Large Diameter Heads
Stability Against Dislocation
Dislocation has been reported at rates of 2-10% for primary hip replacement and up to 20% in revision hips. 12 13 14 15 16 17 Dislocation is physically destructive, damaging abductor tissue. Up to two out of three recur, requiring reoperation. 15 16 17 Clinical reports have demonstrated lower dislocation rates for larger heads. 18 19 20 21
One goal in using larger heads is to reduce neck-cup impingement. Larger heads increase the range of motion before impingement, thus decreasing the chance of subsequent lever-out dislocation.
Larger diameter heads also increase the “jump distance” or the distance the head must displace before dislocation. For instance, the jump distance for a 28mm head is 14mm, while the jump distance for a 44mm head improves to 22mm.
Range of Motion of X-alt Highly Crosslinked Polyethylene Liners
Arc of motion of the Linear® hip stem in flexion-extension for the smallest FMP Acetabular shell size available at each head size.
Fig. 1. Tapered distal stem of the Linear hip stem.
Fig. 2. Linear Hip with a 40mm CoCr head and a FMP 56mm Hemispherical Shell.
| Head Size (mm) | 28 | 32 | 34 | 36 | 40 | 44 |
| ROM (°) | 130° | 133° | 135° | 138° | 144° | 147° |