KNEE · MCL, LCL AND POSTEROLATERAL CORNER

Inner side heals. Outer side often does not. The difference decides the treatment.

The two sides of the knee behave in opposite ways after injury. The MCL on the inner side heals reliably and rarely needs surgery. The outer side heals poorly and is the structure most often missed, and missing it is the commonest reason an ACL reconstruction stretches out and fails.

Dr. Prashant Parate · Shoulder, Knee & Sports Injury Surgeon · In orthopaedic practice since 2010, DNB-qualified since 2012 · Jupiter Hospital (Baner) & Sainath Hospital (Moshi, PCMC) · Reviewed July 2026

Why the two sides differ so much

The MCL on the inner side is a broad, flat ligament with a generous blood supply, lying against the joint capsule. Torn ends stay close together and it heals well, which is why most MCL injuries are treated in a brace and never see an operating theatre.

The outer side is not one ligament but a group of structures working together, the LCL, the popliteus tendon and the popliteofibular ligament, collectively called the posterolateral corner. They are narrow, cord-like, and when torn the ends retract and scar in the wrong position. They do not reliably heal, and once healed badly they cannot simply be repaired later.

The result is an asymmetry that governs everything: inner side, be patient. Outer side, act early.

The four knee ligaments showing the MCL on the inner side and the LCL with the posterolateral corner on the outer side

MCL injury

Typically a blow to the outside of the knee forcing it inward, or a twist with the foot planted. Pain and tenderness along the inner side, sometimes with a sense of the knee opening up on that side.

Grade	What it means	Treatment
Grade I	Stretched, tender, no opening on testing	Brace briefly, early movement, back to activity in a few weeks
Grade II	Partial tear, some opening with a firm endpoint	Hinged brace for around six weeks, progressive rehabilitation
Grade III	Complete tear, clear opening with no endpoint	Usually still non-operative in a brace, and most heal. Surgery for specific situations only

When an MCL does need surgery: when the torn end flips outside the tendons and cannot reach its bed to heal, when it is part of a multi-ligament injury, when a bony avulsion has occurred, or when significant looseness persists after a proper trial of bracing.

With an ACL tear: the usual approach is to brace the MCL and let it heal first, then reconstruct the ACL once the inner side is stable and the knee has regained movement. Operating on both at once increases the risk of stiffness.

The posterolateral corner: the one that gets missed

Injury to the outer side comes from a blow to the inner side of the knee forcing it outward, from hyperextension, or from a twisting injury, and it very often accompanies a cruciate tear rather than occurring alone.

Two clinical clues matter. Foot drop or numbness on the top of the foot, because the common peroneal nerve runs right through this region and is injured in a significant proportion of these cases. And a knee that hyperextends and rotates outward, or that feels unstable walking on uneven ground in a way that does not fit a simple ACL tear.

Why missing it matters so much

An ACL reconstruction in a knee with an unrecognised posterolateral corner injury is being asked to resist forces it was never designed to take. The graft stretches, and the reconstruction fails, often months later. Unrecognised posterolateral corner injury is one of the leading causes of failed ACL reconstruction, and the failure is usually blamed on the graft or on the patient rather than on the structure nobody examined.

How it is assessed

Specific tests are needed, and they are not part of a routine knee examination unless the examiner is looking for this. Opening on the outer side with the knee slightly bent tests the LCL. The dial test, comparing how far each foot rotates outward with the knee bent to thirty and to ninety degrees, tests the rotational component and helps establish whether the PCL is also involved. Both knees are always compared.

MRI shows these structures well in the first few weeks after injury, which is another argument for early assessment, since the findings become much harder to interpret once scarring sets in. Standing alignment films matter in chronic cases, because a bow-legged limb overloads exactly the structures that are already failing.

The decision pathway

1. Which side is injured?

Inner side: expect to treat it in a brace and expect it to heal. Outer side: expect to need surgery, and expect the timing to matter. This single question changes the whole plan.

2. Is anything else torn?

An isolated collateral injury is a different problem from the same injury alongside a cruciate tear. Combined injuries shift the balance firmly towards surgery, and towards doing it sooner. Assess every structure before planning anything.

3. If the outer side is torn, how long ago?

Within roughly the first two to three weeks, the torn structures can often be repaired directly, which uses your own tissue and generally gives a better result. Beyond that they have scarred in the wrong position and reconstruction with graft is needed instead. Early repair beats late reconstruction.

4. Is the limb bow-legged, in a chronic case?

A varus limb loads the outer side constantly and will stretch out any reconstruction. Realignment by osteotomy is done first, and in some patients that alone restores enough stability that no ligament surgery is needed at all. Correct the bone before the ligament.

5. Has an ACL reconstruction already failed?

Then the outer side and the alignment both need assessing before revision surgery is planned. Repeating the same ACL reconstruction without addressing why the first one stretched will produce the same outcome. Find the cause before revising.

Recovery

MCL, treated in a brace: a hinged brace for around six weeks depending on grade, with movement encouraged throughout. Most patients return to sport somewhere between six weeks and three months.

Posterolateral corner surgery: protected weight-bearing and a brace for around six weeks, then progressive range and strengthening. Return to demanding activity is generally around nine months, and longer when the cruciates were reconstructed at the same time.

Frequently asked questions

My MCL is completely torn. Why am I not being operated on?

Because complete MCL tears heal well in a brace in the large majority of cases, and the results of non-operative treatment are as good as surgery with none of the risk. This is one of the situations in orthopaedics where doing less is genuinely better. What is important is that the rest of the knee was examined, since an MCL tear frequently accompanies other injuries.

Why does the outer side need surgery when the inner side does not?

Blood supply and anatomy. The MCL is broad, well supplied and lies against the capsule, so torn ends stay in contact and heal. The outer structures are narrow cords with poorer supply, and when torn they retract and heal in a lengthened, scarred position that never regains proper function.

My ACL reconstruction failed and nobody can say why. What should be checked?

Three things in particular: whether the posterolateral corner was injured and never addressed, whether the limb alignment is driving load through the graft, and whether the original tunnels were positioned correctly. All three are assessable, and revision surgery planned without answering them tends to fail the same way.

I have numbness on the top of my foot after a knee injury. Is that related?

Very likely. The common peroneal nerve runs around the outer side of the knee and is injured in a meaningful proportion of posterolateral corner injuries. It should be documented and monitored, and its presence raises the suspicion of an outer-side injury even before any scan.

Can these be treated with physiotherapy alone?

MCL injuries, generally yes. Isolated low-grade outer-side injuries, sometimes. Complete posterolateral corner injuries, generally no, because physiotherapy cannot restore a structure that has healed in a lengthened position. Strengthening remains essential either way, but it is not a substitute for reconstruction where reconstruction is needed.

Key takeaways

MCL heals. The posterolateral corner usually does not. The plans are opposite.
Even complete MCL tears are mostly braced rather than operated.
Outer-side injuries repair better within the first two to three weeks than they reconstruct later.
Unrecognised posterolateral corner injury is a leading cause of failed ACL reconstruction.
Foot drop or numbness on the top of the foot points to an outer-side injury.
In chronic bow-legged knees, correct the alignment before reconstructing anything.

Knee still unstable, or an ACL reconstruction that failed?

Bring your scans and any previous operation notes. The outer side and the limb alignment are the two things most often missed, and both are assessable in a single consultation.

Appointments by phone

Sainath Hospital, Moshi, PCMC
86000 05886
Mon to Fri, 10 am to 4 pm · Sat, 10 am to 12 noon

Jupiter Hospital, Baner, Pune
020 2799 2150
Mon to Sat, 5.30 pm to 6.30 pm · OPD 5

This page is patient education, not a substitute for consultation. Treatment decisions require clinical examination and imaging review. Last medically reviewed: July 2026 by Dr. Prashant Parate, DNB Orthopaedics, Fellowship in Arthroscopy & Sports Medicine (Thammasat University Hospital, Thailand). MMC Reg. 2006042129.