Buffalo Horn Sign – A New Finding on MRI for Meniscal Bucket-Handle Tears

ABSTRACT:

OBJECTIVE To describe a new sign on magnetic resonance imaging (MRI) axial images of patients with bucket-handle meniscal tears.
METHODS Of 610 consecutive patients with a surgical diagnosis of meniscal tear, those with a bucket-handle pattern were chosen, and 28 met the inclusion criteria. The most frequent mechanism was a twisting injury with or without a coronal stress (16 patients), and the injury was sports-related in 12 cases. All patients were symptomatic and had X-rays showing a preserved joint line. Next, their MRI examinations were analyzed.
RESULTS The buffalo horn pattern was found in 13 patients (46.4%), occurring in either the medial or the lateral meniscus. It was the 3^rd most prevalent sign, after the fragment within the intercondylar notch (n = 21; 75.0%) and the absent bow tie sign (n = 17; 60.7%). We observed that it had a significant association with other signs of displaced meniscal handle. The sign was neither found on the healthy menisci, nor was affected by the occurrence of an anterior cruciate ligament tear.
CONCLUSION The buffalo horn is a new finding for displaced meniscal bucket-handle tears; it is easy to identify and relevant in the interpretation of axial MRI images. Its recognition is very important to determine the type of treatment and the surgical plan.

Keywords: diagnostic imaging; knee; magnetic resonance imaging; tibial meniscus injuries.

RESUMO:

OBJETIVO Descrever um novo sinal em imagens axiais de ressonância magnética (RM) de pacientes com rupturas em alça de balde do menisco.
MÉTODOS De 610 pacientes consecutivos com diagnóstico cirúrgico de ruptura do menisco, aqueles com padrão em alça de balde foram escolhidos, e 28 atenderam aos critérios de inclusão. O mecanismo de lesão mais frequente foi a torção com ou sem estresse coronal (16 pacientes). Além disso, a lesão foi relacionada ao esporte em 12 casos. Todos os pacientes eram sintomáticos e tinham radiografias que mostravam a preservação da linha articular. Em seguida, seus exames de RM foram analisados.
RESULTADOS O padrão de chifre de búfalo foi encontrado em 13 pacientes (46,4%) no menisco medial ou lateral. Foi o 3^o sinal mais prevalente, depois do fragmento no interior da incisura intercondilar (n = 21; 75,0%) e da ausência do sinal da gravata borboleta (n = 17; 60,7%). Observamos uma associação significativa a outros sinais de deslocamento da alça do menisco. O sinal não foi encontrado em meniscos saudáveis, nem foi afetado pela ocorrência de ruptura do ligamento cruzado anterior.
CONCLUSÃO O chifre de búfalo é um novo achado para rupturas em alça de balde do menisco com deslocamento; é fácil de identificar e relevante na interpretação de imagens de RM de corte axial. Seu reconhecimento é muito importante para determinar o tipo de tratamento e o plano cirúrgico.

Palavras-chave: diagnóstico por imagem; imagem por ressonância magnética; joelho; lesões do menisco tibial.

FIGURES

INTRODUCTION

Menisci are crescent-shaped intracapsular fibrocartilagineous laminae with a role in load transmission, shock absorption, stability, lubrication, nutrient diffusion, sensory perception, and proprioception.¹ Meniscal tears are a common problem, with a reported incidence of ~60 per 100 thousand inhabitants in the United States.² There are several patterns. Buckethandle tears consist in a full-thickness longitudinal tear that propagates anteriorly and posteriorly, creating an inner fragment - the “handle”- that can displace into the intercondylar notch.³ These lesions represent ~10% of all tears;^3,4 they occur mainly in the medial meniscus,^4,5 but they can also affect the lateral meniscus.^6-8 Since surgical treatment is often required, the correct preoperative diagnosis is important to optimize treatment and save meniscal tissue.⁹ Magnetic resonance imaging (MRI) is the gold-standard imaging method with an overall reported sensitivity of up to 90.0% and specificity of up to 89.0%.^10-13 On MRI, meniscal bucket-handle tears usually display some well-known signs, mainly in the coronal and sagittal sections: absent bow tie sign,¹⁴ double posterior cruciate ligament (PCL) sign,^15,16 double anterior horn sign,¹⁷ flipped meniscus sign,¹⁸ disproportional posterior horn sign,¹⁹ double anterior cruciate ligament (ACL) sign,²⁰ triple PCL sign,⁷ triple cruciate sign,⁸ quadruple cruciate sign,⁶ and thepresenceof a fragment within theintercondylar notch⁴ (►Appendix A). The reported sensitivity and specificity for the diagnosis of meniscal bucket-handle tears varies in a wide range, from 64.0 to 93.0%^4,5,21 and from 64.0 to 100%^10,22 respectively - but it improves if more signs are known.^5,22

To the best of our knowledge, there was only one sign previously described in MRI axial view: the V sign.²³ The present paper aims to report a new sign to be identified in that view, which is perceived as a buffalo horn and appears in patients with meniscal bucket-handle tears. The sensitivity of this finding will be determined and compared with the presence of the other already-known signs.

MATERIALS AND METHODS

Compliance with Ethical Standards

The current study follows the ethical standards of the institutional Research Committee and of the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. It received approval n. 215/22 from the Ethics Board of Centro Acadêmico Médico de Lisboa. Informed consent was exempted as long as personal data protection standards were met, but additional written informed consent was obtained from all patients for whom MRI scans are included in the present article.

This is a retrospective study based on MRI scans of patients diagnosed during surgery with meniscal bucket-handle tears, disregarding age, sex, mechanism, the time between trauma and MRI and between trauma and surgery, treatment, surgeon, ACL injury, and affected knee or meniscus.

A consecutive sample of 1,767 patients operated at the Orthopedics Service of a university hospital for any knee pathology between 2012 and 2021 was obtained (►Fig. 1). This period was chosen because contains the highest number of patients available at our institution due to the limited computerization of medical files in the previous years. Only 610 patients had surgically proven meniscal tears, and 49 showed a bucket-handle pattern. Next, the patients with the following criteria were excluded: history of knee surgery, other patterns of meniscal tears, absent meniscal tear, unknown MRI protocol, absent axial sequences or menisci not acquired, and refusal to participate in the research. The collected sample was of 28 patients, 22 men and 6 women, with a mean age of 34.2 (± 14.0; range: 9-63) years. The most frequent mechanism of injury reported by patients was torsion with or without coronal load (16 patients). Four patients experienced the injury after a knee flexion or a squat. Two cases suffered a fall from their height, and one described a complex trauma during surf practice. Five patients could not identify any trauma.

Fig. 1. Flowchart illustrating the search strategy and selection criteria.

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Fig. 1. Flowchart illustrating the search strategy and selection criteria.

Twelve of these injuries were sports-related. Every patient showed, at some degree, knee pain, joint effusion, a sensation of locking or looseness, or loss of extension. All the X-rays showed a preserved joint line.

In total, 24 MRI scans were performed at the hospital where this research was conducted, and 4 were performed elsewhere, but with a similar protocol. Seventeen were performed with Philips Intera 1.5 T scanner, and 11 were performed with the Philips Achieva 3.0 T scanner. The receive-only specific knee coil provided by the manufacturer for each scanner was used. The standard acquisitions are summarized in ►Table 1, and they include: a coronal T1, T2 fast field echo (FFE) and short tau inversion recovery (STIR); a sagittal proton density (PD) with and without fat suppression; and an axial T2 FFE. Some exams had an additional coronal T2 spectral attenuated inversion recovery (SPAIR) and an axial SPAIR. A slightly different protocol for the Intera scanner was used to assess the pediatric patient, using a slice of 3 mm and a gap of 0.3 mm.

Table 1. Magnetic resonance imaging acquisition protocols

All surgeries were performed by senior knee surgeons, with fellowship training. Meniscal bucket-handle tear was defined as “a longitudinal tear with central migration of the ‘inner’ handle fragment.”³

A senior knee surgeon and an orthopedics resident were instructed on MRI interpretation of bucket-handle tears and then prospectively and blindly assessed the images. Consensus was established by the senior musculoskeletal radiologist. Each MRI scan was assessed for the presence of an absent bow tie sign,¹⁴ double PCL sign,^15,16 double anterior horn sign,¹⁷ flipped meniscus sign,¹⁸ disproportional posterior horn sign,¹⁹ double ACL sign,²⁰ triple PCL sign,⁷ triple cruciate sign,⁸ quadruple cruciate sign,⁶and the presence of a fragmentwithin the intercondylar notch.⁴ When in cross-section, the V sign²³ and the new sign were recorded. The buffalo horn sign is the presence of a low signal intensity area projecting from the anterior border of the medial tibial plateau, resembling a horn which can be appreciated in the MRI axial view, as demonstrated in ►Fig. 2. If the lateral meniscus is affected, the sign appears like a low-intensity horn-shaped band lying parallel to the anterior border of the lateral tibial plateau in two consecutive axial images (►Fig. 3). The other definitions used are reviewed in ►Appendix A. The presence of joint effusion,²⁴ parameniscal cyst,²⁵ meniscal extrusion,²⁶ bone marrow edema and its location^27,28 were also recorded. The cases with MRI suspicion of hemibucket-handle tear of the meniscus were counted.²⁹ Surgical evidence of rupture of the ACL was reported.

Fig. 2. (A) Preoperative MRI scan of a 54 year-old male patient with a surgically-reported bucket-handle tear of the medial meniscus of the right knee showing the buffalo horn sign (arrow) in the axial cross-section. This finding is a low signal intensity area projecting from the anterior border of the medial tibial plateau, resembling a horn. (B) Right knee arthroscopy using an anterolateral portal, showing the meniscal fragment of the same patient displaced anteriorly to the femoral condyle. Axial spectral attenuated inversion recovery (SPAIR) sequence: matrix, 250 X 200; repetition time/echo time (RT/ET), 3,000/30 milliseconds; slice, 3,5 mm; gap, 0,35; average, 4; field of view (FOV), 180 mm; flip angle (FA), 90°.

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Fig. 2. (A) Preoperative MRI scan of a 54 year-old male patient with a surgically-reported bucket-handle tear of the medial meniscus of the right knee showing the buffalo horn sign (arrow) in the axial cross-section. This finding is a low signal intensity area projecting from the anterior border of the medial tibial plateau, resembling a horn. (B) Right knee arthroscopy using an anterolateral portal, showing the meniscal fragment of the same patient displaced anteriorly to the femoral condyle. Axial spectral attenuated inversion recovery (SPAIR) sequence: matrix, 250 X 200; repetition time/echo time (RT/ET), 3,000/30 milliseconds; slice, 3,5 mm; gap, 0,35; average, 4; field of view (FOV), 180 mm; flip angle (FA), 90°.

Fig. 3. Preoperative MRI scan of a 17 year-old male subject with a surgically-reported bucket-handle tear of the lateral meniscus of the left knee. (A,B) Axial cross-section in T2 fast field echo (FFE) showing the buffalo horn sign (arrow) in lateral meniscus appearing as a low signal intensity horn-shaped band lying parallel to the anterior border of lateral tibial plateau in two consecutive axial images. Axial T2 FFE: matrix, 300 X 250; RT/ET, 500/12 milliseconds; slice, 3 mm; FOV, 180 mm; FA, 20°.

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Fig. 3. Preoperative MRI scan of a 17 year-old male subject with a surgically-reported bucket-handle tear of the lateral meniscus of the left knee. (A,B) Axial cross-section in T2 fast field echo (FFE) showing the buffalo horn sign (arrow) in lateral meniscus appearing as a low signal intensity horn-shaped band lying parallel to the anterior border of lateral tibial plateau in two consecutive axial images. Axial T2 FFE: matrix, 300 X 250; RT/ET, 500/12 milliseconds; slice, 3 mm; FOV, 180 mm; FA, 20°.

Fig. 4. The dislocated bucket-handle fragment of the medial meniscus of a right knee is sliced in axial section (orange box), appearing as a horn.

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Fig. 4. The dislocated bucket-handle fragment of the medial meniscus of a right knee is sliced in axial section (orange box), appearing as a horn.

A literature search for the terms and expressions menisci, bucket handle and MRI scan was conducted on the PubMed search engine. Only studies written in English and involving human subjects were considered. No reports regarding a buffalo horn sign could be found.

Statistical Analysis

For the statistical analysis, version 18.0 of the software PASW Statistics for Windows was used. Values of p lower than 0.05 were considered significant. The nominal variables were analyzed with the Chi-squared (χ²) test or the Fisher’s Exact test when > 20% of the cells had an expected count lower than 5.

RESULTS

The buffalo horn sign was found in 13 out of 28 cases (46.4%), which made it the third most prevalent sign, after the fragment within the intercondylar notch (n = 21; 75.0%) and the absent bow tie sign (n = 17; 60.7%). It occurred in 7/19 medial menisci (36.8%) (►Fig. 2) and 6/9 lateral menisci (66.7%) (►Fig. 3). If findings suggesting an anteriorly displaced handle were present (n = 9), the new sign appeared in 7 cases (77.8%), 3 medial menisci (33.3%), and 4 lateral menisci (66.7%). If findings suggesting a posteriorly displaced fragment were present (n = 4), the buffalo sign was observed in 2 patients (50.0%), 1 medial and 1 lateral meniscus. None of the aforementioned findings were statistically significant.

A significant association was observed between the new sign and a fragment within the intercondylar notch (p = 0.01) and a double anterior horn sign (p = 0.01) (►Table 2). There were no other statistically significant findings. The buffalo horn sign was observed regardless of an ACL tear (►Table 3).

Table 2. Frequency of the buffalo horn sign

Table 3. Anterior cruciate ligament tear and the frequency of signs of bucket-handle tear

Looking at the findings of the axial sections, the V sign was identified in 9 cases, a sensitivity of 32.1%. Of these nine patients, three presented only the V sign, and six also presented the buffalo horn sign. Of 13 patients with the latter, 7 had only the buffalo horn sign, but not the V sign. None of those signs appeared as the only sign for the tear, and all cases had the fragment within the intercondylar notch and the sign of absence of the bowtie.

The medial meniscus was torn in 19 cases (67.8%) and the lateral, in 9 (32.2%) cases. Three cases exhibit the hemibucket-handle pattern,²⁷ which could not be confirmed through the surgical records, and none of these patients presented the buffalo horn sign. Six MRI scans missed the diagnosis of bucket-handle tear, but all presented at least one indirect sign. The new sign was never found on an unaffected meniscus.

DISCUSSION

The most relevant finding of the present study was that the buffalo horn could be a new sign to diagnose meniscal bucket-handle tears. It is useful for both menisci in fragments displaced anterior^18,20 or posterior¹⁹ dislocated fragments. It was found in 7/19 medial menisci (36.8%) and 6/9 lateral menisci (66.7%), without statistically significant differences. All cases with a double anterior horn presented the buffalo horn sign (p = 0.01). In 8/28 (28.6%) cases with a flipped meniscus, the proposed sign appeared in 6 (75.0%). Both patients with double ACL presented it (100%). The studied finding also appeared in 2/4 disproportional posterior horn cases. Additionally, all patients with the new finding presented a fragment within the intercondylar notch (p = 0.01), and the new sign never appeared alone, but always in the presence of other signs for a displaced fragment. Thus, one can conclude that the buffalo horn sign means a displaced handle of a meniscal tear, and the horn appears when the axial section bisects it (►Fig. 4).

This new finding was observed because the assessment of cross-sectional images is gaining importance in clinical practice, and there was only one sign previously described for this condition in these images: the V sign.²³ Despite being close to each other, these two findings are not the same. The buffalo horn uses a sequence of images, appears anterior, and does not need to have the handle and the meniscus in the same section. Seven patients had the buffalo horn sign but not the V sign, and 3 had the latter but not the former. Six out of nine patients had both findings simultaneously (►Fig. 5). In this sample, the V sign had 32.1% of sensitivity, which is lower than the rate previously reported by Rao et al.,²³ of 72.0%, and it is also lower than the sensitivity of the buffalo horn sign in the sample of the present study (46.4%).

Fig. 5. Cross-sections of a preoperative MRI scan of a 27 year-old male patient with a bucket-handle tear of the medial meniscus of the left knee. (A) The V sign (^*) and the buffalo horn sign (arrow) are visible. (B) In the following section, only the buffalo horn sign (arrow) is found. Axial T2 SPAIR: matrix, 300 X 250; RT/ET, 3800/65 milliseconds; slice, 3 mm; gap, 0.3; average, 2; FOV, 180 mm; FA, 90°.

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Fig. 5. Cross-sections of a preoperative MRI scan of a 27 year-old male patient with a bucket-handle tear of the medial meniscus of the left knee. (A) The V sign (*) and the buffalo horn sign (arrow) are visible. (B) In the following section, only the buffalo horn sign (arrow) is found. Axial T2 SPAIR: matrix, 300 X 250; RT/ET, 3800/65 milliseconds; slice, 3 mm; gap, 0.3; average, 2; FOV, 180 mm; FA, 90°.

The overall sensitivity of MRI in the current study was low, corresponding to the lower half of the previously-reported range of 64 to 93%.^{4,5,12,13,21,22} In total, 6 out of 28 MRI scans missed any sign of a meniscal bucket-handle tear, which translates to an overall sensitivity of only 78.6%. This fact may explain the low sensitivity of the buffalo horn sign (46.4%), even though it was the third most relevant sign, with higher sensitivity than the other 6 widely-spread findings. Its sensitivity was surpassed by the fragment within the intercondylar notch (21/28; 75.0%) and the absent bow tie sign (17/28; 60.7%). That is consistent with the reported sensitivity by Dorsay and Helms for the fragment within the intercondylar notch of 76.7%,²²but is lower than the 88.4% described for the absence of the bowtie sign in the same study. In the present research, we do not intend to calculate either the specificity or reproducibility of the sign, but we never found the buffalo horn on an unaffected meniscus; moreover, the observation of the sign requires a displaced fragment to show up. Therefore, we believe that it is specific. Additionally, the proposed definition and the appearance of the buffalo horn sign are easy to spot, maybe more than the fragment within the intercondylar notch or the absence of the bow tie, whose definitions are vague.

The present study has several limitations. The sample is small due to the obstacles faced, such as the limited computerization of medical files before 2012 and a low volume of treated patients. This prevented us from obtaining a larger sample, but we believe that it does not threaten the aim of the current paper. The unaffected menisci were used as controls, and the time elapsed from the trauma to the MRI scan and surgery may be a source of bias. The patients were chosen based on a surgical diagnosis, which did not enable the performance of a test for the specificity. The reliability of the sign was not calculated. The sample gathers different magnetic field intensities, but Van Dyck et al.³⁰ showed, in a controlled prospective study, that diagnostic accuracy of 3.0 T for meniscal and ACL tears is not significantly higher than 1.5 T, so it seems reasonable to believe that this issue does not compromise the identification of a new sign. Knees with ACL deficiency (acute or chronic; 11/28; 39.3%) were studied aside from ACL-competent knees, but there are controversies regarding their effect on the accuracy of MRI.^10,12 Although all the surgeries were performed by experienced knee surgeons working in the same department, the surgical reports had not been standardized, and some missed the subtype classification of the bucket-handle tear.

CONCLUSION

In conclusion, the buffalo horn sign could be present in the medial and lateral torn menisci, with and without ACL tear. A dislocated fragment is the cause of the sign, so it is statistically associated with thefragment within the intercondylar notch or the double anterior horn sign. Recognizing this situation is very important to determine the type of treatment and to be able to plan the surgery. Moreover, we believe that this finding is easy to identify, and it is specific for bucket-handle tears, because it was not found on unaffected menisci.

Herein, the buffalo horn sign appears as a new finding that can be relevant to diagnose meniscal bucket-handle tears using axial MR imaging.

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Appendix A. Definition of the characteristic findings on magnetic resonance imaging of meniscal bucket-handle tears