It’s hard to believe that the world’s most famous sports injury — the ACL tear — is still so misunderstood, even among world-leading experts. Despite billions being spent on research, thousands of papers published, regular conferences with robust discussion and ACL societies in every country – this one simple principle is not understood or spoken about.
I’m not just talking about whether the ACL can heal. Thankfully, the tide is turning, and perhaps 30% of sports medicine professionals now accept that it can.
The real secret — the simple fact that almost no one knows — is this:
👉 The ACL heals best when the knee is kept in a bent position.
I’d estimate that fewer than 1% of healthcare professionals understand this principle. Walk into almost any Accident & Emergency department with a fresh ACL rupture, and you’ll likely leave in a long Zimmer splint keeping the knee straight. The position that puts the ACL under stretch — pulling the stumps apart and reducing the chance of healing.
Contrast this with the posterior cruciate ligament (PCL). It’s widely accepted that the PCL heals well if the knee is straight, and bracing in the straightened position during the healing window is standard management. Surgery is seldom performed. As the names suggests, the PCL and ACL are opposites: Anterior Vs Posterior — when one is taut, the other is slack. The ACL is taut in extension (straight) and slack in flexion (bent). And we want the tissues held in the lax position so the separated tissues can meet, and form an ‘exuberant bridge of healing’. And if the healing process is initiated during the first three weeks, it continues for years.
The principle of approximation — holding damaged tissues close together to promote healing — is a cornerstone of orthopaedic care. It’s what screws, plates, and pins achieve in surgery – the hardware doesn’t provide the long term stability – it simply holds the tissue close enough together for the body to do its work.
This information on the taut/lax positions of ACL has been available for years. It’s been published in research papers (Jordon et al. AJSM (2007) – see graphs in the comments), it’s also easily observed in cadaver studies, and by simply positioning the knee in different angles and strumming an uninjured ACL during surgery.
Yet this knowledge was lost in the rush to surgically reconstruct every ACL tear and hushed up by postulating that the ACL did not have blood supply and therefore could not heal. Surgeons rightly focused on their techniques (graft types, tunnel angles, screw selection etc); and we physios focused on post-op rehab. Both approaches paid the bills, so few questioned the system.
But if we truly want to give the body a chance to heal an ACL, maybe we should listen to my old Coach Klaus’s simple wisdom that he shouted at us for years:
“Bend ze kneeeeees”