Floating Knee

Etiology

As with most complex fractures, floating knee injuries are associated with high-energy mechanisms of injury and are often accompanied by other injuries to other parts of the body, including severe soft tissue injury. High-energy trauma such as motor vehicle accidents falls from an extreme height, pedestrian-versus-auto accidents, cyclist-versus-auto accidents, and other mechanisms that involve blunt trauma to the area.[3][5][6] The associated injuries are significantly severe, with a reported mean injury severity score (ISS) of over 16.[7]

Severe Associated Injuries

• Severe head injuries (14% of cases) [8] 
• Chest, abdomen, and concomitant severe soft tissue injuries [9] 
• Knee ligamentous injuries with laxity (19% of cases) [9] 
• Concomitant fractures (up to 44% of cases [10]

Limb and Life-threatening Injuries

• Vascular injuries, mainly involving the popliteal artery (7% of the cases); up to 69% of cases reported to have open fractures
• Popliteal artery injuries and severely mangled limbs or open fractures can result in amputation (up to 9% in the first 24 hours of admission) [11]
• Compartment syndrome and fat embolism syndrome are commonly reported [2][5][12] 
• Death rate on admission reported in up to 10% of cases [13][14]

Epidemiology

Floating knee injuries are rare, and there are insufficient data to definitively estimate the prevalence amongst populations. The largest series reported in the literature included 222 patients over a duration of 11 years.[4] There is a strong predominance of floating knee injuries in males over females. This male-to-female preference may be related to the high velocity and strenuous activities seen more in males than females. Also, there seems to be a higher prevalence in males over females engaging in more risk-taking behaviors and activities.

History and Physical

Patients with floating knee injuries typically will have incurred multiple trauma injuries to that region and should not be overlooked. Furthermore, patients will most likely present with a history of trauma (eg, a motor vehicle accident, a fall from a height, a pedestrian-versus-auto accident, etc). Patients with an isolated floating knee injury will present with complaints of severe leg pain, inability to bear weight, and potentially some knee instability (due to ligamentous disruption, which often accompanies these injuries).

On physical examination, there will usually be tenderness to palpation at both the tibial and femoral fracture sites, along with gross deformities and shortening of the affected limb. More advanced classification fractures may also be present in one or more of the fractured components. The neurovascular status may or may not be compromised and requires a thorough examination.

Evaluation

Patients with floating knee injuries should undergo evaluation according to standard protocols for trauma and as per clinical presentation. If polytrauma was involved, the basic Advanced Trauma Life Support^® (ATLS^®) protocol should be implemented first. Therefore, the most urgent needs should be addressed before any intervention elsewhere, including using the Airway, Breathing, Circulation, Disability, Exposure (ABCDE) approach. Generally, standard trauma workup will be a requirement in most instances, and stabilization of a critically ill patient must be first before any intervention to musculoskeletal complaints. Once the patient is stable and other life-threatening injuries have been dealt with or ruled out, focusing on other injuries, such as the floating knee, is indicated. 

Ensuring the patient is neurovascularly intact is one of the most important things to check for. Examining sensation in all dermatomes of the lower extremity, as well as pulses of the dorsalis pedis and posterior tibial artery, should be tested first after addressing any circulation matters. Posterior displacement of the distal femur may also cause damage to the adjacent popliteal artery. If this is suspected, a Doppler ultrasound should be conducted to rule out intimal tears.[15] Monitoring patients for signs of fat embolism is in order due to the significant skeletal trauma that usually accompanies these injuries. If any sign of fat embolism is detected, postponement of the surgical management of the fractures is necessary until the patient is stable.[16] 

Imaging in the form of plain film radiography is the best initial tool for diagnosis. X-rays will allow an initial gestalt view of the affected limb to rule in or out any obvious fractures. A CT scan is a diagnostic option to further investigate any complicated fractures, including the severity of comminution and bone loss, and can assist in the planning of fixation techniques and predicting any complications that may arise. Due to the high incidence of ligamentous injuries associated with floating knee–type injuries, an MRI may also be necessary.[16][17][18]

Treatment / Management

The management of patients presenting with floating knee injuries should be based on the concept of "damage control orthopedics."[19][20] Head, chest, abdominal injuries, popliteal artery injuries, and open fractures should be treated first. Meanwhile, femur and tibia fractures should be addressed with temporary stabilization, either traction or external fixation. This will reduce the biological load of surgery. Conversely, immediate reduction and fixation should be attempted in hemodynamically stable patients. Surgical treatment of both fractures yields better results, and there are multiple series reported in the literature demonstrating the good outcomes with both antegrade grade nailing and retrograde nailing.[10][14][21](B2)

Treatment and management of the floating knee injury and each fracture is dependent upon multiple variables and factors: whether the fracture is open or closed, the type of fracture pattern, the location of the fracture, the comminution of fracture, and skeletal maturity. However, due to the rarity of the floating knee injury, management is more based on experience than evidence-based. There are multiple fracture patterns in the femur and the tibia, and in multiple cases, it would be challenging to achieve optimal fixation in both using the same implant. Additionally, the status of the soft tissue envelope and the presence of prosthetic implants either in the hip or the knee would affect the implant choice.[1]

Nonoperative Management

Skeletally immature patients are more likely to be treated non-operatively with a long leg cast than skeletally mature patients with minimally displaced fractures.[3] "Pediatric floating knee," classified as isolated physeal fractures of the distal femur and proximal tibia, may be treated operatively by fixation with K-wires followed by casting for 6 weeks.[15]  (A1)

Operative Management: Floating Knee

Intramedullary Nailing 

The femur and tibia fractures could be both managed with intramedullary nailing (IMN). IMN is typically the choice for diaphyseal fractures where a functional reduction is more indicated. This approach allows for stability of the fracture while still allowing for callus formation that occurs with secondary bone healing. IMN has been the treatment of choice for the true floating knee since the description of "floating knee" injuries by Blake and McBride.[2] Ideally, femoral fixation should be done first, followed by tibial fixation. Unless the tibia fracture is open, it should take priority to be fixed first.[1][21] 

In type II fractures, nailing is not generally the best option. However, in some cases, the intra-articular component can be addressed first, followed by intramedullary nailing of the shaft. Segmental femur fractures can be managed with a combination of retrograde nailing and screws or a sliding hip screw.

Plating 

Plating is considered a better option in type IIc fractures (Fraser classification), where there are intraarticular distal femur or proximal tibia fractures. Plating is advantageous as the approach allows simultaneous management of associated soft tissue injuries, such as in cases of associated meniscal tears that can be addressed through the same approach. There are series in the literature with concomitant plating of both the femur and the tibia, even in extra-articular scenarios and open fractures, that reported overall satisfactory results.[22](B2)

A distal femoral fracture in the setting of a floating knee injury should be managed similarly to an isolated counterpart with a retrograde nail or a locking plate. Both modes of fixation have similar outcomes and complication rates reported.[23] For the proximal tibia, antegrade IMN or locking plates can achieve an adequate fixation. For simple articular fractures, nails with advanced locking options should suffice, whereas, for more complex intra-articular fractures, locking plates with lag screws supplementation should be used for fixation.(A1)

External Fixation

External Fixation is an option that would be indicated in open fractures or as a part of the damage control orthopedics management. It grants sufficient fracture stability and allows soft tissue resuscitation where there is damage to the soft tissue envelope. Other clinical scenarios have been reported in the literature where plating both the femur and tibia was more favorable, such as a floating knee injury with concomitant distal femur and proximal tibia epiphyseal injuries equivalent to Salter-Harris type II injuries.[24](B3)

Operative Management: Floating Total Knee

Knee periprosthetic fractures with combined distal femur and proximal tibia fractures have been reported in the literature. The distal femur is most commonly affected, followed by the proximal tibia and the patella.[25][26](B3)

Plating

Plating can be an option in fractures adjacent to the prosthesis when there is good stability of the prosthesis. Double plating of the distal femur fractures with a lateral and medial plate enhances the stability of the fixation, even in the most distal fractures. Similarly, double plating of proximal tibial fractures provides a robust fixation.[27]

Revision Total Knee Arthroplasty

Revision total knee arthroplasty (TKA) can be considered in cases where fractures adjacent to the prosthesis compromise its stability. Treating surgeons should be aware of scenarios where the TKA implant has a closed box and cases of interprosthetic fractures (femur fractures with total hip and ipsilateral total knee arthroplasties).

Concomitant Ligamentous and Meniscal Injuries

Systematic MRI and arthroscopy examination have resulted in increased reporting of these injuries in the setting of floating knee injuries.[18][28]

Differential Diagnosis

Patients with floating knee injuries are typically polytrauma patients. Because of this, a healthcare provider should always rule out other life-threatening injuries. Also, complications can arise from such traumas. One must monitor for deep vein thrombosis and fat embolus formation that can occur secondary to skeletal trauma. Patients may also have ligamentous injuries (ACL, PCL, meniscal, etc). These are non–life-threatening injuries and are manageable after the stabilization of the critically ill patient and the reduction of associated fractures. 

Staging

Blake and McBryde Classification (1975)

• Type I (71%): True floating; none of the femur or tibia fractures extends to the knee or the hip joints
• Type II ( 29%): At least one fracture extends to the knee joint [2]

Fraser Classification (1978)

This is a further classification of Type II Blake and McBryde based on the knee injury pattern.

• Type II a (80%): Tibial plateau fracture and femoral shaft fracture
• Type II b (12%): Distal femur articular fracture with a femoral shaft fracture
• Type II c (9%): Tibial plateau and distal femur articular fracture

Prognosis

Predictors of Poor Prognosis

The reported factors accounting for poor outcomes in floating knee injuries included open tibial fractures, segmental fractures, fractures involving the articular surface of the knee joint, and the requirement of initial external fixation or additional surgeries to address the injuries.[29] Patients with operative treatment were found to have statistically significantly shorter hospital stays than those treated conservatively.[3]

Additionally, advanced age was reported to be associated with delayed bony union and full weight-bearing ability. Increased pack-years of smoking at the injury time were more likely associated with increased knee stiffness, delayed bony union, and full weight-bearing ability. Similarly, open fractures were more likely to predict knee stiffness and delayed full weight bearing. And higher ISS was found to be associated with delayed full weight bearing.[7] Additionally, knee joint involvement and higher grades of pretibial soft tissue injuries were associated with poorer outcomes in floating knee injuries.[30]

Complications

Malunion and Delayed Union

Malunion and delayed union are always possible complications of fractures. In the setting of floating knee injuries, comminuted fractures have been reported to be associated with malunion, whereas segmental fractures are more associated with delayed bony union.[7] The average union time for tibial and femur fractures in floating knee injuries was reported to be 9.52 (+/-6.6) and 10.5 (+/-7.37) months, respectively, in a population of 89 patients aged 34.34 (+/- 12.28) years.[29] Segmental femur fractures showed a delay in union time by about 6 months. This delay did not manifest in segmental tibial fractures. Over 50% of these patients had either an excellent or good outcome.[29]

Limb Length Discrepancy

Limb length discrepancy is another possible complication, whether it is due to lengthening or shortening of the affected limb. Limb length discrepancies may also occur in skeletally immature individuals if there is disruption of the growth plate resulting in premature physeal closure.

Infection 

infection risk is higher in open fractures and with the increased number of surgical interventions.[3] The high infection risk in a floating knee with an open femur fracture is mainly attributed to the fact that a floating knee is associated with more severe soft tissue injuries.[21]

Other Complications

Other complications found to occur with floating knee injuries include compartment syndrome, loss of joint motion, and requirement of limb amputation. The complication rate decreases when one or both of the fractures are in the femoral and tibial diaphysis.[16] One study found treatment with a retrograde intramedullary nail resulted in an increased risk of heterotopic ossification around the knee compared with anterograde femoral nails. The explanation for this result is the fact that anterograde femoral nails never penetrate the knee joint.[31]