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THR: Femoral Head Size

- Discussion:
       - polyethylene liner: (traditional design)
              - references:
                      - Effect of femoral head size on wear of the polyethylene acetabular component.
                      - Independent contribution of elevated-rim acetabular liner and femoral head size to the stability of total hip implants.
       - 22 mm head
              - produces too much linear wear or creep;
              - smaller diameter head (22 mm) allow less stress/torque but may result in increased central acetabular wear and dislocation;
       - 28 mm head:
              - less acetabular strain and lower revision rates are associated w/ use of a twenty-six or 28 mm head;
              - smaller diameter head (22-28 mm) allow less stress/torque but may result in increased central acetabular wear / dislocation;
              - ref: Dislocation rate after hip arthroplasty within the first postoperative year: 36 mm versus 28 mm femoral heads.
       - 32 mm head:
              - 32 mm allow increased ROM before it impinges against acetabulum, and therefore theoretically has less dislocation;
              - main disadvantage is that it has less net wall thickness, and produces too large a volume of wear debris;
       - 36 mm head and larger;
              - larger head sizes (32-36 mm) allow increased ROM and reduced dislocation, but have less net wall thickness for
                       long term wear (unless the liner is lateralized); 
              - psoas tendonitis
              - references:
                     - Is There a Benefit to Head Size Greater Than 36 mm in Total Hip Arthroplasty?
                     - No functional benefit of larger femoral heads and alternative bearings at 6 months following primary hip replacement
                     - Large diameter heads: is bigger always better?
                     - Dislocation rate after hip arthroplasty within the first postoperative year: 36 mm versus 28 mm femoral heads.

       - metal on metal designs begin at 36 mm and go upto greater than 50 mm;
              - the larger femoral head size allows for increased stability and less impingement;
              - references:
                   - Why large-head metal-on-metal hip replacements are painful: the anatomical basis of psoas impingement on the femoral head-neck junction.

     - references:
                   - Size of the femoral head and acetabular revision in total hip-replacement arthroplasty.
                   - Range of Motion and Stability in Total Hip Arthroplasty With 28-, 32-, 38-, and 44-mm Femoral Head Sizes.
                   - Effect of Femoral Head Diameter and Operative Approach on Risk of Dislocation After Primary Total Hip Arthroplasty.
                   - Relationship of femoral head and acetabular size to the prevalence of dislocation.