclops lesion formation involved a relatively large PL graft and the PL femoral tunnel was slightly shallow; extension loss was likely to occur when the position of the AM bone tunnels was offset towards the position of “non-anatomical reconstruction.” Cyclops lesions increase the risk of knee extension loss and consequent weakening of knee extension muscle strength for one year after ACL reconstruction; therefore, it is desirable to reduce the incidence of cyclops lesions as much as possible.

CRediT authorship contribution statement

Takafumi Mizuno: Formal analysis, Writing – original draft. Shinya Ishizuka: Methodology, Writing – review & editing. Kazutoshi Kurokouchi: Conceptualization, Project administration. Junichiro Yasui: Investigation, Data curation. Hiroki Oba: Writing – review & editing. Takefumi Sakaguchi: Visualization, Investigation. Shigeo Takahashi: Investigation, Resources. Shiro Imagama: Supervision.

Informed consent

Informed consent was obtained from all participants or from their parents if they were less than <16 years in age.

Ethical approval

This study was approved by the Institutional Review Board and Ethics Committee. All methods were performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments.

Data statement

Due to the nature of this research, participants of this study did not agree for their data to be shared publicly, so supporting data is not available.

Funding

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