A common strategy of definitive fixation for intraarticular distal femur fractures is lateral locked plating. Proper plate placement requires an understanding of the distal femur anatomy and the design of modern precontoured plates. The distal femur articular block is shaped as a trapezoid in the axial plane and is narrower anteriorly than posteriorly. The lateral cortex has a 10° slope, and the medial cortex has a 25° slope in the axial plane. Because of this trapezoidal shape, it can be relatively easy to place the plate too posterior on the wider part of the trapezoid and therefore medialize the articular block relative to the shaft, known as a "golf club" deformity. Indeed, the prevalence of this technical error has led some authors to state that these plates do not fit the "normal anatomy." However, these plates were designed for normal anatomy determined by the averaging of adult populations, and although exceptions exist, precontoured plates closely approximate the "fit" of most adult distal femurs when they are placed in the correct location. In a paper by Campbell et al, for example, plate fit was assessed by overlaying plate templates on AP radiographs of the femur and was determined to be imperfect. Because of the slight internal rotation of the lateral distal femur, however, the proper plate position is anterolateral, and a template of the plate against an AP radiograph will show an imperfect fit even if the plate does fit clinically.
When the plate is applied distally and remains elevated off the shaft of the femur proximally, this can mean that the plate is placed too distal, too posterior, or the fracture is not adequately reduced. Distal or posterior placement can cause the plate to be lateralized relative to its ideal position, causing the plate to sit off the shaft. When the plate is then brought flush with the shaft, this will medialize the articular block relative to the shaft (ie, "golf clubbing"). A similar problem can arise with malreduction. If the distal femoral block is hyperextended or internally rotated relative to its anatomic position, this will bring the wider posterior condyles more anterior and thereby similarly lateralize the plate. This again increases the distance of the plate from the shaft of the bone and results in medializing the articular block relative to the shaft when the plate is drawn to the shaft proximally.
One method for optimal plate placement can be reproduced with an overlapping femoral condyle lateral fluoroscopy image (Figure 5). The plate should be placed on the articular block with the posterior distal hole of the plate above Blumensaat's line and the anterior distal hole cranial to the trochlear groove on the lateral image. On the AP image, the plate is typically 1 to 1.5 cm cranial to the joint line, with the lateral contour of the plate matching the supracondylar flare. Proximally, the plate typically is seated anterolaterally on the shaft of the femur.
Alternatively, the plate can be applied on an optimal AP image, with a joint axis reference wire used. This wire is placed in the articular cluster of the plate and should be parallel to the joint axis and articular line of the femoral condyles. The supracondylar flare should be matched with the contour of the plate, and a lateral image is used to ensure that the distal articular screw cluster is centered within the subchondral margin of the trochlea and Blumensaat's line.
With the intended anterolateral position of the plate on the femoral shaft, the screws are designed to have a slight anterior-to-posterior trajectory to remain bicortical. It is imperative to avoid placement of the plate too far anteriorly. Overly, anterior placement on the shaft results in eccentrically placed screws with resultant unicortical fixation.[3,16,17] Overly, anterior placement distally may lead to painful encroachment of the plate on the extensor mechanism or screw placement into the patellofemoral joint. This can be avoided with either visual inspection through the approach (if permitted) or the use of an overlapping femoral condyle lateral view to ensure that there is no distal or anterior extension of the plate beyond the radiographic outline of the lateral femoral condyle.
A properly placed plate that is flush with the anterolateral cortex of the articular block and along the anterolateral surface of the shaft will not appear in profile on an AP fluoroscopy image but will instead be seen obliquely in the AP image due to the plate's internal rotation. If the plate is in fact visualized in profile with an AP image of the knee, this often means that the plate is externally rotated relative to its ideal position and not optimally abutting the lateral cortex of the articular block.
One additional assessment of the reduction involves using a bovie cord to evaluate the mechanical alignment of the limb. With the knee in extension, the bovie cord is elongated from the femoral head to the middle of the ankle joint on AP images and its position relative to the center of the knee joint is assessed on an additional AP image. Gross medial or lateral displacement of the bovie cord relative to the center of the knee should raise suspicion for malreduction of the fracture.
The optimal length of the definitive plate is dependent on the intended method of stabilization, absolute or relative stability. The use of longer plates with at least eight plate holes proximal to the fracture has been recommended regardless.[12,18] Typically, four bicortical screws are placed in the shaft, with as many distal screws in the articular block as possible. In noncomminuted patterns where anatomic reduction can be achieved and absolute stability is intended, the surgeon should consider applying compression with four bicortical screws in the shaft. Comminuted fractures are typically treated with longer bridging constructs, where the plate length is at least twice the zone of comminution, and four well-spaced bicortical screws are placed in the shaft.
It is important to assess the position of articular block screws on fluoroscopy relative to the intercondylar notch and the medial cortex. A "notch" view can be obtained with the knee flexed to show the intercondylar notch in profile (Figure 3, top left). This can demonstrate whether screws cross the intercondylar notch, which may cause impingement on the cruciate ligaments or limit knee motion. A 20° to 25° internally rotated view can alert the surgeon about screws that breach the medial cortex, a complication that can cause notable irritation (Figure 7). In both cases, screws should be exchanged to avoid violating the notch or medial cortex.
Radiograph showing the assessment of screw length for the medial cortex of the distal femur. The AP fluoroscopic image on the left shows a distal cannulated screw that appears to be contained within the medial cortex, but when a 20° to 25° internally rotated image is obtained (right), the screw appears long.
J Am Acad Orthop Surg. 2021;29(18):770-779. © 2021 American Academy of Orthopaedic Surgeons