Molar Tooth in the Knee - A Case of Simultaneous Medial and Lateral Bucket Handle Tear with Arthroscopic Correlation and a Review of Literature.

Introduction: Simultaneous bucket handle tear of both lateral and medial menisci is a rarely encountered entity. MR imaging helps in pre-operative diagnosis for optimal operative planning. However, as this condition is not seen in routine practice, awareness of various imaging signs is essential to identify this condition. Few signs on coronal and sagittal images have been described in the literature but none so far on axial images. Case Presentation: We propose a novel "molar tooth sign" in the knee on axial MRI images, in which we identified in the case of a young male with recurrent knee locking after a fall from height.
Conclusion: Different modes of injury can lead to different patterns of meniscal and ligament tears. Having an allied sign which is, furthermore, easy to imagine will help, not only the radiologists but also the surgeons, to identify rare types of meniscal tears similar to the index case. Copyright: © Indian Orthopaedic Research Group.

A bucket handle tear is rare, with simultaneous bucket handle tears of both the medial and lateral menisci rarer, an unmissable entity for accurate preoperative diagnosis for timely management.

Introduction

“Molar tooth” sign has been talked about in the radiological literature for pathologies in the brain and abdomen. Because the eyes do not see what the brain does not know, it is essential not only to be aware of rare entities such as simultaneous bicompartmental bucket handle tears but also be able to diagnose them preoperatively for optimal surgical planning. Signs help the human brain to use the power of imagination for identifying conditions that they might not have yet encountered. We present a rare case and point out an imaginable sign to identify the condition on planning magnetic resonance imaging (MRI) and also validate our finding on arthroscopy.

Case Report

A 39-year-old male presented with complaints of recurrent knee locking and pain in the left knee following a fall from a height of 7 feet, 4 years back, presented to the orthopedic clinic for management. On examination, there were evident quadriceps wasting of 3 cm with a positive Lachman test, anterior drawer test, pivot shift test, McMurray test, and Apley’s grinding test. There was no laxity in the coronal plane and the dial test was negative. A clinical diagnosis of anterior cruciate ligament (ACL) tear with meniscal tear was made and the patient was advised radiological examinations. Kellgren-Lawrence Grade II osteoarthritis was noted on a plain radiograph of the knee which did not reveal any fracture. His Lysholm knee score was 62/100.

MRI was performed on a 3T scanner using a dedicated knee coil. Proton density fat-saturated images were acquired in all planes in addition to a T1-weighted image and gradient echo image. The MRI was diagnostic of a complete chronic ACL tear, medial, and lateral meniscal tears with bucket handle components giving rise to the “triple posterior cruciate ligament (PCL)” sign, double anterior horn sign, and an osteochondral defect in the medial femoral condyle (Fig. 1, 2). While some signs have been described on coronal and sagittal images in the literature to help identify a bicompartmental meniscal tear, no sign has been described on axial images. We propose a novel “molar tooth sign” on axial images to assist in identification of this rare entity which necessitates pre-operative identification for appropriate procedural planning (Fig. 3).

Figure 1

Coronal proton density fat-saturated MRI images show the quadruple cruciate sign in the intercondylar notch (a) formed by the menisci (arrowheads), PCL (yellow arrow), and ACL (green arrow). (b) The truncated meniscus (encircles) with medial meniscus double anterior horn sign (red arrow).

Figure 2

Sagittal proton density fat-saturated MRI images show the double anterior horn sign (red arrow) and absent bow tie sign (red arrowhead) in (a) and the triple PCL sign in (b) formed by the PCL (yellow arrowhead) and flipped menisci (red arrowhead).

Figure 3

Proposed novel MRI sign – the molar tooth sign can be seen on this axial image with the roots of the tooth formed by the flipped menisci (red arrowhead) in the intercondylar notch and the crown formed by the intact PCL (yellow arrow).

A gait analysis was also performed for the patient using BTS bioengineering 3D gait lab (Italy). His gait examination revealed decreased percentage of stance phase of affected limb, reduced stride length slight, and increase in peak flexion of affected limb.

Based on the clinical and radiological findings, the patient was advised arthroscopic ACL reconstruction with meniscal repair/menisectomy. Intraoperatively, diagnostic arthroscopy confirmed a complete ACL tear with an avulsed femoral attachment, bucket handle component arising from meniscal tears of both medial and lateral menisci, and osteochondral defect in medial femoral condyle and an intact PCL. Arthroscopic single bundle ACL reconstruction was done using a five-strand semitendinosus and gracilis autograft. The bucket handle component of medial meniscus (MM) had a complex morphology due to which subtotal menisectomy was performed. For the lateral meniscus, the bucket handle component was reduced and repaired using two sharp-shooters by inside-out method (Fig. 4). Postoperatively, the patient was allowed a range of motion exercises with non-weight bearing protocol for 6 weeks. At 2 months follow-up, the patient had a painless, full range of motion.

Figure 4

Arthroscopic image shows flipped meniscal components – lateral meniscus (red arrowhead), medial meniscus with complex tear anatomy (black arrowhead), and torn remnant ACL fibers (green arrowhead).

Discussion

Simultaneous occurrence of bucket handle tears of both menisci of the same knee is a rare occurrence with interesting and seldom viewed signs on MRI imaging. Few case reports of the same have been published in the literature (Table 1) which shows that a sudden complex twisting injury of the knee results in this condition [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. In our case, the patient fell from a height, probably twisting his knee with the torque tearing both menisci of his left knee. Most common associated injury is a tear of the ACL, same as our case. Koukoulias et al. [7] reported a case with an accompanying medial cruciate ligament tear, while Lykissas et al. [8] discussed a case with an associated chondral lesion of the medial femoral condyle with the latter resembling the finding in the index case. The youngest patient reported was a 16-year-old male reported by Sales et al. [1] and the oldest were aged 38 years Koukoulias et al. [7] and Wright et al. [9]. Our patient is 1 year older.

Table 1

A brief literature review of all cases of bicompartmental bucket handle tear.

Radiological diagnosis in each case was performed by an MR examination which displayed novel and interesting signs – triple PCL sign, quadruple cruciate sign, double anterior horn sign, truncated meniscus sign, and absent bow-tie sign [1, 12]. With this case, we propose a new radiological sign on axial MRI images – the molar tooth sign which has not been previously described in the literature (Fig. 3, 5, 6). In addition, a medial femoral condyle osteochondral avulsion was also present. We also performed a gait analysis for our patient which has not been done previously in the literature. Gait analysis helps in understanding deranged knee kinematics and in post-procedural rehabilitation.

Figure 5

Diagrammatic representation of signs of bicompartmental bucket handle tears as seen on coronal MRI image – quadruple cruciate sign (encircled in grey), truncated meniscus sign (encircled in black), and double anterior horn sign (white arrowhead).

Figure 6

Diagrammatic representation of signs of bicompartmental bucket handle tears as seen on sagittal MRI image – triple PCL sign and double anterior horn sign.

The surgical intervention was performed 4 years following injury in the index case. In the literature, operative intervention has been done as early as within 8 h of injury [10] up to 8 years after injury [11]. Partial or subtotal menisectomy was performed in majority of the cases, while meniscal repair was reserved for those cases which on arthroscopy exhibited salvageable, reducible, and non-complex tear anatomy. In our case, the MM was excised, while the lateral was reduced and repaired. Simultaneous ACL reconstruction was done for the patient. Some authors such as Shepherd et al. [6] and Zabrzynski et al. [4] performed a two-step procedure addressing the meniscus pathology before reconstructing the concurrent torn ACL.

Conclusion

Bi-compartmental bucket handle tear of the meniscus is a rare entity precipitated by a sudden torque to the knee joint. Very few cases have been described in the literature. This article provides a brief summary of all similar cases described in the literature with their imaging diagnosis and operative management. Some signs have been described in the literature to aid diagnosis on MRI. We propose a novel “molar tooth sign” on axial MRI imaging which has not been described before to add to diagnostic confidence.

Rare conditions tend not to have a defined course of management. They are usually dictated by the treating physician’s preference and it is essential to be aware of possible rare occurrences before entering the surgical field. We suggest to look for a “molar tooth” in axial MRI images of complex meniscal tears with evidence of loss of meniscal volume to ensure to not miss bicompartmental bucket handle tears. The literature review lays out the various surgical approaches used in the past and provides a path for possible future choices. Because this condition is rare, the various signs described in the literature and the novel sign suggested by the authors will continue to be scrutinized and validated over time.