Abstract
Introduction
The use of larger femoral heads in Total Hip Arthroplasty has increased in order to reduce the risk of dislocation and to improve the range of motion of the joint. In 2003, within the UK, the “standard” head size of 28mm was used in 73% of all hip procedures, whereas by 2012, this figure dropped to 36%. Concerns regarding the impact of this increment in head size on the cement and bone stresses have arisen; however, this has yet to be clearly determined.
Methods
To understand the relationship between femoral head size and cement mantle and bone stress in cemented hip arthroplasty, 3D-Finite-Element models of a hemipelvis with cemented cup[tb6] (50mm outer-diameter) were developed. Loading conditions of single-leg-stance (average and overweight) were simulated for three head sizes (28, 32 and 36mm). The models were validated with an in-vitro experiment using the average loading condition.
Results
Stresses were evaluated at the periacetabular bone and cement mantle. In the pelvic bone the peak von Mises stress value presented no change in magnitude due to change in head size for the average patient; for the overweight patient, there was a small increment. In the cement mantle, there was a noticeable difference in the pattern distribution and magnitude of the stresses for the two loading conditions[tb7]. For the average patient, average stresses in the cement were 1.7MPa, 1.8MPa and 1.9MPa for 28, 32 and 36mm heads, respectively; whereas for the overweight patient the stresses were 3.4MPa, 3.6MPa and 3.8MPa.
Conclusions
Pelvic bone remained largely unaffected by the changes in femoral head size. The major effect of femoral head size occurs in the stress level and stress distribution pattern in the cement mantle. The current predicted cement stresses are below the cement endurance limit, this indicates that the cement fatigue life is not affected by the increasing head size.
