Floating Knee Injury

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

Biomechanical Considerations

The differential table and the Meccariello series both invoke a paediatric-specific system, because a child's floating knee is a genuinely different injury governed by the physis, remodelling potential and growth. Fraser (an adult, articular-versus-diaphyseal scheme) does not apply. The standard descriptive system is the Letts classification (Letts, Vincent and Gouw, 1986; sometimes cited in the literature as "Letts-Ran").

Letts classification (five types, in increasing severity):

• Type A — both fractures closed and diaphyseal
• Type B — one fracture open
• Type C — intra-articular (or physeal/epiphyseal) extension of one fracture
• Type D — an open fracture with associated intra-articular involvement
• Type E — both fractures open

Open patterns (Letts D and E) carry the worst prognosis, with type E (both fractures open) the most severe.

Why children differ:

• Physeal injury — the distal femoral and proximal tibial physes may be involved, risking growth arrest, progressive angular deformity and limb-length discrepancy; these must be tracked to skeletal maturity.
• Length changes in both directions — post-fracture overgrowth (physeal stimulation) and shortening (arrest or malunion) are both described, so serial length assessment is essential.
• Remodelling potential allows more diaphyseal fractures to be managed non-operatively than in adults, but Letts' original series showed poor results when BOTH fractures were treated non-operatively — so at least one fracture should be rigidly stabilised.
• Implant choice favours flexible intramedullary (elastic stable) nailing and physeal-sparing fixation over the rigid reamed antegrade nails used in adults, to protect the open physis.

Contemporary paediatric outcome data (Tontanahal et al., 2023) show that a high Injury Severity Score and bone loss greater than 4 cm — not age, open status or comminution alone — are the strongest predictors of poorer sports and transfer/mobility function.

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.

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

Late Complications

Immediate Postoperative

Weight-Bearing Protocol

Rehabilitation Phases

Almost every floating knee series in this topic — including the Hegazy cohort and the contemporary Kenmegne series — reports results as "excellent / good / acceptable / fair / poor," but the underlying instrument is rarely named. That instrument is the Karlström-Olerud functional grading, described by Karlström and Olerud in their original 1977 ipsilateral femur-and-tibia series (the same paper that first showed rigid fixation of both bones gives fewer complications, shorter hospital stay and better return to work).

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.

• 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.

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.

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.