ACL Surgery or Physiotherapy? What the SNNAP Trial Found
Key points
- SNNAP was a large UK trial (316 patients across 29 NHS hospitals) comparing two treatments for longer-standing (non-acute) ACL injuries with ongoing instability: surgical reconstruction first, or a course of physiotherapy first.
- At 18 months, patients who had surgery first had better knee scores for pain, function, sport and quality of life than those who started with physiotherapy.
- Surgical reconstruction was also found to be cost-effective for the NHS.
- More than a third of those who started with physiotherapy (around 38%) completed treatment without needing an operation; those who later chose surgery had caught up by 18 months.
- Surgery gave the better average result, while trying physiotherapy first remains a reasonable, low-risk option that keeps surgery open for later.
What is an ACL injury, and why is the best treatment debated?
The anterior cruciate ligament (ACL) is one of the main stabilising ligaments inside the knee. When it ruptures – often during a twisting or pivoting movement in sport – the knee can become unstable and give way. This limits activity, affects quality of life, and over time can lead to further damage to the cartilage and meniscus. ACL injuries are common, with an estimated 200,000 each year in the USA, and around 30,000 ACL reconstructions performed each year in England.
There are two broad ways to manage an ACL injury: surgical reconstruction (rebuilding the ligament, usually using the patient's own tendon) or non-surgical treatment (a structured programme of physiotherapy). For many years there has been genuine debate about which is better – particularly for “non-acute” injuries, where the initial injury happened some time ago and the knee remains symptomatic. Earlier trials had largely studied fresh, acute injuries and reached conflicting conclusions, leaving a gap in the evidence for the longer-standing patients commonly seen in the NHS.
What the SNNAP trial set out to do
SNNAP (ACL Surgery Necessity in Non-Acute Patients) was a pragmatic, multicentre randomised controlled trial – the most reliable type of study for comparing treatments – carried out in 29 NHS orthopaedic units across the UK. I was part of the study team for this trial.
Between 2017 and 2020, 316 patients with a non-acute ACL injury and persistent instability were randomly allocated to one of two management strategies:
- Surgical reconstruction first (156 patients), or
- Rehabilitation first (160 patients): a course of physiotherapy over at least three months, with the option of going on to have reconstruction later if the knee remained unstable.
Patients with problems needing urgent surgery (such as a locked knee from a large meniscal tear) or with advanced arthritis were not included. The main measure was the KOOS4 score – a well-validated questionnaire covering knee pain, symptoms, daily activity and quality of life – assessed 18 months after randomisation.
What did the trial find?
Knee function, pain and quality of life
At 18 months, the group who had surgery first scored better than the physiotherapy-first group (an average KOOS4 of 73 versus 65). The difference was around the size considered clinically meaningful, and every individual area measured – pain, symptoms, daily activities, sport and recreation, and knee-related quality of life – favoured surgery.
Avoiding or delaying surgery
Importantly, starting with physiotherapy did not mean missing out. More than a third of the physiotherapy-first group (around 38%) improved and completed treatment without ever needing an operation. A similar proportion (41%) went on to have reconstruction for ongoing instability – and by 18 months their scores had caught up with those who had surgery from the outset. Those who avoided surgery improved quickly but tended to plateau somewhat below the surgical group.
Returning to sport
Recovery from an ACL injury is significant whichever path is taken. Fewer than a third of patients in either group had returned to their pre-injury level of sport or activity by 18 months, underlining how much an ACL injury affects the knee regardless of treatment.
Safety and cost
Complications were uncommon and similar in both groups, with no serious adverse events and only rare graft failures. The trial's health-economic analysis found that surgical reconstruction was cost-effective for the NHS.
So, is surgery better than physiotherapy for an ACL injury?
For this specific group – patients with a non-acute ACL injury and ongoing symptoms of instability – the trial concluded that surgical reconstruction was the clinically superior and more cost-effective strategy. On average, people who had surgery first reported better outcomes at 18 months.
That headline comes with important nuance. Both groups improved over time. A meaningful minority did well with physiotherapy alone and avoided an operation, and those who tried physiotherapy first and later chose surgery were not disadvantaged at 18 months. So the result is best read not as “everyone needs surgery”, but as: for longer-standing ACL injuries with instability, surgery gives the better average result, while non-surgical treatment remains a reasonable first step that keeps surgery available later.
What this means for you
If you have an ACL injury with a knee that keeps giving way, this trial provides high-quality evidence to help you and your surgeon decide together. It supports offering surgical reconstruction for non-acute injuries with persistent instability, while also reassuring patients who would prefer to avoid or delay surgery that a good course of physiotherapy can still help – and that the option of reconstruction stays open if instability continues.
In practice, the right choice depends on your symptoms, your goals (including the activities and sport you want to return to), and your own preferences. Surgical reconstruction and structured rehabilitation are both part of how I manage these injuries, and I will talk through which approach fits your situation. Whichever route is chosen, committed rehabilitation makes a real difference to the outcome, and it is realistic to expect recovery to take time.
Frequently asked questions
Do I need surgery for an ACL injury, or can physiotherapy work?
It depends on your injury and symptoms. For longer-standing (non-acute) ACL injuries with ongoing instability, the SNNAP trial found surgery gave better average results at 18 months. However, more than a third of people who started with physiotherapy completed treatment without needing an operation, so non-surgical treatment is a reasonable first step for many patients, with surgery remaining an option if the knee stays unstable.
Is ACL reconstruction better than physiotherapy?
In the SNNAP trial, patients who had reconstruction first reported better knee scores for pain, function, sport and quality of life at 18 months than those who started with physiotherapy, and surgery was more cost-effective for the NHS. The average difference was around the threshold considered clinically meaningful.
What does a “non-acute” ACL injury mean?
A non-acute injury is one where some time has passed since the original rupture – often months, sometimes years – rather than a fresh, recent injury. SNNAP studied this longer-standing group with persistent instability, so its findings apply to them rather than to brand-new acute injuries or elite athletes.
If I try physiotherapy first, can I still have surgery later?
Yes. In the trial, patients who started with physiotherapy could go on to have reconstruction if their knee remained unstable. About 41% did, and by 18 months their results were similar to those who had surgery from the start – so trying physiotherapy first did not appear to disadvantage them.
Will I be able to return to sport after an ACL injury?
Recovery varies, and an ACL injury is significant. In SNNAP, fewer than a third of patients in either group had returned to their pre-injury activity level by 18 months. Good rehabilitation is important whichever treatment you have, and returning to sport often takes time and patience.
Is ACL surgery safe?
In the trial, complications were uncommon and similar between the surgical and physiotherapy groups, with no serious adverse events reported and graft failure being rare. As with any operation, your surgeon will discuss the specific risks with you.
Read the Full Paper
This trial is published open access in The Lancet under a Creative Commons licence (CC BY 4.0). You can read the full text below, or on the Lancet website.
Full citation: Beard DJ, Davies L, Cook JA, et al. Rehabilitation versus surgical reconstruction for non-acute anterior cruciate ligament injury (ACL SNNAP): a pragmatic randomised controlled trial. The Lancet. 2022;400(10352):605–615. doi:10.1016/S0140-6736(22)01424-6
This summary is written for general information and explains the research in patient-friendly terms. It does not replace individual medical advice. If you have questions about an ACL injury or your knee, please get in touch to arrange a consultation.
