Floating knee injury describes ipsilateral femur and tibia fractures creating an unstable knee segment disconnected from axial skeleton. It results from high-energy trauma (MVA, motorcycle accidents) with associated injuries in 80% and mortality of 5-15%. The Fraser classification determines prognosis: Type I (both extra-articular, best prognosis), Type IIA (tibial articular), Type IIB (femoral articular), Type IIC (both articular, worst prognosis). Management involves stabilization of both fractures, typically with intramedullary nailing if extra-articular. Critical associated injuries include ligamentous knee injury (greater than 50%, often missed initially), vascular injury requiring angiography, and compartment syndrome in both thigh and leg.

Floating knee injury describes ipsilateral fractures of the femur and tibia, creating an unstable knee segment that is disconnected from the axial skeleton. The term reflects the biomechanical reality that the knee "floats" free between two fracture sites.

Epidemiology

• Incidence: Accounts for 2-4% of lower extremity fractures
• Demographics: Male predominance (3:1), peak age 20-40 years
• Mechanism: High-energy trauma (MVA 70%, motorcycle 20%, pedestrian vs vehicle 5%)
• Polytrauma association: Present in 70-80% of cases
• Mortality: 5-15% (usually from associated injuries)
• Open fracture rate: 30-40%

Associated Injuries

Relevant Anatomy

Knee as the "Floating" Segment:

• The knee joint and surrounding soft tissues become an unstable segment
• Disconnected superiorly by femur fracture and inferiorly by tibia fracture
• Popliteal vessels and nerves traverse this zone

Critical Vascular Anatomy:

• Popliteal artery: Fixed at adductor hiatus proximally and soleus arch distally
• Tethered position makes it vulnerable to traction injury
• Intimal tears may cause delayed thrombosis

Muscular Attachments:

• Quadriceps mechanism spans the femur fracture
• Gastrocnemius origin crosses the knee
• These contribute to deforming forces and stiffness

Biomechanical Considerations

Deforming Forces:

• Femur: Proximal fragment abducted and flexed (iliopsoas, abductors)
• Tibia: Variable depending on fracture level
• Knee tends toward flexion contracture if not mobilized early

Load Transmission:

• Normal: Axial load through femur → knee → tibia
• Floating knee: Complete loss of axial stability
• Requires fixation of BOTH fractures for weight-bearing

The diagnosis of a floating knee is radiographic (ipsilateral femur AND tibia fracture), so the real "differential" is recognising which adjacent injury patterns can masquerade as, accompany, or be mistaken for it. The table below contrasts the floating knee with the patterns it is most often confused with on initial trauma imaging.

The single commonest error is under-imaging: an obvious femoral shaft fracture distracts from a subtle proximal tibial or plateau fracture. Always obtain full-length femur AND tibia films plus dedicated knee views before excluding a floating knee.

Damage Control Phase (Unstable Patient):

• Spanning external fixation of both femur and tibia
• Temporary stabilisation to allow resuscitation
• Definitive fixation once physiological parameters normalise

Definitive Fixation (Stable Patient):

• Address both fractures at same sitting if possible
• Reduces hospital stay and allows early motion

Fixation Strategies by Fraser Type:

• Fraser Type I: Double IM nailing (Antegrade femur + Tibia)
• Fraser Type IIA: Femur nailing + Tibial plateau ORIF
• Fraser Type IIB: Distal femur ORIF + Tibial nailing
• Fraser Type IIC: Dual ORIF (Distal femur + Tibial plateau)

Fixation strategy must be tailored to specific fracture patterns.

Most surgeons prefer femur first:

1. Restores limb length and gross alignment
2. Facilitates subsequent tibial reduction
3. More ergonomic patient positioning

Tibial fixation follows immediately to finalize the construct stability.

Early Complications

Compartment Syndrome (10-20%):

• Affects both thigh AND leg compartments
• Thigh contains 3 compartments (Often overlooked)
• Maintain high index of suspicion with serial evaluations
• Threshold for prophylactic fasciotomy should be low

Vascular Injury (5-10%):

• Popliteal artery most at risk due to tethered anatomy
• May present delayed due to intimal injury
• Requires immediate revascularization if diagnosed
• Limb viability window: 6-8 hours warm ischemia

Infection:

• Higher rates than isolated fractures (10-15%)
• Open fractures require debridement and antibiotics per Gustilo protocol
• External fixation pin site infections

Late Complications

Knee Stiffness (20-50%):

• Most common long-term problem
• Due to periarticular scarring and quadriceps adhesions
• Prevention: Early ROM, avoid prolonged immobilization
• Treatment: Aggressive physiotherapy, consider manipulation under anesthesia

Malunion/Nonunion:

• Both fracture sites at risk
• Smoking, diabetes, and infection increase risk
• May require revision fixation or bone grafting

Post-traumatic Arthritis:

• Particularly prevalent in Fraser Type II injuries
• Articular damage + ligament instability contribute
• May require total knee arthroplasty in the long term

Immediate Postoperative

Day 0-3:

• DVT prophylaxis (LMWH)
• Pain management (multimodal analgesia)
• Neurovascular monitoring
• Wound inspection

Early Mobilization:

• CPM or active-assisted ROM from day 1 if stable fixation
• Quadriceps setting exercises
• Ankle pumps

Weight-Bearing Protocol

Rehabilitation Phases

Phase 1 (0-6 weeks):

• ROM exercises (goal: 0-90° knee flexion)
• Quadriceps strengthening (isometric initially)
• Gait training with assistive devices

Phase 2 (6-12 weeks):

• Progressive weight-bearing
• Active ROM to full
• Closed chain exercises
• Aquatic therapy if available

Phase 3 (12+ weeks):

• Full weight-bearing
• Functional training
• Return to activity assessment
• Address remaining ligamentous instability

• Is Fraser still fit for purpose? Fraser describes only the bony pattern. A 2024 multicentre study of 168 floating knees found Fraser severity did not reliably predict function, whereas soft-tissue and extensor-mechanism damage did, and proposed a new prognostic system. Fraser remains the lingua franca for exams and communication, but is increasingly seen as prognostically weak.
• Early total care vs damage control. Single-stage fixation in the stable patient is well supported, but the threshold for damage control in the "borderline" physiology patient is debated. The EPOFF biological data favour temporising external fixation in borderline patients, while some high-performing trauma centres report good results with early total care in carefully selected borderline cases.
• Femur-first vs tibia-first. Femur-first (length and alignment) is the common teaching, but no high-level evidence proves superiority; some surgeons fix the simpler or more accessible fracture first, or fix the tibia first to allow knee extension for femoral nailing.
• Routine MRI of the knee. Occult ligament and meniscal injuries are common (and may dominate long-term disability), but the optimal timing and whether routine MRI changes management in the acute setting is unresolved; many units image selectively once the knee can be examined.
• How "best" is the best subtype? Even Fraser type I (both extra-articular) carries large isokinetic strength deficits and unsatisfactory function in some series, challenging the assumption that extra-articular equals good outcome.

Global Epidemiology

• The floating knee is uncommon, accounting for roughly 2-4% of lower-limb trauma; a contemporary single-centre series reported an incidence of 2.32% of all lower-limb injuries.
• Road traffic and motorcycle trauma dominate the mechanism worldwide; the burden is disproportionately high in low- and middle-income countries with rising motorisation and a young, male-predominant cohort.
• Outcomes are governed less by geography than by soft-tissue status and associated injuries, which are consistent across published series.

Side-by-Side Guideline Framework

Where these differ, the divergence is mainly on timing (early total care versus damage control) and is reconciled by patient physiology rather than by national preference.

Registry and Trauma-System Notes

• National joint replacement registries (NJR, AJRR, AOANJRR, SHAR, NZJR) capture the downstream burden of post-traumatic arthritis requiring arthroplasty after Fraser type II injuries, but no registry tracks the acute floating knee itself.
• Mature trauma systems with rapid pre-hospital retrieval and Level 1 centres favour single-stage early total care in stable patients; the EPOFF biological data underpin damage control where physiology is borderline.

High- vs Limited-Resource Practice Variation

• High-resource settings: combined ortho-plastic teams, CT angiography on demand, IM nailing of both bones in one sitting, and early MRI of the knee.
• Limited-resource settings: greater reliance on external fixation (limited implants, contaminated open injuries, delayed presentation), staged conversion, and clinical rather than imaging-based vascular assessment. The principle of stabilising both bones to permit early knee motion remains universal.

1. Fraser RD, Hunter GA, Waddell JP. Ipsilateral fracture of the femur and tibia. J Bone Joint Surg Br. 1978;60-B(4):510-515.
2. Veith RG, Winquist RA, Hansen ST Jr. Ipsilateral fractures of the femur and tibia. J Bone Joint Surg Am. 1984;66(7):991-1002. PMID 6480657.
3. Hegazy AM. Surgical management of ipsilateral fracture of the femur and tibia in adults (the floating knee). Clin Orthop Surg. 2011;3(2):133-139. PMID 21629474.
4. Andrade-Silva FB, et al. Functional results and isokinetic muscle strength in Fraser type I floating knee. Injury. 2017;48 Suppl 4:S2-S5. PMID 29145963.
5. Kenmegne GR, et al. The current issues and challenges in the management of floating knee injury. Front Surg. 2023;10:1164032. PMID 37206352.
6. Pape HC, et al. (EPOFF Study Group). Impact of intramedullary instrumentation versus damage control for femoral fractures on immunoinflammatory parameters. J Trauma. 2003;55(1):7-13. PMID 12855874.
7. Chouhan D, et al. Comparison of functional outcomes among subtypes of Fraser's type II floating knee. Chin J Traumatol. 2020;24(1):25-29. PMID 33339679.
8. Meccariello L, et al. Floating knee: a new prognostic classification. Injury. 2024;55 Suppl 4:111471. PMID 39542575.