This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
This is the strongest ligament in the body. The single stem of the Y ligament is attached to the upper edge of the rim of the acetabulum just below the anterior inferior spine. Its two branches are attached below to the anterior intertrochanteric line. Its upper edge is reinforced by a band from the ilium to the trochanter, the iliotrochanteric band, and one from the reflected tendon of the rectus, the tendinotrochanteric band (Henry Morris) (Fig. 509).
The pubofemoral ligament, also called the pectineofemoral ligament, runs outward into the capsule from the horizontal ramus of the pubes. It is quite weak.
Allis describes this ligament as follows: "It arises from the ischial portion of the rim of the socket and sends its fibres to the capsule to be blended with them. As its fibres extend upward they separate like two fingers or terminal processes, the one extending forward to the base of the oblique (posterior) line, the other running backward to the digital fossa (Fig. 510)".
It will be observed that this makes it a posterior Y ligament with a distinct bony attachment for its two arms (like the external lateral ligament of the elbow - see page 283). The web of the two arms is half way down the posterior surface of the neck of the femur.
The capsule of the joint is composed of a thin sac strengthened by the band-like ligaments just described. Wherever there is no reinforcing band the capsule is weak. The posterior and lower portion is weaker than the anterior and upper portion. There is a weak spot between the arms of the iliofemoral ligament anteriorly, a branch of the circumflex artery usually entering here. Between the pubofemoral and inner edge of the iliofemoral ligament is another weak point. A bursa here separates the iliopsoas from the joint and often communicates with the joint. A third weak spot is on the lower posterior part of the neck between the two branches of the ischiofemoral ligament (Fig. 511). Injections into the joint protrude very markedly at this point. The weakest part of the joint is the lower anterior, below the pubofemoral ligament and opposite the cotyloid notch; the strongest part is the upper anterior part.
Fig. 510. - The ischiofemoral or posterior y-ligament. The stem of the Y is attached at the base of the tuberosity of the ischium and one branch is seen going toward the greater trochanter and the other toward the lesser, leaving a weak spot between them half-way down the neck of the bone.
Fig. 511. - Hip-joint distended with wax; the capsule ends posteriorly half-way down the neck and is seen distended by the injection material protruding between the two arms of the ischiofemoral ligament.