Abstract
In the anatomical position the tendon of this powerful muscle curls round the inferior aspect of the neck of the femur to its insertion into the greater trochanter. A component of its action is therefore one of external rotation. However in flexion the tendon eventually comes to lie parallel with the neck of the femur; it is then purely an external rotator.
During an action as when rising from a sitting posture, it is commonly thought that the head of the femur is stressed backwards so as to produce an internal rotation strain at the shaft neck junction and thereby, in a prosthetic replacement, a potentially prejudicial torque is created around the shaft of the femoral component. It is suggested that the action of the obturator externus, in particular, prevents this by converting the torque into compression along the neck of the natural femur or of a prosthesis; this is a condition which is suited to the material properties of bone in the natural state.
This paper contrives to demonstrate the evidence for the mechanism described.
Obturator externus is less frequently violated than other muscles around the hip; however, it is suggested that all of these should be carefully preserved or repaired effectively because they all contribute to a similar conversion of otherwise bending forces around the neck, to compression. In our quest to identify minutiae that contribute to long term survival of hip replacements, this may be another factor for consideration.
Theses abstracts were prepared by Mr Peter Kay. Correspondence should be address to him at The Hip Centre, Wrightington Hospital, Appley Bridge, Wigan, Lancashire WN6 9EP.
