Stress Knee Radiograph cover

Stress Radiographs of the Knee

Stress radiographs of the knee are used for many reasons during workup or recovery for a potential ligamentous injury or osteoarthritis. The most common stress radiographs include varus stress which stresses the lateral compartment ligaments, valgus stress for the medial compartment ligaments or posterior stress for the meniscofemoral or posterior knee ligaments. Most of the time a manual stress is used, or a stress provided by a healthcare professional and many times this is the ordering physician. There are issues performing stress radiographs if these are not a common part of the practice and setting the patient up can be difficult. There have been attempts recently to have patient directed stresses using certain props [1].

The most commonly injured area of the knee is the MCL, or medial collateral knee ligament, and stabilizers in the medial knee [3,4]. Physical examination is one of the first steps in diagnosis of knee injuries. This will usually include a valgus stress to the medial knee ligaments at both full extension and 30 degrees of knee flexion. Many providers will grade MCL injuries depending on this. The grading of these injuries depends both on relaxation of the patient and clinician’s ability to detect an endpoint. Many also describe laxity at full extension to be indicative of a and MCL and cruciate ligament injury [4,12]. Clinician experience, patient pain tolerance or guarding, and concurrent ligamentous injuries may skew physical examination interpretation, detracting in certain situations from its clinical use. This provides a more objective method of measuring medial knee injuries.

Stress radiographs can be used to judge the degree of instability with a measurement of medial compartment gapping. The medial collateral ligament is most commonly affected, but the medial knee does have additional stabilizers. There is a proximal and distal portion of the superficial MCL and a meniscofemoral and meniscotibial division of the deep MCL. There is also a capsular arm of the posterior oblique ligament, or POL [5,6].

Image 1. Illustration of the lateral side of the knee including the LCL, popliteus tendon and more (adopted from [12])

Current data has compared bilateral valgus stress radiographs in normal subjects and reported minimal physiologic side to side difference [7,8]. The value for gapping significant for superficial medial collateral ligament injury presents is 3.2 mm or greater. The value for involvement of most or all medial knee structures was found to be 9.8 mm. Both were done at 20 degrees knee flexion [9]. Valgus stress has also been used for workup of varus osteoarthritis in attempts to quantify cartilage thickness and was found to predict cartilage thickness, but not cartilage damage [10]. It has also been used for pre-operative planning of soft tissue balancing during TKA for mainly varus osteoarthritis and evaluation of medial knee stability during the postoperative period [11].
Image 2. Illustration of xray of the knee performed at 20 degrees of knee flexion (adopted from [9])
Image 3. Xrays showing valgus stress views (adopted from [12])
There are similar ideas and uses for lateral knee injuries and varus stress radiographs [13]. Although MRI is nearly universally ordered, recent studies have questioned the utility of this modality, with some studies reporting a sensitivity of only 58% for diagnosing FCL tears on MRI [19]. Lateral knee injuries can be difficult to diagnose and there seems to be more variability among lateral gapping when compared to the medial side [15,16]. Reconstructed cruciate ligaments may also fail with unrecognized or undertreated varus instability [13,14]. The main lateral stabilizers to the posterolateral knee are the fibular collateral ligament, the popliteus tendon and the popliteofibular ligament.
The physical examination usually is done with a varus stress test at full extension and at 20 or 30 degrees of knee flexion. A dial test is also routinely performed to test for a posterolateral corner or posterior cruciate ligament injury [13]. The dial test is done at 30 degrees and 90 degrees of knee flexion and is measured by external rotation. Valgus stress radiographs can aid and provide an objective measure. Side to side stress x-ray studies demonstrated a complete fibular collateral ligament tear demonstrating 2.7 mm of increased gapping. More recent studies have predicted this number to be closer to 2.0 [17]. An increase of 4.0 mm is felt to indicate an injury to three main static stabilizers for the posterolateral aspect of the knee. These tests, similar to valgus stress views, were performed at 20 degrees knee flexion [13]. Varus stress radiographs have also been used for planning of unicompartmental knee arthroplasty or measuring knee stabilizers after total knee arthroplasty.

Image 4. Demonstration of the flexed knee view at 20 degrees.

Image 5. Xrays showing varus stress views (adopted from [14])
Posterior cruciate ligament, or PCL, injuries likely also benefit from stress radiographs. Incidence of these injuries is variable and injuries mostly occur in the athletic population. The PCL has two bundles, the anterolateral and posteromedial bundles, and lies between the anterior meniscofemoral ligament (ligament of Humphrey) and posterior meniscofemoral ligament (ligament of Wrisberg). Grading PCL injuries can be very difficult with physical examination and PCL injuries are often missed at initial evaluation [20].

Image 6. Anatomy illustration of the ACL, PCL and other stabilizing structures (adopted from [21])

The posterior drawer test can be highly subjective and is one of the current tools for grading PCL injuries. The grading system is based on posterior subluxation of tibia relative to the femoral condyles at 90 degrees flexion. A grade I injury has a 1-5 mm posterior tibial translation and the tibia remains anterior to the femoral condyles. Grade II injury has a 6-10 mm translation and anterior tibia is flush with the femoral condyles. 

Grade III has greater than 10 mm of posterior tibial translation, tbia is posterior to the femoral condyles and usually indicated a combined PCL or capsuloligamentous injury. Varus and Valgus stress are also routinely performed with suspected PCL injuries. Posterior sag sign may also be documented and is performed by having the patient supine with hips and knees flexed to 90 and supporting the ankles. The clinician observes for a posterior shift of the tibia when compared to the opposite knee.
In order to allow a better objective quantification of abnormal compartment knee motion, stress radiographs have been recommended. Stress radiographs have been shown to be superior in quantifying posterior translation compared to clinical or arthrometer evaluation [22,23]. For kneeling stress radiographs, the patient kneels on a jig or radiopaque pad with the knee in 90 degrees flexion. The pad or bench supports the lower leg up to the tibial tubercle and the patella and the femoral condyles remained unsupported. It is important to ask the patient to place their weight down and relax while the radiographs are being done. The patient may also be in too much pain to perform these in the case of an acute injury.

Image 7. Demonstration of kneeling stress radiographs toe valuation the stability of the PCL (adopted from [21])

Image 8. Xray of kneeling stress radiographs showing increased posterior translation suggesting a PCL tear (Adopted from [21])

To perform the measurements, first a line was traced across the tibial plateau, then two perpendicular lines were traced passing through the midpoint between the most posterior edges of the medial and lateral condyles and the tibial plateau, respectively, parallel to the posterior tibial cortex. Then, the degree of posterior tibial displacement (PTD) is measured as the distance in mm between the two lines. A diagnostic algorithm has been validated where (1) 0–7 mm of side-to-side difference in posterior displacement constitutes a partial PCL tear, (2) 8–11 mm constitutes an isolated complete constitutes a combined PCL and posterolateral corner or posteromedial corner knee injury [24].
Lateral stress views for the PCL may also be performed and compared to the unaffected side. These are performed with the patient supine and the knee flexed to 70 degrees. A manual stress is applied to the anterior tibia and radiographs are taken. This is repeated for the opposite side and compared. An asymmetric posterior tibial displacement indicates PCL injury and contralateral knee differences >12 mm on stress views suggest a combined PCL and PLC injury.

Image 9. Anterior stress radiographs can also evaluate PCL integrity

In conclusion, stress radiographs can provide important objective measure for both isolated and multiligamentous knee injuries. They can be used for diagnosis, preoperative planning and postoperative management. These radiographs should be performed by specialists who are comfortable interpreting the results and additional training may be needed for radiology technicians or other healthcare professionals involved in the patient’s care. Values for instability have been quantified for lateral, medial and posterior knee injuries, although more studies are needed to confirm these values. Stress radiographs of the knee are as a tool to help with diagnosis and management of both acute and chronic injuries.
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