Overview
ACL reconstruction is a well-established surgery with high success rates. However, like any surgery, complications can occur. Understanding these possibilities helps you:
- Recognize warning signs early
- Seek appropriate care promptly
- Ask informed questions
- Take preventive measures when possible
Important Perspective
Most complications are uncommon. The vast majority of ACL reconstructions succeed without significant problems. This guide exists to educate, not to alarm. If you're concerned about any symptoms, contact your surgeon's office.
Arthrofibrosis (Stiffness/Scar Tissue)
What Is It?
Arthrofibrosis is the formation of excessive scar tissue in the knee joint, leading to loss of range of motion (ROM). It's one of the most common complications after ACL surgery.
Signs and Symptoms
- Inability to fully straighten the knee (extension deficit)
- Inability to fully bend the knee (flexion deficit)
- Stiffness that doesn't improve with therapy
- Pain at end-range of motion
- Feeling of tightness or "blockage" in the joint
Risk Factors
- Surgery performed too soon after injury (while knee is still inflamed)
- Loss of extension before surgery
- Inadequate early rehabilitation
- Excessive swelling post-surgery
- Genetic predisposition to scar tissue formation
Prevention
- Regain full extension before surgery when possible
- Prioritize extension exercises immediately post-surgery
- Aggressive swelling management
- Consistent physical therapy attendance
Treatment
- Physical therapy: Aggressive stretching, mobilization, low-load prolonged stretching
- Extension splinting: Wearing a splint to gradually improve extension
- Manipulation under anesthesia (MUA): The surgeon moves the knee through ROM while you're asleep
- Arthroscopic lysis of adhesions: Surgical removal of scar tissue
Key Point: Extension is Priority
Full knee extension (0° or better) is critical. Even small extension deficits (3-5°) can cause long-term problems including anterior knee pain and abnormal gait. Don't accept "almost full" extension—work until it matches your other knee.
Graft Failure (Re-Rupture)
What Is It?
Graft failure occurs when the reconstructed ACL tears or stretches out, resulting in knee instability. This may require revision surgery.
Types of Graft Failure
- Traumatic re-rupture: New injury tears the graft
- Graft stretching: Graft elongates over time, causing gradual instability
- Failure of incorporation: Graft doesn't heal properly into bone tunnels
- Technical failure: Issues with tunnel placement or graft fixation
Signs and Symptoms
- New giving-way episodes
- Feeling of instability, especially with cutting/pivoting
- A "pop" during activity (if acute re-rupture)
- Swelling after the incident
- Positive Lachman or anterior drawer test on exam
Risk Factors
- Early return to sport: Returning before graft fully matured
- Inadequate rehabilitation: Weak quads, poor neuromuscular control
- Poor landing mechanics: Knee valgus, stiff landings
- Young age: Highest re-injury rates in athletes under 25
- Return to high-risk sports: Soccer, basketball, football have highest rates
- Allograft in young athletes: Higher failure rates than autografts in active young patients
Prevention
- Wait until meeting all return-to-sport criteria (not just time)
- Achieve ≥90% quad strength symmetry
- Pass hop tests with ≥90% LSI
- Complete sport-specific training before full return
- Address landing mechanics and neuromuscular control
- Maintain ongoing strength training after return
Treatment
If graft failure occurs and instability affects function:
- Revision ACL reconstruction: More complex than primary surgery
- May require staged procedure: Bone grafting of tunnels before reconstruction
- Different graft choice: Often uses autograft if allograft failed
- Longer recovery: Often 18-24 months for return to sport
Re-injury Statistics
Young athletes returning to Level I sports have a 15-25% chance of second ACL injury (either knee) within 2 years. This risk is highest in the first 2 years after return to sport. Meeting all criteria and maintaining prevention programs reduces this risk.
Infection
What Is It?
Surgical site infection is uncommon after ACL surgery (1-2% incidence) but serious. Infections can be superficial (skin level) or deep (within the joint).
Signs and Symptoms
- Increasing redness: Around incisions or spreading
- Warmth: Knee significantly warmer than other side
- Swelling: Increasing or not improving as expected
- Drainage: Pus or cloudy fluid from incisions
- Fever: Temperature over 101°F (38.3°C)
- Worsening pain: Pain increasing rather than improving
- Feeling unwell: General malaise, fatigue, chills
When to Seek Immediate Care
Contact your surgeon immediately if you have:
- Fever over 101°F
- Drainage from incision sites
- Rapidly spreading redness
- Severe pain not controlled by medications
Prevention
- Follow wound care instructions carefully
- Keep incisions clean and dry until cleared
- Avoid submerging in water (pools, baths) until healed
- Take prescribed antibiotics as directed
- Don't remove dressings early
Treatment
- Antibiotics: IV or oral depending on severity
- Surgical washout: Arthroscopic irrigation and debridement
- Graft retention vs. removal: Depending on timing and severity, graft may need removal
- Staged reconstruction: If graft removed, reconstruction after infection clears
Cyclops Lesion
What Is It?
A cyclops lesion is a nodule of fibrous scar tissue that forms in front of the ACL graft. It gets its name from its appearance on MRI—it looks like a single eye. This nodule can block full knee extension.
Signs and Symptoms
- Loss of terminal extension (last few degrees)
- Mechanical block—feels like something stopping extension
- Pain at end-range extension
- Clicking or clunking sensation
- Extension loss that doesn't improve with therapy
Diagnosis
- Clinical exam showing extension deficit with mechanical block
- MRI showing nodule anterior to ACL graft
- Confirmed arthroscopically
Treatment
- Arthroscopic excision: The standard treatment—surgically remove the nodule
- Usually outpatient: Minor procedure compared to original surgery
- Good outcomes: Most patients regain full extension after excision
- Early aggressive extension: Important post-excision to prevent recurrence
Not Your Fault
Cyclops lesions aren't caused by anything you did wrong. They result from how your body healed. If you have a persistent extension block despite good therapy effort, bring it up with your surgeon—a quick arthroscopic procedure often solves the problem.
Other Potential Complications
Donor Site Pain
If you had an autograft (your own tissue), the harvest site may cause symptoms:
- Patellar tendon (BTB): Anterior knee pain, kneeling discomfort, patellar fracture risk (rare)
- Hamstring: Hamstring weakness, cramping, numbness on inner leg
- Quad tendon: Quad weakness, suprapatellar discomfort
Most donor site symptoms improve significantly by 1-2 years, though some residual effects may persist.
Numbness
Numbness around incision sites is common due to small sensory nerves cut during surgery. Most common areas:
- Outer knee below kneecap
- Inner shin (hamstring harvest)
Some sensation may return over 1-2 years, but some permanent numbness is common and usually not problematic.
Blood Clot (DVT)
Deep vein thrombosis is uncommon but serious. Signs include:
- Calf pain and tenderness
- Swelling in lower leg (more than expected)
- Warmth and redness of calf
If suspected, seek medical care immediately. Prevention includes early mobilization and blood thinners if prescribed.
Hardware Problems
Screws and other fixation devices occasionally cause issues:
- Hardware irritation (pain over screw site)
- Screw migration (rare)
- Hardware removal may be needed if problematic
When to Contact Your Surgeon
Seek Immediate Care If:
- Fever over 101°F (38.3°C)
- Drainage (pus) from incision
- Rapidly spreading redness
- Severe pain not controlled by medications
- Calf pain with swelling (possible blood clot)
- Chest pain or difficulty breathing
Contact Within 24-48 Hours If:
- Increasing redness around incision
- Swelling that's worsening rather than improving
- New giving-way episodes
- Mechanical block preventing full motion
- Pain that's worsening rather than gradually improving
Discuss at Next Appointment:
- Range of motion concerns
- Persistent numbness
- Donor site discomfort
- Progress concerns