Evaluation of Instability by Stress Radiography in Total Knee Arthroplasty

TKR patients will be invited to participate when the decision of primary knee replacement is made. These patients will be scheduled for stress radiography and clinical evaluation at 12 months. Additional patients that are referred to the outpatient clinic due to painful TKR will also be asked to participate. Patients will be categorized as either "satisfied" (control) or "not satisfied" (case). A minimum of 20:80 and a maximum of 50:50 case control ratio is accepted. This ratio will be monitored during inclusion.

Patients are included after informed consent. The primary outcome is to determine diagnostic accuracy of knee laxity measurement by stress radiography in predicting dissatisfaction after total knee arthroplasty. The primary dichotomous outcome variable is patient satisfaction. Patient satisfaction is defined as an Oxford Knee Score (OKS) above the Patient Acceptable Symptom Threshold (PASS) of 30. Patient satisfaction status will be tested against three predictor variables: Coronal knee laxity during varus and valgus stress, and sagittal laxity defined as total anteroposterior translation. Knee laxity will be measured in both the TKR knee and the contralateral healthy knee. The side to side difference in laxity will be used as the primary predictor variable in order to mitigate individual variability. The absolute opening angle will also be analysed separately in order to include those with contralateral knees with substantial Knee laxity measured by stress radiography Coronal joint laxity is measured as the gapping between the femoral and tibial joint line during varus and valgus stress on an anteroposterior radiograph. The joint convergence angle (JCA) is used to quantify the amount of gapping.The knees will be subjected to 150 newton (N) varus and valgus stress in 30 degrees of flexion. A 30-degree knee bolster is used to ensure a consistent flexion angle. The limb is placed in a Telos frame to ensure consistent force application during stress testing.

Sagittal laxity is quantified as the total anteroposterior (AP) translation of the tibia relative to the femur during anterior and posterior stress on a lateral radiograph. In the prosthetic knee, the distance from the most posterior point on the femoral condyles to the posterior margin of the tibial plateau will serve as a reference. The difference of this distance in anterior and posterior stressed position will reflect the total AP translation. The Blumensaat-Anterior Tibia (BAT) method will be used as reference distance in the native knee. The test will be performed with 90-degree flexion angle of the knee, again using the Telos frame to perform the anterior and posterior drawer test with a force of 150N. To ensure consistent rotational position of the femur, the femoral condyles must be superimposed on the lateral stress image.

Reliability of the acquisition and interpretation of images will be evaluated by three parameters: Precision under repeatability conditions, inter- and intra-observer reliability.

Secondary outcomes A) Agreement between the clinical examination of knee laxity and laxity measured by stress radiographs The examiner will evaluate the laxity as "normal" or "excessive". The examiner is blinded for both patient satisfaction and stress radiograph results. Coronal laxity is measured at 0 and 30 degrees of flexion. Anteroposterior laxity is evaluated by anterior and posterior drawer test, as well as the Clunk test. Correlation between the clinical examination and the dichotomized results of normal or excessive in stress radiography will be explored.