Hinged-Knee External Fixator Used to Reduce and Maintain Subacute Tibiofemoral Coronal Subluxation
Dislocation of the knee is a rare phenomenon that is becoming increasingly recognized with the expansion of its definition to include knees presenting with multiligament compromise. Hinged external fixators are now considered a viable supplementary treatment option in the management of acute ligament repair or reconstruction but their use in the management of subacute or chronic tibiofemoral dislocations or subluxations is less well defined. We report a case of a hinged-knee external fixator used to facilitate and maintain reduction of a chronic coronal tibial subluxation that presented after repair of an acute knee dislocation with lateral ligament injury secondary to a motor vehicle accident. At 5-year follow-up, the patient treated with hinged external fixation had a stable joint, was able to tolerate regular aerobic exercise, was minimally symptomatic, and did not require more extensive ligament reconstruction. Although there are reports on postoperative use of hinged external fixation to maintain the reduction of chronic or subacute knee dislocations in the sagittal plane after cruciate ligament repair, there are no reports on management of subacute tibiofemoral subluxation in the coronal plane.
Dislocation of the knee is a severe injury that usually results from high-energy blunt trauma.1 Recognition of knee dislocations has increased with expansion of the definition beyond radiographically confirmed loss of tibiofemoral articulation to include injury of multiple knee ligaments with multidirectional joint instability, or the rupture of the anterior and posterior cruciate ligaments (ACL, PCL) when no gross dislocation can be identified2 (though knee dislocations without rupture of either ligament have been reported3,4). Knee dislocations account for 0.02% to 0.2% of orthopedic injuries.5 These multiligamentous injuries are rare, but their clinical outcomes are often complicated by arthrofibrosis, pain, and instability, as surgeons contend with the competing interests of long-term joint stability and range of motion (ROM).6-9
Whereas treatment standards for acute knee dislocations are becoming clearer, treatment of subacute and chronic tibiofemoral dislocations and subluxations is less defined.5 Success with articulated external fixation originally across the ankle and elbow inspired interest in its use for the knee.10-12 Richter and Lobenhoffer13 and Simonian and colleagues14 were the first to report on the postoperative use of a hinged external fixation device to help maintain the reduction of chronic fixed posterior knee dislocations. The literature has even supported nonoperative reduction of small fixed anterior or posterior (sagittal) subluxations with knee bracing alone.15,16 However, there are no reports on treatment of chronic tibial subluxation in the coronal plane.
We report a case of a hinged-knee external fixator (HEF) used alone to reduce a chronic medial tibia subluxation that presented after initial repair of a knee dislocation sustained in a motor vehicle accident. The patient provided written informed consent for print and electronic publication of this case report.
Case Report
A 51-year-old healthy woman who was traveling out of state sustained multiple orthopedic injuries in a motor vehicle accident. She had a pelvic fracture, a contralateral femoral shaft fracture, significant multiligamentous damage to the right knee, and a cavitary impaction fracture of the tibial eminence with resultant coronal tibial subluxation. Initial magnetic resonance imaging (MRI) showed the tibia injury likely was the result of varus translation, as the medial femoral condyle impacted the tibial spine, disrupting the ACL (Figures 1A, 1B). 
On initial presentation to our clinic 5 weeks after injury, x-rays showed progressive medial subluxation of the tibia in relation to the femur with translation of about a third of the tibial width medially (Figures 2A, 2B). 
Given the worsening tibial subluxation and resultant instability, the patient was taken to the operating room for examination under anesthesia, and planned closed reduction and spanning external fixation. Fluoroscopy of the lateral translation and external rotation of the tibia allowed us to reduce the joint, with the lateral tibial plateau and lateral femoral condyle relatively but not completely concentric. A rigid spanning multiplanar external fixator was then placed to maintain the knee joint in a more reduced position.
A week later, the patient was taken back to the operating room for arthroscopic evaluation of the knee joint. At the time of her index operation at the outside institution, she had undergone arthroscopic débridement of intra-articular loose bodies and lateral meniscus repair. Now it was found that the meniscus was not healed but had displaced. A bucket-handle lateral meniscus tear appeared to be blocking lateral translation of the tibia, thus impeding complete reduction.
Given the meniscus deformity that resulted from the chronicity of the injury and the resultant subluxation, a sub-total lateral meniscectomy was performed. As the patient was now noted to have an intact medial collateral ligament and an intact en masse lateral repair, we converted the spanning external fixator to a Compass Universal Hinge (Smith & Nephew) to maintain reduction without further ligamentous reconstruction (Figure 4). 
After HEF placement, the patient spent a short time recovering at an inpatient rehabilitation facility before starting aggressive twice-a-week outpatient physical therapy. Initially after HEF placement, she could not actively flex the knee to about 40° or fully extend it concentrically. Given these limitations and concern about interval development of arthrofibrosis, manipulation under anesthesia was performed, 3 weeks after surgery, and 90° of flexion was obtained. 
Six weeks after HEF removal, the patient was ambulating well with a cane, pain was minimal, and knee ROM was up to 110° of flexion. Tibiofemoral stability remained constant—no change in medial or lateral joint space opening. Full-extension radiographs showed medial translation of about 5 mm, which decreased to 1 mm on Rosenberg view. This represents marked improvement over the severe subluxation on initial presentation.
Follow-up over the next months revealed continued improvement in the right lower extremity strength, increased tolerance for physical activity, and stable right medial tibial translation.
At 5-year follow-up, the patient was asymptomatic, had continued coronal and sagittal stability, and was tolerating regular aerobic exercise, including hiking, weight training, and cycling. Physical examination revealed grade 1B Lachman, grade 0 pivot shift, and grade 0 posterior drawer. There was 3 mm increased lateral compartment opening in full extension, which increased to about 6 mm at 30° with endpoint. 
