Both the Medium and Narrow Bambach Saddle Seats are 17” across at the rear, but this measurement is irrelevant because the body and thighs extend beyond the edges of the seat itself. The real issue is the spread of the knees (hip abduction). Unfortunately, the width of the front part of the seat (the pommel) is also irrelevant, because the thighs drape across the seat at an angle which can vary depending upon seat height and the user’s anatomy (e.g., muscular or thick thighs vs. thin or soft thighs). It’s the front dimension (the pommel) that affects the spread of the knees (hip abduction) and the position can vary from individual to individual in the same seat.
People who have had hip surgery rarely have problems fitting into the Bambach Saddle Seat. However, there can be problems when the joint itself is sensitive to direct pressure. This occurs when the muscles surrounding the hip joint are atrophied to the point they no longer cushion the hip joint, or when the joint is inflamed (e.g., active arthritis). In these cases, any pressure to the upper thigh/inner buttock area can be uncomfortable.
The only way to know for sure is to try the seat in person. If you are in the San Francisco Bay Area, are welcome to trial the various saddle seats in our office.

