Knee Dislocation, Knee Instability, and Arthroscopy Principles

Introduction

The knee dislocation is one of the most dramatic and most dangerous injuries in orthopedic practice, combining the disruption of the principal ligamentous restraints of the joint with the immediate threat of popliteal artery injury and limb loss. The classical incidence is reported as low (less than 0.2 percent of orthopedic admissions in older series) but the true incidence is substantially higher than historical figures suggest, with as many as 50 percent of knee dislocations reducing spontaneously before presentation and being missed unless specifically considered. The knee instability spectrum extends from this acute multiligamentous disruption to the chronic instabilities (ACL deficiency, PCL deficiency, posterolateral corner instability) that are the principal subject of contemporary sports medicine. Arthroscopy of the knee, the diagnostic and therapeutic modality that revolutionized knee surgery in the 1970s and 1980s, deserves brief consideration here as both a diagnostic tool in the trauma setting and the platform for reconstructive surgery in chronic instability. This chapter draws on Rockwood and Green’s Fractures in Adults, Apley & Solomon’s, Miller’s Review of Orthopaedics, and Dutton’s Orthopaedic Examination.

Knee Stability — The Principal Restraints

The knee is stabilized by a complex of structures that the orthopedic surgeon must understand to evaluate ligamentous injury: The anterior cruciate ligament (ACL) is the primary restraint to anterior tibial translation on the femur (95 percent of restraint to anterior translation) and a secondary restraint to internal tibial rotation. The ACL has two functional bundles: the anteromedial bundle (tight in flexion) and the posterolateral bundle (tight in extension). The posterior cruciate ligament (PCL) is the primary restraint to posterior tibial translation on the femur (94 percent of restraint to posterior translation at 90 degrees of flexion). The PCL has anterolateral (larger, tight in flexion) and posteromedial (smaller, tight in extension) bundles. The medial collateral ligament (MCL) complex includes a superficial component (the principal valgus restraint) and deep components (the meniscofemoral and meniscotibial portions of the deep MCL). The lateral collateral ligament (LCL, also called fibular collateral ligament) is the primary varus restraint and is part of the broader posterolateral corner (PLC) complex, which also includes the popliteus tendon and muscle, the popliteofibular ligament, and the arcuate ligament complex. PLC injury is critical to recognize because untreated PLC injury produces ACL or PCL reconstruction failure as the secondary stabilizer overloading. The menisci contribute to stability as secondary stabilizers and shock absorbers, but their primary role is load distribution.

The iliotibial band anterolaterally and the pes anserinus tendons medially contribute dynamic stability through their muscular attachments.

Knee Dislocation — Classification and Mechanism

The Schenck anatomic classification (1992) organizes knee dislocations by the pattern of ligamentous injury, which is more clinically useful than the older positional classification (anterior, posterior, medial, lateral, rotatory): KD-I: Multiligamentous injury with one cruciate intact (ACL + collateral injury with PCL intact, or PCL + collateral with ACL intact). KD-II: Bicruciate injury with both collaterals intact (rare). KD-III: Bicruciate injury plus one collateral injury. Subdivided into KD-IIIM (medial-sided injury, more common) and KD-IIIL (lateral-sided injury, with PLC component). KD-IV: Bicruciate injury plus both collateral injuries. KD-V: Knee dislocation with periarticular fracture. The Schenck classification correlates with management complexity, with KD-IV and KD-V injuries having the most challenging reconstruction needs and the highest rates of complications. The classical positional classification (anterior, posterior, medial, lateral, rotatory) describes the direction of tibial displacement and provides additional information about mechanism but is less useful for treatment planning. The mechanism of knee dislocation includes high-energy injuries (motor vehicle accident, fall from height, sporting injury — particularly contact sports) and low-energy or “ultra- low-energy” injuries in obese patients (where the substantial body mass produces force sufficient to dislocate the knee with relatively minor mechanism). The obese patient with knee dislocation has been recognized as a distinct entity with particularly high rates of complications.

Acute Evaluation of the Dislocated Knee

The clinical examination of the obviously dislocated knee identifies the position of the tibia relative to the femur and assesses for the immediate concerns of vascular injury, neurological injury, and skin compromise. The spontaneously reduced knee (which may be the more common presentation) is more difficult to recognize but should be considered in any patient with significant knee trauma and multiligamentous instability on examination. The neurovascular examination is central and must be repeated serially: Vascular examination: Distal pulses (anterior tibial, posterior tibial, dorsalis pedis), capillary refill, skin color and temperature, and the appearance of the foot. The ankle- brachial index (ABI) with a value less than 0.9 has been validated as predicting vascular injury and is a strong indication for further evaluation. Bilateral asymmetry of pulses is similarly concerning.

Neurological examination: Specifically the common peroneal nerve (motor: dorsiflexion of foot and great toe, eversion; sensory: dorsum of foot and first webspace) and the tibial nerve (motor: plantar flexion, toe flexion; sensory: plantar surface of foot). Peroneal nerve injury is common (10 to 40 percent of knee dislocations, particularly with lateral or posterolateral disruption); tibial nerve injury is less common. Skin and soft-tissue examination: Open wounds, blistering, abrasions, and signs of imminent skin necrosis from continued displacement. The reduction of the obviously dislocated knee is performed urgently in the emergency department with appropriate analgesia and muscle relaxation. The reduction maneuver depends on the direction of displacement but generally involves traction with countertraction in line with the limb. Post-reduction examination confirms reduction by clinical alignment and immediate neurovascular reassessment.

Vascular Evaluation — The Central Concern

The popliteal artery is at substantial risk in knee dislocation because of its tethering at the adductor hiatus proximally and at the entry into the anterior compartment distally; the artery has limited capacity to “give” with the displacement and is consequently injured in 5 to 60 percent of knee dislocations (depending on series and direction of dislocation, with posterior dislocations historically considered the highest-risk pattern). The injury can range from intimal disruption (with delayed thrombosis), to partial laceration (with continuing or recurring hemorrhage), to complete transection. The classical teaching was that routine angiography was indicated in all knee dislocations because of the high incidence of arterial injury and the catastrophic consequences of missed injury. The contemporary approach is more nuanced: Hard signs of vascular injury (absent pulses, expanding hematoma, pulsatile bleeding, bruit, distal ischemia) mandate immediate operative exploration and vascular surgery consultation, with intraoperative angiography or direct exploration of the popliteal artery. Soft signs or abnormal ABI (less than 0.9) mandate CT angiography or formal angiography for definitive evaluation. Normal pulses with normal ABI in the awake examinable patient permit serial examination as the principal monitoring strategy, with CT angiography reserved for changes in clinical status. The NORMAL pulse with NORMAL ABI combined with serial examination over 24 to 48 hours has been validated as appropriate in carefully selected patients. The clinical principle is the time-dependent nature of vascular injury — limb salvage rates after popliteal artery injury fall dramatically with delays exceeding 6 to 8 hours. The orthopedic surgeon’s threshold for vascular surgery consultation should be low in any patient with concerning examination findings.

Operative Management of Knee Dislocation

Timing of Surgery The timing of definitive ligamentous reconstruction after knee dislocation has been an active area of debate. The principal approaches are: Acute reconstruction (within 3 weeks): Allows direct repair of avulsion injuries and primary repair of ligaments where structurally feasible. Provides better outcomes for some ligamentous injuries (particularly PLC and MCL avulsions). May have higher rates of arthrofibrosis because of the inflammatory state. Subacute reconstruction (3 to 6 weeks): A compromise allowing some inflammation to subside while preserving the option of primary repair for some injuries. Delayed reconstruction (after 6 weeks): Allows full resolution of swelling and inflammation, facilitates physiotherapy to restore range of motion, but requires graft reconstruction of all ligaments since primary repair is no longer feasible. The contemporary trend in many centers is acute or early reconstruction for the multiligamentous knee dislocation, with simultaneous reconstruction of cruciate and collateral injuries through arthroscopic and open techniques. The single-stage approach reduces the total rehabilitation burden and provides early stability for graft incorporation. Surgical Approach The surgical approach typically combines arthroscopic cruciate reconstruction (ACL and/or PCL through standard anteromedial and anterolateral portals with reconstruction using bone-tendon-bone or hamstring autografts, or allograft) with open collateral repair or reconstruction (medial- or lateral-sided approaches addressing the MCL or LCL/PLC). The complex multiligamentous reconstruction may require multiple staged or simultaneous procedures. The PLC reconstruction deserves particular emphasis because of its technical complexity and the central role of PLC integrity in the success of cruciate reconstructions. Anatomical reconstruction techniques (such as the LaPrade technique with separate reconstruction of the LCL, popliteofibular ligament, and popliteus tendon using allograft) have produced the best contemporary outcomes. External Fixation Bridging For the patient with knee dislocation and severe soft-tissue compromise, vascular injury requiring repair, or polytrauma precluding immediate definitive ligamentous reconstruction, knee spanning external fixation may be applied as temporary stabilization. The fixator maintains reduction, protects the soft tissues, and provides stability for vascular repair when needed. Definitive ligamentous reconstruction is delayed until conditions permit.

Outcomes and Complications

The functional outcomes of knee dislocation are generally less than perfect, with most patients retaining some level of pain, stiffness, or instability despite optimal management. Patient-reported outcome scores (Lysholm, IKDC, KOOS) are typically 60 to 80 percent of normal in well-managed multiligamentous knee dislocations. The principal complications include: Vascular complications: Limb loss (5 to 15 percent in classic series, lower with prompt recognition and treatment in contemporary practice), delayed thrombosis with subsequent ischemia, compartment syndrome secondary to reperfusion injury. Neurological complications: Permanent peroneal nerve injury (50 to 80 percent of acute peroneal nerve injuries do not recover satisfactorily); peroneal nerve recovery, when it occurs, takes 6 to 12 months. Arthrofibrosis and stiffness: A substantial concern with knee dislocation, particularly with prolonged immobilization. Aggressive early protected range of motion, manipulation under anesthesia, and arthroscopic lysis of adhesions are used to address stiffness. Ligamentous reconstruction failures: Particularly common in PLC reconstructions; secondary stabilizer loading can produce ACL or PCL graft failure. Post-traumatic arthritis: Common in the long term, particularly with associated articular injury or persistent instability.

Chronic Knee Instability

The chronic knee instabilities represent a distinct topic with their own substantial literature. The principal entities relevant here are: Chronic ACL Deficiency Chronic ACL deficiency produces giving-way, particularly with cutting and pivoting activities, and progressive secondary injury (meniscal tears, articular cartilage degeneration). Reconstruction is performed when functional instability or risk of progressive meniscal injury is present. The standard graft choices are bone-patellar tendon-bone autograft (the historical gold standard), hamstring autograft (the modern most common choice), and quadriceps tendon autograft (gaining popularity). Allograft is reserved for revision and multiligament cases. Outcomes are generally good in single- ligament ACL reconstruction, with 80 to 90 percent return to pre-injury activity level. Chronic PCL Deficiency Chronic PCL deficiency is often better tolerated than chronic ACL deficiency, with many patients functioning well without reconstruction. Reconstruction is indicated for grade III injuries with persistent symptoms, multiligamentous injuries, and high-demand patients. Outcomes after isolated PCL reconstruction are less predictable than ACL reconstruction.

Posterolateral Corner Deficiency Untreated PLC injury produces varus and external rotation instability with poor functional outcomes, and is a recognized cause of ACL and PCL reconstruction failure when combined injuries are not addressed. Anatomical PLC reconstruction is the standard for grade III injuries.

Recurrent Patellar Instability Recurrent patellar instability after first dislocation depends on the underlying anatomical risk factors — patella alta (high-riding patella, measured by Insall-Salvati or Caton- Deschamps ratio), trochlear dysplasia (Dejour classification), increased TT-TG distance (tibial tuberosity to trochlear groove), and rotational malalignment of the tibia or femur. Management is MPFL reconstruction (medial patellofemoral ligament reconstruction with hamstring graft) for primary instability without other anatomical anomalies; addition of tibial tuberosity transfer (Fulkerson osteotomy, anteromedialization) or trochleoplasty for more complex anatomical situations.

Knee Arthroscopy — Principles

Knee arthroscopy is the diagnostic and therapeutic modality that revolutionized knee surgery in the 1970s and 1980s and provides the foundation for contemporary knee surgery. The principles relevant to the trauma context include: Diagnostic Arthroscopy Arthroscopy provides direct visualization of the intra-articular knee structures with sensitivity superior to MRI for some pathologies (loose bodies, articular cartilage defects, subtle meniscal tears). In the trauma setting, arthroscopy may be used as a diagnostic and therapeutic adjunct in tibial plateau fractures, in suspected internal derangement after acute injury, and in the workup of recurrent instability. Therapeutic Arthroscopy Therapeutic procedures performed arthroscopically include: Meniscal procedures: Partial meniscectomy (resection of the torn fragment with preservation of as much rim as possible), meniscal repair (suture repair of repairable tears in the vascular zone — the “red-red” and “red-white” zones in the peripheral 25 to 33 percent of the meniscus), and meniscal transplantation (replacement of the entire meniscus with allograft in the meniscal-deficient knee). Cartilage procedures: Microfracture (creating perforations in the subchondral bone to allow marrow stem cells to populate a clot at the chondral defect), chondroplasty (debridement of damaged cartilage to stable rim), osteochondral autograft transfer (OATS — taking plugs of bone and cartilage from non-weight-bearing donor sites), autologous chondrocyte implantation (ACI — culture-expanded chondrocytes implanted under a covered graft).

Loose body removal: Common in trauma after osteochondral fracture or anterior cruciate ligament injury. Synovectomy: Partial or total for inflammatory or pigmented villonodular synovitis. Ligament reconstruction: The cruciate ligament reconstructions are performed arthroscopically with portals on either side of the patellar tendon for instrument access and visualization. Portals and Approach The standard portals for diagnostic knee arthroscopy are the anterolateral portal (just lateral to the patellar tendon, at the level of the joint line) and anteromedial portal (just medial to the patellar tendon, established under direct vision through the lateral portal). Accessory portals (superolateral, posteromedial, posterolateral) are added as needed for specific procedures. The arthroscopic examination systematically evaluates the suprapatellar pouch, the patellofemoral joint, the medial and lateral gutters, the medial and lateral compartments (meniscus and articular surfaces), the intercondylar notch (ACL, PCL, intermeniscal ligament), and the posteromedial and posterolateral compartments through specific portal access. The arthroscopic assessment of the ACL includes inspection of the femoral and tibial attachments, probing of the tibial attachment, and assessment of fiber integrity; for the PCL, the inspection is more limited and may require the posterior approach.

Summary and Take-Home Points

The knee dislocation is one of the most dangerous orthopedic injuries because of the immediate threat of popliteal artery injury and limb loss. The Schenck anatomic classification (KD-I through KD-V) organizes injuries by ligamentous pattern and is more useful than the older positional classification. The vascular evaluation is central: hard signs of vascular injury mandate immediate operative exploration; abnormal ABI mandates CT angiography; normal pulses with normal ABI permit serial examination as the monitoring strategy. The time-dependent nature of vascular injury with falling limb salvage rates after 6 to 8 hours of ischemia makes the low threshold for vascular surgical involvement essential. The peroneal nerve is injured in 10 to 40 percent of knee dislocations, with poor recovery prospects when complete. The operative management of multiligamentous knee dislocation increasingly favors acute or early reconstruction (within 3 weeks) of all injured structures in a coordinated single or staged surgery, combining arthroscopic cruciate reconstruction with open collateral and posterolateral corner repair or reconstruction. The Schenck KD-IV and KD-V patterns have the most complex reconstruction needs. The chronic knee instabilities (ACL, PCL, PLC, patellar) form a distinct topic with established reconstructive techniques and generally good outcomes for single-ligament injuries. The arthroscopy of the knee is the platform for both diagnostic and therapeutic procedures across the spectrum of acute and chronic knee pathology.

The principal complications of knee dislocation — vascular and neurological injury, arthrofibrosis, ligamentous reconstruction failure, and post-traumatic arthritis — highlight the limited functional outcomes that even optimal management can achieve. The chapter that follows turns to the tibia and fibula shaft, completing the lower extremity diaphyseal fracture sequence.