CASE 8131 Published on 21.02.2010

ACL Tunnel Cyst following ACL reconstruction

Section

Musculoskeletal system

Case Type

Clinical Cases

Authors

Madden M, Wong IK, Munk PL, Kelly MJ

Patient

42 years, male

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Clinical History

42-year-old Caucasian male, with a history of right ACL reconstruction. The patient presents with a painless mass over his right proximal anterior tibia.

Imaging Findings

Initial ultrasound showed a well-defined, mixed hypoechoic to anechoic mass measuring 2.6 x 1.4 x 3.2 cm. It was located inferior to the tibial tubercle and patellar tendon insertion, with an anechoic neck which abuts the tibial cortex. No flow was present on colour Doppler imaging. It was not compressible.

Subsequent MRI shows a cystic lesion in the tibial tunnel, extending through the distal aspect of the tunnel into the subcutaneous tissues. The lesion extends proximally in the tibial tunnel to the level of the tibial plateau. There was no evidence of ACL graft failure.

Discussion

An ACL tunnel cyst is a known complication of an ACL repair [1-9]. It is a cystic lesion that usually communicates with the tibial tunnel. Patient's typically present with a palpable mass if the cyst protrudes through the tunnel into the thin subcutaneous tissues of the anterior shin. The estimated incidence is 1.4-2.2% of patients with ACL reconstruction [2, 6]. The time of onset following surgery has ranged between 1-44 months [6].

The aetiology of ACL tunnel cysts is also unknown, with several possible theories.

The first theory is that a traumatic event leads to graft tear or graft necrosis, with subsequent formation of a synovialised tract in the tunnel, followed by cyst formation [2, 5]. A second theory suggests that micromotion within the graft leads to incomplete remodelling, which then leads to a synovialised tract. Finally, a third theory proposes that the tunnel is simply too large for the graft, which leads to a direct connection between the anterior shin and the knee 5.

A small prospective study of 22 people found that 64% of patients had tibial tunnel fluid in the first post-operative year. Most collections resolved by 18 months. None of the fluid filled tunnels progressed to cysts [5].

On ultrasound, a thin walled hypoechoic cyst anterior to the bony cortex is the most common appearance. No flow should be present on colour Doppler imaging.

On MRI, a cystic structure is present, typically with a communication to the tibial tunnel. MRI is the optimum imaging modality as it can demonstrate communication of the cystic lesion with the tibial tunnel, as well as detect potential communication with the knee joint.

No consensus for management of tibial tunnel cysts exists in the literature. Cyst excision is done on a case by case basis, with an emphasis on removal if the patient is symptomatic or if the graft has failed. It is important to determine if the cyst communicates with the knee joint.

Non-communicating cysts are thought to be due to local necrosis of the graft underneath the hardware. These can be treated by cyst excision. Communicating cysts require more thorough debridement and bone grafting [6].

Differential Diagnosis List

ACL tunnel cyst following ACL reconstruction.

Final Diagnosis

ACL tunnel cyst following ACL reconstruction.

References

[1] Bining J, Andrews G, and Forster BB (2009) The ABCs of the Anterior Cruciate Ligament: a Primer for Magnetic Resonance Imaging Assessment of the Normal, Injured and Surgically Repaired Anterior Cruciate Ligament. Br J Sports Med 43:856-862 (PMID: 19864590)

[2] Deie M, et al (2000) Pretibial Cyst Formation After Anterior Cruciate Ligament Reconstruction Using Auto Hamstring Grafts: Two Case Reports in a Prospective Study of 89 Cases. Magnetic Resonance Imaging18:973-977 (PMID: 11121700)

[3] Malhan K, Kumar A, and Rees D (2002) Tibial Cyst Formation After Anterior Curciate Ligament Reconstruction Using a New Bioabsorbable Screw. The Knee 9:73-75 (PMID: 11830385)

[4] Martinek V, Friederich N (1999) Tibial and Pretibial Cyst Formation After Anterior Cruciate Ligament Reconstruction With Bioabsorbable Interference Screw Fixation. Arthroscopy 15:317-320 (PMID: 10231113)

[5] Sanders TG, Tall MA, Mulloy JP, and Leis HT (2002) Fluid Collections in the Osseus Tunnel During the First Year After Anterior Cruciate Ligament Repair Using an Autologous Hamstring Graft: Natural History and Clinical Correlation. Journal of Computer Assisted Tomography 26:617-321 (PMID: 12218830)

[6] Simonian PT et al (1998) Pretibial Cyst Formation After Anterior Cruciate Ligament Surgery With Soft Tissue Autografts. Arthroscopy 14:215-220 (PMID: 9531136)

[7] Thaumat M, Chambat P (2007) Pretibial Ganglion-like Cyst Formation After Anterior Cruciate Ligament Reconstruction: A Consequence of the Incomplete Bony Integration of the Graft?. Knee Surg Sports Traumatol Arthrosc 15:522-524 (PMID: 17103004)

[8] Tsuda E et al (2006) Pretibial Cyst Formation After Anterior Cruciate Ligament Reconstruction With a Hamstring Tendon Autograft. Arthroscopy 22:691.e1-691.e6 (PMID: 16762723)

[9] Victoroff BN, et al (1995) Subcutaneous Pretibial Cyst Formation Associated With Anterior Cruciate Ligament Allografts: A Report of Four Cases and Literature Review. Arthroscopy 11:486-494 (PMID: 7575885)

Case information

URL:	https://www.eurorad.org/case/8131
DOI:	10.1594/EURORAD/CASE.8131
ISSN:	1563-4086

Figure 1

Ultrasound

Sagittal ultrasound demonstrating a subcutaneous mixed hypoechoic to anechoic lesion anterior to the proximal tibia. There is a tract extending to the tibial cortex.