Screw Fixation of Acetabular Components : Wheeless' Textbook of Orthopaedics

- See: Acetabular Component Menu

Discussion

it is controversial as to whether screws are necessary in THR, and there 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:
advantages of 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;
biomechanical considerations: (design of components)
- references:
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;
- in order to reduce the chance of osteolysis some surgeons insert the least number of screws possible in order 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;
- Dorr et al 1998 and by Latimer 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;

Radiographic Evaluation of Screw Position

Judet views;
references:
- Radiographic Evaluation of Screw Position in Revision Total Hip Arthroplasty

Screw Placement Considerations

main issue is to avoid vascular injuries:
note that a relatively retroverted, vertically inserted cup will be at most risk for neurovascular injury;
hence, an optimally anteverted and positioned acetabular component will have the least risk of injury;
when in question, screws aimed just anterior to the sciatic notch, will find safe passage in strong bone;
quadrant system:
- based on line from ASIS through center of acetabulum;
- safe quadrant is defined by two lines from the anterior-inferior iliac spine through the center of the acetabulum and posterior by a line from the sciatic notch to the center of the 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 sciatic notch is especially strong);
  - usually do not need screws greater than 35 mm long are placed in the posterosuperior quadrant
  - 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;
  - fixation of screws placed posterosuperiorly may be achieved without drilling through the second cortex, bicortical fixation is usually unnecessary;
  - with osteoporotic bone, firm fixation of screws placed superiorly often requires that the screws reach proximally enough to contact both cortices;
  - avoid bicortical screw placement with patients on anticoagulants;
    - Retroperitoneal hematoma with bone resorption around the acetabular component after total hip arthroplasty: a case report and review of the literature
- posteroinferior quadrant:
  - in posteroinferior quad, screws are rarely longer than 20-25 mm;
  - need to ensure that screw does not impale sciatic nerve;
  - ref: Does ischial screw fixation improve mechanical stability in revision total hip arthroplasty?
- anterior-superior quadrant:
  - external iliac vein is most at risk w/ anterosuperior screws, but the iliac artery may be damaged as well;
    - located within 7 mm of the anterior column of the pelvis at the anterior inferior iliac spine and within 4 mm at the acetabular dome;
  - external iliac artery: within 10 mm of the bone at the anterior inferior iliac spine and within 7 mm at the acetabular dome
  - references:
    - Iliopsoas Tendonitis due to the Protrusion of an Acetabular Component Fixation Screw After Total Hip Arthroplasty.
    - Screw-augmented fixation of acetabular components. A mechanical model to determine optimal screw placement.
- anterior-inferior quadrant:
  - obturator artery is most at risk with anteroinferior screws;
  - screws may injure obturator nerve, artery, and vein;
  - quadrilateral surface: interposition of the obturator internus muscle between the obturator vessels;

Other Considerations

if drill holes are placed eccentrically, then screw insertion may lift cup up out of the bony bed;
screw heads left pround wont allow liner to be fully seated