Knee Surgery, Sports Traumatology, Arthroscopy

, Volume 26, Issue 4, pp 1174–1181 | Cite as

Lateral meniscus posterior root tear contributes to anterolateral rotational instability and meniscus extrusion in anterior cruciate ligament-injured patients

  • Takao Minami
  • Takeshi Muneta
  • Ichiro Sekiya
  • Toshifumi Watanabe
  • Tomoyuki Mochizuki
  • Masafumi Horie
  • Hiroki Katagiri
  • Koji Otabe
  • Toshiyuki Ohara
  • Mai Katakura
  • Hideyuki Koga
Knee

Abstract

Purpose

The purposes of this study were to investigate (1) meniscus status and clinical findings in anterior cruciate ligament (ACL)-injured patients to clarify associations between the meniscus posterior root tear (PRT) and knee instability, and (2) magnetic resonance imaging (MRI) findings of the PRT to clarify sensitivity and specificity of MRI and prevalence of meniscus extrusion.

Methods

Three hundred and seventeen patients with primary ACL reconstruction were included. PRTs for both medial and lateral sides were confirmed by reviewing surgical records. Preoperative MRI was reviewed to evaluate sensitivity and specificity of the PRT and meniscus extrusion width (MEW). Clinical information regarding the number of giving-way episodes, preoperative KT-1000 measurements and preoperative pivot shift was also assessed.

Results

Thirty-nine patients had a lateral meniscus (LM) PRT, whereas only four patients had a medial meniscus PRT. One hundred and seventeen patients had no meniscus tear (control). Twenty-eight patients (71.8%) showed positive signs of the LMPRT based on at least one view of MR images, with the coronal view showing the highest sensitivity. MEW in the LMPRT group was significantly larger than that in the control group. The preoperative pivot shift test grade in the LMPRT group was significantly greater than that in the control group. There were no significant differences in other parameters.

Conclusions

In ACL-injured patients, the LMPRT was associated with ALRI as well as with meniscus extrusion. The coronal view of MRI was useful in identifying the LMPRT, although its sensitivity was not high. Therefore, surgeons should prepare to repair PRTs at the time of ACL reconstruction regardless of MRI findings, and they should make every effort to repair the LMPRT.

Level of evidence

III.

Keywords

Anterior cruciate ligament Meniscus Posterior root tear Extrusion Knee instability 

Introduction

Instability in anterior cruciate ligament (ACL)-injured knees can be described as two types of instabilities, anterior instability and anterolateral rotational instability (ALRI). Anterior instability can be quantitatively evaluated by, for instance, a KT-1000 arthrometer (MEDmetric, San Diego, CA, USA) [8, 21]. On the other hand, residual ALRI as revealed by a positive pivot shift test after ACL reconstruction has been shown to correlate with worsening functional outcomes [2] and patient dissatisfaction [20], as well as with development of osteoarthritis [18], indicating that controlling ALRI is one of the keys to improving outcomes after ACL reconstruction.

However, surgeons still sometimes detect residual instability even after anatomic ACL reconstruction. In such cases, concomitant secondary stabilizer injuries, such as other ligaments, capsular and meniscus injuries, are often identified. Among them, the importance of the posterior root of the meniscus has been particularly recognized. A recent cadaveric study revealed that the posterior root of the lateral meniscus (LM) plays an important role in ALRI in ACL deficient knees [43]. On the other hand, the medial meniscus (MM) acts as a secondary restraint for anterior stability [30], and a biomechanical study revealed the importance of the posterior root of the MM in anterior stability [31]. However, such contributions of the posterior root of the meniscus on knee stability have not been thoroughly investigated in clinical settings.

The posterior root tear (PRT) of the meniscus has also been shown to be associated with meniscus extrusion [1]. Biomechanical studies have shown that meniscus extrusion caused by PRTs leads to increased tibiofemoral contact pressure [11, 12, 26, 32], and it has also been reported that meniscus extrusion is associated with the development of osteoarthritis [3, 14, 27].

As the posterior root of the meniscus plays an important role in knee stability and load distribution, such meniscal lesions associated with ACL reconstructions should be repaired. In order to perform repair of the PRT, surgeons should prepare necessary instruments based on preoperative diagnosis. Magnetic resonance imaging (MRI) has improved the accuracy of diagnosing these lesions; however, its sensitivity still remains controversial [9, 15, 29, 38].

Therefore, the purposes of this study were to investigate 1) meniscus status and clinical findings in ACL-injured patients in order to clarify associations between the meniscus PRT and knee instability in ACL-injured patients, and 2) MRI findings of the PRT to clarify sensitivity and specificity of MRI as well as prevalence of meniscus extrusion. The hypotheses underlying this study were that the LMPRT would be more associated with ALRI in ACL-injured patients, whereas the MMPRT would be more associated with anterior instability. It was also hypothesized that sensitivity and specificity of MRI to the PRT would be high, and the PRT would be associated with meniscus extrusion. This study will help surgeons recognize the importance of the PRT in better restoring knee instability as well as in preventing secondary osteoarthritis after ACL reconstruction.

Materials and methods

Patients who underwent primary ACL reconstruction between 2012 and 2016 were included and retrospectively reviewed. Exclusion criteria included revision surgery, knees with a locked meniscus, concomitant ligament tears, history of injuries in the ipsilateral knee, history of ligamentous injuries in the contralateral knee and knees with osteoarthritis.

Meniscus status evaluations

PRTs for both medial and lateral sides were retrospectively confirmed by reviewing surgical records at the time of primary ACL reconstruction. All surgeries were performed by two experienced attending surgeons. Preoperative magnetic resonance imaging (MRI) was then retrospectively reviewed. Coronal, axial and sagittal planes of the T2 images were obtained from all patients. Positive signs of both MMPRT and LMPRT in the MRI were defined as (1) vertical linear defect in the coronal plane, (2) radial linear defect in the axial plane, and (3) defect of meniscus in the sagittal plane, known as “Ghost sign” [6, 15, 28] (Fig. 1). Sensitivity and specificity of the meniscus PRT in each plane of the MRI were evaluated using arthroscopic findings as the gold standard. In addition, after assessing all coronal slices for each knee, meniscus extrusion was measured on the image showing maximum extrusion. Meniscus extrusion width (MEW), which was defined as the distance from the most peripheral aspect of the meniscus to the border of the tibia, excluding any osteophytes, was measured in 0.1 mm increments [22] (Fig. 2).
Fig. 1

Positive signs of the LMPRT in the MRI. a Vertical linear defect in the coronal plane. b Radial linear defect in the axial plane. c Ghost sign in the sagittal plane

Fig. 2

Extrusion of the lateral meniscus caused by the LMPRT. In this case, 3.0 mm of extrusion is observed

All MRI measurements were taken by 2 orthopaedic surgeons, who were blinded to any other medical records, independently and blindly to each other’s measurement. Analysis of inter-observer reliability yielded an intra-class correlation coefficient (ICC) of 0.82 (95% confidence interval; 0.55–0.93) for positive signs of the PRT and 0.91 (95% confidence interval; 0.70–0.98) for MEW.

Clinical evaluations

Preoperative medical records of all patients were retrospectively reviewed, and information regarding (1) number of giving-way episodes, (2) preoperative anterior instability, and (3) preoperative ALRI revealed by the pivot shift phenomenon was obtained. Preoperative knee instabilities were routinely evaluated under anaesthesia before surgery. A full evaluation was performed by two attending surgeons blindly and independently, and if differences were found between the two examiners, a re-evaluation was performed together until consensus was obtained. Anterior instability was evaluated with the KT-1000 arthrometer (MEDmetric, San Diego, CA) at manual maximum pull, being expressed as the difference between the injured and uninjured legs in 0.5 mm increments. To evaluate the pivot shift phenomenon, we used an N test [36]. Namely, with the patient under anaesthesia lying supine on an operating table, the examined extremity was lifted from the table, and the leg was extended from a position of 90° of flexion to full extension while a valgus and internal rotation moment was applied to the leg. In order to more strictly evaluate the pivot shift phenomenon, the grading system we used was the modification of the International Knee Documentation Committee (IKDC) criteria [16] (Grade 0 = negative; Grade 1 = subtle glide, but not negative; Grade 2 = glide, Grade 3 = between grade 2 and 4; Grade 4 = clunk; Grade 5 = between grade 4 and 6; Grade 6 = gross), which has been shown to have high inter-observer reliability with an ICC of 0.97 (95% confidence interval; 0.94–0.98) [23].

This study was approved by the institutional Review Board in Tokyo Medical and Dental University (research protocol identification number: 1146), and all patients provided informed written consent.

Statistical analysis

Statistical analysis was performed using the SPSS 21.0 software package (SPSS, Chicago, IL, USA). A Kruskal–Wallis test was used to compare the differences among the three groups, and an additional Steel–Dwass test was used to compare the differences between each group. A Mann–Whitney U test was used to compare the differences between the control and LMPRT groups based on the MRI and clinical findings. Case 2 intra-class correlation coefficients were used to evaluate the inter-observer reliability. All data were reported as the mean ± standard deviation if not described particularly. For all analyses, statistical significance was set at P < 0.05. Post hoc power analysis revealed that, with an alpha of 0.05, a power of 0.98 was achieved for the differences between the control and the LMPRT group in preoperative pivot shift test grading.

Results

Three hundred and seventeen patients were included according to inclusion and exclusion criteria. They comprised 171 males and 146 females with an average age of 25 years (range 12–62 years) at the time of surgery. All patients had an ACL-injured knee with a mean period of 52 months (range 0.5–300 months) from injury to surgery. One hundred and seventeen patients (39.1%) had no meniscus tear (Control group), whereas 69 patients had a MM tear, 52 patients had a LM tear and 36 patients had both MM and LM tears. Detailed meniscus tear morphologies are described in Table 1. Among them, 39 patients (12.3%) had a LMPRT (LMPRT group), whereas only 4 patients (1.3%) had a MMPRT (MMPRT group). There were no significant differences in the demographic data among the three groups, as can be seen in Table 2. However, because of such a small number of patients in the MMPRT group, we did not include the MMPRT group for further statistical comparisons.
Table 1

Meniscus tear morphologies

 

MM

LM

Longitudinal tear (n)

82

55

Bucket handle tear (n)

7

4

Flap tear (n)

11

4

Horizontal tear (n)

3

5

Radial tear including posterior root tear (n)

4

41

MM medial meniscus, LM lateral meniscus

Table 2

Patients’ demographic data

 

Control (n = 117)

LMPRT (n = 39)

MMPRT (n = 4)

P value

Mean age, year (range)

25 (13–53)

24 (12–62)

30 (20–48)

n.s.

Gender, male/female

60/57

19/23

1/3

n.s.

Mean BMI, kg/m2 (range)

23.5 (17.7–34.5)

23.3 (18.9–32.4)

23.5 (20.1–28.2)

n.s.

Laterality, right/left

51/66

17/22

2/2

n.s.

Period from injury, weeks (range)

39 (2–1200)

36 (2–528)

45 (4–144)

n.s.

LMPRT lateral meniscus posterior root tear, MMPRT medial meniscus posterior root tear, BMI body mass index, n.s. not significant

MRI findings

Sensitivities and specificities of the MRI in PRT are shown in Table 3. The sensitivity of at least one view of MR images in the LMPRT group was 71.8%, with the coronal view showing the highest value. The sensitivity in the axial and sagittal views was not as high as that in the coronal view. In terms of meniscus extrusion, 48.7% of patients showed MEW of 1.0 mm and more. The sensitivity in at least one view of MR images in the MMPRT group was 100.0%, with the coronal and sagittal views showing the highest value. 100.0% of patients showed MEW of 1.0 mm and more. Specificities in the LMPRT were generally high, although 14.5% of the patients in the control group showed MEW of 1.0–3.0 mm. Specificities in the MMPRT were also high, although in the control group, 15.3% of the patients showed MEW of 1.0–3.0 mm and 1 patient showed MEW of 3.0 mm and more.
Table 3

Sensitivities and specificities of the PRT in MRI

 

LMPRT

MMPRT

Sensitivity (%)

Specificity (%)

Sensitivity (%)

Specificity (%)

Vertical linear defect in coronal plane

69.2

93.2

75.0

93.2

Radial linear defect in axial plane

28.2

98.3

50.0

95.7

Ghost sign in sagittal plane

10.3

96.6

75.0

99.1

Meniscus extrusion width ≥1.0 mm

48.7

85.5

100.0

84.7

Meniscus extrusion width ≥3.0 mm

12.8

100.0

25.0

99.1

Comparison of MEW among the groups revealed that MEW in the LMPRT group was significantly larger than that in the control group (Table 4).
Table 4

Clinical evaluation outcomes

 

Control

LMPRT

MMPRT

P value, control versus LMPRT

Number of giving-way episodes (range)

2.3 (0–30)

1.6 (0–10)

1.5 (0–4)

n.s

Meniscus extrusion width (mm)

LM, 0.5 ± 0.5

MM, 0.6 ± 0.5

1.3 ± 1.1

2.7 ± 2.5

<0.001

Preoperative KT measurements (mm)

6.7 ± 2.9

6.8 ± 2.7

6.8 ± 3.0

n.s

Preoperative pivot shift test grading

   

<0.001

 Grade 1

3

0

0

 

 Grade 2

3

1

0

 

 Grade 3

34

1

0

 

 Grade 4

58

24

3

 

 Grade 5

17

0

1

 

 Grade 6

2

3

0

 

Clinical evaluations

Preoperative pivot shift grading in the LMPRT group was significantly greater than that in the control group (Table 4). On the other hand, there was no significant difference in the number of giving-way episodes or preoperative KT-1000 measurements. A representative case is shown in Fig. 3.
Fig. 3

MRI findings of a representative case. An 18-year-old female basketball player had an ACL injury concomitant with a LMPRT 6 weeks prior to her surgery. She had experienced only two giving-way episodes. Side-to-side difference in KT measurement was 7.0 mm, whereas the pivot shift test was graded as 6. a The coronal MRI at the level of LM posterior root showed vertical linear defect. b The axial MRI showed radial linear defect. c Sagittal MRI showed Ghost sign. d The coronal MRI just anterior to the popliteal hiatus showed 3.0 mm extrusion of the LM

Discussion

The most important findings of the current study were that the LMPRT contributed to ALRI in ACL-injured patients, and that the LMPRT was associated with meniscus extrusion. The first hypothesis, that the LMPRT would be more associated with ALRI, whereas the MMPRT would be more associated with anterior instability, was partly validated, as the pivot shift test grading in the LMPRT group was significantly greater than that in the control group, whereas there was no significant difference in anterior instability by KT measurements between the LMPRT group and the control group. However, any associations between the MMPRT and either ALRI or anterior instability could not be evaluated because of the small number of patients with the MMPRT. The second hypothesis, that sensitivity and specificity of MRI to the PRT would be high and the PRT would be associated with meniscus extrusion, was also partly validated. Specificity of the MRI to the LMPRT was high, and MEW in the LMPRT group was significantly larger than that in the control group. However, sensitivity of MRI was not as high as expected. In terms of the MMPRT, both sensitivity and specificity of MRI seemed high. However, the MEW between the MMPRT group and the control group could not be compared because of the small number of patients in the MMPRT group.

The current study revealed that the LMPRT was associated with ALRI in ACL-injured patients. Residual ALRI after ACL reconstruction has been shown to correlate with worsening functional outcomes [2] and patient dissatisfaction [20], as well as with development of osteoarthritis [18]. Past studies have discussed several factors associated with high grade pivot shift phenomenon, such as lateral meniscus [34], anterolateral structures including anterolateral capsule [33], anterolateral ligament [39] and iliotibial tract [19], and posterior slope of tibial plateau [4]. Among them, the importance of the LMPRT in ALRI has been particularly recognized. Musahl et al. [34] described that the anterior translation of the lateral compartment significantly increased by lateral meniscectomy in ACL-deficit knee during the pivot shift test, and concluded that the LM was an important restraint for rotational instability. A cadaveric study showed that release of the LM posterior root increases ALRI in ACL resected knees [43]. A recent case–control study revealed that combined lateral meniscal lesion was one of the independent risk factors for grade 3 pivot shift after acute ACL injuries [44]. A more recent cross-sectional study also showed that ACL-injured patients with LM injuries had significantly greater ALRI compared with patients with isolated ACL injury [35]. The results from the current study are in good agreement with the previous studies. Therefore, surgeons should endeavour to repair LMPRTs associated with ACL reconstructions. It has been reported that patients undergoing ACL reconstruction were 4.9 times more likely to fail if they had a deficient meniscus compared to those with an intact meniscus, and those patients who underwent meniscal repair did not demonstrate any increased risk of failure [40].

On the other hand, associations between the MMPRT and knee instabilities could not be evaluated because of the small number of MMPRT patients in this study. Prevalence of the MMPRT concomitant with an acute ACL injury has been reported to be relatively rare. The MMPRT is rather commonly associated with degenerative changes, with reported risk factors being increased age, female gender, higher BMI, varus mechanical axis and lower sports activity level [17]. Whereas the LMPRT has been reported to be identifiable in 8% of ACL injuries [9], the MMPRT with ACL injuries has been reported to be about 3% [5]. The prevalence of the LMPRT and MMPRT in the current study was 12.3% and 1.3%, respectively, similar to previous reports. Therefore, it may be difficult to investigate associations between the MMPRT and knee instabilities in ACL-injured knees.

Although diagnosis of PRTs remains difficult, MRI has improved the accuracy of diagnosing these lesions [38]. Still, the most reliable MRI technique and image cut for detecting meniscus root tears remain controversial [29]. De Smet et al. [9] reported that the highest sensitivity for diagnosis of the LMPRT was found in coronal and sagittal planes. As for MRI signs of PRTs, the Ghost sign, defined as complete absence of the normal meniscal signal as a direct sign, and meniscus extrusion as an indirect sign have been described [6, 15, 28]. In this study, the coronal view of MRI was the most useful in identifying the LMPRT (69.2%), although its sensitivity was not as high as expected. In addition, sensitivities of other views were lower than the coronal image. There are several causes for the low accuracy of MRI diagnosis, including pulsation artefact from the popliteal artery and complex anatomy [10]. Furthermore, previous studies describing high sensitivity of the Ghost sign in the sagittal view were only in the MMPRT [6, 27], which is rather commonly associated with degenerative changes, and the most frequent tear morphology is an avulsion tear [25]. Although the current study was not classified according to the tear morphologies, one can speculate that the low sensitivity of the Ghost sign in the LMPRT might be because of the few patients with complete avulsion tear. In terms of the MMPRT, sensitivity of MRI to the MMPRT in this study was relatively high. However, because of the small number of MMPRT patients, the validity of this study cannot be ensured.

Meniscus extrusion is considered pathologic when greater than 3 mm [7]. In this study, only 12.8% showed meniscus extrusion of 3 mm and more in the LMPRT group. This might be due to the fact that MRI was taken under non-weight bearing conditions. Still, the LMPRT group showed significantly greater meniscus extrusion than the control group. Meniscus extrusion leads to meniscal dysfunction, and to a diminishing of the load bearing mechanism. A recent biomechanical study demonstrated that the LMPRT increased tibiofemoral contact pressures and decreased contact areas in the lateral compartment [42]. Consequently, meniscus extrusion has been reported to be associated with progression of osteoarthritis [3, 27]. The current study suggests that the LMPRT causes meniscus extrusion, which would lead to osteoarthritis. Furthermore, from this perspective, surgeons should make every effort to repair LMPRTs associated with ACL reconstructions.

There were some limitations in this study. First, as this is a retrospective study, morphology as well as classification of all LMPRTs was not sufficiently evaluated. A recent study [25] proposed a detailed classification system for the PRTs; however, it was not possible to classify according to the system from the surgical records. The current study also did not assess the status of the meniscofemoral ligament (MFL). Forkel et al. [11] demonstrated that the MFL was associated with stability of the lateral meniscus posterior root, and classified the LMPRT based on presence of the MFL injury [13]. Brody et al. [5] also described that, among patients with the LMPRT concomitant with the ACL injury, meniscus extrusion was significantly more common when injured MFL was present. Secondly, patients who had other risk factors associated with high grade pivot shift, such as increased tibial posterior slope, anterolateral structure injuries, hyperlaxity and increased knee hyperextension, were not excluded. These factors might have affected our analyses. Finally, the pivot shift test in this study was not quantitatively evaluated. Quantitative evaluation of the pivot shift [24, 37, 41] is necessary to further prove contribution of the LMPRT to knee stability.

The clinical relevance of this study is that the LMPRT was associated with ALRI as well as with meniscus extrusion in ACL patients. The coronal view of MRI was the most useful to identify the LMPRT, although its sensitivity was not very high. These findings will help surgeons recognize the importance of the PRT in better restoring residual ALRI as well as in preventing secondary osteoarthritis after ACL reconstruction. Therefore, surgeons should prepare to repair PRTs at the time of ACL reconstruction regardless of MRI findings, and they should make every effort to repair the LMPRT.

Conclusion

In ACL-injured patients, the LMPRT was associated with ALRI, whereas contribution of the PRT to anterior instability seemed to be limited. The LMPRT was also associated with meniscus extrusion. The coronal view of MRI was the most useful in identifying the LMPRT, although its sensitivity was not high.

Notes

Authors’ contributions

TMi analysed the data and drafted the manuscript. TMu conducted the study. IS conducted the study. TW, TMo, MH, HKa and MK collected the data. KO and TO analysed the data. HKo designed the initial plan, conducted the study and completed the final manuscript. All authors read and approved the final manuscript.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Funding

No funding was received for this study.

Ethical approval

This study was approved by the institutional Review Board in Tokyo Medical and Dental University (research protocol identification number: 1146).

Informed consent

All participants provided informed written consent.

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Copyright information

© European Society of Sports Traumatology, Knee Surgery, Arthroscopy (ESSKA) 2017

Authors and Affiliations

  • Takao Minami
    • 1
    • 2
  • Takeshi Muneta
    • 1
  • Ichiro Sekiya
    • 1
  • Toshifumi Watanabe
    • 1
  • Tomoyuki Mochizuki
    • 1
  • Masafumi Horie
    • 1
  • Hiroki Katagiri
    • 1
  • Koji Otabe
    • 1
  • Toshiyuki Ohara
    • 1
  • Mai Katakura
    • 1
  • Hideyuki Koga
    • 1
  1. 1.Department of Orthopaedic SurgeryTokyo Medical and Dental University Hospital of MedicineBunkyo-kuJapan
  2. 2.Department of Orthopaedic SurgeryWakayama Medical UniversityWakayama CityJapan

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