Femoral Head Size: Bigger Is Better
At ICJR’s Pan Pacific Orthopaedic Congress, Dr. Adolph Lombardi discussed the benefits of using larger femoral heads in primary total hip arthroplasty. Following is the abstract of his presentation.
Dislocation continues to be an unfortunate, frequent complication of primary total hip arthroplasty (THA). Reported incidence ranges from less than 1% to 5% with a recent Medicare claims data analysis of 58,521 patients reporting 3.9% dislocation in the first 26 weeks postoperative. 
Numerous patient factors may contribute to increased dislocation risk:
- Female gender
- Lower muscular disorders
- Cognitive dysfunction
- Previous femoral neck fracture
- Rheumatoid arthritis
The correlation between higher surgeon and hospital volume and lower dislocation rates has also been established. 
Recent advances in implant design allow for use of larger prosthetic heads that more accurately reconstruct native femoral head size and improve head-neck ratio. The desire for larger heads is based on literature that shows a direct relationship between increasing femoral head size improving implant stability. [3-10] Because the neck of a femoral component is much smaller than the native human femoral neck, matching head size markedly improves head/neck ratio – a benefit in younger patients and those with high-demand lifestyle.
Increased volumetric polyethylene wear has previously limited femoral head size to 32 mm or less. However, the development of highly cross-linked polyethylene and its superior wear characteristics allows use of larger – greater than 32 mm – femoral heads, with several reports documenting improved wear characteristics independent of head size. [11-20] Large femoral heads offer the benefit of increasing the ROM before component-to-component impingement, while increasing the displacement necessary before dislocation.
It has been recognized that by increasing femoral head component size, a corresponding increase in head-neck ratio occurs, thereby allowing greater ROM.  A study conducted on an anatomic full size hip model found femoral heads greater than 32 mm virtually eliminate component-to-component impingement. 
The benefit of increased head size was seen in a study of 22-mm and 40-mm femoral heads; the displacement required for dislocation increased by approximately 5 mm with 40-mm heads when the acetabular component is in 45 ̊ of abduction. 
A number of studies have shown increasing femoral head size increases implant stability, thereby reducing postoperative dislocations. [3,5-8,23] We previously reported an incidence of 12 dislocations in 1,518 primary THAs (0.8%) done with femoral heads 32 mm or less through a standard direct lateral approach.  More recently, we reported on dislocation rates in primary THA with large-heads ≥36mm in diameter in several material combinations in 1748 patients (2020 hips).  With mean follow up of 2.8 years only 1 dislocation occurred, for a rate of 0.05%.
Dr. Lombardi’s presentation can be found here.
Adolph V. Lombardi Jr., MD, FACS, is from Joint Implant Surgeons, Inc., New Albany, Ohio.
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