- See:
Vascular Injuries in THR:
- Discussion:
- it is controversial as to whether screws are necessary in THR, and their are relative advantages and disadvantages;
- cups should be designed with only 2-3 holes so as to maximize the porous coating surface and to minimize conduit for osteolysis;
-
acetabular cup insertion without screw insertion:
- advantages:
- shorter operative time;
- ability to adjust cup position if hip instability is found to be present;
- absence of screw related vascular complications;
- easier cup removal at the time of revision (should it be necessary);
- technical considerations:
- cup should be oversized (or acetabulum should be under-reamed);
- note that insertion of oversized cups may result in fracture, especially in over-sized bone;
-
acetabular cup insertion with screw insertion:
- theoretically, screw fixation can close any gap created by non congrous reaming;
- prevents the loss of compressive stresses and resultant shear stresses;
- may be indicated in osteoporotic bone (when insertion of over-sized components may not be wise) and when additional fixation is required;
- screw insertion is also indicated in situations in which there is insufficient bony coverage;
-
controversies: Do acetabular screws promote
osteolysis?
- although screws may provide excellent initial fixation, later they may be sources of fretting, which could produce wear debris, and provide a conduit
for migration of the polyethylene debris;
- inorder to reduce the chance of
osteolysis some surgeons insert the least number of screws possible inorder to achieve solid fixation;
- while screws may provide a conduit for wear debris, unfilled screw-holes may also provide access for
wear debris from ultra-high molecular weight
polyethylene liner to area behind ingrowth cup;
- ultra-high molecular weight polyethylene may cold-flow (creep) into these holes, resulting in early failure of the polyethylene liner;
- hence, it remains unclear as to whether screws strongly influence the formation of osteolysis in acetabular components which contain screw holes;
- as pointed out by Dorr et al 1998 and by Latimer and Lachiewicz 1996, there is a paucity of evidence to support notion
that screw fixation leads to osteolysis;
- certainly, if a screw does not achieve a rigid bite it should be removed - due to the risk of osteolysis;
- Screw Placement Considerations:
- if drill holes are placed eccentrically, then screw insertion may lift cup up out of the bony bed;
- also screw heads left pround wont allow liner to be fully seated;
-
quadrant system:
- based on line from
ASIS thru center of acetabulum;
- screws placed thru posterosuperior & posteroinferior quadrants do not emerge within the pelvis;
-
posterosuperior quadrant is the safest;
- in posterosuperior quad, screws > 25 mm frequently are placed thru strong bone in this area (bone around the sciatic notch is especially strong);
- screws may pass into
sciatic notch and endanger
sciatic nerve and
superior gluteal vessels;
- protect sciatic nerve durig placement of screws, w/ finger along sciatic notch to prevent penetration of drill or screw;
-
posteroinferior quadrant:
- in posteroinferior quad, screws are rarely longer than 20 mm;
- need to ensure that screw does not impale
sciatic nerve;
-
anterior-superior quadrant:
- iliac vein is most at risk w/ anterosuperior screws, but the iliac artery may be damaged as well;
-
anterior-inferior quadrant::
-
obturator artery is most at risk with anteroinferior screws;
- screws may injure
obturator nerve,
artery, and vein;
Structures at risk from medially placed acetabular screws.
The relationship of the intrapelvic vasculature to the acetabulum. Implications in screw-fixation acetabular components.
Acetabular anatomy and the transacetabular fixation of screws in total hip arthroplasty.
The Harris Galante porous coated acetabular component with screw fixation: Radiographic analysis of 83 primary hip replacements at a minimum of five years. TP Schmalzried et al.
JBJS. Vol 74-A. 1992. p 1130-1139.
Hemispheric titanium porous coated acetabular component without screw fixation. LD Dorr et al.
CORR. No 351. p 158-168.
1998.
Screw Augmentation: the gold standard for cementless cup fixation. JJ Callaghan MD. Orthopedics. March 2000. Vol 23. No 3. p 204.
Extra-large press-fit cups without screws for acetabular revision
Radiographic Evaluation of Screw Position in Revision Total Hip Arthroplasty
Neurovascular Injuries in Acetabular Reconstruction Cage Surgery. An Anatomical Study.
