RESURFACING HIP REPLACEMENT: THE EFFECTS OF BONE DENSITY

Abstract

Introduction: The Resurfacing Hip has been increasingly popular for younger patients. Femoral neck fractures are still the main complication. The problems associated with cement such as thermal necrosis, cement debris and lack of long-term biological fixation, combined with the general use of cementless fixation in young patients invite the question whether a cementless component can be used for resurfacing hip replacement. Given that the cement may reinforce the femoral head preventing collapse, an additional question regarding the effect of bone density in cemented and cementless fixation can be asked. The hypotheses of the study are that:

1. High bone density will increase the yield point and stiffness of the femoral head and therefore improve the implant fixation.

2. Cement fixation will increase the yield point and stiffness of the femoral head, especially for the lower density bone compared with cementless fixation.

Materials and Methods: Thirty-six femoral head specimens were obtained from consented patients receiving routine hip arthroplasty. The heads were stored frozen at −20oC until use. pQCT was used to analyse trabecular bone density within each head. Specimens were ranked according to bone density and were assigned to high and low bone density groups. Cemented and cementless fixations were then alternatively assigned to individual heads in each group. Thus the 4 groups included in the study were: High density cemented, high density cementless, low density cemented, and low density cementless. Implantation of Birmingham resurfacing hips was carried out according to recommended surgical procedures. For cementing groups, surgical simplex P bone cement was used. Each sample was potted in a cylindrical polyethylene block for testing. A compressive load up to 5 or 10 KN using a Hounsfield Universal Testing Machine were applied on each sample at a rate of 1 mm min-1. Load versus displacement graphs were plotted for all tests. Yield point and stiffness were measured for each sample.

Results:

1. For yield point, there is no significant difference between cemented or cementless resurfacing (4169 ± 1420 N vs. 3789 ± 1461 N; P = 0.434). However, the high density heads provide a significantly higher yield point than low density heads (4749 ± 1145 N vs. 3208 ± 1287 N; P = 0.01).

2. The addition of cement significantly contributes to femoral head stiffness compared to cementless resurfacing (5174 ± 1730 N/mm vs. 3678 ± 1630 N/mm; P = 0.012).

Discussion: Bone density plays an important role in resurfacing hip arthroplasty. Higher bone density will reduce the incidence of fractures comparing with lower density. Therefore, resurfacing THR for the older patients and those with sub-optimal bone density should be used with caution. Consequently, it is suggested that a bone density scan should be routinely applied for those patients who are considered for resurfacing hip replacement. There is no difference between the cemented and cementless fixation in reducing femoral head failure, though cement will increase the stiffness of the bone. The study suggests that cementless resurfacing hip could be an alternative design with its clinical advantages of long-term osseointegration if implant is coated with bio-active materials.