The anterior cruciate ligament (ACL) is one of the key stabilising ligaments in the knee joint. ACL ruptures are a common injury, particularly in pivoting sports such as netball, AFL, soccer and basketball. This injury can have a significant impact on mobility and activity levels, particularly in younger and active individuals. Understanding the anatomy, causes, diagnosis and management options is essential to making informed decisions about treatment and recovery.
Anatomical Overview
The ACL is one of four major ligaments in the knee, along with the posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL). The ACL runs diagonally through the centre of the knee, connecting the femur (thigh bone) to the tibia (shin bone).
Its primary function is to prevent the tibia from moving too far in front of the femur (anterior translation). It also provides rotational stability during pivoting movements and helps control hyperextension of the knee
The ACL is composed of collagen fibres and some areas of the ligament have limited blood supply compared to others. For this reason, surgical management has often been the only option. Recent research has demonstrated that the proximal (top) portion of the ACL does have capacity for healing, changing management options.
Mechanisms of Injury
Most ACL ruptures occur via non-contact mechanisms, which account for around 70% of cases. Common scenarios include:
- Hyperextension of the knee (often landing from a jump).
- Sudden deceleration combined with a change in direction (cutting)
- Landing from a jump with poor knee alignment (e.g. knee collapsing inwards)
- Pivoting on a planted foot
Contact injuries may also cause ACL rupture, particularly from a direct blow to the outer knee when the foot is planted, pushing the knee inward and creating excessive rotational force.
Risk factors include:
- Poor neuromuscular control
- Weakness in hip or core muscles
- Previous ACL injury
- Anatomical differences (e.g. narrower intercondylar notch)
- Female sex (due to anatomical and hormonal factors)
Signs & Symptoms
An ACL rupture is often associated with the following symptoms:
- A “pop” sound or sensation at the time of injury
- Immediate swelling (within 1–2 hours) due to haemarthrosis (bleeding into the joint)
- Instability or a feeling of the knee “giving way” during pivoting
- Pain, particularly in the initial phase
- Reduced range of motion
- Difficulty bearing weight
Diagnosis
Diagnosis involves a combination of clinical examination and history initially, followed by imaging for confirmation.
Clinical tests used are primarily completed by a physiotherapists. These tests may include Lachman, anterior drawer and Lever’s sign. The Physiotherapist will use the results of these tests, as well as your symptoms and injury mechanism to determine if your ACL may be at risk. You will likely then be referred to your GP, who can provide referral for a bulk billed MRI if there is suspicion of an ACL rupture.
ACL injuries are best confirmed via MRI, which demonstrate bony and soft tissue injuries.
X-rays may be used to exclude fractures, particularly in adolescents (to rule out tibial spine avulsion).
Grading
ACL injuries are typically classified as:
- Grade I – Mild sprain with no significant tearing
- Grade II – Partial tear
- Grade III – Complete rupture (focus of this guide)
Complete ruptures result in functional instability and are most likely to require surgical or very structured conservative management.
Prognosis
Regardless of the chosen management option, return to sport is likely to take at least 9-12 months.
Long-term prognosis of the knee depends on several factors including age, activity level, associated injuries, and management approach. Without appropriate rehabilitation, individuals with ACL rupture are at increased risk of:
- Recurrent instability episodes
- Secondary meniscal damage
- Early onset osteoarthritis
However, most individuals can return to sport or function, either with or without surgery, provided a structured and well-supervised rehab program is followed.
Management Options
Surgical Reconstruction
Surgical management involves reconstructing the torn ACL, rather than repairing it. The damaged ligament is replaced with a graft, usually harvested from the patient (autograft) or a donor (allograft). Common graft options include:
- Hamstring tendon – Most common in Australia; minimal impact on kneecap pain; good strength outcomes
- Patellar tendon – Strong graft; may lead to anterior knee pain or kneeling discomfort
- Quadriceps tendon – Increasing in popularity; robust size; fewer donor-site issues
Surgery is typically followed by 9–12 months of structured Physiotherapy. The timing of surgery may vary depending on knee swelling, range of motion and prehab status. Many clinicians advocate a “prehabilitation” phase, including 4–6 weeks of physio to optimise outcomes. Evidence demonstrates that those who complete a prehabilitation phase have a much better outcome when returning to sport.
Surgical reconstruction is preferred by those who plan to return to pivoting sports and high impact activities. It is the management option with best clinical evidence for long term effectiveness.
Some reasons to not have surgical reconstruction include; surgical risks (Infection, graft failure, stiffness), as well as high levels of cost in comparison to other interventions. Surgery is often interpreted as a ‘fix’, however it still requires an extensive rehab period of at least 9-12 months for return to sport. Those that do not appropriately complete these rehab period are at high risk of re-rupture.
Conservative (Non-Surgical) Management
Conservative treatment focuses on physiotherapy-based rehabilitation to strengthen other areas which can compensate for the lack of an ACL. As we know, the role of the ACL is to prevent the anterior movement of the tibia in relation to the femur. This action can also be completed by what is known as ‘co-contraction’ of the quadriceps and hamstrings. This involves the two muscles activating at similar times to control the tibia. If co-contraction, or muscle control is good enough, the ACL is not required, as long as activities are not too demanding.
This option is increasingly considered viable, especially for older or less active individuals, those not involved in pivoting or cutting sports and patients who respond well during early rehab (e.g. no giving way episodes).
Key components of conservative rehab include:
- Strengthening of quadriceps, hamstrings, glutes, and core
- Neuromuscular training to improve joint control
- Balance and proprioceptive exercises
Some individuals may initially trial conservative management and choose surgery later if instability persists — often referred to as the “delayed surgical” approach.
Benefits of the conservative approach include avoiding surgery, which reduces cost and risk. It also enables an earlier return to daily activities. Negative aspects include a reduction in knee stability, potential secondary meniscal injuries if control is not good enough, and no guarantee that surgery will not be required eventually.
Long-term research (e.g. the KANON trial) has shown that with proper rehabilitation, non-surgical outcomes can be comparable to surgery for selected individuals.
Cross Bracing Protocol
The cross bracing protocol is a promising recent area of research. It is a conservative approach that aims to promote biological healing of the ACL rather than replacing it, via bracing and immobilisation. It involves:
- Wearing a range of motion brace that holds the knee locked in a flexed position (90°) for around 4 weeks, followed by a gradual 8 week program of increase range of motion (total of 12 weeks in a brace).
- Gradual unlocking of the brace in stages
- A structured physiotherapy program alongside bracing
The cross bracing protocol is only suitable for certain patients. Patients must begin the program in the acute stage (within 4 weeks of injury), have minimal damage to surrounding structures (cannot be done if any MCL/LCL tears) and the tear must be in a certain location which has capacity to heal.
Evidence is quite new, and minimal long term studies have been conducted. Research shows extremely promising outcomes of ACL healing and return to sport in the short and medium term time-frames (1-2 years).
Benefits include natural ACL healing, with preserved original ligament and proprioception (rather than a donor site in surgery). Negatives include limited availability and suitable patient criteria, immobilisation, and limited long term evidence comparatively to surgery.
Click here for more details on the Cross Bracing Protocol.
Comparison Table: ACL Management Options
| Feature | Surgical Reconstruction | Conservative Management | Cross Bracing Protocol |
|---|---|---|---|
| Primary Goal | Replace ACL with graft | Strengthen and stabilise via rehab | Heal native ACL through immobilisation |
| Who it’s for | Young, active individuals in pivoting sports | Older, less active, or good initial stability | Acute, proximal tears in compliant patients |
| Invasiveness | Invasive (requires surgery) | Non-invasive | Non-invasive (brace only) |
| Rehab Duration | 9–12 months | 9–12 months | 9–12 months |
| Return to Sport | Often successful in pivoting sports | Possible, with restrictions for pivoting | Emerging evidence in short term, more data needed long term |
| Risk of Re-Injury | Moderate (especially under 20s) | Higher if instability persists | Unknown (still under study) |
| Cost | High (surgical + rehab) | Lower (rehab only) | Moderate (brace + rehab) |
| Access | Widely available | Widely available | Limited access (mostly research/early adopters) |
| Evidence Base | Strong; long-term follow-up available | Moderate; long-term comparable in some cases | Promising early studies (e.g. from Norway, Australia) |
| Potential Complications | Infection, stiffness, graft failure | Meniscal damage if unstable | Stiffness, incomplete healing, compliance issues |
Physiotherapy and Rehabilitation
Physiotherapy is essential in all management approaches – pre-operative, post-operative, conservative and cross-bracing. The goals include restoring range of motion, improving strength (especially quadriceps and hamstrings), neuromuscular control, balance, and functional movement.
Most physiotherapists will follow the Melbourne ACL Guide for rehabilitation. This a structured, evidence based program that begins from initial injury, all the way until return to sport and preventing re-injury.
Melbourne ACL Rehabilitation Guide
Developed by La Trobe University researchers and clinicians, this evidence-based protocol outlines a criteria-based progression through rehab, rather than a time-based approach. Key features include:
- Five rehabilitation phases, from acute injury to return to performance
- Objective testing to progress between phases, including:
- Range of motion
- Strength ratios (e.g. limb symmetry)
- Hop and agility tests
- Emphasis on high-quality strength training, particularly quadriceps
- Running mechanics and jump-landing retraining
- Psychological readiness assessment before return to sport
This approach helps reduce the risk of re-injury and improves long-term knee health. The protocol can be used following any management path (surgical or conservative).
Phases of Rehab
Pre-Op
The primary focus of this stage is to get ready to have surgery for cases who have chosen this route. Research demonstrates that those who achieve the below goals before surgery have much better outcomes after surgery.
Key Aims:
- Eliminate swelling and reduce pain
- Achieve full range of motion
- Restore hamstrings and quadriceps strength (>90% of other side)
- Begin normalising walking (with gait aids if required)
Phase 1: Recovery from Surgery / Acute Management
This stage is aimed at recovery after surgical cases, as well as acute injury management in cases that do not have surgery. This is similar to the pre-op phase, and is aiming to reduce swelling, pain, improve range of motion and get muscles turning on again.
Key Aims:
- Regain full range of motion
- Reduce swelling
- Improve quadriceps activation
Phase 2: Strength and Control
Phase 2 involves beginning to restore neuromuscular control and facilitate co-contraction. Rehab will likely involve body weight exercises, focusing balance, coordination and control.
Key Aims:
- Regain single leg balance
- Return muscle strength within 85% of the other side (quadriceps, hamstrings, glutes, calves etc).
- Single leg squats with good mechanics and control
Phase 3: Running and Agility
Once you have satisfied the goals of phase 2, and we are confident you have full strength and good knee control, it’s time to begin some running and jumping. This phase should be pain-free, and there is a large focus on correct technique with agility, jumping and hopping exercises.
Key Aims:
- Regain full strength and balance
- Jump and land with great technique and power and control.
- Complete agility drills with no pain.
Phase 4: Return to Sport
The return to sport phase is different for each client, depending on their goals. Patient-specific rehab is the priority of this stage, and you should be getting back to most pre-injury activities. Neuromuscular control is the main goal, however it is also important to build confidence in your knee as you prepare to get back into sport. Return to sport will typically not happen before the 9 month mark, and may take longer.
Key Aims:
- Score >95 on the Melbourne Return to Sport Score
- Be confident and feel ready to return to sport/activity
- Begin implementing an injury prevention program
Phase 5: Prevent Re-Injury
It is important to maintain the great strength and control you have built up over your rehabilitation. If prevention programs are not followed, risk of re-rupture increases. There is no specific program to follow, as long as it includes plyometric, balance and strength exercises. This program should be completed before every training and game for >10 minutes.
Program Examples:
- Sportsmetric Program
- The FIFA 11+ Warmup
- PEP program
- KNEE Program – Netball Australia
- FootyFirst Program – AFL
References
- Grindem, H. et al. (2016). “Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction.” British Journal of Sports Medicine.
- Culvenor, A. G. et al. (2021). “Melbourne ACL Rehabilitation Guide.” La Trobe University.
- Webster, K. E. & Feller, J. A. (2019). “ACL injury and reconstruction: what we have learned from the last 30 years.” Journal of Orthopaedics and Traumatology.
- Eitzen, I. et al. (2010). “A progressive 5-week exercise therapy program leads to significant improvement in knee function early after ACL injury.” British Journal of Sports Medicine.