Utilizing acetabular screws in total hip arthroplasty is
controversial. Screws can compress the cup and close any gap. Screws could be
helpful in osteoporotic bone and when the bone coverage is not sufficient.
Screws and its holes can be conduit for migration of polyethylene debris that
can cause osteolysis (The surgeon should insert the least number of screws).
Many surgeons prefer to use screws in the acetabular cup. The quadrant
technique is a safe technique of screw placement. It provides the surgeon with
a simple, basic guide during surgery for safe placement of the screws during the
primary and revision surgery.
In order to find the safe zone for the placement of
acetabular screws and avoid the danger zone, you will need to draw a line from
the Anterior Superior Iliac Spine (ASIS). Then you will draw a second line,
perpendicular to the first line. Drawing these lines will divide the acetabulum
into four quarters. The posterior-superior quarter is considered the safe zone.
There is minimal risk to the sciatic nerve, superior gluteal nerve, and
vessels. It is important to direct the screws away from the notch. The
anterior-superior and anterior-inferior quarters are considered to be the
danger zone due to the external iliac artery and vein as well as the obturator
nerve, artery, and vein are at risk. The posterior-inferior quarter is
considered the caution zone. Placing screws here can be safe if the screws are
less than 20mm. There are still several risks when placing screws in this area
and these risks include: the sciatic nerve, inferior gluteal nerve and vessels,
internal pudendal nerve and vessels.
A retroverted vertical cup will be at most risk for injury
to neurovascular structures. The structures can be at risk in a high hip center,
which is reamed superiorly ½ the native acetabular diameter. The new hip will
have different anatomic zones than what the surgeon might think. The
antero-superior and the antero-inferior quadrants are positioned in the
postero-superior quadrant (Be aware of the shift in the zones). The insertion
of the screws as guided by the quadrant method is based on a study done by Dr.
Ray Wasielewski and his colleagues. When in doubt, aim the screws anteriorly to
the greater sciatic notch. The bone will be stronger and thicker. Screws longer
than 35mm are usually not needed. Avoid drilling eccentrically because it may
lift up the liner. The screw must be fully seated and the head of the screw
should not be proud. If the screw is proud, the liner may not be seated
properly.
Vascular injuries occur more in females and more often in the
left hip.
