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.
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.
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].
ACL tunnel cyst following ACL reconstruction.
Based on ultrasound and MRI images, there is a thin-walled cystic lesion located in the anteroproximal region of the right tibia, adjacent to the tibial cortex. Ultrasound shows a superficial, well-defined anechoic or hypoechoic lesion with no apparent blood flow signal inside. MRI indicates a cystic lesion possibly communicating with the tibial tunnel, demonstrating high signal intensity on T2-weighted sequences, suggesting fluid or cystic content. Furthermore, no significant abnormal signal changes are seen in the surrounding soft tissue or the tibial bone, and there is no apparent soft tissue edema or evidence of damage to other structures.
Considering the location, morphology, and the possible communication with the tibial tunnel, the most consistent diagnosis is an ACL tunnel cyst.
Comprehensive evaluation of a 42-year-old male with a history of right knee ACL reconstruction, presenting with a palpable mass in the anteroproximal tibia without significant pain, along with imaging features showing a cystic lesion communicating with the surgical tunnel, strongly suggests: Anterior Cruciate Ligament (ACL) Tunnel Cyst.
1. Treatment Options:
If the cyst is small, with mild or no symptoms, observation or conservative management could be considered, with periodic follow-up to monitor size changes. If the cyst enlarges significantly or causes notable symptoms (e.g., local pain, discomfort, or compression of surrounding tissues), or if there is potential compromise of the ACL graft function, surgical intervention may be indicated. Surgery could involve cyst excision, debridement of necrotic tissue, inspection and repair of the graft integrity, and bone grafting if needed to fill an excessively large tunnel.
2. Rehabilitation/Exercise Prescription Recommendations (FITT-VP Principle):
Whether there is no clear indication for surgery, or postoperatively during rehabilitation, it is crucial to restore knee function gradually and monitor during exercise. Recommendations include:
If moving into a postoperative rehabilitation phase, the goals of each stage can be summarized as:
• Early Stage: Swelling and pain control, maintaining the baseline tension of the quadriceps, and preserving knee range of motion.
• Mid Stage: Strengthening quadriceps through eccentric and concentric exercises, enhancing knee joint stability.
• Late Stage: Gradually returning to higher-level activities such as running and jumping. If necessary, receive professional guidance for posture and movement training to prevent re-injury.
Precautions: For patients with postoperative bone density loss, knee cartilage lesions, or other comorbidities (e.g., cardiovascular issues), exercise volume should be adjusted under professional supervision. Protect the joint and ensure overall safety.
Disclaimer: This report provides a reference-based analysis based on current imaging findings and medical history. It does not substitute for in-person consultation or professional medical advice. Specific diagnosis and treatment plans should be finalized by combining clinical examinations, intraoperative findings, and other diagnostic tests.
ACL tunnel cyst following ACL reconstruction.
