Medial-compartment partial knee arthroplasty (unicompartmental replacement) is an accepted surgical intervention for anteromedial osteoarthritis of the knee.1 The radiographic investigations required in the workup of these patients should include weight-bearing standing anteroposterior (AP), lateral, and sunrise (Merchant) views, as well as a valgus stress AP radiograph to assess the functionality of the lateral compartment. The method of properly obtaining the valgus stress film has been well described by the Oxford Group.2 Its recommended radiographic technique requires that a surgeon or a radiologic technologist perform the valgus stress maneuver, manually, while another technologist shoots the film. The 2 consequences of this technique are that it requires 2 individuals to obtain the film, and it subjects the individual who is applying the stress to some level of radiation exposure, which is undesirable. Because of this and the time inconvenience, many surgeons omit the valgus stress radiograph, which can lead to the adverse outcome of missing a lateral compartment that is functionally incompetent, resulting in the potential for early lateral compartment progression of osteoarthritis and the need for revision surgery, usually to a total knee arthroplasty.
In an attempt to mitigate these barriers to obtaining the necessary valgus stress radiograph, Dr. Mauerhan’s team developed a technique that could be done with the assistance of the patient and would require only 1 technologist to perform. Additionally, this project was a quality improvement initiative, because it lowered radiation exposure to all personnel involved in obtaining the correct films.
Materials and Methods
We initiated the project using weight-bearing strategies to impart the valgus stress view of the knee. After trying several different wedges and blocks, and varying patient instructions, we realized a different approach to this problem would be required to find an acceptable solution. We redirected our efforts to effectively performing the stress view with the patient in a supine position on the radiograph table. Ultimately, we decided that a much stiffer wedge and a denser object to squeeze would facilitate obtaining a proper film. Considering all available options, a youth size 4 soccer ball (diameter, 11 in) was introduced along with a slightly larger positioning wedge. The soccer ball was wrapped with 4-in Coban wrap (3M) to create a nonslip surface. This change in patient positioning, along with a standardized 7º to 10º cephalic radiographic tube angulation, helped to correct issues with tibial plateau visualization. Once these changes were enacted, we obtained fairly consistent positive results, and we instituted this patient-directed valgus stress view of the knee, along with a manual valgus stress view for comparison.
The protocol for obtaining the patient-directed valgus stress view of the knee is as follows: The patient lays supine with a dense 45º spine-positioning wedge (Burlington Medical Supplies) placed under both knees and the patient’s heels on the examining table. The radiographic tube is angled cephalad 7º to 10º centered on the inferior pole of the patella, using a 40-in source to image-receptor distance, collimated to part; the image receptor is placed under the affected knee, below the positioning wedge. The affected knee is rotated to the “true” AP position (the patella will be centered between the femoral condyles on the AP exposure), and the ball is placed between the patient’s legs just above the ankle joint. The technologist demonstrates to the patient how to squeeze the ball while maintaining contact of heels with the table. The technologist can exit the room and obtain the exposure, which is taken while the patient is squeezing the ball, as shown in Figures 1A and 1B. Examples of the standing AP, manual stress, and patient-directed valgus radiographs are shown in Figures 2A-2C. The entire technique is demonstrated in the Video.
During the 9 months of this quality improvement project, 78 examinations were performed. Five studies did not show complete correction of the varus deformity. Of these, 3 showed complete correction on a manual valgus stress radiograph, and 2 did not, contraindicating the use of partial knee replacement. Three patients displayed collapse of the lateral compartment, indicating a nonfunctional lateral compartment, and, therefore, were also a contraindication to partial knee arthroplasty. The remaining 70 patients had identical radiographic results with both the manual and patient-directed valgus stress tests. There was no instance of examination failure or need to repeat as a result of difficulty of the examination for the patient. Repeat films because of positioning errors were very rare, usually early in the learning curve, and no more prevalent than when using the manual stress method. The technique was reproducible and easy to teach and adopt.