|
Year |
2002 |
|---|---|
|
Question Number |
271 |
|
Category |
Hip and Knee Reconstruction |
Question
The rationale for using a larger diameter femoral head in THA is a lower
1) incidence of dislocation
2) incidence of poly component dissociation from the metal shell
3) manufacturing cost
4) rate of volumetric wear
5) rate of acetabular component loosening
Figures
Answer (click for answer)
The correct answer is 1.
There are several reasons why using a larger femoral head decreases the incidence of dislocation. The primary arc range of the hip defines the amount of arc ball and cup articulation moves before impinging and levering out. The major determinant of primary arc range is the head to neck ratio. A larger head to neck ratio will allow for further motion before impingement will occur. A second factor is once impingement occurs the head will start levering out of the socket. The distance the head must travel is call excursion. The greater the distance the head must travel to dislocate the more stable it will be. By using a larger femoral head you increase the excursion distance which is defined as one half the diameter of the head. The downside of using a larger head is increased volumetric wear. Acetabular loosening is not affected nor is dissociation from the metal shell.
Reference: Bartz RL, Nobel PC, Kadakia NR, Tullos HS: The Effect of femoral component size on posterior dislocation of the artificial hip joint. J Bone Join Surg Am 2000;82:1300-7