As expected with the progression of hip OA, the hip begins to lose mobility in all directions along with a capsular pattern of restriction. During a FABERs test, the patient typically demonstrates a loss of mobility with the knee not being able to fall out as much, sticking up a lot more than normal.
One way to improve hip multiplanar mobility is to perform a mobilization technique in what I call a Modified FABERs. Place the patient prone with a pillow under their waist for unloading lumbar spine and place the involved hip into a relative upside-down FABERs position. Initially, you may have to keep them in less relative external rotation using your hand on the involved foot/ankle as this technique can feel very "vulnerable and sensitive" to the patient. Over time, they will be able to better position the foot/ankle on their popliteal fossa area for more relative ER.
***CAUTION *** Begin with much lighter pressure and lower grades I and II to help the patient get accustomed to the position and direction of force before progressing to grades III and IV.
As stated above, this is a relatively more advanced technique that can feel quite "vulnerable and sensitive" for the patient, which is completely normal. Choose the sets and reps as you would typically for mobilization. Personally, I like to perform approximately 2-4 sets of 20-40 oscillations, grades III and IV. Remember, this technique is typically performed after the patient is already comfortable with the beginning techniques of lateral distraction in neutral, IR, and ER as well as anterior for extension and inferior for flexion. This is not a technique that you would start with on a more sensitive and restricted patient.