Editorial Open Access
Copyright ©The Author(s) 2024. Published by Baishideng Publishing Group Inc. All rights reserved.
World J Orthop. Jun 18, 2024; 15(6): 495-497
Published online Jun 18, 2024. doi: 10.5312/wjo.v15.i6.495
Recurrent cyclops lesion after primary resection of fibroreactive nodule following anterior cruciate ligament reconstruction
Cadence Lee, Farid Amirouche, Department of Orthopedic Surgery, University of Illinois College of Medicine, Chicago, IL 60612, United States
ORCID number: Cadence Lee (0000-0002-5437-4273); Farid Amirouche (0000-0002-3002-4199).
Author contributions: Amirouche F performed supervision, conception, and design; Lee C contributed to literature review and writing; Amirouche F and Lee C performed editing, analysis, and revision.
Conflict-of-interest statement: The authors have declared that no conflicts of interest exist.
Open-Access: This article is an open-access article that was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution NonCommercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: https://creativecommons.org/Licenses/by-nc/4.0/
Corresponding author: Farid Amirouche, PhD, Professor, Department of Orthopedic Surgery, University of Illinois College of Medicine, 835 S. Wolcott Ave, Chicago, IL 60612, United States. amirouch@uic.edu
Received: January 9, 2024
Revised: May 9, 2024
Accepted: May 27, 2024
Published online: June 18, 2024
Processing time: 155 Days and 8.2 Hours


In this case report featured in World Journal of Orthopedics, Kelmer et al describe a rare finding of a 28-year-old female patient who presented with a recurrent fibroreactive nodule 7 months following the resection of a primary cyclops lesion, suggesting recurrent cyclops syndrome. The patient had undergone an initial anterior cruciate ligament reconstruction for a non-contact right knee injury and reported successful recovery. Two years later, the patient sustained a repeat right knee injury followed by a positive McMurray test and acute pain with terminal extension. Arthroscopic synovectomy confirmed magnetic resonance imaging (MRI) finding of a cyclops lesion, which was surgically removed. Seven months postoperatively, the patient reported stiffness and difficulty with terminal extension. Repeat MRI indicated a recurrent cyclops lesion, which was surgically resected. Following resection of the second lesion, the patient underwent physical therapy and achieved full range of motion, maintaining complete recovery 19 months postoperatively. Recurrent cyclops lesions have rarely been reported in the literature, and this article is novel in its report of recurrent cyclops syndrome following a bone-patellar tendon-bone allograft. The presentation of this unusual finding exposes a need for further investigation of cyclops lesion pathology, which will aid its prevention and treatment.

Key Words: Cyclops lesion, Anterior cruciate ligament reconstruction, Fibrous nodule, Bone-patellar-tendon-bone allograft, Cyclops syndrome

Core Tip: This editorial outlines the case study described by Kelmer et al, whereby a 28-year-old female presented with a secondary, recurrent fibroreactive nodule 7 months following the resection of a primary cyclops lesion. Here, we describe the pathology and history of the cyclops lesion and expound on the case study findings by the original manuscript authors. We further provide potential explanations for the observations.


Cyclops syndrome is characterized by loss of full knee extension following anterior cruciate ligament reconstruction (ACLR) due to the development of a fibrous nodule of granulation tissue along the anterior cruciate ligament (ACL) graft, known as a cyclops lesion[1,2]. Symptomatic cyclops lesions have been reported in 1.9% to 10.9% of patients following ACLR, though incidence is likely greater considering asymptomatic cases[3]. Risk factors for the development of cyclops lesions include female sex, narrow intercondylar notch, surgical use of quadricipital tendon autograft in double-bundle reconstruction (as opposed to hamstring tendon autograft), post-operative hamstring spasm, and timing of operation following ACL injury[2,4,5]. Though the exact mechanism of cyclops lesion development is unknown, histological analysis suggests a pathology involving reactive fibroproliferation following ACL fiber rupture and scar formation that occurs at the time of ligament damage rather than during ACLR procedure[6]. The etiopathogenesis is theorized to involve either inflammatory response due to misplacement of the neoligament and intercondylar notch, fibrogenesis from remnants of the native or graft ACL, or connective tissue remnants following tunnel drilling[7]. However, the exact pathogenesis and risk factors underlying cyclops syndrome are still yet to be elucidated and require further investigation.

In this article of World Journal of Orthopedics, Kelmer et al[8] describe an unusual finding of a patient with a recurrent cyclops lesion following prior successful resection and rehabilitation of a primary cyclops lesion following ACLR. While the incidence of cyclops lesions have been well-documented, Kelmer et al[8] describe a novel incidence of recurrence of a cyclops lesion in a 28-year-old female patient within 7 months of the primary resection. Following an initial, non-contact right leg ACL tear, arthroscopic reconstruction was performed using a bone-patellar tendon-bone allograft. The procedure was well-tolerated and the patient recovered through physical therapy with no significant complaints. Two years following the initial procedure, the patient sustained a repeat right knee injury accompanied by knee extension pain and stiffness, warranting arthroscopic synovectomy. Intraoperatively, a cyclops lesion was discovered at the insertion of the ACL on the proximal tibia, restricting the intercondylar notch at terminal extension. Seven months following the resection of this lesion, the patient reported recurrent stiffness of the knee and difficulty achieving terminal extension, at which time an additional cyclops lesion was found on magnetic resonance imaging (MRI) and subsequently resected during surgical joint debridement. Following this second resection, the patient was enrolled in physical therapy and successfully achieved full active and passive range of motion, no recurrent stiffness, and return to normal daily activities, which were maintained 19 months postoperatively.

The authors describe several risk factors that may have predisposed this patient to cyclops lesion recurrence, notably the bone-patellar tendon-bone allograft and elevated body mass index (BMI), although previous studies have conflicting evidence as to whether bone-patellar tendon-bone grafts and different surgical techniques increase risk of cyclops syndrome[9,10]. The authors also consider the risk factor reported in the literature, whereby cyclops lesions occur following postoperative ACLR extension deficit. However, this finding was not contributory in this clinical scenario as this patient achieved full recovery following the bone-patellar tendon-bone allograft ACLR. Though the authors briefly mention the nature of the patient’s first and second injuries, this should be further expounded upon as it is a potential explanation for the cyclops lesion recurrence. The patient’s history describes a noncontact initial injury and a second presentation with a “pop and medial-sided knee pain,” which may have prompted the second development of a fibroreactive nodule. Mechanistic explanation for cyclops lesions may involve rupture of native ACL fibers causing fibroproliferative scar formation6, which may be accounted for by the patient’s second injury. A detailed history is important in differentiating the recurrence of a primary cyclops lesion from a repeat injury prompting the development of a separate, novel nodule. Given the discrepancy in size and timing of these cyclops lesions, the second nodule may have developed from cellular inflammation that was undetectable at the time of first resection. Pathologic analysis of the second lesion biopsy was “composed of synovial tissue with fibrosis and reactive fibrocartilage,” but origin of the novel nodule remains unclear. Additionally, further research must confirm whether the ACLR procedure itself can provoke inflammation within the ligament to cause the development of a cyclops lesion.

A key limitation of this work is the study design, a clinical case report, representing the lowest level of evidence. Without a causative explanation or similar clinical case presentations, it is difficult to interpret the value of this finding. However, case reports are used as a first line of evidence and essential for identification of clinical novelties. Recurrent cyclops lesions are seldom reported in the literature, with only one case reported recently, in a cohort of 33 MRI’s evaluating cyclops lesions[11]. Therefore, this finding is significant to report and provides further inquiry regarding the pathogenesis of cyclops syndrome. Especially given the recency of its recognition in 1990 by Jackson and Schaefer[1], cyclops syndrome is a relatively new clinical discovery that requires further investigation.


In summary, Kelmer et al[8] have presented an intriguing clinical case suggesting recurrence of a cyclops lesion following ACLR. The patient meets several of the risk factors previously described for cyclops syndrome (female sex, increased BMI, bone-patellar tendon-bone allograft), yet the recurrence demonstrated in this instance is a clinical novelty. Additional information would benefit this specific analysis, such as quality of primary and secondary injury, biomechanical distribution of weight-bearing within the knee connective tissues, and investigation of whether graft type and surgical technique influence risk. Further research is needed to elucidate the cellular mechanism underlying the hypothesized inflammatory pathogenesis and whether immunomodulatory processes drive the development and redevelopment of fibroreactive nodules. This case report exposes a need for a better understanding of the development and treatment of Cyclops syndrome.


Provenance and peer review: Invited article; Externally peer reviewed.

Peer-review model: Single blind

Specialty type: Orthopedics

Country of origin: United States

Peer-review report’s classification

Scientific Quality: Grade C

Novelty: Grade B

Creativity or Innovation: Grade C

Scientific Significance: Grade B

P-Reviewer: Li JM, China S-Editor: Qu XL L-Editor: A P-Editor: Zhao YQ

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