Multi-Ligament Knee Injuries

A multi-ligament knee injury (MLKI) is injury to two or more of the four major knee ligaments: the ACL, PCL, MCL and LCL/PLC. A knee dislocation is complete loss of tibiofemoral contact, and a knee dislocation is a multi-ligament injury until proven otherwise.

Who. Men outnumber women 3:1 and the peak age is 20-40 years. Motor vehicle accidents are the most common cause (55%), then sports injuries (25%) and falls and industrial accidents (20%).

Mechanism. High-energy trauma (motor vehicle accidents, pedestrian versus car, motorcycle) carries a higher vascular injury rate and polytrauma. Low-energy injuries, from sport (cutting, pivoting, contact, hyperextension) or a simple fall, are usually isolated and the soft tissues are in better condition; the vascular risk is lower but is still checked.

Ultra-low velocity dislocation. Obese patients (BMI greater than 40) can dislocate the knee with minimal trauma, and these injuries are often missed because the mechanism seems trivial. They have similar rates of vascular injury to high-energy mechanisms, so the trivial mechanism is no reassurance: keep a high index of suspicion in an obese patient with multiligament laxity, and complete the vascular assessment.

The reduced dislocation. Up to 50% of dislocations reduce spontaneously before imaging, which is why a high index of suspicion is needed: any multiligament laxity after knee trauma means a dislocation is assumed to have occurred.

Associated injuries. The rates reported with the dislocation:

• Popliteal artery injury: 5-32%
• Common peroneal nerve palsy: 25-40%
• Meniscal tears: 40-50%
• Periarticular fractures (Schenck KD-V): 15-20%
• Tibial plateau or femoral condyle involvement

Four ligaments. The knee relies on four major ligaments working in concert, and each fails in a recognisable way:

The PCL. The PCL is the central pillar of knee stability: it is stronger than the ACL and provides the primary restraint to posterior translation. In a multi-ligament injury the quality of the PCL reconstruction correlates strongly with outcome, which is why it is the priority at surgery.

The posterolateral corner. The PLC is a complex of structures providing varus and rotational stability:

• Fibular collateral ligament (FCL/LCL): primary varus restraint
• Popliteus tendon: primary restraint to external rotation at 30 degrees of flexion
• Popliteofibular ligament: links popliteus to the fibular head; a secondary stabiliser
• Lateral capsule: secondary stabiliser
• Arcuate ligament: variable importance

Why the PLC must not be missed. It is the most commonly missed structure, and chronic PLC deficiency causes failure of ACL and PCL reconstructions by increasing the stress on the grafts. It has to be assessed clinically, with the dial test and external rotation recurvatum, and addressed surgically.

The popliteal artery. The artery is at risk because it is tethered at two points, the adductor hiatus proximally and the soleal arch distally, and it travels close to the posterior capsule, so it cannot move away from a displacing tibia. Intimal tears may not be immediately evident.

Two vascular figures, two populations. Popliteal artery injury was reported in 32% of the high-energy dislocations in Green and Allen's 1977 collected series; across all ICD-coded knee dislocations in a modern US database the figure is 1.1%. Both are true of different populations - the low number is diluted by subluxations and spontaneously reduced injuries - so treat the patient in front of you as high risk.

The common peroneal nerve. It wraps around the fibular neck, which is what puts it at risk in lateral dislocations and PLC injuries.

Schenck's anatomic classification is the most commonly used and the most surgically relevant. Kennedy's, by direction of tibial displacement, is useful for understanding the mechanism, and the time from injury decides whether repair is still possible.

History. The mechanism, and the direction of force if it was witnessed, is what tells you the likely pattern. Ask whether the knee was felt to go out or snap back, whether there was gross deformity at the time, and how long ago it happened.

Examination priorities. Before any ligament is tested, the findings that change what happens next:

Vascular assessment. Life before limb, limb before function. Palpate the popliteal, dorsalis pedis and posterior tibial pulses, check capillary refill and calculate the ankle-brachial index, which is normal at more than 0.9. What happens next depends on the finding:

The ABI threshold. An ABI below 0.9 is abnormal and requires further investigation. In Mills's prospective series an ABI of 0.9 or above had a 100% negative predictive value, but that was 27 patients with no missed injury, and the confidence interval around a perfect result on numbers that small is wide, so it does not mean the test never misses.

Why a normal ABI buys observation, not discharge. Two things make the point practical. Those patients were verified by serial examination and duplex rather than by arteriography, so a non-occlusive intimal flap could have been present and silent; and the ABI is unreliable in exactly the patients you worry about - peripheral arterial disease, diabetic medial calcification, or an ipsilateral tibial or ankle injury. Serial examination is therefore mandatory, and intimal tears can progress.

Ligament examination. Once the vascular status is confirmed, each ligament is tested in turn:

Nerve examination. Three nerves are tested and documented:

• Common peroneal: ankle dorsiflexion, toe extension, lateral leg sensation
• Tibial nerve: ankle plantarflexion, toe flexion, plantar sensation
• Saphenous: medial leg and ankle sensation

Differential diagnosis. The acutely unstable or grossly swollen knee after trauma has several mimics. The cardinal point is that a multiligament injury or spontaneously reduced dislocation must be actively excluded, because the others do not carry the same limb-threatening vascular risk.

Radiographs. AP and lateral views of the knee, which may show the joint reduced or still dislocated. Look for periarticular fractures and calculate the tibial subluxation. An avulsion fragment names the ligament that pulled it off:

• Fibular head - PLC
• Segond fragment - ACL/PLC
• PCL avulsion

CT. If the reduction is maintained, CT is helpful for fracture assessment. CT angiography is the study of choice for vascular assessment and has largely replaced conventional angiography for initial assessment: it is non-invasive, rapid and highly sensitive, with a sensitivity conventionally quoted as greater than 95% for significant arterial injury in extremity trauma, although no knee-dislocation-specific series cited here measures it. Conventional angiography is reserved for planned intervention.

MRI. MRI is essential for surgical planning. It defines which ligaments are injured, separates repairable from reconstructable injuries, and shows meniscal tears (common), cartilage injury and PLC involvement. It is obtained once the patient is stable and the vascular status confirmed, usually within the first week, and may be limited by swelling initially.

Algorithm

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Priorities in the emergency department. Vascular assessment comes first, as set out under assessment, and everything else waits on it. Reduction is second, stabilisation third, and the ligaments are planned last.

Reduction. If the knee is still dislocated, reduce it urgently under sedation with longitudinal traction and reversal of the deformity, then take post-reduction radiographs and check the pulses again.

Stabilisation. Splint in slight flexion, 20-30 degrees, and avoid hyperextension, which stretches a vascular repair. A grossly unstable knee may need a spanning external fixator.

After reduction. Serial vascular examination continues, with ice, elevation and DVT prophylaxis. MRI follows when the swelling allows, and surgical planning follows the MRI.

Timing. The optimal window is 1-3 weeks from injury: the swelling has settled, the tissues remain identifiable for repair or reconstruction, the capsule has not yet scarred or contracted, and there has been time for MRI planning. Operating in the first week finds identifiable structures but at the cost of swelling and the risk of arthrofibrosis; waiting beyond six weeks allows definitive planning but the scarring means repair is no longer possible. The early window does require a stable patient. Levy's "early" (within three weeks) and Fanelli's "delayed" (two to three weeks) in the evidence base both land on this same window.

Some injuries cannot wait for the window:

• Open injury (urgent washout)
• Irreducible dislocation
• Vascular repair requiring a stable knee
• Trapped nerve requiring release
• Locked knee from a meniscal tear

Others justify delay beyond it: the polytrauma patient requiring stabilisation, severe soft tissue injury requiring a staged approach, or medical comorbidities requiring optimisation.

Goals and principles. Surgery aims to restore knee stability, allow early rehabilitation, prevent arthrofibrosis and optimise long-term function. Every injured structure is addressed, especially the PCL and PLC, with the PCL as the priority. The MCL and some PLC injuries can be repaired; the ACL, PCL and most PLC injuries are reconstructed; and the reconstruction is staged where severe injury, a stiff knee or a revision demands it.

Vascular. Where arterial repair was delayed beyond eight hours, 86% came to amputation (Green & Allen 1977, PMID 845209). That is a historical figure from before duplex, CT angiography and endovascular repair, so it is not a contemporary prognosis, but it is the origin of the eight-hour rule and the reason the clock starts at injury rather than at diagnosis. Missed injuries are due to intimal tears, compartment syndrome may follow reperfusion, and a stenosed vessel can leave long-term claudication.

Nerve. Recovery of a common peroneal nerve palsy is variable, and only 50% make a meaningful recovery; consider tendon transfers if there is no recovery by 12 months. Posterior tibial nerve injury is less common but more devastating.

Stiffness. Arthrofibrosis is the most common complication affecting function. Prevention is optimal surgical timing, early range of motion and sometimes a hinged knee brace allowing protected motion; if more than 90 degrees is lost, consider manipulation under anaesthesia at 6-12 weeks.

Persistent instability. Usually a missed PLC injury or an inadequate PCL reconstruction, and it requires revision surgery, addressing all structures and considering alignment.

The protocol. Progression is slower than after an isolated ACL reconstruction and return to sport is later than after an isolated ligament injury; motion restoration is balanced against stability, and the pace is set by tissue quality and compliance.

Protecting the PCL graft. The hamstrings cause posterior tibial translation, which stresses the PCL graft, so active hamstring work is avoided in the early postoperative period and the programme is built on quadriceps-dominant exercises.

By pattern. The Schenck type predicts the outcome:

Prognostic factors. Early surgery, complete reconstruction and good compliance predict a good result; vascular injury, nerve injury, delayed surgery and a missed PLC predict a poor one. A missed or inadequately treated PLC is the most common cause of failure.

Return to sport. Reported at 60-80% overall, better for KD-I and KD-II (70-80%) and guarded for KD-IV (40-60%). Return to high-level sport is often not achieved.

Long-term concerns. Post-traumatic osteoarthritis develops in 20-50%. Residual laxity is often acceptable if the knee is functional; stiffness needs intervention in 10-20% and chronic pain affects 10-15%.

Global epidemiology. Knee dislocation is rare. In a US national all-payer dataset of 99,688 coded knee dislocations (2010-2022), associated vascular injury occurred in only 1.1%, of which one quarter required repair (Dubin et al, J Orthop 2024). This contrasts sharply with the historical 30% figure derived from high-energy true dislocations, and the gap reflects case mix rather than contradiction. Prospective national series confirm a young, male-predominant population (mean age around 37, roughly 80% male) with common peroneal nerve injury in the order of 18% (Lustig et al, Orthop Traumatol Surg Res 2009). Obesity, including ultra-low velocity dislocation, is an independent risk factor for vascular injury and for missed diagnosis worldwide.

Major guidance, side by side. No single high-level society guideline governs the multiligament knee; practice is driven by expert consensus and systematic-review evidence, which is remarkably consistent across regions:

The areas of genuine international agreement are: vascular assessment first with ABI 0.90 as the action threshold; early surgery (within roughly 3 weeks) beats delayed surgery; and the posterolateral corner should be reconstructed rather than repaired in isolation (Levy et al, Arthroscopy 2009).

Registry evidence. There is no dedicated multiligament knee registry; this is a ligament-reconstruction rather than an implant-arthroplasty problem, so the major joint registries (NJR, AJRR, AOANJRR, Swedish/SHAR, Norwegian, NZJR) do not capture it. The relevant population-level data come instead from national administrative datasets and prospective multicentre cohorts (e.g. the French Society of Orthopaedic Surgery series and US national database studies cited above). Where end-stage post-traumatic arthritis later requires arthroplasty, those joint registries become relevant for implant selection and survivorship.

Global practice variation.

• High-resource settings favour MRI-planned, single-stage anatomic reconstruction of all torn ligaments at 1-3 weeks, often with allograft.
• Limited-resource settings more often rely on staged surgery, autograft (allograft and graft banks may be unavailable), bracing of selected collateral injuries, and acceptance of non-operative management in older or comorbid patients.
• Allograft availability is the single largest driver of technique differences worldwide; autograft-only strategies (hamstrings, quadriceps, contralateral grafts) are well described where banks are limited.