# Knee Dislocations and Multiligament Injuries

## Overview

Knee dislocation is a limb-threatening emergency because of the risk of popliteal artery injury. It is defined as disruption of at least two of the four major knee ligaments (ACL, PCL, MCL, LCL/PLC). Many knee dislocations spontaneously reduce before presentation, so a normal-appearing radiograph does not rule out a prior dislocation. Vascular assessment is the immediate priority.

## Anatomy

### Ligamentous Stabilizers

The ACL is the primary restraint to anterior tibial translation and also resists internal rotation and valgus. The PCL is the primary restraint to posterior tibial translation and the strongest knee ligament, consisting of an anterolateral bundle (tight in flexion, dominant) and a posteromedial bundle (tight in extension). The superficial MCL is the primary valgus restraint, with the deep MCL serving as a capsular ligament. The LCL (fibular collateral ligament) is the primary restraint to varus stress. The posterolateral corner (PLC) consists of the LCL, popliteus tendon, and popliteofibular ligament, resisting varus, external rotation, and posterior translation in combination with the PCL. Failure to address PLC injuries leads to failure of both PCL and ACL reconstructions.

### Neurovascular Structures at Risk

The popliteal artery is tethered at the adductor hiatus proximally and the soleal arch distally, limiting its mobility and making it vulnerable to stretch or intimal injury during dislocation. The injury rate ranges from 5 to 40%, with higher rates in anterior and posterior dislocations. The common peroneal nerve courses around the fibular neck and is injured in 15 to 40% of knee dislocations. It controls ankle dorsiflexion (foot drop), toe extension, and ankle eversion, with sensation to the dorsum of the foot and first web space. Recovery is poor, at best 20 to 40%.

## Classification

### Schenck Classification (Anatomic)

| Type | Ligaments Injured | Key Features |
|------|------------------|--------------|
| KD I | Single cruciate + one collateral | Least severe pattern |
| KD II | ACL + PCL (bicruciate) | Collaterals intact |
| KD IIIM | ACL + PCL + MCL | Medial-sided injury |
| KD IIIL | ACL + PCL + LCL/PLC | Highest peroneal nerve injury risk |
| KD IV | All four ligaments | Most severe; highest vascular risk |
| KD V | Fracture-dislocation | Any pattern + periarticular fracture |
| Modifier C | Vascular injury requiring repair | |
| Modifier N | Nerve injury | |

KD I involves a single cruciate plus one collateral. KD II is bicruciate (ACL plus PCL) with intact collaterals. KD III is bicruciate plus MCL (IIIM) or LCL/PLC (IIIL). KD IV involves all four ligaments. KD V is a fracture-dislocation. Modifiers include C for vascular injury requiring repair and N for nerve injury.

### Direction of Dislocation

Anterior dislocations are the most common, caused by a hyperextension mechanism. Posterior dislocations result from a dashboard injury. Lateral, medial, and rotatory patterns (posterolateral being the most common rotatory type) also occur.

<image>Schenck classification of knee dislocations showing KD I through KD V patterns with associated ligament injuries</image>

## Initial Evaluation and Management

### Emergency Department Assessment

#### Vascular Assessment -- CRITICAL

Pulses (dorsalis pedis and posterior tibial) must be checked. The ankle-brachial index (ABI) is measured: an ABI of 0.9 or greater indicates low probability of significant vascular injury and warrants serial monitoring, while an ABI below 0.9 indicates likely arterial injury requiring immediate CTA or surgical exploration. Hard signs of vascular injury include a pulseless limb, active hemorrhage, expanding hematoma, and palpable thrill or bruit, all of which mandate immediate vascular surgery consultation. If pulses are absent after reduction, emergent surgical exploration is required without delaying for imaging.

#### Vascular Imaging Protocol

Selective angiography with CTA is obtained for ABI below 0.9 or a concerning clinical examination. CTA has replaced conventional angiography in most centers due to its high sensitivity and specificity. Whether routine CTA should be performed for all knee dislocations remains controversial. Intimal flap injuries may be initially subclinical and progress to thrombosis within 6 to 48 hours, making serial vascular checks mandatory. The golden period for vascular repair is revascularization within 6 to 8 hours to prevent irreversible ischemia and potential amputation.

#### Nerve Assessment

The common peroneal nerve is tested by assessing ankle dorsiflexion, great toe extension, and sensation of the first dorsal web space. Nerve status must be documented before and after reduction. Peroneal nerve palsy is typically associated with lateral-sided injuries (KD IIIL).

### Reduction

Gentle longitudinal traction under sedation reduces most dislocations easily. Irreducible dislocations may have interposed soft tissue, such as medial structures "button-holing" through the medial capsule, with the MCL blocking reduction. After reduction, a repeat vascular examination and radiographs confirming concentric reduction are obtained. The knee is splinted or placed in a hinged knee brace at 20 to 30 degrees of flexion.

### Temporary Stabilization

A spanning external fixator is used if the knee is grossly unstable or if the patient requires vascular repair. Pins are placed away from planned future surgical approaches for ligament reconstruction.

## Definitive Management

### Timing of Surgical Reconstruction

Most surgeons prefer acute repair or reconstruction within 2 to 3 weeks because tissue planes are still identifiable, mobilization is easier, and scarring and arthrofibrosis are avoided. Surgery may be performed as early as 7 to 14 days once soft tissues allow. Delayed reconstruction beyond 3 weeks is considered when there are soft tissue concerns, polytrauma, or a vascular repair that requires a stability period. The MCL and medial side may heal with bracing, but the LCL and PLC do not heal reliably and nearly always require surgical repair or reconstruction. Staged reconstruction is an option, addressing the most critical structures (PLC and PCL) first and then the ACL at a later date.

### Surgical Principles

#### Posterolateral Corner (PLC)

The PLC must be addressed first because failure to reconstruct it leads to failure of cruciate reconstructions. Acute injuries (within 3 weeks) may undergo direct repair of the LCL, popliteus tendon, and popliteofibular ligament using suture anchors. Chronic injuries (beyond 3 weeks) require anatomic reconstruction using allograft, such as the Larson or LaPrade technique, reconstructing the LCL, popliteofibular ligament, and popliteus tendon. The fibular head is the key landmark, as all three structures converge near the fibular styloid.

#### PCL Reconstruction

Options include single-bundle (anterolateral bundle) or double-bundle techniques. The tibial inlay technique avoids the "killer turn" at the posterior tibial cortex, while the transtibial tunnel is an alternative. Graft options include Achilles tendon allograft, tibialis anterior allograft, and quadriceps autograft.

#### ACL Reconstruction

Standard techniques using BPTB, hamstring, quadriceps autograft, or allograft are employed. In multiligament cases, allograft is frequently used to avoid excessive autograft harvest. ACL reconstruction may be staged as a second procedure if PCL and PLC are addressed first.

#### MCL

Isolated MCL injuries in the setting of a bicruciate injury are often treated with bracing because they may heal spontaneously. If the MCL does not heal or the injury is part of a KD IV, surgical repair or reconstruction is indicated. Deep MCL and capsular repair may be necessary for posteromedial instability.

<image>Surgical reconstruction of multiligament knee injury showing PCL and PLC reconstruction with allograft tendons</image>

### Hinged External Fixation

Hinged external fixation is indicated when gross instability persists after ligament repair or reconstruction, when vascular repair requires knee stability, or when soft tissues are too compromised for internal surgery. It is applied with the axis of rotation at the medial epicondyle, allows controlled range of motion while protecting repairs, and is removed after 6 to 8 weeks.

## Rehabilitation

Rehabilitation follows a prolonged and structured protocol. A hinged knee brace is locked in extension for 2 to 4 weeks initially, with progressive range of motion to 0 to 90 degrees by 6 weeks and full motion by 10 to 12 weeks. Active hamstring exercises are avoided for 3 months to protect PCL reconstruction, and varus or valgus stress is avoided for 6 weeks to protect collateral repairs. Return to sport requires a minimum of 9 to 12 months, and many patients do not return to their pre-injury activity level.

## Outcomes

Overall functional outcomes are 60 to 80% satisfactory with modern surgical techniques. Chronic instability persists in 10 to 20% despite reconstruction. Arthrofibrosis is a significant risk, especially with delayed reconstruction or prolonged immobilization. Post-traumatic arthritis develops in a majority over 10 to 20 years. Peroneal nerve injury recovers poorly and may require an ankle-foot orthosis or tendon transfer (posterior tibial tendon transfer) for definitive management.

## Clinical Pearls

A knee dislocation that spontaneously reduces can look deceptively normal on radiographs; if clinical examination reveals bicruciate instability with positive anterior and posterior drawer tests, it should be treated as a knee dislocation. Serial vascular examinations with pulse checks and ABI are mandatory for the first 48 hours because delayed arterial thrombosis from intimal flap injury can occur. The popliteal artery injury threshold is approximately 50% intimal damage, and CTA may show a normal vessel initially that thromboses hours later. The PLC is the "keystone" of knee stability, and an unrepaired PLC injury will cause failure of both ACL and PCL reconstructions. Common peroneal nerve palsy has a poor prognosis, and patients should be counseled early with consideration of early AFO fitting. Obesity is a major challenge in knee dislocation management, as ultra-low-velocity dislocations from simply standing or twisting in morbidly obese patients are increasingly common and carry similar vascular risk. Always examine the contralateral knee for comparison, since laxity assessment is meaningless without a baseline.

## References
- Schenck RC. Classification of knee dislocations. *Oper Tech Sports Med*. 2003;11(3):193-198.
- Stannard JP, et al. Vascular injuries in knee dislocations: the role of physical examination in determining the need for arteriography. *JBJS Am*. 2004;86(5):910-915.
- Levy BA, et al. Controversies in the treatment of knee dislocations and multiligament reconstruction. *JAAOS*. 2009;17(4):197-206.
- LaPrade RF, et al. Posterolateral corner of the knee: anatomy and surgical treatment. *Arthroscopy*. 2007;23(2):225.e1-225.e4.
- Medina O, et al. Vascular and nerve injury after knee dislocation: a systematic review. *Clin Orthop Relat Res*. 2014;472(9):2621-2629.