Schnell und einfach für eine Untersuchung anmelden:

Zum Kontaktformular



The “Cyclops Syndrome” with the clinical result of loss of motion, due to an extension deficit, was first described by Jackson et al in 1990.

T. Muellner, R. Kdolsky, K. Großschmidt *, R. Schabus, O. Kwasny, H. Plenk Jr.* 

University Clinic of Traumatology, * Department of Bone and Biomaterials Research, Histological-Embryological Institute, University of Vienna, Austria

ACL Reconstruction - Cyclops Syndrome – Histomorphology – Tibial Tunnel Placement


 The “Cyclops Syndrome” with the clinical result of loss of motion, due to an extension deficit, was first described by Jackson et al in 1990 [5]. He postulated that the post-arthroscopic ACL reconstruction development of an intraarticular nodule (= cyclops), leads to impingement of the patellar tendon graft in the intercondylar notch. The major hypotheses discussing the etiological origin; granulation tissue, arising from collagen fibers of the patellar tendon graft, or a prolonged deficit of motion are controversially discussed. [5, 8]

The aim of this prospective study was to record the incidence of the „Cyclops Syndrome“ in our patients after ACL-reconstruction, to histologically evaluate the type and origin of tissues constituting the cyclops, to correlate cyclops formation to tibial tunnel placement, and to define the improvement associated with excision of the nodule after arthroscopic debridement. 


Between January 1992 and December 1993, 120 patients (66 men, 54 women, average age: 26 years) underwent and acute or delayed arthroscopic reconstruction of a torn ACL with an ipsilateral bone-patellar tendon-bone graft, and augmented by a synthetic implant (LAD™, 3M-Comp., St.Paul, MN) in a divergent route and rigid double-end fixation technique [6]. Acute reconstruction was defined as within 6 weeks after ACL injury (n=28). In 27 patients a tear of the medial meniscus was found, while 11 patients showed a tear of the lateral meniscus. The medial collateral ligament was ruptured in 2 patients, the lateral collateral ligament was ruptured in 3 patients, and the posterior cruciate ligament was ruptured in 2 patients. In 21 patients chondral lesions were found arthroscopically, and in 48 patients a notchplasty was performed. Postoperatively, the knee was fixed in a brace, and after 10 days S 0-0-90 by full weight bearing was allowed. All patients underwent the same physiotherapeutic program, including postoperative continuous passive motion, isometric exercises, and a proprioceptive rehabilitation program. Competitive sports were allowed after 3 to 6 months, if the obtained thigh circumference had reached that of the contralateral level.

Patients were prospectively followed for the occurrence of a „Cyclops Syndrome“. One patient was excluded after moving abroad. Follow-up-arthroscopy was performed, when either an early deficit in range of motion demanded arthrolysis by debridement (group 1), or approximately 6 months later during LAD™ fixation device removal (group 2). After detection of intraarticular nodulous scar formation, and measurement with a scaled probe, it was totally excised during debridement. Intensive physiotherapy was prolonged to treat persistent range of motion deficit.

The retrieved specimens were fixed in 2.5% buffered glutaraldehyde, for histologic evaluation. After rinsing in the buffer and dehydration in graded ethanol they were embedded in methylmethacrylate, and 5 µm thick undecalcified microtome sections were prepared ("K-microtome", Reichert-Jung, Vienna, Austria). After appropriate staining [15], the sections were examined under transmitted and polarized light microscopes.

Placement of the tibial tunnel was examined on lateral radiographs taken in maximal extension. The index between the distance between the ventral edge of the tibia and the tunnel on joint line level (A) and the diameter of the proximal tibia (A+B) was calculated (Fig.1) and compared between the groups. For the statistical comparison of the results the Students t-test was used regarding a p-value of less than 0.05 as statistically significant.

After a follow-up period of at least 18 months patients were called for a checkup. The investigation included clinical examination, subjective symptom and activity level analysis, functional testing, and radiographic control. Range of motion was measured goniometrically with the patient supine. The clinical outcome of all patients was assessed with the evaluation sheet of the Orthopaedische Arbeitsgemeinschaft Knie (OAK) [11].


In twenty-one of the 119 patients a nodulous scar formation was detected at follow-up-arthroscopy (Tab. 1). However, only ten of these patients (Group 1) had shown the clinical symptoms of a „Cyclops Syndrome“ (i.e. limited extension, crepitus at terminal extension) before follow-up-arthroscopy, which was performed an average of 5.9 months (range 4 to 14 months) after ACL-reconstruction. There were seven males and three females. The mean age was 30 years (range: 24 - 50 yr.). Five patients underwent acute ACL reconstruction. One patient underwent repair of a grade III medial collateral ligament tear during ACL reconstruction. Meniscal lesions were present in six patients. One medial meniscus tear was repaired, three lateral and one medial meniscus tear required partial menisectomy, and one medial and one lateral meniscus tear were judged as stable. In these ten patients, the hard nodulous formation found in front of the intercondylar notch, had a mean size of 14mm (±4mm) by 8 mm (±2mm). Removal of the nodule resulted in extensional improvement from a preoperative loss of 19°, to 7° after surgery (Tab. 2)

Six male and five female patients, averaging 27 years of age (range: 15 - 48 yr), showed no clinical symptoms (i.e. free range of motion) prior to follow-up- arthroscopy(Group 2) and yet scar formation with similar macroscopic appearance to the cyclops, yet softer and smaller ( mean size: 8mm±3mm by 6mm±2mm) was detected in front of the intercondylar notch. We named this arthroscopic finding "Cyclopoid". Four patients were operated on in the acute phase. Five patients had meniscal lesions at the time of ACL reconstruction. Two lateral and three medial meniscus tears required partial menisectomy, and one medial meniscus was totally resected. Follow-up-arthroscopy was done on an average of 9.5 months (range 5 to 25 months) after the index operation.

A histomorphological examination was possible in 10 patients (six from group 1, four from group 2). In the other 11 patients only small fragments of the nodules were retrieved, and they did not allow identification or orientation in the biopsies. In five patients from group 1 the calcified core of the cyclops could already be seen on the lateral radiographs (Fig. 1)

Histomorphologically the specimens of group 1 showed chondral and membranous ossification. There were signs of new bone formation with activity of osteoblasts and osteoclasts. Tissues showed evidence of good vascular ingrowth. There was no inflammatory cell infiltration, nor granulation tissue (Fig. 2). In the cyclopoid formation excised from group 2, no bony formation could be seen in any of the specimens. The cyclopoid scar formations were built up of fibrous tissue showing elements of granulation tissue and in two specimens areas of cartilaginous tissue. A hypervascular granulation tissue was surrounded by a mature, well organized fibrous capsule. None of the specimens contained elements of the tendinous graft (Fig. 3).

The radiological investigation measurements of the tibial tunnel placement revealed no statistically significant differences between groups 1 and 2. The ratio of A/(A+B) was on average 0.33 ± 0.06 for group 1, and 0.36 ±0.08 for Group 2.

The follow-up period was similar; 22 months (18-29 months) in group 1 and 21 months (18-29 months) in group 2. In group 1 two patients achieved an excellent result, three patients achieved a good result, three patients achieved a fair result, and two patients achieved a poor result. In group 2, eight patients were graded as excellent, 2 patients as good, and 1 patient as fair. 


Arthrofibrosis is defined as a continuous loss of knee extension of more than 10 degrees and as a loss of flexion of 15 or more degrees [9, 14, 16]. Primary arthrofibrosis with generalized adhesions still remains an unpredictable phenomenon [14]. The cyclops syndrome, a rare cause of secondary arthrofibrosis, is due to an impingement of scar formation anterior to the tibial tunnel in the intercondylar notch. The two development theories for this lesion are: 1) ventral impingement of the graft and ventral fiber breakage which forms the fibrous nodule as described by Marzo et al [8] and Fullerton et al [3]; 2) cartilaginous or bony fragments which were left around the tibial tunnel as described by Jackson et al [5].

In the present study only specimens of adequate size, allowing definition of orientation and depth of biopsies were interpreted in the histomorphological examination. Delcogliano et al [2] didn’t observe any newly formed bone or necrotic lamellae nor cartilaginous or fibrocartilaginous tissue in cyclops formations investigated under light and scanning electron microscopes. Marzo et al [8] stated that disorganized fibrous connective tissue, which underwent fibrocartilaginous metaplasia with time, was the most common finding in the cyclops formations examined microscopically. In the present study the cyclopoid scar formations excised from group 2 consisted of connective tissue containing intermittent small areas of cartilaginous tissue. The smaller average size of the excised nodule than in the study of Marzo et al [8], could explain the lack of knee motion limitations in the 11 patients. However, their average time interval between ACL reconstruction and arthroscopic debridement was longer (11 as opposed to 9.5 months). In the present study symptomatic patients with an extension loss underwent arthroscopic debridement after an average of 5.9 months postoperatively. All these patients revealed bigger nodules consisting of bony cores with areas of cartilaginous tissue.

To exclude a overly anterior tibial tunnel placement, its position was measured on lateral radiographs. This was not performed in the two studies cited above. However, the radiological investigation showed no evidence of incorrect tunnel placement (i.e. ideal tunnel placement in almost all patients), nor any differences between the cyclops (group 1) and the cyclopoid scar formation (group 2) groups. These results can be interpreted as exact tunnel placement and are comparable to those of other investigators [4, 7].

The occurrence rate of a cyclops syndrome varies in literature from 2.2% [2] to 21% [1]. In this study an occurrence rate of 8.4% was found. Due to the small number of patients developing a cyclops formation after ACL reconstruction, only trends regarding predictive factors can be observed. A primary notchplasty was carried out in group 1 twice as often as in group 2. A predisposition for arthrofibrotic changes may be the stimulation of connective tissue proliferation. However, it is evident that the graft needs enough space. It is also possible that some of these problems are due to augmentation with a synthetic device. Considering recent studies have shown that the LAD™ augmentation of a patellar tendon graft has no benefit [12, 13], the treatment protocol of our clinic was changed and only multiple suture repair is augmented with the LAD™ to achieve good results, as reported previously [6, 17]. With the improvement in surgical techniques for ACL reconstruction and an aggressive rehabilitation concept [10], less problems could have been observed in the postoperative course than previously.

As our study revealed there are two histomorphologically different forms of nodulous scar formations. We believe that the neither of these lesions are stimulated by the debris raised from the drilling and preparation of the tibial tunnel. Neither remnants of tendon graft fibers, nor old bone particles were found in the specimens of both groups, so it can be concluded that both the hard cyclops and the soft “cyclopoid” are de- novo scar formations, seemingly subjected to different biomechanical stresses. As not only the extension was limited in the cyclops group, but also flexion, we further believe that the development of this nodulous scar formation is only the expression of a generalized inclination to fibrotic healing. It is still unknown whether a cyclopoid scar formation can develop into a cyclops by itself or perhaps by cellular modulation due to compressive forces.

Presented in part at the ESSKA Meeting in Budapest, 1996


1.         D. J. Dandy, D. J. Edwards (1994) Problems in regaining full extension of the knee after anterior cruciate ligament reconstruction: does arthrofibrosis exist? Knee Surg Sports Traumatol Arthrosc 22 :76-9

2.         A. Delcogliano, S. Franzese, A. Branca, M. Magi, C. Fabbriciani (1996) Light and scan electron microscopic analysis of cyclops syndrome: etiopathogenic hypothesis and technical solutions. Knee Surg Sports Traumatol Arthrosc 44 :194-9

3.         Fullerton LR, Andrews JR (1984) Mechanical block to extension following augmentation of the anterior cruciate ligament. Am J Sports Med 12:166 - 68

4.         Jackson DW, Gasser SI (1994) Tibial tunnel placement in ACL reconstruction. Arthroscopy 10:124-131

5.         Jackson DW, Schaefer RK (1990) Cyclops Syndrom: loss of extension following intra-articular anterior cruciate ligament reconstruction. Arthroscopy 6: 171-178

6.         Kdolsky R, Kwasny O, Schabus R (1993) Synthetic augmented repair of proximal ruptures of the anterior cruciate ligament. Long term results of 66 patients. Clin. Orthop 295:183-

7.         Lemos MJ, Albert J, Simon T, Jackson DW (1993) Radiographic analysis of femoral interference screw placement during ACL reconstruction: Endoscopic versus open technique. Arthroscopy 9:154-158

8.         Marzo JM, Bowen MK, Warren RF, Wickiewicz TL, Altchek DW (1992) Intraarticular fibrous nodule as a cause of loss of extension following anterior cruciate ligament reconstruction. Arthroscopy 8:10-18

9.         Mohtadi NGH, Webster Bogaert S, Fowler PJ (1991) Limitation of motion following anterior cruciate reconstruction. A case control study. Am J Sports Med 19:620-625

10.       Muellner T, Alacamlioglu Y, Nikolic A, Schabus R (1998) No benefit of bracing on the early outcome after anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 6:88-9

11.       Müller W, Biedert R, Hefti F, Jakob RP, Munzinger U, Stäubli HU (1988) OAK Knee Evaluation: A new way to assess knee ligament injuries. Clin Orthop 232:37-50

12.       Muren O, Dahlstedt L, Dalén N (1995) Reconstructions of old anterior cruciate ligament injuries. No difference between Kennedy LAD-method and traditional patellar tendon graft in a prospective randomized study in 40 patients with 4-year follow up. Acta Orthop Scand 66:118-22

13.       Noyes FR, Mangine RE, Barber S (1987) Early knee motion after open and arthroscopic anterior cruciate ligament reconstruction. Am J Sports Med 15:149-160

14.       Paulos LE, Rosenberg TD, Drawbert J, Manning J, Abbott P (1987) Infrapatellar contracture syndrom: an unrecognized cause of knee stiffness with patella entrapment and patella infera.  Am J Sports Med 15:331-341

15.       Plenk H. jr (1989) in Mikroskopische Technik Romeis B, Böck P, Eds. (Urban & Schwarzenberg, München-Wien-Baltimore), vol. 17, pp. 527.

16.       Raymond A, Sachs RA, Reznik A, Daniel DM, Stone ML (1990) in Knee ligaments. Structure, Function, Injury and Repair Daniel DM, Akeson WH, O´Connor JJ, Eds. (Raven Press, Ltd,  New York) pp. 505-520.

17.       Schabus R (1988) Die Bedeutung der Augmentation für die Rekonstruktion des vorderen Kreuzbandes. Acta Chir. Austriaca 76suppl. :1-48 


Table 1 



m : f

Age ø (range)

Acute ACL

Chronic ACL


Group 1

7 : 3

30y (24- 50)




Group 2

6 : 5

27y (15- 48)





Table 2 



Before Follow-up-AS

After Follow-up-AS










Group 1

19 ± 10

30 ± 16

7 ± 4

16 ± 9

4 ± 4

13 ± 9





Light and electron microscopic study of stress-shielding effects on rat patellar tendon

In this second part of our study, the histomorphologic changes occurring in the patellar tendon (PT) of rats after sole stress-shielding were evaluated.
» Mehr


Twisting of patellar tendon grafts does not reduce their mechanical properties

The purpose of this study was to investigate whether the twisting of a patellar tendon (PT) graft improves or reduces its mechanical properties.
» Mehr


Cyclops and cyclopoid formation after anterior cruciate ligament reconstruction: clinical and histomorphological differences

Prospectively, 119 patientswere pursued clinically and by follow-up-arthroscopy for the occurrence of a “cyclops syndrome” afterACL reconstruction with a patellar tendon autograft, augmented by LAD.
» Mehr


Inter- and intratester comparison of the Rolimeter knee tester: effect of tester’s experience and the examination technique

Clinical knee examination is imperative for the prognosis, follow- up, and scientific comparison of anterior cruciate ligament surgery results.
» Mehr


Die Arthroskopie des Hüftgelenks

Die Arthroskopie des Hüftgelenks wurde erstmals bereits 1931 von Burman beschrieben. Im Gegensatz zur Arthroskopie des Kniegelenks oder des Schultergelenks...
» Mehr