CORTICAL VOLUME MEASUREMENT IN UNCEMENTED TOTAL HIP REPLACEMENTS- A CT BASED ANALYSIS

Abstract

Periprosthetic bone remodeling after uncemented hip replacement has always been a matter of research and debate. DEXA analysis of bone density was studied by previous groups but not the cross sectional cortical volume. We report a validated CT based algorithm for accurate measurement of cortical volume in these group of patients.

Twenty two patients who have undergone Uncemented Furlong total hip replacement agreed to undergo CT scan of their hips for our study. The mean age was 74.6 yrs. The mean follow up was 5.4 yrs. Using software adapted for the specific purpose, femoral cortical volume was measured at three different levels at a fixed distance from the lower border of the lesser trochanter on both sides:

• 6mm distal to the tip of the prosthesis (z),

• At the top of the cylindrical portion(x)

• Midway between x and z (y).

Accuracy of the method was assessed by measuring the volume of artificial cavities created on a polyurethane pelvis. Assessment of precision of method was done by calculating the level of agreement between two observers.

The mean cortical volume in the proximal cylindrical portion (x), midpoint(y) and the portion of bone distal to the prosthesis (z) were 458 mm3, 466 mm3, 504 mm3 respectively. The corresponding cortical volumes in the contralateral native femur in unilateral hip replacements were 530 mm3(x), 511 mm3(y), 522 mm3 (z) giving a ratios of 0.86(x), 0.91(y) and 0.97(z). The mean cortical volumes on the left side of bilateral hips were 490 mm3(x), 499 mm3(y) and 528 mm3 (z). The mean cortical volumes on the right side were 456 mm3(x), 463 mm3 (y) and 516 mm3 (z). No significant trend was noted with change of volume of bone with time. In the three cases who had cemented hips on their other side, the cemented hips exhibited substantially more stress shielding than their cementless controls (ratios of 0.82, 0.74 and 0.85). A high correlation between the test and standard measurements was noted. The interobserver agreement between two observers was also good.

In a fully coated uncemented femoral component, with documented long term results, it is to be expected that load will be shed steadily along the length of the prosthesis. In this study we have confirmed this supposition, with volumetric data, by showing that an almost normal bone just below the tip of the stem (97% volume) reduces to a bone volume of 91% by the middle of the stem and then 86% by the shoulder of the prosthesis. This decrease in the volume of cortical bone effectively normal at the tip of the prosthesis while not optimal appears to stabilize early with no trend of continued reduction over a decade. The effect of cementation on stress shielding was only examined incidentally in this study but appears to contribute to more marked bone loss.